INDONESIAN THROUGHFLOW VARIABILITY AND ITS INFLUENCE ON THE CONTINENTAL CLIMATE HISTORY OF NW AUSTRALIA DURING THE PLIO-PLEISTOCENE

Plio-Pleistocene sediments collected on IODP Expedition 356 from Site U1463 off the coast of Northwest Australia contain organic biomarkers capable of recording variability in land and sea surface temperatures (SSTs) near the outlet of the Indonesian Throughflow (ITF). The ITF affects global thermohaline circulation by regulating the movement of water from the Pacific Ocean into the Indian Ocean. Despite the importance of ITF variability on ocean circulation and global climate, few studies have been able to reconstruct the land and SSTs of this region from the Mid-Pliocene through Early Pleistocene. Here we constrain ITF variability across this interval by applying several organic geochemical proxies to sediments from Site U1463 spanning 3.5-1.5 Ma, which encompasses the mid-Pliocene warm period, characterized by comparable CO₂ concentrations and higher global temperatures relative to modern values (e.g. Dowsett et al., 1992; Raymo et al., 1996). We reconstructed NW Australian SSTs using the long chain diol index (LDI), based on ratios of diols produced by marine diatoms (Rampen et al., 2012), and the TEX₈₆ proxy, based on glycerol dialkyl glycerol tetraethers (iGDGTs) produced by mesophilic archaea (Schouten et al., 2002). We determined continental air temperatures across this interval using the MBT’_5ME proxy, based on branched GDGTs (brGDGTs) produced by a suite of unknown bacteria formed in soil sediments and likely deposited to offshore Site U1463 via aeolian transport (De Jonge et al., 2014; Weijers et al., 2007). We confirmed the continental origin of brGDGTs at Site U1463 using the #rings_tetra index (Sinninnghe Damsté, 2016). All proxy records suggest cooling from 3.5-1.5 Ma, with a more pronounced decrease in temperature from 2.4-1.5 Ma, corresponding to an arid interval identified on the same core by Christensen et al. (2017). The TEX₈₆ record strongly corresponds to the LR04 global benthic d¹⁸O stack (Lisiecki and Raymo, 2005), with strong cooling noted during Marine Isotope Stage M2. Our multi-proxy record constrains shifts in the ITF from the Mid-Pliocene to the Early Pleistocene, provides a new continental temperature reconstruction from NW Australia, and provides insight on climate forcing during the mid-Pliocene warm period.