Reef Growth Research Articles

Bioerosion of reef-building crustose coralline algae by endolithic invertebrates in an upwelling-influenced reef

Coral reef growth is primarily determined by constructive and bioerosive processes acting on key reef-building organisms. Among them, corals are major contributors to the construction of reef frameworks, while crustose coralline algae (CCA) primarily take part in reef cementation. Despite the significance of CCA for reef ecology and functioning, there is very little information on CCA bioerosion rates, in particular in reefs influenced by coastal upwelling. Therefore, the aim of this study was to examine the percentage and rates of internal bioerosion by macroborer invertebrates in two dominant CCA species and to explore whether the oceanographic variability influences the nature of bioerosion in coral reefs of the Tayrona Natural National Park (Colombian Caribbean). Annual rates of gross calcium carbonate production for Lithoplyllum kaiseri and Porolithon antillarum were 0.556 (± 0.38) and 0.883 (± 1.0) g CaCO3 cm−2 year−1, respectively, and estimates of percent area covered by macroborer boreholes showed values of 29.7% for L. kaiseri and 18.0% for P. antillarum. Rates of calcium carbonate removal by internal macroborers in L. kaiseri (0.19 ± 0.17 g CaCO3 cm−2 year−1) were higher than those in P. antillarum (0.14 ± 0.17 g CaCO3 cm−2 year−1). The percentage of internal bioerosion did not vary significantly across climatic/oceanographic seasons. Boreholes produced by mollusks, polychaetes, sponges and sipunculids were identified, with mollusks having the highest erosion activity. A total of 2,095 individuals of boring and opportunistic organisms were identified and grouped into 29 families (17 polychaetes, 4 mollusks, 2 sipunculids and 6 crustaceans). The composition of the macroborer invertebrate community also varied between CCA species, with higher boreholes attributed to vermetids in L. kaiseri than in P. antillarum. Although there is no clear influence of climatic seasons on internal bioerosion, the high rates of CCA bioerosion may reduce reef consolidation in the region.

Symbiont shuffling across environmental gradients aligns with changes in carbon uptake and translocation in the reef-building coral Pocillopora acuta

Symbiosis between reef-building corals and unicellular algae (Symbiodiniaceae) fuels the growth and productivity of corals reefs. Capacity for Symbiodiniaceae to fix inorganic carbon (Ci) and translocate carbon compounds to the host is central to coral health, but how these processes change for corals thriving in environmental extremes remains largely unresolved. We investigate how a model coral—Pocillopora acuta—persists from a reef habitat into an adjacent extreme mangrove lagoon on the Great Barrier Reef. We combine respirometry and photophysiology measurements, Symbiodiniaceae genotyping, and 13C labelling to compare P. acuta metabolic performance across habitats, in relation to the Ci uptake and translocation capacity by symbionts’ autotrophy. We show that differences in P. acuta metabolic strategies across habitats align with a shift in dominant host-associated Symbiodiniaceae taxon, from Cladocopium in the reef to Durusdinium in the mangroves. This shift corresponded with a change in “photosynthetic strategy”, with P. acuta in the mangroves utilising absorbed light for photochemistry over non-photochemical quenching. Mangrove corals translocated similar proportions of carbon compared to the reefs, despite a lower Ci uptake. These trends indicate that coral survival in mangrove environments occurs through sustained translocation rate of organic compounds from coral symbionts to host.

