Paleoproductivity changes in the upwelling system of Socotra (Somali Basin, NW Indian Ocean) during the last 72,000 years: evidence from biological signatures

High resolution analyses of foraminiferal and radiolarian assemblages were performed on core MD 85682 (10 degrees 53'5N-52 degrees 23'E, 3092 m depth, 7.23 m length), located close to the Socotra Island, beneath the 10 degree sN gyre of the upwelling area of the Somali Basin (NW Indian Ocean), in ord er to reconstruct the paleoproductivity changes during the last 72 kyr. Cor respondence and cluster analyses of combined planktonic foraminiferal and r adiolarian quantitative data show that the distribution pattern of the micr ofauna is partly controlled by temperature and hydrographic structures and their interrelations on water mass fertility. The continuous occurrence of high abundances of foraminiferal species well known in fertile areas such a s Globigerinita glutinata, Globigerina bulloides and Globigerina falconensi s, as well as radiolarian species characteristic of upwelling systems, test ifies to a sustained activity of the upwelling during the last 72 kyr. Down core evolution of these biological proxies suggests an increase in paleopro ductivity during isotope stages 1 and 3, which reflects a stronger upwellin g activity during these periods. These observations are in agreement with t he results previously obtained by geochemical markers such as biogenic bari um, phosphorus, organic matter, and also trace elements related to the orga nic matter and indicators of redox conditions (vanadium). Samples from isot ope stage 2 record a more complex biological and geochemical signature, res ulting from increased terrigenous input. Comparison between the biological signatures obtained both under the Socotra (10 degreesN) and Somali (5 degr eesN) upwelling areas indicates higher productivity throughout the last 72 kyr record of the Socotra upwelling system. In both areas, a similar evolut ion of biological proxies indicative of high fertility is observed during i sotope stages 1 and 3. However, minor differences in the biological and geo chemical signatures are related to local conditions prevailing beneath the two gyres. (C) 2000 Elsevier Science B.V. All rights reserved.