Distinguishing the combined vegetation and soil component of <i>δ</i>¹³C 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 δ44Ca to remove the overprinting effect of PCP on measured δ13C to infer the temporal variations in the initial δ13C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ44Ca covary as expected. However, Sr / Ca does not show consistent relationships with δ44Ca so PCP is not the dominant control on Sr / Ca. From δ44Ca 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 δ44Ca fractionation factor and inferred variation in DMg. Uncertainty in the effective fractionation of δ13C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ13C. Nonetheless, the trends in initial δ13C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82 ka interval, trends in calculated initial δ13C are more similar than those in measured δ13C and reveal a common positive-anomaly initial δ13C during a stadial cooling event. During deglaciations, calculated initial δ13C implies a trend of greater respiration rates and higher soil CO2, although the higher interglacial drip water saturation favors more extensive PCP. Initial δ13C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or δ44Ca 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.