Coral Aragonite Research Articles

13C– 18O isotope signatures and ‘clumped isotope’ thermometry in foraminifera and coccoliths

Accurate constraints on past ocean temperatures and compositions are critical for documenting climate change and resolving its causes. Most proxies for temperature are not thermodynamically based, appear to be subject to biological processes, require regional calibrations, and/or are influenced by fluid composition. As a result, their interpretation becomes uncertain when they are applied in settings not necessarily resembling those in which they were empirically calibrated. Independent proxies for past temperature could provide an important means of testing and/or expanding on existing reconstructions. Here we report measurements of abundances of stable isotopologues of calcitic and aragonitic benthic and planktic foraminifera and coccoliths, relate those abundances to independently estimated growth temperatures, and discuss the possible scope of equilibrium and kinetic isotope effects. The proportions of 13C– 18O bonds in these samples exhibits a temperature dependence that is generally similar to that previously been reported for inorganic calcite and other biologically precipitated carbonate-containing minerals (apatite from fish, reptile, and mammal teeth; calcitic brachiopods and molluscs; aragonitic coral and mollusks). Most species that exhibit non-equilibrium 18O/ 16O (δ 18O) and 13C/ 12C (δ 13C) ratios are characterized by 13C– 18O bond abundances that are similar to inorganic calcite and are generally indistinguishable from apparent equilibrium, with possible exceptions among benthic foraminiferal samples from the Arctic Ocean where temperatures are near-freezing. Observed isotope ratios in biogenic carbonates can be explained if carbonate minerals generally preserve a state of ordering that reflects the extent of isotopic equilibration of the dissolved inorganic carbon species.

River runoff reconstructions from novel spectral luminescence scanning of massive coral skeletons

Inshore massive corals often display bright luminescent lines that have been linked to river flood plumes into coastal catchments and hence have the potential to provide a long-term record of hinterland precipitation. Coral luminescence is thought to result from the incorporation of soil-derived humic acids transported to the reef during major flood events. Corals far from terrestrial sources generally only exhibit dull relatively broad luminescence bands, which are attributed to seasonal changes in coral density. We therefore tested the hypothesis that spectral ratios rather than conventional luminescence intensity provide a quantitative proxy record of river runoff without the confounding effects of seasonal density changes. For this purpose, we have developed a new, rapid spectral luminescence scanning (SLS) technique that splits emission intensities into red, green and blue domains (RGB) for entire cores with an unprecedented linear resolution of 71.4 μm. Since humic acids have longer emission wavelength than the coral aragonite, normalisation of spectral emissions should yield a sensitive optical humic acid/aragonite ratio for humic acid runoff, i.e., G/B ratio. Indeed, G/B ratios rather than intensities are well correlated with Ba/Ca, a geochemical coral proxy for sediment runoff, and with rainfall data, as exemplified for coral records from Madagascar. Coral cores also display recent declining trends in luminescence intensity, which are also reported in corals elsewhere. Such trends appear to be associated with a modern decline in skeletal densities. By contrast, G/B spectral ratios not only mark the impact of individual cyclones but also imply that humic acid runoff increased in Madagascar over the past few decades while coral skeletal densities decreased. Consequently, the SLS technique deconvolves the long-term interplay between humic acid incorporation and coral density that have confounded earlier attempts to use luminescence intensities as a proxy for river runoff.

Diagenetic processes in carbonate mound sediments at the south-west Rockall Trough margin

Cold water coral covered carbonate mounds at the south-west margin of the Rockall Trough form ridges several kilometres long and up to 380 m high. Piston cores obtained at three mound crests reveal the complex internal structure of the mound build up, with alternating unlithified coral-dominated intervals and lithified intervals. The most recent lithified interval is covered by corals embedded in a fine-grained matrix, comprising ca 11 000 years of continuous mound evolution. Before this time 230Th/U dating shows the presence of several hiatuses in mound build-up. Aragonitic coral material is absent or only present as mouldic porosity in the lithified intervals and coccoliths display widespread overgrowth. Downcore X-ray fluorescence scanning, computer tomography scan images and petrographic observations indicate different degrees of diagenetic alteration. The upper boundary of the most recent lithified interval shows some erosional features, but petrographic observations indicate that initial lithification of the sediments is not related to this erosive event or to long-term non-sedimentation, but to earlier sub-surface diagenesis. Organic matter oxidation and the subsequent lowering of the saturation state of the carbonate system drives dissolution of the unstable aragonitic coral skeletons. Depending on the openness of the system, this can lead to precipitation of a more stable low-magnesium carbonate. A model is presented describing the sedimentary and diagenetic processes leading to the formation of lithified intervals.

