Paleoceanography for the last 195,000 years in the Solomon Sea (ODP Site 1109) by means of calcareous nannofossils

Abstract

The paleoceanography of the Solomon Sea for the last 195,000 years was investigated, based on the observations of 21 taxonomic groups of calcareous nannofossils in 144 samples taken from ocean drilling program (ODP) cores 1109A-1H and 1109C-2H. Oxygen isotope ratios in tests of the planktonic foraminifer Globigerinoides ruber allow us to assign the cores to oxygen isotope stages 7–1. Past sea-surface temperatures (SST) were estimated qualitatively using the composition of three Gephyrocapsa coccolith morphotypes; the high-, intermediate- and low-angle types that are classified based on the bridge angle of coccolith, can be used as proxies for warm, intermediate and cold temperatures, respectively. Abundance of the high-angle type coccoliths increased and decreased during the interglacial and glacial periods, respectively. This tendency is consistent with the expected change of the Western Pacific Warm Pool (WPWP) development. However, a large shift in the abundance of the high-angle type coccolith recorded during early stage 3 did not correspond to the oxygen isotope record. The ratio of small (<2.5 μm) placoliths of Reticulofenestra and Gephyrocapsa to the lower-photic species was higher during stage 2 than late stage 3 and 1. As the reduced blanketed effect of the WPWP would cause an increase in the number of small placoliths, a reduction of the WPWP in stage 2 is indicated by the increase of small placoliths and by the lower SST deduced from the composition of the Gephyrocapsa morphotypes. The decreasing trend of small placoliths after ∼40 ka is similar to the results reported in several previous studies in the surrounding regions of Australia.