Plio-Pleistocene Perth Basin water temperatures and Leeuwin Current dynamics (Indian Ocean) derived from oxygen and clumped-isotope paleothermometry
Authors: De Vleeschouwer David , Peral Marion , Marchegiano Marta , Füllberg Angelina , Meinicke Niklas , Pälike Heiko , Auer Gerald , Petrick Benjamin , Snoeck Christophe , Goderis Steven , Claeys Philippe 
Published: June 2022

Abstract. The Pliocene sedimentary record provides a window into Earth's climate dynamics under warmer-than-present boundary conditions. However, the Pliocene cannot be considered a stable warm climate that constitutes a solid baseline for middle-of-the-road future climate projections. The increasing availability of time-continuous sedimentary archives (e.g., marine sediment cores) reveals complex temporal and spatial patterns of
Pliocene ocean and climate variability on astronomical timescales. The Perth
Basin is particularly interesting in that respect because it remains unclear
if and how the Leeuwin Current sustained the comparably wet Pliocene climate
in Western Australia, as well as how it influenced Southern Hemisphere
paleoclimate variability. To constrain Leeuwin Current dynamics in time and
space, this project obtained eight clumped-isotope Δ47
paleotemperatures and constructed a new orbitally resolved planktonic
foraminifera (Trilobatus sacculifer) stable isotope record (δ18O) for the Plio-Pleistocene (4–2 Ma) interval of International Ocean Discovery Program (IODP) Site U1459. These new data complement an existing TEX86 record from the same site and similar planktonic isotope records from the Northern Carnarvon Basin (Ocean Drilling Program (ODP) Site 763 and IODP Site U1463). The comparison
of TEX86 and Δ47 paleothermometers reveals that TEX86
likely reflects sea surface temperatures (SSTs) with a seasonal warm bias
(23.8–28.9 ∘C), whereas T. sacculifer Δ47 calcification temperatures probably echo mixed-layer temperatures at the studied Site U1459 (18.9–23.2 ∘C). The isotopic δ18O gradient along a 19–29∘ S latitudinal transect, between 3.9
and 2.2 Ma, displays large variability, ranging between 0.5 ‰ and 2.0 ‰. We use the latitudinal δ18O gradient
as a proxy for Leeuwin Current strength, with an inverse relationship
between both. The new results challenge the interpretation that suggested a
tectonic event in the Indonesian Throughflow as the cause for the rapid
steepening of the isotopic gradient (0.9 ‰ to 1.5 ‰)
around 3.7 Ma. The tectonic interpretation appears obsolete as it is now
clear that the 3.7 Ma steepening of the isotopic gradient is intermittent,
with flat latitudinal gradients (∼0.5 ‰) restored in the latest Pliocene (2.9–2.6 Ma). Still, the new analysis affirms that a combination of astronomical forcing of wind patterns and eustatic sea level controlled Leeuwin Current intensity. On secular timescales, a period of relatively weak Leeuwin Current is observed between 3.7 and 3.1 Ma. Notably, this interval is marked by cooler conditions
throughout the Southern Hemisphere. In conclusion, the intensity of the
Leeuwin Current and the latitudinal position of the subtropical front are
both long-range effects of the same forcing: heat transport through the
Indonesian Throughflow (ITF) valve and its propagation through Indian Ocean
poleward heat transport. The common ITF forcing explains the observed
coherence of Southern Hemisphere ocean and climate records.