High-precision analysis of planktonic foraminiferal shell weights reveals an abrupt decrease in mass at the onset of the Paleocene-Eocene thermal maximum (PETM) at Site 690 in the Weddell Sea. The decline observed in the dominant species Acarinina soldadoensis occurs at the onset of the carbon isotope excursion (CIE) interval. Comparison of the mass and inferred volume suggests a thinning of A. soldadoensis shell walls. The event does not correlate with the significant decrease in calcium carbonate, likely a result of chemical erosion, suggesting that the mass reduction is a primary feature. We speculate that the mass decrease represents a thinning of A. soldadoensis shell walls in response to lower carbonate saturation at the onset of the PETM.