Robust chronological reconstruction for young speleothems using radiocarbon

We have studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean) and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia). Attempts to date these speleothems by the Th/U method have proved unsuccessful with some age reversals for SC4 due to multiple sources of non-authigenic Th. This method has also resulted in imprecise ages for WM7 because of low U concentrations (<10 ppb) and consequently very low levels of authigenic ²³⁰Th relative even to the very low levels of detrital ²³⁰Th present. Here, we present an alternative method for reliable dating of these young speleothems using radiocarbon. Approximately 100 carbonate samples from SC4 and WM7 were analysed for ¹⁴C by accelerator mass spectrometry (AMS). The AMS results indicate that bomb ¹⁴C was evident in the youngest parts of both stalagmites. Two different approaches were used to estimate dead carbon fraction (DCF) values for these stalagmites for the pre-bomb period. For SC4, the DCF values were estimated based on the timing of ¹⁴C dates for that period determined by high-resolution δ¹⁸O recorded in the speleothem, and the timing of the onset of bomb ¹⁴C. For WM7, a “maximum” range of pre-bomb DCF was determined. Chronologies of these speleothems were built based on a dense sequence of DCF-corrected ages using three different age-depth models: Clam (Classical method), and Bacon and OxCal (Bayesian statistical approach). Good agreement between these age-depth models were observed indicating that the top 170 mm of SC4 and the top 50 mm of WM7 grew during the past 550–750 years and 1360–1740 years, respectively.     

Accurate chronological construction for two young stalagmites from the tropical South Pacific

Modern to Holocene tropical Pacific stalagmites are commonly difficult to date with the U-series, the most commonly used dating method for speleothems. When U-series does not provide robust age models, due to multiple sources of ²³⁰Th or little U, radiocarbon is, potentially, the best alternative. The ¹⁴C content of two stalagmites (Pu17 and Nu16) collected from Pouatea and Nurau caves in the Cook Island Archipelago of the South Pacific were measured to obtain accurate chronology for their most modern parts. The bomb-pulse soil continuum modelling indicates that bomb radiocarbon in Pu17 onsets in 1956 and reaches its maximum in 1966 CE, suggesting a fast transfer of atmospheric carbon to the stalagmite of <1 year. The modelling for Pu17 suggests a 20% contribution from C₁ - an instantaneous carbon source, which renders possible an immediate transfer of atmospheric signal into the cave. Nu16 shows a slower transfer of atmospheric carbon to the stalagmite than Pu17, with bomb radiocarbon onsetting in 1957 CE and peaking in 1972 CE. The less negative δ¹³C values in Nu16 than Pu17, and also the modelling corroborated this, which points out no contribution from the instantaneous carbon source. The radiocarbon age models and laminae counting age models were then spliced to achieve a single master chronology for the top part of each stalagmite. This study is an example of ¹⁴C age modelling combined with visible physical and chemical laminae counting and how it can improve the accuracy and precision of dating for otherwise hard-to-date tropical Pacific speleothems. Such accurate and precise age models pave the way to obtain sub-annually resolved paleoclimate records by further improving the calibration of climate proxy data with the current and instrumental weather parameters.     

