Introduction
The southern segment (41.5°S–46°S) of the Southern Volcanic Zone (SVZ) includes 13 active volcanic centers in Chile, but until some years ago, only the Michinmahuida and Hudson volcanoes had documented historical eruptions (Naranjo and Stern, Reference Naranjo and Stern1998; Stern, Reference Stern2004). The unexpected eruption of Chaitén in 2008, however, exposed significant gaps in the understanding of the eruptive history from the SVZ in general and from Chaitén in particular (Carn et al., Reference Carn, Pallister, Lara, Ewert, Watt, Prata and Thomas2009; Alfano et al., Reference Alfano, Bonadonna, Volentik, Connor, Watt, Pyle and Connor2011). Initial studies by Naranjo and Stern (Reference Stern2004) identified and mapped tephra deposits from Chaitén, including the Cha-1 layer as the most recent eruptive event up to that moment, dated to <9370 ± 60 1⁴C yr BP (10,533 cal yr BP). The 2008 Chaitén event, consequently, prompted renewed field investigations to assess the volcano’s past activity, including other volcanic centers, and its potential hazards (e.g., Lara, Reference Lara2009; Amigo et al., Reference Amigo, Lara and Smith2013; Watt et al., Reference Watt, Pyle and Mather2013; Alloway et al., Reference Alloway, Pearce, Moreno, Villarosa, Jara, De Pol-Holz and Outes2017).
To address these gaps, researchers have turned to alternative methods for reconstructing eruptive histories. Explosive volcanic eruptions generate tephra layers that serve as valuable chronostratigraphic markers, allowing for the correlation of sedimentary sequences across extensive spatial and temporal scales (Fisher and Schmincke, Reference Fisher and Schmincke1984). Additionally, high-resolution sedimentary records from lake cores have been highly promising in documenting the frequency and magnitude of past explosive eruptions around the world, including globally important projects (Lowe, Reference Lowe2011). Such studies on lacustrine records have been conducted within the SVZ, particularly in its southern segment, where studies performed over the last decades have provided valuable insights into previously unknown volcanic events, especially for volcanoes such as Chaitén and Michinmahuida (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014; Daga et al., Reference Daga, Guevara and Arribére2016; Serra et al., Reference Serra, Massaferro and Villarosa2021). Tephra layers preserved in lacustrine cores provide crucial information on past volcanic activity from multiple sources, while their mineralogy, texture, and mainly geochemical analyses help to establish stratigraphic correlations over large areas (Lowe et al., Reference Lowe, Pearce, Jorgensen, Kuehn, Tryon and Hayward2017). Combined with radiocarbon dating and volcanic source identification, these studies have significantly advanced our understanding of tephrochronology.
Lakes located on the eastern slopes of the Andes are particularly valuable due to their position relative to the SVZ and the prevailing westward wind direction (westerlies) at these latitudes (Garreaud et al., Reference Garreaud, Vuille, Compagnucci and Marengo2009), and because they provide ideal conditions for tephra preservation (e.g., Fontijn et al., Reference Fontijn, Lachowycz, Rawson, Pyle, Mather, Naranjo and Moreno-Roa2014). Most large explosive eruptions during the Late Quaternary resulted in distal tephra fallout deposits across the Argentine Patagonian steppe, driven by the dominant westerlies. These lakes (east of the Andes) provide detailed records of past eruptions, which are essential for understanding the long-term behavior of SVZ volcanoes and for assessing future volcanic hazards. Furthermore, tephra deposits also contribute valuable multiproxy data for disciplines such as archaeology, paleoclimatology, and geomorphology, offering insights into past environmental changes and providing important chronostratigraphic tools (Lowe, Reference Lowe2011; Fontijn et al., Reference Fontijn, Lachowycz, Rawson, Pyle, Mather, Naranjo and Moreno-Roa2014).
Despite the recognized importance of tephra layers as chronostratigraphic markers, comprehensive studies on the eruptive frequency of SVZ volcanoes remain limited. This gap in knowledge hinders efforts to accurately predict the recurrence of future eruptions and to assess the potential hazards they pose. Moreover, key tephra horizons from moderate to large eruptions of other volcanoes in the region are also being found in coarser and thicker subaerial sedimentary records, offering significant insights and possible correlations (Naranjo and Stern, Reference Stern2004; Fontijn et al., Reference Fontijn, Lachowycz, Rawson, Pyle, Mather, Naranjo and Moreno-Roa2014).
In this study, we aim to correlate tephra layers from different sediment cores to better understand their volcanic sources, their spatial distribution, and the chronology of eruptive events that produced them. To accomplish this, we analyzed mainly the geochemical signatures together with mineralogy and morphology of tephra components (including pumice, glass shards, scoria) from four short lacustrine cores. The study sites—Lakes Rivadavia, La Zeta, Brychan, and Theobald—are located on the eastern slope of the Andes (Argentina), downwind to the east of the SVZ. Characteristic multielement patterns of pumice and glassy fraction were identified for each tephra layer, forming the basis for our correlation efforts, supported also by mineralogy and morphology/texture of components. Additionally, radiocarbon dating was employed to refine the chronological framework of the tephra deposits. This approach allowed us to assemble a geochemical inventory of key tephra marker horizons, both newly identified and previously described, with significant potential for regional and interregional stratigraphic correlation within the broader SVZ sedimentary records.
Geological background
The study area is situated on the eastern slope of the Southern Andean Cordillera, near the SVZ (33°S–46°S). This SVZ encompasses at least 60 active and potentially active volcanoes, 3 giant silicic caldera systems, and minor eruptive centers (Stern, Reference Stern2004). The area belongs to the Northern Patagonian Cordillera geological province, which comprises the Patagonian Andes developed in Argentina and Chile between 39°S and 45°S. The relief is characterized by two geographically distinct regions: the Andean region belonging to the Northern Patagonian Cordillera, and the extra-Andean region. Holocene glaciation variations have significantly reshaped the landscape of the region, enlarging and deepening existing river valleys and resulting in the contemporary numerous lakes found in Patagonia.
In Patagonia, the climate is dominated by the seasonal air mass movements from the Pacific Ocean and the presence of the Andes as a topographic barrier along the Pacific coast (Paruelo et al., Reference Paruelo, Jobbágy and Sala1998). This combination generates strong and persistent westerly winds across the region, while the location of the Andes creates a pronounced west-to-east precipitation gradient. Fall–winter precipitation exceeds 2000 mm/yr at the eastern base of the Andes but decreases exponentially to less than 200 mm/yr on the steppe. The mean annual temperature ranges approximately between 6°C and 8°C (Paruelo et al., Reference Paruelo, Jobbágy and Sala1998; Carabelli et al., Reference Carabelli, Scoz, Claverie, Jaramillo and Gómez2006; Roig and Villalba, Reference Roig and Villalba2008). Consequently, the region under study, located in the northwest of Patagonia, features numerous permanent lakes and the persistent westerly winds, factors that make it a suitable site for studying pyroclastic eruptions originating from the SVZ (Fig. 1).
Location of sampling sites: Lakes Rivadavia, La Zeta, Brychan, and Theobald (Argentina), and their positions in relation to the nearest volcanoes (Chile) in the Southern Volcanic Zone (SVZ).
Study site
Lakes under study span a latitudinal range between 42.5°S and 43.5°S, covering around 90 km (Fig. 1) in a region with limited tephrostratigraphic records, particularly for the last millennia. Lake Rivadavia (42.56°S, 71.65°W) is situated within Los Alerces National Park and is part of the Futaleufú basin, which drains into the Pacific Ocean. Located at an altitude of 527 meters above sea level (m asl), it covers an area of 21.7 km2 with a maximum length of 13 km, a maximum width of 2.5 km, and a maximum depth of 147 m. It exhibits a warm monomictic thermal regime (Modenutti et al., Reference Modenutti, Queimaliños, Balseiro and Reissig2003). Lake La Zeta (42.89°S, 71.35°W) is within the La Zeta Urban Natural Reserve, 7 km west of Esquel city. This oligotrophic waterbody (Serra et al., Reference Serra, Massaferro and Villarosa2021) is located at 774 m asl and covers an area of 0.6 km2. Lake depth reported in the literature varies, with maximum depth ranging from 6.75 m (Iglesias et al., Reference Iglesias, Whitlock, Markgraf and Bianchi2014) to 11 m (Martínez Peck, Reference Martínez Peck1994), and 8.9 m during the present fieldwork. The lake has low oxygen concentrations and lacks thermal stratification (Baigún et al., Reference Baigún, Mugni and Bonetto2006). The contributing watershed, originally about 12 km2, was extended to approximately 18 km2 following the construction of a diversion channel in the 1980s. Stream inflows are scarce and temporary, primarily during rainy or thaw periods, discharging by Arroyo Seco (Araqué, Reference Araqué2015). Lake Brychan (43.05°S, 71.48°W) is a small waterbody (0.27 km2 surface area) currently located within the Brychan Natural Reserve. Situated at an altitude of 450 m asl and 2.8 km northwest of the town of Trevelin, it features a permanent water regime. The maximum depth recorded in this study using an echo sounder, was 8.8 m. Lake Theobald (43.43°S, 71.55°W) is situated adjacent to Provincial Route 17 in Chubut Province, 40 km south of Trevelin and 12 km north of the town of Corcovado (Fig. 1). This waterbody covers 0.92 km2 and is located at an elevation of 678 m asl. The lake has a maximum depth of 6.1 m (this study) and is situated within a late Pleistocene glacial depression (Iglesias et al., Reference Iglesias, Whitlock, Markgraf and Bianchi2014). Its watershed covers approximately 8 km2.
