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13 pages, 4 figures, supporting Information https://doi.org/10.1111/ecog.05606.-- Data availability statement: Data available from the Dryad Digital Repository: https://dx.doi.org/10.5061/dryad.zs7h44j8s (Figuerola et al. 2021), Coastal hypoxia has become an increasingly acknowledged threat to coral reefs that is potentially intensifying because of increased input of anthropogenic nutrients. Almirante Bay (Caribbean Panama) is a semi-enclosed system that experiences hypoxia in deeper waters which occasionally shoals onto shallow coral reefs, suffocating most aerobic benthic life. To explore the long-term history of reefs in the bay we extracted reef matrix cores from two reefs that today experience contrasting patterns of oxygenation. We constructed a 1800-year-long record of gastropod assemblages and isotope compositions from six U-Th chronologically-constrained reef matrix cores. We extracted two cores from each reef at 3 m water depth and two additional cores from a deeper part (4.8 m) of the hypoxia-exposed reef. Results show that the deeper part of the hypoxic reef slowed in growth and stopped accreting approximately 1500 years BP while the shallow part of the reef continued to accrete to the present day, in agreement with a model of expanding hypoxia at this time. Our proxy-based approach suggests that the patterns of increasing herbivores and decreasing carbon isotope values in the deeper part of the hypoxic reef may have been driven by an increase in hypoxia via eutrophication caused by either natural changes or human impacts. Similar patterns in these paleoindicators occurred in the shallow part of the hypoxic reef during the last few decades. This suggests that the deep water hypoxia may be expanding to depths as shallow as 3 m and that shallow reefs are at greater risk due to increased human activity, This project and the research visit of BF to Texas A&M Univ. were financially supported by the Secretaría Nacional de Ciencia y Tecnología e Innovación (SENACYT; 47-2017-4-FID16-239), Sistema Nacional de Investigación (SENACYT), the Scholarly Studies Program (Smithsonian Institution), Smithsonian Tropical Research Institute (STRI), Texas A&M University and donations from M. Selin and family, J. Bilyk, V. and B. Anders, J. and M. Bytnar and the Young Presidents Organization (Los Angeles Gold Chapter). BF was supported by a STRI postdoctoral fellowship and the postdoctoral fellowships programme Beatriu de Pinós funded by the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 programme of research and innovation of the European Union under the Marie Skłodowska-Curie grant agreement no. 801370 (Incorporation grant 2019 BP 00183) and the Juan de la Cierva programme funded by the Ministry of Science and Innovation (Incorporation grant IJCI-2017-31478). With the institutional support of the ‘Severo Ochoa Centre of Excellence' accreditation (CEX2019-000928-S)

High‐precision uranium‐thorium (U‐Th) dating of dead branching corals from Luhuitou reef, Sanya, northern South China Sea indicates that the reef framework grew episodically over the past 7,000 years. Episodes of coral reef growth (“switch‐on”phases) occurred in response to regional warming during the mid‐Holocene Climate Optimum, Medieval Warm Period, and Current Warm Period, when the East Asian summer monsoon was strong and the East Asian winter monsoon was weak. In contrast,episodes when reef growth dramatically slowed or ceased (“switch‐off”phases) occurred during comparatively cold periods (e.g., Dark Age Cold Period and Little Ice Age) and are linked to abrupt weakening of the East Asian summer monsoon and concurrent strengthening of the East Asian winter monsoon. In the context of global warming, the northern South China Sea may become an important refugium for coral reef growth of up to 2 °C above present, but only if local anthropogenic pressures are reduced.

Evidence from the fossil coral record has shown that coral assemblages were able to extend their geographical range to higher latitudes during past global warming events. In the face of future global warming scenarios, we investigate the potential for China's subtropical coral communities to act as a refuge for corals as ocean temperatures continue to warm. Using uranium-thorium dating to chronologically constrain the age of dead corals, we reveal two distinct periods of coral growth between 6.85 and 5.51 ka B.P. and 0.11 to -0.05 ka B.P. (relative to A.D. 1950). The former coincides with the mid-Holocene Warm Period when temperatures in South China were similar to 1-2 degrees C warmer than present. Very few ages (13%) were obtained for the similar to 5.6 k.y. that followed. An increased frequency of corals with Th-230 ages dated to the past century suggests an increase in abundance coinciding with rising global temperatures. Nevertheless, modern monitoring programs have reported a recent dramatic decline in coral cover over the past 34 yr attributable to human influences. Our results suggest that although coral communities existed around the subtropical islands of Daya Bay (southeast China) in the past, their continued presence in the region largely depends on appropriate management of the adjacent coastline and coupled ocean-atmosphere conditions similar to those experienced in the mid-Holocene.