Towards radiocarbon calibration beyond 28 ka using speleothems from the Bahamas

We present a new speleothem record of atmospheric Δ¹⁴C between 28 and 44 ka that offers considerable promise for resolving some of the uncertainty associated with existing radiocarbon calibration curves for this time period. The record is based on a comprehensive suite of AMS ¹⁴C ages, using new low-blank protocols, and U–Th ages using high precision MC-ICPMS procedures. Atmospheric Δ¹⁴C was calculated by correcting ¹⁴C ages with a constant dead carbon fraction (DCF) of 22.7 ± 5.9%, based on a comparison of stalagmite ¹⁴C ages with the IntCal04 (Reimer et al., 2004) calibration curve between 15 and 11 ka. The new Δ¹⁴C speleothem record shows similar structure and amplitude to that derived from Cariaco Basin foraminifera (Hughen et al., 2004, 2006), and the match is further improved if the latter is tied to the most recent Greenland ice core chronology (Svensson et al., 2008). These data are however in conflict with a previously published ¹⁴C data set for a stalagmite record from the Bahamas — GB-89-24-1 (Beck et al., 2001), which likely suffered from ¹⁴C analytical blank subtraction issues in the older part of the record. The new Bahamas speleothem ?¹⁴C data do not show the extreme shifts between 44 and 40 ka reported in the previous study (Beck et al., 2001). Causes for the observed structure in derived atmospheric Δ¹⁴C variation based on the new speleothem data are investigated with a suite of simulations using an earth system model of intermediate complexity. Data-model comparison indicates that major fluctuations in atmospheric ?¹⁴C during marine isotope stage 3 is primarily a function of changes in geomagnetic field intensity, although ocean–atmosphere system reorganisation also played a supporting role.     

Lower Groundwater 14C Age by Atmospheric CO2 Uptake During Sampling and Analysis

Uptake of atmospheric CO₂ during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO₂ can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated ¹⁴C measurements. Kinetic isotope fractionation during atmospheric CO₂ uptake may also result in decrease of δ¹³C, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured ¹⁴C values of groundwater should not be used for age estimation without corresponding δ¹³C values, and historical ¹⁴C data in the range of 1 to 10% modern Carbon should be re‐evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for ¹⁴C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if ¹⁴C measurements are made with an accelerator mass spectrometer.     

Reconstructions of the heliospheric modulation potential and Wolf numbers based on the content of the <Superscript>14</Superscript>C isotope in tree rings during the Maunder and Spörer minimums

Data on variations in the content of the 14C cosmogenic isotope in tree rings and the Earth’s atmosphere (Δ¹⁴C) make it possible to study the behavior of solar activity (SA) in previous centuries and millenniums. The latter is related to the fact that SA temporal variations result in a change in the IMF (Interplanetary Magnetic Field) parameters and, as a consequence, in the galactic cosmic ray (GCR) flux, under the action of which the ¹⁴C isotope is produced in the Earth’s atmosphere. This makes it possible to study SA history based on data on the ¹⁴C isotope content in tree rings. However, in this case we have several difficulties related to climate change. Climate changes result in carbon redistribution between natural reservoirs, which is reflected in radiocarbon data and results in solar signal distortion. The effect of variations in the global temperature and carbon dioxide concentration on the reconstruction of the heliospheric modulation potential and Wolf numbers from the late 14th century to the early 19th century is considered. It has been shown that the radiocarbon data do not make it possible to conclude that SA during the Maunder minimum was extremely low as compared to SA during the Dalton minimum.     

Sub-Milankovitch climatic cycles in Holocene stalagmites from Sauerland, Germany

Calcitic stalagmites from caves in the Sauerland, Germany, prove the existence of sub-Milankovitch cycles in precipitation during the last 6000 yr. The δ¹⁸O record dated with Th/U is interpreted as an indicator of paleohumidity. Spectral analysis of δ¹⁸O from 6000 a BP up to the recent top of a stalagmite from the Atta cave yields statistically significant peaks at 1450, 117, 64 and 57 a. Additionally we find a good correlation of the stalagmite’s δ¹⁸O and Δ¹⁴C from European tree rings. The 1450 a cycle in the stalagmite probably is analogous to the pervasive millennial scale climate cycle described by Bond et al. [Science 278 (1997) 1257-1266; 294 (2001) 2130-2136] derived from the amount of ice rafted debris in deep sediments from the North Atlantic. Our results suggest that the centennial to millennial shifts observed in the North Atlantic are accompanied by synchronous shifts of the climate in Northern and Central Europe, which most probably can be attributed to solar irradiation variations.     

