Deciphering the hydroclimatic significance of dripwater δ13CDIC variations in monsoonal China based on modern cave monitoring

Abstract

Stable carbon isotopic composition (δ13C) of speleothems has often been recognized as a proxy of vegetation and soil processes in many climatic regimes. In monsoonal regions, the speleothem δ13C records are thought to be able to document changes in local and/or regional precipitation as well. Due to the complexity of carbon isotopic evolution within heterogeneous karst aquifers, accurate interpretations of this proxy are challenging. It is essential to carry out detailed monitoring of cave dripwater δ13CDIC to disentangle various processes governing carbon isotopic evolution and decipher the hydroclimatic constraints. Here, we reported the results of a five-year monitoring of the δ13CDIC as well as hydrochemical compositions of dripwaters in the Maomaotou Big Cave, Guilin, South China. Great spatiotemporal variations in the dripwater δ13CDIC values were observed: (i) seepage flow-fed drips commonly had lower δ13CDIC values, with smaller temporal variation; (ii) the lowest δ13CDIC values were found at fracture-fed drips that had a large supply of soil CO2, whereas the fracture-fed drips with less, episodic CO2 recharge had much more positive δ13CDIC; (iii) the δ13CDIC exhibits remarkable season changes, particularly at fracture-fed drips where higher δ13CDIC values were observed in winter seasons; and (iv) an increasing trend in mean value of δ13CDIC at individual sites during 2015–2019 was found. Both hydrochemical and isotopic analyses revealed that the dripwater δ13CDIC is mainly controlled by spatiotemporal variations in water-CO2-rock interactions in association with hydrological processes, coupled with soil CO2 dynamics, which are closely linked to changes in local rainfall at monthly to annual timescales. Prior calcite precipitation (PCP) along the flow path above the cave could also affect the δ13CDIC of fracture-fed dripwaters in dry periods. Comparisons of the dripwater δ13CDIC records with local climate showed that the change of summer monsoonal rainfall, particularly the August-September-October precipitation, significantly influences the annual mean δ13CDIC value of dripwaters in caves of South China. Further comparison analyses of dripwater δ13CDIC records from both southern and northern China suggested that variations of dripwater δ13CDIC could reflect changes of regional monsoonal precipitation over inter-annual (and maybe decadal) timescale. This demonstrates that the δ13C of speleothems, if precipitated at isotopic equilibrium and not greatly influenced by the CO2 degassing, is likely to be a valuable proxy of monsoon rainfall variability across East Asia.