Rising sea level in the coming century is of significant concern, yet predicting relative sea level change in response to eustatic sea level variability is complex. Potential analogues are provided by the recent geological past but, until recently, many sea level reconstructions have been limited to millennial scale interpretations due to age uncertainties and paucity in proxy derived records. Here we present a sea level history for the tectonically stable “far-field” Great Barrier Reef, Australia, derived from 94 high precision uranium–thorium dates of sub-fossil coral microatolls. Our results provide evidence for at least two periods of relative sea level instability during the Holocene. These sea level oscillations are broadly synchronous with Indo-Pacific negative sea surface temperature anomalies, rapid global cooling events and glacial advances. We propose that the pace and magnitude of these oscillations are suggestive of eustatic/thermosteric processes operating in conjunction with regional climatic controls.

Understanding the marine radiocarbon reservoir effect (i.e., marine radiocarbon reservoir age (R) and/or correction (ΔR)) is important for the construction of robust radiocarbon chronologies for marine archives for various research areas including archaeology, palaeoecology, paleoceanography, Quaternary research and climate change studies. In this study, we examined temporal ΔR variability for the South China Sea (SCS) and the Great Barrier Reef (GBR) during the past ∼8.1 ka based on 14C analysis of 230Th-dated corals. Results show large ΔR variations of ∼410 yr and ∼490 yr for the SCS and the northern GBR for ∼5.5–8.1 ka and ∼5.5–7 ka, respectively, and a smaller ΔR variability of ∼200 yr for the SCS for ∼2–3.5 ka. Our data, together with those previously published for the Tropical and South Pacific, indicate that variability in ocean upwelling bringing old subsurface waters to the surface and/or changes in the sources (or 14C level) of the upwelled waters are responsible for seeing large ΔR variations in coastal areas along the eastern Pacific and the Tropical East Pacific (TEP) of several hundred to a thousand years mostly during the Early to Middle Holocene. ΔR variations in the central and western Pacific of several and a couple of hundred years during the Middle and Late Holocene, respectively, might be due to variability in Pacific-wide ocean circulation associated with climatic changes controlling the spread of upwelled waters from the TEP to the west. This mechanism together with local/regional effects, such as changes in ocean upwelling in the SCS resulting from East Asian monsoon variability and changes in upwelling and/or horizontal advection in the northern GBR associated with variability in the El Nino Southern Oscillation, might be responsible for the observed ΔR variations in these areas. The results of our study also indicate the need for regional marine radiocarbon calibration curves for improved radiocarbon dating of marine samples as the observed Holocene ΔR values for the Tropical and South Pacific are not fully reproduced by a recent modelling work using a 3D ocean model, which takes into account climate change effects. Ocean circulation changes were included in the model for the period of 11.5–50 cal kyr BP but possibly not considered or not well represented for the Holocene, which might explain the differences between the observed and modelled ΔR values.

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.

Prograded coral rubble ridges have been widely used as archives for reconstructing long-term storm or storminess history. Chronologies of ridge systems in previous studies are often based on a limited number of low-resolution radiocarbon or optically-stimulated luminescence (OSL) ages per ridge (usually only one age per ridge), which carry intrinsic age uncertainties and make interpretation of storm histories problematic. To test the fidelity of storm ridges as palaeo-storm archives, we used high-precision U-Th dating to examine whether different samples from a single ridge are temporally constrained. We surveyed three transects of ridge systems from two continental islands (Normanby Island and High Island) within the Frankland Islands, Great Barrier Reef (GBR), and obtained 96 U-Th dates from coral rubble samples collected from within and between different ridges. Our results revealed significant differences in age ranges between the two islands. The steeper and more defined rubble ridges present on Normanby Island revealed that the majority of U-Th ages (over 60%) from a single ridge clustered within a narrow age range (∼100 years). By contrast, the lower and less defined ridges on High Island, which were more likely formed during both storm and non-storm high-energy events, revealed significant scatter in age distribution (>>200 years) with no notable clustering. The narrower age ranges obtained from the steeper and more defined rubble ridges suggest that previous approaches of using either limited samples from a single ridge or low-precision dating methods to establish chronologies are generally valid at centennial to millennial timescales, although caution must be taken to use such approaches for storm history reconstruction on shorter timescales (e.g. decadal). The correlation between U-Th mortality ages of coral rubble and historical stormy periods highlights the possibility of using coral rubble age distribution from rubble ridges to reconstruct the long-term regional storminess history. U-Th age distribution of all dated coral rubble samples, combined with previously published storm-transported coral block data from the same region, revealed at least six relatively stormy periods in the past 500 years in the Frankland Islands region: 1570–1590, 1760–1780, 1800–1830, 1860–1880, 1940–1960, 1980–2010 CE. Wavelet power analysis of the relative probability of U-Th age distribution indicates that the Pacific Decadal Oscillation (PDO) and El Nino-Southern Oscillation (ENSO) may be partially responsible for the long-term variability in storminess.

