Coral Reef Development Research Articles

The impact of macroalgae on reef-building corals depends on their species, density, and contact status

Coral reefs are severely threatened by global and local disturbances that can shift reefs from coral to algal dominance. Coral-macroalgae competition is expected to exacerbate coral decline as the interactions increase in frequency. Whereas numerous studies over the last decade have aimed to characterize the interactions and impacts on coral growth and physiology, the underlying mechanisms remain controversial. This study tested the impact of different macroalgal species, densities, and contact status with corals on the physiological response of corals in Sanya Reefs. The results revealed that direct contact with increasing densities of fleshy macroalgae had a negative impact on the photosynthesis, growth rate, and tissue biomass of the two common corals. However, calcified macroalgae did not significantly affect the corals, regardless of whether there was direct contact or not. Under the same conditions, Acropora intermedia appeared to be more susceptible to fleshy macroalgae compared to Porites lutea. This suggested that different corals varied in their susceptibility to various macroalgae. Additionally, the results of the generalized linear mixed model revealed that macroalgal species and contact status with corals were the most important predictors of the impacts of macroalgae on corals, and macroalgal density was another nonnegligible parameter. Overall, macroalgae may have caused a potential functional shift in the composition of coral assemblages on the Sanya reefs by further reducing the already depauperate reef-building coral populations. The negative impacts of macroalgae in Sanya Reefs may serve as an early warning that the persistence of the invaluable ecological functions provided by coral reefs will be increasingly threatened throughout the South China Sea. Our findings could contribute to improving the scientific and effective management practices, fostering sustainable coral reef development in China and beyond.

Early Jurassic coral reef development outside Tethys: an example from western Argentina

Early Jurassic coral reef development outside Tethys: an example from western Argentina

Implementing the iCORAL (version 1.0) coral reef CaCO3 production module in the iLOVECLIM climate model

Abstract. Coral reef development is intricately linked to both climate and the concentration of atmospheric CO2, specifically through temperature and carbonate chemistry in the upper ocean. In turn, the calcification of corals modifies the concentration of dissolved inorganic carbon (DIC) and total alkalinity in the ocean, impacting air–sea gas exchange, atmospheric CO2 concentration, and ultimately the climate. This feedback between atmospheric conditions and coral biogeochemistry can only be accounted for with a coupled coral–carbon–climate model. Here we present the implementation of a coral reef calcification module into an Earth system model. Simulated coral reef production of the calcium carbonate mineral aragonite depends on photosynthetically active radiation, nutrient concentrations, salinity, temperature, and the aragonite saturation state. An ensemble of 210 parameter perturbation simulations was performed to identify carbonate production parameter values that optimize the simulated distribution of coral reefs and associated carbonate production. The tuned model simulates the presence of coral reefs and regional-to-global carbonate production values in good agreement with data-based estimates, despite some limitations due to the imperfect simulation of climatic and biogeochemical fields driving the simulation of coral reef development. When used in association with methods accounting for bathymetry changes resulting from different sea levels, the model enables assessment of past and future coral–climate coupling on seasonal to millennial timescales, highlighting how climatic trends and variability may affect reef development and the resulting climate–carbon feedback.

HIARA study protocol: impacts of artificial coral reef development on fisheries, human livelihoods and health in southwestern Madagascar.