Methods and codes for reservoir–atmosphere 14C age offset calculations

Reservoir ¹⁴C age offsets are invaluable tracers for past changes in carbon cycle and oceanic circulation. Reconstruction of reservoir age offsets with time is also required for calibration purposes (reconstruction of atmospheric calibration curve, calibration of non-atmospheric radiocarbon ages). Thus, properly propagating the various uncertainties linked to reservoir age offset is important for proper interpretation. However, approaches for reservoir age offset calculation – especially when considering pairs of reservoir-derived ¹⁴C and calendar ages – are usually not detailed and inadequate for proper propagation of uncertainties. Here, the various ways to properly calculate reservoir age offsets are described with a focus on a new approach when considering pairs of ¹⁴C and calendar ages. This approach maps the calendar age distribution onto the ¹⁴C time scale prior to reservoir age offset calculation – the “uncalibration-convolution process”. R codes computing reservoir age offsets based on available data are presented. Finally, a case study focusing on the reconstruction of the speleothem-atmosphere ¹⁴C age offsets of speleothem ¹⁴C data used in the latest release of the atmospheric calibration curve is discussed.     

An isotopic calibration study of precipitation,cave dripwater,and climate in west‐central Florida

A calibration study of oxygen and hydrogen isotopic composition from precipitation and cave dripwater was conducted in west‐central Florida at Legend Cave during 2007–2008. This study was performed to better understand how modern precipitation patterns can be discerned through examination of cave dripwater and speleothem calcite for paleoclimate reconstruction. The ‘amount effect’ was shown to be a dominant control on the oxygen isotopic composition of precipitation for the study area. A meteoric water line with a slope of 6·7 suggests evaporative effects occur either during precipitation or subsequent hydrological processes. However, δ¹⁸O values of cave dripwater averaged near the mean annual amount‐weighted average of precipitation, suggesting that the isotopic composition of dripwater tracks the long‐term average of rainfall. An observed weak seasonal influence occurred in the d‐excess values, with summer precipitation being more enriched due to increased evaporative effects. Comparison of precipitation δ¹⁸O values to synoptic weather data shows the dominant amount effect influence occurs due to strong convective storms producing highly ¹⁸O‐depleted rainfall at greater amounts during the year. Constant δ¹⁸O values of the dripwater indicate that paleoclimate reconstructions using speleothems from this area would record changes in annual to interannual shifts in precipitation amount above the cave. Copyright © 2011 John Wiley & Sons, Ltd.     

A 53 year seasonally resolved oxygen and carbon isotope record from a modern Gibraltar speleothem: Reconstructed drip water and relationship to local precipitation

The response of a climate proxy against measured temperature, rainfall and atmospheric circulation patterns at sub-annual resolution is the ultimate test of proxy fidelity but very few data exist showing the level of correspondence between speleothem climate proxies and the instrumental climate record. Cave sites on the Gibraltar peninsula provide a unique opportunity to calibrate speleothem climate proxies with the longest known available precipitation isotopes and instrumental records. An actively growing speleothem sampled from New St. Michaels Cave in 2004 is composed of paired laminae consisting of light columnar calcite and a darker microsparitic calcite. Stable isotope analysis of samples micromilled in 100 μm steps at the equivalent of bi-monthly intervals reveals fabric-correlated annual cycles in carbon isotopes, oxygen isotopes and trace elements responding to seasonal changes in cave microclimate, hydrology and ventilation patterns. Calcite δ¹³C values reach a minimum in the light columnar fabric and evidence from trace element behaviour and cave monitoring indicates that this grows under cave ‘winter’ conditions of highest pCO₂, whereas the dark microsparitic calcite, characterised by elevated δ¹³C and δ¹⁸O values grows under low ‘summer’ pCO₂ conditions. Drip water δ¹³C_DIC reaches a minimum in March–April, at which time the attenuated δ¹⁸O signal becomes most representative of winter precipitation. An age model based on cycle counting and the position of the ¹⁴C bomb carbon spike yields a precisely dated winter oxygen isotope proxy of cave seepage water for comparison with the GNIP and instrumental climate record for Gibraltar. The δ¹⁸O characteristics of calcite deposited from drip water representing winter precipitation for each year can be derived from the seasonally resolved record and allows reconstruction of the δ¹⁸O drip water representing winter precipitation for each year from 1951–2004. These data show an encouraging level of correspondence (r² = 0.47) with the δ¹⁸O of rainfall falling each year between October and March and on a decadal scale the δ¹⁸O of reconstructed winter drip water mirrors secular change in mean winter temperatures.     

