RESEARCH ARTICLES CURRENT SCIENCE, VOL. 91, NO. 11, 10 DECEMBER 2006 1495 *For correspondence. (e-mail: rahulmohan@ncaor.org) Spatial distribution of diatoms in surface sediments from the Indian sector of Southern Ocean Rahul Mohan*, S. Shanvas, M. Thamban and M. Sudhakar National Centre for Antarctic and Ocean Research, Headland Sada, Goa 403 804, India A multidisciplinary scientific expedition to the Southern Ocean (Pilot Expedition to the Southern Ocean – PESO) onboard ORV Sagar Kanya during the austral summer of 2004 collected various physical, chemical, biological and geological data/samples. From the sedi- ment cores collected during the expedition, six repre- sentative core-top samples were studied along a lati- tudinal transect from 28° to 56° south to ascertain the modern variation in distribution of siliceous microfossils called diatoms. This is the first Indian attempt to un- derstand the latitudinal variation in the distribution of diatom species in Southern Ocean, its relationship with the changing nutrient availability and/or supply, and its utility in palaeoceanographic reconstruction. In all, 24 diatom species were identified. The diatom population seems to be dominated by seven species namely Fragilariopsis kerguelensis, Fragilariopsis separanda, Thalassionema nitzschioides, Thalassiothrix spp. , Thalassiosira lentiginosa, Eucampia antarctica and Azpeitia tabularis. Of these, F. kerguelensis and T. lentiginosa dominate the diatom community in the Southern Ocean sediments. The spatial distribution of most of the diatoms in surface sediments seems to be controlled by physicochemical parameters like sea sur- face temperature, salinity, silicate, nitrate and phos- phate concentrations. Keywords: Diatoms, nutrient, sediment, sea surface tempe- rature, Southern Ocean. THE Southern Ocean being a sink and source for major global oceanic water masses there is lot of attention to study not only the modern changes, but also the past vari- ability using sedimentary records. The Indian sector of the Southern Ocean is yet to be studied in detail to under- stand the past climatic oscillations and its implications for the future changes 1–3 . For the last several decades, scien- tists have extensively used faunal and floral records of Southern Ocean sediments to infer the past changes in biological productivity as well as changes in sea surface temperature (SST) and salinity 4,5 . Since the accuracy of the climatic reconstructions greatly depends on the qual- ity of proxy database, it is necessary to assess the re- sponse of such proxies to modern environmental changes before carrying out any reconstructions. Due to the uni- que characteristics of Southern Ocean, the most important and predominant biogenic proxies preserved in the sedi- mentary records of Southern Ocean are the siliceous micro- fossils called ‘diatoms’. Diatoms are unicellular algae made of siliceous skeleton called frustules and are found in almost every aquatic en- vironment including fresh and marine waters. Their useful- ness in Southern Ocean is because of its extreme sensitivity to changes in salinity, temperature, nutrient supply and other environmental factors. Because their cell wall is composed of hydrated silica [Si(H 2 O) n ], they are well preserved in the sediments. Diatoms contribute more than 70% of the primary production in Southern Ocean and play a major role in global silica and carbon cycling 6 . Their size ranges from 2 to 200 μm and they exhibit a wide variety of shapes. The preservation and abundance of diatoms in marine sedimentary records find its use in palaeoceanographic reconstruction, particu- larly in the reconstruction of past SST 7,8 , and more re- cently in the estimation of past sea ice extent 9–12 . The distribution of diatoms is chiefly influenced by the pres- ence of nutrients like silicate, nitrate, phosphate 13 , SST 13,14 and the stability of the water column 15 . In the present study, we identified the distribution patterns of fossil diatoms in core-top samples recovered from the Indian sector of Southern Ocean between 28 and 55°S lat. The relative abundances of seven relevant diatom species from the core-top samples were studied to document the distribution patterns of diatoms with respect to various oceanic fronts prevalent in the Southern Ocean, its relation to the SST, salinity, nutrients as well as the extent of the winter sea-ice. Oceanographic setting The dynamics of the Southern Ocean is governed by the strong westerly winds resulting in the clockwise Antarctic Circumpolar Current (ACC) 16 . The ACC, sometimes called the ‘great ocean conveyor’, connects the three major ocean basins – Atlantic, Pacific and Indian – allowing water, heat, salt and other properties to ‘flow’ from one to the other.