The Health Impacts of Artificial Reef Advancement (HIARA; in the Malagasy language, "together") study cohort was set up in December 2022 to assess the economic and nutritional importance of seafood for the coastal Malagasy population living along the Bay of Ranobe in southwestern Madagascar. Over the course of the research, which will continue until at least 2026, the primary question we seek to answer is whether the creation of artificial coral reefs can rehabilitate fish biomass, increase fish catch, and positively influence fisher livelihoods, community nutrition, and mental health. Through prospective, longitudinal monitoring of the ecological and social systems of Bay of Ranobe, we aim to understand the influence of seasonal and long-term shifts in marine ecological resources and their benefits to human livelihoods and health. Fourteen communities (12 coastal and two inland) were enrolled into the study including 450 households across both the coastal (n = 360 households) and inland (n = 90 households) ecosystems. In the ecological component, we quantify the extent and health of coral reef ecosystems and collect data on the diversity and abundance of fisheries resources. In the social component, we collect data on the diets, resource acquisition strategies, fisheries and agricultural practices, and other social, demographic and economic indicators, repeated every 3 months. At these visits, clinical measures are collected including anthropometric measures, blood pressure, and mental health diagnostic screening. By analyzing changes in fish catch and consumption arising from varying distances to artificial reef construction and associated impacts on fish biomass, our cohort study could provide valuable insights into the public health impacts of artificial coral reef construction on local populations. Specifically, we aim to assess the impact of changes in fish catch (caused by artificial reefs) on various health outcomes, such as stunting, underweight, wasting, nutrient intake, hypertension, anxiety, and depression.

Sustainable artificial coral reef restoration using nanoclays and composite hydrogel microcapsules

Nanoclays, nanocomposite hydrogel microcapsules, and encapsulated bacteria can be considered as cement replacements for artificial coral reef development.

A Geo-Itinerary to Foster Sustainable Tourism in West African Islands: Storytelling the Evolution of the Ancient Cameroon Volcanic Line Coral Reefs

This study characterizes the submerged geomorphology around Annobón, São Tomé, and Príncipe Islands, and the De Santarém–Escobar seamount in the Cameroon Volcanic Line or CVL (Gulf of Guinea, West Africa) through analyses of topographic relief and coastal bathymetry, and data from fieldwork and historical fossil collections. The clear relation between each submerged island and the seamount morphology of the CVL and the various stages of coral reef development according to Darwin’s subsidence hypothesis meets the concept of intangible geoheritage. This type of geoheritage is related to phenomena rather than to a thing, and it is usually highly evaluated by scientific communities. Academics tend to use science-based discourse to explain this theory, but it is rarely understood by non-specialist audiences. This work proposes a virtual geo-itinerary along the submerged geomorphology of the CVL islands and seamounts, which aims at telling the geohistory of the coral reefs’ origin and evolution according to Darwin’s theory, and disclosing its geoheritage contents for further sustainable uses. The resulting narrative can be used to support geotourism initiatives and to support the United Nations’ objectives for Small Island Developing States.

Local compositional change and regional stability across 3000 years of coral reef development

AbstractAimMeasuring changes in ecological diversity over time is core to understanding and managing human impacts on natural systems. Observational data do not capture pre‐modern community states or multiple generations of long‐lived taxa, so interpretation of current diversity trends requires comparison with the past. We aimed to estimate diversity trends over long time periods in a series of fossil coral terraces.LocationHuon peninsula, Papua New Guinea.Time Period9817–6433 years before 1950 AD.Major Taxa StudiedThirty‐five genera of Sceleractinian coral.MethodsWe estimated trends in standing richness (α diversity) and turnover (temporal β diversity) in palaeoecological time series of fossil reef terraces. We compared taxonomic and functional group composition estimates of α diversity, using Hill numbers, and β diversity, measured both sequentially along each time series and to the earliest community.ResultsWe found regional‐scale trends of slowly increasing α diversity and directional turnover driven by increased evenness over time. These regional trends were averages of local‐scale trends in α and β diversity that were largely synchronous, even as coral composition diverged between spatial clusters. Our two β diversity estimates provided opposing, but complementary, reference frames to understand the magnitude and rate of compositional change in ecological time series.Main ConclusionsWe provide evidence that prior to strong anthropogenic influence, natural communities were experiencing directional diversity and compositional change. Our results provide a valuable perspective of diversity dynamics in the recent geological past, including how choices in the measurement of diversity can occlude and influence results.