Role of Future Reef Growth on Morphological Response of Coral Reef Islands to Sea‐Level Rise

AbstractCoral reefs are widely recognized for providing a natural breakwater effect that modulates erosion and flooding hazards on low‐lying sedimentary reef islands. Increased water depth across reef platforms due sea‐level rise (SLR) can compromise this breakwater effect and enhance island exposure to these hazards, but reef accretion in response to SLR may positively contribute to island resilience. Morphodynamic studies suggest that reef islands can adjust to SLR by maintaining freeboard (island crest elevation above still water level) through overwash deposition and island accretion, but the impact of different future reef accretion trajectories on the morphological response of islands remains unknown. Here we show, using a process‐based morphodynamic model, that, although reef growth significantly affects wave transformation processes and island morphology, it does not lead to decreased coastal flooding and island inundation. According to the model, reef islands evolve during SLR by attuning their elevation to the maximum wave runup and islands fronted by a growing reef platform attain lower elevations than those without reef growth, but have similar overwash regimes. The mean overwash discharge Qover across the island crest plays a key role in the ability of islands to keep up with SLR and maintain freeboard, with a Qover value of O (10 l m−1 s−1) separating island construction from destruction. Islands, therefore, can grow vertically to keep up with SLR via flooding and overwash if specific forcing and sediment supply conditions are met, offering hope for uninhabited and sparely populated islands. However, this physical island response will negatively impact infrastructure and assets on developed islands.

Influence of reef flat submergence on infra-gravity wave energy and resonance over the fringing reef

Fringing reefs which are common nearshore islands with coral reef growth have special topography of very steep slope on the fore-reef and mild slope on the wide flat. When incident waves propagate from a very deep water region (from hundreds of meters to thousands of meters of depth) to approaching the reef they abruptly commence a very shallow water (only few centimeters to several meters of depth) and create strong hydrodynamic processes on the reef flat. Due to shallow depth, waves feel the bottom and break in the area of fore-reef slope and reef crest and partial reef flat. Infra-gravity waves (IG), other name as bound long waves or surf beat, which belong to low-frequency wave type (0.002Hz ¸ 0.04Hz) are generated by the varying-breaking point mechanism on the shallow reef flat. On the flat, short wave energy is almost dissipated; low-frequency waves are strongly dominated over the surf zone till swash zone. Wave set-up causing an increase of water level on the flat combines with the run-up at the shoreline which can lead to coastal flooding. Besides, if the reef flat length is in the order of one fourth of wavelength the first oscillation resonant mode with standing wave occurs. This component is resonantly amplified at the shoreline relative to the incident infra-gravity wave energy at the reef crest.

Microfossil and strontium isotope chronology used to identify the controls of Miocene reefs and related facies in NW Cyprus

The existing chronostratigraphic framework in NW Cyprus of two-phase, Early and Late Miocene reef and associated facies development is tested and improved using a combination of calcareous nannofossil, benthic and planktonic foraminiferal, and also Sr isotope dating. Following localized Late Oligocene neritic carbonate deposition (e.g. benthic foraminiferal shoals), reefs and related facies (Terra Member) began to develop c. 24 Ma (Aquitanian) and terminated c. 16 Ma (end-Burdigalian). Early Miocene reef and marginal facies were then extensively redeposited as multiple debris-flow deposits until c. 13.7 Ma, influenced by a combination of global sea-level fall (related to growth of the East Antarctic Ice Sheet) and local- to regional-scale tectonics. Reef growth and related deposition resumed (Koronia Member) c. 9.1 Ma (Tortonian), then terminated by c. 6.1 Ma (Middle Messinian), followed by the Messinian salinity crisis. Neritic accumulation in NW Cyprus began earlier (Late Oligocene), than in southern Cyprus (Early Miocene). The Early Miocene reefs developed on a roughly north–south-trending structural high in the west (Akamas Peninsula area) whereas the Late Miocene reefs developed on both flanks of the neotectonic Polis graben. The two-phase reef development is mirrored in SE Cyprus and in some other Mediterranean areas (e.g. southern Turkey, Israel, Italy, southern Spain). Supplementary material: Global positioning system locations of dated samples, the Sr isotope method and the samples examined for planktonic foraminifera biostratigraphy are available at https://doi.org/10.6084/m9.figshare.c.5205315

Brackish water algal reefs – facies analysis as a tool to identify palaeoenvironmental variations in Miocene deposits (Mainz‐Weisenau, Germany)

