THE RELATIONSHIP BETWEEN DELTA-C-13 OF ORGANIC-MATTER AND [CO2(AQ)] IN OCEAN SURFACE-WATER - DATA FROM A JGOFS SITE IN THE NORTHEAST ATLANTIC-OCEAN AND A MODEL

The delta-C-13 of suspended particulate organic matter (SPOM) in surface waters increased from -22.9 to -18.1 parts per thousand during April 25-May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47-degrees-N, 20-degrees-W). During the same period, nearly parallel increases in sinking POM-delta-C-13 were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton-delta-C-13 and [CO2(aq)] are inversely related, the increases in both sinking and suspended POM-delta-C-13 were highly negatively correlated with mixed-layer [CO2(aq)] that generally decreased from 13.2-10.1-mu-moles/kg during the five weeks. The change in SPOM-delta-C-13 per change in [CO2(aq)], however, appears to be somewhat greater than that expected from previous, though less direct, ocean and laboratory evidence. By adapting a model of plant delta-C-13 by FARQUHAR et al. (1982), it is shown that under a constant phytoplankton demand for CO2 an inverse, nonlinear SPOM-delta-C-13 response to ambient [CO2(aq)] is expected. Such trends are unlike the negative linear relationships indicated by data from the NABE Site and or from Southern Hemisphere waters. Such differences between predicted and observed SPOM-delta-C-13 vs. [CO2(aq)] trends and among observed relationships can be reconciled, however, if biological CO2 demand is allowed to vary. This has significant implications for the use of the delta-C-13 of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO2 concentrations and biological productivity.