INTRODUCTION
The most widespread severe anthropogenic influences on estuarine and marine ecosystems are hypoxia and anoxia. The number of dead zones (or hypoxic areas with dissolved oxygen below 2 mg/L) in the coastal oceans has been increasing since the 1960s, mainly due to increases in primary production and consequent coastal eutrophication fueled by riverine runoff of fertilizers and sewage (Diaz and Rosenberg, Reference Diaz and Rosenberg2008; Irby et al., Reference Irby, Friedrichs, Da and Hinson2018). With regard to anticipating global warming, deoxygenation driven by anthropogenic influences is expected to have greater impacts as ocean warming and acidification increase, leading to greater challenges for aquaculture and fisheries from hypoxia. The expected hypoxia and anoxia in coastal areas are mainly associated with a semi-enclosed hydro-geomorphology, hindering water exchange and resulting in water column stratification (Diaz and Rosenberg, Reference Diaz and Rosenberg2008; Lee et al., Reference Lee, Park, Lim, Yoon and Kim2018). However, there have been limited studies on the past coastal hypoxia or anoxia variability at centennial to millennial timescales and their controlling factors from the viewpoint of natural climate change and geomorphological evolution, which makes it difficult to constrain future predictions.
Past coastal evolution can be understood in terms of changes in long-term morphology and seawater-influenced bottom-water conditions (hypoxic, anoxic, euxinic, and so forth). Many previous studies have suggested that current coastal morphological evolution is the result of local or regional responses to past climate and sea-level changes since the last glacial maximum and deglacial periods (Stanley and Warne, Reference Stanley and Warne1994; Kim and Kennett, Reference Kim and Kennett1998; Dellwig et al., Reference Dellwig, Watermann, Brumsack, Gerdes and Krumbein2001; Chen et al., Reference Chen, Liu, Shieh and Liu2004; Hori et al., Reference Hori, Tanabe, Saito, Karuyama, Nguyen and Kitamura2004; Mackie et al., Reference Mackie, Leng, Lloyd and Arrowsmith2005; Lamb et al., Reference Lamb, Vane, Wilson, Rees and Moss-Hayes2007; Shin et al., Reference Shin, Chough, Kim and Woo2007; Nahm et al., Reference Nahm, Kim, Bong, Kim, Yang and Yu2008; Yang et al., Reference Yang, Kim, Nahm, Ryu, Yi, Kim, Lee and Kim2008; Yu et al., Reference Yu, Zong, Lloyd, Huang, Leng, Kendrick, Lamb and Yim2010, Reference Yu, Zong, Lloyd, Leng, Switzer and Yim2011; Kim et al., Reference Kim, Eum, Yi, Kim, Hong and Lee2012; Ishihara et al., Reference Ishihara, Sugai and Hachinohe2012). These studies clearly demonstrated Holocene transgression in various regions in terms of millennial timescales, but there has been little study regarding coastal bottom water and sediment conditions due to the lack of a suitable index and compatible sedimentary records with high-resolution dating ages. Recent geochemical and isotopic data from coastal sediments from the past have shown potential to provide information about such coastal environmental changes (e.g., Pasquier et al., Reference Pasquier, Sansjofre, Rabineau, Revillon, Houghton and Fike2017; Lim et al., Reference Lim, Lee, Hong, Park, Lee and Yi2019; Liu et al., Reference Liu, Fike, Li, Dong, Xu, Zhuang, Rendle-Buhring and Wan2019).
δ13CTOC values in coastal sediments
Transgression represents a remarkable coastal change during the Holocene, although its timing was asynchronous in each present coastal area due to different geomorphological settings and elevations. This coastal change, including proximity to coasts responsible for the first-order riverine organic input, has been reconstructed using organic carbon isotope records preserved in estuary and coastal sediments (Mackie et al., Reference Mackie, Leng, Lloyd and Arrowsmith2005; Lamb et al., Reference Lamb, Wilson and Leng2006, Reference Lamb, Vane, Wilson, Rees and Moss-Hayes2007; Yu et al., Reference Yu, Zong, Lloyd, Huang, Leng, Kendrick, Lamb and Yim2010, Reference Yu, Zong, Lloyd, Leng, Switzer and Yim2011; Williams et al., Reference Williams, Dellapenna, Lee and Louchourarn2014; Lim et al., Reference Lim, Lee, Kim, Hong and Yang2015). In general, marine aquatic algae and plants have δ13C values between −17.3 and −21.7‰, whereas freshwater algae and terrestrial plants (e.g., C3 plants) have δ13C values between −24.9 and −32.5‰ (e.g., Meyers, Reference Meyers1994; Mackie et al., Reference Mackie, Leng, Lloyd and Arrowsmith2005; Lamb et al., Reference Lamb, Wilson and Leng2006, Reference Lamb, Vane, Wilson, Rees and Moss-Hayes2007), forming two groups that can be clearly distinguished by their different δ13C values. This distinct distribution has been confirmed in different environmental settings. For example, Yu et al. (Reference Yu, Zong, Lloyd, Huang, Leng, Kendrick, Lamb and Yim2010) tested the applicability of δ13C values as indicators for sources of organic matter in deltaic and estuarine sediments based on the geochemical information from organic matter from source to sink in the Pearl River catchment, delta, and estuary. They found that the average δ13C of estuarine surface sediment increases from −25.0‰ ± 1.3‰ in the freshwater environment to −21.0‰ ± 0.2‰ in the marine environment. Based on these variances in the δ13C value, past relative sea-level changes and terrigenous input changes driven by freshwater (e.g., summer monsoons) have been reconstructed in various areas (Mackie et al., Reference Mackie, Leng, Lloyd and Arrowsmith2005; Lamb et al., Reference Lamb, Wilson and Leng2006, Reference Lamb, Vane, Wilson, Rees and Moss-Hayes2007; Zong et al., Reference Zong, Lloyd, Leng, Yim and Huang2006, Reference Zong, Huang, Yu, Zheng, Switzer, Huang, Wang and Tang2012; Yu et al., Reference Yu, Zong, Lloyd, Leng, Switzer and Yim2011; Lim et al., Reference Lim, Lee, Kim, Hong and Yang2015).
