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The tropical and subtropical oceans of the Southern Hemisphere are poorly represented in present-day climate models, necessitating an increased number of paleoclimate records from this key region to both understand the Earth's climate system and help constrain model simulations. Here we present a site-specific calibration of live collected massive Porites Sr/Ca records against concomitant in situ instrumental water temperature data from the fore-reef slope of Heron Reef, southern Great Barrier Reef (GBR). The resultant calibration, and a previously published Acropora calibration from the same site, was applied to subfossil coral material to investigate Holocene water temperatures at Heron Reef. U-Th-dated samples of massive Porites suggest cooler water temperatures with reduced seasonal amplitude at ~5.2 ka (2.76–1.31i?½C cooler than present) and ~7 ka (1.26i?½C cooler than present) at Heron Reef. These results contrast the previous suggestion of a mid-Holocene Thermal Maximum in the central GBR around 5.35 ka and 4.48 ka, yet may be explained by differences in temperature of the shallow ponded reef flat (central GBR) and the deeper reef slope waters (this study) and potential large reservoir correction errors associated with early radiocarbon dates. Combining coral-based water temperature anomaly reconstructions from the tropical and subtropical western Pacific indicates a coherent temperature response across the meridional gradient from Indonesia and Papua New Guinea down to the southern GBR. This similarity in reconstructed temperature anomalies suggests a high probability of an earlier expression of a mid-Holocene Thermal Maximum on the GBR between ~6.8 and 6.0 ka.

This study was funded by the National Environmental Research Programme (NERP) Tropical Ecosystems Hub Project 1.3 "Characterising the cumulative impacts of global, regional and local stressors on the present and past bio-diversity of the GBR" to J-xZ, JMP, SGS, TRC, Y-xF, and others and Australian Research Council LIEF grant (LE0989067 for the purchase and installation of the MC-ICP-MS essential for this study) to JxZ, JMP, Y-xF, and others, as well as an Australian Postgraduate Award (APA) to NDL. We are grateful to Eric Matson from the Australian Institute of Marine Science (AIMS) for technical assistance in obtaining the fossil coral cores and to Hard Rock Earth Works (Rod Morelli and associates) in Townsville for allowing access to the corals. We also thank St. Vincent's Private Hospital, Brisbane for CT scans, and Queensland Diagnostic Imaging, Indooroopilly for X-rays. We thank an anonymous reviewer for helpful comments on an earlier manuscript. The data used in this study are available through the cited references, figures, and tables as well as in the supporting information to this manuscript.