Ocean gateways facilitate water circulation between ocean basins, and therefore directly impact thermohaline circulation and global climate. In order to better predict the effects of future climate change, it is critical to constrain past changes in ocean gateway behavior, and corresponding changes in thermohaline circulation, particularly during analogue periods for modern climate change. The Indonesian Throughflow (ITF) is a primary ocean gateway and vital component of the global conveyor that transports water from the Pacific Ocean into the Indian Ocean, however due to a lack of long and continuous sedimentary records from locations under its influence, changes in ITF behavior remain poorly constrained. In this study organic geochemical biomarkers preserved in marine sediments are used to reconstruct both sea surface and continental air temperatures in Northwest (NW) Australia from sediments spanning the mid-Pliocene Warm Period (mPWP), a critical carbon dioxide (CO2) and temperature analogue period for modern climate change spanning 3.3-3.0 Ma. These sediments were collected during IODP Expedition 356 from Site U1463, located near the outlet of the ITF, and are therefore sensitive to changes in ITF behavior over time. Here, NW Australian air temperatures were reconstructed from 1.5-3.5 Ma using the MBT’5ME proxy (Weijers et al., 2007a; De Jonge et al., 2014a), and offshore sea surface temperatures (SSTs) were reconstructed using both the TEX86 proxy (Schouten et al., 2002; Tierney & Tingley, 2014) and the Long Chain Diol Index (LDI; Rampen et al., 2012). Global climate events, including Marine Isotope Stages (MIS) 55, 63, 64, 82, 84, 88, 92, G10, G18, G20, G22, and M2 (Lisiecki & Raymo, 2005) are apparent in all of our records. TEX86 SSTs suggest a stronger cooling signal during MIS Stages G18, G20 and G22 relative to cooling during MIS M2, however LDI SSTs do not yield the same result. Overall, all three proxies indicate higher temperatures across the Pliocene and a cooling trend from ~1.7-1.5 Ma. Cooling occurs during an arid interval identified by Christensen et al. (2017), from 2.4-1.0 Ma, which suggests that offshore cooling contributed to shifts in NW Australian continental hydrology. Cooling from 3.5-1.5 Ma at Site U1463 was likely a reflection of 1) constriction of the ITF from 5-2 Ma and a switch from warm South Pacific to cool North Pacific source waters, and 2) an increase in meridional SST gradients at 1.8 Ma; the particularly strong cooling signal identified in all three records at 1.7 Ma is likely a direct response to the latter. This study helps elucidate ITF variability and shifts in thermohaline circulation across the Plio-Pleistocene and the mPWP, which will help modelers better predict the effects of future climate change.

Yes / The Mg/Ca ratios of biogenic calcite is commonly seen as a valuable palaeo-proxy for reconstructing past ocean temperatures. The temperature dependence of Mg/Ca ratios in bivalve calcite has been the subject of contradictory observations. The palaeoceanographic use of a geochemical proxy is dependent on initial, rigorous calibration and validation of relationships between the proxy and the ambient environmental variable to be reconstructed. Shell Mg/Ca ratio data are reported for the calcite of two bivalve species, Mytilus edulis (common mussel) and Pecten maximus (king scallop), which were grown in laboratory culturing experiments at controlled and constant aquarium seawater temperatures over a range from 10 to 20 C. Furthermore, Mg/Ca ratio data of laboratory- and fieldgrown M. edulis specimens were compared. Only a weak, albeit significant, shell Mg/Ca ratio¿temperature relationship was observed in the two bivalve species: M. edulis (r2=0.37, p<0.001 for laboratory-cultured specimens and r2=0.50, p<0.001 for field-cultured specimens) and P. maximus (r2=0.21, p<0.001 for laboratory-cultured specimens only). In the two species, shell Mg/Ca ratios were not found to be controlled by shell growth rate or salinity. The Mg/Ca ratios in the shells exhibited a large degree of variability among and within species and individuals. The results suggest that the use of bivalve calcite Mg/Ca ratios as a temperature proxy is limited, at least in the species studied to date. Such limitations are most likely due to the presence of physiological effects on Mg incorporation in bivalve calcite. The utilization is further limited by the great variability both within and among shells of the same species that were precipitated under the same ambient conditions