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.