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.

中文翻译：

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两个冰期周期中中纬度环境中洞穴岩初始 234U/238U 比值的气候控制


尽管对地下水中 234U/238U 进行了早期水文研究，但它们作为石笋中古气候指标的利用仍然零星。本研究探讨了伊比利亚东北部 Ejulve 洞穴中 6 个石笋的 235 个测年（第 230 个）中的铀同位素比值，涵盖了最后的 260 ka。观察到的 234U 富集归因于 234U 从受损晶格位点选择性浸出，这与滴落路线中的微裂缝数量和湿润频率有关，在某些条件下，这可能导致 234U 反冲的积累。这种选择性浸出过程随着基岩溶解的增强而减弱，从而导致低 δ234U。温度变化显著影响基岩溶蚀强度。在稳定期和冰期极大期，较低的温度可能会降低植被和呼吸速率，从而降低土壤 CO2 和整体岩石溶解速率。由于块状岩石溶解较低，这种减少可能会增强 234U 从基岩表面的优先浸出。此外，寒冷时期的温度范围可能促进了更频繁的冻融循环，导致微破裂和新鲜表面暴露。相反，当低 δ234U 与高基岩溶解速率一致时，温暖的温度增加了土壤呼吸速率和土壤 CO2，加快了间冰期和间冰期的岩石溶解速率。许多对基岩溶解和湿频率过程敏感的变量的贡献成功地解释了在 MIS 3-4 和 MIS 5b-5e 期间仙女座石笋中 δ234U 中观察到的 57% 和 74% 的变化。 在这些变量中，生长速率已成为解释 δ234U 变异性的关键，突出了土壤呼吸和土壤 CO2 通过基岩溶解在 δ234U 中的重要作用。I-STAL 模拟提供了将先验方解石沉淀 （PCP） 指标（如 Mg/Ca）与基岩溶解的 PCP 不敏感指标（如 δ234U）以及生长速率数据相结合的可能性，可能有助于诊断 PCP 变化何时主要反映滴灌间隔的变化以及基岩溶解强度的变化何时起作用。石笋 δ234U、基岩溶解和初始滴水过度饱和之间的关系表明古气候替代物的两项重大进展。首先，δ234U 可以作为 δ13C 的宝贵补充，因为它受土壤呼吸和土壤 CO2 的显著影响，从而反映了土壤和植被对湿度和温度敏感的生产力。其次，由于 PCP 不分馏铀同位素，因此 δ234U 可以与 Mg/Ca 或 δ44Ca 结合使用，以解卷积由于滴落速率变化引起的 PCP 变化。本研究强调对 δ234U 的首要气候控制，而不管样品之间的绝对 234U/238U 活度比及其与长期平衡的接近或距离如何，并倡导将其应用于其他洞穴遗址。