Studying the variations of iron (Fe) and other trace elements (TE) such as lead (Ld) and gallium (Ga) together with nutrients such as dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved silicate (DSI) in the marginal seas of Asia can improve our understanding of the biogeochemical cycle of chemical elements between land, atmosphere, and the Pacific Ocean, as well as contribute to our knowledge of the functions of marine ecosystems in a changing climate.
Previous studies have shown that the main sources of Fe, Ld, and Ga in the surface waters of the open ocean are precipitation and partial dissolution of mineral aerosols (dust), while river runoff and volcanic activity may have an important role near continents. Similarly, the comprehensive nitrogen (N) cycle in the ocean is significantly controlled by atmospheric fluxes, as well as currents, biological processes, and river runoff near continents.
This study is based on the results of the R/V Akademik Oparin cruise No. 74 implemented in September-October 2024 and has three main objectives: 1) obtaining knowledge about the concentration of total dissolved Fe, Ld and Ga (TDFe, TDPb, TDGa) in the surface water layer of the Sea of Okhotsk along its entire length from the Kuril Islands to the northern coasts and from the western to the eastern coast; 2) obtaining basic knowledge about the spatial distribution of isotopic compositions δ¹⁵N– , δ¹⁸O– to understand the sources, ways and transformations of DIN, as well as other nutrients from the Pacific Ocean to the northern coasts; 3) analysis and discussion of the current variability of the biogeochemistry of the water column of the Sea of Okhotsk in the climatic scale.
21 stations for sampling surface water (depth 7-10 m) for TDFe, TDPb, and TDGa were distributed across the Sea of Okhotsk in two diagonal lines to ensure uniform coverage and study areas, such as the Amur River runoff, tidal mixing in the Kuril Strait, and in the Shelikhov Gulf in the north part of the sea. A Niskin-X bottle (with a Teflon inner coating and a spring on the outside) with an “X-Vane” sampling system was used to collect surface water samples. On the open deck, water from the Niskin-X bottles were transferred to centrifuge tubes (50 ml, Corning®) cleaned using 2 mol/l hydrochloric acid (analytical grade). Then samples were stored at a room temperature and delivered for measuring to laboratory of SKLEC ECNU, Shanghai.
Subsequent acidification to pH ∼ 1.7 was performed in a laboratory polyethylene clean bench with purified air using MAC 200E Air Clean Unit with HEPA Filter. We did not have filtration of the samples in this study.
Samples for δ¹⁵N– , δ¹⁸O– were collected at five stations in contrasting areas for the main water masses from the surface to the bottom in the Sea of Okhotsk and down to a depth of 3,600 m in the Pacific Ocean. Water samples were filtered through a Whatman PP 0.45 µM filter, frozen, and delivered for measurements to laboratory SKLEC ECNU, Shanghai.
The main results are as follows. The highest concentrations of all three elements are observed in the area influenced by the Amur River runoff in area of the north of Sakhalin Island. The main features in the distribution of TE is a general increase in the south and north and decrease concentrations in the central part of the Sea of Okhotsk. This similarly distribution was observed for concentrations of Chl-a, phytoplankton cells, and particular organic carbon, as observed in satellite and cruise data. The dependence of TDFe on TDPb showed a good correlation of 0.57, while this was not observed for TDGa. The concentration of TDFe and TDPb is significantly higher in Shelikhov Gulf which suggests a possible influence of Kamchatka volcanism.
Isotopic compositions δ¹⁵N– , δ¹⁸O–  show variation for the surface waters of the Sea of Okhotsk with significant differences for the adjacent Pacific Ocean. A decreasing isotopic weight of  is observed with increasing depth from the surface to intermediate waters. While the variability of the isotopic composition is almost similarly in the intermediate water mass and deep water. But the concentration of varies greatly depending on the proximity to the core of the oxygen minimum zone of the study area. Comparing DIN, DIP, and DSI in the oxygen minimum zone of the Sea of Okhotsk and the adjacent part of the Pacific Ocean, a general increase has been observed over the past thirty years.
Thus, this study provides new information about the distribution of TDFe, TDPb, and TDGa on the surface of the Sea of Okhotsk, as well as a complex of nutrients and isotopes δ¹⁵N– , δ¹⁸O– in the water column of this sea and the adjacent part of the Pacific Ocean, using the date of the cruise in autumn 2024.
This work was supported by the Russian Science Foundation (Project No. 23-77-10001) and Interdepartmental Integrated Program for Scientific Research of the Kamchatka Peninsula and Adjacent waters in 2024-2026, Reg. No. 124072200009-5 at V.I. Il’ichev Pacific Oceanological Institute, Far Eastern Branch Russian Academy of Sciences (Reg. No. 124022100077-0). Ship time of the R/V Akademik Oparin was funded by the Ministry of Science and Higher Education of the Russian Federation.
 

Sea of Okhotsk,Trace elements,stable isotopes,nutrients