C/S ratio and δ34S value in coastal sediments
Pyrite is generally considered a final product of the early diagenesis of sulfur in marine sediments through metabolic processes starting from seawater sulfate (SO42−) with +20‰ of δ34S value, including microbial sulfate reduction and sulfur isotopic fractionation up to 70‰ (e.g., Canfield et al., Reference Canfield, Farquhar and Zerkle2010; Sim et al., Reference Sim, Bosak and Ono2011). Pyrite can be formed under the two different processes (diagenetic and syngenetic) (Raiswell and Berner, Reference Raiswell and Berner1985; Werne et al., Reference Werne, Lyons, Hollander, Formolo and Damsté2003; Lyons et al., Reference Lyons, Anbar, Severmann, Scott and Gill2009). In normal marine environments (oxygenated bottom water) the H2S needed for pyrite formation is produced diagenetically (after deposition) by sulfate-reducing bacteria only below the sediment–water interface. In euxinic environments (anoxic with H2S in the water column), pyrite forms above the sediment–water interface syngenetically (before burial) as well as in the sediments. It is clear that the pyrite from H2S is enriched in the light sulfur isotope (32S), resulting in enrichment of 34S in the parent sulfate (SO42−). In the euxinic marine setting, the δ34S value is determined by the combination of fractionation through bacterial sulfate reduction (more than −50‰) and reservoir effect, which can suppress net 34S depletion in H2S and pyrite due to limited sulfate supply in the local reservoir. This reservoir effect will be amplified in the syngenetic process. In euxinic environments, pyrite forms in the water column isolated from the upper oxygenated seawater. As pyrite increases, the heavy 34S isotope in the euxinic water column gradually becomes enriched, resulting in a decrease in the net 34S depletion in H2S and pyrite due to limited sulfate supply (Lyons et al., Reference Lyons, Anbar, Severmann, Scott and Gill2009).
Compared with the use of carbon isotope analysis for the reconstruction of past sea level–related coastal changes, sulfur isotope analysis of coastal sediments has not been widely used. But several past studies have pointed out the potential for reconstructing alternating marine and nonmarine environments through sedimentary sulfur isotope values. For example, Middelburg (Reference Middelburg1991) suggested that freshwater sediments underlying the marine sediments can be recognized on the basis of their δ34S values. The Holocene marine sediments in Kau Bay, Indonesia, showed isotopically lighter values (δ34S = −20‰) but underlying freshwater sediments had a δ34S value of +15‰. Similarly, Wilkin and Arthur (Reference Wilkin and Arthur2001) reported different δ34S values in each different depositing environment. The δ34S values observed in shallow-water sedimentary cores from the Black Sea covering the past 15 ka ranged widely between −38.0‰ in the stratified anoxic–sulfidic water column and +11‰ in a freshwater lake. The δ34S value in coastal sediments has been reported to vary considerably between inorganic (predominantly pyrite) and organic components. For organic sulfur, δ34S values from −2.0 to +10.0‰ have been reported from the late Pleistocene and Holocene coastal plain sediments in Taiwan (Chen et al., Reference Chen, Liu, Shieh and Liu2004). For pyrite, which is the main form of inorganic sulfur typically formed during bacterial sulfate reduction, much lower values have been reported, varying from +15.2 to −27.6‰ in coastal plain sediments in Taiwan (Chen et al., Reference Chen, Liu, Shieh and Liu2004). Dellwig et al. (Reference Dellwig, Watermann, Brumsack, Gerdes and Krumbein2001) investigated marshland sediment in northwest Germany covering the entire Holocene and showed that the peat layers were characterized by significant enrichment of pyrite. The bulk δ34S values of the peat layer sediments ranged from −2.6 to −26.7‰ and were attributed to microbial sulfate reduction following the input of seawater sulfate. Furthermore Chen et al. (Reference Chen, Liu, Shieh and Liu2004) found a close relationship between the δ34S of pyrite and the late Pleistocene and Holocene sea-level curve in the coastal plain sediments in Taiwan and suggested that sulfur analysis is a useful technique for reconstructing alternating marine and nonmarine environments.
Furthermore, the ratio of organic carbon to pyrite sulfur (C/S ratio) is thought to be affected by the salinity at the time of deposition, as diagenetic pyrite forms more readily in marine than freshwater sediments due to the relatively high availability of dissolved sulfate in seawater (Berner, Reference Berner1984; Berner and Raiswell, Reference Berner and Raiswell1984; Morse and Berner, Reference Morse and Berner1995). For example, C/S ratios were 17–34 at salinities <1‰, whereas at salinities of 19–21‰, C/S ratios were 1.4–1.8 (Berner and Raiswell, Reference Berner and Raiswell1984 and references therein). Based on the relationship between salinity and C/S ratio, it was suggested that freshwater and marine sediments have high (>10) and low C/S values (0.5–5), respectively, providing two endmembers (Berner, Reference Berner1984; Berner and Raiswell, Reference Berner and Raiswell1984; Woolfe et al., Reference Woolfe, Dale and Brunskill1995, and references therein). Furthermore, it has been suggested that the C/S ratio is 2.8 ± 0.8 in normal marine sediments. In addition, deviations toward higher C/S ratios indicate a freshwater environment, while deviations toward lower C/S ratios suggest burial under a euxinic environment (Berner and Raiswell, Reference Berner and Raiswell1984).
