In situ δ13C measurements in planktic foraminiferal shells from the PETM: New constraints on the CIE magnitude preserved in the marine record

Earth surface temperatures warmed by ~5-8°C during an ancient (~55.5 Ma) global warming event referred to as the Paleocene-Eocene thermal maximum (PETM). This transient (~200 ka) 'hyperthermal' climate state is widely touted as an ancient analog for future climate change. A hallmark feature of the PETM is a negative carbon isotope excursion (CIE) that has been documented in various terrestrial and marine substrates from numerous locations around the world. The ubiquity of the CIE signals the rapid release (≤30 ka) of massive quantities (≥2000 x 10¹⁵ g C) of isotopically depleted carbon into the exogenic carbon cycle; however, it has long been recognized that the magnitude of the CIE in terrestrial records is substantially greater than that measured in marine carbonates. For instance, estimates indicate that the average CIE magnitude in terrestrial records is ~4.7‰ while the mean CIE magnitude in marine carbonates is only ~2.8‰. Resolving this conundrum is paramount for constraining the size and source(s) of carbon input, determining the fate of the released carbon and gauging climate sensitivity to greenhouse gas forcing. Here, the effects of post-depositional diagenesis on planktic foraminiferal records is assessed by in situ δ¹³C measurements of alteration-resistant domains within planktic foraminiferal shells and diagenetic crystallites from ODP Site 865 using secondary ion mass spectrometry with an analytical precision of ×0.7‰ (7 μm beam). The analyses yielded mean δ¹³C values of ~2.7‰, ~4.7‰, and 0.0‰ for the crystallites, pre-CIE biogenic, and CIE biogenic calcites, respectively. The δ¹³C values of the diagenetic crystallites are ~3‰ higher than those of biogenic calcite in foraminiferal shells bearing the CIE signal, but ~2‰ lower than the δ¹³C of biogenic calcite in foraminiferal shells bearing the pre-CIE signal. These data indicate that the magnitude of the CIE at the central Pacific ODP Site 865, originally reported as being only ~2.5‰, is actually ~5‰ and highly congruous with the CIE magnitude seen in terrestrial records. Moreover, a CIE of comparable magnitude (~5‰) has been reproduced in the high-latitude (Weddell Sea) Site 690 PETM record using the same in situ analytical approach. These results indicate that the addition of modest amounts of diagenetic carbonate can attenuate the magnitude of the CIE when the whole foraminiferal shell is analyzed by conventional analytical approaches. Therefore, we conclude that diagenetic overprinting on foraminiferal shells is a viable explanation for the discrepant magnitudes seen between terrestrial and marine CIE records.