In an effort to investigate the controlling factors behind Holocene δ13Corg changes in East Antarctica we report high-resolution down-core records of bulk organic matter carbon isotopic ratios (δ13Corg), diatom census counts, total organic carbon, and biogenic silica content taken from one core recovered in the Ad¿lie Trough. A good correspondence between the δ13Corg record and records of small/large and pennate/centric ratios in many of the core segments indicates that diatom species composition affected Holocene δ13Corg variations, possibly via diatom shape and size effects. Variations in the surface water CO2 concentration and in the isotopic composition of the source during the Holocene cannot be ruled out although they cannot explain rapid and large-amplitude δ13Corg changes. Within the limit of our investigation, our results argue against active carbon acquisition through a carbon concentration mechanism as evidenced in low-latitude upwelling systems. The bulk organic matter thus represents a mixing of diatom taxa having different fractionation affinity to aqueous CO2. Our results confirm previous evidence which demonstrates that δ13Corg down-core records should be used with great caution to reconstruct past CO2 content in surface waters.