Response of CO2 Sink and Biogeochemistry to Sea-Ice Loss in the Western Arctic Ocean

Abstract

The oceanic uptake of atmospheric CO2 is of global importance as it affects the pace of climate change. The Arctic Ocean acts as a carbon sink for atmospheric CO2, benefiting from high solubility of CO2 in cold seawater and high summer biological production. It has been known that amplified warming and accelerated sea ice loss in the Arctic Ocean since 1980s have profoundly altered the Arctic Ocean environment and related biogeochemical processes. However, less is known about how oceanic CO2 uptake and biological production changes in different biogeochemical provinces in respond to warming and sea ice loss and how fast are these changes. Based on results from two cruises conducted in the western Arctic Ocean in 2016 and 2018, we examined seasonal and regional variabilities in metabolic status and the coupling of biological production and oceanic CO2 uptake, which provided a mechanistic view of the summer evolution of net community production and CO2 flux in the various stages of ice-melt and nutrient status. By compiling historical datasets of underway measurements of sea surface partial pressure of CO2 (pCO2), we found that despite the western Arctic Ocean as a whole continuing to act as an oceanic carbon sink, regional carbon flux dynamics differ greatly; the Chukchi Sea continues to absorb CO2 at pace with the atmospheric CO2 increase, whereas Beaufort Sea and Canada Basin become a weakened or diminishing CO2 sink as the sea surface CO2 increased at more than twice the rate of CO2 in the atmosphere. In addition to examination of the long-term trend of sea surface CO2, we further assessed seasonal and interannual variations in CO2 uptake between 1994 and 2019. Two complementary approaches (observation-based and model-based) were conducted. Our results suggest that CO2 uptake in the Chukchi Sea significantly increased at a rate of 1.4 ±0.4 Tg C decade-1, which was primarily due to a longer ice-free period with a larger open area and increased primary production and partially due to enhanced wind. However, no significant change in CO2 uptake was found in the Canada Basin and Beaufort Sea. Our model results further revealed that the greatly decreased sea ice extent in summer indeed promoted CO2 uptake and resulted in a weak increased CO2 sink by 0.6±0.3 Tg C decade-1 in the Canada Basin, but this increasing sink was counteracted by a rapidly decreasing air-sea CO2 gradient.