Baluk cave (NW China) Holocene speleothem trace element ratios

The interpretation of trace element/calcium ratios of speleothems as indicators of local hydroclimatic variability in the vicinity of caves has led to controversy in reconstructing the evolution of moisture conditions in arid central Asia (ACA) during the Holocene. Here we present records of Mg/Ca, Sr/Ca, Ba/Ca, and U/Ca from precisely dated stalagmites from Baluk cave in Xinjiang (northwest China), spanning the past 9370 years. The co-variations of the trace element ratios, together with the slopes of the regression lines of the corresponding logarithmically transformed data, suggest that they are dominated by prior calcite precipitation (PCP) and thus can be used as reliable proxies of changes in moisture/precipitation. The trace element ratios are relatively high during ~9 to 5 ka and lower from 5 ka to the present, indicating a trend of increasing mid–late Holocene moisture in ACA. The long-term trend of variation of the trace element ratios is correlative with two other records of speleothem trace element ratios from caves in ACA: Kesang cave (western Xinjiang) and Ton cave (Uzbekistan). This spatial coherency of the trend of inferred moisture conditions from three caves that are separated by hundreds of kilometers demonstrates that speleothem trace element ratios are indicative of large spatial scale rather than local hydroclimatic variability in ACA during the Holocene. However, the long-term trend of variation of the trace element ratios is the inverse of the corresponding oxygen isotope (δ18O) records from the same cave sites, which implies that Holocene speleothem δ18O records do not represent changes in the precipitation amount in ACA; rather, they most likely reflect moisture sources and related water vapor transport controlled by Northern Hemisphere summer insolation (NHSI). Our findings provide new evidence for a ‘westerlies-dominated climatic regime’, which influenced hydroclimatic changes in ACA during the Holocene.将石笋（speleothems）的微量元素/钙比值作为洞穴周边局地水文气候变率的代用指标，这一研究思路在重建全新世（Holocene）干旱中亚（ACA）湿度条件演化的工作中曾引发广泛争议。本研究报道了来自中国新疆（中国西北）巴卢克洞（Baluk cave）两根精确定年石笋的Mg/Ca、Sr/Ca、Ba/Ca及U/Ca比值记录，时间跨度覆盖过去9370年。微量元素比值的协同变化，结合对应对数转换数据的回归线斜率特征，表明这些比值主要受控于方解石先期沉淀（PCP）过程，因此可作为湿度/降水变化的可靠代用指标。上述微量元素比值在约9~5 ka期间相对偏高，而在5 ka至今的时段内偏低，这指示干旱中亚全新世中晚期湿度呈逐渐升高的趋势。微量元素比值的长期变化趋势，与干旱中亚另外两处洞穴的石笋微量元素比值记录——新疆西部的克桑洞（Kesang cave）以及乌兹别克斯坦的通洞（Ton cave）——具有显著相关性。这三处间距达数百公里的洞穴所重建的湿度趋势呈现空间一致性，这表明全新世时期干旱中亚的石笋微量元素比值可指示大尺度气候信号，而非局地水文气候变率。但微量元素比值的长期变化趋势，与同一洞穴位点的对应氧同位素（δ¹⁸O）记录恰好相反，这意味着全新世石笋δ¹⁸O记录并非指示干旱中亚的降水量变化；相反，其更可能反映受北半球夏季日照（NHSI）调控的水汽来源及相关水汽输送过程。本研究结果为“西风主导的气候体系”提供了新证据，该体系在全新世时期调控了干旱中亚的水文气候变化。