A TIMESCALE FOR DISSOLVED ORGANIC-CARBON PRODUCTION IN EQUATORIAL PACIFIC SURFACE WATERS

Concentrations of dissolved organic carbon (DOC) in the equatorial Pac ific Ocean are high (roughly 80 mu M) in sunlit surface waters and low (similar to 40 mu M) in subsurface waters. Concentrations in recently upwelled waters in the central equatorial region and in the coastal u pwelling zone near Peru are intermediate between the surface water and deep water values (similar to 65 mu M). The data appear to be consist ent with the existence of a short-lived ''semilabile'' DOC component, generated in surface waters, superimposed on a long-lived ''refractory '' DOC concentration of similar to 40 mu M. We assume that the oligotr ophic 80 mu M 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 stat e. The steady state assumption for oligotrophic surface waters can be used to couple the kinetic rate constants of DOC production and consum ption, so that the only tunable parameter in the model is the e-foldin g timescale for the approach to steady state. We tested and assessed t he implications of this model by embedding surface ocean production an d concentration-dependent consumption of DOC into the three-dimensiona l flow field of a high resolution equatorial circulation model. The di stribution of DOC near the equator can be reproduced using a grow-in t imescale of 30-120 days, consistent with inferred values from field da ta and with direct field measurements. Implied gross DOC production ra tes 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 mag nitude higher than previous estimates [Siegenthaler and Sarmiento, 199 3]. The DOC export production rate predicted by the model from the equ atorial region (5 degrees N to 5 degrees S and 90 degrees W to the dat eline) is 0.2-0.4 Gton C yr(-1), which is equivalent to 20-40% of the non-El Nino (NO3)-N-15-uptake based new production estimate from the e quatorial region [McCarthy et al., 1996; Murray et al., 1997].