Fluoride in coral aragonite related to seawater carbonate

In order to elucidate the controlling factors for fluoride incorporation into coral aragonite (CaCO3), we determined the ratio of fluorine to calcium (F/Ca) in coral skeletons collected from Pohnpei (Micronesia), Khang Khao (Thailand), Cebu (The Philippines), Tosa Bay (Kochi, Japan), Sakai Port (Wakayama, Japan), Tateyama (Chiba, Japan), Sesoko, Rukan-sho and Mizugama (Okinawa, Japan) using the ion-chromatograph method. The F/Ca (mmol/mol) ratio was highest in Tateyama with an average ratio of 6.04, whereas coral collected from Sakai Port, Tosa Bay, Okinawa, Cebu, Pohnpei and Khang Khao exhibited F/Ca ratios of 5.57, 5.02, 4.49, 4.24, 4.00 and 3.75, respectively. The factor controlling F/Ca ratio is discussed based on the proposed coprecipitation process of F in coral aragonite as the ion-exchange reaction: CaCO3(s) + 2F-(aq) = CaF2(s) + CO32-(aq). The equilibrium constant for the above equation can be expressed as: KF´ = [CaF2][CO32-]/[CaCO3][F-]2. In this paper, analyzed F/Ca ratio in coral shows a proportional change to calculated seawater [F-]2/[CO32-]. This finding suggests that F/Ca in coral skeletons is possibly controlled by carbonate ion concentration in seawater.

Thermochemistry of strontium incorporation in aragonite from atomistic simulations

We have investigated the thermodynamics of mixing between aragonite (orthorhombic CaCO3) and strontianite (SrCO3). In agreement with experiment, our simulations predict that there is a miscibility gap between the two solids at ambient conditions. All SrxCa1−xCO3 solids with compositions 0.12 < x < 0.87 are metastable with respect to separation into a Ca-rich and a Sr-rich phase. The concentration of Sr in coral aragonites (x ∼ 0.01) lies in the miscibility region of the phase diagram, and therefore formation of separated Sr-rich phases in coral aragonites is not thermodynamically favorable. The miscibility gap disappears at around 380 K. The enthalpy of mixing, which is positive and nearly symmetric with respect to x = 0.5, is the dominant contribution to the excess free energy, while the vibrational and configurational entropic contributions are small and of opposite sign. We provide a detailed comparison of our simulation results with available experimental data.

Speciation of Mg in biogenic calcium carbonates

A selection of marine biominerals, mostly aragonitic coral skeletons were probed at the Mg K-edge by XANES spectroscopy coupled to μXRF methods and compared to an extensive set of relevant model compounds (silicates, carbonates, oxides and organic). Extensive methodologies are required to better describe the speciation of Mg in those minerals. A combination of ab-initio XANES calculations for defective clusters around Mg in aragonite together with wavelets analyzes of the XANES region are required to robustly interpret the spectra. When using those methodologies, the speciation of Mg ranges from a magnesite-type environment in some scleractinian corals to an organic-type environment. In all environments, the Mg-domains probed appear to be less than 1 nm in size.

Sea-level changes as controlling factor of early diagenesis: the reefal limestones of Adnet (Late Triassic, Northern Calcareous Alps, Austria)