Materials and methods
Sediment recovery
Short sediment cores were obtained from Lakes Rivadavia (RI, 75 cm length, at 90 m water depth), La Zeta (ZE, 66 cm length, at 8.2 m depth), Brychan (Bry, 83 cm length, at 8 m depth), and Theobald (TH, 82 cm length, at 6.1 m depth) (Supplementary Fig. S1) using a messenger-activated gravity-type corer. Sampling sites were carefully selected in flat regions of the lake bottom by inspecting bathymetric maps and using echo soundings, ensuring minimal influence from terrigenous material or lake-bottom topography on hemipelagic sedimentation.
Core processing
Sediment cores were longitudinally cut, opened, photographed, and described (color, grain size, contacts), and volumetric magnetic susceptibility (MS) profiles were obtained using a Bartington MS3-MS2E point sensor meter every 0.5 cm. Subsequently, the cores were subsampled at 1 cm intervals or following observed lamination boundaries. Each subsampled sediment layer was freeze-dried until it reached a constant weight. Based on the recorded weights, it was possible to calculate the bulk dry density of each subsample.
Chronology
A Bayesian age–depth model for each lake under study was constructed using 210Pb and 137Cs techniques (Joshi and Shukla, Reference Joshi and Shukla1991; Robbins and Herche, Reference Robbins and Herche1993; Ribeiro Guevara and Arribére, Reference Ribeiro Guevara and Arribére2002; Ribeiro Guevara et al., Reference Ribeiro Guevara, Rizzo, Sánchez and Arribére2003) and accelerator mass spectrometry (AMS) radiocarbon dating. Specific activity profiles of 210Pb, 226Ra, and 137Cs were measured using high-resolution gamma-ray spectrometry (Ribeiro Guevara and Arribére, Reference Ribeiro Guevara and Arribére2002; Ribeiro Guevara et al., Reference Ribeiro Guevara, Rizzo, Sánchez and Arribére2003) with self-shielding corrections according to Furci et al. (Reference Furci, Arribére and Guevara2013). As macrofossils were not identified in all cores, AMS radiocarbon dating was performed on bulk sediments from two or three layers at each core at the Silesian University of Technology (Poland) (Ustrzycka et al., Reference Ustrzycka, Piotrowska, Kłusek, Pawełczyk, Michczyńska, Michczyński, Kozioł and Jędrzejowski2025). The age–depth model was performed using OxCal 4.4.4 (Bronk Ramsey, Reference Bronk Ramsey2021; https://c14.arch.ox.ac.uk/oxcal/OxCal.html), with the latest Southern Hemisphere radiocarbon calibration data SHCal20 (Hogg et al., Reference Hogg, Heaton, Hua, Palmer, Turney, Southon and Bayliss2020). The OxCal 4.4.4 Bayesian model (Bronk Ramsey, Reference Bronk Ramsey2008) employed to constrain the chronology and interpolate between dated levels incorporated prior information about the sequence (core boundaries, depositional order, and depths) along with measurements (uncalibrated 14C ages and calibrated 137Cs points) (Bronk Ramsey, Reference Bronk Ramsey2008). The P_Sequence model (Bronk Ramsey, Reference Bronk Ramsey2008) was applied with the k-parameter set to 2/cm, adjusted for cores with minimal sedimentation rate changes to decrease uncertainty between dated depths and reduce “bowing” effects (Bronk Ramsey, Reference Bronk Ramsey2008; Parnell et al., Reference Parnell, Buck and Doan2011). The depth of the cores was corrected by subtracting tephra layers >0.5 cm thick, which are assumed to have been instantaneously deposited. Only the most recent tephra identified in the core was included in the model due to its confirmed correspondence with the 2008 Chaitén eruption. Ages are reported as calibrated calendar years CE at the 95.4% (2σ) confidence level (Table 1).
Calibrated radiocarbon dates from lakes under study using OxCal 4.4 (Bronk Ramsey, Reference Bronk Ramsey2021) and the Southern Hemisphere radiocarbon calibration data SHCal20 (Hogg et al., Reference Hogg, Heaton, Hua, Palmer, Turney, Southon and Bayliss2020).
a Adjusted depth corresponds to depth discarding the thickness of tephra layers.
Tephra identification, granulometry, and morphological characterization
Coarse tephra layers were first visually identified with the naked eye based on color and grain size, and their pyroclastic origin was confirmed by examining componentry using a binocular magnifying glass. In other instances, magnetic susceptibility (and later density depth curves) was used to identify cryptotephras without any discernible visual feature on the open core, later confirmed by a higher concentration of vitric fragments compared with adjacent layers. The identified tephra deposits were dry-sieved using ASTM mesh sieves with openings of 16, 8, 4, 2, 1, 0.5, 0.25, 0.125, and 0.063 mm, and classified according to the criteria of McPhie et al. (Reference McPhie, Doyle and Allen1993).
The tephra layers were first described under a binocular magnifying glass to identify macroscopic characteristics and classify the different components in each layer. Juvenile volcanic components, such as white pumice, scoria fragments, and glass shards were identified based on characteristics such as shape, color, and texture. From this characterization, some layers were reassigned as volcaniclastic layers considering the componentry and morphology of particles (see “Sedimentary Record” section). Afterward, relevant particle types from each pyroclastic and volcaniclastic layer were mounted on sample holders using adhesive film and sputtering for morphological characterization with a scanning electron microscope (SEM FEI Inspect-S50) at the Departamento de Caracterización de Materiales, Centro Atómico Bariloche. Additionally, polished resin sections were prepared by embedding selected pumice, glass shards, scoria, and crystals in epoxy, with particles isolated manually under a binocular magnifying glass. The stubs were polished and subsequently analyzed under an SEM in backscattered electron (BSE) mode at 20 kV to examine vesicularity, internal textures, mineral phases, glass matrix presence, and semiquantitaive compositional characteristics.
Geochemistry
To evaluate the elemental concentrations of tephra layers, separated samples of volcanic components (i.e., pumice with and without phenocrysts, glass shards of different morphologies and color, and scoria fragments) were meticulously prepared for geochemical analysis to enable correlation with volcanic sources. Subsamples of each component (from the different layers and different grain sizes when possible), ranging from 2.5 to 10 mg, were separated under a magnifying glass. These subsamples were then analyzed for bulk composition using instrumental neutron activation analysis (INAA) at the RA-6 nuclear research reactor (Centro Atómico Bariloche). Each sample underwent dual irradiations: a short one lasting 1 to 2 minutes, and a longer one spanning 6 hours. Following irradiation, gamma-ray spectra were collected at different decay times. These spectra were collected using a high-purity germanium detector and a multichannel analyzer. After the 6 hour irradiation, samples were transferred to non-irradiated containers to avoid interference due to irradiation vial impurities. In cases where the sample mass was low, measurements were taken without transfer, and corrections were applied for interference associated with irradiation vial impurities. The comprehensive elemental analysis included major (Al, Ca, Fe, Mg, Mn, Na, K, Ti), rare earth (La, Ce, Nd, Sm, Eu, Tb, Tm, Yb, Lu), and trace elements (Sb, As, Ba, Br, Cs, Zn, Co, Cr, Hf, Sc, Sr, Ta, Th, U, V). Major elements were converted to oxides, and SiO₂ content was estimated by mass difference for comparison (Alfonzo et al., Reference Alfonzo, Daga, Demichelis, Goldmann and Guevara2025). Analytical errors varied for each element and sample, depending on nuclear parameters, irradiation conditions, and sample composition, typically ranging from 5% to 15%. Certified reference material CRM 2709a San Joaquin Soil was analyzed alongside the samples for analytical quality control (Supplementary Tables S1 and S2).
Age–depth models for the sequences under study using OxCal 4.4 (Bronk Ramsey, Reference Bronk Ramsey2021), with Southern Hemisphere radiocarbon calibration data SHCal20 (Hogg et al., Reference Hogg, Heaton, Hua, Palmer, Turney, Southon and Bayliss2020).
Results
Age–depth modeling
The age–depth models for the lacustrine sequences from Lakes Rivadavia, La Zeta, Brychan, and Theobald were obtained by Bayesian models combining the age information provided by different methods (Fig. 2). For each lake, 210Pb and 137Cs dating was performed by evaluation of the specific activity in the upper layers. In all sites, the 210Pb-specific activity profile does not exhibit the exponential profile required for dating, sometimes due to the presence of tephra layers or reworked material, thus not allowing the sequence dating (Supplementary Fig. S2). Therefore, it is not considered for the analysis. The 137Cs-specific activity, on the other hand, provides helpful information. The most intense fallout peaks of 137Cs for these southern latitudes correspond to 1964 and 1966, coming mostly from delayed Northern Hemisphere nuclear tests (maximum fallouts in 1963) and the South Pacific nuclear tests starting in 1966, respectively (Ribeiro Guevara and Arribére, Reference Ribeiro Guevara and Arribére2002). The 137Cs-specific activity peak occurred at 10.5–11.5 cm depth (4.058–4.403 g/cm2 depth) in Lake Rivadavia, at 16–17 cm depth (2.050–2.205 g/cm2 depth) in Lake La Zeta, at 16–17 cm depth (1.5602–1.6450 g/cm2 depth) in Lake Theobald, and it was not identified above 10 cm depth (0.968 g/cm2 depth) in Lake Brychan (Supplementary Fig. S2).