Brackish‐water carbonates are far less studied than their marine or limnic counterparts. However, their association with few, specialized species enables the documentation of fine‐scale changes in the depositional environment. The Cenozoic Mainz Basin (Germany) was only sporadically connected to the North Sea and the Paratethys, exposing several transitions from marine to fresh water influence. Focusing on one outcrop of the Rüssingen Formation of Mainz‐Weisenau (Aquitanian, Miocene), we present a detailed analysis of the faunal and sedimentological responses to changing salinities and water depth, including algal reef growth and facies development. The deposits include allochthonous limestones surrounding an autochthonous reef complex and several smaller reef patches. The allochthonous facies is dominated by the gastropod Hydrobia inflata, and the reef facies is mainly made up by the green alga Cladophorites sp. The algal thalli are overgrown by cryptocrystalline, organic precipitations, and laminated, chemical precipitations. Locally, quiver‐shaped structures of Trichoptera sp. protective cases occur. The depositional setting was a shallow, low energy, and brackish environment supersaturated by carbonate. We could not confirm a general trend of reducing salinities as reported for the Rüssingen Formation. Our results question previously reported episodic desiccation events, because apparent caliche horizons actually represent thin beds of increased Cladophorites growth. Set‐up, distribution of the reef facies, and reef debris indicate short‐time variations of temperature, salinity and water depth. We conclude that these variations are based on the geographic position at the edge of an algal reef barrier, separating the Mainz Basin from the Rhine Rift Valley.

Stratigraphy of the Gorstian and Ludfordian (upper Silurian) Hemse Group reefs on Gotland, Sweden

ABSTRACT The Hemse Group is one of the least understood stratigraphic units of the Silurian sequence of Gotland, Sweden. New results from airborne transient electromagnetic (ATEM) measurements in combination with previously published data from field studies and geophysical investigations shed new light on carbonate platform development during the early- to mid-Ludlow Hemse Group. ATEM reveals a transgressive phase that began near the Wenlock-Ludlow boundary, which resulted in deposition of marls and corresponds roughly to the Hemse limestone units a-c and the Hemse Marl NW. In this phase little or no reef development occurs. The end of the transgressive phase coincides with the weak Linde Event. The following highstand favoured extensive reef growth forming a reef barrier system of both fringing reefs and more rampiform settings with stromatoporoid biostromes and occasional biohermal buildups. The Kuppen-Snabben Unconformity Complex marks an erosional (karstic) sequence boundary and rocky shoreline and the transition from a rampiform setting with reef biostromes towards a more rimmed setting with patch reefs.

EARLY–MIDDLE ORDOVICIAN SEASCAPE-SCALE AGGREGATION PATTERN OF SPONGE-RICH REEFS ACROSS THE LAURENTIA PALEOCONTINENT

ABSTRACTDuring the late Cambrian–Early Ordovician interval the predominant non-microbial reef builders were sponges or sponge-like metazoans. The lithological and faunal composition of Cambro-Ordovician sponge-dominated reefs have previously been analyzed and reviewed. Here we take the relationship between reef aggregation pattern at reef to seascape scale into account, and look for changes during the Early–Middle Ordovician interval, in which metazoans became dominant reef builders. In a comparison of sponge-rich reefs from eight sites of the Laurentia paleocontinent three different seascape level reef growth patterns can be distinguished: (1) mosaic mode of reef growth, where reefs form a complex spatial mosaic dependent on hard substrate; (2) episodic mode, where patch reefs grew exclusively in distinct unconformity bounded horizons within non-reefal lithological units that have a much larger thickness; and (3) belt-and-bank mode, where reefs and reef complexes grew vertically and laterally as dispersed patches largely independent from truncation surfaces. The distinct modes of growth likely represent specific reef forming paleocommunities, because they differ in content and abundance of skeletal metazoan framebuilders, bioturbation intensity of non-skeletal reef sediment matrix, and in association of reef growth with underlying hard substrate. We suggest, based on a review of Laurentian reef occurrences, that the mosaic mode dominated in Early Ordovician strata and that the dominance shifted toward the belt and bank mode from Middle Ordovician strata onward.

