Isotopic fingerprinting of dissolved iron sources in the deep western Pacific since the late Miocene
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otopic fingerprinting of dissolved iron sources in the deep western Pacific since the late Miocene 1
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Ruolin LIU , Bai GUO , Maoyu WANG , Weiqiang LI , Tao YANG , 1 1,2* Hongfei LING & Tianyu CHEN 1
Center for Marine Geochemistry Research, State Key Laboratory for Mineral Deposit Research, School of Earth Sciences and Engineering, 2
Nanjing University, Nanjing 210023, China; Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266071, China Received February 7, 2020; revised June 5, 2020; accepted June 22, 2020; published online August 5, 2020
Abstract Iron (Fe) is a productivity-limiting nutrient in the ocean. However, the sources of dissolved Fe (dFe) in the deep ocean and how they respond to tectonic and climate changes are still poorly understood. In the northern hemisphere, dust flux to the low-latitude western Pacific has increased dramatically since the late Miocene associated with intense aridification of the Asian inland. Meanwhile, the terrigenous material supply to the open ocean might have also changed as a result of the reorganization of the Pacific circulation due to the gradual closure of seaways in the low latitudes. Therefore, the western Pacific is a characteristic region for understanding the sources of dFe in the deep ocean and their responses to long term climate changes. Here, we present data on isotopic evolution of dFe and dissolved Pb since ~8 Ma based on ferromanganese crust METG-03 (16.0°N, 152.0°E, 56 3850 m water depth) in the western Pacific deep water. Our results show that δ Fe of the crust remains fairly stable since the late 56 Miocene, i.e., about −0.32±0.08‰ (2SD). We infer that δ Fe of dFe in the deep western Pacific is relatively invariant at ~0.45 ±0.1‰ based on the Fe isotopic fractionation between hydrogenetic crust and the seawater dissolved component. The re56 constructed isotope signature is similar to the measured δ Fe value (0.37±0.15‰) of the intermediate to deep waters in the modern low-latitude western Pacific region close to the island arcs, but is significantly higher than that of the eastern Pacific deep waters near South America which is controlled by the reductive dissolution of continental shelf sediments and the hydrothermal 56 206 204 inputs (δ Fe>5 noml kg close to the hydrothermal vents) are indicative of the hydrothermal sources (Resing et al., 2015; Wu et al., 2011). Away from these typical regions, it is still very challenging to differentiate the sources of dFe in the seawater. By assuming 56 δ Fe of the 4 dFe sources in the North Atlantic, i.e., the non-reductive and reductive dissolution of continental margin sediments, hydrothermal fluids, and dust dissolution, Conway and John (2014) carried out the first study in quantifying the contribution of different dFe sources in the deep waters of this region. In areas where Fe is intensively scavenged onto particles, such as the upper ocean or restricted basins, the isotope composition of dFe is controlled not only by end-member mixing, but also b
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