长江中游石笋环境磁学参数的古降水意义

The middle reaches of Yangtze River has frequently suffered extreme flooding and droughts caused by the dramatic variation of monsoon precipitation in history and present days. The reconstruction of high resolution precipitation since Holocene is therefore crucial to the disaster preparedness and reduction in this region. Stalagmites are perfect archives for high resolution studies of paleoclimate and many important relevant results have been widely documented in the last decade. Although more and more climatic proxies have been successively extracted, there is no specific proxy of paleohydrology from single stalagmite. Our previous study from the middle reaches of Yangtze River indicated that the environmental magnetic parameters of stalagmites revealing the grain size distribution and concnetrations of magnetic minerals could record paleohydrology，including the storms. Unfortunately, the response mechanism remains unclear which will retard the application and development of this promise precipitation proxy index. In order to identify the response mechanism of stalagmite magnetic parameters to paleohydrology, this proposal will monitor the concentrations, grain size distributuion, and species, as well as environmental magnetic properties of magnetic minerals in dripping water , atmospheric deposition and soil above Heshang cave, and the results will be compared with the precipitation monitoring data. Meanwhile, two stalagmites will be determined by a u-channel magnetometer in high resolution and the data will undergo deconvolution analysis. In addition, the section of the stalagmites with frequent variation in paleohydrology will be measured by a Scanning SQuID Microscope to obtain the annual variation. Finally, the reconstruction of the high resolution variation of precipitation in the middle reaches of Yangtze River over the past nine thousand years will be achieved.

长江中游地区由降水变化引起的自然灾害频繁，重建该区全新世以来高分辨率的降水量并掌握其演变规律是防洪减灾的关键。洞穴石笋是高分辨率古气候重建的重要载体，虽然古气候指标丰富，但目前还未能从单点石笋中获取古降水信息的可靠代用指标。前期研究发现洞穴石笋中反映磁性颗粒粒度及浓度的环境磁学参数能够记录包括暴雨事件在内的古降水信息，但因无确凿的解释机制而限制了这一颇具有潜力指标的推广。本项目拟通过土壤、滴水、洞内大气沉降物及降雨量的现代过程监测，揭示洞穴沉积物中磁性颗粒特征与区域降水量的关系，为其古降水意义提供直接证据。同时，通过岩石磁学和矿物形貌学分析，探寻石笋中磁性颗粒的来源及磁学参数变化的矿物学原因，从而为石笋环境磁学参数的古降水意义寻求合理的解释机制。在此基础上对石笋超导长岩芯扫描数据进行去卷积化处理以及高分辨率的超导量子干涉扫描显微镜测试，以期重建长江中游地区9000年高分辨率的古降量。

长江中游地区由降水变化引起的自然灾害频繁，重建该区全新世以来高分辨率的降水量并掌握其演变规律是防洪减灾的关键。洞穴石笋是高分辨率古气候重建的重要载体，虽然古气候指标丰富，但目前还未能从单点石笋中获取古降水信息的可靠代用指标。本项目计划通过土壤、滴水、洞内大气沉降物及降雨量的现代过程监测，揭示洞穴沉积物中磁性颗粒特征与区域降水量的关系，为其古降水意义提供直接证据；在此基础上对鄂西和尚洞HS4石笋开展系统的环境磁学分析，以期重建长江中游地区9000年高分辨率的古降量。. 本项目自主设计滴水中磁性颗粒收集装置开展现代过程监测。发现洞穴滴水中的磁性颗粒均以软磁组分磁铁矿为主，同时还含有不同浓度的针铁矿，初步显示夏季针铁矿浓度较高，冬季较低。鄂西和尚洞和豪猪子洞的监测显示，11-12月和5-6月份洞穴滴水中磁性颗粒浓度较高，同时高矫顽力的赤铁矿含量也相对增加，两个洞7-8月份滴水中磁性颗粒浓度均比较低。.通过对HS4石笋的环境磁学研究，发现该石笋中来自于洞顶土壤的软磁组分通量（IRMsoft_flux）能够反映降雨量变化，以此重建了长江中游地区过去9000年以来降雨量特征，揭示出该区全新世以来的古水文表现出千年尺度的三段式变化：6.7-3.4kaBP是相对比较干的时期，而在这之前和之后都表现出相对潮湿的气候，并与千年尺度的ENSO活动强度相关。更为重要的是，这一参数还表现出许多更短时间尺度的变化，而且其峰值与已知的该区古洪水事件在时间上基本吻合，证实该参数响应了短时间尺度的区域强降雨（暴雨）事件。这些强降雨事件呈现显著的500年周期变化（置信度大于99%），并与ENSO变化和太阳活动周期存在显著相关性，表现为暴雨事件主要发生在强ENSO活动和弱太阳活时期。本成果是依据石笋记录，第一次报道了暴雨事件及其驱动机制（Zhu et al., 2017, PNAS）。