Actualize The Coastal Ecosystem Resilience : Determining The Location Of Artficial Reef

Oceanographic parameters that affect coral reef growth are clarity, temperature, salinity and water currents taken at 20 stations using AAQ Rinko 1183 and FLOWATCH FL-03 current meter. The data processing using the IDW (Inverse Distance Weighted) interpolation method and the determination of the suitability area through scoring and weighting. The identification value consist of: (a) Clarity ranges from 2 - 6 meters, (b) Water turbidity ranges from 0.3 - 4.32 NTU, (c) Bottom substrate type is fine black sand, (d) Current velocity ranges from 0.1 m/s - 0.4 m/s, and (e) TSS ranges from 35.6 - 351 mg / L. The suitability of placing artificial reefs is classified at the appropriate level (S2), as a support for the survival of fishermen and local communities in Damas Beach, Trenggalek Regency

First report of reef formation from the Balkanide type of Triassic: the Tran Formation (Upper Triassic) near the village of Gorna Sekirna, SW Bulgaria

Sphinctozoa skeletons were the main builders in microbial-sponge boundstones and cementstones. Syndepositional lithification of associated automicrite and marine phreatic cementation by radial fibrous and radiaxial calcite favored stabilization and growth of skeleton-cement reefs across a central reef area (close to a platform margin), but without forming a wave-resistant reef framework. Storm-influenced destruction of the buildups and subsequent reworking by waves and currents produced bioclastic-intraclastic grainstones.

Late Pleistocene Carbon Cycle Revisited by Considering Solid Earth Processes

AbstractThe importance of volcanic CO2 release, continental weathering, and coral reef growth on the global carbon cycle has been highlighted by several different studies. Based on these independent approaches, we here revisit the last 800 kyr with the box model BICYCLE, which has been extended to be able to address these solid Earth contributions to the carbon cycle. We show that the volcanic outgassing of CO2 as a function of sea level change from mid‐ocean ridges and hot spot island volcanoes cannot be the generic process that leads during phases of falling obliquity to a sea level‐CO2 decoupling as has been suggested before. The combined contribution from continental and marine volcanism, if both lagging sea level change by 4 kyr, might have added up to 13 ppm to the glacial/interglacial CO2 rise. Coral reef growth as suggested by an independent model is during glacial terminations about an order of magnitude too high to be reconciled with meaningful carbon cycle dynamics. Global riverine input of bicarbonate caused by silicate and carbonate weathering is suggested to have been stable over Termination I. However, if weathering fluxes are changed by up to 50% in sensitivity experiments, the corresponding bicarbonate input might contribute less than 20 ppm to the deglacial atmospheric CO2 rise. The overall agreement of results with the new process‐based sediment module and the previously applied time‐delayed response function to mimic carbonate compensation gives confidence in the results obtained in previous applications of the BICYCLE model without solid Earth processes.

Campaign Design About Oxybenzone and Octinoxate-Based Sunblock Hazards for Survival of Coral Reefs in Bali

The condition of coral reefs in several locations in Indonesia is indeed in critical condition and the impact of damage continues to grow every year. Sunscreen, which is one of the mandatory requirements used for skin protection to prevent sunburn, has dangerous substances oxybenzone and octinoxate, causing unconscious damage. Substances in sunscreens make coral reefs lose their adaptability to climate change which also damages coral reefs and prevents the growth of coral reefs. This topic is important because coral reefs have many ecological, social and economic influences and benefits that are very useful for life. Therefore, it is necessary to design a campaign that is made with the aim that this topic can increase awareness of campaign targets and readers with information that can be accounted for so that the risks of damage to coral reefs can be reduced. This design is carried out through the process of collecting data, formulating strategies, thinking ideas, designing visuals, and applying them to campaigns.

