Quaternary Reef Research Articles

Competition between Coral and Algae in Tertiary and Quaternary Reefs: Greenhouse to Icehouse Transitions

Competition between Coral and Algae in Tertiary and Quaternary Reefs: Greenhouse to Icehouse Transitions

Fibrous dolomite formation at a Miocene methane seep may reflect Neoproterozoic aragonite-dolomite sea conditions

Fibrous dolomite widely formed in Neoproterozoic marine sedimentary environments, but apparently disappeared in the Phanerozoic. Here, fibrous dolomite is recognised in a Miocene methane seep limestone (Marmorito, Italy) by synchrotron X-ray diffraction, length slow crystal optics and primary zonation under cathodoluminescence, which is unexpected. Low δ13C values and their negative correlation with MgCO3 contents indicate a formation driven by highly alkaline pore waters and catalysis of dissolved sulphide generated by sulphate-driven anaerobic oxidation of methane. Cementing cavities of reefal carbonate, Neoproterozoic fibrous dolomite might have formed under sulphate-reducing conditions like Quaternary reef microbialites. Since the cavities of Neoproterozoic reefs were restricted microenvironments, the formation of fibrous dolomite was possibly favoured by catalysis similar to its Miocene seep counterpart. Our findings reinforce the concept of penecontemporaneous dolomite formation by sulphide catalysis and contribute to our understanding of the environmental conditions of the Neoproterozoic.

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.

Three common sampling techniques in Pleistocene coral reefs of the Red Sea: a comparison.

Line Intercept Transects (LIT), Point Intercept Transects (PIT), and Photoquadrats (PQ) are the most common quantitative sampling techniques in modern and fossil coral reefs. Data from coral reefs obtained by the different methods are generally compared between various reef ages and localities. Quaternary reefs from warmer interglacial periods, which represent climate scenarios projected for the future, are particularly interesting for comparisons with modern reefs. Importantly, fossil reefs differ from modern reefs because they are diagenetically altered and time averaged. While several studies have compared different quantitative methods in modern reefs, very few have dealt with the comparability among fossil and between fossil and modern reefs. Here, we compare LIT, PIT at 10, 20 and 50 cm intervals, and PQ in two Pleistocene reef localities in Egypt. We find that alpha diversity, reef cover and community composition are dependent on the method. Results gained with plotless methods (LIT, PIT) differ strongly from results gained with plot methods (PQ). However, coral cover results are similar between LIT and PIT, and community composition is indistinguishable between the two, but alpha diversity depends on the interval used for PIT. We discuss the implications of our findings for comparing coral reefs of various ages and localities. We recommend surveying Pleistocene reefs with PIT at 20 cm intervals. This is because A) alpha diversity is well captured, B) the amount of time averaging recorded by PIT is reduced compared to PQ, C) the PIT results can be directly compared to reefs analyzed by LIT, and D) the method is less time consuming than LIT and PQ.

Neogene and Quaternary reef terraces of Mangaia, Rarotonga and Aitutaki (southern Cook Islands, South Pacific) revisited: gauges of sea-level change and island uplift?

New sedimentological data of facies and diagenesis as well as chronological data including strontium (87Sr/86Sr)-isotope ratios and uranium (U)-series dating, radiocarbon (14C) accelerator mass spectrometry (AMS) dating and biostratigraphy from elevated reef terraces (makatea) in the southern Cook Islands of Mangaia, Rarotonga and Aitutaki contribute to controversial discussions regarding age and sea-level relationships of these occurrences during the Neogene and Quaternary. The oldest limestones of the uplifted makatea island of Mangaia include reef-related facies which are mid-Miocene in age, based on new Sr-isotope and biostratigraphical data. In between these older deposits and the lowest coastal reef terrace of marine isotope stage (MIS) 5e, various older Pleistocene reef-related facies were identified. Based on Sr-isotope ratios, these were deposited during earlier Pleistocene highstands (as old as 2.28 Ma). Rare reef terraces on Rarotonga belong to the Plio-Pleistocene and the late Miocene, according to 87Sr/86Sr ratios. The late Miocene age is enigmatic as it exceeds the age of subaerially exposed volcanic rocks of Rarotonga island. The fossil reef could have formed on an older submarine volcanic high that was later displaced by younger volcanism to its present position, or the Sr-age could be too old due to diagenetic resetting. The Plio-Pleistocene Rarotonga reef terraces are overlain irregularly by Holocene reef deposits that are interpreted as storm rubble. Reef terraces on Aitutaki represent evidence of a higher-than-present (up to 1 m) sea-level during the late Holocene, based on 14C AMS age data. They are very similar to elevated late Holocene reefs of adjacent French Polynesia with regard to composition, elevation and age.

