Climate mediated size variability of diatom Fragilariopsis kerguelensis in the Southern Ocean

• Fragilariopsis kerguelensis average valve areas strongly linked to iron availability.
• Strong differences in average valve areas in Atlantic and Indian APF since 18 cal ka BP.
• Largest valves were recorded in Indian APF during the Holocene due to upwelling.
• Larger valves in Atlantic APF during deglacial due to iron from Antarctic Peninsula.

Fragilariopsis kerguelensis (O'Meara) Hustedt is the most abundant open ocean diatom species in Southern Ocean sediments and its average valve area has recently been used to infer glacial–interglacial paleoceanographic conditions. Studies from the Atlantic sector of the Southern Ocean demonstrated how larger average valve area of F. kerguelensis during the Last Glacial compared to the interglacial possibly relate to greater availability of iron (through wider sea ice coverage and higher eolian dust input). We present here data on average valve area of F. kerguelensis from three sediment cores covering the last ∼42 cal ka BP from different zones of the Southern Ocean. Our records confirm previous results from the Atlantic sector, but highlight a different pattern from the Indian sector where the largest valves of F. kerguelensis are encountered during the Holocene. Fragilariopsis kerguelensis average valve area variations in the Antarctic Polar Front (APF) of the Atlantic sector and Subantarctic Front of the Indian sector are in phase with records of opal burial while this correlation does not hold for the APF of the Indian sector. Variations in circum-polar upwelling were suggested as the main controlling factor of opal production during the last 20,000 years. We here hypothesize that high nutrient input from the Antarctic Peninsula during the last deglaciation may have exerted a stronger control on F. kerguelensis average valve area and opal export in the Atlantic sector of the Southern Ocean than inferred changes in circum-polar upwelling.