The influence of hydrogeology and carbon sources on speleothem δ13C based on long-term monitoring in Furong Cave, southwest China

Abstract

The stable carbon isotopic composition of speleothems is an important but incompletely understood parameter in paleoclimatological and paleoenvironmental reconstruction, recording changes in vegetation and hydrology. This study systematically assesses the influences of karst hydrology, cave environment, and carbon sources on the δ¹³C values of farmed calcite in Furong Cave, located in Chongqing, southwest China. Six drip sites (MP1-MP5) were monitored between 2009 and 2019, and Δ¹⁴C data (Δ¹⁴C is the difference in the ¹⁴C/¹²C ratio between a sample and a standard, expressed in permille, ‰) of soil organic matter, drip water and farmed calcite (calcite precipitated on a glass substrate) were obtained. A linear relationship was found between the seasonal variability of pCO₂ in cave air and δ¹³C_Cc of the farmed calcite in “Great Hall” characterised by a relatively stable microenvironment. The growth rate and δ¹³C_Cc values of the farmed calcite was not be affected by the drip rate unless the drip rate decreased to <1 drip/min for a long time, leading to a decrease in growth rate and higher δ¹³C_Cc values because of longer CO₂ degassing. MP2 and MP9 show faster drip rates, higher δ¹³C_DIC and lower Δ¹⁴C than MP1 and MP3, which suggests that the “old carbon” in MP9 is derived from the host rock. MP4 and MP5 are characterised by slower drip rates and lower δ¹³C_DIC and δ¹³C_Cc, and the Δ¹⁴C values of the farmed calcite are lower than those of MP9. The slow drip rate indicates that less infiltration water reaches MP4 and MP5, and soil CO₂ derived from the decomposition of “old” organic matter enters the fractures that lead to these drip sites. The fast drip rate of MP9 suggests that the fissures feeding this drip site are mostly water-saturated, limiting the exchange between soil CO₂ and CO₃²⁻ and allowing more “old carbon” from the bedrock to dissolve leading to higher δ¹³C_DIC. This study emphasizes that in addition to changes in the cave environment, the source(s) of the carbon may be a more important factor controlling the δ¹³C values of drip water (and speleothem) than drip rate.