Correlation between the carbon isotopic composition of planktonic foraminifera-bound organic matter and surface water pCO2 across the equatorial Pacific

For times prior to those represented by the air trapped in Antarctic ice core records, the concentration of CO2 in the atmosphere must be reconstructed using geochemical proxies. The δ13C of particulate organic carbon (POC) produced in ocean surface waters has previously been observed to covary with the concentration of CO2 in the water. Relative to bulk sedimentary organic carbon, the minute quantity of organic matter trapped within the shell walls of planktonic foraminifera, “foraminifera-bound organic matter” (FBOM), has the potential advantage of being protected from diagenetic alteration and contamination by allochthonous organic matter. Here, with new protocols and instrumentation, FBOM-δ13C is investigated as a potential proxy for the aqueous CO2 concentration ([CO2(aq)]) and thus the partial pressure of CO2 (pCO2) in past surface waters. We achieve a full method precision for FBOM-δ13C of 0.4‰ (1SD) on sample sizes of 20 nanomole C by adding a cryofocus step, a helium sheath flow to the oxidation and reduction reactors, and other modifications to an elemental analyzer and by introducing new approaches to reduce contamination during sample preparation. FBOM-δ13C was analyzed in core-top sediments from the eastern tropical Pacific that span the equatorial maximum in surface water [CO2(aq)]. FBOM-δ13C is lower than predicted for marine phytoplankton, consistent with a substantial lipid component in FBOM. Anticorrelation is observed between FBOM-δ13C and climatological [CO2(aq)]; the relationship is statistically significant (P <0.05) in mixed-species samples and in 4 out of 5 picked species. The slope of the FBOM-δ13C:[CO2(aq)] anticorrelation is equivalent to or greater than has been measured for the δ13C of suspended POC in the surface ocean. Based on the few species analyzed in this study, endosymbiont-bearing and -barren species do not clearly differ from each other either in terms of their average value of FBOM-δ13C or the strength of the FBOM-δ13C:[CO2(aq)] anticorrelation, despite the recognized importance of the photosynthetic endosymbionts as a source of organic carbon for the symbiotic species. Correcting for variations in the δ13C of CO2(aq) and phytoplankton growth rate did not improve the significance of the FBOM-δ13C:[CO2(aq)] anticorrelation. The findings support the possibility that FBOM-δ13C can be used as a paleoceanographic proxy for surface water [CO2(aq)] and thus atmospheric pCO2.