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The Hendy Test is widely used for assessing whether isotopic equilibrium was reached in speleothems by examining the δ18O and δ13C correlation along a single growth layer. However, stalagmite micro-layers are typically only a few micrometers thick and taper off from the center towards the sides, making it challenging to sample within the same growth layer in practice. To address this, we selected three caves in the monsoon region of China (Shihua Cave in the north, Heshang Cave in the central, and Baojinggong Cave in the south) to verify whether the modern cave calcite has reached equilibrium fractionation with drip water. We examined the spatial variations in the δ18O and δ13C values of farmed calcite on glass plates, which are analogous to a single growth layer. The δ18O and δ13C correlations of farmed calcite from different cave sites are consistently strong, suggesting that kinetic fractionation effects are prevalent, especially at the drip sites with lower drip rates due to longer CO2 degassing. The δ18O–δ13C covariations can also occur along speleothem growth axes on short time scales, while isotopic variations over longer time scales are still in response to climate change. We propose that the Hendy Test criteria might not be prerequisites to isotopic equilibrium, and a Replication Test provides a more reliable indication of the integrity of isotopic proxies in paleoclimate research. [ABSTRACT FROM AUTHOR]

© 2015 Elsevier Ltd. Several recent studies have used records of the radiocarbon (14C) bomb peak in speleothems to inversely model the soil a 14CO2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14CO2 at non-karst sites, which indicate that soil CO2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon.

Organic matter preserved in speleothems has the potential to reflect the changes of the overlying vegetation and soil ecosystem. Here we report the first near 50-yr-resolution acid-soluble organic matter (ASOM) carbon isotope (δ 13 C ASOM ) sequence derived from a laminated stalagmite (HS-4) spanning the last 9 ky in Heshang Cave, central China. The δ 13 C ASOM values vary between − 25.8‰ and − 22.0‰, with lower values from 9 to 4 ka BP and less negative values from 4 to 0 ka BP. We postulate that the δ 13 C ASOM sequence is mainly controlled by temperature and water balance. Temperature could affect both vegetation physiology and microbial degradation in soil horizons. The influence of temperature on the δ 13 C ASOM is supported by the negative correlation ( r = − 0.48, p < 0.001) between the δ 13 C ASOM record and the paleotemperature record in the nearby Dajiuhu peatland. The water balance can affect the retention time of organic matter in soils. Under drier conditions, the soil organic matter will be retained longer and is more likely to be biologically degraded, resulting in more negative δ 13 C ASOM values. Our results reveal that δ 13 C ASOM in speleothems has the potential to reflect the response of vegetation and/or soil processes to paleoclimate changes. [ABSTRACT FROM AUTHOR]

Abstract: Recently, it has been shown that U–Th dated speleothems may provide a valuable archive of atmospheric radiocarbon (14C), but the reliability of these records is dependent upon the stability of the dead carbon proportion (DCP) derived from the soil and bedrock. In order to assess climatic influences on speleothem DCP, we have investigated DCP variability over the Holocene interval where atmospheric 14C is well known based on dendrochronologically dated tree rings by conducting 14C measurements on a U–Th dated stalagmite (HS4) from Heshang Cave, Hubei Province, China ( , ; 294 m) spanning 0.5–9.6 ka. We investigated climatic controls on DCP, and found that DCP in HS4 has an average value over the Holocene of , with an average age offset from atmospheric radiocarbon of , and displays a response to both precipitation increases and decreases. HS4 DCP increases during the wetter mid-Holocene interval (∼5.5–7.1 ka), likely reflecting a shift to more closed-system dissolution in response to increased soil moisture. DCP decreases during the 8.2 ka event, a time period of dry conditions at Heshang Cave, though the lower amplitude of this shift indicates that DCP may be less sensitive to dry events. Speleothems are potentially valuable archives of atmospheric radiocarbon, especially in older portions of the 14C calibration curve where knowledge of atmospheric 14C is limited, however minor climatic influences on DCP could introduce uncertainties of several hundred years to calibrated ages. [Copyright &y& Elsevier]

Several recent studies have used records of the radiocarbon ( 14 C) bomb peak in speleothems to inversely model the soil a 14 CO 2 and the age distribution of soil organic material (SOM) above caves, in part to investigate the potential of speleothems as sensitive records of past SOM dynamics. The results of these modeling studies have suggested that soil CO 2 at karst sites is derived primarily from the decomposition of SOM with turnover times on the order of decades to centuries. This result is in stark contrast with observations of soil a 14 CO 2 at non-karst sites, which indicate that soil CO 2 is derived primarily from root respiration and decomposition of SOM with much shorter turnover times. This discrepancy suggests that SOM in karst settings may have a very different age distribution than sites that have been studied previously and/or that soil CO 2 is not the main source of speleothem carbon. To help resolve this discrepancy, we present an improved inverse model which we use to estimate the age of CO 2 above several caves. We also present results from a detailed case study of soil carbon dynamics at Heshang Cave, China. This work demonstrates that SOM in karst sites may be much older than SOM in non-karst soils that have been studied previously, but that CO 2 produced in the shallow soil zone is unlikely to be the main source of speleothem carbon. A review of the literature suggests that the most likely explanation for the aforementioned discrepancy is that decomposition of down-washed SOM in the vadose zone is the dominant source of speleothem carbon. [ABSTRACT FROM AUTHOR]

Abstract: Analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. Earlier Mesoproterozoic successions, however, have received comparatively little attention due to the relatively subdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profiles from three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of the early Mesoproterozoic (1600–1400Ma, Calymmian period) Jixian Group. High-resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better constrain both the pattern and origin of isotopic variation in the Mesoproterozoic. Marine carbonate rocks of the Jixian Group show generally muted isotopic variation with average values near 0‰, consistent with previous observations from the early Mesoproterozoic. Data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long-term decrease in pCO2 and, consequently, in the isotopic buffering capacity of marine dissolved inorganic carbon (DIC). By contrast, the isotopic composition of marine organic matter suggests facies-dependent differences in carbon cycling. Organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decomposition in shallow-water environments, and increased remineralization by anaerobic heterotrophs in deeper-water environments. Correlation between organic carbon composition and depositional environment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters. [Copyright &y& Elsevier]