In an effort to investigate the controlling factors behind Holocene d 13 C org changes in East Antarctica we report high-resolution down-core records of bulk organic matter carbon isotopic ratios (d 13 C org), diatom census counts, total organic carbon, and biogenic silica content taken from one core recovered in the Adélie Trough. A good correspondence between the d 13 C org record and records of small/large and pennate/centric ratios in many of the core segments indicates that diatom species composition affected Holocene d 13 C org variations, possibly via diatom shape and size effects. Variations in the surface water CO 2 concentration and in the isotopic composition of the source during the Holocene cannot be ruled out although they cannot explain rapid and large-amplitude d 13 C org 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 CO 2. Our results confirm previous evidence which demonstrates that d 13 C org down-core records should be used with great caution to reconstruct past CO 2 content in surface waters.