Physical behavior of the SEEDS iron-fertilized patch by sulphur hexafluoride tracer release |
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| Authors: | Daisuke Tsumune Jun Nishioka Akifumi Shimamoto Shigenobu Takeda Atsushi Tsuda |
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| Institution: | aCentral Research Institute of Electric Power Industry, Environmental Science Department, 1646 Abiko, Abiko-shi, Chiba-ken 270-1194, Japan;bThe General Environmental Technos Co., Ltd., Chuou, Osaka 541-0052, Japan;cDepartment of Aquatic Bioscience, University of Tokyo, Bunkyo, Tokyo 113-8657, Japan;dOcean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano, Tokyo 164-8639, Japan |
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| Abstract: | The first iron (Fe) – fertilization experiment in the western North Pacific was carried out using SF6 to trace the Fe-fertilized water mass. A solution in 10,800 liters of seawater of 350 kg of Fe and 0.48 M of SF6 tracer was released into the mixed layer over a 8 × 10 km area. On the first underway transects through the patch after the Fe release, we observed a significant increase of dissolved Fe (ave. 2.89 nM). The fertilized patch was traced for 14 days by on-board SF6 analysis. A Lagrangian frame of reference was maintained by the use of a drogued GPS buoy released at the center of the patch. The patch moved westward at a rate of 6.8 km d−1. Mixed layer depth increased from 8.5 to 15 m during the experiment. Horizontal diffusivity was determined by the change of SF6 concentration in the patch. The horizontal diffusivity increased during the experiment. We evaluate here the fate of Fe in a Fe-fertilized patch using the dilution rate determined from sulphur hexafluoride (SF6) concentration. Dissolved Fe concentrations subsequently decreased rapidly to 0.15 nM on Day 13. However, the dissolved Fe half-life of 43 h was relatively longer than in previous Fe-enrichment studies, and we observed a larger increase of the centric diatom standing stock and corresponding drawdown of macro-nutrients and carbon dioxide than in the previous studies. The most important reason for the larger response was the phytoplankton species in the western North Pacific. In addition, the smaller diffusivity and shallower mixed layer were effective to sustain the higher dissolved Fe concentration compared to previous experiments. This might be one reason for the larger response of diatoms in SEEDS. |
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| Keywords: | SF6 tracer Dilution rate Experimental methods Patch Fe behavior |
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