Mg isotopic composition of carbonate: insight from speleothem formation

Simultaneous high-precision measurement of ²⁴Mg, ²⁵Mg and ²⁶Mg isotopic compositions were made by multiple collector inductively coupled mass spectrometry (MC-ICP-MS) relative to the international standard SRM980. Data are presented on low-Mg calcite speleothems and their associated host rocks and waters from four caves, one in the French Alps and three in Israel, covering various climate conditions. In addition, data are presented on three dolostones and three limestones from the Himalaya. The overall variation is 4.13‰ and 2.14‰ in δ²⁶Mg and δ²⁵Mg, respectively. This is 35 times the uncertainty of the measurements and clearly demonstrates that the terrestrial isotopic composition of Mg is not unique. Each speleothem shows a characteristic range of δ²⁶Mg values that are attributed to the isotopic composition of the local water. Differences between the isotopic composition of Mg in the water dripping from stalactites and that of the modern speleothem are interpreted as being due to Mg isotopic fractionation during carbonate precipitation in the temperature range of 4-18°C. The low-Mg calcite is enriched in light isotopes by 1.35‰/AMU and the dependence on temperature has been found to be less than 0.02‰/AMU/°C. Despite various geological settings, the δ²⁶Mg of the studied dolostones is 2.0±1.2‰ higher than the δ²⁶Mg of the limestones. All together, these results suggest a strong mineralogical control and a weak temperature effect on the Mg isotopic composition of carbonate.     

Later Pleistocene/Holocene climate conditions of Qinghai-Xizhang Plateau (Tibet) based on carbon and oxygen stable isotopes of Zabuye Lake sediments

We present a time series of carbon and oxygen stable isotope records of the last 30?000 ¹⁴C years throughout the last glacial-postglacial cycle from western Qinghai-Xizhang (Tibet) Plateau. A 20-m core drilled in the south basin of Zabuye Salt Lake was analyzed for inorganic and organic carbon and total sulfur contents, δ¹³C and δ¹⁸O values of carbonates. Our results indicate that climatic changes have led to a drastic negative shift of stable isotope ratios at the transition between the Last Full Glacial and the postglacial phase during Later Pleistocene times (∼16.2 kyr BP), and a rapid positive shift at the transition from Pleistocene to Holocene (∼10.6 kyr BP). The first shift is marked by the drop of δ¹⁸O_carb values of about 10‰ (from +2 to −8‰) and δ¹³C_carb values of about 3‰ (from 5 to 2‰). The second shift which occurred at the transition from Pleistocene to Holocene was of similar magnitude but in the opposite direction. Isotope data, combined with total organic and inorganic carbon contents and the lithological composition of the core, suggest this lake was an alluvial pre-lake environment prior to ca. 28 ¹⁴C kyr BP. During ca. 28-16.2 ¹⁴C kyr BP, Zabuye Lake was likely a moderately deep lake with limited outflow. The cool and arid glacial climate led the lake level to drop drastically. Extended residence time overwhelmed the lower temperature and caused a steady increase of δ¹³C_carb and δ¹⁸O_carb values and total inorganic carbon content in the sediments. During ca. 16.2-10.6 ¹⁴C kyr BP, this lake probably overflowed and received abundant recharge from melting glaciers when the deglaciation was in its full speed. A spike of markedly enhanced δ¹³C_carb and δ¹⁸O_carb is seen at ∼11.5 kyr BP, probably due to the isotopic effects left behind by the short but severe Younger Dryas (YD) event. After ca. 10.6 ¹⁴C kyr BP, Zabuye Lake probably closed its surface outflow, due to strong desiccation and drastic climate warming. The Early and Middle Holocene were characterized by unstable climatic conditions with alternating warmer/cooler episodes as indicated by the severe fluctuations of total organic carbon, δ¹³C and δ¹⁸O values. A hypersaline salt lake environment was finally formed at Zabuye after ∼5 ¹⁴C kyr BP when the mirabilite and halite concentrations steadily increased and became the dominant minerals in the sediments. Severe imbalance of inflow/outflow resulted in the drastic increase of total sulfur, δ¹³C_carb and δ¹⁸O_carb values and dominance of halite in the lake since ca. 3.8 kyr BP to present.     

