Distributions of oxygen, argon, nitrogen, and radon in the upper ocean of the subarctic Pacific distinguish the fluxes controlling the oxygen mass balance during the summers of 1987 and 1988. The difference between the net O2 flux (in mmol m−2 d−1) to the atmosphere via gas exchange (32) and the integrated decrease with time (−14) is balanced by biological production (13‐17), air injection by bubble entrainment (5), and O2 flux to the thermocline −(0‐4). Nitrogen/argon and oxygen/argon ratios reveal that ˜15% of the oxygen supersaturation in summer is produced by air injection and ˜40% by biological production, with the rest induced by surface water warming. Our estimate of biologically induced oxygen production when translated stoichiometrically to nitrogen uptake agrees to within error estimates with both the particulate and dissolved nitrogen mass balances for the upper ocean determined in the SUPER program during the same time period. The oxygen mass balance requires a net carbon production in the euphotic zone of ˜140 mg C m−2 d−1 (PQ=1.5), which is 20–30% of the level of 14C primary production determined by SUPER investigators.
The 18O/16O of dissolved oxygen was measured in the upper ocean of the subarctic Pacific in 1988. In May and August, at stations Papa (50°N, 145°W) and R (53°N, 145°W), the mean δ18O in the mixed layer was 23.84±0.20 and 24.00±0.24 ‰ (versus standard mean ocean water) consistently more depleted than atmospheric saturation levels by about 0.4 ‰. This relative depletion is caused by input of photosynthetically produced O2. A value for the isotopic fractionation effect during respiration (αT) of 0.978±0.006 was determined from the time rate of change of the concentration and δ18O of O2 in the mixed layer measured during August 1988. Below the mixed layer (100–280 m) the O2 concentration decreased with a corresponding increase in δ18O. Model derived values for αT over this depth region ranged from 0.980 to 0.988 and depended on the mixing model. The difference between αT determinations for the surface layer versus upper thermocline likely results from mixing model inaccuracies or different isotope fractionation effects during plankton and bacterial respiration. If the calculated mixed layer αT values apply oceanwide, then photosynthesis and respiration by the marine biota have a similar effect to land plants in maintaining the δ18O of atmospheric O2 at 23.5 ‰.
We determine annual rates of net biological oxygen production in the euphotic zone and respiration in the upper thermocline of the subtropical North Pacific ocean using mass balances of oxygen, argon, and nitrogen measured at the U.S. Joint Global Ocean Flux Study time series station ALOHA. Net evasion of nitrogen and argon to the atmosphere caused by warming of surface waters is balanced by supply primarily from cross‐isopycnal transport. Mixing rates between the euphotic zone and the top of the permanent thermocline required to balance the inert gas flux are 1–2 cm2 s−1 when transformed to units of an eddy diffusion coefficient. Application of mixing rates derived from the inert gas mass balance to the oxygen field yields a net annual euphotic zone production rate of 1.4±1.0 moles O2 m−2 yr−1, one half of which is lost to the atmosphere, with most of the rest mixed into the top of the thermocline. Since cross‐isopycnal gradients of dissolved organic carbon (DOC) are about half to those of oxygen, we estimate that at least one quarter of the carbon flux out of the euphotic zone is via DOC. Because surface ocean dissolved organic matter has a relatively high C/N ratio, the stoichiometry among O2, C, and inorganic N in the upper ocean should be different than that observed in deeper waters.
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