The Gulf of Alaska coastal ocean as an atmospheric CO2 sink

A new data set of directly measured surface seawater carbon dioxide partial pressures (pCO2) was compiled for the Gulf of Alaska (GOA) coastal ocean. Using this information, along with reconstructed atmospheric pCO2 data, we calculate sea–air CO2 fluxes over two interconnected domains: the coastal ocean defined by the Surface Ocean CO2 Atlas (SOCAT) Continental Margin Mask, and the continental margin shoreward of the 1500 m isobath. The continental margin in this region lies within the coastal ocean. Climatological sea–air CO2 fluxes were calculated by constructing monthly climatologies of sea–air pCO2 difference (ΔpCO2), sea surface temperature, salinity, and CO2 solubility, coupled with the monthly second moment of wind speeds from the Scatterometer Climatology of Ocean Winds (SCOW; http://cioss.coas.oregonstate.edu/scow). Climatological sea–air CO2 fluxes showed instances of atmospheric CO2 uptake and outgassing in both domains for nearly all months; however, uptake dominated from April through November, with distinct spring and autumn peaks that coincided with periods of strong winds and undersaturated surface seawater pCO2 with respect to atmospheric levels. Atmospheric CO2 uptake during the spring and autumn peaks was stronger on the continental margin compared with the coastal ocean. Annual mean area-weighted fluxes for the coastal ocean and continental margin were −2.5 and −4 mmol CO2 m−2 d−1, respectively. Scaling these annual means by the respective surface areas of each domain resulted in estimates of substantial atmospheric CO2 uptake between 34 and 14 Tg C yr−1. This region is a large sink for atmospheric CO2, which impacts the current view of weak net CO2 emission from coastal waters surrounding North America.