Optimizing radiocarbon chronologies in peat profiles with examples from Xinjiang,China

Reliable chronological frameworks are crucial to paleoenvironmental studies, and high precision ¹⁴C dating is the foundation, but many factors, such as dating materials, surficial deposition (influenced by nuclear bomb), and the ¹⁴C age plateau, will affect the reliability of the ¹⁴C ages and chronology frameworks. In this paper, we present 87 ¹⁴C dates of different peat fractions from three peat sites in Xinjiang, China. Plant macrofossils, rootlets, the fine fraction of <90 μm, the mid-size fraction of 90–250 μm and the coarse fraction of >250 μm from selected peat samples were measured to investigate the alternative suitable fraction for dating except for plant macrofossils. We discovered that the 90–250 μm component of peat can provide alternative and reliable results in case of plant macrofossils are not available. Additionally, more dating samples from surficial peat deposition were collected, and accurate surface chronological control points were produced by comparing ¹⁴C results of plant macrofossils with atmospheric ¹⁴C bomb data. Furthermore, multiple data sets with wiggle matching were used along the radiocarbon age plateau to minimize calibrated errors when dates on the ¹⁴C age plateau were shown. Finally, radiocarbon chronology frameworks in peat profiles were optimized. In conclusion, we not only focus on the reliable dating materials, but also highlight that the importance of surficial deposition (after 1950AD) and the anomalous ¹⁴C dates when establishing the dating framework in peat profiles. Furthermore, we propose that the obtaining chronological control points of surficial peat is an important part of the establishing and improving of peat chronological framework in future research.     

Kinetic processes and stable isotopes in cave dripwaters as indicators of winter severity

We examine how the stable isotope composition of meteoric water is transmitted through soil and epikarst to dripwaters in a cave in western Romania. δ²H and δ¹⁸O in precipitation at this site are influenced by temperature and moisture sources (Atlantic and Mediterranean), with lower δ¹⁸O in winter and higher in summer. The stable isotope composition of cave dripwaters mimics this seasonal pattern of low and high δ¹⁸O, but the onset and end of freezing conditions in the winter season are marked by sharp transitions in the isotopic signature of cave dripwaters of approximately 1 ‰. We interpret these shifts as the result of kinetic isotopic fractionation during the transition phase from water to ice at the onset of freezing conditions and the input of meltwater to the cave at the beginning of the spring season. This process is captured in dripwaters and therefore speleothems from Ur?ilor Cave, which grew under such dripping points, may have the potential to record past changes in the severity of winters. Similar isotopic changes in dripwaters driven by freeze–thaw processes can affect other caves in areas with winter snow cover, and cave monitoring during such changes is essential in linking the isotopic variability in dripwaters and speleothems to surface climate.     

Reconstructions of the <Superscript>14</Superscript>С cosmogenic isotope content from natural archives after the last glacial termination

Data on the content of the ¹⁴C cosmogenic isotope in tree rings, which were obtained as a result of laboratory measurements, are often used when solar activity (SA) is reconstructed for previous epochs, in which direct observations are absent. However, these data contain information not only about SA variations but also about changes in the Earth climatic parameters, such as the global temperature and the CO₂ content in the Earth’s atmosphere. The effect of these variations on the ¹⁴C isotope content in different natural reservoirs after the last glacial termination to the middle of the Holocene is considered. The global temperature and the CO₂ content increased on this time interval. In this case the ¹⁴C absolute content in the atmosphere increased on this time interval, even though the ¹⁴С to ¹²С isotope concentration ratio (as described by the Δ¹⁴С parameter) decreased. These variations in the radiocarbon absolute content can be caused by its redistribution between natural reservoirs. It has been indicated that such a redistribution is possible only when the rate of carbon exchange between the ocean and atmosphere depends on temperature. The values of the corresponding temperature coefficient for the 17–10 ka BC time interval, which make it possible to describe the carbon redistribution between the ocean and atmosphere, have been obtained.     