Coral reef development influences the spatial pattern of echinoderms in the Central Mexican Pacific

Coral reef development influences the spatial pattern of echinoderms in the Central Mexican Pacific

Ecological vulnerability assessment of coral islands and reefs in the South China Sea based on remote sensing and reanalysis data

Coral reefs are ecosystems that are highly vulnerable to external environmental impacts, including changes associated with ocean acidification and global warming. Assessing the vulnerability of coral reef growth environments over large areas of the sea is a difficult and complex process, as it is influenced by many variables. There are few studies on environmental vulnerability assessment of coral islands and reefs in the South China Sea. It is therefore particularly important to understand the environmental sensitivity of corals and how coral communities respond to changes in climate-related environmental variables. In this study, indicators were selected mainly from natural environmental factors that hinder the development of coral reefs. The sea surface temperature (SST), sea surface salinity (SSS), wind velocity (WV) and direction, sea level height (SL), ocean currents (OC), and chlorophyll concentration (Chl) of coral reefs in South China Sea Island were integrated to calculate the coral reef environmental vulnerability region. In a GIS environment, Spatial Principal Component Analysis (SPCA) was used to develop sensitivity models and evaluate the ecological vulnerability of coral reefs. Based on the Environmental vulnerability indicator (EVI) values, the study area was classified as 5 grades of ecological vulnerability: Potential (0.000–0.577), Light (0.577–0.780), Medium (0.780–0.886), Heavy (0.886–0.993) and Very Heavy (0.993–1.131). Sensitivity models identified regional gradients of environmental stress and found that some coral reefs in western Malaysia and southwestern Philippines have higher vulnerability. Meanwhile, the study found that the reefs of Paracel Islands and Macclesfield Bank areas of medium vulnerability. Future use of high-precision data from long time series will allow better estimates of site-specific vulnerability and allow for the precise establishment of marine protected areas so that the ecological diversity of coral reefs can be sustained.

Coral Detection, Ranging, and Assessment (CDRA) algorithm-based automatic estimation of coral reef coverage

Climate change poses significant threats to the health of coral reefs. The detection of coral coverage is a quantitative method for assessing the health of coral reefs. Valuable insights into the effects of climate change on coral reef ecosystems and the trends in coral reef development can be gained through the monitoring of coral coverage. However, traditional manual methods for estimating coral coverage are time-consuming and labor-intensive, limiting the scalability and efficiency of coral surveys. The field of automated processing for coral video imagery is currently limited, impeding the development of effective techniques for comprehensive coral monitoring and further hindering progress in coral ecosystem monitoring. To cope with these challenges, there is a crucial need for automated systems to efficiently handle large volumes of video data, thereby transforming imaging devices, such as underwater robots, into autonomous sensors capable of conducting reliable and comprehensive surveys. In response to this pressing issue, this study presents a novel video analysis approach, termed the Coral Detection, Ranging, and Assessment (CDRA) algorithm, which combines cutting-edge computer vision techniques including You Only Look Once version 5 (YOLOv5) object detection, binocular stereo vision, and tracking algorithms. The CDRA algorithm aims to analyze continuous video segments of coral reefs, enabling the identification, counting, and estimation of size and location of individual coral colonies. Experimental evaluations conducted in controlled pool environments demonstrated the high accuracy of the proposed method, with an average accuracy of 90.7% in estimating coral coverage at different locations. Moreover, field trials conducted in Xidao Island of China verified the effectiveness and robustness of the CDRA algorithm under varying water quality and lighting conditions. The findings of this study represent a significant step towards the development of reliable and automated techniques for coral reef detection, contributing to the advancement of coral reef conservation efforts.

