Delays, Discrepancies, and Distortions: Size-Dependent Sediment Mixing and the Deep-Sea Record of the Paleocene-Eocene Thermal Maximum From ODP Site 690 (Weddell Sea)

The deep-sea sedimentary record of Ocean Drilling Program Site 690 (Weddell Sea) has figured prominently in the study of an ancient (similar to 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 (delta C-13 and delta O-18) records of this reference section. A recent study invoked size-dependent sediment mixing (SDSM) to reconcile the discrepancies between these parallel delta C-13 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 delta C-13 signatures in photosymbiotic taxa (acarininids) at Site 690 is an artifact of SDSM. Synthetic bulk-carbonate delta C-13 records generated with two end-member (pre-CIE versus CIE materials) sediment-mixing curves readily reproduce the series of "steps" seen in published bulk-carbonate delta C-13 records of the CIE, suggesting that these fine-scale features may also be artifacts of SDSM and varying rates of carbonate sedimentation.