A Cenozoic Great Barrier Reef on Australia's North West shelf

Abstract The Great Barrier Reef of Australia is the largest modern coral reef system on Earth. However, a similar-sized barrier reef (~2000 km long) existed on Australia's north-western margin in the middle Miocene and to some extent, still exists today. Seismic profiles reveal that this reef system was first initiated in the late Paleogene (~34–28 Ma) and reached its zenith as a prograding barrier reef during the middle Miocene (16–15 Ma). This giant reef system backstepped in the late Miocene (~10 Ma), forming a series of isolated atolls and pinnacle reefs. These remaining reefs were gradually extinguished, leaving only a few surviving isolated atolls today (Rowley Shoals, Scott Reef, Seringapatam Reef and Ashmore Reef). The distribution of these extant reefs therefore outlines the former presence of the mid-Miocene Great Barrier Reef of the Australian North West Shelf. This middle Miocene shallow-water rimmed-reefal shelf grew contemporaneously on a deeper-water heterozoan ramp, producing an unusual mixed heterozoan/photozoan carbonate system for most of its length. The initiation of reef growth appears related to Australia's northward drift into lower latitudes, combined with warm mid-Miocene climatic optimum conditions. However, the sudden and simultaneous initiation of reef growth in the mid-Miocene across the North West Shelf was probably also related to changes in paleoceanography (increased aragonite saturation and/or nutrient-depletion) perhaps influenced by changing climatic conditions (increased aridity). From the late Miocene (~10 Ma) to Holocene, high rates of subsidence across the North West Shelf probably set the scene for ultimate demise of this giant barrier reef system and led to widespread backstepping with the development of a series of isolated atolls and pinnacle reefs. It is likely that other paleoceanographic and climatic events (like nutrient supply, carbonate saturation, eustasy etc) combined with continued high subsidence rates acted to gradually extinguish the reef system in a series of steps over a period of 10 million years. The onset (~1 Ma) of a strong Leeuwin Current (with the delivery of warm and nutrient depleted water from the north) and its effect on the Quaternary reefs of the Western Australian coast is a good example of a paleoceanographic process that would be difficult to recognize in an ancient setting. It is likely that similar paleoceanographic events have controlled the rich and complex history of reef growth and drowning since the mid-Miocene on the North West Shelf.

Microbialites in Last Glacial Maximum and deglacial reefs of the Great Barrier Reef (IODP Expedition 325, NE Australia)

Microbialites are volumetrically abundant components in Last Glacial Maximum and deglacial reefs in the Australian Great Barrier Reef sampled by IODP Expedition 325 in 34 holes from 17 sites (M0030–M0058), along four transects on the shelf edge. Detailed radiometric datings show that four distinct reef phases developed between 28and 10 ka, displaying offlapping and then backstepping patterns. The reef boundstone facies include coralgal, coralgal-microbialite and microbialite boundstone. The microbialite consists of combinations of micrite/microspar, bioclasts, siliciclastic grains (up to 14.5%), fenestrae and encrusting epibionts. The micrite/microspar is high-magnesian calcite commonly irregularly clotted, fenestral and peloidal. Mesoscale microbialite fabrics include laminated, structureless, digitate, intraskeletal and boring-filling, and coatings on debris. Intraskeletal and boring-filling is the first fabric to develop in skeletal voids and borings. It is usually followed by structureless and laminated microbialite, locally overlain by digitate fabric. Microbialite-coated debris can occur at any stage in this succession, including in bioclastic accumulations where the scarce in situ framework builders are mainly encrusting corals. Lipid biomarkers of intermediate to high specificity for sulfate-reducing bacteria, together with δ13C values of these lipids, indicate that microbialite formation was favored by sulfate-reducing bacteria in anoxic microenvironments, probably under high nutrient levels. The microbialite in fore-reef deposits accumulated in the photic zone in water depths of a few to several tens of meters, within small spaces generated by large bioclasts and encrusting corals in the topmost centimeters of the sediment. These crusts that formed on the illuminated surface constitute a previously unrecognized style of microbialite formation in Quaternary reefs. As with the cryptic crusts described from other reef locations, its greatest development occurred during the Last Glacial Maximum and early deglaciation. Microbial carbonate formation during this interval may reflect elevated seawater carbonate saturation corresponding with relatively low levels of atmospheric CO2.