Three emergent subfossil reef flats from the inshore Keppel Islands, Great Barrier Reef (GBR), Australia, were used to reconstruct relative sea level (RSL). Forty-two high-precision uranium–thorium (U–Th) dates obtained from coral microatolls and coral colonies (2σ age errors from ±8 to 37 yr) in conjunction with elevation surveys provide evidence in support of a nonlinear RSL regression throughout the Holocene. RSL was as least 0.75 m above present from ~6500 to 5500 yr before present (yr BP; where “present” is 1950). Following this highstand, two sites indicated a coeval lowering of RSL of at least 0.4 m from 5500 to 5300 yr BP which was maintained for ~200 yr. After the lowstand, RSL returned to higher levels before a 2000-yr hiatus in reef flat corals after 4600 yr BP at all three sites. A second possible RSL lowering event of ~0.3 m from ~2800 to 1600 yr BP was detected before RSL stabilised ~0.2 m above present levels by 900 yr BP. While the mechanism of the RSL instability is still uncertain, the alignment with previously reported RSL oscillations, rapid global climate changes and mid-Holocene reef “turn-off” on the GBR are discussed.

In the face of changing global climate the future of corals reefs is uncertain. High latitude reefs may offer potential refugia for corals under projected increasing sea surface temperatures (SSTs). To understand the reef growth potential of modern high latitude reefs it is first necessary to understand past reef growth and response to climatic and environmental changes. The history of the Great Barrier Reef (GBR) has been shown to be punctuated by a multi-millennial Mid-Holocene reef “turn-off” or initiation hiatus in the Northern and Southern GBR (∼5500–2500 years before present [yr. BP]). Here we present the results of chronologically constrained reef matrix cores from five continental island fringing reefs and coral communities in the Keppel Islands to revaluate the timing, extent and possible drivers of the Mid-Holocene hiatus in the Southern GBR. Earliest initiation occurred at Wedge and Halfway Islands at 7773 ± 19 yr. BP and 7455 ± 20 yr. BP, respectively. Following initiation, vertical reef accretion at Halfway was comparable to previously reported rates for the GBR of ∼3.4 mm yr−1, increasing to ∼8.0 mm yr−1 between 6100 and 5500 yr. BP. Conversely, the coral community at Wedge Island, located closer to mainland terrestrial influence, accreted

Carbonate successions are principal geochemical archives of the extraordinary environmental changes that occurred during the Neoproterozoic Era. In this study, we present a stratigraphic record of carbonate rare earth element (REE) compositions from Neoproterozoic and Cambrian strata in central and southern Australia. The new REE results provide insight into the redox conditions of the Neoproterozoic-Cambrian ocean, the influence of glacial meltwater on carbonate precipitation, and Neoproterozoic-Cambrian paleoenvironmental changes. High Ce/Ce* and low Y/Ho suggest that the major Neoproterozoic and Cambrian carbonate accumulations of central and southern Australia formed in restricted marine environments that were largely dysoxic. Elevated ∑REE of mid-Cryogenian and early Ediacaran carbonate from the Amadeus Basin (central Australia) and King Island (Tasmania), respectively, are interpreted to reflect increased detrital mineral flux following periods of Cryogenian glaciation. REE data from the basal Ediacaran Cumberland Creek Dolostone of King Island suggest a transient episode of ocean oxidation in the wake of the terminal Cryogenian glaciation, but the balance of evidence suggests dominantly low oxygen ocean conditions during carbonate deposition in the remainder of the Neoproterozoic Era. An apparent mismatch in timing between carbonate deposition in central and southern Australia versus previously proposed mid-to-late-Ediacaran oxygenation events based on geochemical results from strata of South China highlights what may be important factors in reconciling varied geochemical records of Neoproterozoic-Paleozoic ocean oxygenation.

Long-term data with high-precision chronology are essential to elucidate past ecological changes on coral reefs beyond the period of modern-day monitoring programs. In 2012 we revisited two inshore reefs within the central Great Barrier Reef, where a series of historical photographs document a loss of hard coral cover between c.1890–1994 AD. Here we use an integrated approach that includes high-precision U-Th dating specifically tailored for determining the age of extremely young corals to provide a robust, objective characterisation of ecological transition. The timing of mortality for most of the dead in situ corals sampled from the historical photograph locations was found to coincide with major flood events in 1990–1991 at Bramston Reef and 1970 and 2008 at Stone Island. Evidence of some recovery was found at Bramston Reef with living coral genera similar to what was described in c.1890 present in 2012. In contrast, very little sign of coral re-establishment was found at Stone Island suggesting delayed recovery. These results provide a valuable reference point for managers to continue monitoring the recovery (or lack thereof) of coral communities at these reefs.