Assemblage Composition and Glacial-Interglacial Variations in Deep-Sea Ostracoda from the Eastern Equatorial Pacific (odp Site 1238) Over the Last 460 KA

We examine the benthic ostracod assemblages and their glacial-interglacial variations over the last 460 ka at ODP Site 1238 located ~200 km west of Ecuador at a water depth of 2203 m. Ages for Site 1238 were estimated by correlating its carbonate profile to cores v19-28 and v19-29, which have oxygen isotope age control. The CaCO3 profile for Site 1238 was generated by XRF scanning. For the 25-28 mcd interval of the core 1238 age was obtained by linear interpolation using a tie-point of 460 ka at 27.44 mcd. A total of three different assemblages were distinguished: glacial, interglacial and background. Glacial assemblage is characterized by higher total ostracod abundance, ostracod valve accumulation rates, and diversity. The typical glacial taxa here are Krithe spp., Legitimocythere castanea, Bradleya mesembrina, Cytheropteron spp. and Apatihowella (Fallacihowella) sol. Interglacial intervals are characterized by low abundance and diversity and the characteristic taxa are Pseudobosquetina mucronalata, Bradleya dictyon, Agrenocythere hazelae, Poseidonamicus major. A background assemblage is found throughout the entire record and consists of Krithe spp., Parakrithe sp., Ambocythere cf. sturgio and Rugocythereis sp. Taxonomic composition of the glacial assemblage suggests a deep sea environment with stronger influence of cold water from Antarctica (Circumpolar Deep Water) and higher oxygen content. Higher Corg MAR during glacials result in overall higher total abundance levels. The interglacial assemblage appears to be related to dissolution intervals. Although the characteristic taxa are not restricted to corrosive bottom waters, it is possible that only they can grow and reproduce in calcite undersaturated bottom waters. In the Equatorial Pacific interglacials are associated with increased dissolution and corrosiveness of bottom water. So, it seems natural that the low abundance low diversity interglacial assemblage of ostracods at Site 1238 reflects the interglacial climate warming and bottom water calcite undersaturation. The background assemblage is not linked to oceanographic and climate changes and possibly represents an opportunistic group of taxa, tolerant to changes in oxygen level and food availability.