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

研究海洋沉积物是评估过去影响海洋某些区域的环境和气候变化的最佳方法之一，最终有助于更好地了解地球的整体气候系统，并有可能更好地了解未来气候变化的影响。在气候变化的背景下，碳和其他基本元素在大气、生物圈、海洋和海洋沉积物之间的循环具有重要意义。海洋初级生产者将大气中的二氧化碳吸收到它们的生物量中，并最终进入海底的海洋沉积物中。在源、汇和一些基本元素（如碳、磷、铁）之间的转化之间存在着微妙的平衡，这些元素控制着大气中有多少碳被埋在海底。通过使用无机地球化学方法分析过去环境条件的档案，可以重建这种平衡及其与全球气候变化的联系。这是综合海洋钻探计划的主要目标之一，该计划致力于提供高质量的海洋沉积物记录，涵盖地球气候历史的关键时期，来自世界各地的海洋。在这一全球性和长期事业的背景下，IODP第341号考察队将从目前尚未充分研究的东北太平洋边缘地区恢复长沉积物岩心。选定的钻探地点覆盖了一定的水深范围和距离海岸的距离，这有望允许沿着深度/距离样带进行古环境重建。此外，阿拉斯加湾位于白令海附近，该项目的PI参加了IODP第323次考察，并在随后的研究中研究了各种古环境和生物地球化学参数，这些参数可以理想地与阿拉斯加湾的各自数据进行比较。在那里回收的沉积物非常适合解决一些高度相关的研究问题。首先，北太平洋的生物蛋白石产量通常被认为在上新世-更新世边界“崩溃”，但来自白令海的新数据与这一被广泛接受的假设相矛盾。来自阿拉斯加湾的可比数据将为这一讨论提供更多的信息，并将使我们更好地了解北半球变冷对整个北太平洋初级生产力的影响。其次，阿拉斯加湾的初级生产力是有限的，冰川沙尘暴和大陆架上的成岩作用都被用来向开放的海洋浮游植物群落提供迫切需要的活性铁。研究沉积物中各种铁相和P相的分布将使我们能够估计这些铁源在过去是如何和何时活跃的，以及它们如何影响初级生产力和有机碳埋藏。第三，在过去的数百万年里，北太平洋边缘经历了海底氧化还原条件的强烈变化，这可能与有机碳向海底输出的增强和/或深水通风程度的变化有关。利用痕量金属记录，重建了阿拉斯加湾不同水团的氧化还原历史。第四，阿拉斯加湾沉积物为研究白令海中新发现的各种P相的来源、汇和生物地球化学循环提供了理想的场所。认识到这些P相及其与生物地球化学循环的关系，可能会改变我们目前对全球海洋P循环的认识，并可能有助于建立更好的全球生物地球化学模型。

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.