Determination of the Anthropogenic Carbon Signal to the Total Change in Dissolved Carbon in the Coastal Upwelling Region Along the Washington-Oregon-California Continental Margin

The continental shelf region off the Washington-Oregon-California coast is seasonally exposed to water with a low aragonite saturation state by coastal upwelling of CO₂-rich waters. To date, the spatial and temporal distribution of anthropogenic CO₂ (C_anthro) contribution to the CO₂-rich waters is largely unknown. Here we use an adaptation of the linear regression approach described in Feely et al (2008) along with the GO-SHIP Repeat Hydrography data sets from the northeast Pacific to establish an annually updated relationship between C_anthro and potential density. This relationship was then used with the NOAA Ocean Acidification Program west coast cruise data sets from 2007, 2011, 2012 and 2013 to determine the spatial variations of C_anthro in the upwelled water. Our results show large spatial differences in C_anthro in surface waters along the coast with the lowest surface values (40-45 µmol kg^-1) in strong upwelling regions of off northern California and southern Oregon and higher values (50-70 µmol kg^-1) to the north and south. C_anthro contributes an average of about 70% of the increased amount of dissolved inorganic carbon in the upwelled waters at the surface. In contrast, at 50 m the C_anthro contribution is approximately 31% and at 100 m it averages about 16%. The remaining contributions are primarily due to respiration processes in the water that was upwelled and transported to coastal regions or underwent respiration processes that occurred locally during the course of the upwelling season. The uptake of C_anthro has caused the aragonite saturation horizon to shoal by approximately 30-50 m since preindustrial period so that the undersaturated waters are well within the regions that affect the biological communities on the continental shelf.