INTRODUCTION
Radiocarbon (14C) is a cosmogenic radioisotope continuously produced in the Earth’s atmosphere due to nuclear reaction 14N(n,p)14C. In the next step the new formed radioisotope is oxidized to produce 14CO2, and in this form through many different processes is incorporated into the global carbon cycle. Plants incorporate 14CO2 due to photosynthesis, and 14C concentration in biogenic material reflects its concentration in the atmosphere during the growing season.
The production rate changes with latitude and depends on the intensity of the cosmic ray flux and the global average value is in order of 2 atoms·cm−2·s−1 (Castagnoli and Lal Reference Castagnoli and Lal1980; Kvaltsov et al. Reference Kovaltsov, Mishev and Usoskin2012). Natural changes in the intensity of solar, interplanetary and Earth’s magnetic field are the main factors modulating the intensity of cosmic ray flux reaching the Earth’s atmosphere. The extraterrestrial high-energy events, such as solar proton events (SPEs), supernovae explosions (SNe), gamma-ray bursts (GRBs), have a significant impact on the 14C production rate. Such phenomena lead to a short-term increase in 14C concentration in the atmosphere, and through carbon exchange mechanisms, in other carbon reservoirs, and can be observed i.e. in annual tree rings.
Miyake et al. (Reference Miyake, Nagaya, Masuda and Nakamura2012) were the first to describe sharp and short-lasting increase of 14C (14C) concentration in the annual tree rings of the Japanese cedar (Cryptomeria japonica) between AD 774/775. The increase of 14C concentration for this event (henceforth M12) was about 12‰ between AD 774 and 775, and has been confirmed by several authors (Jull et al. Reference Jull, Panyushkina, Lange, Kukarskih, Myglan, Clark, Salzer, Burr and Leavitt2014; Güttler et al. Reference Güttler, Adolphi, Beer, Bleicher, Boswijk, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wunder2015; Rakowski et al. Reference Rakowski, Krapiec, Huels, Pawlyta, Dreves and Meadows2015; Büntgen et al. Reference Büntgen, Wacker, Galván, Arnold, Arseneault, Baillie, Beer, Bernabei, Bleicher and Boswijk2018). Results from samples of annual tree rings from Siberia (Jull et al. Reference Jull, Panyushkina, Lange, Kukarskih, Myglan, Clark, Salzer, Burr and Leavitt2014) indicate that the increase of 14C concentration in the atmosphere already began in AD 774, and continued to AD 776. This suggests that the reason for the increase in 14C concentration was not a single phenomenon, but rather a series of similar phenomena occurring over a longer period of time. The maximum 14C concentration for this event recorded in samples of the kauri tree (Agathis australis) from New Zealand is observed half a year later (Güttler et al. Reference Güttler, Adolphi, Beer, Bleicher, Boswijk, Christl, Hogg, Palmer, Vockenhuber, Wacker and Wunder2015), probably due to the Southern Hemisphere offset (Rodgers et al. Reference Rogers, Mikaloff-Flecher, Bianchi, Beaulieu, Galbraith, Gnannadesikan, Hogg, Iudicone, Lintner, Naegler, Reimer, Sarmiento and Slater2011).
Since then, other events similar to the M12 have been observed at various time intervals. Miyake et al. (Reference Miyake, Masuda and Nakamura2013, Reference Miyake, Masuda, Hakozaki, Nakamura, Tokanai, Kato, Kimura and Mitsutani2014) have observed another rapid increase of 14C content between 9.1‰ and 11.3‰ in the period AD 993–994. This has also been confirmed by other research teams (Fogtmann-Schultz et al. Reference Fogtmann-Schulz, Ostbo, Nielsen, Olsen, Karoff and Knudsen2017; Rakowski et al. Reference Rakowski, Krąpiec, Huels, Pawlyta and Boudin2018). Park et al. (Reference Park, Southon, Fahrni, Creasman and Mewaldt2017) have reported such event around 660 BC, Jull et al. (Reference Jull, Panyushkina, Miyake, Masuda, Nakamura, Mitsutani, Lange, Cruz, Baisan, Janovics, Varga and Molnar2018) observed 14C increase at 814–815 BC, Wang et al. (Reference Wang, Yu, Zou, Dai and Cheng2017) have observed rapid increase of 14C concentration at 3372–3371 BC, and Miyake et al. (Reference Miyake, Jull, Panyushkina, Wacker, Salzer, Baisan, Lange, Cruz, Masuda and Nakamura2017) have found a large 14C excursion in mid-Holocene around 5480 BC.
