| Summary: | 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 δ<sup>44</sup>Ca to remove the overprinting effect of PCP on measured δ<sup>13</sup>C to infer the temporal variations in the initial δ<sup>13</sup>C of drip water prior to degassing and PCP. In nine examined stalagmites, the most widely utilized PCP indicators Mg/Ca and δ<sup>44</sup>Ca covary as expected. However, Sr /Ca does not show consistent relationships with δ<sup>44</sup>Ca so PCP is not the dominant control on Sr /Ca. From δ<sup>44</sup>Ca and Mg/Ca, our calculation of PCP as <i>f</i><sub>Ca</sub>, the fraction of initial Ca remaining in solution at the time the stalagmite layer is deposited, yields multiple viable solutions depending on the assumed δ<sup>44</sup>Ca fractionation factor and inferred variation in <i>D</i><sub>Mg</sub>. Uncertainty in the effective fractionation of δ<sup>13</sup>C during degassing and precipitation contributes to uncertainty in the absolute value of estimated initial δ<sup>13</sup>C. Nonetheless, the trends in initial δ<sup>13</sup>C are less sensitive to these uncertainties. In coeval stalagmites from the same cave spanning the 94 to 82 ka interval, trends in calculated initial δ<sup>13</sup>C are more similar than those in measured δ<sup>13</sup>C and reveal a common positive-anomaly initial δ<sup>13</sup>C during a stadial cooling event. During deglaciations, calculated initial δ<sup>13</sup>C implies a trend of greater respiration rates and higher soil CO<sub>2</sub>, although the higher interglacial drip water saturation favors more extensive PCP. Initial δ<sup>13</sup>C can be estimated for active and fossil speleothems from a range of settings, wherever there is confidence that Mg/Ca and/or δ<sup>44</sup>Ca 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.
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