Planktonic Foraminiferal δ13Corg as a novel proxy for Carbon Cycling

It has been hypothesized that lower atmospheric CO₂ concentrations and lower temperatures during glacial times caused the enrichment of carbon isotopes of particulate organic material (δ¹³C_org-POM) produced in the surface ocean. Some downcore measurements of organic carbon isotopes of bulk sediments show such a trend, however, others do not. The lack of a coherent picture could be due to issues relating to the bulk sediments, including diagenetic alteration, the nature of the organic material, input of allochthonous material, and sediment redistribution.

Recent work by Hoogakker et al. (2022) shows that planktonic foraminifera-bound organic carbon δ¹³C values (δ¹³C_FBOM) are remarkably similar to those of δ¹³C_org-POM. Here we present the first down-core organic carbon isotope record of planktonic foraminifera-bound organic carbon (δ¹³C_FBOM) from the Southern Ocean (ODP Site 1088), to test for a glacial enrichment in δ¹³C_org-POM. The samples (Globigerina bulloides, Globorotalia truncatulinoides, and G. inflata) cover the last 20,000 years.

Our δ¹³C_FBOM results show a slight positive trend toward the Last Glacial Maximum (LGM), in accordance with the hypothesized δ¹³C_org-POM trend, but not to the extent as shown in some bulk sediments from more tropical latitudes. We discuss our results in the context of predicted past δ¹³C_org-POM using ice core atmospheric pCO₂ concentrations, G. bulloides calcification DIC (from inorganic carbon isotopes), and temperature (using Mg/Ca).