Analysis of water mixing history in high latitude oceans using chemical tracers of the third generation

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The research supported by this grant-in-aid mainly consists of following two parts. One is the analysis of history of the biological activity and water mixing in ocean surface layer using nitrogen isotope ratio of nitrate, and the other is the analysis of water advection and mixing processes in the intermediate and deep layers based on precise measurements of nutrient concentration. We used the surface water samples which were annually collected in Bering Sea and Alaska Bay by a merchant vessel between Japan and Canada for the former study, and the samples collected by the CTD/RMS observations which were carried out in Southern basins of the Sea of Okhotsk by Russian ship were utilized for the latter study. Most part of this grant was applied to construct the vacuum system for nitrogen isotope analysis and the auto-analyzer for nutrient measurement. The concentration & nitrogen isotopic ratio plot of nitrate suggested the possibility that, in summer, the surface water is more rapidly m … More ixed with the subsurface water in Bering Sea than in Alaska Bay. By the analysis of intermediate and deep waters based on the precise measurements of nutrients, a new chemical quantity, N* (=([NO_3-] - 16. [PO_4^3-] + 2.9) 0.87), was found to be very useful to trace the origin and mixing process of water masses. This N* is a quasi-conservative tracer which does not change in the ordinary water column, but decrease and increase by denitrification and N_2-fixation, respectively, occurring in the bottom sediment or some special water column. The detailed observation of N* in all over the Southern basin area in the Sea of Okhotsk revealed that the low N* water was not only located on the sea floor and continental slope where the denitrification probably occurs, but also penetrated from the continental slope area into the intermediate layer along the low potential vorticity water between 200 and 400m depths. This provides an evidence for the mechanism that the Okhotsk Intermediate Water, which is the origin of North Pacific Intermediate Water, is a water mass which is originally formed on the continental shelf area (= low N* region) in the northwestern of Sea of Okhotsk and horizontally flowed out along intermediate layers. Less