Downcore diatom data from a deep basin in Granite Harbor, Antarctica, provide new, high-resolution information concerning changes in oceanographic and depositional processes that have occurred during approximately the last 1250 years. The influence of global-scale climatic change, during the time of the Medieval Warm Period and possibly the Little Ice Age, is revealed by long-term changes in floral assemblages, as recorded by changes in the relative concentration of Nitzschia cylindrus. The diatom data suggest that Granite Harbor experienced less annual ice and pack ice cover during both these periods, possibly because of higher temperatures during the Medieval Warm Period and stronger offshore winds during the Little Ice Age. The sedimentary record reveals several distinct, short-lived events, in which nearly monospecific blooms of Corethron criophilum and Chaetoceros spp. occurred, possibly the result of strong stabilization of the upper water column and were most likely a response to regional, rather than global conditions. Although short-lived, these events may play a significant role in the delivery of organic carbon and biogenic silica to the seafloor. ¿ American Geophysical Union 1993