Paleoceanographic and paleoclimatic history of the Somali Basin during the Pliocene-Pleistocene; multivariate analyses of benthic foraminifera from DSDP Site 241

Abstract

This study quantitatively examines benthic foraminifera (>125mu m) from 40 Pliocene-Pleistocene samples from Deep Sea Drilling Project (DSDP) Site 241, Somali Basin. R-mode factor and Q-mode cluster analyses of the 26 highest ranked species define five significant factor assemblages and five clusters reflecting different environmental characteristics in the Plio-Pleistocene sequence. Assemblage 1, consisting of Bulimina marginata, Nonionella japonicum, Cassidulina carinata, and Uvigerina proboscidea, reflects high flux of organic matter from high surface productivity and dominates Custer II. Eggerella bradyi, Melonis pompilioides, and Oridorsalis umbonatus constitute Assemblage 2, which is inferred to reflect intermediate flux of relatively degraded organic matter (Cluster IV). Assemblage 3, characterized by Cibicides wuellerstorfi and Epistominella exigua, coincides with Cluster I. This assemblage indicates increased oxygenation and strongly pulsed food supply. Assemblage 4, marked by Loxostomum karrerianum (= Bolivina karrerianum), Trifarina angulosa and Bolivina spathulata, corresponds to Cluster III and suggests an oxygen-deficient, organic carbon-rich environment. The principal species of Assemblage 5 is Nuttallides umbonifera, with Astrononion umbilicatulum and Epistominella exigua as secondary species. This assemblage, which corresponds to Cluster V, suggests low food, high oxygen and increased bottom water corrosiveness. Mean test diameters of the planktic foraminifer Orbulina universa were used to interpret sea-surface temperature history at Site 241. Based on earlier observations that a positive correlation exists between the mean test diameter of O. universa and surface water temperature, several intervals of cool and warm sea-surface temperatures have been interpreted in the Pleistocene. The Pliocene surface water temperatures were less variable than those of the Pleistocene, indicating increased monsoonal activity in the younger interval.