The Late Quaternary evolution of water masses in the eastern Indian Ocean between Australia and Indonesia, based on benthic foraminifera faunal and carbon isotopes analyses

Abstract

Benthic foraminifera and carbon isotopes from closely spaced samples taken from three deep-sea cores were analysed to reconstruct the palaeoceanographic evolution of the eastern Indian Ocean for the last 30 kyrs, with an extension back to 60 kyrs based on one core. Benthic foraminifera were studied by means of Q-mode Factor Analysis. The benthic foraminifera accumulation rate (BFAR) and the accumulation rates (AR) of Bulimina aculeata, Epistominella exigua and Uvigerina proboscidea were calculated for determining episodes of increased organic matter supply to the sea floor. The δ 18O and δ 13C records of Cibicidoides wuellerstorfi were measured from all 3 cores to gather information about past intermediate- and deep-water circulation and changes in sea-surface palaeoproductivity. The co-variance of the organic matter supply and dissolved-oxygen levels affected the distribution of benthic foraminifera. Below a depth of 1800 m, reduced deep-water circulation was recognised by a low δ 13C of C. wuellerstorfi and increased carbon-flux rate by a high BFAR and B. aculeata, E. exigua and U. proboscidea AR, as well as by a B. aculeata faunal dominance. A more oligotrophic environment was characterised by a low BFAR and B. aculeata, E. exigua and U. proboscidea AR. Active deep-water circulation was postulated with high δ 13C values for C. wuellerstorfi and by a faunal dominance by C. wuellerstorfi. At intermediate depths (∼ 1000 m) and south of 20°S, the presence of strong bottom currents and the lateral advection of small amounts of organic matter, favoured the suspension feeder C. wuellerstorfi. Under extremely high dissolved-oxygen levels, determined by the increased influence of the Antarctic Intermediate Water (seen through high δ 13C of C. wuellerstorfi) and a reduced supply of organic matter, Nummoloculina irregularis and Globocassidulina subglobosa dominated the benthic foraminifera assemblage. The reduction of oxygen levels, a more stratified water column and the Leeuwin Current flow, along its present pattern, favoured the species U. proboscidea and Bolivina robusta. Based on these observations, the following palaeoceanographic evolution of the eastern Indian Ocean is proposed: • For 60–35 kyr BP, conditions of higher productivity (compared to the Present) at the sea surface were suggested for the Banda Sea. • For 35– 15 kyr BP, high productivity still characterised the Banda Sea. Strong and oxygenated bottom currents were present offshore Western Australia. For the Last Glacial Maximum, a reduction of deep-water circulation characterised the eastern Indian Ocean, while more active circulation was recorded at intermediate depths. Productivity also increased offshore the north coast of Western Australia • For 15–5 kyr BP, a reduction in productivity levels over the Banda Sea is related to increased atmospheric precipitation, which led to the formation of a low-salinity water cap at the sea surface. Off the north coast of Western Australia, productivity levels remained similar to those recorded for the LGM, probably due to the nutrients injected into the sea by rivers. Off the northwest coast of Western Australia, a deepened nutricline prevented any increase of organic matter supply to the sea floor. • For 5 kyr BP–Present, in the Banda Sea, productivity levels are similar to the Present. Off the Western Australian coast, an increased influence of the oxygen-depleted Indonesian Intermediate Water and the Leeuwin Current engendered a more stratified water column, characterised by lower dissolved-oxygen levels.