Geodynamic control on Pleistocene coral reef development: Insights from northwest Sumba Island (Indonesia)

AbstractThe fossil record of Quaternary reef systems, as expressed in uplifted regions by sequences of stacked terraces, has been extensively used either to understand their morphodynamics or to unravel sea level variations. Yet, because these two aspects are intimately linked, Quaternary reef analysis is often underdetermined because the analysis often focuses on single sequences, along one‐dimensional profiles. Here, we take advantage of the lateral variations of coral reef sequences by documenting the morphological variations of the reef sequence on Sumba Island. Near Tambolaka, northwest Sumba, we analysed a reef transect, topography, and associated sedimentological record to obtain a precise coral reef stratigraphy and geomorphic patterns that can be compared with the well‐documented eastern counterpart. In Tambolaka, the reef sequence displays four lower layers of bedded chalky limestone units with a weakly cemented sandy matrix, which we attribute to the Middle Miocene to Pliocene Wakabukak formation based on calcareous nannofossils and planktonic foraminifers. The uppermost layer is a calcretized reefal limestone unit with a well‐lithified sandy matrix, which we attribute to the Plio–Pleistocene reef sequence of the Kalianga formation. Seven marine terraces imprint the regional morphology, four of which we correlate with Marine Isotope Stage (MIS) 5e, MIS 7e, MIS 9e, and MIS 11c terraces of Cape Laundi, northeast Sumba. When scrutinized at the light of numerical models of reef development, these results indicate that the morphodynamics of reefal sequences is strongly impacted by the tectonic evolution. The geodynamic context sets both the extrinsic conditions of reef development, such as the morphology of the basement and hydrodynamics, and the intrinsic properties, in particular reef growth rate. While the morphodynamic evolution of the sequence is at first‐order representative of the interplay between uplift rates and sea level oscillations, the detailed assemblage of the reef units drastically varies along the coastline.

Effects of Marine Microalgae on the Developmental Growth of the Sea Urchin Larviculture Anthocidaris crassipina

The sea urchin is a very important aquatic economic organism in many countries and has high food value. However, it has recently been heavily fished, and it would be of great importance to the sustainable development of coral reefs to develop large-scale aquaculture of sea urchins. Sea urchins are prone to death during larval development. Therefore, in this study, three kinds of microalgae were used as the initial food for sea urchins to evaluate whether different kinds of microalgae can improve their survival and growth rates. Chaetoceros muelleri (C), Isochrysis galbana tml (I), and Dunaliella salina (D) were fed to A. crassipina at concentrations of 5000, 10,000, 20,000, and 30,000 cell mL−1. A fasted group was used as control (N). The final body length, final body width, final stomach length, rudiment length, survival rate and morphology were measured to evaluate development and growth. The results showed that feeding with C. muelleri resulted in better growth and survival. After 9 days of feeding with C(20,000 cells mL−1), the rudiment length reached 203.33 ± 12.47 µm. The onset of metamorphosis was observed 12 days post-feeding. The survival rate after feeding C was also significantly higher than that after feeding I and D. In summary, when sea urchins are breeding, it is recommended to choose C(20,000 cells mL−1) or C(30,000 cells mL−1) as the initial feed for larvae to increase the growth and survival of sea urchin seedlings.

Climate Change and The Persistence of Coral-Reef Development in Upwelling Zones

Upwelling exerts a major control on coral-reef development in the eastern tropical Pacific (ETP). Upwelling zones exhibit conditions that are detrimental to coral growth, such as low sea-surface temperatures and high levels of turbidity. During the late Holocene, the reefs in the strongly upwelling Gulf of Panamá (GoP) and the weakly upwelling Gulf of Chiriquí (GoC) experienced a climate-driven hiatus in coral growth and reef development, and strong upwelling exacerbated this hiatus in the GoP. Strong upwelling in the GoP is now acting as a buffer against thermal stress, providing a refuge from climatic warming, whereas corals in the GoC are highly vulnerable to increased thermal stress. Using ecological surveys and paleoecological data, we quantified calcification and bioerosion processes for the reefs in these two gulfs to develop carbonate-budget models. We determined the reef-accretion potential (RAP) for reefs in each gulf to project their capacity to keep pace with current and predicted future rates of sea-level rise. On average, reefs in the GoP exhibited an average RAP of 5.5 mm yr-1, which would be enough to keep pace with future rates of sea-level rise if CO2 emissions were reduced under representative concentration pathways (RCPs) 2.6 and 4.5. In contrast, reefs in the GoC exhibited an average RAP of only 0.3 mm yr-1, which is not even enough to keep pace with contemporary rates of sea-level rise in Panamá (1.4 mm yr-1). Furthermore, even if the reefs in either gulf could achieve 100% coral cover, none of them has the capacity to keep pace with RCP 8.5. Although the GoP should support reef development in the near future, reducing greenhouse-gas emissions will be essential to ensure the persistence of accreting reefs and promote the recovery of those vulnerable to net erosion.

