Late Quaternary palaeoceanography near Tasmania, southern Australia

Abstract

Abstract This study of the palaeoceanographic record south of Australia is based on two cores taken from lower Circumpolar Deep Water depths: core E27-30, located south of Tasmania in water depth of 3552 m, to the north of the present location of the Subtropical Convergence, and core E55-6 from the continental margin off Victoria at 2346 m depth, in a region of very low surface productivity. A preliminary examination of planktonic foraminiferal faunas in core E27-30, located south of Tasmania, provides a record of movement of the Subtropical Convergence through the late Quaternary that is consistent with records elsewhere in the Southeast Indian Ocean. The Convergence has been located equatorward of its present position through much of the late Quaternary, with poleward excursions in the Holocene and at oxygen-isotope stage 5e. Benthic foraminiferal faunas contrast strongly in the two cores, despite their location at similar depths within the same water mass. Core E55-6 is largely dominated by deep infaunal taxa, while core E27-30 has a high proportion throughout of epifaunal taxa. The benthic foraminiferal fauna in core E27-30 is dominated by Epistominella exigua, known to feed directly on phytoplankton detritus in the modern deep-sea. Abundances of that species coincide with the presence of the Subtropical Convergence at or near the site, indicating that surface phytoplankton production has had a major influence on benthic productivity. Dissolution records in the two cores also differ markedly. Core E55-6 records a strong dissolution event in oxygen-isotope stage 4. The cause of the event is not clear, although it appears to be associated with increased productivity levels. This pronounced dissolution event is not evident in core E27-30, suggesting either the event is localised or that the record in core E27-30 has been overprinted by the Convergence. The extent to which the records in the two cores vary emphasises the influence of frontal structures on core records in this region and the need to take this into account when investigating Southern Ocean history.