Based on the calculation of 14C production rate for the M12 event, Miyake et al. (Reference Miyake, Nagaya, Masuda and Nakamura2012) made a first attempt to explain the possible causes of this phenomena. The 14C production rate 6·108 atoms·cm−2, obtained for this event by applying four-box carbon cycle model, is very high, and could be explain only by a giant solar eruption or supernovae explosion. Since there is no evidence for SNe in this period of time, this possibility can be ignored. Using another carbon cycle model (5-box model), Usoskin et al. (Reference Usoskin, Kromer, Ludlow, Beer, Friedrich, Kovaltsov, Solanki and Wacker2013) had recalculated the 14C production rate for this event. The value obtained (1.3 ± 0.2) ·108 atoms·cm−2 is around 4 times smaller than presented in Miyake et al. (Reference Miyake, Nagaya, Masuda and Nakamura2012). This discrepancy was later explained (Miyake et al. Reference Miyake, Masuda, Hakozaki, Nakamura, Tokanai, Kato, Kimura and Mitsutani2014) as due to calculation error. The production rate for the Miyake event at AD 993/994 is smaller and has value (0.9 ± 0.2) ·108 atoms·cm−2 (Miyake et al. Reference Miyake, Masuda, Hakozaki, Nakamura, Tokanai, Kato, Kimura and Mitsutani2014). The 10Be and 36Cl measurements in ice core from Antarctica and Greenland also show a spike around AD 775 and AD 994, which indicates a large SEP or series of SEP as a potential cause of those events (Mekhaldi et al. Reference Mekhaldi, Muscheler, Adolphi, Aldahan, Beer, McConnell, Possnert, Sigl, Svensson, Synal, Welten and Woodruff2015). Typical GRB energy spectra and fluxes could generate significant amounts of 14C, however, they would not significantly affect 10Be production rate (Pavlov et al. Reference Pavlov, Blinov, Konstantinov, Ostryakov, Vasilyev, Vdovina and Volkov2013; Mekhaldi et al. Reference Mekhaldi, Muscheler, Adolphi, Aldahan, Beer, McConnell, Possnert, Sigl, Svensson, Synal, Welten and Woodruff2015).
Here, we report the measurement of 14C concentration along with stable isotopic composition of carbon, for a sub-fossil oak (Quercus robur) from southern Poland for the period 670–650 BC to search for increase in 14C concentration due to a Miyake effect.
SAMPLE AND METHODS
Sub-fossil oaks (Quercus robur L.) were recovered from a site in a gravel pit by the Vistula river in the village of Grabie, near Krakow (50.0391 N, 19.992 E; Figure 1). Slices of 100 oak trunks were taken for dendrochronological studies. After sample preparation to disclose visible anatomic structure and enable identification of annual growth rings, measurement tracks along 2–4 trunk radii were delineated. Measurements were made with 0.01 mm accuracy using a DENDROLAB 1.0 apparatus, then the ring-width sequences were processed with a set of computer programs TREE-RINGS (Krawczyk and Krąpiec Reference Krawczyk and Krąpiec1995), TSAP (Rinn Reference Rinn2005) and DPL (Holmes Reference Holmes1999).
Location of Grabie village (50.0391 N, 19.992 E).
The oaks from Grabie were felled at inundations in the Vistula river valley within the last two millennia BC. As some of them were long-lived, 300–400 year-old trees, two standards could be produced, which spanned the years 1750–1018 BC (chronology GAA_1U) and 994–612 BC (chronology GAA_3U) (Krąpiec Reference Krąpiec2001). Sample G58 was selected for 14C analysis, spans BC 797-620, dendrochronologically-dated using the standard for southern Poland oak, C_3000E (Krąpiec Reference Krąpiec2001) with t = 15.5, GL = 79%, where the t-value represents the significance of the correlation of two series in relations to their overlap and should not drop to a value below 3.5 (Baillie and Pilcher Reference Baillie and Pilcher1973). The GL was developed as a special tool for cross-dating of tree-ring series. The degree of similarity based on the positive or negative trend of each width is expressed as a percentage of the number of intervals (Eckstein and Bauch Reference Eckstein and Bauch1969).
