Eleven new Antarctic records of diatom-bound Δ13Corg show significantly lighter values during the Last Glacial Maximum than during the Holocene with a general trend of increasing depletions southward of the modern Antarctic Polar Front (MAPF). In contrast, glacial diatoms from cores farther north of the MAPF show Δ13Corg values which are ~1? heavier than during the Holocene. Likely causes for the low glacial Δ13Corg in Antarctic diatoms are higher 2>aq, lower growth rates, and changes in diatom abundance or size. Taken at face value, the glacial depletions of Δ13Corg suggest that Antarctic surface waters south of the MAPF might have been a source of atmospheric pCO2 during the glacial period. Alternatively, the glacial depletions might have been caused by either a relatively small decrease in growth rates or changes in the abundance of the two dominant diatom species or their average cell size. Interpretations of the Subantarctic records are complicated by similar uncertainties. ¿ 2000 American Geophysical Union