Reconstructing Pliocene-Pleistocene paleoenvironmental conditions and biogeochemical element cycling in the Gulf of Alaska (Northeast Pacific)

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Studying marine sediments is one of the best methods to assess changes to the environment and climate that affected certain regions of our ocean in the past, ultimately leading to a better understanding of Erath's climate system as a whole, and potentially a better idea of the effects of future climate change. In the context of climate change, the cycling of carbon and other essential elements between the atmosphere, the biosphere, the ocean and marine sediments is of major importance. Marine primary producers sequester carbon dioxide from the atmophere into their biomass, and ultimately into marine sediments at the seafloor. There is a delicate balance between sources, sinks, and transformations between a number of essential elements (like carbon, phosphorus, iron) that controls how much atmospheric carbon gets buried in the seafloor. Reconstructing this balance, and its linkage to global climate changes, can be done by analysing the archives of past environmental conditions using inorganic geochemical methods. This is one of the major aims of the Integrated Ocean Drilling Program, dedicated to provide high-quality marine sediment records covering critical intervals in Earth's climatic history from various parts of the world ocean.Within the context of this global and longterm undertaking, IODP Expedition 341 will recover long sediment cores from the currently understudied marginal Northeast Pacific. The selected drill sites cover a range of water depths and distances to the coast, which will hopefully allow for paleoenvironmental reconstructions along a depth/distance transect. In addition, the Gulf of Alaska is located close to the Bering Sea, where the PI of this project participated in IODP Expedition 323 and in the following studied various paleoenvironmental and biogeochemical parameters than can ideally be compared to respective data from the Gulf of Alaska. The sediments to be recovered there are well suited to tackle a number of highly relevant research questions. First, biogenic opal productivity in the North Pacific is generally believed to have "crashed" at the Pliocene-Pleistocene boundary, but new data from the Bering Sea contradict this widely accepted assumption. Comparable data from the Gulf of Alaska will provide additional information for this discussion, and will give us a better idea of the development of primary productivity across the wider North Pacific in response to northern hemisphere cooling. Second, primary productivity in the Gulf of Alaska is Fe-limited, and both glacial dust storms as well as diagenetic processes on the shelf have been invoked to deliver the urgently needed reactive Fe to the open marine phytoplankton communities. Studying the distribution of various Fe, but also P phases in the sediments will allow us to estimate how and when these Fe sources were active in the past, and how they affected primary productivity and organic carbon burial. Third, the North Pacific margin has experienced strong variations in bottom water redox conditions over the past millions of years, likely related to enhanced export of organic carbon to the seafloor and/or changes in the degree of deepwater ventilation. Using trace metal records, we will reconstruct the redox history of different water masses in the Gulf of Alaska. Fourth, the Gulf of Alaska sediments provide an ideal location to study the sources, sinks and biogeochemical cycling of various P phases that were newly identified in the Bering Sea. Recognition of these P phases and their involvement into biogeochemical cycles might change our current understanding of the global marine P cycle, and might be instrumental for better global biogeochemical models.