Distinguishing the combined vegetation and soil component of I13C variation in speleothem records from subsequent degassing and prior calcite precipitation effects

Abstract

The carbon isotopic signature inherited from soil and epikarst processes may be modified by degassing and prior calcite precipitation (PCP) before its imprint on speleothem calcite. Despite laboratory demonstration of PCP effects on carbon isotopes and increasingly sophisticated models of the governing processes, to date, there has been limited effort to deconvolve the dual PCP and soil-epikarst components in measured speleothem isotopic time series. In this contribution, we explore the feasibility, advantages, and disadvantages of using trace element ratios and I44Ca to remove the overprinting effect of PCP on measured I13C to infer the temporal variations in the initial I13C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and I44Ca covary as expected. However, Srg/gCa does not show consistent relationships with I44Ca so PCP is not the dominant control on Srg/gCa. From I44Ca and Mg/Ca, our calculation of PCP as fCa, the fraction of initial Ca remaining in solution at the time the stalagmite layer is deposited, yields multiple viable solutions depending on the assumed I44Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of I13C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial I13C. Nonetheless, the trends in initial I13C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82gka interval, trends in calculated initial I13C are more similar than those in measured I13C and reveal a common positive-Anomaly initial I13C during a stadial cooling event. During deglaciations, calculated initial I13C implies a trend of greater respiration rates and higher soil CO2, although the higher interglacial drip water saturation favors more extensive PCP. Initial I13C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or I44Ca provides a quantitative indication of past changes in PCP. Further study of Mg partitioning in speleothems will improve the robustness of Mg/Ca as a PCP proxy.