Each annual tree ring from 670–650 BC was washed in distilled water, and α-cellulose was extracted using the Green (Reference Green and Whistler1963) protocol with some modifications. To accelerate the separation of single cellulose fibers from wood samples, and thus to increase the penetration of reagents, an ultrasonic bath was utilized (Michczyńska et al. Reference Michczyńska, Krapiec, Michczyński, Pawlyta, Goslar, Nawrocka, Piotrowska, Szychowska-Krąpiec, Waliszewska and Zborowska2018). Wood samples were dried and cut into shavings, then weighed and placed into glass tubes. Each sample was putted into glass tube with deionized water, sodium chlorite, and 1% HCl. The tube was then placed in an ultrasonic bath kept at 70°C for 1 hr, and then sodium chlorite and 1% HCl was added to each sample. This procedure was repeated five times, and the samples were then rinsed with hot deionized water up to neutral pH. This process removes most of the lignin and the remaining sample consists mostly of holocellulose. For the next step, 10% NaOH solution was added to the samples and then put into an ultrasonic bath at 70°C for 45 min. After the solution was removed, the sample was rinsed with cold deionizing water. Next, 17% NaOH solution was added and samples were put into ultrasonic bath at room temperature for 45 min. After this time deionizing water was added to the samples. This step removes rest of the remaining lignin and holocellulose. The sample were then rinsed with hot deionized water up to neutral pH. Finally, 1% HCl was added to the samples and the samples were rinsed to neutral pH. The obtained α-cellulose was dried overnight at 70°C.
About 4 mg of α-cellulose extracted from each sample was transferred into a prebaked (900°C) quartz ampoules together with CuO and Ag, evacuated to a pressure of 10−5 mbar, sealed and combusted for 4 hr at 900ºC in a muffle oven (Krapiec et al. Reference Krąpiec, Rakowski, Huels, Wiktorowski and Hamann2018). The resulting CO2 was released under vacuum and cryogenically purified for subsequent graphitization during the reaction with H2 at 600ºC, on an Fe catalyst (Nadeau et al. Reference Nadeau, Grootes, Schleicher, Hasselberg, Rieck and Bitterling1998).
The resulting mixture of graphite and Fe powder was pressed into a target holder for AMS 14C measurements. All prepared targets contain approximately 1 mg of carbon and were measured at either the Leibniz-Laboratory of the University of Kiel (Labcode KIA, Nadeau et al. Reference Nadeau, Grootes, Schleicher, Hasselberg, Rieck and Bitterling1998) or the Center for Applied Isotope Studies at the University of Georgia, USA (Labcode UGAMS; Cherkinsky et al. Reference Cherkinsky, Culp, Dvoracek and Noakes2010), respectively. The possible offset between those two AMS laboratories has been checked before and the results were presented in Krąpiec et al. (Reference Krąpiec, Rakowski, Huels, Wiktorowski and Hamann2018). 14C contents are reported as Δ14C in per mil (‰) deviations from the standard sample, 0.7459 activity of NBS oxalic acid (SRM-4990C). Age correction and isotopic compositions correction were calculated following formulas presented in (Stuiver and Polach, Reference Stuiver and Polach1977). The correction for isotopic composition was made based on δ13C measured with AMS system. The age calculation was presented in Nadeau and Grootes (Reference Nadeau and Grootes2013).
