A Timescale for Dissolved Organic Carbon Production
in Equatorial Pacific Ocean Surface Waters

David Archer
Department of Geophysical Sciences
University of Chicago
Chicago, IL 60637

Edward T. Peltzer*
Marine Chemistry and Geochemistry Department
Woods Hole Oceanographic Institution
Woods Hole, MA 02543

David L. Kirchman
College of Marine Studies
University of Delaware
Lewes, DE 19958

Global Biogeochemical Cycles (1997) 11: 435-452.

Received: 6 August 1996.
Revised: 6 March 1997.
Accepted: 15 April 1997.
Published: September 1997.

*: Present address: Monterey Bay Aquarium Research Institute
PO Box 628, Moss Landing, CA 95039-0628, USA.

ABSTRACT

Concentrations of dissolved organic carbon (DOC) in the equatorial Pacific Ocean are high (roughly 80 µM C) in sunlit waters and low (~ 40 µM C) 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 C). 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 "refratory" DOC concentration of ~ 40 µM C. We assume that the oligotrophic 80 µM C 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 per yr, 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 per yr, which is equivalent to 20-40% of the non-El Niño N15-NO3 uptake based new production estimate from the equatorial region [McCarthy et al., 1996; Murray et al., 1996].

References

Barber, R. T., M. P. Sanderson, S. T. Lindley, F. Chai, J. Newton, C. C. Trees, D. G. Foley and F. Chavez (1996). Primary production in the equatorial Pacific along 140°W. Deep-Sea Res., Part II 43: 933-970.

McCarthy, J. J., Garside, C., J. Nevins and R. T. Barber (1996). New production along 140°W in the equatorial Pacific during and following the 1992 El Niño event. Deep-Sea Res., Part II 43: 1065-1094.

Murray, J. W., J. Young, J. Newton, J. Dunne, T. Chapin, B. Paul and J. J. McCarthy (1996). Export flux of particulate organic carbon from the central equatorial Pacific determined using a combined drifting trap 234-Th approach. Deep-Sea Res., Part II 43: 1095-1132.

Siegenthaler, U. and J. L. Sarmiento (1993). Atmospheric carbon dioxide and the ocean. Nature 359: 119-125.

Acknowledgements

We are grateful to Robbie Toggweiler and Steve Carson for access to their model flow fields and extensive consultation. We also acknowledge helpful discussion with Jorge Sarmiento, Paul Quay and Parker MacCready, and feedback from several anonymous reviewers.

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