(Table 1) Age determination of stalagmites of the Marcello Arévalo Cave in Chile

Stalagmites are important palaeo-climatic archives since their chemical and isotopic signatures have the potential to record high-resolution changes in temperature and precipitation over thousands of years. We present three U/Th-dated records of stalagmites (MA1-MA3) in the superhumid southern Andes, Chile (53°S). They grew simultaneously during the last five thousand years (ka BP) in a cave that developed in schist and granodiorite. Major and trace elements as well as the C and O isotope compositions of the stalagmites were analysed at high spatial and temporal resolution as proxies for palaeo-temperature and palaeo-precipitation. Calibrations are based on data from five years of monitoring the climate and hydrology inside and outside the cave and on data from 100 years of regional weather station records.
Water-insoluble elements such as Y and HREE in the stalagmites indicate the amount of incorporated siliciclastic detritus. Monitoring shows that the quantity of detritus is controlled by the drip water rate once a threshold level has been exceeded. In general, drip rate variations of the stalagmites depend on the amount of rainfall. However, different drip-water pathways above each drip location gave rise to individual drip rate levels. Only one of the three stalagmites (MA1) had sufficiently high drip rates to record detrital proxies over its complete length. Carbonate-compatible element contents (e.g. U, Sr, Mg), which were measured up to sub-annual resolution, document changes in meteoric precipitation and related drip-water dilution. In addition, these soluble elements are controlled by leaching during weathering of the host rock and soils depending on the pH of acidic pore waters in the peaty soils of the cave's catchment area. In general, higher rainfall resulted in a lower concentration of these elements and vice versa. The Mg/Ca record of stalagmite MA1 was calibrated against meteoric precipitation records for the last 100 years from two regional weather stations. Carbonate-compatible soluble elements show similar patterns in the three stalagmites with generally high values when drip rates and detrital tracers were low and vice versa. d13C and d18O values are highly correlated in each stalagmite suggesting a predominantly drip rate dependent kinetic control by evaporation and/or outgassing. Only C and O isotopes from stalagmite MA1 that received the highest drip rates show a good correlation between detrital proxy elements and carbonate-compatible elements. A temperature-related change in rainwater isotope values modified the MA1 record during the Little Ice Age (~0.7-0.1 ka BP) that was ~1.5 °C colder than today. The isotopic composition of the stalagmites MA2 and MA3 that formed at lower drip rates shows a poor correlation with stalagmite MA1 and all other chemical proxies of MA1. 'Hendy tests' indicate that the degassing-controlled isotope fractionation of MA2 and MA3 had already started at the cave roof, especially when drip rates were low. Changing pathways and residence times of the seepage water caused a non-climatically controlled isotope fractionation, which may be generally important in ventilated caves during phases of low drip rates. Our proxies indicate that the Neoglacial cold phases from ~3.5 to 2.5 and from ~0.7 to 0.1 ka BP were characterised by 30% lower precipitation compared with the Medieval Warm Period from 1.2 to 0.8 ka BP, which was extremely humid in this region.

石笋（Stalagmites）是重要的古气候档案（palaeo-climatic archives），因其化学与同位素特征能够记录数千年来温度与降水的高分辨率变化。本文报道了智利安第斯山脉南部（53°S）超湿润区域三处经铀钍（U/Th）测年的石笋（MA1-MA3）记录。该三处石笋均在过去5000年（ka BP，即千年以前）内同步生长，赋存于片岩（schist）与花岗闪长岩（granodiorite）发育的洞穴中。研究人员以高空间与时间分辨率分析了石笋的常量元素、微量元素以及碳、氧同位素组成，将其作为古温度（palaeo-temperature）与古降水（palaeo-precipitation）的代用指标。校准工作基于洞穴内外为期5年的气候与水文监测数据，以及区域100年的气象站观测记录。石笋中的钇（Y）与重稀土元素（HREE, Heavy Rare Earth Elements）等水不溶元素，可反映其混入的硅质碎屑沉积物（siliciclastic detritus）含量。监测结果表明，当滴水速率超过阈值水平（threshold level）后，碎屑沉积物的含量受滴水速率调控。总体而言，石笋的滴水速率变化受降雨量控制，但每个滴水点位上方不同的滴水路径（drip-water pathways）会形成各自的滴水速率水平。三处石笋中仅MA1具备足够高的滴水速率，可在其完整生长序列中记录碎屑代用指标。以亚年分辨率（sub-annual resolution）测定的碳酸盐相容元素（carbonate-compatible elements，如U、Sr、Mg）含量，可反映大气降水（meteoric precipitation）变化及相关的滴水稀释（drip-water dilution）作用。此外，这些可溶性元素的含量受控于洞穴汇水区（catchment area）泥炭土（peaty soils）中酸性孔隙水的pH值，即围岩（host rock）与土壤风化过程中的淋滤（leaching）作用。通常而言，降雨量越高，这些元素的浓度越低，反之则越高。研究人员以两处区域气象站过去100年的大气降水记录，对石笋MA1的镁钙（Mg/Ca）比值记录进行了校准。三处石笋的碳酸盐相容可溶性元素均呈现相似的变化模式：当滴水速率与碎屑示踪剂数值较低时，元素浓度普遍偏高，反之则偏低。每处石笋的δ¹³C与δ¹⁸O值均呈现高度相关，表明其主要受蒸发作用与/或脱气作用的滴水速率依赖性动力学控制。仅滴水速率最高的石笋MA1的碳、氧同位素，与碎屑代用指标元素及碳酸盐相容元素呈现良好的相关性。在小冰期（Little Ice Age，较现代偏冷约1.5℃，对应~0.7-0.1 ka BP）期间，雨水同位素值的温度相关变化改造了MA1的同位素记录。形成于较低滴水速率的石笋MA2与MA3，其同位素组成与MA1及MA1的所有其他化学代用指标相关性均较差。亨迪测试（Hendy tests）结果表明，MA2与MA3的脱气控制同位素分馏（isotope fractionation）作用早在洞顶（cave roof）阶段就已开始，尤其是在滴水速率较低时。渗流水（seepage water）的路径与驻留时间（residence times）的变化会引发非气候控制的同位素分馏，这一现象在滴水速率较低时期的通风洞穴（ventilated caves）中可能普遍存在。本研究的代用指标显示，新冰期冷期（Neoglacial cold phases，~3.5-2.5 ka BP与~0.7-0.1 ka BP）的降水量较中世纪暖期（Medieval Warm Period，1.2-0.8 ka BP）降低30%，而中世纪暖期是该区域极端湿润的时期。