Abstract
The mid-Holocene climate optimum saw warm temperatures in large parts of China, but its impact on seasonal environmental changes is not fully understood yet. Here, we use high-resolution geochemical analyses of 7000 to 6000 year-old oyster shells from the Yangtze River Delta to reconstruct climatic and oceanographic patterns. The stable isotope (δ18O, δ13C) and clumped isotope data reflect prominent seasonal changes in temperature, precipitation, and river discharge. Summer months experienced warm temperatures and a distinct increase in rainfalls and river discharge. In contrast, winter months were characterized by a dry season, which might have been longer than today. Stable isotope data also indicate regular summer upwelling in the study area. These results partly disagree with available climate models raising doubts on the models’ reliability. Thus, our palaeo-proxy data offers the possibility to evaluate and correct climate models and thereby improve predictions for the future considering on-going global warming.
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Introduction
During the mid-Holocene, most parts of China experienced a phase of warm temperatures, the so-called mid-Holocene climate optimum, which lasted from about 8000 to 6000 years BP depending on the exact locality1,2,3,4. In the Yangtze River Delta of eastern China (Fig. 1), the mid-Holocene saw prominent steps in the development of human civilization including the spread of settlements across the growing delta plain, the introduction of rice cultivation, and the formation of larger towns5,6,7,8. The fates of these early humans were strongly influenced by environmental changes, but humans generally prospered on the delta plain in the following millennia, which were characterized by a gradual cooling of up to 2 °C in average temperatures until pre-industrial times4,9. Today, the Yangtze River Delta is a densely populated region with enormous economic importance and a population of more than 100 million people in the delta region itself10. Following the latest IPCC report, global average temperatures are rising by 3 °C or more (compared to pre-industrial times) until the end of the 21st century, if humans are not able to drastically lower emissions of greenhouse gases11. The warm mid-Holocene provides a suitable scenario for East China to observe effects of a warmer climate on environmental parameters such as precipitation and monsoon strength.
The present study uses high-resolution geochemical analyses of fossil oyster shells from the Yangtze River Delta to reconstruct seasonal climate and oceanographic patterns in the mid-Holocene, which were unknown for the region so far. These proxy data are then compared with the results of climate models for the mid-Holocene, the conditions of today, and simulations for the future considering ongoing global warming. In combination, the study provides a better understanding of mid-Holocene environments as well as offers predictions for future environmental changes in the delta region.
Presently, the Yangtze River Delta is characterized by a humid subtropical climate with an annual temperature average of 15–16 °C (see Supplementary Information 1; Fig. S1). Winters are cold and dry with little precipitation. Summers are hot and humid with stronger rainfalls, particularly during the monsoon months (June-August). The discharge of the Yangtze River also follows a well-documented seasonal pattern with much higher water levels in the summer months compared to the dry winter. Oceanographic conditions in the adjacent East China Sea have been studied intensely in the last decades12,13,14,15,16,17, but detailed observations over long, centennial time intervals are lacking. In general, the salinity of coastal surface waters is determined by the seasonal strength in river discharge17,18,19,20,21. In addition, the interplay of the Taiwan Warm Current, the China Coastal Current (also called Zhejiang-Fujian Coastal Current), and the Kuroshio Current together with its branches contribute to the mixing of water masses and the influx of saline waters in the delta region16. Further research in recent years concentrated on the causal relationships and seasonal occurrences of alongshore winds, coastal upwelling, phytoplankton blooms, and hypoxia22,23,24,25,26,27,28,29, but observations spanning long-time intervals are still missing. It is at this point, that comparisons between present-day observations with seasonal reconstructions from the Holocene can add particularly valuable information.
Results
Three shells of the oyster Crassostrea gigas were selected from an oyster reef in the central delta plain, approximately 40 km southwest of Shanghai (Fig. 1). Radiometric dating resulted in conventional radiocarbon ages of 7030, 6550, and 6160 ± 30 years BP for shells 1 to 3 (Supplementary Information 1). The fossils (Fig. 2) were cut through the hinge along the major growth axis. Thin-sections covering the hinge region were produced and examined with petrographic and cathodoluminescence microscopes. Samples were taken with a computer-controlled micromill for stable isotope analysis and with a hand-held dental drill for clumped isotope analysis.