琉球列島,宝島・小宝島の第四紀後期離水サンゴ礁と完新世後期の海水準

琉球列島,宝島・小宝島の第四紀後期離水サンゴ礁と完新世後期の海水準

Pleistocene reef development in Bulukumba, South Sulawesi

Quaternary reefs are commonly studied right now to explain climate change during that time. They act as a good archive of climate change, because their development is influenced by climate condition. The research area is located in the southern tip of Bulukumba Regency, South Sulawesi. The objective of this research is to define the development of the reef. Methods applied in this research are field survey of 4 line transects along reef cliff. Laboratory work is mostly on petrographic and biofacies analyses in order to reconstruct the reef development. Four reef biofacies have developed in this study namely 1) Coralgal framestone - wackestone , 2) Massive coral framestone facies , 3) Platylike coral Bindstone facies, and 4) Branching Coral Bafflestone facies. Based on the facies association and organism accumulation, the reefs are interpreted to be developed within a reef complex in a shallow marine environment.

Quaternary onset and evolution of Kimberley coral reefs (Northwest Australia) revealed by high-resolution seismic imaging

The inner shelf Kimberley Bioregion of Northwest Australia is characterised by a macrotidal setting where prolific coral reefs growth as developed around a complex drowned landscape and is considered a biodiversity “hotspot”. High-resolution shallow seismic studies were conducted across various reef settings in the Kimberley (Buccaneer Archipelago, north of Dampier Peninsula, latitude: between 16°40′S and 16°00′S) to evaluate stratigraphic evolution, interaction with different substrates, morphological patterns and distribution. Reef sites were chosen to assess most of the reef types present, particularly high intertidal planar reefs and fringing reefs. Reef internal acoustic reflectors were identified according to their shape, stratigraphic position and characteristics. Two main seismic horizons were identified marking the boundaries between Holocene reef (Marine Isotope Stage 1, MIS 1, last 12ky), commonly 10–20m thick, and MIS 5 (Last Interglacial, LIG, ~120ky, up to 12m thick) and Proterozoic rock foundation over which Quaternary reef growth occurred. Within the Holocene Reef unit, at least three minor internal reflectors, generally discontinuous, subparallel to the reef flat were recognised and interpreted as either growth hiatuses or a change of the coral framework or sediment matrix. The LIG reefs represent a new northernmost occurrence along the Western Australian coast.The research presented here achieved the first regional geophysical study of the Kimberley reefs. Subbottom profiles demonstrated that the surveyed reefs are characterised by a multi-stage reef buildup, indicating that coral growth occurred in the Kimberley during previous sea level highstands. The data show also that antecedent substrate and regional subsidence have contributed, too, in determining the amount of accommodation available for reef growth and controlling the morphology of the successive reef building stages. Moreover, the study showed that in spite of macrotidal conditions, high-turbidity and frequent high-energy cyclonic events, corals have exhibited prolific reef growth during the Holocene developing significant reef accretionary structures. As a result coral reefs have generating habitat complexity and species diversity in what is a biodiversity hotspot.

