The Gulf of Alaska coastal ocean as an atmospheric CO2 sink

Abstract A new data set of directly measured surface seawater carbon dioxide partial pressures (pCO 2 ) was compiled for the Gulf of Alaska (GOA) coastal ocean. Using this information, along with reconstructed atmospheric pCO 2 data, we calculate sea–air CO 2 fluxes over two interconnected domains: the coastal ocean defined by the Surface Ocean CO 2 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 CO 2 fluxes were calculated by constructing monthly climatologies of sea–air pCO 2 difference (ΔpCO 2 ), sea surface temperature, salinity, and CO 2 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 CO 2 fluxes showed instances of atmospheric CO 2 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 pCO 2 with respect to atmospheric levels. Atmospheric CO 2 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 CO 2  m −2  d −1 , respectively. Scaling these annual means by the respective surface areas of each domain resulted in estimates of substantial atmospheric CO 2 uptake between 34 and 14 Tg C yr −1 . This region is a large sink for atmospheric CO 2 , which impacts the current view of weak net CO 2 emission from coastal waters surrounding North America.

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