Recently, several studies have tested the applicability of C/S ratios by measuring the ratios from present river and inner-continental sediments (e.g., Liu et al., Reference Liu, Zhang, Li, Fan, Dong, Jiao, Chang, Gu, Zhang and Wang2021, Reference Liu, Zhang, Li, Dong, Zhang, Gu, Chang, Zhuang, Li and Wang2022; Chang et al., Reference Chang, Liu, Wang, Zhuang, Ma, Yu and Chen2022). For example, Liu et al. (Reference Liu, Zhang, Li, Fan, Dong, Jiao, Chang, Gu, Zhang and Wang2021) established a new C/S ratio data set from surface sediments of five rivers (Aojiang, Feiyunjiang, Jiaojiang, Oujiang, and Qiantangjiang Rivers), as well as core sediments offshore the Oujiang Estuary. Based on the C/S ratios from the various environmental conditions, they suggested that a C/S ratio of 2.8 can effectively separate freshwater environments (river sediments) from marine environments (Liu et al., Reference Liu, Zhang, Li, Fan, Dong, Jiao, Chang, Gu, Zhang and Wang2021). But these studies suggested that C/S ratios should be interpreted with other geochemical indicators, because sedimentation processes, including influence of sea-level change, can modulate the diagenetic path in the marine sediments (Chang et al., Reference Chang, Liu, Wang, Zhuang, Ma, Yu and Chen2022; Liu et al., Reference Liu, Zhang, Li, Dong, Zhang, Gu, Chang, Zhuang, Li and Wang2022). Thus, the combined information of C/S ratios, the δ34S values, and δ13C values in coastal sediments can be used to diagnose past anoxia and test possible controlling factors.
Here, we report and discuss the content and isotopic composition of organic carbon and total sulfur in southern coastal sediments of South Korea. We trace past changes in C/S ratios, the δ34S values, and δ13C values in coastal sedimentary sequences and interpret these changes in terms of salinity and anoxic condition changes during the Holocene at multicentennial to millennial timescales.
STUDY AREA
The study area, a small paleo-bay with an area of ~3 km × 3 km, is located where the Seomjin River drains into the northern reaches of Gwangyang Bay, forming an estuarine environment, as shown in Figure 1. There are elongated sandbars in the present river mouth on the northwest of Gwangyang Bay. Subaqueous parts of the sandbars are migrating seaward, forming sandy-silt tidal–subtidal flats on the south inlet of the study area, resulting in a semi-isolated bay environment. The present tide at the Gwangyang Bay and its surrounding coastal areas is semidiurnal with an amplitude of 3 m. This coastal area is characterized by low water exchange between the bay and the outside coastal waters due to subaqueous sills and narrow inlets. The surface sediments in the bay mainly consist of mud and sandy mud, and the benthic environment is partly anoxic, as evidenced by the C/S ratios and hydrogen sulfide content (1–367 ppm) (Hyun et al., Reference Hyun, Lee, Choi, Choi and Woo2003).
Study areas (Gwangyang Bay and Goheung Bay) and coring sites on the southern coast of Korea. (A–D) Coring sites: STP17-14 (this study), STP16-20 (Lim et al., Reference Lim, Lee, Hong, Park, Lee and Yi2019), STP17-13 (Park et al., Reference Park, Park, Yi, Kim, Lee and Choi2019), and STP18-05 (Lim et al., Reference Lim, Yi, Han, Park and Kim2022). (E and F) Testing past geomorphological changes between 1920 and 2013. A geomorphological map made in the 1920s showed that study area including a coring site (STP17-14) was a small semi-closed bay characterized by low-lying mountain-fringed coastal lines.
METHODS
Part of the study area has been reclaimed as rice fields. Based on a geomorphological map made in the 1920s showing a small semi-closed bay characterized by low-lying mountain-fringed coastal lines, we selected a coring site (STP17–14) in the central part of the bay. We recovered sedimentary cores 27 m in length in 2017 using a rotary corer, which returns a core sample 1 m in length and 50 mm in diameter in a plastic liner (Fig. 2). After taking pictures, we subsampled the cores at 5 cm intervals. During this process, plant fragments for radiocarbon dating (n = 16) were selected. To test the sedimentation rate (SR) change in core STP17–14 in terms of regional perspective, we collected additional plant fragments (n = 18) from the core STP17–13 located at the mouth of Seomjin River (Park et al., Reference Park, Park, Yi, Kim, Lee and Choi2019). After pretreatment, including application of the acid–alkali–acid method and graphitization, age dating was performed using the accelerator mass spectrometry (AMS) facility at the Korea Institute of Geoscience and Mineral Resources (KIGAM).
(A) Lithological features, (B) radiocarbon dates and sedimentation rate (SR) change, (C) age–depth model generated from the 14C ages using the R code package clam (Blaauw, Reference Blaauw2010), and (D) photograph of core STP17-14 from Gwangyang Bay on the southern coast of Korea.
To analyze the total organic carbon (TOC) content and its isotope (δ13CTOC), bulk subsamples of ~500 mg were treated with 1 N HCl at ~100°C for 1 h to remove carbonate and then rinsed with distilled water. Samples of approximately 3–5 mg HCl-treated subsamples were loaded into tin combustion cups, and TOC contents were determined using a carbon, nitrogen, and sulfur (CNS) elemental analyzer (vario MICRO Cube, Elementar, Langenselbold, Germany). Simultaneously, stable carbon isotope (δ13C) analyses were performed using a continuous-flow isotope ratio mass spectrometer (Isoprime100, G.V. Instruments, Manchester, UK) coupled to the CNS elemental analyzer. The results are expressed in delta (δ) notation relative to the Vienna Pee Dee belemnite standard. The reference material was CH-6 (sucrose, δ13C = −10.45‰ ± 0.033‰), which was obtained from the International Atomic Energy Agency (IAEA). Replicated analyses had precision >0.2‰. Analyses of the total sulfur (TS) content and its isotope (δ34STS) were performed with bulk subsamples using the same continuous-flow isotope ratio mass spectrometer coupled to the CNS elemental analyzer. The results are expressed in delta (δ) notation relative to the Vienna Canyon Diablo Troilite. The reference material used was NBS127 (seawater sulfate, δ34S = 20.3‰ ± 0.4‰) obtained from the IAEA. Replicated analyses had precision >0.6‰.
