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

Abstract

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 fCa, 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 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 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.

Exploring soluble and colloidally transported trace elements in stalagmites: The strontium-yttrium connection

“…A MATLAB script was written to allow for semi-automated estimation of growth rate and associated trace element concentrations, most notably the Sr partition coefficient (D Sr ). First, we assumed a classical model for Sr partitioning behavior, and because Sr/Ca is affected by PCP, we modulated this value by Mg/Ca assuming congruent dissolution of bedrock (assuming negligible soil-derived components), and therefore equivalent bedrock and drip water chemistry (Stoll et al, 2022b; see supplemental methods). The resulting estimate for D Sr can be directly correlated with growth rate, or background-subtracted (de-trended) to account for changes in baseline Mg (Fig.…”

Northern Hemisphere Summer Insolation and Ice Volume Driven Variations in Hydrological Environment in Southwest China

“…The extensive synchronisation of these multi‐indicator records on orbital scales supports our interpretation of X/Ca PC1 as an indicator of regional hydroclimatic variations. Since the underlying PCP process is a non‐linear Rayleigh process, we also use $$\frac{{\text{Mg}/\text{Ca}}_{\min }}{{\text{Mg}/\text{Ca}}_{\text{sample}}}$$ to approximate the remaining fraction f Ca in the drip water, which may serve as a more linear, that is, more direct proxy for regional precipitation amount variation than the X/Ca ratios (Stoll et al., 2022, Figure S6 and Text S2 in Supporting Information ).…”

“…The differential increase in the relative content of trace elements in stalagmites and positive correlation between all three elements was interpreted as prior calcite precipitation (PCP) (Fairchild et al., 2006; Shi et al., 2022; Stoll et al., 2022; Warken et al., 2018). Sinclair, Wassenburg, and their co‐workers infer that predominance of PCP can be tested d through linear regression between ln(Sr/Ca) and ln(Mg/Ca) (Sinclair et al., 2012; Wassenburg et al., 2020).…”

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

“…Measured δ 44 Ca, Mg/Ca, and Sr / Ca, as well as the calculated f Ca for differing 44 Ca and B (bedrock factors). (Stoll et al, 2023).…”