Developing a 3.5-million-year benchmark record of Indian Ocean Dipole variability

印度洋偶极子（IOD）是热带印度洋海表温度年际变化的主导模态。IOD描述了印度洋西部和东部的暖（正）和冷（负）海表温度振荡。从东非的极端洪水到澳大利亚的严重干旱，IOD变化的最相关的社会后果范围广泛。随着温室气体排放的增加，这些由IOD引起的极端气候事件预计将在未来变得更加普遍。然而，尽管这些令人震惊的预测及其对维持20多亿人口的边境地区的深远影响，令人惊讶的是，人们对过去IOD的变化知之甚少。特别是，如果已知二氧化碳含量较高的地质时间间隔内的碘含量变化，则可以进一步测试所提出的碘含量对二氧化碳的敏感性。然而，由于缺乏来自印度洋的适当的沉积和地球化学数据集，对过去IOD变化的认识受到严重阻碍。本文提出的研究旨在阐明过去350万年的IOD变化，这与全球CO2水平的明显变化相吻合；包括上新世中期的暖期，被认为是未来气候变化的类似物。该项目的核心假设是，与iod相关的海温和相关的地面风异常影响了印度洋深经向翻转环流的Ekman输送，从而影响了深水区的通风。根据观测资料，在正（负）IOD阶段，Ekman输运的减弱（加强）有利于深水区通风减少（增加），从而使溶斜层变浅（加深）。这些新的现代水文研究成果尚未应用于过去的地质，因此为研究过去的IOP变化开辟了一条全新的途径。因此，该项目旨在探索在整个地质历史中利用西印度洋深水通风变化作为IOD变化指标的可能性。本项目将具体解决两个假设：1)碳酸盐溶解循环作为西印度洋深水通风变化的指标，可以用作IOD强度指标；2)IOD的振幅与全球CO2水平的变化呈线性相关。为了以前所未有的时间分辨率破译其过去的深水通风变化，将利用基于XRF岩心扫描、稳定同位素和基于底栖和浮游有孔虫的Mg/ ca古温标的新型IOD代理研究位于赤道印度洋西部（IOD强迫的关键区域）的ODP 709沉积物岩心。收集到的数据的综合将用于开发过去350万年中IOD变化的新基准记录。

The Indian Ocean Dipole (IOD) is the leading mode of interannual variability of sea surface temperature (SST) in the tropical Indian Ocean. The IOD describes warm (positive) and cold (negative) sea surface temperatures oscillations in the western and eastern part of the Indian ocean. The societal most relevant consequences of IOD variability range from extreme floods in eastern Africa to severe droughts in Australia. These extreme climate events caused by the IOD are predicted to become more common in the future as greenhouse gas emissions increase. Yet despite these alarming predictions and its profound impact on the bordering regions which sustain more than 2 billion people, surprisingly little is known about IOD variability in the past. In particular, the proposed sensitivity of the IOD to CO2 could be further tested if IOD variability during geological time intervals of higher CO2 contents were known. However, the insights into past IOD variability have been severely hampered by the lack of appropriate sedimentary and geochemical data sets from Indian Ocean.The here proposed research aims to elucidate IOD changes during the last 3.5 million years which coincides with distinct changes in the global CO2 level; including the mid-Pliocene warm period considered an analogue for future climate change. At the heart of this project stands the hypothesis that IOD-related SST and the associated surface wind anomalies influenced the Ekman transport of the deep meridional overturning circulation in the Indian Ocean, and thus the ventilation of the deep-water regime. Based on observational data, the weakening (strengthening) of the Ekman transport during positive (negative) IOD phases facilitates less (more) ventilation of the deep-water regime, and consequently a shoaling (deepening) of the lysocline. These new modern hydrographic findings have not yet been applied to the geological past, and thus open up an entire new avenue to research past IOP changes. Hence, this project aims to ground truth the possibility of utilizing deep-water ventilation changes in the western Indian Ocean throughout the geological past as an index of IOD variability. Two hypotheses will be specifically tackled in this project:1) Carbonate dissolution cycles as an indicator of deep-water ventilation changes in the western Indian Ocean can be used as IOD intensity index,2) The amplitude of the IOD is linear correlated to changes in global CO2 levels.To decipher its past deep-water ventilation variability in unprecedented temporal resolution, a sediment core from Site ODP 709 situated in the western equatorial Indian Ocean - a key region of IOD forcing – will be investigated utilizing a novel IOD proxy based on XRF core scanning, stable-isotope and Mg/Ca-paleothermometry based on benthic and planktic foraminifera. The synthesis of the collected data will then be used to develop a new benchmark record of IOD variability during the last 3.5 million years.