Four Antarctic marine mollusc shells, which were collected alive between 1917 and 1940, were analyzed by accelerator mass spectrometry to provide the first pre-bomb radiocarbon measurements of biogenic carbonates from the Southern Ocean. After correcting for the impact of fossil fuel combustion (Suess Effect), radiocarbon activities of the pre-bomb shells averaged -149.8¿10.4?. In contrast, the Δ14C values for post-bomb molluscs, echinoderms, brachiopods and foraminifera averaged -96.1¿25.2?. These biogenic carbonate Δ14C values are nearly identical to pre-bomb estimates (-148? to -152?) and post-bomb measurements (-98.4¿22.0?) of the surface waters in the Southern Ocean. Average radiocarbon ages of the biogenic carbonates before and after 1950 (1303¿84 years and 811¿205 years, respectively), along with those from seals and penguins, indicate that the Antarctic marine radiocarbon reservoir has decreased in age by nearly 500 years during the second half of the 20th century. Marine species and seawater measurements firmly place the radiocarbon reservoir correction at 1300¿100 years for calcareous marine fossils which are widespread, abundant and well-preserved organic materials for interpreting ice-sheet, climate and sea level impacts on the Antarctic marine ecosystem during the Holocene. ¿ American Geophysical Union 1996