Timing and Pacing of Indonesian Throughflow Restriction and Its Connection to Late Pliocene Climate Shifts

Abstract

The Pliocene was characterized by a gradual shift of global climate toward cooler and drier conditions. This shift fundamentally reorganized Earth's climate from the Miocene state toward conditions similar to the present. During the Pliocene, the progressive restriction of the Indonesian Throughflow (ITF) is suggested to have enhanced this shift toward stronger meridional thermal gradients. Reduced ITF, caused by the northward movement of Australia and uplift of Indonesia, impeded global thermohaline circulation, also contributing to late Pliocene Northern Hemisphere cooling via atmospheric and oceanographic teleconnections. Here we present an orbitally tuned high‐resolution sediment geochemistry, calcareous nannofossil, and X‐ray fluorescence record between 3.65 and 2.97 Ma from the northwest shelf of Australia within the Leeuwin Current. International Ocean Discovery Program Site U1463 provides a record of local surface water conditions and Australian climate in relation to changing ITF connectivity. Modern analogue‐based interpretations of nannofossil assemblages indicate that ITF configuration culminated ~3.54 Ma. A decrease in warm, oligotrophic taxa such as Umbilicosphaera sibogae, with a shift from Gephyrocapsa sp. to Reticulofenestra sp., and an increase of mesotrophic taxa (e.g., Umbilicosphaera jafari and Helicosphaera spp.) suggest that tropical Pacific ITF sources were replaced by cooler, fresher, northern Pacific waters. This initial tectonic reorganization enhanced the Indian Oceans sensitivity to orbitally forced cooling in the southern high latitudes culminating in the M2 glacial event (~3.3 Ma). After 3.3 Ma the restructured ITF established the boundary conditions for the inception of the Sahul‐Indian Ocean Bjerknes mechanism and increased the response to glacio‐eustatic variability.

A 5.3‐Million‐Year History of Monsoonal Precipitation in Northwestern Australia

“…In addition, Andrae et al () in their pollen studies on core ODP 763A reveal the onset of Australian C 4 ‐plant expansion at ~3.5 Ma (Figure ). This timing coincides almost exactly with inferred changes in the Indonesian Throughflow and Leeuwin‐Current dynamics recognized in core U1463 by Auer et al (). Our records show a much earlier shift in the elemental ratio records after 3.8 Ma.…”

“…The main trends interpreted from our records are shown, in comparison with the global benthic δ 18 O stack (Lisiecki & Raymo, ). The major shift in C 4 plants implying onset of summer monsoon climate in Australia at ~3.5 Ma, as described by Andrae et al (), and the reorganization of the ITF and LC north of Australia by Auer et al () due to tectonic changes are also shown. See text for further discussion on the chronology of such change.…”

Toward a Robust Plio‐Pleistocene Chronostratigraphy for ODP Site 762

“…But Stuut et al (2019) place the onset of the aridification in Australia~300 kyr earlier (~3.8 Ma) at Site 762. The authors ascribe the difference to insufficient time resolution at Site 763 (Andrae et al, 2018) or to the fact that the Site U1463 records (Auer et al, 2019) do not extend far enough back in time. Finally, Stuut et al (2019) argue that the smoothed wireline K wt.% long-term Pliocene record at Site U1463 (Christensen et al, 2017) may be too imprecise to pinpoint this shift.…”

“…With this commentary, we highlight the issues that arise in comparative studies when all available chronostratigraphic data are not integrated or updated. We show the importance of an approach that integrates all chronostratigraphic information by comparing several Plio‐Pleistocene paleoclimate archives in the eastern Indian Ocean (Andrae et al, ; Auer et al, ; Christensen et al, ; De Vleeschouwer et al, , ; Karas et al, ; Sniderman et al, ; Stuut et al, ). These records provide insights into the climate evolution of Australia and the role of the Indonesian Throughflow between 2 and 5.5 Myr.…”

Stepwise Weakening of the Pliocene Leeuwin Current

“…Based on Site U1463, Christensen et al (2017) identified a major reorganization of Indian Ocean circulation around 3.3 Ma. Subsequently, De Vleeschouwer et al (2018) used U1463 to detail a significant contraction of the LC at exactly that time and Auer et al (2019) demonstrated the connection with the Sahul-Indian Ocean Bjerknes mechanism (Bjerknes, 1969;Di Nezio et al, 2016). However, the Pliocene variability of the LC south of the North West Cape remains undocumented to date and the effect of ITF variability on Indian Ocean circulation remains debated.…”