Geological Perspectives on the Degradation and Restoration of Coral Reefs

The growth and maintenance of coral-reef structures built over 1000s of years serve as the foundation for the myriad ecosystem services reefs provide to society. Predicting how reef building will change in the future is, therefore, critical to designing effective coral-reef management strategies; however, it is challenging to accurately forecast the long-term process of reef accretion based on short-term ecological studies alone. Geological records, particularly those from sensitive, marginal reef environments such as the subtropical reef system of south Florida, are essential for projecting changes in reef accretion, and for optimizing strategies for coral-reef management. Using a combination of millennial-scale reconstructions of reef accretion and paleoecology from reef cores, and contemporary carbonate budget modeling, I evaluated the past, present, and a possible future of coral-reef development in south Florida. I will show that climate has been the primary control on the rate and extent of regional reef development and, by 3000 years ago, reef accretion was negligible throughout the region. This confined the ecosystem to an unstable equilibrium in which a veneer of living coral was the only barrier to catastrophic reef erosion. In recent decades, climate and other anthropogenic disturbances have pushed many reefs into a novel state characterized by a loss of reef-building corals that is unprecedented in the geological record. These changes have unbalanced Florida’s carbonate budgets, leading to increases in reef-framework erosion. I will show that there is hope for ongoing coral restoration efforts to revive reef growth on a local scale to levels comparable to long-term natural baselines; however, the central role of climate in both the millennial-scale declines in reef building and the modern decline in coral populations suggests that the efficacy of these local efforts will be limited without global-scale action to mitigate anthropogenic climate change.

Upwelling, climate change, and the shifting geography of coral reef development

The eastern tropical Pacific is oceanographically unfavorable for coral-reef development. Nevertheless, reefs have persisted there for the last 7000 years. Rates of vertical accretion during the Holocene have been similar in the strong-upwelling Gulf of Panamá (GoP) and the adjacent, weak-upwelling Gulf of Chiriquí (GoC); however, seasonal upwelling in the GoP exacerbated a climate-driven hiatus in reef development in the late Holocene. The situation is now reversed and seasonal upwelling in the GoP currently buffers thermal stress, creating a refuge for coral growth. We developed carbonate budget models to project the capacity of reefs in both gulfs to keep up with future sea-level rise. On average, the GoP had significantly higher net carbonate production rates than the GoC. With an estimated contemporary reef-accretion potential (RAP) of 5.5 mm year−1, reefs in the GoP are projected to be able to keep up with sea-level rise if CO2 emissions are reduced, but not under current emissions trajectories. With an estimated RAP of just 0.3 mm year−1, reefs in the GoC are likely already unable to keep up with contemporary sea-level rise in Panamá (1.4 mm year−1). Whereas the GoP has the potential to support functional reefs in the near-term, our study indicates that their long-term persistence will depend on reduction of greenhouse gases.

