A timescale for dissolved organic carbon production in equatorial Pacific surface waters

Abstract

Concentrations of dissolved organic carbon (DOC) in the equatorial Pacific Ocean are high (roughly 80 μM) in sunlit surface waters and low (∼40 μM) in subsurface waters. Concentrations in recently upwelled waters in the central equatorial region and in the coastal upwelling zone near Peru are intermediate between the surface water and deep water values (∼65 μM). The data appear to be consistent with the existence of a short‐lived “semilabile” DOC component, generated in surface waters, superimposed on a long‐lived “refractory” DOC concentration of ∼40 μM. We assume that the oligotrophic 80 μM end‐member is close to a biochemical steady state where production is balanced by consumption, and the lower concentrations in recently upwelled water represent a transient approach to steady state. The steady state assumption for oligotrophic surface waters can be used to couple the kinetic rate constants of DOC production and consumption, so that the only tunable parameter in the model is the e‐folding timescale for the approach to steady state. We tested and assessed the implications of this model by embedding surface ocean production and concentration‐dependent consumption of DOC into the three‐dimensional flow field of a high resolution equatorial circulation model. The distribution of DOC near the equator can be reproduced using a grow‐in timescale of 30–120 days, consistent with inferred values from field data and with direct field measurements. Implied gross DOC production rates are roughly half of measured particulate primary production rates [Barber et al., 1996]; this proportion applied globally would imply a global rate of DOC production of 50–100 Gton C yr−1, an order of magnitude higher than previous estimates [Siegenthaler and Sarmiento, 1993]. The DOC export production rate predicted by the model from the equatorial region (5°N to 5°S and 90°W to the dateline) is 0.2–0.4 Gton C yr−1, which is equivalent to 20–40% of the non‐El Niño 15NO3− uptake based new production estimate from the equatorial region [McCarthy et al., 1996; Murray et al., 1997].