δ13C was measured in the subsamples of the same alpha-cellulose extracted for the need of 14C measurements. Composition of stable carbon isotopes was determined using Eurovector 3000 elemental analyzer (Eurovector Srl, Italy) coupled with IsoPrime isotope ratio mass spectrometer (Elementar UK Ltd.). 110–130 µg of α-cellulose, which gives about 60 µg of carbon, was weighed for each sample and packed in a tin capsule. The tin wrapped samples were combusted in the reactor set to 1030°C temperature in the excess of oxygen. Because of the oxidation of tin wrapping in the reactor, effective combustion temperature was a couple of hundreds of centigrade higher. Resulting gases were passing through Cu-filled reactor operating at 650°C and dried in the magnesium perchlorate filled water trap. Gases were then separated in a packed chromatographic column and transferred in a helium stream to open-split interface of IsoPrime. IRMS was working in a continuous-flow mode. IAEA-C3 cellulose sample (IAEA 2014) was used as a reference material, B2213 spruce powder certified material (Elemental Microanalysis Ltd) and WSTA wheat sample (Boettger et al. Reference Boettger, Haupt, Knöller, Weise, Waterhouse, Rinne, Loader, Sonninen, Jungner, Masson-Delmotte, Stievenard, Guillemin, Pierre, Pazdur, Leuenberger, Flot, Saurer, Reynolds, Helle and Schleser2007) were used to check both stability of the EA-CF-IRMS system and isotopic delta calculations. δ13C values were expressed in ‰ versus VPDB scale (Coplen et al. Reference Coplen, Brand, Gehre, Gröning, Meijer, Toman and Verkouteren2006). Samples were randomized in the measurement queue in order to minimize instrument drift effect on the time series data. Uncertainty of δ13C was estimated as a standard deviation of repeated monitoring/standard sample measurements.
RESULTS AND DISCUSSION
The measured results (F14C, Δ14C, and δ13C) with corresponding uncertainties are presented in Table 1. Figures 2 and 3 show Δ14C and δ13C for the period 670–652 BC. The series G58 has been measured at Leibniz Laboratory of the University of Kiel, except for two samples (G58-7 and G58-8), which have been measured at the Center for Applied Isotope Studies at the University of Georgia, USA.
F14C, Δ14C, and δ13CPDB values for annual tree rings of Quercus robur from Grabie village near Kraków (SE Poland). Uncertainty of δ13CIRMS measurements was estimated as 0.20‰.
Δ14C time series for G58 core (crosses) with a fitted sum of three periodic functions (smooth line), IntCal13 (grey-shaded area) (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffman, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013), Park et al. (Reference Park, Southon, Fahrni, Creasman and Mewaldt2017) data (double empty triangle) and raw atmospheric data (empty triangles, empty stars, empty diamonds, and empty rectangles). Raw datasets named as in the database (IntCal13 2013) Parameters of fit are presented in the Table 2.
δ13C time series for G58 core (dots) with a fitted sum of three periodic functions and a constant value. Parameters of fit are presented in the Table 2.
On the broad scale, the Grabie tree rings sequence between 670 BC and 652 BC showed a similar pattern to that seen in German oak reported by Park et al. (Reference Park, Southon, Fahrni, Creasman and Mewaldt2017). We observed a gradual increase in Δ14C of 18.2 ± 3.6‰ (p-value for the t-Student test of the increase is equal 0.055) from 665 BC (Δ14C = −3.1 ± 2.2‰) to 662 BC (Δ14C = 15.1 ± 2.8‰). In the first step, we observed an increase in Δ14C to ∼ 5.2‰ (Δ14C increase of 8.3 ± 2.9‰) between 665 and 664 BC, followed by a second jump in Δ14C to ∼ 15.1‰ (Δ14C increase of 10.3 ± 3.4‰) between 663 and 662 BC. After 662 BC, the high level of 14C concentration (Δ14C ∼ 16‰) lasted for the next several years until 659 BC, and then began to decrease. At the end of the analyzing period at 652 BC, we observed an increase in Δ14C to 14.1 ± 2.5‰, however without precise measurements around this period it is difficult to conclude the reason for such a high value. We found that Δ14C time series may be quite well described as a sum of three periodic functions above a constant level.
