New estimates of Southern Ocean biological production rates from O2/Ar ratios and the triple isotope composition of O2

We report O-2/Ar ratios (a constraint on net community production) and the triple isotopic composition of dissolved 02 (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 degrees 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 O-18 and C-14 incubations along 170 degrees 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 similar to 0.25. Net cominunity/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 O2 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 02 production with other environmental variables, the strongest correlations are between net O-2 production and sea surface temperature (SST) (direct correlation), climatological [NO3-] (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 SiO2 and iron are the leading influence or if lower SSTs promote greater export in this region. (c) 2007 Elsevier Ltd. All rights reserved.