岩溶滴水有机质在石笋中记录的现代过程观测：古环境定量重建的启示

Organic matter preserved into speleothem, which was mainly derived from the leachate of overlying soil, has offered increased potential in areas such as paleo-environment reconstruction. However, organic matter will experience physiochemical and biological alteration during the transport process from overlying soil to dripwater, and the adsorption process from dripwater to associated calcite, which bring uncertainty for interpretation of organic matter signals in speleothem, and it must therefore be corrected by cave monitoring before being interpreted as paleo-environment and paleo-ecology proxies. Previous cave monitoring works about organic matter mainly focused on climate factors driving the transport of organic matter from soil to dripwater, and it is still unclear about how the organic matter in dripwater was recorded into the associated calcite. In this proposal, we plan to conduct cave modern investigation of partition coefficient (Kd) values of organic matter between dripwater and associated calcite, environment factors inside the cave, and crystallization characteristics of associated calcite, and to illustrate the main factors of Kd on seasonal and inter-annual time scales. Combined with the relationship between organic matter in dripwater and annual rainfall amount, we can establish the equation model about the speleothem organic matter in response to the outside climate and environment. This study will helpful for understanding the paleo-environmental significance of organic matter preserved into stalagmite quantitatively.

石笋有机物因其蕴含着洞穴上覆土壤、植被等信息，已被广泛应用于石笋古环境重建中。然而，在土壤-洞穴滴水-石笋运输、记录过程中，有机物会经历复杂的理化和生物作用，导致石笋中的有机替代指标的解译存在很大的不确定性，需要通过长期的洞穴现代过程监测来校正石笋有机质蕴含的古气候、古环境信息。以往的洞穴监测都强调有机质从土壤到洞穴滴水的运输过程，忽视了有机质从滴水到石笋的记录过程，因而对石笋有机质响应气候和环境变化的机制还缺少系统的认识。本项目将选取和尚洞长年滴水点的滴水及其哺育的石笋碳酸盐开展洞穴现代过程监测，调查有机质在水-岩相互作用过程中的分配系数（Kd），探讨其与洞内环境因子以及沉积碳酸盐晶体的沉积特征之间的相互关系，揭示有机质分配系数的主要控制因素。综合有机质的生产、迁移、转换和记录过程的观测结果，建立石笋有机质对外界环境响应的热力学机制，为定量解译石笋有机质的古环境意义提供依据。

石笋有机指标因其可用来反映当地的植被、微生物群体等环境、生态演化信息，已被广泛应用于古环境、古生态重建领域。然而，在土壤-洞穴滴水-石笋运输、记录过程中，有机物会经历复杂的理化和生物作用，导致石笋中的有机替代指标的解译存在很大的不确定性，需要通过长期的洞穴现代过程监测来校正石笋有机质蕴含的古气候、古环境信息。本项目的研究课题来源于湖北清江和尚洞洞穴的长期监测。通过选取两个长期监测点（HS4、D5）的滴水及其对应的沉积碳酸盐中的有机质（NOM）开展长期的洞穴现代过程监测。我们发现滴水中可溶有机质（DOM，<0.22μm）浓度在季节和年际尺度均受大气降雨控制，而胶体有机质（COM,>0.22μm）的年际变化主要受控于前期干旱时长。在水-岩相互作用研究上，我们通过对滴水点D5长达11年的监测数据分析，提出矿物颗粒比表面积是影响有机质分配系数的主要因子，洞内温度变化是矿物颗粒比表面积大小的制约因素。在东亚季风区石笋生长速率呈现出夏季高、冬季低的特征，夏季矿物结晶快，比表面积小，而冬季矿物结晶慢，比表面积大，因而有机质浓度在玻璃板碳酸盐表现的特征为冬季高、夏季低的特征，这与和尚洞石笋纹层有机质的季节性分布是对应的（冬春季高、夏秋季低）。以上认识，可为石笋有机质纹层的形成机制以及定量解译石笋有机质的古环境、古生态意义提供科学依据。