Coral reef response since the Mid-Pleistocene Climate Transition on the North West Shelf, Australia: Insights from fossil coral reef cores

Quaternary coral reef science has limited understanding of coral reef response to the Mid Pleistocene Climate Transition (MPT) (ca. 0.8–0.6 Ma) that triggered the Earth's transition into a dry, arid climate and eventually into a period of glacial-interglacial cycles. During this period many of the extant large reef systems initiated, for example barrier reefs around Pacific atolls and the Great Barrier Reef, however, data availability is low. North and South Scott reefs are isolated carbonate platforms on the North West Shelf of Australia that have outpaced subsidence and oceanographic stressors intermittently since the Miocene. The long-term carbonate platform evolution from ca. 20 Ma to present has been characterised using seismic stratigraphy and seismic geomorphology, integrated with industry well log data, but, high-resolution reconstructions of sedimentological and environmental processes controlling coral reef growth cycles have not been determined prior to the Holocene. Lithologic and chrono-stratigraphic interpretations of four fossil coral reef boreholes extending to 200 m below sea floor provide new insights into the sensitivities of coral reef response to regional (i.e., tectonics, oceanographic) and global (i.e., climate, eustacy) environmental forcing conditions at millennial scale resolution since the MPT. Depositional facies and paleoenvironmental reconstructions are based on; detailed logging and petrologic, mineralogic and sedimentary lithofacies analyses of four cores located on the windward and leeward reef crests of north and south Scott Reef. Coralgal assemblages and other reef biota (e.g., foraminifera) constrain paleowater depths and precise paleoenvironmental settings. New radiometric dating based on sample selection from hyperspectral and neutron scattering data are presented. We recognise distinct reef growth sequences within lithologic and chrono-stratigraphic units that correspond to global high sea levels within Marine Isotope Stages (MIS) 1 (Holocene), 5, 7, 8, 11, 13. Each reef growth sequences established on the drowned platforms during deglacial and interglacial sea levels. Hiatuses in reef growth represented by paleosol horizons in the core that signals subaerial exposure events during glacial (low sea level) periods. Significant variations in elevation, subsidence histories, paleo-coralgal communities and geomorphological features occur between the north and south reef, as well as spatially across each reef. Sequences of coral reef growth kept pace with deglacial sea level rise owing to favourable environmental and oceanographic conditions that allowed the cyclic re-establishment of shallow, moderate and deep-water coral reef development across both platforms since the MPT. A comprehensive review of comparable MPT-triggered coral reef sequences for Ribbon Reef 5 on the NE margin of Australia exposes some significant variations in reef response to eustatic (subsidence and sea level) and environmental controls, including: proximity to land, position along the continental shelf, terrigenous sediment influx, seabed geomorphology, ocean currents and nearby sites for coral larvae recruitment. Analysis of two newly acquired and two undescribed fossil coral reef cores provides a unique insight into the Quaternary history of a key site along the NWS, demonstrating how fossils can be used to reconstruct Earth history to answer broader questions in the bio and geological sciences with respect to coral reef ecological responses to climate change.