New estimates of Southern Ocean biological production rates from O 2/Ar ratios and the triple isotope composition of O 2

Abstract

We report O 2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved O 2 (a constraint on gross primary production) in samples collected from the surface mixed layer on 23 Southern Ocean transits. Samples were collected at 1–2° meridional resolution during the austral summer. Methodological limitations notwithstanding, the results constrain the net/gross production ratio, net O 2 production, and gross O 2 production at unprecedented resolution throughout the Southern Ocean mixed layer. Gross O 2 production rates inferred from the oxygen triple isotopes are greater than production rates calculated from a model based on remotely sensed chlorophyll. This result agrees with previous 18O and 14C incubations along 170°W. O 2/Ar ratios exceeding saturation are consistently observed within the Subantarctic and Polar Frontal Zones south of New Zealand and Australia, showing that a net autotrophic community predominates during austral summer. Lower O 2/Ar values are observed within the Drake Passage and Antarctic Zone, suggesting unresolved influences of low net community production, net heterotrophy, and upwelling of O 2-undersaturated waters. In autotrophic waters of the austral summer mixed layer, ratios of net community production/gross O 2 production scatter about 0.13, corresponding to f ratios of ∼0.25. Net community/gross O 2 production ratios show no meridional gradient across the Antarctic Circumpolar Current, suggesting that an approximately constant fraction of gross primary productivity is regenerated or exported. Our calculated net O 2 production rates are in satisfactory agreement with comparable published estimates. Net and gross O 2 production rates are highest in the Subantarctic and decline to the south, paralleling the well-known trend of chlorophyll a concentrations. In an analysis of variance of net O 2 production and gross O 2 production with other environmental variables, the strongest correlations are between net O 2 production and sea surface temperature (SST) (direct correlation), climatological [NO 3 −] (inverse correlation), and estimates of primary productivity derived from a remote sensing (direct correlation). These trends are as expected if aerosol iron input is the most important influence on production. They are unexpected if upwelling-derived SiO 2 and iron are the leading influence or if lower SSTs promote greater export in this region.