Speleothem records of monsoon intensity: evaluating proxy fidelity via cave process monitoring in the Shillong plateau, NE India

印度季风给南亚带来了强烈的夏季降雨，因此影响了数百万人的生存，然而，决定其年际到十年时间尺度强度的根本原因仍然知之甚少。气候建模者需要了解印度夏季风（ISM）的分布和强度如何在一系列时间尺度上发生变化，并将这些变化与已知的天文和陆地周期进行比较，以确定驱动季风演变的机制。大气过程的地球化学记录，主要是降雨和温度，保存在洞穴内形成的碳酸盐地层中（石笋）；例如，氧同位素的变化提供了降雨原始氧同位素组成的记忆。我们的建议的基础是收集和分析位于ISM核心区的洞穴滴水和石笋。石笋生长的年变化，就像树木年轮的变化一样，提供了温度和降雨的记录，但根据真实的气候过程来解释它们的氧同位素记录，需要详细了解气候信号是如何从降雨通过土壤和地下水系统传递到洞穴的，并在石笋从滴水中生长时被记录下来。印度东北部的西隆高原拥有一个迷宫般的石灰岩洞穴，是一个捕捉从孟加拉湾向北移动的ISM气团早期降雨的地区，是季风代理发展和校准的理想地点。由于永远不能保证恢复可用的洞穴记录，我们正在请求资源进行侦察研究，我们将对至少两个洞穴系统进行全面监测（一个已经确定，其他洞穴系统将在实地活动中进行评估和选择），这些洞穴系统将产生关于热带碳酸盐过程的重要新信息，从而开发可靠的地球化学代用物，作为测量温度变化和季风强度的措施。对洞穴气候记录的定量解释至关重要。这些代用物包括反映气候变化的氧和碳同位素，确定生长年龄的u同位素分析，以及记录土壤和基岩过程的微量元素，如Mg、Sr和P。该项目被设计为风险最小/回报最高的试点研究，将提供有关热带潮湿石灰石系统控制氧同位素从降水向石柱方解石转移的当地过程的基本和迄今为止几乎未知的背景信息。这项建议的及时性不仅源于技术的发展，使洞穴地球化学分析的空间精度更高，还源于最近与当地科学家的合作，他们将监督洞穴监测计划，确保定期可靠地返回高质量的监测数据。我们的项目将使用在直布罗陀开发的成熟的洞穴监测技术，对热带雨-土壤-石灰石环境中的季风气候捕获过程进行首次详细研究。这项工作的成功将导致一个更大的项目对该地区的评估，该项目针对的是最有可能产生长期气候记录的洞穴遗址，在那里气候捕获的机制可以得到适当的理解。

The Indian monsoon delivers intense summer rainfall to southern Asia and so affects the survival of many millions of people, yet the underlying causes that determine its intensity on interannual to decadal timescales remain poorly understood. Climate modellers require a knowledge of how the distribution and intensity of the Indian summer monsoon (ISM) has varied through a range of timescales and to compare these variations with known astronomical and terrestrial cycles in order to identify the mechanisms that drive monsoon evolution. Geochemical records of atmospheric processes, primarily rainfall and temperature, are preserved in carbonate formations formed within caves (stalagmites); for example variations in oxygen isotopes provide a memory of the original oxygen isotope composition of rainfall. The basis of our proposal is the collection and analysis of both cave drip water and stalagmites that are located in the core zone of the ISM. Annual variations in stalagmite growth, like those of tree rings, provides a record of temperature and rainfall, but interpretation of their oxygen isotope record in terms of real climatic processes requires detailed knowledge of how the climate signal is passed from rainfall though the soil and groundwater system and into the cave, to be recorded as a stalagmite grows from the dripwater. The Shillong plateau of NE India hosts a labyrinth of limestone caves and is an area that captures the early rainout of the ISM air mass moving north from the Bay of Bengal and is an ideal location for monsoon proxy development and calibration. Since recovery of useable speleothem records can never be guaranteed we are requesting resources for a reconnaissance study in which we shall carry out comprehensive monitoring of at least two cave systems (one already identified, others to be evaluated and selected during the field campaign) that will generate vital new information on tropical carbonate processes leading to the development of reliable geochemical proxies as a measure of the changing temperatures and intensities of the monsoon, fundamental to the quantitative interpretation of speleothem climate records. These proxies include oxygen and carbon isotopes, which reflect climatic variations, U-isotope analyses which determine the age of growth and trace elements such as Mg, Sr and P which record processes in the soil and bedrock. This project is designed as a least-risk/highest-return pilot study that will provide essential and, to date, virtually unknown, background information on local processes in a tropical, humid limestone system controlling the transfer of oxygen isotopes from precipitation to stalagmite calcite. The timeliness of this proposal stems not only from technological developments allowing greater spatial precision on geochemical analyses of speleothems but also from recently-forged collaborations with local scientists who will supervise a cave monitoring program, ensuring a regular and reliable return of high quality monitoring data. Our project will provide the first detailed study, using proven cave monitoring techniques developed in Gibraltar, of the monsoon climate capture process in the rain-soil-limestone tropical environment. The success of this work will result in an assessment of the area for a larger project targeted at cave sites with the best potential for generating longterm climate records where the mechanisms of climate capture can be properly understood.