The Indonesian Throughflow (ITF) controls the oceanic flux of heat and salt between the Pacific and Indian Oceans and therewith plays an important role in modulating the meridional overturning circulation and low latitude hydrological cycle. Here, we report new sea surface temperature and aridity records from the west coast of Australia (IODP Site U1460), which allow us to assess the sensitivity of the eastern Indian Ocean to the major reorganization of Earth’s climate that occurred during the Mid-Pleistocene Transition. Our records indicate glacial coolings at 1.55 and 0.65 million years ago that are best explained by a weakening of the ITF as a consequence of global sea level and tectonic changes. These coincide with the development of pronounced gradients in the carbon isotope composition of the different ocean basins and with substantial changes in regional aridity, suggesting that the restrictions of the ITF influenced both the evolution of global ocean circulation and the development of the modern hydrological cycle in Western Australia.

Non‐skeletal carbonate grains are classically interpreted to form in shallow, tropical environments. Peloids deposited in deep, subtropical marine conditions are poorly studied. IODP site U1460 on the subtropical Carnarvon Ramp (Southwest Shelf of Australia) recovered a nearly continuous Pliocene to Recent record of outer shelf and slope sediments. The relative abundance of peloids varies between 0% and 67% of the fine to medium sand fraction, and contributes on average ~4% of all grains. The origin and composition of these peloids were investigated using scanning electron microscopy equipped with an energy‐dispersive X‐ray spectrometer, light microscopy, X‐ray diffraction and stable isotope analysis. The peloids have a uniform size and shape and are interpreted as faecal pellets. They are mainly composed of skeletal fragments such as ascidian spicules, planktic foraminifera and sponge spicules in a mud‐sized matrix containing abundant coccolith plates. Mineralogical analysis show that the pellets consist of aragonite, calcite and dolomite. The pellets have an identical mineralogical composition and skeletal assemblage as the surrounding matrix, indicating that they have formed in situ. They occur more abundantly during interglacials when the site was situated in deeper waters below the swell wave base, presumably because the pellets were protected from disintegration and therefore available for cementation. The presence of framboidal pyrite within the pellets indicates bacterial sulphate reduction (BSR). The reduction of iron by hydrogen sulphide produced during BSR decreases the pH and likely explains the observed aragonite dissolution. Aragonite dissolution likely increases the alkalinity, and in consequence causes the precipitation of calcite and dolomite cements. It is suggested here that pellets are hardened due to this early cementation close to the sea floor increasing the potential for preservation in the fossil record.

In the last decades, the understanding of temperate carbonate systems has improved considerably, but their development over glacial-interglacial timescales is still understudied in comparison to their tropical counterparts. A key question is how do temperate carbonate platforms respond to high-amplitude, glacial-interglacial sea-level changes? Integrated Ocean Drilling Program Site U1460 was drilled at the uppermost slope of the Southwest Shelf of Australia at the transition between the subtropical Carnarvon Ramp and the warm-temperate Rottnest Shelf. The origin and composition of the sediments in the upper 25 m below seafloor at Site U1460 were investigated using X-ray diffraction, scanning electron, and light microscopy. The Middle Pleistocene to Holocene sequence at Integrated Ocean Drilling Program Site U1460 contains a record of sea-level controlled sedimentary cycles. Carbonate sediments deposited during interglacial sea-level highstands (Marine Isotope Stages 1, 5, most of 7, 9 and 11) are mainly fine-grained (<63 µm) and dominated by low-Mg calcite from pelagic bioclasts such as planktic foraminifera. The glacial lowstand intervals (Marine Isotope Stages 2 to 4, 6, 8, 7d, 10 and 12), instead are coarsergrained and relatively rich in aragonite and high-Mg calcite from neritic bioclasts, such as bryozoans. These changes in texture, mineralogy and composition are best explained by the deposition of neritic bioclasts closer to the shelf edge during glacial sea-level lowstands. During early transgression, reworking of bioclast-rich coastal dune deposits likely leads to transport and redeposition of neritic clasts on the upper slope. In contrast, dominantly pelagic sediments characterize deposition at the platform edge during interglacial highstands. These results highlight regional differences in the response of temperate carbonate systems to sea-level change: A previously published model developed for early Pleistocene temperate carbonates from the Great Australian Bight indicates that shelfal material was exported to the upper slope during sea-level highstands. It is argued that this difference is related to the change in duration and amplitude of glacial-interglacial sea-level cycles before and after the Mid-Pleistocene transition.

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Aragonite and high‐Mg calcite are abundant in modern, neritic carbonate systems but almost absent in their fossil counterparts. Dissolution of these metastable mineral phases commonly leaves no visible trace in the sedimentary record, compromising the derivation of palaeoenvironmental information from the rock record. The upper 25 m of Integrated Ocean Drilling Program (IODP) Site U1460 on the outer ramp of the western Australian Shelf were investigated to study shallow burial (tens of metres) marine diagenesis in organic‐carbon poor sediments using microscopic, total organic carbon, biomarkers and mineralogical analysis in combination with porewater geochemistry. Aragonite dissolution is negligible at the seafloor but intensifies ca 5 m below, even though bulk porewaters are supersaturated for aragonite. This apparent contradiction likely results from dissolution in undersaturated microenvironments. Aragonite dissolution below 5 to 6 m is on average more intense in interglacial compared to glacial intervals. The presence of disseminated framboidal pyrite and porewater results indicate that minor sulphate reduction is active at IODP Site U1460. Sulphate reduction is probably limited by the low organic matter content (ca 0.2%). It is well‐known from the literature that incipient sulphate reduction can lead to a drop in pH and consequently to carbonate dissolution. It is therefore assumed that the slightly higher concentration of organic matter in the interglacial intervals allowed increased aragonite dissolution during sulphate reduction compared to glacial beds. Low amounts of dolomite cement (<15%) start to form at the same depth (5 to 6 m) as aragonite dissolution intensifies. Dolomite formation and aragonite dissolution also show covariance on a metre‐scale below 5 to 6 m, indicating that a low carbonate saturation state might enhance dolomite formation. This mechanism provides an indirect link between dolomite formation, aragonite dissolution and orbital cycles. The outcome of this study, therefore, contributes to a better understanding of differential diagenesis in marine carbonates.