Distinguishing the vegetation and soil component of δ¹³C variation in speleothem records from degassing and prior calcite precipitation effects

Abstract. The carbon isotopic signature inherited from soil/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 dripwater. In 8 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 universally the dominant control on Sr/Ca. From δ⁴⁴Ca and Mg/Ca, our calculation of PCP as f_Ca, fraction of initial Ca remaining at the deposition of the stalagmite layer, yields multiple viable solutions depending on the assumed δ⁴⁴Ca fractionation factor and inferred variation in D_Mg. 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 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, the trend of greater respiration rates and higher soil CO₂ is captured in the calculated initial δ¹³C, despite the tendency of higher interglacial dripwater situation to favor more extensive PCP.