The uppermost Rhaetian Adnet reef is part of the Dachstein carbonate platform and is situated at the transition to the intrashelf Kossen Basin. Its diagenetic evolution is investigated focusing on dissolution cavities in the Tropfbruch quarry of Adnet (near Salzburg) stratigraphically situated immediately below the Triassic–Jurassic boundary. Sea-level changes due to global eustatic trends and regional tectonics are assumed to be the controlling factors in the development of a manifold diagenetic sequence characterized by phases of meteoric dissolution, marine and burial cementation, and internal sedimentation. Despite small-scale variations of the sequence, a superordinate pattern of diagenetic phases could be elaborated. Small-scale eustatic sea-level falls subordinate to a global regression trend caused subaerial exposures of the Adnet reef in the latest Rhaetian to earliest Hettangian. The result was karstification and meteoric dissolution of aragonitic coral skeletons (Retiophyllia) leading to the formation of biomoldic porosity. Coral septa which escaped dissolution were transformed into neomorphic calcite spar under meteoric–phreatic conditions. A first generation of dog-tooth cements precipitated sporadically on the altered coral skeletons. Eustatic sea-level rise in Early to Mid-Hettangian times caused a renewed flooding of the pore space of the Adnet reef by marine water and the influx of a first generation of internal sediments (IS I), derived from the karstified host rock of the Upper Rhaetian reef limestone. These internal sediments are overgrown by radiaxial-fibrous calcites (RFCs) whose oxygen-isotopic signature (δ18O = −1.3 (±0.7)‰) indicates precipitation in deeper (colder) water (18–21°C) due to a first phase of drowning. An intermediate phase of eustatic sea-level lowstand in the Late Hettangian is expressed by dissolution and corrosion of RFCs. Rapid drowning of the Dachstein carbonate platform due to eustatic sea-level rise and tectonic movements took place in the Early Sinemurian and a second generation of internal sediments (IS II) derived from the Lower Sinemurian Adnet Formation is washed into the dissolution cavities. Where IS II is absent, RFCs are overgrown by a second generation of dog-tooth cements with a bright-luminescent outer rim indicating the transition to negative redox conditions in the pore water during shallow burial. Burial diagenesis is represented by blocky calcite cements which occlude the remaining pore space. Depleted oxygen-isotope values and significant Fe contents indicate precipitation under reducing redox conditions and elevated temperatures of 30–50°C at burial depths of 420–870 m. Locally, replacive saddle dolomite is the latest diagenetic phase in the Adnet reef indicating crystallization under hydrothermal influences related to compressional subduction regimes of the Penninic Ocean.

Late Miocene sea surface salinity variability and paleoclimate conditions in the Eastern Mediterranean inferred from coral aragonite δ18O

Coral skeletons are archives of chemical proxies which enable paleoenvironmental reconstructions to be made at subannual resolution. Stable oxygen isotope ( δ 18O) ratios of these archives reflect sea surface temperature (SST) as well as the δ 18O composition of ambient seawater. The δ 18O seawater composition is not only controlled by global ice build-up, but river discharge and the hydrological balance of evaporation and precipitation, all influencing sea surface salinity (SSS), also play an important role in marginal seas. New sub-annually resolved coral δ 18O data were measured and evaluated together with published data from reef coral communities of Late Miocene age from Crete (Eastern Mediterranean). This time-window is of particular importance for the paleoceanographic evolution of the Mediterranean Sea, because it covers the successive closure of the marine gateways connecting the Mediterranean with the Atlantic Ocean, which culminated in the onset of the Messinian salinity crisis (MSC) at 5.96 Ma. Corals were recovered from eight time-slices dated by 87Sr/ 86Sr chronostratigraphy and cover a time-window from ∼ 10 to ∼ 7 Ma to monitor pre-MSC environmental changes. The oxygen isotope composition of the reef corals Porites and Tarbellastraea document significant changes in mean annual δ 18O as well as in mean δ 18O seasonality during the Late Miocene. Tortonian and Messinian coral mean annual δ 18O values differ by up to 1.72‰ and exhibit substantial variability. Since SSTs can be considered rather constant over the Late Miocene according to lithological, paleobotanical and geochemical evidence, the mean annual δ 18O variations in the corals appear to result from changing SSS during the Late Miocene prior to the MSC. This result is in contrast to earlier concepts that, despite increasing isolation of the Mediterranean basin starting at about 9 Ma ago, SSS did not change until only 300 kyr prior to the deposition of the first MSC evaporites. Mean seasonal δ 18O amplitudes are lower by 0.4‰ in the Messinian compared to those of the Tortonian corals, which may be due to enhanced summer evaporation. Spectral analyses of Tortonian and Messinian coral δ 18O records indicate significant interannual variability with periods of 2–3 and 4–5 years. Such variability is similar to that found in modern records. In the modern case, the Iceland Low and Azores High pressure centers influence climate in the Circum-Mediterranean region. Combined with evidence from other studies, the coral records of this study suggest that a similar pressure field system was already in existence in the early Late Miocene.

