Glacial valve size variation of the Southern Ocean diatom Fragilariopsis kerguelensis preserved in the Benguela Upwelling System, southeastern Atlantic

The valve size variations of the Southern Ocean (SO) diatom Fragilariopsis kerguelensis from the sediments of Antarctic Polar Front and Subantarctic Front were shown to be strongly correlated with iron availability at the glacial-interglacial timescale. However, recent evidence from the subtropical region of the SO has suggested sea surface temperature to be a key controlling factor by mediating silica uptake other than the iron availability. We present here the first record of F. kerguelensis size variation in core GeoB3606-1 collected in the Benguela Upwelling System (BUS), SE Atlantic (25°28′S, 13°05′E) where F. kerguelensis valves were preserved during the glacial period (68-30 ka) as a consequence of silicic acid leakage from polar to mid-latitudes. Our study presents the first record of diatom morphology variations of F. kerguelensis from a mid-latitude core location to test the previous hypotheses proposed for SO. Our results demonstrate that F. kerguelensis valves at GeoB3606-1 were almost half of the mean size of specimens compared to the SO downcore records. Further, F. kerguelensis valves were smaller and more abundant during early Marine Isotope Stage (MIS) 3 and conversely larger and less abundant during MIS4 and late MIS3. However, such patterns are not supported by the linear regression analyses suggesting that iron availability could have mediated the size changes of F. kerguelensis rather than the species productivity during 68-30 ka. These findings agree with the recently gained knowledge on the life cycle patterns of F. kerguelensis related to reproduction strategies, cell size range, and formation of initial cells.