Two and three layers of organic-bearing lake sediments selected for AMS 14C (Table 1) from each core were incorporated into the model. However, one radiocarbon age from Lake Brychan (GdA-7105) and one from Lake Theobald (GdA-7110) were removed due to being erroneously old for their stratigraphic positions. In the case of the Lake Rivadavia core, three boundaries at different depths (at 38, 16, and 5 cm original depths) were included in the model due to the presence of reworking layers between 3 and 8 cm thick, which were also discarded for the adjusted depth of the model. After all these considerations, the OxCal model’s consistency showed agreement indexes (Amodel) for the analyzed sequences between 80% and 98%, far above the acceptance threshold (60%), and convergence values (C) above the suggested 95%, confirming the good-quality performance of the model. The final model allowed us to calculate the age of the tephra layers with 68.2% and 95.4% age probability ranges (1σ and 2σ) and extrapolate the dating to the bottom of the sequence.
Sedimentary record
In the lacustrine cores, the tephra layers identified were intercalated in the brown and dark-gray, predominantly silty clayey–sized sediment (Supplementary Fig. S1), with variable remnants of non-identifiable labile organic matter, charcoal fragments, diatoms, chironomids, pollen grains, copepods, and biogenic sulfides. Some light- to dark-brown aggregates of such silty-clayey sediment of nonvolcanic origin were also identified in nearly all tephra levels.
Once volcanic/volcaniclastic layers were identified, primary tephra fallout deposits were distinguished from those of reworked origin based on the profiles of MS curves, bulk density, the presence of organic matter, the macroscopic characteristics of the clasts, and tephra componentry and composition. Consequently, two layers in Lake Brychan (BRY10 and BRY61), four layers in Lake La Zeta (ZE7, ZE26, ZE34, and ZE65), and six layers in Lake Theobald (TH15, TH40, TH59, TH62, TH70, and TH81) were defined as primary tephra deposits. Lake Rivadavia presented the most complex sedimentary record, with four primary tephra fallout layers (RI23, RI43, RI59, and RI70) and three reworked layers (identified by highly rounded pumice particles with and without phenocrysts, abundant heterogeneous lithics, and large-size organic matter remnants coexisting in the same layers, compared with density and grain size profiles) excluded from the primary tephra fallout deposit analyses (position of reworked layers in Supplementary Fig. S1).
Subsequent analyses will focus exclusively on the primary tephra fallout levels identified, as they are the focus for this study. These tephra fallout deposits comprise coarse to fine ash.
Tephra deposit characterization: volcanic particles
Based on macroscopic optical characteristics and morphologies observed under a binocular magnifying glass, various types of pyroclasts were identified in the four lacustrine cores. These pyroclasts were classified as white pumice particles, scoriaceous particles, glass shards, and crystals and crystal fragments, varying in shape and size within each class. Afterward, surface textural characteristics of each of these were obtained by SEM.
White pumice
This type of particle varies between 0.25 and 2 mm in size. Independently of the size, white pumice particles exhibit irregular to subrounded shapes, influenced by the dominant type of vesicles. Their characteristic feature is a moderate to very high degree of vesicularity. Vesicles can vary in size and shape. Small spherical vesicles, approximately 4–5 µm in diameter with very thin walls, coexist with tortuous-shaped vesicles. Some white pumice fragments are dominated by spherical and subspherical vesicles ranging from 10 µm to 150 µm in diameter within the same particle. These fragments have irregular to subrounded shapes, saccharoidal texture, and a glassy luster. Fragments with elongated and tubular vesicles display irregular but elongated shapes in one direction, creating a marked fluidal texture and a silky sheen on the fragments. Deformed, tortuous vesicles resulting from bubble coalescence are common, producing fragments with irregular shapes. The glassy surfaces of both types of vesicles are characterized by the presence of fresh, smooth glass walls. Apart from the vesicularity, a main difference is marked by the presence of phenocrysts. Three different types are categorized based on the presence of phenocrysts. The white pumice from ZE7, Bry10, and TH15 are generally aphyric or contain scarce plagioclase phenocrysts with resorptions and also present microlites/nanolites of tabular minerals smaller than 40 µm (Fig. 3A–G). The major axis of the vesicles can be pseudo-parallel to the major axis of the crystals. In the second group, the high-vesicular pumice samples from RI43, RI59, ZE65, and TH70 are aphyric, while the less vesicular pumice samples have phenocrysts with a greater diversity of mineral species. Plagioclase crystals larger than 50 µm were found, along with smaller plagioclase crystals and high-Z minerals with glass embayments. In contrast, the third group includes the pumice from RI23, ZE26, and Bry61 with phenocrysts and clusters of phenocrysts mainly of plagioclase, pyroxene, and Fe-Ti oxide minerals larger than 100 µm, giving porphyritic/glomeroporphyritic textures (Fig. 3H–M).
Morphological and textural features of tephra components. A–G, Aphyric to nearly aphyric white pumice; H–M, white pumice with phenocrysts; N–Y, different morphologies of glass shards with internal textures; Z–AD, scoria particles showing recrystallization textures; AE–AG, subhedral prismatic crystal surrounded of volcanic glass. A, H, N, Z: Binocular magnifying glass images; B, C, I, J, O–R, AA, AE: scanning electron microscope (SEM) images; D–G, K–M, S–Y, AB–AD, AF, AG: backscattered electron (BSE) images.
Glass shards
Glass shards have been found in a wide variety of morphologies, even within the same tephra layer, with grain sizes <1000 µm. They are generally light brown, dark brown, colorless or translucent and are characterized by varying vesicularity; the higher the vesicularity and the smaller the particle size, the more translucent they are. Some shards have a pumice-like appearance, others are cuspate, some have fluidal shapes with smooth walls, and others are blocky (Fig. 3N–Y). The vesicularity decreases respectively with each type. All these particles lack observable crystal inclusions under a binocular magnifying glass. Different types of glass shards were identified.
Pumice-like shards: These shards are caramel brown or colorless with a glassy luster and may present isolated vesicles or fluidal forms. They have a lower vesicularity compared with white pumice. Pumice-like shards possess subrounded to irregular shapes, depending on the variation in the size and shape of the dominant vesicles (Fig. 3O and P). The edges are irregular, with concave–convex boundaries corresponding to walls or septa between bubbles, which are very thin in some fragments (less than 20 µm). The more irregular shapes are generated by the presence of tortuous vesicles formed by bubble coalescence. Generally, these particles are aphyric (Fig. 3S) as in RI43 and TH81, or in isolated cases, tabular microcrystals are identified in the glassy matrix.
Cuspate and platy shards: These shards are translucent and have a glassy luster. They correspond to fragments of bubble walls. They have a relatively low vesicularity and can present microporphyritic textures with tabular phenocrysts of plagioclase or mafic minerals with glass embayments or entrapments (Fig. 3U–X). The glass in the matrix is well-preserved. These particles are present in RI43, ZE34, ZE65, and RI70. In contrast, the colorless particles in RI70 present aphyric or scattered microlitic/nanolithic textures.
Fluidal shards with smooth walls: These particles exhibit low vesicularity, usually with vesicles observable inside the particle when broken, rather than on the outer walls. They commonly have branched, aerodynamic fluidal, or teardrop shapes (Fig. 3Q). The walls of the particles have smooth channeling that gives them a fluidal appearance. They were observed in RI43 and RI70.
Blocky shards: These are dense fragments with cuspate edges and smooth surfaces (Fig. 3R). They are found in smaller proportions and sizes compared with the previously characterized glass types. These fragments have irregular shapes with well-defined edges and conchoidal or curviplanar fracture surfaces, exhibiting a resinous sheen in larger fragments, while the smaller shards show a glassy luster. They are present in TH40 and TH62.
Scoria
Scoriaceous particles are generally darker, ranging from dark gray to black, and denser than pumice particles (Fig. 3Z). Although they possess vesicles, they have fewer and more irregularly shaped cavities, showing moderate to low vesicularity (Fig. 3AA). The vesicles can also form convolute arrangements and typically have thicker walls between cavities. The texture of the glass surface exhibits two distinct characteristics. Some scoriaceous particles have fresh glassy walls with hackle lines (radial striations on fracture surfaces [Quinn, Reference Quinn2020]) and surface pitting. On the other hand, other particles are rounded, have low vesicularity, and show greater surface recrystallization. Various internal textural types can be observed under SEM. In TH62, ZE65, and TH70, some particles have well-preserved glass with porphyritic textures of plagioclase and mafic minerals larger than 100 µm along with populations of smaller crystals in the groundmass (Fig. 3AB and AC). In ZE34, TH40, TH59, and TH62, scoria exhibits porphyritic textures with crystal boundary resorption, whereas some have a significant degree of recrystallization in the glassy matrix, with nanometric spherulitic crystals (Fig. 3AD).
Crystals and crystal fragments
Crystals and crystal fragments are found in tephra layers in fractions smaller than 2000 µm, but in lower proportion compared with other types of pyroclasts. In all cases, the crystals are coated with glass of different colors, which imparts a glassy luster and vesicular surface textures to the particles. Despite the crystals being coated with glass, their shape and color can be observed under a binocular microscope. Large crystal fragments were found in RI23, ZE26, and ZE40 (Fig. 3AE–AG). Also, noticeable fragments of a black to dark-green mineral of vitreous luster, with layered structure and basal cleavage resembling micaceous minerals, were found in RI23, ZE26, and Bry61 (see Tephra RI23/ZE26/Bry61 section). In some cases, these fragments reach up to 1000 µm, with rather euhedral (hexagonal) shapes. These fragments are not coated with glass, unlike crystals previously described, but represent a remarkable feature in some tephra deposits.
