Stratigraphic features of the carbon isotope excursion (CIE) marking the Paleocene-Eocene Thermal Maximum (PETM; ca. 55.8 Ma) are used to study ocean-climate change and carbon cycling during this ancient global warming event. Yet discrepancies in its timing and amplitude exist between bulk-carbonate and planktic-foraminifera δ13C records. Here we examine these disparities through the lens of δ13C compositions of size-segregated planktic shells across the pre-CIE to CIE transition in the iconic PETM section of Ocean Drilling Program Site 690 in the Weddell Sea. Our results show that the stratigraphic position of the CIE onset is dependent upon shell size, which we attribute to preferential mixing of smaller shells with pre-CIE δ13C values up into the overlying CIE interval. Hence, the transitory loss of size-dependent δ13C signatures in photosymbiotic planktic foraminifera is a taphonomic artifact, not a geochemical signal of symbiont “bleaching” during the PETM. Our results also indicate that many salient features of the Site 690 bulk-carbonate δ13C record are aberrations caused by size-dependent sediment mixing, and as such, should not be viewed as primary signals of ocean-climate change during what is arguably one of the best ancient analogs for future ocean-climate change.