Light Limitation and Depth-Variable Sedimentation Drives Vertical Reef Compression on Turbid Coral Reefs

Turbid coral reefs experience high suspended sediment loads and low-light conditions that vertically compress the maximum depth of reef growth. Although vertical reef compression is hypothesized to further decrease available coral habitat as environmental conditions on reefs change, its causative processes have not been fully quantified. Here, we present a high-resolution time series of environmental parameters known to influence coral depth distribution (light, turbidity, sedimentation, currents) within reef crest (2–3 m) and reef slope (7 m) habitats on two turbid reefs in Singapore. Light levels on reef crests were low [mean daily light integral (DLI): 13.9 ± 5.6 and 6.4 ± 3.0 mol photons m–2 day–1 at Kusu and Hantu, respectively], and light differences between reefs were driven by a 2-fold increase in turbidity at Hantu (typically 10–50 mg l–1), despite its similar distance offshore. Light attenuation was rapid (KdPAR: 0.49–0.57 m–1) resulting in a shallow euphotic depth of <11 m, and daily fluctuations of up to 8 m. Remote sensing indicates a regional west-to-east gradient in light availability and turbidity across southern Singapore attributed to spatial variability in suspended sediment, chlorophyll-a and colored dissolved organic matter. Net sediment accumulation rates were ∼5% of gross rates on reefs (9.8–22.9 mg cm–2 day–1) due to the resuspension of sediment by tidal currents, which contribute to the ecological stability of reef crest coral communities. Lower current velocities on the reef slope deposit ∼4 kg m2 more silt annually, and result in high soft-sediment benthic cover. Our findings confirm that vertical reef compression is driven from the bottom-up, as the photic zone contracts and fine silt accumulates at depth, reducing available habitat for coral growth. Assuming no further declines in water quality, future sea level rise could decrease the depth distribution of these turbid reefs by a further 8–12%. This highlights the vulnerability of deeper coral communities on turbid reefs to the combined effects of both local anthropogenic inputs and climate-related impacts.

A possible link between coral reef success, crustose coralline algae and the evolution of herbivory

Crustose coralline red algae (CCA) play a key role in the consolidation of many modern tropical coral reefs. It is unclear, however, if their function as reef consolidators was equally pronounced in the geological past. Using a comprehensive database on ancient reefs, we show a strong correlation between the presence of CCA and the formation of true coral reefs throughout the last 150 Ma. We investigated if repeated breakdowns in the potential capacity of CCA to spur reef development were associated with sea level, ocean temperature, CO2 concentration, CCA species diversity, and/or the evolution of major herbivore groups. Model results show that the correlation between the occurrence of CCA and the development of true coral reefs increased with CCA diversity and cooler ocean temperatures while the diversification of herbivores had a transient negative effect. The evolution of novel herbivore groups compromised the interaction between CCA and true reef growth at least three times in the investigated time interval. These crises have been overcome by morphological adaptations of CCA.

Long-term (70-year) monitoring of reef structure through high-resolution multidecadal aerial imagery

Here, we utilise a unique, 70-year-long (1949–2017), high-resolution historical aerial imagery dataset to track changes to coral reef structures in Bill’s Bay on the Ningaloo coast. Reef habitat was distinguished from sand and rubble substrates based on imagery greyscale values and autoclassified using unsupervised image analysis in ArcMap. This approach was validated through comparison of reef structure (defined by bathymetric LiDAR imagery), greyscale values of aerial photography taken during the same survey, and in-water ground-truthed photographs. Long-term (1990–2015) live coral cover data were then compared with changes in reef structure. Reef structure increased from 11 to 38% within Bill’s Bay between 1949 and 1990. A coral spawn-induced hypoxia event in 1989 caused a loss of live coral cover from 50 to 7%, a 25% reduction in reef structure by 2002, and a switch from branching to massive coral-dominated reef. From 2002, reef structure recovered and continued to expand, peaking at 41% of total area (2017). Live coral cover in 2015 (46%) matches this measured increase in reef structure, despite a drop in coral cover in 2008. Discrepancies observed between live coral cover and reef structure during the 2002–2010 period reflect the persistence of the massive dominated coral community and in situ dead reef framework. Across the entire study period, reef growth was most prominent along the south-western edge of the bay, possibly due to an increase in the availability of hard substrate for coral recruitment. This study provides new insights into reef health over decadal timescales by capturing changes in reef geomorphology that go beyond the spatial scale and scope of traditional in-water assessments. These data further highlight that coral reef structure in Bill’s Bay has expanded even with frequent natural disturbances affecting the region over the last 70 years.

