Tara R. Clark

Understanding long-term coral reef dynamics provides essential context for assessing climate change impacts. Using uranium-thorium dating of dead coral in the Great Barrier Reef’s Mackay region, we reconstructed a 150-year history of Acropora spp. coral mortality. Results reveal decadal-scale variability superimposed on a century-long rise in mortality since ~1860, coinciding with expanding anthropogenic pressures. Major mortality episodes occurred between 1905-1970 and 1975-2005, with peaks aligning with El Niño events, cyclones and flooding. Although causality is difficult to isolate, mortality is significantly correlated with Niño 3.4 anomalies and positive phases of the Interdecadal Pacific Oscillation. Synchronous declines in the Palm Islands (~380 km north-west) began around European colonisation, alongside increased rainfall and lagoon freshening. The coherent cross-regional variability suggests regional rather than local drivers. We propose that intensifying marine heatwaves and declining water quality from catchment modification since colonisation together drive this long-term Acropora decline.

In the absence of detailed broad-scale studies, both spatially and temporally, the overall status (disturbed, recovering, or in decline) of many of the reefs that make up the Great Barrier Reef remains uncertain. Moreover, of the numerous and varied threats, their relative role in impacting individual reefs is generally unclear. Here, we adopt a novel approach to reliably reconstruct historical disturbance events at Rib and Davies Reefs, two mid-shelf reefs, using uranium-thorium (U-Th) dating of dead corals. Corrected ²³⁰Th ages obtained from dead Acropora spp. bracket time periods of lowest coral cover observed during independent ecological surveys in the late 1980s and 2000s at 1988.2 ± 1.3 and 2003.9 ± 6.2 CE, shortly after the arrival of crown-of-thorns starfish (CoTS) at Rib Reef in 1983−1985 and 2000−2002, respectively. At Davies Reef, ²³⁰Th ages dated to 1999.2 ± 1.2 CE when coral cover was halved as a result of Cyclone Tessi and an “unknown” disturbance in 2000−2001. Prior to modern surveys, there is remarkable overlap in ²³⁰Th ages between reefs, with repeated peaks in the age distribution having a return period of ∼10−15 years, akin to the periodicity of modern CoTS outbreaks. Our findings suggest that U-Th dating of dead corals can provide a robust foundation for understanding disturbance history and show promise in contributing to the effective monitoring of coral communities by providing a reliable benchmark with which to assess recovery.

Caribbean Acropora spp. corals have undergone a decline in cover since the second half of the twentieth century. Loss of these architecturally complex and fast-growing corals has resulted in significant, cascading changes to the character, diversity, and available eco-spaces of Caribbean reefs. Few thriving Acropora spp. populations exist today in the Caribbean and western North Atlantic seas, and our limited ability to access data from reefs assessed via long-term monitoring efforts means that reef scientists are challenged to determine resilience and longevity of existing Acropora spp. reefs. Here we used multiple dating methods to measure reef longevity and determine whether Coral Gardens Reef, Belize, is a refuge for Acropora cervicornis against the backdrop of wider Caribbean decline. We used a new genetic-aging technique to identify sample sites, and radiocarbon and high-precision uranium-thorium (U-Th) dating techniques to test whether one of the largest populations of extant A. cervicornis in the western Caribbean is newly established after the 1980s, or represents a longer-lived, stable population. We did so with respect for ethical sampling of a threatened species. Our data show corals ranging in age from 1910 (¹⁴C) or 1915 (²³⁰Th) to at least November 2019. While we cannot exclude the possibility of short gaps in the residence of A. cervicornis earlier in the record, the data show consistent and sustained living coral throughout the 1980s and up to at least 2019. We suggest that Coral Gardens has served as a refuge for A. cervicornis and that identifying other, similar sites may be critical to efforts to grow, preserve, conserve, and seed besieged Caribbean reefs.

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.

Coral reefs globally are impacted by natural and anthropogenic stressors that are compounded by climate change. Understanding past reef responses to natural stressors (cyclones, sea-level change, freshwater inputs, and sedimentation) can provide important insights to further understand recent (within the past century) trends in coral cover and diversity. Here we use a compilation of recently published data to investigate the Holocene development of five fringing reefs that are located on a cross-shelf transect on the central Great Barrier Reef, and that are exposed to varying degrees of natural and anthropogenic sedimentation, storm exposure, and Holocene sea-level change. Forty-two reef cores collected using a combination of manual percussion coring and hydraulic drilling techniques, were analysed and dated using uranium-thorium methods. The chronostratigraphic records of reef development established using 105 recently published radiometric ages and seven new uranium-thorium ages from the reef cores and fossil microatolls preserved across the reef flats were compared to investigate cross-shelf variations in reef development. This is the first study to conduct an internal investigation of reef framework across an inshore–offshore gradient to examine the varying levels of influence of sedimentation, sea level and cyclones. Our observations from the central Great Barrier Reef show that reefs furthest offshore from the mainland coast were typically initiated earliest after the post-glacial marine transgression. Reef flat size, morphology, and growth style varied according to constraints placed on reef development by the composition, depth, shape, and relief of the underlying substrate. We establish that terrigenous sedimentation had a marked effect on the development of inshore reefs closest to the mainland (within 10 km of the mainland coast). Periods of relatively high terrigenous sedimentation correspond with enhanced reef accretion rates, and also resulted in a superior record of palaeo-ecological coral composition (i.e., better preservation) at inshore sites. In contrast, mid-Holocene cyclones played a seemingly more important role in the development of reefs >10 km from the mainland; although cyclones clearly affect reefs closer inshore, their geomorphology is affected by a range of controlling factors. Insights provided by these five Holocene reef chronostratigraphies provide useful baseline understanding of reef condition and growth along a cross-shelf transect where the reefs are exposed to variable stressors.