Interior views and sampled longitudinal sections of the three analyzed mid-Holocene shells of the oyster Crassostrea gigas from the Yangtze River Delta (lines indicate position of sections; A, B: Shell 1; C, D: Shell 2; E, F: Shell 3). Thin-sections through the hinges show darker bands reflecting growth increments (arrows indicate the position of winter growth breaks; G, H: Shell 1; I: Shell 3). Sketches of the hinge area of the three shells show the position of samples used for high-resolution stable isotope analysis (simple numbers) and clumped isotope analysis (CL1 to CL10). Dark-grey and brown areas inside the shells indicate discolorations avoided during sampling (J: Shell 1; K: Shell 2; L: Shell 3).
Macroscopic and microscopic observations
Macroscopic and microscopic observations (Fig. 2; compare Supplementary Information 1; Figs. S2–S6) show that the hinge is the most compact part of each shell. Growth increments are visible in most sections and comparatively wide in shells 1 and 2 and narrower in shell 3. The internal surfaces of the hinges are wrinkled, particularly in the older parts of the shells. Similar to earlier observations30, concave depressions on the hinge surface correspond to winter seasons, while convex ridges formed during summer months. However, this pattern is less clear in shell 3 and also seems to be lost in the younger parts of shell 1. Colour changes within growth bands also seem to reflect changing seasonal growth rates (e.g., via crowding of growth layers during winter months). Spawning shocks are not visible in the thin-sections or only cause faint, inconspicuous growth lines (compare Fig. 2G–I).
When examined with a cathodoluminescence microscope, the calcite of the hinge appears largely dark and non-luminescent (Fig. S5). Weak and cloudy orange luminescence can be seen along some of the growth layers. In contrast, strong luminescence characterizes the chalky parts of the shell. Following previous research, a predominantly non-luminescent, calcitic shell indicates no diagenetic alteration30,31,32,33. The comparatively weak luminescence seen along some growth layers is similar to previously published observations on recent shells of C. gigas34, which did not experience any diagenetic alteration35,36.
Following macroscopic and microscopic observations, the hinge areas of shells 1 to 3 were selected as targets for high-resolution stable isotope analysis (Fig. 2J–L; for a more detailed description of the fossils see Supplementary Information 1).
Stable isotope analyses
Results of the high-resolution stable isotope analyses of all three sampled shells show a prominent cyclic pattern in both δ18O and δ13C values (Fig. 3A, B; Fig. S7; Supplementary Data 1). Maxima in δ18O values (reflecting cold winter conditions) are relatively sharp, while minima (corresponding to summer months) are broader. The same pattern is also visible in δ13C values, which show a strong positive correlation to δ18O values (Fig. S8). However, the decrease in δ13C values at the beginning of each cycle is commonly delayed compared to the δ18O values (seen particularly well in cycles 3 to 8 of shell 1 or in cycle 3 of shell 2). Furthermore, almost all cycles show second-order excursions of δ18O and δ13C values towards more positive values situated within the broad minima.
A, B The δ18Oshell and δ13Cshell values show prominent first-order cycles with sharp maxima and broad minima. Furthermore, second-order excursions towards more positive values can be found within the broad minima (shaded). Results of clumped isotope analyses (in black and yellow) often show high standard deviations. The temperature scale for the Δ47 values is calculated based on Meinicke et al.46,47. Yellow shading represents ± 1 standard deviation. Dotted lines for samples CL3 (summer season of cycle 5 in shell 2) and CL10 (winter season between cycles 2 and 3 in shell 1) indicate samples with only one measurement (compare Supplementary Data 2). C Relative river discharge strength estimated based on first-order cycles in δ13Cshell values (B). D Seasonal changes in δ18Owater values estimated for reconstructed river discharge (C) and a proposed general temperature seasonality of 10 to 30 °C. Variations in δ18Owater values can also be connected to seasonal salinity fluctuations41. Shaded areas of the curves indicate the impact of the second-order excursions in δ18Oshell values. E Seasonal water temperatures were calculated based on measured δ18Oshell values (A) and estimated δ18Owater values (D). Shaded areas of the curves indicate the impact of the second-order excursions in δ18Oshell values. F Upwelling strength estimated based on second-order δ18Oshell excursions (A).
The minima and maxima for measured stable isotope values are largely comparable throughout the lifetime of each individual. The amplitude of the signal decreases somewhat in cycles with a lower sampling resolution, which is most likely a sampling artifact and not an original signal. This is particularly a problem for shell 3, which has only one cycle with a higher sampling resolution (cycle 2). Shell 1 shows a decrease in the width of the cycles parallel to the growth direction, especially after cycle 8 (compare Supplementary Information 1)37.
