The Leeuwin Current System during the Early Middle Pleistocene Transition (EMPT): foraminiferal assemblage and sea level reconstruction

The Early Middle Pleistocene Transition (EMPT) represents a fundamental reorganization in Earth’s climate system as the obliquity-dominated glacial/interglacial rhythmicity characterizing the Quaternary got progressively replaced by a high-amplitude, quasi-periodic 100 kyr cyclicity. This critical change in the climatic response to orbital cycles occurred without proportional modifications in the orbital-forcing parameters before or during the EMPT, implying a substantial change internal to the climate system. The EMPT had a severe impact on marine ecosystems. However, the trigger mechanisms and the components of the climate system involved in this global reorganization are still under debate, and high-resolution studies from the equatorial to mid-latitude shelf regions are at present rarely available.

In this study, we analyze the benthic foraminifera assemblage of an expanded section from Site U1460 (eastern Indian Ocean, 27°22.4949′S, 112°55.4296′E, 214.5 meters water depth), collected during International Ocean Discovery Program (IODP) Expedition 356 on the southwestern Australian shelf covering the EMPT. At this site, we provide a new benthic and planktonic foraminifera dataset to better define the response of the Leeuwin Current System during the EMPT on low to mid latitude shelf regions that are strongly sensitive to glacial/interglacial sea-level oscillations. Specifically, benthic foraminifera assemblage and the plankton/benthos (P/B) ratio are used to understand the bottom water community and its reaction to the Leeuwin Current System variations during the EMPT. Additionally, these data will untangle the local impact of eustatic sea-level changes in a highly dynamic setting.

Preliminary data of the microfossil content revealed a polyspecific benthic foraminifera assemblage with high diversity. The most abundant taxa are trochospiral forms (e.g., Cibicides, Cibicidoides, Heterolepa, Nuttallides, Eponides). Triserial and biserial taxa are abundant (e.g., Textularia, Spirotextularia, Gaudryina, Bolivina, Uvigerina). Planispiral tests such as Melonis and Lenticulina are also commonly present, as well as uniserial ones such as Siphogenerina, Lagena, and Cerebrina. Preservation varies significantly between glacial and interglacial intervals. Particularly, benthic foraminifera are poorly to moderately preserved during glacial stages while exhibiting moderate to good preservation in the interglacials. The variations in the P/B ratio allowed to constrain the sea-level changes along the Australian shelf. Specifically, higher and lower values of this ratio indicate highstand and lowstand phases, respectively. In this regard, foraminifera data will be integrated in a multiproxy dataset available for Site U1460 to obtain new insights on sea-level-driven environmental changes in the area during the EMPT. This, in turn, will allow to resolve the impact of local versus global climatic change across the studied interval.