[M-IS10] 南大洋・南極氷床が駆動する全球気候変動

[MIS10-10] Past and present variability of nitrate utilization in the seasonally ice-covered Southern Ocean

*Aymeric Pierre Marie Servettaz¹、Yuta Isaji¹、Chisato Yoshikawa¹、Nanako O. Ogawa¹、Yeongjun Ryu²、Boo-Keun Khim³、Yang-Hee Jang⁴、Daniel M. Sigman²、Francisco J. Jiménez-Espejo^1,5、Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, Yokosuka, Japan、2.Department of Geosciences, Princeton University, Princeton, New Jersey 08544, USA、3.Department of Oceanography and Marine Research Institute, Pusan National University, Busan 46241, Korea、4.Marine Ecosystem Management Department, Marine Environment Research Institute, Korea Marine Environment Management Corporation, Busan 49111, Korea、5. Instituto Andaluz de Ciencias de la Tierra, CSIC-UGR, Spain)

キーワード：Nitrate, Isotope, Sea ice, Ocean, Phytoplankton

Introduction. The Bransfield Strait (BS) in the Southern Ocean is an ecologically active zone with high productivity of phytoplankton that fix inorganic carbon and contribute to its transfer to the deep ocean. Primary productivity is sustained by the upwelling of deep, nutrient-rich water, which enables phytoplankton to grow rapidly as the sea ice melts in spring and summer. The BS receives water from the west through a branching of the Antarctic Circumpolar Current, which is more intense during stronger westerly wind periods, and coastal current brings water from the Weddel Sea, east of BS, where sea ice cover is markedly more persistent. Sea ice exerts multiple control over biology, shading the light entering the ocean and stratifying the upper ocean with the freshening following ice melt. However, productivity remains poorly quantified in this region because satellites are unable to quantify biomass in partially ice-covered ocean, and direct measurements are too scarce to characterize the seasonally varying productivity.

Nitrate drawdown and isotopes. Here we study biological nutrient utilization by assessing removal of nitrate from surface waters, and the associated change in δ¹⁵N of nitrate. We show that sea ice melt date conditions the initiation of nitrate drawdown, but the annual minimum concentration of nitrate is not controlled by sea ice concentration. In the seasonally ice-covered ocean, δ¹⁵N of nitrate increases with nitrate removal, similarly to what has been described for open ocean. The North half of the BS shows minimal nitrate removal and minimum δ¹⁵N of nitrate, despite an intermediate sea ice coverage. This particular situation probably results from light-driven productivity limitation in the deeper mixed layer, due to tidal mixing and density homogenization around the South Shetland Islands. Isotope-enabled ecosystem modelling confirms that mixed layer depth rather than sea-ice shading conditions the light limitation and total nitrogen utilization and export to the deep ocean.

Marine core. δ¹⁵N of chlorophyll synthesized by primary producers reflects the δ¹⁵N of nitrate in surface water with an offset due to biological uptake and molecular synthesis. Furthermore, partial degradation of organic matter does not substantially modify chlorophyll-specific δ¹⁵N during settling and sedimentation. Therefore, we use the δ¹⁵N of chlorophyll extracted from a sediment core retrieved in the BS (61.99°S, 55.09°W), as a proxy for nitrate drawdown and interpret its variability in terms of surface productivity for the past ~1000 years. Lower δ¹⁵N of chlorophyll values during periods of weaker westerly winds are consistent with the interpretation of more intense primary productivity in stratified Weddel Sea surface water entering BS from the east, relative to water branched from the Antarctic Circumpolar Current that traveled along South Shetland Islands and have a deeper mixed layer.