Absolute δ18O values are largely comparable between all three specimens, but δ13C values are on average higher in shell 1, intermediate in shell 2, and more negative in shell 3 (Fig. 3B; Fig. S8).
Clumped isotope analyses
Following the high-resolution stable isotope analysis, areas of the shells representing summer and winter seasons were resampled for clumped isotope analysis (Supplementary Data 2). Because this method requires much larger sample sizes, the ten collected samples have a lower temporal resolution. Results for Δ47 values fluctuate between 0.563 and 0.671 ‰ (I-CDES90), but partly have large analytical uncertainty (Fig. 3A; Table S2; Fig. S9). Large standard deviations are caused by a comparatively low number of repetitions of analyses of single samples. In general, samples were analyzed two or three times, but two samples could be analyzed only once and therefore no standard deviation can be given (see Supplementary Information 1 and Supplementary Data 2). The standard deviations for the clumped isotope analyses are always higher for samples representing summer seasons than those of winter samples. This might indicate a higher heterogeneity in these samples. The clumped isotope samples representing summer seasons show lower values (average: 0.598 ‰) compared to those of winter seasons with higher values (average: 0.651‰).
Discussion
Mid-Holocene seasonal environmental patterns
The stable isotope data of the three analyzed oysters reflect prominent seasonal changes (compare Supplementary Information 1 for further discussion; Figs. S10–S13). In general, δ18O values of mollusk shells (δ18Oshell) are negatively correlated with water temperatures and positively correlated with δ18O values of the surrounding water body (δ18Owater)38,39,40. Because δ18Owater values in river water are much more negative than in ocean water, the measured very low δ18Oshell values clearly suggest a brackish setting for the study area in the mid-Holocene41. Because δ18Oshell values depend on temperature and δ18Owater values, it is not possible to reconstruct both environmental parameters independently. This is particularly difficult in an estuarine setting, where δ18Owater values are known to fluctuate considerably. In such a situation, seasonal temperature extremes might be proposed with a higher certainty than changes in δ18Owater values. In the following, the different factors influencing the recorded geochemical dataset are discussed in detail.
Previous studies showed that C. gigas stops or strongly slows shell growth in cold water during winter months (below ca. 10–12 °C)30,42,43,44,45. This explains the sharp δ18Oshell maxima observed in the record. Samples collected at these maxima and used for clumped isotope analysis result in average water temperatures of ca. 10 °C (calculated with the equation of Meinicke et al.46,47). On the other end of the spectrum, the maximal temperatures tolerated by C. gigas are about 35 °C, but elevated mortality rates have been noted already at temperatures above 30 °C48,49. Present-day average summer water temperatures in the East China Sea adjacent to the Yangtze River Delta fluctuate between 27 and 28 °C. Considering that the summer months might have been slightly warmer in the mid-Holocene compared to today4, the analyzed oysters might have experienced conditions up to around 30 °C. This is close to their upper tolerance limit, but the recorded broad δ18Oshell minima indicate that they did not suffer prominent summer growth breaks. Summer temperatures reconstructed based on clumped isotope analyses fluctuate around an average of 24 °C, but due to a stronger averaging over several months this value might not represent peak temperatures during summer months (see also below). Based on observed growth patterns in C. gigas, reported present-day and mid-Holocene temperature conditions, and the rough temperature data from clumped isotope analyses, it seems realistic to assume that the measured δ18Oshell values reflect seasonal temperature changes somewhere between 10 to 30 °C. Considering measured amplitudes in δ18Oshell cycles up to 6–7‰ (Figs. 3A, S7), it becomes evident that the study area must have experienced additional changes in δ18Owater values (Figs. S15, S16). This can be explained by seasonal changes in the discharge strength of the Yangtze River, which are also observed today (Fig. S1C, D).
