High-resolution study of Antarctic planktonic foraminiferal assemblages (Ocean Drilling Program Site 690, Weddell Sea) shows that these microplankton underwent a stepwise series of changes during the Paleocene--Eocene thermal maximum (PETM). Initiation of this response coincides with the onset of the carbon isotope excursion (CIE) but precedes the benthic foraminiferal mass extinction. The top-to-bottom succession in the biotic response indicates that the surface ocean/atmosphere was affected before the deep sea. The earliest stage of the faunal response entailed a conspicuous turnover within the shallow-dwelling genus Acarinina and a succession of stratigraphic first appearances. The genus Morozovella, large (>180 ¿m) biserial planktonics, and A. wilcoxensis are all restricted to the lower CIE within this PETM section. Acarininid populations crashed as the ocean/climate system ameliorated during the CIE recovery, reflecting atypical surface water conditions. This transient decline in acarininids is paralleled by a marked increase in carbonate content of sediments. It is postulated that this interval of carbonate enrichment, and its unusual microfauna, reflects enhanced carbon storage within reservoirs of the global carbon cycle other than the marine carbonate system Broecker et al., 1993; Ravizza et al., 2001>.