Geosecs North Atlantic radiocarbon and tritium results

Radiocarbon data for 11 stations and tritium data for 16 stations in the North Atlantic Ocean from 74°N to 3°N are presented. For radiocarbon, normal errors inΔ¹⁴C are± 4‰, and in tritium,± 0.09TU or± 3%, whichever is larger. There is a remarkable, but not simply linear, correlation between oceanic bomb transients in¹⁴C and³H. The deep convective mixing in the Greenland Sea is reflected in substantial bomb tracer penetration to all depths, with residence time for the deep, cold core water that seems to be 20 to 30 years. The outflow in the bottom layer southward over the sills of the Denmark Strait and Faroe Passage carries significant tritium concentration, at least to 40°N. Complicated, but coherent, profile structures in the subtropical Atlantic suggest effects of large-scale lateral advection. In particular, a pronounced minimum in both¹⁴C and³H might be associated with the Antarctic intermediate water.     

13C/12C ratios of rocks and inclusions in popping rocks of the Mid-Atlantic Ridge and their bearing on the problem of isotopic composition of deep-seated carbon

CO₂-rich inclusions recovered from “popping” and related tholeiitic rocks from the Mid-Atlantic Ridge have δ¹³C values of ?7.6 ± 0.5%. relative to PDB. δ¹³C values of total carbon in the same rocks range from ?12 to ?13.7‰. These values are discussed in the light of the known δ¹³C variations in rocks of deep-seated origin. The ?7.6‰ value is interpreted as a reasonable estimate of the primary value of δ¹³C of deep-seated carbon in the ridge area.     

A discontinuous climatic record from 187 to 74 ka from a speleothem of the Clamouse Cave (south of France)

This study presents a continental paleoclimatic record in the south of France, based on δ¹⁸O, δ¹³C and the ²³⁴U/²³⁸U activity ratio, dated by the ²³⁰Th/²³⁴U disequilibrium method with thermal ionization mass spectrometry. A stalagmite (Cla4) from the Clamouse Cave offers a discontinuous stable isotopic record between 189 ka and 74 ka which covers marine isotope stages (MIS) 7, 6.4, 5.5, 5.3 and 5.1. The growth phases of the Cla4 stalagmite correspond to high sea level stages, except during MIS 6.4 (169-162 ka). All the growth phases of Cla4 correspond to humid periods, corresponding to the sapropel events observed in the eastern Mediterranean basin. Thus, the influence of a strong hydrological activity in the eastern Mediterranean basin during these periods prevailed as far west as the south of France. Because the karstic system studied strongly buffers the isotopic composition of water, isotopic variations of the calcite deposited in the cave represent mainly global and large-scale environmental variations when isotopic equilibrium conditions prevailed for calcite crystallization. Sub-stage 5.3, the end of 5.5 and MIS 7 were colder by about 4-6°C (calculated temperature) compared to present-day temperature while the growth phases of sub-stages 5.1 and the beginning of 5.5 reflect environmental conditions close to present ones. The δ¹⁸O and δ¹³C values of cave deposits of the sub-stage 6.4 are: (1) strongly marked by kinetic fractionation processes such as evaporation due to moisture deficit within the cave atmosphere during the first step of this growth phase and (2) related to higher humidity due to a second period of enhanced rainfall during the second step of growth. This study shows that even if calcite has not been deposited at isotopic equilibrium, its isotopic composition can give insights into the environmental conditions at the time of deposition.     