Geochemical fingerprint similarities
The particles analyzed, constituting primary tephra fallout deposits, include vesicular glass shards in various morphologies and colors, saccharoidal white pumice (with and without phenocrysts), fluidal pumice without phenocrysts, and scoria. The bulk major and trace element concentrations measured for the different types of particles from variable-size fractions are listed in Supplementary Table S2. Major elements indicated basaltic to rhyolitic compositions for the different components analyzed. To summarize the geochemical information, Figure 4 presents normalized to primitive mantle multielement diagrams (McDonough and Sun, Reference McDonough and Sun1995) that highlight the compositional differences among the tephras under study, allowing better correlations.
Multielement diagram groups representing bulk compositional variations for Rivadavia (RI), La Zeta (ZE), Brychan (Bry), and Theobald (TH) tephra deposits (different symbols refer to certain tephra components), compared with reference data from volcanic sources. A to D represent Groups 1 to 4 described in the text. Michinmahuida A (C) and Michinmahuida B (D) represent different temporal groupings of eruptive products from the Michinmahuida volcano, which in turn correspond to different rock types. Reference data compared in the text are plotted for each layer, all normalized to primitive mantle (McDonough and Sun, Reference McDonough and Sun1995). Data from: aAmigo et al. (Reference Amigo, Lara and Smith2013) (A, C, D); bWatt et al. (Reference Watt, Pyle and Mather2011) (B).
The normalized multielement diagrams show variations in the shape of the patterns of the analyses of the different components of the tephra from all lakes, reflecting different elemental concentrations. Generally, all samples exhibit patterns enriched relative to primitive mantle concentrations, ranging from 1 to approximately 400 times depending on the element analyzed, with higher concentrations of large ion lithophyle elements (LILE) compared with high field strength elements (HFSE). Notably, all samples exhibit a pronounced negative anomaly in Ta and, to varying degrees, in Sr (which is positive in only one group) and Ti (Fig. 4).
Instead, the geochemical fingerprints of the tephras indicate four groups. Group 1 consists of white pumice without phenocrysts from Bry10, ZE7, TH15, RI43, RI59, and colorless glass shards from TH81 (Fig. 4A). This group is characterized by a pattern with the highest differential enrichment between LILE and HFSE. Additionally, group members exhibit a marked negative anomaly in Ba, Ta, and Sr, with Ti, showing variable negative ratio but the lowest relative enrichment in this element compared with following groups. Group 2 includes white pumice with phenocrysts from Bry61, ZE26, and RI23, along with black glass shards and scoria from TH59 and caramel-colored glass shards from TH62 (Fig. 4B). In comparison with Group 1, Group 2 is characterized by a lower enrichment in LILE, a marked negative anomaly in Ta and Ti, a positive Sr anomaly, and an overall relative enrichment in HFSE and rare earth elements (REE). Group 3 is composed of aphyric white pumice from ZE65 and TH70, along with colorless glass shards from RI70 (Fig. 4C). Group 3 components exhibit elemental concentrations that define a spiky geochemical pattern, with higher overall enrichment in LILE, a strong negative anomaly in Sr and Ti, and relatively higher enrichment in heavy REE (Tb, Dy, Yb, and Lu) than Group 2. Finally, Group 4 consists of glass shards and scoria from ZE34, ZE65, RI43, TH40, and scoria from TH70 (Fig. 4D). This group shows an analytical pattern similar to that of Group 3, but is differentiated by a flatter shape due to greater depletion in LILE and REE, variable with lower values of the Sr anomaly, and lower negative Ti anomaly (Fig. 4D).
Discussion
Composite tephrostratigraphic and tephrochronological lacustrine record
The correlation of distal primary tephra layers between different sites or depositional settings, as well as with their proximal equivalents, represents a crucial process for tephrochronology. In the region of interest, where detailed studies like this have only been undertaken during last decades, the volcanic stratigraphy and, as consequence, the characterization of the products are not complete. As is usually the case, the available information is partial and variable, and correlations are made based on different data, such as particle morphology, bulk geochemistry, glass and mineral composition, lithology, stratigraphy, and chronology (e.g., Sulpizio et al., Reference Sulpizio, Zanchetta, D’Orazio, Vogel and Wagner2010; Lowe et al., Reference Lowe, Pearce, Jorgensen, Kuehn, Tryon and Hayward2017).
In the present work, the comparative analysis revealed that the Theobald, La Zeta, and Rivadavia sedimentary sequences contain a higher number of primary tephra layers compared with the Brychan core, with Lake Rivadavia providing the longest temporal record. Overall, tephra deposits are composed of various juvenile components, including white pumice, glass shards with differing morphologies and colors, as well as scoria fragments. The componentry and characteristics of the components, together with the bulk geochemical data obtained for them, allowed the correlation of primary tephras between lakes. Groupings were defined based on the optical characteristics of the tephras, particle sizes, BSE texture analysis, mineralogy, and geochemical similarities in rock type and trace elements, as well as their stratigraphic superpositioning and dating probabilities derived from age–depth models. This approach led to the identification of 10 (9 groups in this section, 1 with a double source) primary tephra fallout layers within the cores from 16 individual pyroclastic layers in total (Fig. 5; Table 2). Comparisons with data from nearby eruptive centers of the SVZ allowed assignment of these tephras to distinct volcanic sources, enabling chronological correlations between the cores. In the following section, primary tephra deposits are described from the youngest to the oldest, allowing the reconstruction of a composite stratigraphic and chronological framework for the region during the last millennia, while integrating these findings with reports from nearby records. Tephras are described according to their correlation, that is, layers from different lakes that were correlated to the same eruptive event are described together, under the notation, for example, ZE7Bry10/TH15.
Stratigraphic correlation of the tephras identified in the Lakes Rivadavia, La Zeta, Brychan, and Theobald sedimentary sequences based on age–depth models and geochemical correlations, with the proposed volcanoes of provenance.
Table summarizing the correlations of tephra layers identified in this study, including their age probabilities, main morphological and geochemical characteristics, the source volcano determined by geochemical fingerprinting, and potential correlations with previous studies.
a Correlation based on recalculated age using the SHCal20 curve in the Bayesian model from Daga et al. (Reference Daga, Guevara and Arribére2016).
Tephra ZE7/Bry10/TH15
The most recent primary tephra fallout deposit within the sedimentary records corresponds to a set of white rhyolitic pumices that exhibit similar optical and textural characteristics (designated tephras ZE7/Bry10/TH15). Furthermore, these characteristics align with the known geochemical signatures of the 2008 eruptives of the Chaitén volcano (Watt et al., Reference Watt, Pyle and Mather2011 and references therein). The Chaitén tephra deposit represents the most recent explosive volcanic event that has directly impacted the region and the first rhyolitic eruption to be recorded by geophysical tools (Lara, Reference Lara2009). This volcano initiated an unprecedented historical eruption in 2008, taking local residents, Chilean and Argentine authorities, and the geological community by surprise. At that time, available data indicated the absence of explosive eruptions from this rhyolitic volcano since a major eruptive event dated to approximately 9400 yr BP (Naranjo and Stern, Reference Stern2004). Lacustrine tephrochronological records for this eruption have been identified at nearby sites, such as in Lake Futalaufquen (level FU1; Daga et al., Reference Daga, Guevara and Arribére2016), and in Lakes La Zeta and Terraplén (ZT3 and TerT4, respectively; Serra et al., Reference Serra, Massaferro and Villarosa2021) (Table 2). Isopach maps report a subaerial deposit of less than 1.5 cm in thickness in the areas of Lakes La Zeta, Brychan, and Theobald (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009). In contrast, the lakes exhibit thicker deposits, likely resulting from postdepositional processes. Conversely, at Lake Rivadavia, located approximately 40 km northwest of Lake La Zeta, the deposit was not identified in the core, consistent with the isopach map, which reports a fallout deposit less than 0.1 cm thick (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009).
Tephra RI23/ZE26/Bry61
The tephra layers RI23, ZE26, and Bry61 are identical in geochemical composition, a crucial factor for correlation. These layers are composed of white pumices containing abundant mafic glomerocrysts, millimeter-sized in RI23 and ZE26, with the size and abundance of mafic crystals decreasing in Bry61 (Fig. 6). Also, the presence of a black mica-like layered mineral, decreasing in size from RI23/ZE26 to Bry61, is a remarkable element of correlation, even though it is probably a non-juvenile component. These three tephra deposits exhibit geochemical signatures consistent with those of the Huequi volcano (Fig. 4B). This geochemical signature encompasses both the andesitic lavas sampled from the central cones within the caldera by Tagiri et al. (Reference Tagiri, Lopez-Escobar and Onuma1985) and the andesitic lava units L2, L3, and L5 and the T1 tephra from the studies by Watt et al. (Reference Watt, Pyle and Mather2011). Watt et al.’s (Reference Watt, Pyle and Mather2011) study of proximal deposits from the Huequi eruptive center revealed that, although the volcano’s activity is typically dominated by repeated lava dome formations and subsequent collapses, the surrounding tephra deposits indicate periods of moderate explosive activity. The T1 layer, consisting of white pumice with characteristics similar to those observed in lacustrine cores, is exposed at the upper sections of a collapse dome scar approximately 600 m northwest of the current summit. This T1 unit, 1 m thick and containing clasts up to 10 cm in diameter, suggests a moderately large explosive event. While the exact date of the T1 eruption remains undetermined, its stratigraphic position suggests it occurred after the last major dome collapse but before the most recent activity (1890–1920 CE cycle; named T2 in Watt et al. [Reference Watt, Pyle and Mather2011]). Moreover, other older lava units from this volcano, along with the more recent scoria and pumice deposits from T2 attributed to the 1890–1920 eruptive period, did not show geochemical concordance with RI23, ZE26, and Bry61.