Ancient hydrocarbon slicks recorded by a coral atoll in the South China Sea

The intensifying oil slicks exert deleterious effects on coral reefs, hence attracting many investigations on coral's tolerance and resilience to anthropogenic oil pollution. We hypothesize that intense seafloor hydrocarbon seepages in the past could also produce oil slicks and subsequently contaminate coral reefs nearby, which may provide clues on their responses to natural oil contamination. Here, a long coral reef core (NK1) drilled in the isolated Meiji Atoll in the southern South China Sea is analyzed for aliphatic hydrocarbons to test this hypothesis. Two depth intervals, centered at 45–65 m and 95–110 m, corresponding to ages of ca. 0.3–0.6 Ma and 1.3–1.5 Ma, respectively, are characterized by geological-configured hydrocarbons indicative of ancient natural oil pollution. Because the Meiji Atoll stands on a volcanic seamount and in isolation in the sea, we deem that natural oils were incorporated into contemporary coral reefs by surface oil slicks, which were derived from hydrocarbon seepages at the seafloor of the adjacent petroliferous basins. The natural hydrocarbon seepages should result from regional tectonic movements that occurred extensively in the middle and late Pleistocene, likely in association with the arc-continent collision in the Taiwan region since 2 Ma. Albeit the two significant contamination events, it appears that they exerted insignificant disturbances on the growth and development of coral reefs, perhaps protected by certain holobiotic oil-degrading bacteria as indicated by excess production of bacterial branched fatty acids during the events. Our study thus suggests a high resilience of coral reefs to oil pollution under natural conditions, which, of course, does not mean that they would survive readily from the abrupt and intense anthropogenic pollution.

Climate-modulated range expansion of reef-building coral communities off southeast Florida during the late Holocene

The Holocene reefs off southeast Florida provide unique insights into the biogeographical and ecological response of western Atlantic coral reefs to past climate change that can be used to evaluate future climate impacts. However, previous studies have focused on millennial-scale change during the stable mid-Holocene, making it difficult to make inferences about the impact of shorter-term variability that is relevant to modern climate warming. Using uranium-series dating of newly discovered subfossil coral rubble deposits, we establish a new high-resolution record of coral community development off southeast Florida during a period of variable climate in the late Holocene. Our results indicate that coral communities dominated by reef-building Acropora palmata and Orbicella spp. persisted in the nearshore environments off southeast Florida ~75 km north of their primary historical ranges between ~3500 and 1800 years before present. This timing coincides with regional warming at the northern extent of the Atlantic Warm Pool, suggesting a likely link between regional oceanographic climate and the expansion of cold-sensitive reef-building coral communities to the high-latitude reefs off southeast Florida. These findings not only extend the record of coral-reef development in southeast Florida into the late Holocene, but they also have important implications for future range expansions of reef-building coral communities in response to modern climate change.

Coral reef development and sea-level changes over the past 50,000 years: new evidence from the north-west shelf of Australia

Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia’s North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps. In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km south-east of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart. High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines and tidal-estuarine channel systems on the adjacent shelf between ~ 90 to 110 mbsl. The authors have completed detailed logging, high-spatial resolution hyperspectral scanning, petrologic, mineralogic and sedimentary facies analysis of these cores, including a precise palaeoenvironmental reconstruction based on coral, algal and larger benthic foraminifera assemblages; and extensive radiometric dating. The authors have observed a complex suite of lithologies including in situ coralgal reef frameworks, well-lithified to friable grainstones, packstones and coralline algal floatstone facies separated by at least two major palaeosol horizons. Together with thirty 14C-AMS and closed-system U/Th ages spanning 10.7 to > 50 ka, the authors define a complex but consistent record of four distinct chrono-stratigraphic units (Units 1 to 4), representing a repeated succession of shallow reef to deep reef-slope depositional settings as the platforms experienced repeated sea-level oscillations (interstadial/stadial to glacial/deglacial) over the last 75,000 yr. Two distinct phases of shallow-water, high-energy reef development are defined. The age of the older, diagenetically distinct reef unit (Unit 3) is unknown but interpreted to have developed before the MIS 4 lowstand (~ 65 ka). However, firm chronological constraints on the MIS 3 development of the younger reef unit (Unit 2), place the position of relative sea-level (RSL) between ~ 63 to 75 + 1.8 mbsl by 45.95 to 39.23 + 0.2 ka, consistent with other predictions and observations for the region. Following its exposure and demise due to sea-level fall to the Last Glacial Maximum (LGM), the platform system was unable to re-establish fully as it was reflooded during the subsequent deglacial sea-level rise. Deeper reef slope (Unit 1) facies dominate the core tops between ~ 13.2 to 10.7 ka, representing a major hiatus in shallow-water reef development on the platforms. Deglacial sea-level rise was either too fast and/or other environmental conditions inadequate (i.e. massive riverine sediment flux due to the strengthening Australian summer monsoon and/or reworking of shelf sediments) to allow re-establishment of shallow-water coral reef development on the platforms apart from a few isolated and distal locations (i.e. Ashmore, Cartier, Adele and Scott Reefs).

