[A-CG45] Biogeochemical linkages between the surface ocean and atmosphere

[ACG45-03] Characteristics of late summer Arctic brash sea ice and its melting effect on the surface water biogeochemistry in the Chukchi Sea

*Ryota Akino¹, Daiki Nomura^1,2,3, Reishi Sahashi¹, Manami Tozawa¹, Mariko Hatta⁴, Kohei Matsuno^1,3, Takuhei Shiozaki⁵, Takeshi Kinase⁴, Tatsuya Kawakami¹, Masato Ito⁶, Akihiko Murata⁴, Amane Fujiwara⁴ (1.Faculty of Fisheries Sciences, Hokkaido University, 2.Field Science Center for Northern Biosphere, Hokkaido University, 3.Arctic Research Center, Hokkaido University, 4.Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 5.Atmosphere and Ocean Research Institute, The University of Tokyo, 6.National Institute of Polar Research)

Keywords:sea ice, brash ice, nutrients, Chukchi sea, Arctic ocean

To determine the biogeochemical characteristic of late summer arctic brash ice (smaller sea ice than 1 m³) in the Chukchi Sea and its melting effect on the surface marine environment, we have collected ice samples with a net during the summer Arctic cruise by R/V Mirai in 2021. Ice samples were melted in the dark under cool conditions to measure physical and biogeochemical components such as salinity, oxygen stable isotopic ratio, turbidity, chlorophyll-a, and nutrients. The brash ice samples collected in our study were expected to be a multi-year ice based on satellite back trajectory analysis and low salinity due to brine drainage. Comparison of brash ice nutrients with those of seawater samples from the temperature minimum layer, corresponding to the water in which the sea ice originated, suggested that the biological activity such as remineralization process was dominated for our brash samples. Extremely high turbidity values were obtained for the brown/green color sediment-laden ice samples which chlorophyll-a, phosphate, and ammonium concentrations were high and N/P ratios were low (about 1) as compared to those of others. This is thought to be results of the combined effects of the accumulation of nutrients due to organic matter decomposition in the biofilm in which phytoplankton accumulated, and differences in the biological and physical behaviors of nitrogen and phosphorus (e.g., decomposition rate and adsorption properties). Based on the nutrient concentrations in the sea ice samples collected in this study, we have estimated changes in nutrient concentrations in the surface waters of the Chukchi Sea due to sea ice melting. It was estimated that sea ice melt supplies nitrate and ammonium and dilutes silicate, suggesting that sea ice melting would be a possible source of nitrogen in the nitrate depleted Chukchi Sea in late summer.