Net and gross O2 production in the Southern Ocean from measurements of biological O2 saturation and its triple isotope composition

Mixed layer seawater samples from t`he Southern Ocean were analyzed for the triple oxygen isotope composition (delta(17)O and delta(18)O) of dissolved O-2 and the ratio of [O-2] to [Ar]. delta(17)O and delta(18)O together constrain the mass independent anomaly in O-2, and hence the fraction of photosynthetic O-2 in the dissolved O-2 pool. Assuming oxygen in the mixed layer is at steady state, we calculate ratios of the gross photosynthetic O-2 production rate to the O-2 air-sea gas exchange rate, and ratios of net to gross O-2 production rates. With estimates of the O-2 gas exchange rate from a wind speed parameterization, we determine absolute in situ rates of gross and net O-2 production. Based on the net/gross production ratios and delta(18)O of dissolved O-2, we calculate a value for the O-18 fractionation factor associated with marine respiration in the Southern Ocean mixed layer of 0.978 (an isotope effect of 22parts per thousand). The study regions cover latitudes between similar to45degreesS and the ice edge at: (1) 175degreesE in December 1999, (2) 145degreesE in December 2000, and (3) 145degreesE in January 2001. At both meridians, gross O-2 production decreases to the south. At 145degreesE, rates of net O-2 production follow the same pattern, while at 175degreesE these rates are consistently low at all latitudes. In December, gross and net O-2 production rates are both higher at 145degreesE than at 175degreesE. Gross O-2 production at 145degreesE was similar in December and January, but net O-2 production decreased by similar to50%. Net/gross C production ratios, calculated by scaling O-2 production rates, are lower than estimates of the N-15 f-ratio, determined as the ratio of (NO3)-N-15 uptake to (NO3-)-N-15 + (NH4+)-N-15 uptake in N-15 incubations. They are also lower than ef-ratios predicted by the model of Laws et al. (Global Biogeochem. Cycles 14 (2000a) 1231) at the temperatures of the Southern Ocean. The differences in the measurements could be due to natural interannual and spatial variability or the differences in metabolic rates measured and modeled, and the fundamental assumptions required by each technique. (C) 2004 Elsevier Ltd. All rights reserved.