| Summary: | Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(2), (2019):821-843, doi:10.1029/2018JC014568. Shifting baselines in the Arctic atmosphere‐sea ice‐ocean system have significant potential to alter biogeochemical cycling and ecosystem dynamics. In particular, the impact of increased open water duration on lower trophic level productivity and biological CO2 sequestration is poorly understood. Using high‐resolution observations of surface seawater dissolved O2/Ar and pCO2 collected in the Pacific Arctic in October 2011 and 2012, we evaluate spatial variability in biological metabolic status (autotrophy vs heterotrophy) as constrained by O2/Ar saturation (∆O2/Ar) as well as the relationship between net biological production and the sea‐air gradient of pCO2 (∆pCO2). We find a robust relationship between ∆pCO2 and ∆O2/Ar (correlation coefficient of −0.74 and −0.61 for 2011 and 2012, respectively), which suggests that biological production in the late open water season is an important determinant of the air‐sea CO2 gradient at a timeframe of maximal ocean uptake for CO2 in this region. Patchiness in biological production as indicated by ∆O2/Ar suggests spatially variable nutrient supply mechanisms supporting late season growth amidst a generally strongly stratified and nutrient‐limited condition. We thank the Captain, crew, and marine technicians of the USCGC Healy for their shipboard support. We also thank anonymous reviewers for providing useful feedback that improved this manuscript. This work was supported by NSF awards 1232856 and 1504394 to L.W.J. T.T. was supported by a grant NA150AR4320064 from Climate Program Office/NOAA and R.P. by NSF PLR‐1504333 and OPP‐1702371. All O2 and O2/Ar data and metadata are available at Arcticdata.io, doi:10.18739/A21G22, and pCO2 data are available at www.ldeo.columbia.edu/CO2 as ...
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