Diagenetic controls on the carbon isotope record of the Paleocene Eocene thermal maximum at Ocean Drilling Program Site 1135 (southern Indian Ocean)

Carbon isotope (13C) records of an ancient global warming event, the Paleocene Eocene Thermal Maximum (PETM), show that the negative carbon isotope excursion (CIE) marking the onset of the event is distinctly smaller in marine records than in terrestrial records. Such disparities have been attributed to secondary mechanisms that either attenuate or amplify the CIE magnitude in marine and terrestrial records, respectively. Here we show planktic foraminifer (Acarinina spp., Morozovella spp.) carbon isotope (13C) records constructed for the PETM section recovered at Ocean Drilling Program (ODP) Site 1135 along the Kerguelen Plateau (southern Indian Ocean). All planktic foraminifers appear opaque (frosty) under reflected light and show signs of carbonate diagenesis such as thickened, blade-like protrusions (muricae) on their shell exteriors. To evaluate the effects of diagenesis on the stratigraphic signature of the CIE, parallel 13C records were generated using in-situ secondary ion mass spectrometry (SIMS) and traditional gas source mass spectrometry (GSMS). The SIMS-based 13C records yield excursion magnitudes (acarininids = 5.3, morozovellids = 3.9) that are ~1.6 larger than in the single-shell GSMS-based 13C records. In-situ measurements on micrometer-scale domains within individual shells suggest that the 13C compositions of planktic foraminifer shells from the CIE interval have been secondarily elevated by post-depositional, chalk diagenesis. We also show that the interiors of foraminifer shells are infilled with fine-fraction, nannofossil carbonate. Incorporation of pre-CIE nannofossils with a higher 13C composition into these infillings would also attenuate the magnitude of the negative CIE marking the PETM in 13C records based on whole-shell GSMS analyses. These findings suggest that carbonate (chalk) diagenesis and secondary infilling of foraminifer shells with extraneous fine-fraction carbonate (nannofossils) combine to attenuate the CIE magnitude in GSMS-based 13C records constructed with whole foraminifer shell analyses.