Indonesian Throughflow and Leeuwin Current dynamics in the Plio-Pleistocene

Abstract:

The modern Leeuwin Current (LC) carries warm equatorial waters along the western coast of Australia to higher latitudes, and therefore serves an important role in thermohaline circulation and global climate change. The strength of the LC is primarily controlled by the Indonesian Gateway and corresponding Indonesian Throughflow (ITF). Tectonic changes have caused constriction and shallowing of the Indonesian Gateway since the Pliocene, however the timing and influence of these changes on both ITF and LC dynamics remain poorly constrained. Here, we investigate ITF and LC dynamics across the Plio-Pleistocene from 3.5 to ~1 Ma, using organic geochemical proxies to reconstruct sea surface temperatures (SST), upwelling conditions and nearby continental air temperatures from the Northwest Australian shelf at IODP Site U1463, downstream of the ITF outlet and under the influence of the LC. We reconstruct SST with multiple proxies, including TEX₈₆, based on isoprenoid glycerol dialkyl glycerol tetraethers (iGDGTs), and the long-chain diol index (LDI), based on the ratio of diols produced by marine diatoms. We constrain upwelling via the Diol Index, and compare these results to total concentration of C₃₇ alkenones, a primary productivity proxy. Finally, we determine Northwest Australian continental air temperatures via the MBT’_5ME proxy, based on branched GDGTs. Results from air and sea surface temperature records confirm that SST exerted a control on Australian continental hydroclimate, as previously hypothesized by Christensen et al. (2017), and indicate significant cooling and an increase in upwelling after ~1.7 Ma. These results suggest a reduction in LC intensity at ~1.7 Ma due to a major step in ongoing Indonesian Gateway constriction and ITF reduction. Our new data suggest that an additional ITF constriction event may have occurred in the Pleistocene.