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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)

Coral reefs are experiencing a dramatic loss of hard coral abundance and associated habitat structure from a myriad of local and global factors. Here, utilizing U–Th radiometric age-dating of coral death assemblages, we investigated patterns of coral mortality from the eastern margin of the Red Sea along a latitudinal gradient (Yanbu, 24o N; Thuwal, 22o N; Al-Lith, 19o N; Farasan Banks, 18o N) in 2018 and 2019. In all four regions, radiometric ages of in situ dead Acropora and Pocillopora colonies were largely confined to the late twentieth and early twenty-first century. During the early twenty-first century, coral mortality was found to be synchronous with previously documented bleaching events in 2010 and 2015 and, at one site (Farasan Banks), an outbreak of crown-of-thorns starfish (COTS) in 2009. The most northern site, Yanbu, had the highest relative percentage of live coral (42 ± 4%) and of living Acropora, and may serve as a refugium under climate warming scenarios. For the three southern regions (Thuwal, Al-Lith, Farasan Banks) benthic structure was mostly comprised of dead corals. The southernmost survey site, Farasan Banks, underwent a dramatic change in coral benthic structure associated with a COTS outbreak in 2009 and a bleaching event in 2015, and had the lowest relative percentage of live coral (6 ± 2%), comprised mostly of massive Porites, with no live Acropora or Pocillopora. Our results highlight the asynchronous impact of disturbance events on eastern Red Sea coral reefs and emphasize regional differences in recovery and ecosystem state.

Reefs are biogenic structures that result in three-dimensional accumulations of calcium carbonate. Over geological timescales, a positive balance between the production and accumulation of calcium carbonate versus erosional and off-reef transport processes maintains positive net accretion on reefs. Yet, how ecological processes occurring over decadal timescales translate to the accumulation of geological structures is poorly understood, in part due to a lack of studies with detailed time-constrained chronologies of reef accretion over decades to centuries. Here, we combined ecological surveys of living reefs with palaeoecological reconstructions and high-precision radiometric (U-Th) age-dating of fossil reefs represented in both reef sediment cores and surficial dead in situ corals, to reconstruct the history of community composition and carbonate accumulation across the central and southern Saudi Arabian Red Sea throughout the late Holocene. We found that reefs were primarily comprised of thermally tolerant massive Porites colonies, creating a consolidated coral framework, with unconsolidated branching coral rubble accumulating among massive corals on shallow (5–8 m depth) exposed (windward), and gently sloping reef slopes. These unconsolidated reef rubble fields were formed primarily from ex situ Acropora and Pocillopora coral fragments, infilled post deposition within a sedimentary matrix. Bayesian age-depth models revealed a process of punctuated deposition of post-mortem coral fragments transported from adjacent reef environments. That a large portion of Saudi Arabian Red Sea reef slopes is driven by allochthonous deposition (transportation) has important implications for modeling carbonate budgets and reef growth. In addition, a multi-decadal lag exists between the time of death for branching in situ coral and incorporation into the unconsolidated reef rubble. This indicates that recent climate related degradation in the 21st century has not had an immediately negative effect on reef building processes affecting a large portion of the reef area in the Saudi Arabian Red Sea.

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.

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 expand into 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 differences among these palaeoindicators in the two reefs may have been driven by an increase in hypoxia via eutrophication caused by either natural changes or human impacts. Similar patterns of increasing herbivores and decreasing carbon isotope values occurred in the shallow part of the hypoxic reef during the last few decades. This suggests that hypoxia may be expanding to depths as shallow as 3 m and that shallow reefs are experiencing greater risk due to increased human activity.

Reconstructing coral reef histories provides a window of understanding into reef response to changing environmental and climatic conditions over various temporal scales. Here we present the results of 117 U-Th dates from emergent reef flat and slope cores and surface death assemblages, combined with previously published fossil microatoll data, to capture the entire sequence of reef growth at Mazie Bay, inshore Southern Great Barrier Reef (GBR). Coral U-Th dates indicate that Mazie Bay reef initiated ~6,900 years before present (yr. BP) quickly filling accommodation space. While rates of vertical reef accretion (5.3 ± 1.0 mm year⁻¹) were comparable to the GBR average during the mid-Holocene (~5.0 mm year⁻¹), reef flat progradation occurred at a rate 1.5- to 6-fold previous GBR rates until 5,100 yr. BP (~70.4 cm year⁻¹). Average progradation slowed to ~7.1 cm year⁻¹ in the subsequent ~4,000 years and reef slope cores indicate this reef had largely "turned-off" by 400 yr. BP, with modern coral communities existing as a veneer over the largely senescent framework. Death assemblage dates highlight coral disturbance and recovery regimes in response to increased cyclone activity 1960-1985 AD and recent extreme sea surface temperature and flood events post 2000 AD. U-Th dating of mid-Holocene to modern coral deposits from Mazie Bay reef provides a unique insight into past reef development, response to recent disturbance regimes, and potential for future reef growth. In the case of Mazie Bay, our data suggest limited accommodation space and increased occurrence of sea surface temperature extremes will restrict future reef growth at this site.