The pyrite content of sediments (n = 22) was determined at 3.4, 4.0, 4.8, 5.6, 6.2, 6.8, 8.2, 9.2, 9.8, 10.8, 11.6, 12.2, 13.2, 13.8, 14.2, 14.8, 16.8, 18.2, 19.0, 19.6, 20.2, and 20.4 m, using finely ground samples with a Panalytical X'Pert3 powder diffractometer (Cu Kα radiation; 45 kV; 40 mA). SIROQUANT (v. 3.0, Sietronics, Canberra, Australia) software was used to quantify mineralogy.
RESULTS
Age dating and lithological features
The sedimentary cores from the paleo-bay were formed mainly during the Holocene period based on the 14C dating results (Tables 1 and 2, Figs. 2 and 3). The sedimentary core STP17-14 can be divided into six units based on lithological features: Unit 1, gray silty sand layer overlaid on the weathered bedrock, located at 26.5–25.8 m; Unit 2, gray coarse-grained sand layer with little structure, located at 25.8–24.0 m; Unit 3, gray silty sand unit with clear sand layers corresponding to ca. 9600–8900 cal yr BP and located at 24.0–19.6 m; Unit 4, green-gray silty mud grains with no clear layers corresponding to ca. 8900–500 cal yr BP and located at 19.6–3.2 m; Unit 5, silty sand sediment, corresponding to ca. 500 cal yr BP to present, located between 3.2 and 1.1 m; and Unit 6, the uppermost layer, consisting of reworked gravel and sandy deposits formed by reclamation.
Results of accelerator mass spectrometry (AMS) 14C dating and calibrated dates for core STP17-14.
a Calibrated with Radiocarbon Calibration Program (CalPal), http://c14.arch.ox.ac.uk/embed.php?File=oxcal.html.
b Excluded from SR calculation.
Results of accelerator mass spectrometry (AMS) 14C dating and calibrated dates for core STP17-13.
a Calibrated with Radiocarbon Calibration Program (CalPal), http://c14.arch.ox.ac.uk/embed.php?File=oxcal.html.
b From Park et al. (Reference Park, Park, Yi, Kim, Lee and Choi2019).
Radiocarbon dates, high sedimentation rate (SR) events, and photograph of core STP17-13 recovered in the Seomjin River estuary on the southern coast of Korea (modified from Park et al., Reference Park, Park, Yi, Kim, Lee and Choi2019). The age–depth model for core STP17-13 was generated from the 14C ages by using the R code package clam (Blaauw, Reference Blaauw2010).
This study revealed significant SR changes during the Holocene based on the dating results (n = 16). As shown in Figure 2, in the Early Holocene, SR was not stable and fluctuated with higher SR periods of 9400–8900 cal yr BP and 8300–7500 cal yr BP. In the Middle Holocene, SR showed a decreasing trend, with one SR peak at 5800–5500 cal yr BP. The gradual decrease in SR during the Late Holocene was disturbed by a recent SR increase at 520–150 cal yr BP. The age–depth model for core STP17-14 was generated from the 14C ages by using the R code package clam (Blaauw, Reference Blaauw2010; Fig. 2).
The use of a combination of additional ages (n = 18) dated in this study and previously reported ages in sedimentary core STP17-13 (Park et al., Reference Park, Park, Yi, Kim, Lee and Choi2019) indicated rapid SR changes during the Early Holocene, including SR events centered at 9800, 9100, 8200, and 7900 cal yr BP (Table 2 and Fig. 3). These SR changes were compared with those of core STP17–14 to test whether they were local to the Seomjin River estuary or at least regional.
Geochemical/isotopic analyses of TOC, TS, and their relationships
No information is available regarding TOC% in Units 1 and 2, because we analyzed samples from Units 3–5 based on age dating results (Fig. 4). In Unit 3, which consisted of a silty sand layer, TOC% showed relatively low values of about 0.3–0.7%. In Unit 4, TOC% increased, with a long-term trend from 0.4% at 19.6 m to 1.6% at 5.7 m, and then decreased. The decreasing trend in the upper part of Unit 4 was continued in Unit 5, reaching 0.15% at 1.4 m. TS% showed a slight increasing trend in Unit 3, and the lower part of Unit 4, reaching 0.55% at 14.4 m. An abrupt increase in TS% was found at 14.2 m with a value of 1.8%, corresponding to the maximum value during the Holocene, and then a gradual decreasing trend was dominant in the rest of Unit 4 and Unit 5. The δ13CTOC values showed rather simple patterns following the features of each unit. In Unit 3, δ13CTOC values were relatively low, fluctuating between −25 and −23‰. At the starting point of Unit 4, the δ13CTOC value increased rapidly to −22‰ and then remained stable, although there were slight fluctuations of less than 1‰, except for one anomaly of −19.8‰ at 5.8 m. Unit 5 showed decreasing δ13CTOC values from −22.3‰ at 3.2 m to −24.5‰ at 1.4 m.
Lithological unit features, radiocarbon dates, and results of geochemical (TOC, total organic carbon; TS, total sulfur), isotopic (δ13CTOC and δ34STS), and mineral analyses of sedimentary core STP17-14 from Gwangyang Bay on the southern coast of Korea. The depth profile was divided into five stages according to the geochemical and isotopic features, mainly based on their averages.
As shown in Figure 4, δ34STS values were characterized by significant fluctuations. The δ34STS values showed a wide range of variation between −32 and 5‰. In Unit 3, there were three remarkable peaks with δ34STS values of ca. −11 to −8‰. Unit 4 showed clustering of higher δ34STS values up to 6‰ located at depths of 14.2 to 13.2 m. Then, a gradual decreasing trend was observed in order of increasing depth, reaching a minimum δ34STS value of −32‰. In Unit 5, δ34STS values increased to −7‰. C/S ratios in the depth profile changed within a narrow range of 0.3–4.8, showing relatively low values in Unit 4 compared with Units 3 and 5. These elemental and isotopic data showed a long-term change consisting of five stages that reveal geochemical features of the deviation from the averages of the sedimentary δ13CTOC values, TS%, δ34STS values, and C/S ratios (Fig. 4).
