Thalassiosira lentiginosa size variation and associated biogenic silica burial in the Southern Ocean over the last 42 kyrs
Résumé
The diatom Fragilariopsis kerguelensis (O′Meara) Hustedt is thought to be the main carrier of biogenic silica to the Southern Ocean sea-floor, thereby attracting most investigations of diatom biometry. Thalassiosira lentiginosa (Janisch) Fryxell, a large centric diatom which is the second most abundant species in the Southern Ocean, conversely received very little attention to date. We here present the first study on modern sediments to explore the variability of the mean valve area of T. lentiginosa in relation to productivity and modern environmental conditions. Larger T. lentiginosa are observed around the Polar Front Zone with size decreasing both northward and southward as previously observed for F. kerguelensis. Such a pattern neither corresponds to the species productivity changes nor with the iron concentration in surface waters. Conversely, this pattern may be partly due to the low efficiency of Si uptake, internal transport and deposition of silicic acid under conditions which are outside of the species specific temperature range, i.e. at the lower and upper limit of the diatom ecological preferences. We also propose that T. lentiginosa mean valve area variations in four deep-sea cores across the frontal zones in the Atlantic and Indian sectors of the Southern Ocean resulted from changes in sea-surface temperatures and sea-ice presence modulating the species ability to make use nutrient stocks and controlling the length of its growing season, respectively. Our results also indicate that T. lentiginosa exported more biogenic silica to the sea-floor than F. kerguelensis over the last 42 kyrs.