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