Mg structural state in coral aragonite and implications for the paleoenvironmental proxy

Thermodynamic calculations and inorganic precipitation experiments indicate a relationship between aragonite Mg/Ca and water temperature. This offers a route to reconstruct seawater temperatures from fossil corals. Fundamental to this is the assumption that Mg2+ exchanges for Ca2+ within carbonate. We present X‐ray Absorption Fine Structure (XAFS) data to indicate the structural state of Mg in modern Porites coral skeletons. We show Mg is not in aragonite, but hosted by a disordered Mg‐bearing material. Mg may be predominantly hosted in organic materials or as a highly disordered inorganic phase, e.g., a nanoparticulate form of Mg carbonate or hydroxide. Reported correlations between seawater temperature and coral Mg/Ca are unlikely to be consistent between corals and hence analysis of Mg/Ca in fossils is unlikely to produce accurate climate reconstructions. We anticipate XAFS will be applied widely to environmental proxies and become an important tool in identifying those that reconstruct accurate climates.

Caribbean chronostratigraphy refined with U-Pb dating of a Miocene coral

An exceptionally well-preserved aragonitic coral of the extinct species Goniopora hilli was collected from late Cenozoic sedimentary deposits in the Dominican Republic and dated using U-Pb techniques. Nine coralline subsamples yielded a 238 U/ 206 Pb– 207 Pb/ 206 Pb three-dimensional (3-D) inverse linear concordia age of 5.52 ± 0.15 (2σ) Ma, which, when coupled with 87 Sr/ 86 Sr ratios obtained from the same coral, allows for tighter constraints on temporal variability of marine species diversity prior to closure of the Central American Seaway. The recognition that pre-Quaternary aragonitic corals can be suitable for U-Pb dating creates new possibilities for refining the chronologies of late Cenozoic marine sedimentary sequences.

Palaeoenvironmental records from fossil corals: The effects of submarine diagenesis on temperature and climate estimates

The geochemistry of coral skeletons may reflect seawater conditions at the time of deposition and the analysis of fossil skeletons offers a method to reconstruct past climate. However the precipitation of cements in the primary coral skeleton during diagenesis may significantly affect bulk skeletal geochemistry. We used secondary ion mass spectrometry (SIMS) to measure Sr, Mg, B, U and Ba concentrations in primary coral aragonite and aragonite and calcite cements in fossil Porites corals from submerged reefs around the Hawaiian Islands. Cement and primary coral geochemistry were significantly different in all corals. We estimate the effects of cement inclusion on climate estimates from drilled coral samples, which combine cements and primary coral aragonite. Secondary 1% calcite or ∼2% aragonite cement contamination significantly affects Sr/Ca SST estimates by +1 °C and −0.4 to −0.9 °C, respectively. Cement inclusion also significantly affects Mg/Ca, B/Ca and U/Ca SST estimates in some corals. X-ray diffraction (XRD) will not detect secondary aragonite cements and significant calcite contamination may be below the limit of detection (∼1%) of the technique. Thorough petrographic examination of fossils is therefore essential to confirm that they are pristine before bulk drilled samples are analysed. To confirm that the geochemistry of the original coral structures is not affected by the precipitation of cements in adjacent pore spaces we analysed the primary coral aragonite in cemented and uncemented areas of the skeleton. Sr/Ca, B/Ca and U/Ca of primary coral aragonite is not affected by the presence of cements in adjacent interskeletal pore spaces i.e. the coral structures maintain their original composition and selective SIMS analysis of these structures offers a route to the reconstruction of accurate SSTs from altered coral skeletons. However, Mg/Ca and Ba/Ca of primary coral aragonite are significantly higher in parts of skeletons infilled with high Mg calcite cement. We hypothesise this reflects cement infilling of intraskeletal pore spaces in the primary coral structure.