Quaternary reef response to sea‐level and environmental change in the western Atlantic

AbstractThe tropical–subtropical western Atlantic realm harbours abundant fringing reefs, two major barrier reefs (Belize, Florida), several atolls and large reef‐fringed carbonate platforms (Bahamas). Data from Belize and south Florida indicate that major barrier reefs were established during the high sea‐levels of the long and warm marine isotope stage 11. Reefs formed during Pleistocene sea‐level highstands, whereas erosion, non‐deposition and the formation of subaerial exposure surfaces prevailed during lowstands of sea‐level (Florida, Bahamas); exposed reefs and platforms with siliciclastic hinterlands were characterized by deposition of clays and sands on fluvial plains (Belize). Elevated terraces and outcrops of reefs of the last interglacial of marine isotope stage 5e are evidence of higher than present sea‐level and temperature that sustained large reef‐fringed carbonate platforms and ramps in the Florida–Bahamas region and Central America (Belize, Mexico), including ooid deposition in warm and shallow waters. During the highstands of marine isotope stages 5c and 5a, at sea‐levels several metres lower than today, fringing reefs developed in parts of the Caribbean, for example, in south Florida (‘outlier reefs’). Data from the island of Barbados support the contention that the postglacial sea‐level rise from about −120 m was rapid and punctuated by two meltwater pulses Ia and Ib, which bracket the beginning and end of the Younger Dryas, respectively. After reflooding of shelves in the early Holocene, reefs developed preferentially on highs of the underlying karstified Pleistocene limestone surface and on former river channel bars. Many reefs in the western Atlantic region kept pace with Holocene sea‐level rise whereas some were drowned. Reasons for reef drowning are debated and include sea‐level rise, elevation of antecedent topography, and deterioration of environmental conditions such as high nutrient input. Apart from sea‐level and antecedent topography, factors such as exposure to waves and currents, storms, and sedimentation have controlled reef development. The historical decline of acroporid corals in the Caribbean realm has been attributed to cooling following the Holocene thermal optimum; however, historical die‐offs of reef‐building corals and other reef organisms suggest that some if not all recent declines were unprecedented during the Holocene. Rising temperatures, decreasing carbonate saturation, increased frequency of major storms, high nutrient concentrations and sedimentation, overfishing, and disease certainly played a role in decline, but systematic studies relating these environmental factors to Holocene reef‐building are only in the early stages.

Towards an improved understanding of erosion rates and tidal notch development on limestone coasts in the Tropics: 10 years of micro‐erosion meter measurements, Phang Nga Bay, Thailand

AbstractKnowledge and understanding of shore platform erosion and tidal notch development in the tropics and subtropics relies mainly on short‐term studies conducted on recently deposited carbonate rocks, predominantly Holocene and Quaternary reef limestones and aeolianites. This paper presents erosion rates, measured over a 10 year period on notches and platforms developed on the Permian, Ratburi limestone at Phang Nga Bay, Thailand. In so doing it contributes to informing a particular knowledge gap in our understanding of the erosion dynamics of shore platform and tidal notch development in the tropics and subtropics – notch erosion rates on relatively hard, ancient limestones measured directly on the rock surface using a micro‐erosion meter (MEM) over time periods of a decade or more.The average intertidal erosion rate of 0.231 mm/yr is lower than erosion rates measured over 2–3 years on recent, weaker carbonate rocks. Average erosion rates at Phang Nga vary according to location and site and are, in rank order from highest to lowest: Mid‐platform (0.324 mm/yr) > Notch floor (0.289 mm/yr) > Rear notch wall (0.228 mm/yr) > Lower platform (0.140 mm/yr) > Notch roof (0.107 mm/yr) and Supratidal (0.095 mm/yr). The micro‐relief of the eroding rock surfaces in each of these positions exhibits marked differences that are seemingly associated with differences in dominant physical and bio‐erosion processes. The results begin to help inform knowledge of longer term shore platform erosion dynamics, models of marine notch development and have implications for the use of marine notches as indicators of changes in sea level and the duration of past sea levels. Copyright © 2014 John Wiley & Sons, Ltd.