Sr isotope variations in Oligocene–Miocene and modern biogenic carbonate formations of Koko Guyot (Emperor Seamount Chain, Pacific Ocean)

The Hawaiian–Emperor Seamount Chain, a major topographic feature of the Pacific Ocean floor, is composed of seamounts capped with fossil coral reef deposits that originally formed close to sea level but are now covered by hundreds of meters of water owing to prolonged subsidence. These fossil reef deposits are important archives of paleoenvironmental change and yield information on the subsidence history of the seamounts. We studied the Sr isotope compositions of Oligocene–Miocene coral reef limestone from Koko Guyot in the southern Emperor Seamount Chain to assess the dynamics of the subsidence. The ages of the studied samples containing coral fragments established by Sr isotope stratigraphy vary from 26.3 to 20.1 Ma. In contrast, the youngest samples (15.3 Ma), which were deposited in water depths of >120 m, are barren of corals and are composed exclusively of bryozoans and coralline algae. The subsidence rate of the Koko Guyot volcanic structure was not constant over time. Integration of our new data with the results of previous studies reveals that the subsidence rate was 0.046 ± 0.005 mm/yr during the first 25–30 Myr (from 49–44 to 20 Ma). During this period, Koko Guyot was in a bathymetric interval favorable for coral reef development, and its subsidence was compensated by rapid vertical growth of the reef. Subsequently, the subsidence rate decreased to an average value of 0.019 ± 0.003 mm/yr from 20 to 15 Ma. The decrease in the rate of bottom subsidence coincided with unfavorable environmental conditions for coral reef development, leading to the disappearance of corals. The average subsidence rate has been 0.015 ± 0.002 mm/yr since 15 Ma, comparable to the present-day subsidence rate. We also analyzed the stable Sr isotope ratios (δ88/86Sr) of warm-water coral samples formed at 25–20 Ma (0.32‰ ± 0.1‰), as well as carbonate of large benthic foraminifera, coralline algae, and other non-coral species for the period 20–15 Ma (0.10‰ ± 0.09‰). We suggest that the large difference in carbonate δ88/86Sr between 25–20 and 20–15 Ma corresponds to a difference in the fractionation factor caused by environmental and benthic community change.

Formation and development of coral reefs in the South China Sea

Coral reefs are unique geologic bodies that are formed by biological activity in tropical and subtropical oceans and coastal areas. When and how these reefs came into being and reached their present scales in the ocean remain controversial. Based on strontium, carbon and oxygen isotopes and mineralogical, micropaleontological and elemental analyses of the coral Meiji Reef in the Nansha Islands, South China Sea, we reconstructed the formation and evolutionary history of the island. The results show that the coral reef developed on top of a volcanic basement under the combined influence of diverse factors, especially regional tectonic movement and long-term climatic change. The evolution of the reef was divided into eight stages based on these factors. Extensive exposure was frequently found on the island, and an almost 1000-m thick coral reef eroded simultaneously. The entire development process of coral reefs on the island may be explained by a phased erosion deposition model, which was also found to be suitable for studying the development of coral reefs in the Xisha Islands. We suggest that many coral reefs in the tropics may not flourish continuously over long periods of time, and sedimentary hiatuses normally occur in the layers of coral reefs.