Discussion on Pleistocene calcified cyanobacterial mounds, Perachora peninsula, central Greece: a controversy of growth and history Geological Society, London, Special Publications , Vol. 255 , 2006, 53–69

J. E. Andrews, M. R. Leeder, C. Portman, P. J. Rowe & J. Smith write: we were very interested to read Kershaw & Guo's (2006) analysis of the conflicting views on the growth and history of Pleistocene cyanobacterial mounds (bioherms) from the Perachora peninsula in the eastern Gulf of Corinth rift, central Greece. A new field guide to this classic geological area and these spectacular exposures will soon be available (Leeder et al . 2007) and the issues we debate below have an important bearing on regional uplift rates, the role of faulting, and the environmental conditions required for cyanobacterial calcification. We welcome Kershaw & Guo's (2006) fair and balanced account of competing hypotheses for bioherm evolution, and particularly commend and accept their new interpretation of the cavity-dwelling, pendant coralline algae. However, we dispute the more fundamental basis of their interpretations and take this opportunity to discuss them further. In the vicinity of Cape Heraion (Perachora peninsula) uplifted Pleistocene shorelines of marine isotope stage (MIS) 5a/c, MIS 5e, MIS 7a/c and MIS 7e age have been mapped (Leeder et al . 2005), and where possible U/Th dated using coral aragonite (Vita-Finzi 1993; Dia et al . 1997; Leeder et al . 2005). The upper part of a prominent terrace, at c . +25 to +30 m above modern sea level, is composed of marine bioclastic marls and carbonates of proven MIS 5e age (Vita-Finzi 1993; Leeder et al . 2005). However, directly below these sediments is a horizon of spectacular bioherms, up to 10 m high (Fig. 1) constructed by calcified cyanobacteria ( Rivularia haematites ) and coralline algae ( Lithophyllum pustulatum ). There is controversy regarding the age, stratigraphic relations and environment of formation of these bioherms. They have variously been interpreted as brackish lake deposits forming immediately before the …

Calibration of the carbonate ‘clumped isotope’ paleothermometer for otoliths

Paleothermometry is an essential tool for understanding past changes in climate. The ‘carbonate clumped isotope thermometer’ is a temperature proxy related to ordering of 13C and 18O in the carbonate lattice (based on measurements of 13C 18O 16O in CO 2 produced by acid digestion of carbonate). This thermometer has been previously calibrated for inorganic calcite and aragonitic corals [Ghosh P., Adkins J., Affek H., Balta B., Guo W. F., Schauble E. A., Schrag D., and Eiler J. M. (2006) C-13–O-18 bonds in carbonate minerals: a new kind of paleothermometer. Geochim. Cosmochim. Acta 70 (6), 1439–1456]. Here we determine the relationship between growth temperatures of aragonitic fish otoliths and abundances of 13C 18O 16O produced by acid digestion of those otoliths. Our calibration is based on analyses of otoliths from six species from four genera of modern fish sampled from a latitudinal transect of the Atlantic Ocean between 54° S and 65° N, plus one species from the tropical western Pacific. The temperatures at which fish otoliths precipitated were estimated by the mean temperature in the waters in which they lived, averaged over their estimated lifetimes. Estimated growth temperatures of our samples vary between 2 and 25 °C. Our results show that the abundance of 13C 18O 16O in CO 2 produced by acid digestion of fish otolith aragonite is a function of growth temperature, following the relationship: Δ 47 = 0.0568 × 10 6 T 2 - 0.0045 , where Δ 47 is the enrichment, in per mil, of 13C 18O 16O in CO 2 relative to the amount expected for a stochastic (random) distribution of isotopes among all CO 2 isotopologues, and T is the temperature in Kelvin. This relationship closely approaches that previously documented for inorganic calcite and aragonitic coral (Ghosh et al., 2006).

High-resolution synchrotron radiation studies on natural and thermally annealed scleractinian coral biominerals

The structural phase transition from aragonite to calcite in biogenic samples extracted from the skeletons of selected scleractinian corals has been studied by synchrotron radiation diffraction. Biogenic aragonite samples were extracteden blocwithout pulverization from two ecologically different scleractinian taxa:Desmophyllum(deep-water, solitary and azooxanthellate) andFavia(shallow-water, colonial, zooxanthellate). It was found that natural (not pulverized) samples contribute to narrow Bragg peaks with Δd/dvalues as low as 1 × 10−3, which allows the exploitation of the high resolution of synchrotron radiation diffraction. A precise determination of the lattice parameters of biogenic scleractinian coral aragonite shows the same type of changes of thea,b,clattice parameter ratios as that reported for aragonite extracted from other invertebrates [Pokroy, Quintana, Caspi, Berner &amp; Zolotoyabko (2004).Nat. Mater.3, 900–902]. It is believed that the crystal structure of biogenic samples is influenced by interactions with organic molecules that are initially present in the biomineralization hydrogel. The calcite phase obtained by annealing the coral samples has a considerably different unit-cell volume and lattice parameter ratioc/aas compared with reference geological calcite and annealed synthetic aragonite. The internal strain in the calcite structure obtained by thermal annealing of the biomineral samples is about two times larger than that found in the natural aragonite structure. This effect is observed despite slow heating and cooling of the sample.

