Summertime CO 2 sinks in shelf and slope waters of the western Arctic Ocean

Abstract

To evaluate summertime sinks for atmospheric CO 2 in shelf and slope waters of the western Arctic Ocean, we measured atmospheric and surface water partial pressures of CO 2 ( pCO 2), and surface water total CO 2 (TCO 2) on board the R/V Mirai in the summers of 1998–2000. Total alkalinity (TAlk) was calculated from the measured pCO 2 and TCO 2. The shipboard observations showed that in the shelf waters, surface water pCO 2 was as low as 190 μatm (Δ pCO 2≅−180 μatm). In the slope waters, surface water pCO 2 showed relatively little spatial variation (±17 μatm), and the water was invariably undersaturated with atmospheric CO 2 (average Δ pCO 2 ≅−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 pCO 2 (±50 μatm). The estimated air–sea fluxes of CO 2 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 Δ pCO 2 and CO 2 fluxes show that this high-latitude continental margin acts as a moderate to strong sink for atmospheric CO 2 in the summer. The contribution of biological production to the low surface water pCO 2 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 pCO 2, TCO 2, TAlk, water temperature, and salinity. In the shelf water, the air–sea exchange of CO 2 also significantly affected the spatial variations in pCO 2. Furthermore, differences in the interrelationships determined the spatial variability of surface water pCO 2 in the shelf and slope waters. We discuss a possible CO 2 sink mechanism, emphasizing the importance of a ‘solubility pump’ in the shelf waters of the western Arctic Ocean.