Triple oxygen isotope composition of dissolved O2 in the equatorial Pacific: A tracer of mixing, production, and respiration

Abstract

As a contribution to the study of equatorial Pacific biogeochemistry, we measured the O2/Ar ratio and the triple isotope composition (18O, 17O, and 16O) of O2 along six meridional lines in the equatorial Pacific (8°N–8°S at 95°W, 110°W, 125°W, 140°W, 155°W, and 170°W). O2/Ar ratios and δ18O were close to equilibrium values within the mixed layer and followed the general trend of increasing δ18O with decreasing O2/Ar at greater depths. The 17Δ (≈δ17O–0.5δ18O) constrains the fraction of photosynthetic O2; 17Δ was slightly elevated with respect to equilibrium within the mixed layer due to local photosynthetic production. In aphotic zone waters above 250 m depth the average 17Δ values were higher than in the mixed layer. There are four sources of this photosynthetic signal in the dark ocean: production in the euphotic zone prior to subduction in the distant source regions, production below the mixed layer during travel to the equatorial zone, diapycnal mixing with shallower waters bearing photosynthetic O2, and accumulation of photosynthetic O2 produced at very low rates below the 1% light level. Our results also constrain biological production rates within the mixed layer at several locations along 95°W and 110°W. Our average rate of 14C production (53 ± 34 mmol C m−2 d−1) agreed well with other estimates in the equatorial Pacific, while our average rate of net C production (6.9 ± 6.2 mmol C m−2 d−1) and f ratio (0.12 ± 0.11) were somewhat lower than other estimates. Adding δ18O and 17Δ as tracers to three‐dimensional biogeochemical ocean GCMs and comparing results with observations will extend our understanding of metabolic rates in the study region.