Interconnected Network of Ganglion-Like Neural Cell Spheres Formed on Hydrozoan Skeleton

Identifying scaffolds supporting in vitro reconstruction of active neuronal tissues in their 3-dimensional (3D) conformation is a major challenge in tissue engineering. We have previously shown that aragonite coral exoskeletons support the development of neuronal tissue from hippocampal neurons and astrocytes. Here we show for the first time that the porous aragonite skeleton obtained from bio-fabricated hydrozoan Millepora dichotoma supports the spontaneous organization of dissociated hippocampal cells into highly interconnected 3D ganglion-like tissue formations. The ganglion-like cell spheres expanded hundreds of microns across and included hundreds to thousands of astrocytes and mature neurons, most of them having only cell-cell and no cell-surface interactions. The spheres were linked to the surface directly or through a neck of cells and were interconnected through thick bundles of dendrites, varicosity-bearing axons, and astrocytic processes. Thus, M. dichotoma exoskeleton is a novel scaffold with the unprecedented ability to support a highly ordered organization of neuronal tissue. This unexpected organization opens new opportunities for neuronal tissue regeneration, because the spheres resemble in vivo nervous tissue having high volume of cells associated primarily through cell-cell rather than cell-matrix interactions.

Sulfate Content in Coral Aragonite

Sulfate Content in Coral Aragonite

Fluoride Content in Coral Aragonite Associated with Carbonate Concentration in Seawater

Fluoride Content in Coral Aragonite Associated with Carbonate Concentration in Seawater

A multi-trace element coral record of land-use changes in the Burdekin River catchment, NE Australia

Previous studies have provided evidence for greater sediment input into the inner Great Barrier Reef (GBR) since the mid-nineteenth century, presumably caused by an increase in erosion due to intensive animal grazing after European settlement. Here we report a new trace element (Ba, Y, Mn) study of a Porites coral from Magnetic Island, northeastern Australia that grew continuously for 175 yr from 1813–1986. Increasing Ba and Y concentrations in the 1850–1890 coral growth bands are attributed to increased soil erosion caused by the introduction of livestock, mainly cattle, into the adjacent Burdekin River catchment, the largest contributor of suspended sediment to the GBR lagoon. In our study Y levels continued to rise with agricultural intensification; however, Ba concentrations reach a plateau after 1890, indicating that Ba/Ca ratios in near-shore corals may reflect additional factors than sediment input. Mn concentrations in the coral follow a more complicated pattern, which is clearly related to changes in catchment land-use and related erosion. Before the initiation of European settlement and the introduction of sheep into the Burdekin River catchment in 1854, coral Mn concentrations were uniformly low (< 1 ppm), yielding typical values for coral aragonite. However, after 1854, coral Mn levels rose dramatically, fluctuating in parallel with livestock numbers but with ever smaller amplitudes before reaching more normal values again by 1900. The coincidence in timing between the initial rise in coral Mn and the introduction of grazing animals in the Suttor and Belyando sub-catchments pinpoints the trace element source as the Burdekin River. A decrease in Mn levels since 1865 against a background of generally increasing land clearing, livestock numbers and sediment flux points to the existence of a transient, surficial Mn reservoir in the southern Burdekin River catchment, which is tentatively identified here as the pre-European topsoil on the Peak Range basaltic province. A brief return to moderate Mn levels between 1945–1960 is attributed to the expansion of the cattle industry into the upper Burdekin, Bowen and Suttor sub-catchments after World War II. This geochemical record confirms that trace elements in massive corals can be used to trace sediment input from neighbouring river catchments. In our study, Y concentrations are closely correlated with predicted increases in sediment flux due to land-use changes and prolonged droughts and so may be more useful than Ba for this purpose. Our Mn results attest to the rapidity of the ecosystem imbalance caused by the introduction of grazing animals, in particular, sheep into NE Australia. Within 30 yr, sheep had efficiently reworked and entirely removed the Mn-rich topsoil, which had conceivably accumulated during thousands of years of regular burning by Australia's indigenous population.