Pyrite% was measured at irregular intervals to check its content in the sediments of low or high δ34STS values. For example, the sediments at 3.4 m with the minimum δ34STS value of −32‰ had a pyrite content of 3.4%, and at 14.2 m with the maximum δ34STS value of 5.2‰, the pyrite content was 3.1%, with values of 1.8–3.8% in Unit 3 and the upper part of Unit 4.
DISCUSSION
Long-term coastal response to Holocene sea-level change
As indicated in many previous studies, it is clear there were significant sea-level changes of up to 20 m during the Holocene (Tanigawa et al., Reference Tanigawa, Hyodo and Sato2013; Lambeck et al., Reference Lambeck, Rouby, Purcell, Sun and Sambridge2014). To specify past long-term coastal environmental changes, it is essential to constrain water depth in relation to the sea level at that time. There are few reconstruction records of Holocene sea-level changes on the southern coast of Korea, including the present study area. We estimated past water-depth changes in the coring site (Fig. 5) based on the reconstructed sea-level changes from the Yellow Sea and East China Sea (Lambeck et al., Reference Lambeck, Rouby, Purcell, Sun and Sambridge2014 and references therein). Based on these estimated water depths, the present tidal range (~3 m), and geochemical information, reconstructions of past coastal environmental changes in the paleo-bay located in the northern part of Gwangyang Bay were created and tested in terms of spatial coastal changes by comparison with other coastal changes on the southern coast of Korea.
Changes in total organic carbon (TOC), total sulfur (TS), C/S ratios, their isotopes (δ13CTOC and δ34STS), pyrite, and sedimentation rate (SR) of core STP17-14 from Gwangyang Bay on the southern coast of Korea during the Holocene. Past water depth in the coring site was estimated based on the reconstructed past sea levels from the Yellow Sea and East China Sea (Lambeck et al., Reference Lambeck, Rouby, Purcell, Sun and Sambridge2014 and references therein). The Holocene environmental change in the study area was divided into five stages according to the geochemical and isotopic features.
Water depth at the coring site during the Holocene was controlled by both sea-level change and SR (or sediment accumulation rate) (Bird et al., Reference Bird, Fifield, Teh, Chang, Shirlaw and Lambeck2007). Water depth can be approximated by subtracting the elevation of the sediment–water interface (or seafloor) in the core from the elevation of reconstructed sea-level curve (Lambeck et al., Reference Lambeck, Rouby, Purcell, Sun and Sambridge2014 and references therein). As shown in Figure 5, the reconstructed sea level at ~9600 cal yr BP was −18 m, and the sediment–water interface elevation in the core site was −24 m, suggesting a water depth of ~6 m at that time. Considering the present tidal range with an amplitude of ~3 m in Gwangyang Bay, the coring site seems to have been included in a subtidal environment, indicating that the coring site experienced transgression before 9600 cal yr BP. This water depth increased according to the subsequent sea-level rise and reached up to 17 m at 8200 cal yr BP, suggesting that the coring site became a small semi-closed bay. After that, it is likely that the water depth decreased, and the study site then had an intertidal to subtidal environment at 2200 cal yr BP. This long-term coastal evolution traced by simple comparison with the reconstructed sea-level changes can be supported by geochemical indicators, including sedimentary δ13CTOC values, δ34S values, C/S ratios, and SRs.
The present surficial δ13CTOC values along the river and coastal areas in China and Korea show the endmember to trace past coastal environments (e.g., Zhan et al., Reference Zhan, Wang, Xie, Xie and He2011; Williams et al., Reference Williams, Dellapenna, Lee and Louchourarn2014). For example, Zhan et al. (Reference Zhan, Wang, Xie, Xie and He2011) reported different δ13C values according to modern sedimentary environments from the lower Yangtze River to the East China Sea. In the river, the δ13C values from suspended particulate matter and sediments range from −28 to −24.4‰, and values in the mid- to lower river mouth decrease to −24.3 and −20.5‰. Samples from shallow-marine deposits show values between −22.7 and −20‰. A similar pattern was reported from modern surficial sedimentary δ13C values along the Yeongsan Estuary, Korea (Williams et al., Reference Williams, Dellapenna, Lee and Louchourarn2014). The δ13C of modern lake surficial sediments located in the upper part of the Yeongsan Estuary varied from −26 to −24‰. Inner estuary sediments ranged between −23.5 and −18‰, whereas outer estuary and coastal sediments had the highest values, varying between −18 and −16.5‰ (Williams et al., Reference Williams, Dellapenna, Lee and Louchourarn2014), suggesting clear spatial differences in the δ13C values.