Rapid localized decline of a French Polynesian coral reef following a climatic irregularity

Coral reefs are among the most biologically diverse and complex marine ecosystems on Earth. However, anthropogenic impacts continue to degrade coral reefs worldwide, potentially increasing the ability for localized climatic irregularities or fluctuations to cause rapid changes to habitats over limited spatial scales. Bora-Bora Island was impacted by one such irregularity between 02th and January 05, 2020, with adjacent reefs subjected to a combination of low tides, high temperature and algal bloom. Our monitoring surveys showed that after only four days of these conditions, live coral cover on some Bora-Bora reefs rapidly declined from 80 ± 7% to 20 ± 7%. In addition, the density of fishes and giant clams also decreased (i.e., respectively, from 3.6 to 6.1 individuals per m2 before climatic anomaly to 2.6 and 2.2 individuals after climatic anomaly). This study suggests that, as the mean environmental conditions continue to move towards to abiotic limits on coral reef growth and persistence, even small fluctuations above these limits could have dramatic impacts on the resilience of reef communities.

Analysis of Relationship between Chemical Oceanography Conditions and Coral Reef Ecosystems in Damas Waters, Trenggalek, East Java

Indonesia is known as one of the world's marine biodiversity centers with its rich coral reefs. Coral growth depends on environmental conditions, which in reality do not always remain due to disruptions originating from nature or human activities. The growth of coral reefs in a sea water is strongly influenced by the quality of its waters such as chemical oceanographic factors namely salinity, pH, DO, nitrate and phosphate. The data collection was carried out twice in September and November 2019 in Damas Waters, Trenggalek, East Java. The waters of Damas Beach are located in Karanggandu Village, Watulimo District. The purpose of this study was to determine the condition of coral reefs in Damas Waters, to determine the effect of water quality on artificial reefs in chemistry in Damas Waters and to determine the relationship of quality parameters of chemical waters with artificial coral reefs in Damas Waters, Trenggalek, East Java. The sampling method is done by purposive random sampling that is determining the sample with certain considerations. The location points used by 20 stations are spread, namely on artificial reefs, natural coral reefs, open seas and the area around the harbor. Measurements made include in situ water quality with a multiparameter measurement tool, namely AAQ. The results obtained that in all stations found good water conditions for the life of coral reefs, and also the analysis of the relationship of each parameter and coral reefs influence each other.

Reef Evolution Model of Dongsha Uplift in Pearl River Mouth Basin, South China Sea