A number of factors can influence δ13Cshell values (Fig. 3B) in fossil shells, including taxonomic differences, ontogenetic trends, metabolic changes, variations in primary productivity, upwelling, sea-level changes, ocean currents, or even water temperatures43,50,51,52. In the present study, the measured δ13Cshell values show a prominent seasonal signal with a strong amplitude, thereby excluding most of the mentioned processes. Because river water transports dissolved inorganic carbon, which is generally isotopically lighter compared to oceanic dissolved inorganic carbon53, it seems reasonable to explain the recorded first-order δ13Cshell cycles by seasonal changes in fluvial influx as observed today54. It is therefore plausible to use the first-order δ13Cshell cycles as a proxy for river discharge strength (Fig. 3C; Fig. S18)54. Furthermore, the amount of river influx directly determines the δ18Owater value.
Combining the assumed seasonal temperature minima and maxima recorded by C. gigas with δ13Cshell values as a proxy for river discharge, it is possible to estimate a reasonable curve for δ18Owater values during the lifetime of the three analyzed oyster shells (Fig. 3D; Table S4; Figs. S19, S20). Following these considerations (see Supplementary Information 1 for further details), the reconstructed δ18Owater values show more negative values in summer (when river discharge is highest) and more positive values during winter. Broadly, these δ18Owater cycles also reflect seasonal salinity changes in agreement with present-day oceanographic studies (Fig. S1E)41,55. Figure 3E shows the water temperatures calculated for the measured δ18Oshell values and the estimated δ18Owater values. Of course it needs to be highlighted that these calculated temperatures are already based on the proposed seasonal temperature extremes of 10 and 30 °C (Fig. S19). In summary, the first-order stable isotope cycles can be used to reconstruct seasonal changes in δ18Owater cycles and therefore river discharge strength (reflecting precipitation). In contrast, the assumed seasonal water temperatures mainly rely on growth patterns in C. gigas, present-day observations, earlier literature information for the mid-Holocene, and the rough data from clumped isotope analyses.
Apart from first-order cycles, the δ18Oshell and δ13Cshell records show second-order excursions towards higher values within the broad minima (Fig. 3A, B). These excursions are too strong to be explained by simple variations in air temperature or river discharge, as there is no reason to propose strong cooling of surface waters by 5 to 7 °C in the study area or drastic decreases in river discharge during the summer months (compare discussion in Supplementary Information 1). Instead, the most likely explanation is regular summer upwelling in the study area, which leads to an influx of saline ocean water with higher δ18Owater values. In addition, this upwelling might lead to higher δ13Cwater values and slightly lower water temperatures at the sea floor (possibly also indicated by the comparatively low summer temperature averages based on clumped isotopes; see above). The second-order fluctuations in δ18Oshell values can thus be used as a proxy for upwelling (Fig. 3F; Figs. S21, S22), but the second-order fluctuations in δ13Cshell values might additionally record the influence of seasonal phytoplankton blooms and their subsequent decomposition43. Such phytoplankton blooms during summer months have been recorded for the Yangtze River Delta in recent years and can be explained by the interplay of nutrient input from river discharge and influx of phosphates from upwelled water29,56,57. In agreement with this interpretation, upwelling along the coasts of the East China Sea has been recorded in the last decades and also proposed for the Holocene (Supplementary Information 1)58.
Comparisons with present-day conditions and future predictions
The stable isotope record of the analyzed shells generally shows a regular seasonal pattern, but for most cycles the decrease in δ13Cshell values is delayed compared to the decrease in δ18Oshell values (Fig. 3A, B; Figs. S7, S23–S25). This is particularly striking in shell 1, prominent in cycle 3 of shell 2, and less visible in shell 3. The presence of a delayed decrease in δ13Cshell values can also be demonstrated when applying an internal age model (either based on constant growth rates or dated with the help of ShellChron59; compare Supplementary Information 1). This indicates that while water temperatures are already rising in spring, river discharge still remains low for a considerable time. This is different to present-day conditions and points to a comparatively longer dry season extending into spring or early summer and increasing river discharge and rainfalls particularly during the summer months. Preliminary geochemical data of subrecent oyster shells from the coast of Jiangsu north of the Yangtze Delta show synchronous δ18Oshell and δ13Cshell cycles, which would be expected under present-day conditions (see Supplementary Information 1; Figs. S13, S14). A stronger seasonality in precipitation (and consequently river discharge) due to a larger impact of the summer monsoon has been proposed for warmer climate conditions such as the mid-Holocene interval and the future following on-going global warming60,61. A stronger seasonality with more pronounced monsoonal rainfalls, river discharge, and wind strength can also explain a regular occurrence of summer upwelling (and phytoplankton blooms) in the mid-Holocene. Consequently, the presented data is in agreement with earlier proposals suggesting that the on-going and future global warming will cause stronger upwelling62,63 and stronger summer precipitation11. In the Yangtze River Delta and the adjacent East China Sea, this might cause more common phytoplankton blooms and thereby more pronounced hypoxia. This finding has important repercussions for the region, impacting, for example, aqua- and agriculture or flood management. Ideally, further research on more Holocene oyster shells from East Asian coasts could elucidate regional patterns more clearly.