Sedimentary δ13CTOC values in this study varied between −25 and −21.5‰ with an average of −22.5‰ (except for a sample with −19.0‰) and showed a long-term change consisting of five stages that reveal geochemical features of the deviation from the average, for example, the δ13CTOC values, TS%, δ34STS values, and C/S ratios (Fig. 5). Based on the present surficial δ13CTOC values along the river and coastal areas in China and Korea (e.g., Zhan et al., Reference Zhan, Wang, Xie, Xie and He2011; Williams et al., Reference Williams, Dellapenna, Lee and Louchourarn2014), during the period between 9600 and 8400 cal yr BP corresponding to Stages 1 and 2, the sedimentary δ13CTOC values fluctuated between −25 and −23‰, suggesting a significant terrestrial organic input. In Stage 1, there were significant changes in SR and δ34STS values. Stage 1, which covers the period 9600–8900 cal yr BP, showed a relative increase in SR up to 7.1 mm/yr and enriched peaks of δ34STS values at 8900, 9200, and 9500 cal yr BP. In Stage 2, which corresponds to the period 8900–8400 cal yr BP, both SR and δ34STS values decreased. This stage was followed by a clear shift to a bay environment with a water depth of 16 m in response to the Early Holocene sea-level rise, as shown by the increase in δ13CTOC value from −24 to −22‰. This bay environment of Stage 3 in the period 8400–7500 cal yr BP was characterized by a remarkable increase in SR up to 20.4 mm/yr and enrichment of δ34STS up to +5‰ coupled to the minimum C/S ratio (0.4) and maximum pyrite content (3.8%), suggesting the frequent occurrence of significant anoxic conditions. In Stage 4, the stable δ13C values during the Middle to Late Holocene indicated relatively stable coastal environments in the study area. During this stage, SR and δ34STS values decreased gradually, reaching 0.7 mm/yr and −30‰, respectively. Since 1500 cal yr BP (late Stage 4), the shallow coastal environments shifted to tidal-influenced environments, as shown by the deceased δ13CTOC value of −24‰ and water depth of ~3 m. In the relatively recent period from 500 cal yr BP to the present (Stage 5), SR increased to 5.4 mm/yr and δ34STS increased to +10‰.
Changes in the δ34S values and sedimentation rate during the Early Holocene
The sedimentary isotope records in this study clearly indicated changes in the δ34STS values and related coastal environmental changes in a semi-isolated bay during the Early Holocene. As pointed out by Chen et al. (Reference Chen, Liu, Shieh and Liu2004), the δ34S value in coastal sediments shows different ranges between inorganic (predominantly pyrite) and organic components. For example, organic δ34S values vary from −2.0 to +10.0‰ and inorganic sulfur values vary from +15.2 to −27.6‰ (Chen et al., Reference Chen, Liu, Shieh and Liu2004). Compared with these values, the δ34STS values in core STP17-14 showing much depleted variation between −32 and +5‰ may have been affected mainly by inorganic components, although the influence of organic sulfur cannot be excluded. This can be supported by the high pyrite content (2–3.5%) in Figure 5, suggesting that TS mainly consisted of pyrite. Thus, the δ34STS values may provide information mainly about pyrite formation in the coastal sediments and this assumption will be tested in the following discussion.
Isotopic fractionation during the metabolic processes of pyrite formation can be influenced by a variety of biochemical and environmental conditions, including limitation of sulfate, organic matter, iron minerals, and depositional environment (e.g., SR) (e.g., Berner, Reference Berner1984; Dellwig et al., Reference Dellwig, Watermann, Brumsack, Gerdes and Krumbein2001; Werne et al., Reference Werne, Lyons, Hollander, Formolo and Damsté2003; Pasquier et al., Reference Pasquier, Sansjofre, Rabineau, Revillon, Houghton and Fike2017; Liu et al., Reference Liu, Fike, Li, Dong, Xu, Zhuang, Rendle-Buhring and Wan2019). The δ34S values in the sedimentary cores in this study were mainly formed after transgression, indicating that there was sufficient sulfate in the paleo-bay provided by seawater. As shown in Figure 5, there was no significant covariance between TOC content and δ34S value, suggesting that organic matter input was not the main factor regulating the δ34S values in this study. The coring site in this study is located near the river mouth, and iron was available during the process of pyrite formation.
Recently, a possible linkage between the δ34S values of pyrite and SR change was reported from a study using a 300 m drill core of Mediterranean sediments deposited over the past 500,000 yr (Pasquier et al., Reference Pasquier, Sansjofre, Rabineau, Revillon, Houghton and Fike2017). This study revealed that the fluctuations of the δ34S values of pyrite in glacial–interglacial timescales have been affected by sea-level and temperature changes controlling SR change in the study area. Pasquier et al. (Reference Pasquier, Sansjofre, Rabineau, Revillon, Houghton and Fike2017) suggested that the change in the δ34S values of pyrite could be influenced by local depositional conditions, especially changes in SR that control connectivity with the overlying water column. Related to a possible millennial-timescale linkage is the report by Liu et al. (Reference Liu, Fike, Li, Dong, Xu, Zhuang, Rendle-Buhring and Wan2019), who suggested that the δ34S values of pyrite ranging between −38.2 and +15‰ during the last 12,500 cal yr BP were influenced by SR changes based on geochemical and isotopic studies using the 60 m drilled core sediments from the inner shelf of the East China Sea. This study clearly showed the δ34S values have been controlled by SR change in the inner shelf linked to the winter monsoon–driven coastal current intensity. For example, the δ34S values are markedly depleted during the intervals with lower SR (δ34S values of pyrite < −30‰) relative to that observed during the intervals with higher SR (δ34S values of pyrite >0‰). These results were attributed to limited connectivity between porewaters and overlying seawater, in which biological fractionation is oppressed, resulting in enriched δ34S values.
As shown in Figures 5 and 6, the SR change in the study area was closely correlated with the long-term sedimentary δ34S values during the Early Holocene. The remarkable variability superimposed on the long-term change indicated fluctuation of the δ34STS values on multi-centennial timescales (indicated by the letters c–e in Fig. 6). These occurrences of higher δ34STS values were consistent with the higher SR and increased pyrite deposition rate of 3.8%. To examine whether the observed synchronous changes in SR and δ34STS values were local or regional, we performed additional high-resolution age dating using sedimentary cores recovered from the Seomjin River estuary (Park et al., Reference Park, Park, Yi, Kim, Lee and Choi2019) and calculated SR changes in the sedimentary core STP17–13 (Fig. 3). As shown in Figure 6, SR in core STP17–13 was similar to the fluctuations of SR and δ34STS values of core STP17–14. For example, the enriched δ34STS events at 8900, 8100, and 7900 cal yr BP in core STP17–14 corresponded to increased SR in core STP17–13 within a dating error of ~100 yr. Similarly high SR has been found in a river mouth located on the southern coast of Korea at around 8000 cal yr BP and attributed to seawater spilling onto the site as a result of sea-level rise (Lim et al., Reference Lim, Lee, Kim, Hong and Yang2015). This suggests that the SR changes responsible for the enriched δ34STS events in core STP17–14 were not driven by local geomorphological features in the paleo-bay. The similarity between sedimentary δ34S values and SR changes identified in this study from the southern coast of Korea supports the recent results showing SR-controlled δ34S values in ocean-bottom sediments, resulting in open or closed diagenetic processes (Pasquier et al., Reference Pasquier, Sansjofre, Rabineau, Revillon, Houghton and Fike2017; Liu et al., Reference Liu, Fike, Li, Dong, Xu, Zhuang, Rendle-Buhring and Wan2019).
