Subtropical Front fluctuations south of Australia (45°09′S, 146°17′E) for the last 130 ka years based on calcareous nannoplankton

Abstract

Calcareous nannoplankton assemblages from a Late Quaternary deep-sea core (GC07; 46°09′S, 146°17′E) south of Australia provide information on regional palaeoceanography and palaeoclimate changes in the Southern Ocean, in particular the movement of the Subtropical Front for the past 130 ka years. Marine Isotope Stages 1–5 are identified through changes in calcareous nannoplankton assemblages, supported by 14C dates, and oxygen isotope and %CaCO data. Two distinct assemblages are recognised: a warm water assemblage with higher abundances of Calcidiscus leptoporus, Emiliania huxleyi, Helicosphaera.carteri, Syracosphaera pulchra, Gephyrocapsa caribbeanica and Gephyrocapsa oceanica; and, a cold water assemblage with higher abundances of Gephyrocapsa muellerae and Coccolithus pelagicus. Alternation between these two assemblages downcore in GC07 reflect movement of the Subtropical Front across the location and can be correlated to Marine Isotope Stages (MIS) 1–5. Sediments with a cold water assemblage indicate the position of the Subtropical Front equatorward of the site when transitional to sub-antarctic waters were overlying the site. Conversely sediments with a warm water assemblage indicate the Subtropical Front was poleward of GC07 when warmer, subtropical waters were over the site. MIS 1 and 5 are interpreted as warmer than MIS 3 (based on species composition) with the Subtropical Front more poleward than for MIS 3. During MIS 3 the Subtropical Front is interpreted as adjacent to or immediately poleward of GC07. Some species including C. leptoporus and C. pelagicus show negative covariance and are considered to be reliable species in identifying glacial and interglacial intervals in this region. Comparison with established biostratigraphy based on calcareous nannoplankton showed the datum event for the reversal between E. huxleyi and G. muellerae of 73 ka in transitional waters is not applicable in this region. The reversal between these two species occurs between 48 and 30 cm downcore in GC07 with a 14C date of 11 020 year BP at 49–48 cm, i.e. the reversal event is younger than this date.