RI23, ZE26, and Bry61 tephra deposits showing similar componentry. RI23 and ZE26 also have similar grain size, and they are composed of white pumice (WP) with phenocrysts (mainly Px, Amp) and plagioclase (Pl) crystals coated by glass, and other fragments of Amp and mica-like minerals, without glass coating, being non-juvenile particles. Bry61, on the other hand, has the same components with finer grain size, in agreement with a more distant location from Huequi volcano.
As we mentioned earlier, the geochemistry and componentry undoubtedly show that RI23, ZE26, and Bry61 deposits are correlatives. The probable age of the event that generated them is uncertain, however, because of a discrepancy between cores. ZE26 was assigned to 1772–1908 CE, while Bry61 was assigned to the period 1482–1667 CE. On the other hand, the estimated age for RI23 (1087–1678 CE) showed the largest range in age due to the effect that the removal of the reworked layers in the sequence and the distance to the 14C dating cause in the age–depth model, adding considerably wider ranges of probabilities of age (Fig. 7), and it is not considered. Close to the lakes under study, similar findings were reported by Daga et al. (Reference Daga, Guevara and Arribére2016) at Lake Futalaufquen, where the authors identified a tephra layer (FU18; 1547–1819 CE) with geochemical and textural features similar to ZE26/RI23/Bry61, which was also correlated with the T1 deposit described by Watt et al. (Reference Watt, Pyle and Mather2011). Furthermore, we propose that the macroscopic optical characteristics of the tephra layers ZT2 (1600–1672 CE) and TerT3 (1617–1709 CE) described by Serra (Reference Serra2017) and Serra et al. (Reference Serra, Massaferro and Villarosa2021), which were extracted from Lakes La Zeta (the same lake as in the present study) and Terraplén, respectively, may correlate with the tephra deposit identified in this study. The ages assigned for such tephras are close the age for Bry61 age. However, we propose the Huequi volcano as the source of such tephra deposits (ZT2 and TerT3) instead of the Chaitén volcano, which was suggested solely on the basis of age (Serra, Reference Serra2017; Serra et al., Reference Serra, Massaferro and Villarosa2021), as neither the components nor the geochemistry align with the characteristics of Chaitén’s Holocene eruptions. Even though the lack of geochemical data for the tephra layers studied by Serra et al. (Reference Serra, Massaferro and Villarosa2021) prevents direct comparisons with their work, the optical characterization (Serra, Reference Serra2017) suggested the correlation.
Summary of the tephra ages from this work combined with available information from references, ordered from youngest (upper part) to oldest (lower part), grouped according the cores and bibliographic correlations. Squares group the correlated deposits, and the source volcano (provenance) is specified next to different tephra layers. More correlations were possible for the youngest tephra layers due to the scarcity of data further back in time. Red vertical rectangles represent the inferred ages of tephra deposits according to our work; in the two upper squares (corresponding to Huequi and Michinmahuida deposits), the light red vertical rectangles show the ages from previous works (see text for detailed explanation). Blue horizontal lines represent ages from previous works, and blue and red circles correspond to historical reports. References: aDaga et al. (Reference Daga, Guevara and Arribére2016); bSerra et al. (Reference Serra, Massaferro and Villarosa2021); cMoreno et al. (Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014).
Regarding the age discrepancies, we infer that they are the result of the age–depth modeling of each sequence rather than the alternative interpretation that the tephras derive from separate eruption events. The factors affecting the age depth model include: (1) the Bayesian modeling approach used to integrate radiocarbon ages, (2) the calibration curves available during previous work (SHCal04/SHCal13), (3) the lack of information concerning whether tephra layers were discounted or not for the final age–depth models in some referenced works, or (4) the kind of material used for 14C. Concerning the calibration curve used in the Bayesian models, the SHCal20 (under use currently) presents improvements regarding the previous version (SHCal13), particularly for the most recent period (2140–0 cal yr BP), by introducing new tree-ring datasets for the calibration curve construction (Hogg et al., Reference Hogg, Heaton, Hua, Palmer, Turney, Southon and Bayliss2020), representing better reconstructions mostly for the last millennia. In the case of FU18 from Daga et al. (Reference Daga, Guevara and Arribére2016), for example, the age probability (originally based on SHCal13) was recalibrated using the SHCal20 version, resulting in an age probability range slightly wider and younger than reported previously (Fig. 7). Regarding the other ages (Serra et al., Reference Serra, Massaferro and Villarosa2021) calibrated using the SHCal13 version, our ages are systematically slightly younger (both this tephra and the lower one; Fig. 7).
Following our data, we consider the results from ZE26 to be more adequate, considering the more consistent age–depth model of La Zeta compared with the Brychan sequence. Bry61 was found to be older than ZE26, and even though the Bry61 age probability is closer to the previously reported ages (Fig. 7), one 14C date was finally used in the Brychan core due to an age inversion in an upper date (see “Materials and Methods” section), giving less consistency to the results. Otherwise, an older age for this Huequi eruption conflicts with the lower eruption (ZE34, see Tephra ZE34/TH40 section) recorded in the La Zeta sequence, because the ages would be reversed (Fig. 7); also, ZE34 correlates with an event in Theobald Lake (ZE34/TH40), offering another tie point. Even though extra identifications in nearby lakes would help to refine the ages, we propose here, in summary, the correlatives RI23/ZE26/Bry61 as a tephra originated by a Huequi volcano eruption, probably during the first half of the nineteenth century (before the 1890–1920 CE Huequi cycle), with distinctive componentry and geochemical composition.
Tephra ZE34/TH40
The tephra layer observed at ZE34 and TH40 is characterized by two basic particle types: brown glass shards and black scoria (Fig. 8). The brown glass shards exhibit well-preserved glass with a microporphyritic texture, containing plagioclase, pyroxene, and Fe-Ti oxide crystals. Resorption textures have been observed in the oxides (Fig. 8). Additionally, the scoria shows greater abundance of crystals, with near-complete recrystallization of the glass matrix and the formation of newly developed acicular or nanometric minerals, which are challenging to identify (Fig. 8). This layer is dated to a range between 1634–1798 CE and 1640–1764 CE, respectively (Table 2). These particles exhibit geochemical signatures similar to some of the Michinmahuida volcano deposits. In particular, ZE34 and TH40 are comparable in composition to the ancient MIC1 tephra described by Naranjo and Stern (Reference Stern2004), as well as to the >11 ka Amarillo ignimbrite and recent scoria tephras reported by Amigo et al. (Reference Amigo, Lara and Smith2013), who referred to them as 1st and 2nd scoria. Even though the geochemical composition is similar to that of ZE34 and TH40, the eruptions related both to MIC1 and Amarillo ignimbrite are temporally unsuitable as the possible origin for the deposit under consideration. On the other hand, Amigo et al. (Reference Amigo, Lara and Smith2013) identified several scoria deposits associated with scoria cones located on the flanks of the Michinmahuida volcano and along the Liquiñe-Ofqui Fault Zone (LOFZ). The two youngest scoria deposits, 1st and 2nd scoria in that study, are basaltic trachyandesites, and their geochemical signatures are consistent with those of the black glass and scoria samples from ZE34-TH40 (Fig. 4D). The 2nd scoria deposit is older than these deposits and was therefore excluded as a correlative on this basis. However, a paleosol on top of the 1st scoria deposit has a calibrated age within the last 300 yr, showing possible temporal correlation with our results. Although the components are not described in detail by Amigo et al. (Reference Amigo, Lara and Smith2013), the geochemical characteristics of the 1st scoria deposit are comparable to those of both ZE34 and TH40.
Binocular magnifying glass and backscattered electron (BSE) images of brown glass shards and black scoria from tephra ZE34 and TH40. BSE images and their magnifications (black boxes) reveal distinct textural characteristics. The different minerals and textures are indicated with lines in the figure. The brown glass shards exhibit fresh juvenile glass with well-preserved surfaces, indicative of primary volcanic material. In contrast, the black scoria displays features of recrystallization, suggesting postdepositional alteration or sintering processes.
Currently, the Michinmahuida volcano has a less-documented eruptive history compared with its neighboring Chaitén volcano and is not as renowned for recent explosive eruptions. Nonetheless, historical records indicate explosive eruptions in 1656 CE, 1742 CE, and another episode of explosive activity between 1834 and 1835 CE, during which a lava flow was reported to have been extruded from a crater on the volcano’s flank (Petit-Breuilh Sepúlveda, Reference Petit-Breuilh Sepúlveda2004; Naranjo and Stern, Reference Stern2004 and references therein). Even though information about the eruptions is limited, and samples to compare with the historical observations are unavailable, the historical explosive eruptions can be considered to be a possible origin for the ZE34-TH40 deposits. Moreover, it is possible those in the historical record of 1656 CE relate to the 1st scoria from Amigo et al. (Reference Amigo, Lara and Smith2013).
Moreno et al. (Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014) suggest that, despite its lower frequency of eruptions, the Michinmahuida volcano has been a significant source of pyroclastic deposits in the past. Some tephras identified in their sediment core studies from Lake Teo (Chile) may originate from Michinmahuida, indicating notable historical eruptive activity, albeit less frequent than that of Chaitén. Unlike Chaitén, which is predominantly associated with rhyolitic eruptions, Michinmahuida is primarily linked with andesitic to dacitic eruptions. Although Moreno et al. (Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014) did not obtain multielemental geochemical signatures for robust comparisons with the tephra from ZE34 and TH40, their analyses of glass compositions based on major elements revealed basic compositions similar to those of the samples in this study. Chronologically, the LTT−3 tephra layer (1450−1550 CE) is slightly older than ZE34-TH40. However, considering possible dating uncertainties, it could represent a deposit correlative to the one analyzed in this work.
