The δ¹⁸O of dissolved O₂ in the surface waters of the subarctic Pacific: A tracer of biological productivity

The 18 O/ 16 O 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 δ 18 O 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 O 2 . A value for the isotopic fractionation effect during respiration (α T ) of 0.978±0.006 was determined from the time rate of change of the concentration and δ 18 O of O 2 in the mixed layer measured during August 1988. Below the mixed layer (100–280 m) the O 2 concentration decreased with a corresponding increase in δ 18 O. Model derived values for α T over this depth region ranged from 0.980 to 0.988 and depended on the mixing model. The difference between α T 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 α T values apply oceanwide, then photosynthesis and respiration by the marine biota have a similar effect to land plants in maintaining the δ 18 O of atmospheric O 2 at 23.5 ‰.