An identification guide to some major Quaternary fossil reef‐building coral genera (Acropora, Isopora, Montipora, and Porites)

AbstractFossil coral identification is essential for paleoenvironment and sea level reconstructions and in paleoecological and evolutionary studies. It follows the taxonomy of their modern analogues and therefore is based on the external skeleton macromorphology. However, for geologists examining outcrops or drill cores, identification largely relies on cross sections. The coral surface is usually obscured by encrusters or by sediments, or because corals are broken or eroded. It is therefore important to understand the relationship between the internal structure of corals and their surface morphology. We provide a comprehensive description of the internal structure of four common coral genera (Acropora, Isopora, Montipora, and Porites). This identification guide is addressed to reef geologists who wish to identify Quaternary reef corals at the generic level based on cross sections. The massive basal part of Acropora is composed of an interbranch space filled with a highly regular mesh‐like network of skeletal elements parallel and perpendicular to growth direction. Corallites have conspicuous dissepiments with a ladder‐like appearance in longitudinal section. Corallites of Isopora are structurally similar but they do not show the same differentiation into axial and radial corallites. Corallites are generally sinuous and the coenosteum is less regular. Montipora has smaller corallites which appear as tubes lined with spiny septa. The coenosteum is characterized by long rod‐like skeletal elements parallel to growth direction projecting upward at the colony surface to form spinules or larger ornementations. These vertical rods are connected laterally by short bar‐like skeletal elements. In Porites, the size of corallites is similar to Montipora but they are juxtaposed and completely filled with skeletal elements. Longitudinal cross sections display a dense network of corallites' skeletal components parallel and perpendicular to growth direction. This new taxonomic guide provides a useful reference material for those who attempt to identify reef‐building corals in the fossil record.

Pleistocene Coral Assemblages on Irabu-Jima, South Ryukyu Islands, Japan

Abstract. We identified 40 genera and 83 species of scleractinian corals from middle Pleistocene reef limestones on Irabu-jima, located in the South Ryukyu Islands (South Ryukyus); these data were then combined with previously published data sets on Pleistocene coral assemblages on Okinawa-jima (southern Central Ryukyus), and Kikai-jima (northern Central Ryukyus) to perform statistical analyses, including cluster analysis and multidimensional scaling (MDS) ordination of Bray—Curtis (BC) similarity coefficients applied to coral taxa (R-type analysis) and coral assemblages (Q-type analysis). Results of the analyses, combined with field descriptions of the coral assemblages, allowed interpretations of reef paleoenvironments. Outcrops of Quaternary reef deposits in the Ryukyus comprise both shallow- and deep-water coral assemblages. Generally, coral diversity increases southward along the island chain. The statistical analyses highlight differences in the taxonomic compositions of fossil coral assemblages in ...

Subsidence of the Miyako‐Sone submarine carbonate platform, east of Miyako‐jima Island, northwestern Pacific Ocean

AbstractBathymetric mapping and observations of the seafloor using a remotely operated vehicle (ROV, Hyper‐Dolphin 3K) were carried out on the slopes of the Miyako‐Sone submarine platform, east of Miyako‐jima in the Ryukyu Islands, northwestern Pacific Ocean. The bathymetric map indicates that terraces are present at water depths of approximately 140 m, 330 m, 400 m, and 680 m on the northwestern slope of the platform. A number of NW–SE trending lineaments, probably faults, extend perpendicular to the axis of the Ryukyu Island Arc. Two ROV surveys were conducted at water depths ranging from 519 m (on the slope) to 121 m (shallowest part of the platform). The surveys revealed that well‐indurated carbonate rocks are exposed at terrace margins and on upper slopes, and that the lower slopes are covered with modern sediments consisting of unconsolidated, coarse‐sand‐sized bioclastic carbonates. Calcareous nannofossils from the well‐indurated carbonate rocks indicate a Middle–Late Pleistocene age, which suggests that the rocks correlate with the Quaternary reef and fore‐reef deposits of the Ryukyu Group (Ryukyu Limestone) on the Ryukyu Islands. No siliciclastic deposits corresponding to the upper Miocene–lower Pleistocene Shimajiri Group (as exposed on Okinawa‐jima and Miyako‐jima islands) were recovered during the surveys. Coeval well‐indurated carbonate rocks, all of which formed in a similar sedimentary environment, have been downthrown towards the west due to displacements on the western sides of normal faults. Subsidence of the Miyako‐Sone submarine platform was the result of large vertical displacements on such normal faults. The timing of initial subsidence cannot be tightly constrained, but the presence of the youngest limestone at progressively lower levels towards the west suggests the subsidence continued until after 0.265 Ma.