Comparisons with climate models’ simulations
The geochemical results show prominent seasonal patterns in temperature, river discharge, and upwelling. The differences in the shape of the δ18Oshell and δ13Cshell curves can be interpreted to indicate a stronger seasonality in river discharge and precipitation with stronger monsoon rainfalls and/or drier winter months compared to today. This interpretation of the palaeo-proxy data can be used to test existing climate models and their reliability for reconstructing conditions in the mid-Holocene (and the future). The use of high-resolution archives such as bivalve shells has the additional advantage of enabling the reconstruction of seasonal environmental patterns.
The majority of used climate models predict drier conditions in the mid-Holocene compared to today64, which does not agree with most climate reconstructions based on fossil data65. Nine state-of-the-art climate models (Fig. 4A; Table S5) were used to simulate climate patterns for the Yangtze River Delta and adjacent East China Sea for the mid-Holocene (PMIP4), historical (CMIP6 DECK) and future time interval based on the SSP5-8.5 scenario (ScenarioMIP)66,67,68. The SSP5-8.5 scenario was chosen, because it shows the most distinct differences to conditions today. Results from these climate models were examined separately as well as combined on a comparable 2° x 2° grid resolution (Figs. S26–S34). All used models simulate much stronger East Asian summer monsoon wind circulation during the mid-Holocene compared to today (Fig. 4A, B)69. All of them fail to simulate higher air temperatures for the mid-Holocene compared to today, thereby contradicting a large amount of fossil evidence and raising doubts on how well these models can predict future climate changes in the region4,9,64. In more detail, the models vary considerably among themselves in the amount of precipitation simulated for the mid-Holocene in eastern China (Supplementary Information 1). A comparison between the model and palaeo-proxy data might help to determine, which of the climate models represent mid-Holocene rainfall patterns better and therefore might be better suited to predicting future changes in the region. In the present case, this comparison is qualitative with a seasonal resolution, because precise internal age models for the analyzed oysters are not available. Based on the data in the present study, three models (GISS-E2-1-G, IPSL-CM6A-LR, and NESM3) were found to be a good match by simulating less precipitation in winter and more rainfall during the summer compared to the historical period (Fig. 4C). Concerning future projections these models simulate monthly precipitation values similar to the historical period in the winter and spring seasons (December to May), but propose considerably more precipitation in the summer similar to the mid-Holocene simulation and the palaeo-proxy data (Fig. 4C). In addition, the same three models simulate stronger upwelling during the mid-Holocene in the East China Sea adjacent to the Yangtze River Delta, which also agrees well with the palaeo-proxy data (Fig. 4D). Consequently, it is suggested here that simulations based on these three climate models might be more realistic.
The difference in the averaged boreal summer (June to August) precipitation and 850-hPa wind components between the mid-Holocene and historical simulations (A), and between the SSP5-8.5 and historical simulations (B) based on multi-model ensembles. The contour interval is 0.5 mm per day. The red rectangles are the Yangtze River Valley region. C Long-term mean monthly precipitation spatially averaged over the Yangtze River Valley region defined within the red rectangle of A and B. The red, black, and green dashed lines represent the mid-Holocene, the historical, and the SSP5-8.5 simulations, respectively. The model values are obtained from the multi-model ensemble using GISS-E2-1-G, IPSL-CM6A-LR, and NESM3. D. The difference in the averaged boreal summer (June to August) upwelling between the mid-Holocene and historical simulations for the multi-model ensemble using GISS-E2-1-G, IPSL-CM6A-LR, and NESM3. The upwelling is computed by averaging the oceanic vertical velocity over the upper 100 m. The contour interval is 0.03 mm per day.
