Iron Isotope Systematics in Rivers and Estuaries: Processes Controlling the Fractionation between Continental Run-Off and the Oceans

铁(Fe)同位素可以提供一种新的方法来限制铁源对海洋的相对贡献，并提高我们对铁如何从源区(即风化制度)动员并运往海洋(即溶解或颗粒态)的理解。然而，铁同位素在水体系中作为来源指示剂和生物地球化学示踪剂的应用在很大程度上仍未得到探索。为此，伍兹霍尔海洋研究所的研究人员将进行实地和实验室研究，以确定控制大陆径流和海洋之间铁同位素分馏的过程。具体地说，科学家计划检验以下三个假设：(1)化学风化在河流中产生不同于颗粒铁池的可溶性和胶体铁池；(2)溶解池中铁同位素组成的季节性变化显著，反映了由于水文或温度变化可能发生的氧化还原反应；(3)河流中溶解铁的大规模清除可以显著改变陆源铁到达沿海水域的铁同位素特征。溶解、胶体和颗粒物质的铁同位素组成将从美国东北部两个截然不同的环境中确定，一个是富含有机和铁的系统(穆利卡河及其河口)，另一个是携带致石颗粒的大河(康涅狄格河及其河口)。还将讨论具有全球意义的河流系统(密西西比河)中铁同位素组成季节性变化的可能性，在该系统中，氧化还原反应已被建议控制溶解的铁浓度。实验室实验将模拟海水对自然产生的腐植酸-铁-胶体的凝聚，并将结果与来自穆利卡和康涅狄格州河口的天然样品进行比较。超滤技术将被用来测量不同大小胶体的铁同位素组成，并确定河流的铁同位素特征是来自风化还是来自与胶体相关的铁的原位循环。至于更广泛的影响，这项研究将提高我们对海洋环境中铁的地球化学循环的理解。一名本科生将在夏季的几个月里参加研究的实地和实验室部分。

Iron (Fe) isotopes may provide a new approach to constrain the relative contribution of Fe sources to the ocean and to improve our understanding of how Fe is mobilized from source regions (i.e., weathering regimes) and transported into the ocean (i.e., dissolved or particulate species). However, the application of Fe isotopes as indicators of provenance and as biogeochemical tracers in aqueous systems remains largely unexplored. For this reason, researchers from Woods Hole Oceanographic Institution will carry out a field and laboratory study to determine the processes controlling the fractionation of Fe isotopes between continental run-off and the oceans. Specifically, the scientists plan to test the following three hypotheses: (1) chemical weathering yields soluble and colloidal Fe pools in rivers that are isotopically distinct from particulate Fe pools; (2) seasonal variability of Fe isotope composition of the dissolved pool is significant, reflecting possible redox reactions due to hydrological or temperature changes; and (3) the large scale removal of river-borne dissolved Fe can significantly modify the Fe isotopic signature of terrestrial Fe reaching coastal waters. The Fe isotope composition of dissolved, colloidal, and particulate matter will be determined from two contrasting environments along the northeast of the United States, an organic-rich and Fe-rich system (Mullica River and its estuary) and a large river carrying lithogenic particles (Connecticut River and its estuary). The possibility of seasonal variations of Fe isotope composition in a river system of global significance (Mississippi River) where redox reactions have been suggested to control dissolved Fe concentrations will also be addressed. Laboratory experiments will simulate the coagulation of naturally occurring humic-Fe-colloids by seawater, and results will be compared with natural samples from the Mullica and Connecticut estuaries. Ultrafiltration techniques will be used to measure the Fe isotope composition of different sizes of colloids and determine if the Fe isotope signatures of rivers are inherited from weathering or from in situ cycling of Fe associated with colloids. As regards broader impacts, this study will improve our understanding of the iron geochemical cycle in the marine environment. One undergraduate student will participate in the field and laboratory components of the study during the summer months.