The Early-Middle Pleistocene Transition in the eastern Indian Ocean: foraminiferal assemblage, sea-level fluctuations and climate variability on the SW Australian shelf

The Early-Middle Pleistocene Transition (EMPT; 1.4-0.4 Myrs) stands out as one of the most studied intervals in Earth’s recent climate history. Specifically, during the EMPT an increase in the amplitude of climatic fluctuations is registered without proportional changes in the orbital cycles. Nevertheless, the mechanisms and the climatic components responsible for the onset of the EMPT are still under debate.

As high-resolution studies on equatorial to mid-latitude microfossil assemblages during the EMPT are still limited, we performed a detailed benthic foraminifera assemblage work on samples from Site U1460 (eastern Indian Ocean, 27°S, 112°E, 214.5 m water depth), cored during IODP Expedition 356 on the SW sector of the Australian shelf. This site is interested by the presence of warm, calcium carbonate-supersaturated waters, and thus is strongly affected by early-marine diagenesis. Moreover, the studied cores are under the influence of the Leeuwin Current (LC), which, in turn, represents a warm and oligotrophic surface water mass, flowing poleward along the Australian coastline.

Preliminary data record a polyspecific benthic foraminiferal assemblage with high diversity. The first part of the record (MIS 27-23) is dominated by Cibicidoides spp., Heterolepa spp., Trifarina spp., and nodosarids, whereas the most recent cores (MIS 22-16) recorded abundant agglutinated tests (e.g., Gaudryina spp., Spiroplectinella spp., Textularia spp., Spirotextularia spp.), Cibicidoides spp., Siphogenerina spp., Uvigerina spp., and bolivinids. Other common taxa detected throughout the whole record are Lenticulina spp. and lagenids.

Foraminiferal tests resulted to be severely encrusted and their preservation strongly varies between glacial and interglacial intervals. Specifically, benthic specimens show poor to moderate preservation during glacials whereas their preservation increases during interglacial stages.

In addition, planktonic foraminifera were also picked to record the variations of the plankton/benthos (P/B) ratio during the studied time interval. This ratio reflects the eustatic fluctuations of the sea level in the region. Particularly, highstand and lowstand stages correspond to higher and lower values of the P/B ratio, respectively. Furthermore, constraining sea-level variability at Site U1460 will allow detailed reconstructions of LC current behavior during the EMPT. The abovementioned data, together with the evaluation of the benthic assemblage, will permit the determination of the paleobathymetry, as well as of the bottom water conditions at the studied site. The environmental reconstruction based on the benthic foraminifera assemblage during the glacial/interglacial phases will be used to assess the local climatic variations during the EMPT.