The sum of three periodic functions: Δi(t) = Ai·sin(2·π·t/Ti + 2·π·ϕAi/Ti) was fitted to the Δ14C time series. Levenberg-Marquardt routine from mpfit library version 1.3 and Fityk version 1.3.1 software (Wojdyr Reference Wojdyr2010) were used for periodic function fitting. Ti were chosen arbitrary to reflect three main observed Sun periodicities: 11, 22, and 88 yr (Usoskin Reference Usoskin2017, Peristykh and Damon Reference Peristykh and Damon2003). Similarly, an attempt was made to fit the sum of three periodic functions: δ13C(t) = δi·sin(2·π·t/Ti + 2·π·ϕi/Ti) plus a constant value to δ13C time series. Local maxima and minima for fitted functions for the period ranging from 700 BC to 600 BC were found using Scilab version 6.01 software.
Results of the periodic functions fitting are given in Table 2 and in Figure 2. The maximum of the fitted functions to the Δ14C time series in the time range from 700 BC to 600 BC is equal 20.71‰ (for 648.8 BC) while minimum is equal to −19.64‰ (for 699.2 BC). The estimated amplitude of the semi-periodic fluctuations fitted to Δ14C data is equal to about 20‰. Most of the Δ14C fluctuations (about 13‰) may be explained by 88-yr solar cycle (Table 2), up to about 7.2‰ may be described by 11-yr cycle. Influence of the 11-yr cycle on the fluctuations is small (less than 0.3‰).
Parameters of the sum of three periodic function fitting to the Δ14C time series and δ13C time series.
The results of our previous studies regarding the Miyake effect in AD 774/775 (Rakowski et al. Reference Rakowski, Krapiec, Huels, Pawlyta, Dreves and Meadows2015) and AD 993/994 (Rakowski et al. Reference Rakowski, Krąpiec, Huels, Pawlyta and Boudin2018) also indicated that they are a consequence of elevated solar activity. Similar to the results obtained for this study, our previous results also show an increase in 14C concentration, which took place gradually over several consecutive years. Park et al. (Reference Park, Southon, Fahrni, Creasman and Mewaldt2017) suggested that a combination of coronal mass ejection events (CME) and associated SPEs could have been a common cause for Miyake events around 660BC, AD 774/775 and AD 993/994. Single-year data and decadal data (from different places) included in IntCal13 (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffman, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013) show also rapid increase in 14C concentration around 660 BC, which suggests that this effect, similar to Miyake effects in AD 774/775 and AD 993/994, has a global character.
Increased solar activity, in addition to increased production of 14C and other cosmogenic isotopes, can directly affect the climate on the Earth. The results of study by Calisto et al. (Reference Calisto, Verronen, Rozanov and Peter2012) suggest that even smaller SEPs observed in 1859 (Carrington event; Clauer and Sicoe Reference Clauer and Siscoe2006) could lead to ozone depletion in atmosphere and to significant climate cooling. If such an effect occurred, it may be visible in stable isotopes composition of carbon and oxygen in cellulose from tree rings.
CONCLUSIONS
The results of our study confirm that the abrupt increase in 14C concentration around 660 BC is shown in annual tree rings of oak (Quercus robur) from southern Poland. The rapid increase in Δ14C between 663 and 662 BC of 10.3 ± 3.4‰ corresponds well with value reported by Park et al. (Reference Park, Southon, Fahrni, Creasman and Mewaldt2017). Also, the IntCal13 curve (Reimer et al. Reference Reimer, Bard, Bayliss, Beck, Blackwell, Bronk Ramsey, Buck, Cheng, Edwards, Friedrich, Grootes, Guilderson, Haflidason, Hajdas, Hatté, Heaton, Hoffman, Hogg, Hughen, Kaiser, Kromer, Manning, Niu, Reimer, Richards, Scott, Southon, Staff, Turney and van der Plicht2013) shows a large excursion in 14C concentration around this period. Those data have been obtained from annual rings of trees growing in different locations. This support the hypothesis that we dealing with global phenomena that has its sources in changes in the solar activity. The identification of such events is of great use for precise dating, for example by using the wiggle-match method. The δ13C data does not show clear periodic fluctuations, which suggests that these particular changes of solar activity did not induce abrupt climatic changes.
ACKNOWLEDGMENTS
We express our thanks to all the staff of the Radiocarbon Laboratory of Silesian University of Technology for their kind support. This work was supported by National Science Centre, Poland, grant UMO-2017/25/B/ST10/02329 and partly by AGH grant 11.11.140.005.
