The relationship between δ13C of organic matter and [CO 2(aq)] in ocean surface water: Data from a JGOFS site in the northeast Atlantic Ocean and a model

Abstract

The δ 13 C of suspended particulate organic matter (SPOM) in surface waters increased from −22.9 to −18.1‰ during April 25–May 31, 1989 at the JGOFS North Atlantic Bloom Experiment Site (NABE Site; 47°N, 20°W). During the same period, nearly parallel increases in sinking POM δ 13 C were also found, although these values were usually lower than those of the corresponding SPOM. Consistent with the hypothesis that plankton δ 13 C and [CO 2(aq)] are inversely related, the increases in both sinking and suspended POM δ 13 C were highly negatively correlated with mixed-layer [CO 2(aq)] that generally decreased from 10.1–13.2 μmoles/kg during the five weeks. The change in SPOM δ 13 C per change in [CO 2(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 δ 13 C by Farquhar et al. (1982), it is shown that under a constant phytoplankton demand for CO 2 an inverse, nonlinear SPOM δ 13 C response to ambient [CO 2(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 δ 13 C vs. [CO 2(aq)] trends and among observed relationships can be reconciled, however, if biological CO 2 demand is allowed to vary. This has significant implications for the use of the δ 13 C of plankton (or their organic subfractions or sedimentary remains) as a proxy for past or present ocean CO 2 concentrations and biological productivity.