Conclusions
Stable isotope (δ18O, δ13C) data from three mid-Holocene oysters from the Yangtze River Delta in eastern China reflect prominent seasonal changes in temperature, precipitation, and river discharge. Results indicate regular summer upwelling in the study area and a longer dry season in winter compared to today. Because the mid-Holocene time interval was characterized by an up to 2 °C warmer climate in China compared to pre-industrial times, the results comprise a potential future scenario for the region considering on-going global warming. A comparison of available climate models shows that their simulations underestimate temperatures in East China during the mid-Holocene and that precipitation patterns vary considerably between climate models. All models simulate stronger monsoon winds in the mid-Holocene, which agrees with the regular presence of summer upwelling. Three models (GISS-E2-1-G, IPSL-CM6A-LR, and NESM3) were identified, which agree with the presented palaeo-proxy data in simulating more rainfalls in summer and less rainfalls in winter compared to today. Thus, the presented palaeo-proxy data can be used to control, correct, and improve existing climate models.
Methods
Geochemical analyses
The oyster shells used in the present study originate from collections of the Shanghai Natural History Museum and are now permanently stored in the collections of Nanjing University (School of Earth Sciences and Engineering). They have a mid-Holocene age, were found in the central Yangtze River Delta (Fig. 1; 31°01’33.0” N, 121°03’13.0”E), and could be assigned to the genus Crassostrea. Today, this genus includes a series of similar species in East Asia with highly variable shell morphologies and sometimes not completely resolved taxonomic status42,49,70. Some of the species described in literature produce identical shells and might be in fact invalid or can only be differentiated genetically71,72,73,74,75,76. Despite these uncertainties, the collected fossil shells were assigned to Crassostrea gigas (Thunberg, 1793) based on general shell morphology and size42. This well-known species is the most common comparable oyster along the coasts of eastern Asia and has been used for stable isotope research previously30,43,77,78,79. Based on macroscopic observations, three right valves were selected for this study (shells 1, 2, 3). The shells were cleaned and photographed externally (Fig. 2). Radiometric dating was conducted on extracted shell material for all three specimens at the Beta Analytic Testing Laboratory in Miami, USA.
Following external examinations, the shells were embedded in resin and cut through the hinge perpendicular to the growth lines (Fig. 2). Thin-sections were produced for the surfaces opposite the selected sampling areas. After examination with a petrographic microscope, the same sections were studied using a cold-cathodoluminescence microscope at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, China.
Sampling for stable isotope analyses was conducted using a computer-controlled micromill at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, China. Sampling tracks were placed parallel to the growth lines within the hinge at distances between 0.3 to 0.5 mm (Fig. 2). Stable isotope analyses were performed using a GasBench II IRMS system with a DELTA PLUS XP mass spectrometer at the School of Earth Sciences and Engineering, Nanjing University, China. Laboratory standards were analyzed at regular intervals to control the precision of the measured values. Based on the performance of standards, the analytical precision was better than ±0.10 ‰ (1 SD) for δ13C and ±0.18 ‰ (1 SD) for δ18O values. All measured values are reported in per mil relative to the Vienna Pee Dee belemnite (VPDB) scale using NBS-19.
The stable isotope data show strong cycles with sharp maxima and broad minima. Because δ18O values of bivalve shells are negatively correlated to water temperature, the maxima are believed to indicate cold winter temperatures and the minima correspond to warm summer seasons. Following the identification of seasonal cycles in the stable isotope data, areas of the shells representing these summer and winter seasons were resampled for clumped isotope analyses at the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, China. The acid hydrolysis, purification, and collection of the CO2 gas were carried out in the automatic clumped isotope preparation instrument Isotope Batch Extraction System (IBEX 2). For each measurement, about 5 mg of the powder samples were weighed in a glass tube and placed in an autosampler dropping into a common acid bath. In this bath, the powder was fully reacted with phosphoric acid with a density of 1.92 g/cm3 at constant temperatures of 90 °C. The produced CO2 was then carried by a helium flow current through a liquid nitrogen cold Trap and GC Trap for purification. The purified CO2 was imported into a ThermoFisher Scientific 253 Plus Mass spectrometer for clumped isotopes measurement. This mass spectrometer is equipped with six Faraday cups measuring m/z 44–49 and an additional 47.5 cup for monitoring background baselines. Peak scans were performed for several days for the pressure baseline correction80. Clumped isotope results are reported relative to I-CDES through an empirical transfer function (ETF) based on measurements of ETH standards and their given I-CDES values (ETH-1 = 0.2052‰; ETH-2 = 0.2085‰; ETH-3 = 0.6132‰; ETH-4 = 0.4505‰)81. In addition, the carbonate standard IAEA603 (i.e. IAEA-C1) was analyzed together with the samples to monitor long-term accuracy and reproducibility of Δ47 values. Multiple analyses yielded an average value of 0.3113 ‰ (n = 16; SD = 0.038), which is indistinguishable from the accepted value for IAEA-C1 of 0.3018 ± 0.0025 ‰ (95% CL)81. Isotopic values were calculated using the latest International Union of Pure and Applied Chemistry values82,83. AFF was needless following the I-CDES standardization since the carbonate standards used for the ETF undergo the same acid reaction as the samples81. All corrections and calibrations were conducted using the Easotope software package84. The mineral growth temperatures are calculated following the equation of Meinicke et al.46,47 and alternative temperatures based on Anderson et al.85 are presented as well in the Supplementary Information 1 and Supplementary Data 2.
