In order to make predictions regarding future climate change it is necessary to understand past climatic conditions. Yet how can past climates, which cannot be measured directly, be reconstructed? Evidence from environmental sources which can give indirect evidence of the climate of the past provide "proxy" data. This proxy evidence can then be calibrated and used to estimate climatic conditions beyond the instrumental record. Thus it is possible to separate anthropogenic influences from the normal climatic variation. Therefore it is essential that the proxies used are reliable. The low values and great of the C isotopic composition of particulate organic carbon (δ13Corg) in the Southern Ocean have long been a puzzle and an impediment to using δ13Corg as a paleoceanographic proxy. To ascertain the exact cause(s) of the anomalously low and variable Southern Ocean δ13Corg, we analyzed a suite of POC samples collected from the chlorophyll maximum during transects of the Antarctic sector of the Southern Ocean during GEOTRACES cruise "Zero and Drake" (ANTXXIV/3). Results suggest that strongly depleted δ13Corg values are a general feature of Antarctic phytoplankton, but that variability in δ13Corg may be largely driven by the abundance of the diatom, F. kerguelensis, which tends to have relatively less depleted δ13Corg values.