Summertime CO2 sinks in shelf and slope waters of the western Arctic Ocean

Abstract To evaluate summertime sinks for atmospheric CO2 in shelf and slope waters of the western Arctic Ocean, we measured atmospheric and surface water partial pressures of CO2 (pCO2), and surface water total CO2 (TCO2) on board the R/V Mirai in the summers of 1998–2000. Total alkalinity (TAlk) was calculated from the measured pCO2 and TCO2. The shipboard observations showed that in the shelf waters, surface water pCO2 was as low as 190 μatm (ΔpCO2≅−180 μatm). In the slope waters, surface water pCO2 showed relatively little spatial variation (±17 μatm), and the water was invariably undersaturated with atmospheric CO2 (average ΔpCO2 ≅−80 μatm). The area of lat. 71–72°N and long. 158–160°W, where the shelf and slope waters intersected, showed high spatial variations in surface water pCO2 (±50 μatm). The estimated air–sea fluxes of CO2 ranged between −17.8 and 3.0 mmol m−2 d−1 for the shelf waters and between −16.9 and −0.1 mmol m−2 d−1 for the slope waters. These ΔpCO2 and CO2 fluxes show that this high-latitude continental margin acts as a moderate to strong sink for atmospheric CO2 in the summer. The contribution of biological production to the low surface water pCO2 and to the high spatial variability was small. Instead, physical factors such as water mixing and cooling were involved, as determined by examining the interrelationships among pCO2, TCO2, TAlk, water temperature, and salinity. In the shelf water, the air–sea exchange of CO2 also significantly affected the spatial variations in pCO2. Furthermore, differences in the interrelationships determined the spatial variability of surface water pCO2 in the shelf and slope waters. We discuss a possible CO2 sink mechanism, emphasizing the importance of a ‘solubility pump’ in the shelf waters of the western Arctic Ocean.

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