The transport mechanism of carbon isotopes based on 10 years of cave monitoring: Implications for paleoclimate reconstruction

Abstract

Cave monitoring is crucial to unraveling the factors controlling carbon isotope compositions and their transportation in karst systems. Here, we report a 10-year (2010 ~ 2019) cave monitoring of carbon isotopes from the plants, soil, cave water, and speleothems in Jiguan Cave, Central China, located on the Chinese north–south divide in an area that is sensitive to the Asian monsoon. The results show that the cave ventilation and soil air CO2 controlled by the local temperature and humidity conditions are responsible for the seasonal variability in the δ13CDIC values. The variation in the δ13CDIC values and trace element ratios of the cave water are sensitive to the local hydrological conditions. Although there is the lag time from precipitation to cave water in drought year, the δ13CDIC and trace element ratios (Mg/Ca, Ba/Ca and Sr/Ca) of cave water are strongly coupled and reflect the extreme drought due to the influence of prior calcite precipitation (PCP) and water–rock interaction. The 10-year monitoring period covered two complete El Niño Southern Oscillation (ENSO) cycles. During the two strong El Niño years (2010 and 2015), the cave drip water δ13CDIC values were negative but the δ18O values were positive. This opposite trend is mainly due to their different controlling mechanisms. The rainfall amount and moisture sources are responsible for the δ13CDIC and δ18O values of the drip water, respectively. Throughout the karst cave system, the carbon isotopes in Jiguan Cave are continuously enriched when migrating from the vegetation to the soil to the cave drip water to the modern speleothems. Comparisons with the stalagmite δ13C records for the nearby Jiguan Cave suggest that stalagmite carbon isotopes may reflect the regional precipitation changes induced by the ENSO on the decadal scale. Overall, we demonstrate that the δ13C is a potentially reliable proxy that records the changes in the regional environment, especially precipitation changes, and is also a sensitive indictor that indirectly reflects ENSO.