Climate controls on speleothem initial 234U/238U ratios in midlatitude settings over two glacial cycles

Abstract

Despite early hydrological studies of ²³⁴U/²³⁸U in groundwaters, their utilization as a paleoclimatic proxy in stalagmites has remained sporadic. This study explores uranium isotope ratios in 235 datings (²³⁰Th) from six stalagmites in Ejulve cave, northeastern Iberia, covering the last 260 ka. The observed ²³⁴U enrichment is attributed to selective leaching of ²³⁴U from damaged lattice sites, linked to the number of microfractures in the drip route and wetness frequency, which under certain conditions, may result in the accumulation of ²³⁴U recoils. This selective leaching process diminishes with enhanced bedrock dissolution, leading to low δ²³⁴U. Temperature variations significantly influence bedrock dissolution intensity. During stadial periods and glacial maxima, lower temperatures likely reduced vegetation and respiration rates, thereby decreasing soil CO₂ and overall rock dissolution rates. This reduction could enhance the preferential leaching of ²³⁴U from bedrock surfaces due to lower bulk rock dissolution. Additionally, the temperature regime during cold periods may have facilitated more frequent freeze–thaw cycles, resulting in microfracturing and exposure of fresh surfaces. Conversely, warmer temperatures increased soil respiration rates and soil CO₂, accelerating rock dissolution rates during interstadials and interglacials, when low δ²³⁴U is consistent with high bedrock dissolution rates. The contribution of a number of variables sensitive to bedrock dissolution and wetness frequency processes successfully explains 57% and 74% of the variability observed in the δ²³⁴U in Andromeda stalagmite during MIS 3–4 and MIS 5b-5e, respectively. Among these variables, the growth rate has emerged as crucial to explain δ²³⁴U variability, highlighting the fundamental role of soil respiration and soil CO₂ in δ²³⁴U through bedrock dissolution. I-STAL simulations provides the potential for a combination of Prior Calcite Precipitation (PCP) indicators like Mg/Ca with PCP-insensitive indicators of bedrock dissolution such as δ²³⁴U, along with growth rate data, may be useful to diagnose when PCP variations reflect predominantly changes in drip intervals and when changes in bedrock dissolution intensity contribute. The relationship between stalagmite δ²³⁴U, bedrock dissolution, and initial dripwater oversaturation suggests two significant advancements in paleoclimate proxies. First, δ²³⁴U could serve as a valuable complement to δ¹³C since it is significantly influenced by soil respiration and soil CO₂, thereby reflecting soil and vegetation productivity sensitive to both humidity and temperature. Secondly, since PCP does not fractionate uranium isotopes, δ²³⁴U could be used in combination with Mg/Ca or δ⁴⁴Ca to deconvolve PCP variations due to changing drip rates from those due to changes in initial saturation state. This study emphasizes the overriding climatic control on δ²³⁴U, regardless of the absolute ²³⁴U/²³⁸U activity ratios among samples and their proximity or distance from secular equilibrium, and advocates for its application in other cave sites.