Comparison of total sulfur (TS), C/S ratios, δ13CTOC and δ34STS, and sedimentation rate (SR) of core STP17-14 with SR change in core STP17-13 on the southern coast of Korea during the Early to Middle Holocene. Past water depth in the coring site was estimated based on the reconstructed past sea levels from the Yellow Sea and East China Sea (Lambeck et al., Reference Lambeck, Rouby, Purcell, Sun and Sambridge2014 and references therein).
Furthermore, significantly increased (more than ~10 times) SR could have supplied increased organic matter to the water column, encouraging oxygen consumption, although relative organic matter content (TOC%) was not high. Under this marine setting, the δ34S value may have been influenced by the combination of fractionation through bacterial sulfate reduction (> −50‰) and a reservoir effect from the water column, which can suppress net 34S depletion in H2S and pyrite due to limited sulfate supply in the local reservoir. Given that the study site is characterized by its semi-enclosed geomorphology and limited water circulation resulting in strong seasonal stratification (Lee et al., Reference Lee, Park, Lim, Yoon and Kim2018), this reservoir effect coupled with significantly increased SR could have been amplified in the syngenetic process, although it is difficult to specify relative contribution of pyrites formed syngenetically (before burial) from dominant ones formed diagenetically (after burial) in this study.
Centennial- to millennial-timescale variability of salinity and anoxic conditions during the Holocene
As shown in Figures 5 and 6, the time series of the δ34STS values and C/S ratios revealed significant fluctuations during the Holocene, suggesting possible centennial to millennial variability in salinity and anoxic conditions.
To examine whether these fluctuations occurred locally or regionally, we compared the time series of C/S ratios in this study with those from Goheung Bay on the southern coast of Korea (Lim et al., Reference Lim, Lee, Hong, Park, Lee and Yi2019) and from the previously seawater-filled Daesan Basin (paleo-Daesan Bay) located in the middle reach of the present Nakdong River, Korea (Lim et al., Reference Lim, Yi, Han, Park and Kim2022). Based on the relationship between salinity and C/S ratio (Berner, Reference Berner1984; Berner and Raiswell, Reference Berner and Raiswell1984; Woolfe et al., Reference Woolfe, Dale and Brunskill1995 and references therein) and differences from present river and inner-continental sediments (e.g., Liu et al., Reference Liu, Zhang, Li, Fan, Dong, Jiao, Chang, Gu, Zhang and Wang2021, Reference Liu, Zhang, Li, Dong, Zhang, Gu, Chang, Zhuang, Li and Wang2022; Chang et al., Reference Chang, Liu, Wang, Zhuang, Ma, Yu and Chen2022), the C/S ratios at the three sites (cores STP16-20 [Lim et al., 2019], STP17-14 [this study], and STP18-06 [Lim et al., 2022]) revealed frequent coastal environmental changes in terms of anoxic conditions (Fig. 7). Given that core STP18-06 is located in the middle reach of Nakdong River, the similar variability found among these cores suggests simultaneous responses to seawater-related environmental changes at that time. Based on the average values, these fluctuations can be roughly divided into five periods with reduced C/S ratios, suggesting higher salinity and more intensified anoxic conditions: 8900–8200, 7950–6500, 5200–4300, 3500–2600, and 2000–1100 cal yr BP.
(A–C) Comparison of changes in the C/S ratio of core STP17-14 (this study) with those of core STP16-20 from Goheung Bay on the southern coast of Korea (Lim et al., Reference Lim, Lee, Hong, Park, Lee and Yi2019) and core STP18-06 from Daesan Plain (paleo-Daesan Bay), located at the middle reach of the Nakdong River, Korea (Lim et al., Reference Lim, Yi, Han, Park and Kim2022). See Fig. 1 for details. (D) Air temperature during the Holocene reconstructed by using argon and nitrogen isotopes within trapped air in a Greenland ice core (Kobashi et al., Reference Kobashi, Menviel, Jeltsch-Thömmes, Vinther, Box, Muscheler and Nakaegawa2017). (E) Holocene Mg/Ca-derived sea-surface temperature (SST) from foraminifers in the sediment cores in the western tropical Pacific Ocean (Stott et al., Reference Stott, Cannariato, Thunell, Haug, Koutavas and Lund2004). (F) Asian summer monsoon index (Dykoski et al., Reference Dykoski, Edwards, Cheng, Yuan, Cai, Zhang, Lin, Qing, An and Revenaugh2005).
At present, the hypoxic event on the southern coast of Korea is a chronic problem that occurs every summer (Lim et al., Reference Lim, Diaz, Hong and Schaffner2006). For example, Jinhae Bay, where physical energy is low due to its semi-enclosed geomorphology and limited water circulation, experiences strong seasonal stratification (Lee et al., Reference Lee, Park, Lim, Yoon and Kim2018). During March to May, thermal stratification is formed due to solar heating, decreasing bottom-water dissolved oxygen. These hypoxic conditions are greatly intensified by thermal stratification due to summer solar heating. Furthermore, this water column stability is intermittently strengthened by freshwater discharges. From the end of summer in late August, the stratification is destroyed by active vertical mixing between surface and bottom waters due to thermal cooling and elevated wind forcing (Lee et al., Reference Lee, Park, Lim, Yoon and Kim2018). Past anoxic conditions in the study area could have been influenced mainly by temperature, and the factors responsible for controlling possible anoxic conditions in the southern coastal areas in Korea may have been limited water circulation and thermal stratification linked to temperature.
