Stepwise Weakening of the Pliocene Leeuwin Current

The Indonesian Throughflow (ITF) operates as an important link in global thermohaline circulation, and ITF variability probably modulated Pliocene climate change. Yet, whether ITF variability accounted for oceanographic change south of Northwest Cape remains controversial. Here, we present a multiproxy oceanographic reconstruction from the Perth Basin and reconstruct the Pliocene history of the Leeuwin Current (LC). We show that the LC was active throughout the Pliocene, albeit with fluctuations in intensity and scope. Three main factors controlled LC strength. First, a tectonic ITF reorganization caused an abrupt and permanent LC reduction at 3.7 Ma. On shorter timescales, eustatic sea level and direct orbital forcing of wind patterns hampered or promoted the LC. At 3.3 Ma, for instance, LC intensity plunged in response to a eustatic ITF restriction. Site U1459 then fell outside the extent of a weakened LC, and the latitudinal sea surface temperature gradient along West Australia doubled its steepness. Plain Language Summary The Leeuwin Current (LC) transports warm, low-salinity, nutrient-deficient water poleward along Australia's west coast. The current is remarkable because eastern boundary currents usually flow equatorward (e.g., Benguela, California Current). The LC extends modern coral reef development to 29 degrees S, but its geological history remains controversial. We use a sediment core (International Ocean Discovery Program Site U1459) from the Perth basin to reconstruct the Pliocene history of the LC. Based on variations in the multiproxy geochemical records of Site U1459 and comparison with contemporaneous sediment cores off northwest Australia, we show that poleward flowing warm waters reached the site throughout the Pliocene. The current's intensity exhibits a stepwise decline at 3.7 Ma though, in response to a tectonic reorganization around Indonesia, and a short-lived dramatic downturn during global glaciation event marine isotope stage M2 (3.3 Ma), in response to a sea level drop.