ZE34-TH40 tephra is also similar in componentry and geochemistry to tephra deposits identified in previous works performed in nearby lakes, namely the FU29 tephra (1389−1694 CE) identified by Daga et al. (Reference Daga, Guevara and Arribére2016), and which was also assigned to the 1st scoria from Michinmahuida. Although without geochemical characterization, a tephra layer in Lake La Zeta named ZT1 for the period 1420−1583 CE was assigned also to the Michinmahuida volcano by Serra et al. (Reference Serra, Massaferro and Villarosa2021). Regarding these records, we observe slightly younger ages for the tephra ZE34-TH40 than the others (Fig. 7), as was also observed for the upper described tephra. For FU29 from Daga et al. (Reference Daga, Guevara and Arribére2016), for example, the recalibrated age probability (using SHCal20) also slightly enlarged the range to younger ages, partially overlapping with our data. To summarize, considering the geochemical correlation of ZE34-TH40 with the 1st scoria deposit, the historical records during the seventeenth to eighteenth centuries, and the possibility that older tephra ages of previous works result from modeling artifacts or uncertainties, we propose that the primary tephra deposit ZE34-TH40 originated in Michinmahuida volcano, probably between the late seventeenth and early eighteenth centuries (Fig. 7).
Tephra TH59/TH62
Tephra layers TH59/TH62 include black glass shards and trachybasaltic to basaltic scoria and are dated to 1358−1523 CE and 1299−1457 CE (Table 2), respectively. Given the close stratigraphic proximity and similar componentry and geochemical signatures of TH59 and TH62, it is possible that TH59 represents reworked TH62, although there is no direct evidence to support or discard this hypothesis. Therefore, following a conservative criterion, tephra TH62 is considered to be the primary tephra fallout in this work. These tephra deposits exhibit slight discrepancies in a general similar multielement pattern with those from the Huequi volcano (Fig. 4B) recorded in the upper parts of the cores (RI23/ZE26/Bry61: white pumice with mafic glomerocrysts), but their pyroclastic types are distinct (Fig. 9). Specifically, the TH59 and TH62 deposits show a higher positive Sr anomaly, a lower negative Ti anomaly, and generally higher U, Th, and REE contents compared with those from Huequi volcano (Fig. 4B). Additionally, they exhibit higher Sr and Ti contents compared with the RI23/ZE26/Bry61 grouping, Huequi volcano, and other southern eruptive centers within the SVZ, such as Corcovado, Yanteles, and Melimoyu (López-Escobar et al., Reference López-Escobar, Kilian, Kempton and Tagiri1993; Geoffroy et al., Reference Geoffroy, Alloway, Amigo, Parada, Gutierrez, Castruccio and Pearce2018) (Fig. 9A). Moreover, they are characterized by a more basic composition and elevated V concentrations (Fig. 9B). On the other hand, the most recent eruptive cycle from the Huequi volcano during 1890−1920 CE generated basaltic andesitic crystal-rich dense pumice and black scoria (T2 in Watt et al. [Reference Watt, Pyle and Mather2011]), indistinguishable in composition, showing relevant compositional variations in Huequi volcano products. Of note, the most recent tephra layers identified in proximal location in Huequi volcano (T1 and T2 in Watt et al. [Reference Watt, Pyle and Mather2011]) represent the compositional extremes in a coherent suite of rocks: the incompatible element contents increase with increasing magmatic evolution, from basaltic andesite (T2) to dacite (T1), with intermediate lavas (Watt et al., Reference Watt, Pyle and Mather2011). In this case, however, TH59/TH62 have some incompatible trace elements slightly higher than other Huequi products with a more basic composition, opposite to the behavior observed by Watt et al. (Reference Watt, Pyle and Mather2011). On the other hand, TH59/TH62 was compared with the composition of eruptives from volcanoes from the southern segment of the SVZ, including Corcovado, Yanteles, and Melimoyu volcanoes (Fig. 1) located between 100 and 150 km west of Lake Theobald. These volcanoes have limited historical records, but the comparison with available information also did not allow any correlation with the tephra layers under study.
Bivariate plots of TiO2, Sr, SiO2, and V concentrations of pumice, glass shards, and scoria fragments of TH59 and TH62, alongside RI23, ZE26, and Bry61, as well as the Huequi, Corcovado and Yanteles, and Melimoyu volcanoes. At the bottom left, macroscopic differences between the white pumice with phenocrysts and the brown glass shards and black scoria from the various sample groups are shown. References: aWatt et al., Reference Watt, Pyle and Mather2011; bLópez-Escobar et al., Reference López-Escobar, Kilian, Kempton and Tagiri1993; cGeoffroy et al., Reference Geoffroy, Alloway, Amigo, Parada, Gutierrez, Castruccio and Pearce2018.
In Lake Terraplén (located ∼50 km north from Lake Theobald), a tephra named Ter-T2, dated to 1000−1538 CE, shows componentry similarities (Serra, Reference Serra2017; Serra et al., Reference Serra, Massaferro and Villarosa2021) with the pyroclasts observed in the TH59/TH62 layers. Even though the authors correlated the Ter-T2 deposit with Chaitén volcano according to the assigned age, the geochemistries of similar glass shards identified in TH59/TH62 do not correspond with those of Chaitén products. Due to the differences from previously described material from the Huequi volcano, the limited information available for tephras from this volcano during this period, and the lack of correlation with other volcanoes from the SVZ (Fig. 9), this tephra might either represent an unknown event from the Huequi volcano or originate from an unidentified eruptive center from the SVZ occurring during the fourteenth century (Fig. 7; Table 2).
Tephra ZE65/TH70
ZE65 and TH70 are dated to 1176−1274 CE and 1216−1340 CE, respectively, and correspond to a mix of white or colorless pumiceous glass shards together with accessory brown glass shards and black scoria. These synchronous levels are composed of two geochemical signatures that denote the same eruptive center, the Michinmahuida volcano. On one hand, the white or colorless glass shards are andesitic to trachyandesitic in composition, which shows macroscopic and geochemical correspondence with the dacitic pumice from the youngest Michinmahuida eruption, dated to 1450–1650 CE (Amigo et al., Reference Amigo, Lara and Smith2013), despite exhibiting a slightly younger age than ZE65/TH70 (Fig. 7). On the other hand, the basaltic black scoria and brown basaltic glass shards correspond to a set of geochemical signatures similar to those identified in ZE34/TH40, which were attributed to the 1st scoria deposit of Michinmahuida (Amigo et al., Reference Amigo, Lara and Smith2013). Given that ZE65 and TH70 are older than the 1st scoria deposit, it is suggested that the basaltic components may be cognate fragments from older eruptive units of this volcano, such as MIC1 (5120−6350 yr BP) (Naranjo and Stern, Reference Stern2004). Supporting this hypothesis, these types of pyroclasts are less abundant than the dacitic pumice. Although the textures of the brown glass shards closely resemble those of juvenile pyroclasts, the scoria exhibits recrystallization and the formation of neominerals in the glassy matrix and on its surface. These features suggest that the scoria may be a product of reheating or weathering processes, further reinforcing their cognate origin, while the colorless glass shards can correspond to juvenile components. Therefore, this primary tephra deposit is associated with an eruption from the Michinmahuida volcano during the thirteenth century, possibly corresponding to the youngest Michinmahuida eruption (1450−1650 CE [Watt et al., Reference Watt, Pyle and Mather2011]) or a slightly older unknown eruption.
Tephra TH81
Tephra TH81, dated to 1063−1229 CE, is composed of white to colorless rhyolitic glass shards, matching the geochemical signature of Chaitén volcano. For this period, volcanic historical records become scarce. Instead, this tephra could correlate with the levels observed in the lacustrine core from Lake Teo (Chile; Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014), where three tephra layers identified between 1000 and 1390 CE were attributed to the Chaitén eruptive center based on major element compositions of glass. These three tephra deposits also overlap in time between them, but specifically, LTT−6 layer dated at 1007−1195 CE, seems to be temporally closer to TH81 (Fig. 7).
Tephra RI43
The RI43 deposit, dated at 679–934 CE, contains two distinct geochemical signatures from different eruptive centers. On one hand, it includes white rhyolitic pumice whose geochemical composition matches that of the Chaitén volcano. The nearest temporally recorded event is LTT−6 (1007−1195 CE) from Lake Teo in Chile (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014), which we propose could be responsible for erupting either RI43 or TH81 (Fig. 7). On the other hand, RI43 also features brown and black basaltic glass shards with well-preserved glassy textures, matching the more basic and older rock patterns of the Michinmahuida volcano. Within this temporal range, tephra LTT-7 (351–669 CE) from Lake Teo could be linked to the Michinmahuida volcano based on its location and major element glass composition (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014), potentially correlating with the brown and basaltic shards from RI43. However, the age difference between the possible correlating deposits (LTT-6 and LTT-7 from Moreno et al. [Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014]) and the stratigraphic separation between them in Lake Teo are evidence that some of the events recorded in RI43 do not correspond to those mentioned. In any case, RI43 represents a deposit encompassing two volcanic events from different sources around the eighth to ninth centuries, possibly including previously unknown events from these volcanoes. However, it cannot be excluded that some of these particle groups, although with primary juvenile features, may be the result of partial reworking of previous tephra deposits.
