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

Abstract

High resolution analyses of foraminiferal and radiolarian assemblages were performed on core MD 85682 (10°53′5N–52°23′E, 3092 m depth, 7.23 m length), located close to the Socotra Island, beneath the 10°N gyre of the upwelling area of the Somali Basin (NW Indian Ocean), in order to reconstruct the paleoproductivity changes during the last 72 kyr. Correspondence and cluster analyses of combined planktonic foraminiferal and radiolarian quantitative data show that the distribution pattern of the microfauna 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 as Globigerinita glutinata, Globigerina bulloides and Globigerina falconensis, as well as radiolarian species characteristic of upwelling systems, testifies to a sustained activity of the upwelling during the last 72 kyr. Downcore evolution of these biological proxies suggests an increase in paleoproductivity during isotope stages 1 and 3, which reflects a stronger upwelling activity during these periods. These observations are in agreement with the results previously obtained by geochemical markers such as biogenic barium, phosphorus, organic matter, and also trace elements related to the organic matter and indicators of redox conditions (vanadium). Samples from isotope stage 2 record a more complex biological and geochemical signature, resulting from increased terrigenous input. Comparison between the biological signatures obtained both under the Socotra (10°N) and Somali (5°N) upwelling areas indicates higher productivity throughout the last 72 kyr record of the Socotra upwelling system. In both areas, a similar evolution of biological proxies indicative of high fertility is observed during isotope stages 1 and 3. However, minor differences in the biological and geochemical signatures are related to local conditions prevailing beneath the two gyres.