EFFECTS OF SIZE-SELECTIVE SEDIMENT MIXING ON DEEP-SEA RECORDS OF THE PALEOCENE-EOCENE THERMAL MAXIMUM

A distinctive geochemical fingerprint of an ancient (~55.8 Ma) global warming event referred to as the Paleocene-Eocene thermal maximum (PETM) is a negative carbon isotope excursion (CIE) signaling the input of massive quantities of isotopically depleted carbon into the ocean-atmosphere system. Pinpointing the CIE onset in stratigraphies is crucial to constructing a chronological framework for PETM records, yet stratigraphic offsets in the position of the CIE onset exist between bulk-carbonate and planktic foraminifer δ¹³C records in many deep-sea records. Here, we investigate this discrepancy by measuring the δ¹³C compositions of size-segregated planktic foraminifers (Acarinina subsphaerica, A. soldadoensis) across the pre-CIE to CIE transition at Ocean Drilling Program Site 690. The results show that the stratigraphic position of the CIE onset is strongly dependent upon shell size, with larger shells registering CIE values lower in the stratigraphy than smaller shells. We attribute this stratigraphic offset to size-selective sediment mixing where smaller shells are preferentially displaced upward in the stratigraphy. The time averaging effects of sediment mixing attenuates the amplitude of the CIE, and the size-selective nature of this process may account for the “stepped” structure seen in some bulk-carbonate δ¹³C records of the CIE. Moreover, differential mixing of small pre-CIE shells and larger CIE shells obfuscates the δ¹³C/size relationship of photosymbiont-bearing planktic foraminifers within the CIE interval. We therefore consider the temporary loss of the δ¹³C/size relationship in symbiotic planktic foraminifers within the Site 690 PETM section to be a taphonomic artifact, not an actual paleoecologial signal of symbiont ‘bleaching’.