To examine the possible influence of temperature on past anoxic condition changes in the study area, we performed comparisons with air temperatures reconstructed by using argon and nitrogen isotopes within trapped air in a Greenland ice core (Kobashi et al., Reference Kobashi, Menviel, Jeltsch-Thömmes, Vinther, Box, Muscheler and Nakaegawa2017). Interestingly, the fluctuation of the C/S ratios was quite similar to the temperature changes in Greenland Summit during the Holocene in terms of centennial to millennial timescales (Fig. 7E). As shown in Figure 7, the periods of higher temperature in Greenland correspond to decreased C/S ratios. For example, the proposed periods with more intensified anoxic conditions seem to have occurred during the synchronous high-temperature periods (7950–6500, 5200–4300, 3400–2600, and 2000–1000 cal yr BP), suggesting possible linkage between them. Kobashi et al. (Reference Kobashi, Menviel, Jeltsch-Thömmes, Vinther, Box, Muscheler and Nakaegawa2017) reported that the reconstructed temperature of Greenland was correlated with significant atmospheric circulation patterns over Greenland and monsoon activity in Oman and China, indicating that the Greenland temperature changes were hemispheric signals. Based on this suggestion, the similarity observed in this study suggests that the fluctuations in C/S ratios may have been influenced by global temperature and atmospheric circulation changes. At present, the hypoxic conditions on the southern coast of Korea are mainly controlled by the intensity and duration of thermal stratification due to solar heating according to seasonal change. It is likely that the higher air temperature conditions linked to higher temperatures in Greenland could have intensified the thermal stratification (less ventilation and mixing) and resultant anoxic conditions, and the higher temperatures could have extended the anoxic conditions in the following autumn by delaying vertical mixing due to higher temperatures and reduced wind forcing.
With regard to the possible influence of sea-surface temperature (SST) on coastal anoxic conditions, the Mg/Ca-derived SST from foraminifers in the sediment cores in the western tropical Pacific Ocean (Stott et al., Reference Stott, Cannariato, Thunell, Haug, Koutavas and Lund2004; Fig. 7F) seem to have affected the anoxic conditions to some extent. For example, the significantly increased anoxic conditions at 8800–8200, 7950–6500, 5200–4300, 4000–3600, 3400–2600, and 2000–1000 cal yr BP corresponded to higher SST periods partly associated with millennial-timescale fluctuations (Fig. 7). Isono et al. (Reference Isono, Yamamoto, Irino, Oba, Murayama, Nakamura and Kawahata2009) reported clear millennial-timescale periodicity (e.g., 1470 yr) in SST changes in the northwestern Pacific off central Japan during the Holocene. They suggested a climatic link between the North Pacific gyre system responsible for changes in the SST and the pathway of the Kuroshio Current and the high-latitude North Atlantic thermohaline circulation. The similarity in fluctuations between C/S ratios and SST in the western tropical Pacific suggest that the millennial-timescale changes in anoxic conditions in coastal areas of East Asia may have been linked to SST in the western tropical Pacific through atmospheric–oceanic circulation changes. In addition, the increased SST signals shown in Figure 7 may have been transferred to the southern coast of Korea by the Kuroshio Current and Tsushima Current, which are responsible for transporting heat energy in the western Pacific Ocean, influencing the chances of thermal stratification between the surface and bottom waters in the study area.
The C/S ratios in the bay in this study could have been affected by freshwater input mainly driven by the East Asian summer monsoon. Lim et al. (Reference Lim, Lee, Kim, Hong and Yang2015) suggested that sedimentary C/S ratios in the river mouth and surrounding areas could have been controlled by past freshwater input events during the Holocene. To test this, we compared the C/S ratios with the change in the Asian summer monsoon intensity reconstructed from the δ18O values of a speleothem in Dongge Cave, South China (Dykoski et al., Reference Dykoski, Edwards, Cheng, Yuan, Cai, Zhang, Lin, Qing, An and Revenaugh2005; Fig. 7F). The two data sets are quite different in terms of the long-term change and their millennial-timescale variability. This suggests that the influence of summer monsoonal rainfall on the C/S ratios in the study area (a semi-closed bay environment) was very weak and supports possible strong influences of atmospheric temperature and SST in the western tropical Pacific on the past coastal hypoxic and anoxic events.
CONCLUSIONS
To test possible change in coastal anoxic conditions driven by natural climate change, we reconstructed past coastal evolution and anoxic conditions of bottom water based on TOC%, TS%, C/S ratios, and isotopic values of δ13CTOC and δ34STS of coastal sedimentary cores. We identified five possible periods with intensified anoxic condition variability at millennial timescales characterized by enriched δ34STS values, increased SR, and decreased C/S ratio: 8900–8200, 7950–6500, 5200–4300, 3500–2600, and 2000–1100 cal yr BP. These intensified anoxic conditions seem to have been influenced mainly by global air temperature and SST conditions at these times, which could have intensified thermal stratification and resultant anoxic conditions. Our results demonstrate the possible utility of δ13CTOC, δ34STS, C/S ratios, and high-resolution SR records for tracing past coastal environment changes, including anoxic conditions. With regard to anticipation of global warming and coastal responses, temperature increases in the atmosphere and ocean will exacerbate the bottom-water conditions and increase hypoxic conditions, causing frequent anoxic conditions. Further studies in various areas using proxies for coastal environments (e.g., δ13CTOC, δ34STS, and C/S ratio) are required in terms of regional- and global-scale responses to climate change.
Acknowledgments
This research was supported by the Basic Research Project (GP2017-013, GP23-3111-3) of the KIGAM funded by the Ministry of Knowledge Economy of Korea.
Competing Interests
The authors declare that they have no competing interests.
