Modeling speleothem δ 13C variability in a central Sierra Nevada cave using 14C and 87Sr/ 86Sr

Abstract

Carbon isotopes in speleothems can vary in response to a number of complex processes active in cave systems that are both directly and indirectly related to climate. Progressing downward from the soil zone overlying the cave, these processes include soil respiration, fluid–rock interaction in the host limestone, degassing of CO 2 and precipitation of calcite upflow from the speleothem drip site, and calcite precipitation at the drip site. Here we develop a new approach to independently constrain the roles of water–rock interaction and soil processes in controlling stalagmite δ 13C. This approach uses the dead carbon proportion (dcp) estimated from coupled 14C and 230Th/U measurements, in conjunction with Sr isotope analyses on stalagmite calcite from a central Sierra Nevada foothills cave in California, a region characterized by a highly seasonal Mediterranean-type climate, to determine the roles of water–rock interaction and soil processes in determining stalagmite δ 13C. Increases in stalagmite dcp between 16.5 and 8.8 ka are coincident with decreased δ 13C, indicating a varying yet substantial contribution from the soil organic matter (SOM) reservoir, likely due to significantly increased average age of SOM in the soil veneer above the cave during wet climatic intervals. We use geochemical and isotope mixing models to estimate the host-carbonate contribution throughout the δ 13C time series and determine the degree of degassing and calcite precipitation that occurred prior to precipitation of stalagmite calcite. The degree of degassing and prior calcite precipitation we calculate varies systematically with other climate indicators, with less degassing and prior calcite precipitation occurring during wetter climatic intervals and more during drier intervals. Modeled δ 13C values and degassing calculations suggest that some degree of prior calcite precipitation is necessary at all time intervals to explain measured stalagmite δ 13C values, even during relatively wet intervals. These results illustrate the importance of constraining degassing and prior calcite precipitation in the interpretation of speleothem δ 13C records, particularly those from caves that formed in seasonal semi-arid to arid environments.