Tephra RI59
RI59 is dated to 261–541 CE and contains white rhyolitic pumice with the same geochemical signature as the Chaitén volcano. This tephra does not have a corresponding layer in any nearby cores, suggesting the possibility of an unknown eruptive event from this volcanic center.
Tephra RI70
Finally, tephra RI70, dated to 39–259 CE, contains white and black glass shards with the same geochemical signature, matching with the most recent products of the Michinmahuida volcano. The LTT-8 layer (52–334 BCE) found in Lake Teo and attributed to the Michinmahuida volcano (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014) is a candidate for correlation, even though it is slightly older (Fig. 7). This multielemental signature is the same as that obtained in pumiceous fraction in ZE65/TH70 tephra (correlating with the dacitic gray pumice of the 1450–1650 CE youngest Michinmahuida eruption [Amigo et al., Reference Amigo, Lara and Smith2013]). This event, dated at around the second century, represents an older volcanic eruption with the same composition as the youngest Michinmahuida eruption, extending the known records with this composition of this volcano.
Potential isochrons/chronostratigraphic layers
The use of the primary tephra fallout deposits as isochronous marker beds to link and synchronize different depositional sequences is the basis of tephrochronology. Once the tephra deposits identified in the present work are well characterized with multiple parameters (morphology, mineralogy, geochemistry) and the relative or numerical age is established, such layers provide a tie point transferable to other sites or depositional sequences (Lowe et al., Reference Lowe, Pearce, Jorgensen, Kuehn, Tryon and Hayward2017). In light of current knowledge, several of the newly identified tephra deposits are regarded as significant time markers.
The occurrence of several tephra deposits in two or three of the four cores under study supports the primary character of the deposit and indicates its usefulness as a stratigraphic marker. This is the case of the youngest recorded tephra in the present work, corresponding to the eruption from the Chaitén volcano from 2008. Regarding the volcano’s location, the deposit is preserved in lakes to the east and southeast, at distances up to at least 100 km, while it was not represented by a well-defined deposit in Lake Rivadavia, to the east-northeast from the volcano. This distribution pattern is supported by the isopach map reported for the eruption, denoting a relatively complex distribution of the deposit, as a result of the multiple days of eruption and changing wind conditions (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009; Alfano et al., Reference Alfano, Bonadonna, Volentik, Connor, Watt, Pyle and Connor2011). Due to the highly explosive character of the eruption and the reconstruction of the ash-dispersion patterns, the Chaitén eruption in 2008 is a potential tephra layer to be found for thousands of kilometers, as occurred with several rhyolitic records from other parts of the world (e.g., Davies et al., Reference Davies, Albert, Bourne, Owen, Svensson, Bolton and Cook2024; Kearney et al., Reference Kearney, Albert, Staff, Magyari, Pál, Veres and Lane2024). It becomes a high-potential continental marker tephra layer for comparison, to be preserved mainly in waterbodies, as the fine grain size and relatively thin deposits may not be preserved over such long distances in subaerial environments or the original deposits strongly affected (Dugmore et al., Reference Dugmore, Thompson, Streeter, Cutler, Newton and Kirkbride2020; Panebianco et al., Reference Panebianco, Mendez, Buschiazzo, Bran and Gaitán2024). Furthermore, this is a very useful example of the dispersion that previous eruptions of Chaitén could have reached during the Holocene. In this sense, more studies of lake sequences covering the last millennia are needed to increase the geological evidence of older events, such as those recorded in the present study around the twelfth century (TH81) and some older ones between the eighth to ninth (RI43) and fourth to fifth (RI59) centuries, with the aim of considering them as chronostratigraphic markers.
Another potential stratigraphic marker that becomes of great importance, considering previous and present reports, is the primary tephra fallout deposit generated by the Huequi volcano (RI23/ZE26/Bry61 tephra layers). Here we identified differences in the grain size and macroscopic features on the products (related to variable distances to the volcano), while the geochemical composition supports the correlation. This tephra deposit was found in Lakes Futalaufquen (Daga et al., Reference Daga, Guevara and Arribére2016), La Zeta, and Terraplén (Serra et al., Reference Serra, Massaferro and Villarosa2021), and now it has been identified to the north as far as Lake Rivadavia; it is also geochemically confirmed in Lake La Zeta to the south. Even though the relatively coarse grain size deposit in Lake La Zeta (inferring possible high explosiveness of the eruption and consequent high dispersion) and the highly possible presence of this tephra in Lake Terraplén, the deposit was not identified in Lake Brychan, located just 10 km south of Lake Terraplén, or in Lake Theobald (around 40 km south of Lake Brychan). An apparent limit on the dispersion plume can be inferred between Lakes Terraplén and Brychan, following a northeast-southeast direction (Fig. 10). Regarding tephra dispersion to the north, lakes located north of Lake Rivadavia need to be studied to improve the knowledge of the dispersion pattern of this Huequi volcano tephra; and the same holds for eastern locations to delimit the impact of the eruption as an indicator of possible future effects. Until new data contribute to improve the event age, we propose here the beginning of nineteenth century as the age of occurrence due to the results obtained following updated Bayesian models.
Dispersion map summarizing the identified tephra layers, their correlations with each other, the different sampled lakes, and their respective source eruptive centers. The TH59 and TH62 layers were not assigned to any eruptive center.
The other two deposits covering Lake La Zeta to Lake Theobald correspond to the primary tephra fallout that originated from the Michinmahuida volcano (and accessory cones) (Fig. 10) and can be proposed as potential isochrons. The most recent of these deposits corresponds to ZE34 and TH40 (late seventeenth to early eighteenth centuries), which is slightly younger than correlative previous records (∼1450–1680 CE, including FU29, ZT1, LTT−3) (Fig. 7). Of note, the age shown for the FU29 tephra level (Daga et al., Reference Daga, Guevara and Arribére2016) was recalibrated in the present work (using the original information but updating the calibration curve) and resulted in an age slightly younger than originally estimated, suggesting that results in previous works underestimate the tephra age. The older primary tephra from the Michinmahuida volcano, recorded also in Lakes La Zeta and Theobald (ZE65/TH70), corresponds to the thirteenth century (Fig. 7). There are no nearby records associated with this eruption for comparison, other than the youngest Michinmahuida eruption defined at 1450−1650 CE (Amigo et al., Reference Amigo, Lara and Smith2013), which is most probably not associated with our record. Future studies will help to constrain the dispersion and age of this event.
Contribution to the eruptive history and volcanic hazard
Tephrostratigraphic studies in Patagonia have increased in recent decades and have become even more relevant since the occurrence of the Chaitén eruption in 2008, directly affecting the present study zone, as well as the eruptions of the Puyehue-Cordón Caulle volcanic complex in 2011 and that of the Calbuco volcano in 2015, just 200–250 km to the north. Since then, significant improvements have been achieved in the regional tephrostratigraphy of high-intensity volcanic eruptions in north Patagonia (e.g., Watt et al., Reference Watt, Pyle and Mather2011, Reference Watt, Pyle and Mather2013; Fontijn et al., Reference Fontijn, Lachowycz, Rawson, Pyle, Mather, Naranjo and Moreno-Roa2014, Reference Fontijn, Rawson, Van Daele, Moernaut, Abarzúa, Heirman and Bertrand2016; Rawson et al., Reference Rawson, Naranjo, Smith, Fontijn, Pyle, Mather and Moreno2015; Naranjo et al., Reference Naranjo, Singer, Jicha, Moreno and Lara2017; Alloway et al., Reference Alloway, Pearce, Moreno, Villarosa, Jara, De Pol-Holz and Outes2017, Reference Alloway, Pearce, Moreno, Villarosa, Jara, Henríquez and Outes2022; Romero et al., Reference Romero, Alloway, Gutiérrez, Bertín, Castruccio, Villarosa and Schipper2021). Important dispersed tephras were identified as potential regional markers for postglacial times in southern Chile and Argentina. However, it was concluded that the estimated frequency of large explosive eruptions (Volcanic Explosivity Index, VEI ≥ 5) at these southern latitudes is probably significantly underestimated (Fontijn et al., Reference Fontijn, Lachowycz, Rawson, Pyle, Mather, Naranjo and Moreno-Roa2014).
Beyond these advances in the regional tephra markers, all those works that have focused on the refinement of the medium- to low-intensity volcanic eruptions that occurred during the last centuries also have the same limitations: scarce historical volcanic records in these latitudes that allow correlation and the lack of geological evidence and proximal characterization of the deposits for eruptions in such times (e.g., Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014; Daga et al., Reference Daga, Guevara and Arribére2016; Serra et al., Reference Serra, Massaferro and Villarosa2021; Sosa et al., Reference Sosa, Daga, Demichelis, Alfonzo and Guevara2024).
