Major factors controlling Holocene δ 13 C org changes in a seasonal sea‐ice environment, Adélie Land, East Antarctica

In an effort to investigate the controlling factors behind Holocene δ 13 C org changes in East Antarctica we report high‐resolution down‐core records of bulk organic matter carbon isotopic ratios (δ 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 δ 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 δ 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 δ 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 δ 13 C org down‐core records should be used with great caution to reconstruct past CO 2 content in surface waters.