洞穴石笋的^14C年代学研究——石花洞研究系列之二

由于受“死碳”的影响,年轻洞穴碳酸盐的＾１４Ｃ测年始终受到限制,为此,建立了＾１４Ｃ法测定石笋生长速率及校正“死碳”影响的理论模式,从而将石笋的测年时限和精度大大提高。这一理论成功地运用于北京石花洞年轻石笋的定年。利用＾１４Ｃ法与石笋生长光性年际纹层推算的平均生长速率十分吻合,表明石笋Ｓ３１２的生长速率为０．０４２ｍｍ／ａ。＾２１０Ｐｂ法测定石笋顶部的年龄小于１００ａ,与＾１４Ｃ结果的外推年龄基本     

Groundwater evolution and mean water age inferred from hydrochemical and isotopic tracers in a tropical confined aquifer

The coastal confined aquifer in the Gulf of Urabá (Colombia) is an important water source for the banana agro‐industry as well as for urban and rural communities. However, the main processes controlling recharge and mixing in the aquifer are still poorly understood. Hydrochemical analyses and stable isotope monitoring were conducted to (a) determine groundwater recharge origin, mean groundwater age, and the main processes governing groundwater chemistry and the potential mixing of marine water and the influence of diffusive processes from the two surrounding aquitard layers. Hydrochemical data indicate that the main processes affecting the dissolved chemical composition include cation exchange, dissolution of carbonated and CO₂, and silicate weathering. δ¹⁸O and δ²H compositions combined with ¹⁴C data highlight the differences in climatic conditions between the recharge zone and the confined section of the aquifer, which is close to the Atlantic Ocean. Groundwater samples with ¹⁴C ages from recent to 28,300 years BP show a depleted isotopic trend ranging from ?6.43‰ to ?9.14‰ in δ¹⁸O and from ?43.2‰ to ?65.7‰ in δ²H. The most depleted δ¹⁸O and δ²H compositions suggest a cooler recharge climate than the current conditions (corresponding to the last glacial period of the late Pleistocene). Depleted δ¹³C values in the total dissolved inorganic carbon indicate the existence of organic material oxidation processes within the geologic formation. These results can be used or transferred to enhance groundwater modelling efforts in other confined coastal aquifers of South America where scarcity of long‐term monitoring data limits water resources planification under a changing climate.     

Radiocarbon dating of small terrestrial gastropod shells in North America

Fossil shells of small terrestrial gastropods are commonly preserved in wetland, alluvial, loess, and glacial deposits, as well as in sediments at many archeological sites. These shells are composed largely of aragonite (CaCO₃) and potentially could be used for radiocarbon dating, but they must meet two criteria before their ¹⁴C ages can be considered to be reliable: (1) when gastropods are alive, the ¹⁴C activity of their shells must be in equilibrium with the ¹⁴C activity of the atmosphere, and (2) after burial, their shells must behave as closed systems with respect to carbon. To evaluate the first criterion, we conducted a comprehensive examination of the ¹⁴C content of the most common small terrestrial gastropods in North America, including 247 AMS measurements of modern shell material (3749 individual shells) from 46 different species. The modern gastropods that we analyzed were all collected from habitats on carbonate terrain and, therefore, the data presented here represent worst-case scenarios. In sum, ～78% of the shell aliquots that we analyzed did not contain dead carbon from limestone or other carbonate rocks even though it was readily available at all sites, 12% of the aliquots contained between 5 and 10% dead carbon, and a few (3% of the total) contained more than 10%. These results are significantly lower than the 20–30% dead carbon that has been reported previously for larger taxa living in carbonate terrain. For the second criterion, we report a case study from the American Midwest in which we analyzed fossil shells of small terrestrial gastropods (7 taxa; 18 AMS measurements; 173 individual shells) recovered from late-Pleistocene sediments. The fossil shells yielded ¹⁴C ages that were statistically indistinguishable from ¹⁴C ages of well-preserved plant macrofossils from the same stratum. Although just one site, these results suggest that small terrestrial gastropod shells may behave as closed systems with respect to carbon over geologic timescales. More work on this subject is needed, but if our case study site is representative of other sites, then fossil shells of some small terrestrial gastropods, including at least five common genera, Catinella, Columella, Discus, Gastrocopta, and Succinea, should yield reliable ¹⁴C ages, regardless of the local geologic substrate.