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

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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.