上新世以来西太平洋沉积铁同位素演化研究

Iron is an essential element for oceanic primary productivity, influencing marine biological community structure, ecological functioning, and carbon cycle. Dust flux to the Northern Hemisphere West Pacific has rapidly increased since the Pliocene, associated with global cooling and Asian aridification. It is generally thought that reactive iron of eolian dust is an important source of dissolved iron in the West Pacific. However, it remains unclear on how the reactive Fe budget responds to the changes in dust flux as well as on the evaluation of influences of other Fe sources such as hydrothermal inputs. To resolve these questions, this project plans to carry out detailed isotope geochemical analysis based on sediments from ODP site 1208 and ferromanganese crusts from seamounts of the West Pacific. The project plans to set up reliable methods to extract and analyze isotopes of reactive iron of the sediments and to clarify the potential influences of micro-mineral facies and chemical composition on Fe isotope fractionation of ferromanganese crust. Based on Fe isotope evolution and fluxes of the reactive fractions of the sediment cores as well as seawater Fe isotopes as revealed by the ferromanganese crusts, this project attempts to clarify the changes in sedimentary Fe budget and its linkage with the source evolution in order to provide essential constraints on the West Pacific Fe cycling since the Pliocene.

铁作为海洋初级生产所必须的元素影响着海洋的生物群落结构、生态功能以及碳循环。上新世以来，随着全球变冷和亚洲干旱化，输入北半球西太平洋的风尘通量快速增加。一般认为风尘携带的活动性Fe是西太平洋海水溶解Fe的重要来源，但该海域活动性Fe收支如何响应风尘通量的变化，以及如何评估其它来源如热液Fe输入的影响至今缺乏系统研究。针对这些问题，本项目拟依托西太平洋沉积钻孔（ODP Site 1208）和海山铁锰结壳开展详细Fe同位素地球化学研究。拟建立沉积物活动性Fe的可靠提取和同位素分析方法、阐明微区矿物相与化学成分对结壳铁同位素分馏的潜在影响。通过获得5Ma以来沉积物钻孔的活动性Fe通量与同位素组成演化，以及铁锰结壳所揭示的海水溶解Fe同位素的时空演化，厘清这一海域沉积Fe的收支与物源演变的联系，为可靠地理解上新世以来西太平洋Fe循环提供基础制约。

铁是海洋生物生产力的限制性营养元素，但海水溶解Fe 的来源一直存在较大争议，且溶解Fe 的输入如何响应构造和气候变化仍知之甚少。本项目针对西太平洋陆架输入、热液输入、粉尘输入对海水溶解Fe的影响在地质时间尺度上如何演变，及其与生物生产力的关系如何开展了研究。研究发现晚中新世以来，西太平洋陆源物质的Fe 在氧化性陆架海水中部分溶解并被有机配体所提取，进而传输到开阔海中，而东亚粉尘和热液可能仅是低纬深水溶解Fe 的次要来源。西太平洋寡营养海中末次冰期比间冰期输出生产力高约40%，与温度变化具有很好的一致性。同时发现风尘Fe输入未驱动西太平洋寡营养海生产力变化，但可能刺激了西太平洋寡营养海固氮，因此具有进一步影响全球N循环的潜力。这些认识为理解Fe在海洋生物地球化学循环中的行为提供了重要约束。