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.

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 sedimentary basins, a better understanding of the controlling effect of accommodation/transfer zones on stratal architecture and facies distribution can improve the success rate of locating hydrocarbon reservoirs. The Fushan Depression is a half-graben rift sub-basin located in the southeast of the Beibuwan Basin, South China Sea. In this study, comparative analysis of seismic reflection, palaeogeomorphology, fault activity and depositional facies distribution indicates that three different types of inner-basin slopes (i.e. multi-level step-fault slope in the western area, slope flexure zone in the accommodation zone area and gentle slope in the eastern area) were developed along the southern slope of the Fushan Depression, together with a large-scale accommodation zone located at the intersection of the western and eastern fault systems. Further analysis shows that the accommodation zone played an important role in controlling not only stratal architectural variability in the southern slope but also depositional facies distribution in the accommodation zone area. During the high-stand stage, the deposition of depositional systems was mainly controlled by sediment supply and the NW-trending transfer faults. Major drainage systems entered the Fushan Depression through the accommodation zone along the direction of sediment supply, and sedimentary flow paths were parallel to the accommodation zone axis with sedimentary flows constrained to the adjacent areas of the NW-trending transfer faults. By contrast, during the low-stand stage, the transfer faults had little control over depositional facies distribution, and the sublacustrine fan sediments diverted away from the accommodation zone and flew down along the northeast, oblique to the accommodation zone axis. The flow division in the lowstand stage might be greatly influenced by flow type and topography. In addition, the accommodation zone area demonstrated unique hydrocarbon accumulation models different from the western and eastern areas, suggesting that the future exploration should be conducted at different levels in the Fushan Depression.

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

Catchment modification and increased sediment loads in coastal regions are impacting many of the world's coral reefs, yet relevant water quality data are limited by the availability of instrumental records. Massive Porites spp. corals offer the potential to reconstruct water quality by use of the geochemical proxies that are incorporated into their skeleton. Here we present ∼ monthly resolved rare earth element (REE), yttrium (Y) and barium/calcium (Ba/Ca) concentrations obtained from four Porites corals collected across a known water quality gradient from the Great Barrier Reef, Australia. Sub-annual REE time series patterns were comparable among corals despite having significantly different concentrations. Whilst Ba/Ca did not reflect the known water quality gradient of the region, Y/holmium ratios reflected broad cross shelf water quality gradients and REE concentrations reflected local scale variance in water quality. This study demonstrates that REE analysis of Porites corals offers great potential for assessing changes in water quality in reef environments.

High-energy storm-transported coral blocks are widespread on the reef flats of the Great Barrier Reef (GBR), Australia, and have the potential to be used as proxies for reconstructing past storm/cyclone events prior to historical or instrumental records. In this study, samples from 42 individual transported coral blocks were collected from the inshore Frankland Islands, northern GBR for high-precision MC-ICPMS U–Th dating with their surface mortality ages recording the timing of individual storms or cyclones responsible for their uplift from their original growth position. The dated mortality ages were found to match well with known historical storm/cyclone events in the last century, with 80% of them falling within episodes of increased storm activity (1910–1915, 1945–1950, 1955–1960, 1975–1990, 1995–2000 AD) captured by instrumental/historic records, confirming that transported coral blocks on inshore reefs can be used as proxies for past storm/cyclone occurrences. Using this approach, this study also identified 17 additional storm/cyclone events that occurred before European settlement in the 1850s, including three oldest events at 758.4 ± 3.7, 777.9 ± 4.9, and 985.2 ± 4.8 AD, respectively. Our results, despite still preliminary, suggest that the storm/cyclone activity in this region tends to broadly correlate with the positive modes of the Pacific Decadal Oscillation (PDO) during the last millennium. In addition, there appears to be a decreasing age trend from the shore to the reef edge (from 758.4 ± 3.7 AD to 1988.3 ± 1.6 AD), which can be attributed to sea-level fall and/or reef/island progradation over the last 2000 years.