Zhao, H.; Wang, C.; Wang, Z., and Liu, J., 2020. Reef evolution model of Dongsha Uplift in Pearl River mouth basin, South China Sea. In: Yang, D.F. and Wang, H. (eds.), Recent Advances in Marine Geology and Environmental Oceanography. Journal of Coastal Research, Special Issue No. 108, pp. 118–124. Coconut Creek (Florida), ISSN 0749-0208.Hydrocarbon resource distribution in the organic reef areas all over the world is so rich that it is estimated to exceed 5 billion tons of oil. With continuous development of oil exploration and production, hydrocarbon resources in bioherm reservoirs will increase greatly. The recent oil exploration situation of bioherm reservoirs in the world is reviewed. The South China Sea is the largest marginal sea in China and various kinds of organic reefs exist and are buried in it. The peculiar geological conditions and the favorable bioherm reservoir conditions contribute to a good prospect for oil exploration. The South China Sea is the biggest marginal sea in the West Pacific and its evolutionary process supplies a proper growing environment to the reef. Dongsha Massif was shallow water and far from provenance supply area in the Miocene; its temperature, salinity, and water depth were fit for reef growth. According to the multidisciplinary analysis of seismic data interpretation, well logging, and core data, combining to the relative sea-level curve of variation, we hold the point that plenty of reef developed in Dongsha Massif, and the growth corresponded to the relative sea-level curve of variation and areal structure evolution period. The relative sea-level cycling variation and the complicated structure evolution made reefs in the study area with good characteristics, and they became potential gas and oil reservoirs.

Crumbling Reefs and Cold-Water Coral Habitat Loss in a Future Ocean: Evidence of “Coralporosis” as an Indicator of Habitat Integrity

Ocean acidification is a threat to the net growth of tropical and deep-sea coral reefs, due to gradual changes in the balance between reef growth and loss processes. Here we go beyond identification of coral dissolution induced by ocean acidification and identify a mechanism that will lead to a loss of habitat in cold-water coral reef habitats on an ecosystem-scale. To quantify this, we present in situ and year-long laboratory evidence detailing the type of habitat shift that can be expected (in situ evidence), the mechanisms underlying this (in situ and laboratory evidence), and the timescale within which the process begins (laboratory evidence). Through application of engineering principals, we detail how increased porosity in structurally critical sections of coral framework will lead to crumbling of load-bearing material, and a potential collapse and loss of complexity of the larger habitat. Importantly, in situ evidence highlights that cold-water corals can survive beneath the aragonite saturation horizon, but in a fundamentally different way to what is currently considered a biogenic cold-water coral reef, with a loss of the majority of reef habitat. The shift from a habitat with high 3-dimensional complexity provided by both live and dead coral framework, to a habitat restricted primarily to live coral colonies with lower 3-dimensional complexity represents the main threat to cold-water coral reefs of the future and the biodiversity they support. Ocean acidification can cause ecosystem-scale habitat loss for the majority of cold-water coral reefs.

Re-evaluating mid-Holocene reef “turn-off” on the inshore Southern Great Barrier Reef

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 <1.0 mm yr−1 to 5500 yr. BP after which a 3000-year stratigraphic hiatus is observed. A negative relative sea level oscillation at 5500 yr. BP coincides with the transition of Halfway reef from vertical accretion to lateral progradation and the “turn-off” of the Wedge Island coral community. The reef at Middle Island appears to have initiated in the midst of the Mid-Holocene hiatus period at ∼3500 yr. BP, with vertical accretion at both Middle and Halfway Islands being the highest for the region’s history at 12.5 ± 3.3 mm yr−1 and 15.0 ± 6.0 mm yr−1, respectively, between 3500 and 3000 yr. BP. A proposed late-Holocene relative sea level highstand of ∼1 m at ∼2000 yr. BP coincides with the first dates identified at near-shore Divided Island, and the re-initiation of coral growth at Wedge Island, although both showed limited net vertical accretion. These near-shore sites “turned-off” again between 1300 and 900 yr. BP, likely associated with a rapid relative sea level fall. Reef progradation also slowed at Halfway Island and Middle Island after 1200 yr. BP until a recent increase in reef growth at Middle Island since the late 1970’s associated with modern increases in average SSTs. Our data suggests that although reef-scale “turn-off” events occurred, there is no evidence for a regional scale hiatus in coral growth or initiation comparable to that found further north on the GBR. A comparison with other Southern and south-Central GBR reef cores suggests that relative sea level oscillations in conjunction with changes to climate, appear to have driven varying, but synchronous, modifications to reefs at 5500, 4600, 2800 and 1200 yr. BP.