Environmentally controlled succession in a late Pleistocene coral reef (Sinai, Egypt)

The concept of ecological succession has been frequently applied in the study of ancient reefs. Whereas Paleozoic and Mesozoic reefs are commonly thought to reveal an autogenic primary—climax zonation, patterns in Neogene and Quaternary reefs are much more diverse. Here, we describe a well-preserved late Pleistocene coral reef from Dahab on Sinai Peninsula (Egypt), which shows a distinct zonation that resembles an ecological succession. In contrast to classical examples of ecological successions, species composition, paleoenvironmental conditions, and coral biodiversity of the Dahab reef indicate an allogenic, sea-level controlled community change, from marginal marine to reef slope and back reef. A review of the literature confirms that autogenic, short-term successions are virtually absent in Quaternary reefs. We predict that long generation times of corals make it unlikely that classical autogenic successions develop in reefs at all, unless environmental conditions are unusually stable.

Magnesium and strontium systematics in tropical speleothems from the Western Pacific

We present bulk average Sr and Mg data for 13 speleothems from different locations in the western equatorial Pacific (Guam, Solomon Islands, and Vanuatu). These data plot on a single straight line in a graph of ln(Sr/Ca) vs ln(Mg/Ca) with a slope of ~0.9. A 22,000year record of Sr and Mg in one of these samples from Guam also plots with the same slope, suggesting that the process partitioning Mg and Sr within the Guam speleothem is the same as the one that partitions Mg and Sr between the different Pacific speleothems. We rule out temperature, growth rate, detrital phases, and sea-spray as likely mechanisms for this correlation. We construct mathematical models of limestone diagenesis and show that this cannot explain the slope of the correlation.Our favoured explanation for the correlation is calcite/water interaction in the form of prior calcite precipitation (PCP) and/or incongruent calcite dissolution (ICD). We present a formal mathematical model of PCP and prove that the slope of a graph of ln(Sr/Ca) and ln(Mg/Ca) is given by KdSr−1KdMg−1. A similar equation is derived for ICD in a companion paper (Sinclair, 2011). Using published values for KdSr and KdMg, this slope is calculated to be ~0.88, in excellent agreement with our observations. Because the slope is independent of solution and host-limestone composition it can be used to diagnose calcite–water interaction in individual cave water studies where host rock composition is unknown, or in speleothems for which no field drip water data or host-rock geochemistry exists. Approximately half of published drip and speleothem Mg+Sr studies plot within error of this slope.We hypothesize that the overall trend in our Pacific speleothem data results from the individual dripwater systems evolving from a roughly similar initial rock composition set by late Quaternary reef limestone. In the Guam speleothem, the broad peak in Mg and Sr centred on the early–mid Holocene reflects a change in hydrology (rainfall), with the most likely scenario being that dry conditions prevailed on Guam at this time.

Late Quaternary reef growth and sea level in the Maldives (Indian Ocean)

Based on rotary drilling and radiometric and U-series dating, we present the first comprehensive data on Holocene reef anatomy and sea-level rise as well as nature and age of underlying Pleistocene limestone in the Maldives. Holocene reefs in Rasdhoo Atoll, central Maldives, are composed of four facies including (1) robust-branching coral facies, (2) coralline algal facies, (3) domal coral facies, and (4) detrital sand and rubble facies. Branching coral and coralline algal facies predominate the marginal reefs and domal corals and detrital facies preferentially occur in a lagoon reef. In addition, microbialite crusts are found in lower core sections of marginal reefs. Microbialites formed during the early Holocene in reef cavities. Holocene reef thickness ranges from 14.5 m to > 22 m. Reef growth started as early as 8.5 kyr BP. Marginal reefs accreted in the keep-up mode with rates of > 15 m/kyr. Rate of sea-level rise significantly slowed down from 7–6 kyr BP and subsequently gradually rose with rates < 1 m/kyr. The lagoon reef accreted in the catch-up mode with rates of around 4 m/kyr. Even though no indications of a higher than present sea level were found during this study, it is not entirely clear from the data whether the sea gradually rose to or exceeded present level in the late Holocene. Submarine cementation in Holocene reefs studied is rather weak, presumably as a consequence of high accretion-rates, i.e., short time available for consolidation. Pleistocene coral grainstone was encountered in one core at 14.5 m below present level and three U-series dates indicate deposition during marine isotope stage 5e ca. 135 kyr BP.