230Th/234U and 14C dating of a lowstand coral reef beneath the insular shelf off Irabu Island, Ryukyus, southwestern Japan

Abstract High‐resolution seismic reflection profiles delineated the distribution of mound‐shaped reflections, which were interpreted as reefs, beneath the insular shelf western off Irabu Island, Ryukyus, southwestern Japan. A sediment core through one of the mounded structures was recovered from the sea floor at a depth of −118.2 m by offshore drilling and was dated by radiometric methods. The lithology and coral fauna of the core indicate that the mounded structure was composed of coral–algal boundstone suggesting a small‐scaled coral reef. High‐precision α‐spectrometric 230Th/234U dating coupled with calibrated accelerator mass spectrometric 14C ages of corals obtained reliable ages of this reef ranging from 22.18 ± 0.63 to 30.47 ± 0.98 ka. This proves that such a submerged reef was formed during the lowstand stage of marine oxygen isotope stages 3–2. The existence of low‐Mg calcite in the aragonitic coral skeleton of 22.18 ± 0.63 ka provides evidence that the reef had once been exposed by lowering of the relative sealevel to at least −126 m during the last glacial maximum in the study area. There is no room for doubt that a coral reef grew during the last glacial period on the shelf off Irabu Island of Ryukyus in the subtropical region of western Pacific.

Open system U-series ages of corals from a subsiding reef in New Caledonia: Implications for sea level changes, and subsidence rate

On the Amédée islet, 4 drill cores were recovered from the barrier reef of Western New Caledonia. The coral reef is slowly subsiding and is thus percolated by sea water during sea level highstands. The cores sample a ∼ 10 m thick Holocene reef overlying a 24 m thick reef of marine isotope stage (MIS) 5.5, which in turn overlies older reef material from MIS 7.5 and beyond. ( 234U/ 238U) and ( 230Th/ 238U) ratios and 232Th were determined by thermal ionization mass spectrometry on aragonitic coral samples that were carefully investigated using X-ray diffraction and scanning electron microscopy. The petrographic study shows an increasing coral weathering with growing coral age that causes different degree of U-series open system behavior and 232Th accumulation. Holocene corals exhibit a small degree of early diagenesis and yield 230Th/ 238U ages according to the Holocene sea level rise from ∼ 8200 years to 5000 years BP. Corals from the last Interglacial section have experienced more frequent replacement of aragonite fibers and minor dissolution, and U-series open system behavior is evident. To estimate the impact of recoil processes and alteration on the U-series system two models by Villemant and Feuillet [B. Villemant, N. Feuillet, Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces, Earth and Planetary Science Letters 210(2003) 105–118.] and Thompson et al. [W. G. Thompson, M. W. Spiegelman, S. L. Goldstein, R. C. Speed, An open-system model for U-series age determinations of fossil corals, Earth and Planetary Science Letters 210(2003) 365–381.] have been tested. These models yield identical ages within uncertainty, which are in agreement to the sea level history of the past 250,000 years, as long as physico-chemical alteration and re-crystallization is small. Consequently, we were able to estimate the subsidence rate from the subsidence observed between the end of MIS 5.5 and the early Holocene, which is ∼ 0.16 ± 0.04 m per 1000 years. In addition, by using this subsidence rate and the sea level highstand of + 6 ± 3 m during MIS 5.5 we can estimate the sea level highstand at 244,000 (MIS 7.5) to be between + 2 m to − 2 m, respectively. Corals from underneath the marine isotope stage 7.5 sequence show more significant dissolution, increasing re-crystallization, elevated 232Th, and excess 234U and 230Th. Here the Villemant and Feuillet [B. Villemant, N. Feuillet, Dating open systems by the 238U– 234U– 230Th method: application to Quaternary reef terraces, Earth and Planetary Science Letters 210(2003) 105–118.] model fails to predict ages, but the Thompson et al. [W.G. Thompson, M. W. Spiegelman, S. L. Goldstein, R. C. Speed, An open-system model for U-series age determinations of fossil corals, Earth and Planetary Science Letters 210(2003) 365–381.] model was applicable predicting ages between 355,000 to > 430,000 years, i.e. coral growth potentially during MIS 9 or 11.