In this sense, the region affected specifically by the volcanoes from the southern segment of the SVZ has historically been poorly documented due to its remoteness and low population density. This lack of knowledge was particularly evident during the unexpected eruption of the Chaitén volcano in 2008 (VEI ∼ 4 [Carn et al., Reference Carn, Pallister, Lara, Ewert, Watt, Prata and Thomas2009]), the last eruption of which was thought to be 9370 14C yr BP (Naranjo and Stern, Reference Stern2004). This eruption enormously affected Chaitén town in Chile, located a few kilometers downslope from the volcano to the south, mainly as a consequence of lahars and ashfall (Martin et al., Reference Martin, Watt, Pyle, Mather, Matthews, Georg and Day2009; Donovan et al., Reference Donovan, Toyos, Amigo, Villarosa, Lanfranco and Rovere2023). Moreover, the eastern territory was the main area affected by the dispersion of volcanic ash deposits during several weeks (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009). The Argentine territory on the Atlantic coast was covered—almost for the first time in the memory of most of the local population—with a centimeter- to millimeter-thick ash layer deposit (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009; Alfano et al., Reference Alfano, Bonadonna, Volentik, Connor, Watt, Pyle and Connor2011). Currently, the Chaitén volcano is considered one of the very high risk volcanic systems in Chile (SERNAGEOMIN, 2023). As we mentioned previously, the deposit of this eruption is the most recent one identified in the cores investigated. Because this eruption affected the area under study, it is possible that previous eruptions of the same volcano could have affected the area as well, and may also have similar impacts in the future with greater effects considering the increasing population of settlements in the area. In total, four tephra deposits related to Chaitén were identified in the area during the last ∼2000 yr. Although distributed unevenly over time, these deposits represent a minimum of eruptions from this volcano that may have affected the area (one every ∼500 yr), for which a median recurrence of ∼200 yr for the last millennium was calculated from a lake-sediment archive taken close to Chaitén volcano (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014). In contrast to this, a recurrence of ∼900 yr was obtained from soils and lake sequences for the ∼18,000 yr BP (Alloway et al., Reference Alloway, Pearce, Moreno, Villarosa, Jara, De Pol-Holz and Outes2017).
On the other hand, some of the events identified here were first identified east of the Andes a little less than one decade ago in several closer lakes (Daga et al., Reference Daga, Guevara and Arribére2016; Serra et al., Reference Serra, Massaferro and Villarosa2021). Such works represented the first distal records of an unknown eruption, even without—or very limited—proximal deposit descriptions and reconstructions. This is the case of one eruption corresponding to the Huequi volcano, possibly during the first half of the nineteenth century (previous to the last 1890−1920 cycle) (Watt et al., Reference Watt, Pyle, Mather, Martin and Matthews2009; Fig. 10). This tephra was not dated in proximal locations, and the first probable ages were inferred by their identification in eastern lake sequences. The identification of the same tephra deposits is reinforced now and extended to several lakes in the region, expanding the evidence of its ash dispersion up to at least 130 km to the southeast from the volcano (Fig. 10). Even though there is no evidence of any effects in the study area of the last eruptive cycle of the Huequi volcano in 1890−1920, there is no doubt that previous eruptions did affect the area, and the geochemical characteristics support their correlation. The current towns of Esquel and Trevelin, dispersed touristic settlements, and rural inhabitants are in the area affected by this eruption (Fig. 10). The most important point at this stage is that this area was affected around the beginning of the nineteenth century by a significant eruption from the Huequi volcano, which is currently considered a moderate-risk volcano due to its low-magnitude but recurrent eruptive events and significant geographic isolation (SERNAGEOMIN, 2023).
Another volcano with important activity during the last centuries is Michinmahuida (and accessory cones), whose eruptive frequency may be underestimated. Even with variation in the products and its geochemistry, at least four tephras that erupted from it reached the study zone, with evidence in several lakes from both Chile and Argentina. The most recent occurred around the seventeenth to eighteenth centuries and the previous one around the thirteenth century, with tephras from both reaching at least 100 km to the east and southeast (Lakes La Zeta and Theobald) of the volcano (Fig. 10). Evidence of older eruptions was recorded as tephra layers in Lake Rivadavia (∼80 km northeast from the volcano) around the eighth to ninth (RI43) and second (RI70) centuries. These lacustrine tephra records are of great importance, because they can be correlated with tephra layers identified in a small lake west of the volcano (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014). Even though greater numbers of records are still necessary to improve the age of occurrence of such events, it is clear that eruptions from the Michinmahuida volcano have affected extended areas, probably with complex patterns of dispersion. In line with this, a high level of risk is currently ascribed to the Michinmahuida volcano (SERNAGEOMIN, 2023). In relation to the proposed correlation between our most recent tephra deposit from Michinmahuida volcano, ZE34-TH40, with the deposit corresponding to the 1st scoria, it was described in proximal location as several layers that vary in thickness and grain size but with similar composition, resembling a strombolian deposit, probably originating from one of the young scoria cones along the LOFZ (Amigo et al., Reference Amigo, Lara and Smith2013). Because the deposit reported in our study was found in Lake Theobald, more than 100 km southeast from the Michinmahuida volcano, it is possible that a more intense eruption than a typical strombolian event, or a violent strombolian eruption, could be considered as the source of these deposits, as was inferred by Daga et al. (Reference Daga, Guevara and Arribére2016).
The core from Lake Rivadavia, in particular, significantly extends the record of tephras in this area, being the only one extending back to the beginning of the Common Era. In this sense, while the record from a nearby lake to the west of the volcano assigns potentially 10 tephras to Michinmahuida over the past ∼10,000 yr (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014), we identify 4 tephra deposits in the last ∼2000 yr in the easterly direction, evidencing a higher volcanic frequency than supposed.
Consequently, given the current state of knowledge, any discussion about the frequency and recurrence of volcanic activity in southern Chile and Argentina should be addressed with a focus on minimum frequencies, whatever the VEI considered. Particularly, small eruptions (i.e., VEI < 3) are likely underreported in comparison to highly explosive eruptions, although frequencies tend to be higher. Comparing records and settings as well, the discrepancy in the reported deposits is further pronounced as the magnitude of the eruption decreases, owing to the progressively diminished preservation potential (Deligne et al., Reference Deligne, Coles and Sparks2010), and it is here that lake sequences play a key role. This implies the need to continue with this kind of study, particularly in well-dated lacustrine systems, which in fact provide the most complete volcanic records. Only then can the volcanic hazard be critically assessed (Moreno et al., Reference Moreno, Alloway, Villarosa, Outes, Henríquez, De Pol-Holz and Pearce2014).
Final remarks
This study corroborates that lacustrine cores serve as high-resolution temporal archives and the tephra found within these cores are well preserved, which is key in the field of tephrochronology. However, establishing clear correlations between lacustrine tephra layers and their volcanic sources within the SVZ has proven to be a complex task. The challenge arises from the combination of a general lack of comprehensive data on volcanic activity and absence of a detailed characterization of known events and their tephra products. Despite those results obtained in geochemical fingerprints, the scarcity of extensive data makes the development of robust correlations between tephra deposits and their volcanic provenance difficult. This deficiency underscores the need for more systematic and detailed data collection to address information gaps and to improve the accuracy of our volcanic records.
The development of consistent age–depth models for the lake sequences has been crucial in identifying and dating distal tephra deposits from the Chaitén, Huequi, and Michinmahuida volcanic complexes, suggesting the possibility of previously unknown eruptions. The identified tephra deposits have affected eastern Patagonia over the past centuries, providing valuable insights into the regional impact of past volcanic eruptions. In summary, during the last ∼2000 yr this sector of north Patagonia was affected by at least four eruptions from Chaitén: 2008 CE (ZE7/Bry10/TH15), ca. twelfth century (TH81), ca. eighth century (RI43), and fourth to fifth centuries (RI59); four eruptions from Michinmahuida: ca. seventeenth to eighteenth centuries (ZE34/TH40), thirteenth century (ZE65-TH70), eighth to ninth centuries (RI43), and ca. second century (RI70); and two eruptions from Huequi: beginning of the nineteenth century (RI23/ZE26/Bry61) and, possibly, the fourteenth (TH62) century (Fig. 7). Furthermore, the deposits corresponding to eruptions in 2008 CE from Chaitén, beginning of the nineteenth century from Huequi, and early eighteenth and thirteenth centuries from Michinmahuida have the potential to act as isochronous marker beds in the region.
To reduce the gaps in the volcanic record, further research is necessary. Investigations on both the west and east sides of the Andes will complement the information concerning eruptive frequency and intensity. Such studies will improve our understanding of the historical volcanic activity of nearby volcanoes and contribute to more accurate volcanic risk assessments.
Finally, the example of the Chaitén volcano, which was considered nearly inactive until its 2008 eruption, highlights the importance of not underestimating potentially hazardous nearby volcanoes. This case reinforces the need for comprehensive monitoring and evaluation of all significant volcanic sources to ensure accurate hazard assessments and preparedness, particularly given the new information now presented. The volcanic activity of this segment of the SVZ is particularly poorly known, even compared with volcanoes located close by to the north. This is evidence of the huge differences in the volcanic records from this part of the world versus other parts of the world, where the eruptions are considerably better known, with records for vastly longer periods of time and reaching hundreds to thousands of kilometers from the sources. This increases the vulnerability of populations in southern latitudes.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/qua.2025.10030.
Acknowledgments
The authors wish to express their gratitude to Micaela Grosso, Romina Ledesma, and Andrea Rizzo for their collaboration in sampling and sample conditioning; the Material Characterization Group of the Centro Atómico Bariloche for their support in SEM studies; and the RA-6 reactor operation staff for their assistance in sample analysis. Special thanks to Juan Otamendi for his comments on geochemistry. We thank reviewers David Lowe and Luis Lara, associate editor Yeong Bae Seong, and senior editor Lewis Owen for their insightful comments, which significantly enhanced the quality of this work. NP received support from EU funds FSD−10.25, Development of Higher Education focused on the needs of the green economy European Funds for Silesia 2021−2027: “The Modern Methods of the Monitoring of the Level and Isotopic Composition of Atmospheric CO2” (project no. FESL.10.25-IZ.01-06C9/23-00). This investigation was funded by the PICT 2019-03000 project.