The Great Barrier Reef (GBR) is a natural, social and economic asset synonymous with Australia; however, there are concerns regarding both the frequency and extent of modern reef decline, especially in regions within close proximity to the coastline. With the future of coral reefs uncertain, elucidating the controls on reef growth and decline in the recent geological past prior to anthropogenic impacts is imperative to future management strategies. Numerous reef cores have revealed a substantial hiatus period or reef “turn-off” event during the mid-Holocene (~4600 years before present; yBP 1950) clearly prior to anthropogenic influence. Previously published research has suggested that changes in sea level, climate, and/or environmental conditions caused this reef “turn-off”, but the exact cause is still tentative. Whether sea level varied significantly during the Holocene has been debated for over half a century, with oscillations generally dismissed as dating artefacts due to large age errors or to the misinterpretation or inaccuracies of the sea level indicator. Coral microatolls, one of the most reliable sea level indicators, were used to test whether relative (RSL) oscillations could be detected during the Holocene. Elevation surveys of sub-fossil coral microatolls (n=32) and non-microatoll reef flat corals (n=10) were conducted on three separate sites in the Keppel Islands, southern GBR and dated using high precision uranium thorium (U-Th) techniques. The resultant palaeo-sea-level reconstruction revealed a rapid lowering of RSL of at least 0.4 m from 5500 to 5300 yBP following a RSL highstand of ~0.75 m above present from ~6500 to 5500 yBP. RSL then returned to higher levels before a 2000-yr hiatus in reef flat corals after 4600 yBP. To determine if this was a local scale response, or part of a broader regional signal, the same methodology was applied to another 8 sites from a wide latitudinal range along the GBR (11˚S to 20˚S). The 94 U-Th dates of sub-fossil microatolls from this research adds support to the RSL lowering event at 5500 yBP, with microatolls close to present sea level found at ~5100 yBP. A second oscillation of ~ -0.3m at 4600 yBP was also detected in the northern GBR, with microatolls at three sites close to modern SL between 4600 – 4000 yBP. The RSL oscillations at 5500 yBP and 4600 yBP coincide with both substantial reduction in reef accretion and wide spread reef “turn-off”, respectively, thereby suggesting that oscillating sea level was the primary driver of reef shut down on the GBR. Understanding the coeval palaeo-climate and -environmental conditions may reveal both the cause of these sea level oscillations and further modes of stress placed on coral reefs prior to the mid-Holocene hiatus. In the first instance one of the simplest and most efficient methods of extracting information from the annual bands of massive Porites sp. coral cores is by using the growth characteristics (i.e. linear extension) and ultra violet (UV) luminescent intensity which are linked to sea surface temperate and river discharge, respectively. As the El Nino Southern Oscillation (ENSO) is recognised as one of the main modulators of rainfall on the GBR, continuous wavelet transforms (CWT) of previously published modern coral luminescence index record was compared to sea surface temperature (SST) anomalies in the Nino 3 and Nino 3.4 regions (an indicator of ENSO). The transformed coral luminescence record matched well with the ENSO signal, so is therefore considered a viable tool for reconstructing ENSO in the Holocene. Continuous wavelet transforms were then applied to luminescence index data of three Porites corals U-Th dated to 5200 yBP, 4900 yBP and 4300 yBP. Results suggest less intense ENSO events during the mid-Holocene with a reduction in ENSO frequency in the 2-7 year band after 5200 y BP. Limited linear extension rates in the fossil corals (

It is increasingly apparent that sea-level data (e.g. microfossil transfer functions, dated coral microatolls and direct observations from satellite and tidal gauges) vary temporally and spatially at regional to local scales, thus limiting our ability to model future sea-level rise for many regions. Understanding sealevel response at ‘far-field’ locations at regional scales is fundamental for formulating more relevant sea-level rise susceptibility models within these regions under future global change projections. Fossil corals and reefs in particular are valuable tools for reconstructing past sea levels and possible environmental phase shifts beyond the temporal constraints of instrumental records. This study used abundant surface geochronological data based on in situ subfossil corals and precise elevation surveys to determine previous sea level in Moreton Bay, eastern Australia, a far-field site. A total of 64 U-Th dates show that relative sea level was at least 1.1 m above modern lowest astronomical tide (LAT) from at least ˜6600 cal. yr BP. Furthermore, a rapid synchronous demise in coral reef growth occurred in Moreton Bay ˜5800 cal. yr BP, coinciding with reported reef hiatus periods in other areas around the Indo-Pacific region. Evaluating past reef growth patterns and phases allows for a better interpretation of anthropogenic forcing versus natural environmental/climatic cycles that effect reef formation and demise at all scales and may allow better prediction of reef response to future global change.

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.