Climate models
The numerical simulations datasets are from models’ outputs provided by the CMIP6 World Climate Research Programme’s Working Group on Coupled Modeling (Eyring et al., 2016). The simulations used are the mid-Holocene experiment (PMIP4), the historical experiment (CMIP DECK), and the SSP5-8.5 future scenario projection (ScenarioMIP)67,68. The first 100 years of the mid-Holocene simulations, the first 100 years of the historical simulations (1850-1949), and the last 50 years of the SSP5-8.5 simulations (2051–2100) were used in the analyses. The SSP5-8.5 future scenario was not available for GISS-E2-1-G. With the exception of GISS-E2-1-G, in which the ensemble member is r1i1p1f2, for all the remaining models the ensemble member r1i1p1f1 is used. Table S5 in Supplementary Information 1 shows the details of the nine climate models used in this study. In the analyses, all models are interpolated to a common 2° × 2° grid resolution. The main differences between the mid-Holocene simulation and the historical and SSP5-8.5 simulations are related to the parametrizations of the Earth’s eccentricity, obliquity, and precession. The mid-Holocene simulation set the eccentricity, obliquity, and precession to 0.018682, 24.105°, and 0.87°, respectively, whilst the historical and SSP5-8.5 simulations are set to 0.016764, 23.459°, and 100.33°, respectively86. The atmospheric CO2 concentration is also different, with 264.4 ppmv for the mid-Holocene period, ~400 ppmv in 2014 for historical simulation, and ~1100 ppmv in 2100 for the SSP5-8.5 future scenario. Notice that for historical and SSP5-8.5 simulations the CO2 concentration increases throughout the experiments.
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Acknowledgements
MA gratefully acknowledges financial support from the State Key Laboratory of Palaeobiology and Stratigraphy of the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (No. 223101). Furthermore, the research was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant no. XDB26000000), the National Natural Science Foundation of China (Grant no. 41922011), and the Fundamental Research Funds for the Central Universities (No. 0206-14380219). SFV is supported by the National Natural Science Foundation of China (Grant no. 42050410322) and the GeoX Interdisciplinary Research Funds for the Frontiers Science Center for Critical Earth Material Cycling, Nanjing University (Grant no. 020714380200). Takaaki Konabe Watanabe (Kiel, Germany) helped with statistical analyses. We thank Niels de Winter, Weizhe Chen, the anonymous reviewer, and the editors for their constructive comments during the peer-review process.
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M.A. contributed to the conceptualization, design, methodology, sample preparation and collection, laboratory work, data analysis, original draft, and revision of the manuscript. S.F.V. contributed to the design, methodology, data analysis, original draft, and revision of the manuscript. B.C. contributed to the design, methodology, laboratory work, data analysis, original draft, and revision of the manuscript. L.H. contributed to the sample preparation and collection, laboratory work, and data analysis. Z.F. contributed to the sample preparation and collection, laboratory work, and data analysis. B.Z. contributed to the conceptualization, design, sample preparation, and collection. Y.P. contributed to the conceptualization, design, methodology, sample preparation and collection, data analysis, original draft, and revision of the manuscript.
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Alberti, M., Veiga, S.F., Chen, B. et al. The Yangtze River Delta experienced strong seasonality and regular summer upwelling during the warm mid-Holocene. Commun Earth Environ 5, 492 (2024). https://doi.org/10.1038/s43247-024-01668-1
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DOI: https://doi.org/10.1038/s43247-024-01668-1
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