Development and demise of the Maldives carbonate platform, Oxygen Minimum Zone and monsoon in the northern Indian Ocean: IODP expedition 359

碳酸盐平台是海洋中的浅水区，藻类、苔藓虫、有孔虫和珊瑚等生物在那里产生大量碳酸盐。碳酸盐台地是生物多样性研究的重要热点。现代碳酸盐平台的例子有巴哈马、大堡礁和马尔代夫。地质记录中还有大量的碳酸盐台地化石。这些都是多样化的生态系统，当它们从海水中浮出时，最终停止形成，或者由于碳酸盐产量跟不上(相对)上升的水位而溺水。人们对这些溺水过程知之甚少。以前的工作提到了下沉、海平面、各种环境条件，如海洋酸化、海洋表面温度和海洋的氧-营养耦合状态，这些都是碳酸盐平台溺水的各种原因。溺水的原因很可能是复杂的。这是目前一个紧迫的问题，因为了解平台是如何溺水的，对于实施现代平台的保护政策至关重要，因为海平面上升伴随着多种环境变化。化石溺水事件可以被描述为现代碳酸盐平台未来可能表现的警告信号。马尔代夫群岛为研究一个相对现代的平台提供了独特的机会。位于赤道印度洋中部的马尔代夫群岛是一个孤立的热带碳酸盐台地。一排南北走向的环礁环绕着马尔代夫内海。地震剖面显示，环礁位于一个部分淹没的平台上，内海部分充斥着沉积物。IODP 359设计用于在沉积盖层和部分下沉的平台上钻孔，以及在环礁之间的内海沉积漂流沉积物上钻孔。该团队的科学家将重建为什么这个特定的平台在中新世部分消失在海浪之下。正是溺水的这种局部性质对于能够调查平台死亡的环境因素至关重要，因为下沉和海平面上升不能解释局部溺水现象。平台生态系统当地不利的环境条件和持续的下沉是理解平台部分溺水原因的关键。所谓的氧气最小区(OMZ)是北印度洋特有的地下水域的一个非常重要的环境特征。OMZ的特点是氧气含量低，营养物质含量高，起源于受夏季风影响的阿拉伯海西部。它像舌头一样延伸到马尔代夫地区，并进一步向南延伸。在季风风的作用下，低氧高营养地下水的局部上升流不利于碳酸盐台地生态系统。OMZ何时扩展到主导北印度洋的鲜为人知，但人们普遍认为这可能发生在中新世。因此，平台的部分溺水可能与禁区的发生有关。在第一个项目中，我提议利用有孔虫生物地层学精确测定IODP探险359期间恢复的沉积层序的日期，以限制马尔代夫碳酸盐平台结构变化的时间。船上没有足够的时间来进行高时间分辨率的约会。在这里，我建议提高分辨率，以精确地确定某些关键时间间隔的日期(样本将从日本高知的核心商店获得)。在第二个项目中，我建议研究内海漂流沉积物的地球化学和微化石含量，我将确定OMZ开始的时间，以及它随时间的演化，特别是关于碳酸盐台地消亡的时间。

Carbonate platforms are shallow water areas in the oceans where large amounts of carbonate are produced by organisms such as algae, bryozoans, foraminifera and corals. Carbonate platforms are important hotspots of biodiversity. Examples of modern carbonate platforms are the Bahamas, Great Barrier Reef and Maldives. There are also numerous fossil carbonate platforms in the geological record. These were diverse ecosystems that eventually ceased forming when they were uplifted out of the seawater, or drowned as carbonate production was unable to keep up with (relatively) rising waters. These drowning processes are poorly understood. Previous work has mentioned subsidence, sea level, various environmental conditions such as acidification of the ocean, sea surface temperature, and the coupled oxygen-nutrient state of the ocean as various reasons why carbonate platforms drown. Complex scenarios with multiple causes of drowning are likely. This is currently a pressing issue, because understanding how platforms drown is critical to implementing conservation policies for modern ones, where sea level rise is accompanied by multiple environmental changes. Fossil drowning events can be portrayed as warning signs for how modern carbonate platforms may behave in the future.The Maldives archipelago offers the unique opportunity to study a relatively modern platform. The Maldives archipelago in the central equatorial Indian Ocean is an isolated tropical carbonate platform. A north-south-oriented double row of atolls encloses the Inner Sea of the Maldives. Seismic profiles indicate that the atolls reside on a partially drowned platform, and that the Inner Sea is partly filled with sediments. IODP excursion 359 is designed to drill bore holes in the sedimentary cover and partly sunk platform, and bore holes in the sedimentary drift deposit in the Inner Sea between the atolls. The scientists on this team will reconstruct why this specific platform partly disappeared beneath the waves in the Miocene. It is this partial nature of the drowning that is critical to enable the environmental factors of platform demise to be investigated, as subsidence and sea level rise cannot explain the phenomenon of partial drowning. Adverse local environmental conditions of the platform ecosystem and continued subsidence are key in understanding why the platform partially drowned.The so-called Oxygen Minimum Zone (OMZ) is a very important environmental feature in the subsurface waters unique to the northern Indian Ocean. The OMZ is characterized by low oxygen and high nutrient content and originates in the western Arabian Sea under the influence of the summer monsoon. It extends as a tongue to the Maldives area and further south. Local upwelling, forced by monsoonal winds, of the coupled low oxygen-high nutrient subsurface waters is unfavourable to the carbonate platform ecosystem. It is poorly known when the OMZ expanded to dominate the northern Indian Ocean but it is widely regarded that this likely happened in the Miocene. Therefore, the partial drowning of the platform may be related to the onset of the OMZ.This Moratorium proposal offers two projects. In the first project, I propose to precisely date the sedimentary sequences recovered during IODP expedition 359 in order to constrain timing of changes in the architecture of the Maldives carbonate platform, using foraminiferal biostratigraphy. There is insufficient time on board to perform dating with a high temporal resolution. Here, I propose to increase the resolution to precisely date certain key intervals (samples will be taken from the core store at Kochi, Japan). In the second project, I propose to examine the geochemistry and microfossil content of the drift sediments in the Inner Sea, I will establish the timing of the onset of the OMZ, and its evolution through time, specifically with respect to the timing of the carbonate platform demise.