NOAA/WDS Paleoclimatology - Weddell Sea d13C and d18O Data during the Paleocene-Eocene Thermal Maximum

The deep-sea sedimentary record of Ocean Drilling Program Site 690 (Weddell Sea) has figured prominently in the study of an ancient (~56 Ma) global warming event referred to as the Paleocene-Eocene Thermal Maximum (PETM). Yet discrepancies in the timing, amplitude, and structure of the isotopic excursions marking the PETM exist between the bulk-carbonate and planktic foraminifer stable isotope (d13C, d18O) records of this reference section. A recent study (Hupp et al., 2019) invoked size-dependent sediment mixing (SDSM) to reconcile the discrepancies between these parallel d13C records. Here we report supplementary stable isotope data compiled from size-segregated planktic foraminifer shells that further elucidate the effects of SDSM on the Site 690 PETM section. Our records reveal a stratigraphic sequence where the carbon (CIE) and oxygen (OIE) isotope excursions marking the onset of PETM conditions are first registered by larger shells, confirming that smaller-sized pre-CIE shells have been more intensely mixed and preferentially displaced up-section into the overlying CIE interval. The size-dependent manner in which the CIE and OIE are recorded by asymbiotic subbotinids indicates that the transitory loss of size-dependent d13C signatures in photosymbiotic taxa (acarininids) at Site 690 is an artifact of SDSM. We also construct two end-member mixing curves to generate synthetic bulk-carbonate d13C records of the CIE by varying the proportions of reworked pre-CIE and in-situ CIE materials through the study section. This suggests that the series of fine-scale "steps" in published bulk-carbonate d13C records may also be artifacts of SDSM.