Nature and biological composition of the New Caledonian outer barrier reef slopes

The Grande Terre of New Caledonia is enclosed by one of the longest barrier reefs in the world. For the first time, the fore-reef slopes of this barrier reef have been sampled by dredging, from 40 to 320 m deep, in order to analyze their sedimentological and biological characteristics. The rocks and sediments can be divided into seven sedimentary facies: bindstones dominated by coralline algal crusts, bindstones dominated by foraminiferal crusts, bindstones dominated by bryozoan crusts, coral framestones, bindstones and interstratified packstones rich in skeletal debris, packstones/wackestones and grainstones rich in rock gravels. Radiocarbon dating performed on encrusting organisms (coralline algae and acervulinids) and corals provide ages relatively young. These ages confirm that the encrusting organisms are modern and the corals mainly dated of Holocene are reworked due to the instability of the fore-reef slopes, especially during storms. Because the biological assemblages are distributed according to a bathymetric range depending on light intensity, a model of distribution of modern encrusting calcareous organisms can be proposed from our observations and analyses. From the upper reef slopes to approximately 90 m, thick coralline algal crusts are dominant and distributed in three groups. Group C, the shallowest parts of the fore slopes, is mainly characterized by mastophorids ( Hydrolithon reinboldii, H. cf. munitum, Lithoporella melobesoides, Aethesolithon cf. problematicum, Neogoniolithon sp. and undetermined species) and lithophylloids ( Lithophyllum sp., L. pustulatum). Group B, composed of lithophylloids ( Lithophyllum sp., L. cf. kotschyanum, L. cf. moluccence, L. pustulatum), Mesophyllum sp. and Peyssonnelia sp. occurs from 15 to 40 m. Group A, rich in Mesophyllum sp., M. cf. mesomorphum, Peyssonnelia sp. and Sporolithon sp. is characteristic of deep reef slopes up to 90 m. Below approximately 90 m, when the light intensity decreases, the encrusting foraminifera acervulinids progressively replace the coralline algal crusts. Such a model is particularly useful to interpret and reconstruct the past Quaternary reef environments rich in crusts of coralline algae and/or foraminifera.

High-precision U-series measurements of more than 500,000 year old fossil corals

Robust, independent age constraints on the absolute timing of climate events based on the U-series dating of fossil coral are sparse before the last glacial cycle. Using multiple-collector inductively coupled plasma mass spectrometry with multiple-Faraday protocols, we are able to date ∼ 600 ka samples with an uncertainty of better than ± 15 ka (2 σ), representing a three-fold improvement in precision compared with previous techniques. Using these methods, we report U-series measurements for a suite of > 500 thousand year old (ka) corals from Henderson Island, an emergent atoll in the south-central Pacific Ocean. The fossil corals show extraordinarily little diagenetic alteration for their age and the best-preserved sample yields a U-series age of 600 ± 15 ka (2 σ), which overlaps with the timing of the warm Marine Isotope Stage (MIS) 15 interglacial. The open-system model of Villemant and Feuillet [Villemant B. and Feuillet N. (2003) Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces. Earth and Planetary Science Letters 210(1–2), 105–118.] and the linear regression (or open-system isochron) is clearly limited for such old samples. However, the open-system model developed by Thompson et al. [Thompson W.G., Spiegelman M.W., Goldstein S.L., and Speed R.C. (2003) An open-system model for U-series age determinations of fossil corals. Earth and Planetary Science Letters 210(1–2), 365–381.] appears to reliably correct for open-system effects in roughly half of the corals, giving a MIS 15 origin for these. Thus the data provide evidence that the systematic addition of 230Th and 234U through α-recoil is a dominant open-system process occurring in the Henderson Island fossil reef system. Several coral samples yield significantly older Thompson et al. open-system ages between 650 and 750 ka. The uncertainty on these ages (typically ± 30 kyrs) is too great for precise assignment but most data overlap with the MIS 17 interglacial. The reliability of these ages is currently unclear. It is shown that separate aliquots of the same coral can yield different Thompson model ages. Therefore, there appear to be additional diagenetic mechanisms that create further anomalous excursions in the U-series systematics, limiting the reliability of the Thompson et al. open-system model.