| The Pacific water carried a large amount of dissolved inorganic carbon(DIC)and nitrate into the Pacific Arctic Ocean,which provided important material basis for phytoplankton photosynthesis,and also affected the change of air-sea carbon dioxide flux.Under rapid environmental changes,it is necessary to clarify the mechanisms of variations in DIC and nitrate in the upper layer to further understand the changes of marine ecosystem and oceanic carbon source/sink pattern in the Pacific Arctic Ocean.This relies on a fully coupled ocean-sea ice-ecosystem-carbon cycling model.In this study,the spatial and temporal distributions of simulated sea ice concentration,hydrologic structure,DIC,nitrate and primary production,derived from the ocean-sea ice-ecosystem-carbon cycling model(NAPA-BGC),are evaluated by observed dataset in the Arctic Ocean.The results show that NAPA-BGC reasonably captures the spatiotemporal distributions of physical and ecological elements in the Pacific Arctic Ocean.Based on the hindcast simulation of NAPA-BGC from 1998 to 2015,environmental changes in the Pacific Arctic Ocean,mechanisms of Pacific-sourced DIC and nitrate expansions,and the impact of material transportation within Pacific inflow on carbon uptake in the upper Pacific Arctic Ocean are analyzed.The main findings are summarized as follows.(1)The environmental conditions in the Pacific Arctic Ocean are obviously different before and after the year of 2007.After 2007,the Arctic sea ice retreat rate slowed down significantly.Meanwhile,the Chukchi slope current and Pacific inflow increased.The layer of Pacific water deepened and expanded to high latitudes in the Pacific Arctic Ocean.(2)Corresponding to the environmental changes,the maximum DIC and nitrate in the subsurface(50-250 m)expended to high latitudes at 85°N and deepened to 300 m in the Pacific Arctic Ocean.The subsurface Chukchi-East Siberian Shelfbreak and Makarov basin are the most significant areas of DIC and nitrate increase.Estimates of the DIC and nitrate budget indicated that the subsurface increment of DIC and nitrate in the Chukchi-East Siberian Shelfbreak were controlled by biological and physical effects,respectively.The increases of primary production and detrital deposition in the surface contributed to the increases of remineralization of organic matter and DIC in the subsurface,while the nitrate increase was attributed to the advection.In the Makarov basin,the subsurface increment of DIC and nitrate was attributable to the advected DIC and nitrate.The enhancement of Chukchi-East Siberian slope current is favorable for DIC and nitrate transport to the Makarov Basin.(3)Followed the changes in DIC and nitrate concentrations in the subsurface layer of the Pacific Arctic Ocean,the material exchange between the subsurface and surface layer also changed.Estimations of vertical exchange between subsurface and surface layers(0-50 m)showed that both DIC and nitrate were transported from surface to subsurface caused by advection in the Chukchi-East Siberian Shelfbreak after 2007,and the ratio of DIC and nitrate in surface supplemented by advection was smaller than Redfield C/N ratio,which favors the sea surface absorb atmospheric carbon dioxide.In the Makarov basin,DIC was advected from subsurface to surface,while the nitrate was replenished to surface by mixing.They resulted in a larger ratio of DIC and nitrate into surface water than Redfield C/N ratio,playing a role in weakening oceanic carbon uptake.The response of the ocean currents to environmental changes in the Pacific Arctic Ocean makes the nutrient-rich Pacific acidic water deepening and moving northward and westward.In the high productivity area,remineralization of the increased precipitated organic matter contributes to the subsurface acidification.The changes in the nutrient-rich and acidic water exchange direction and the C/N ratio at the surface-subsurface interface play an important role in the efficiency of carbon sink in the Shelfbreak and basin. |
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