The sedimentological and chronological study of Holocene reef sequences recovered in drill cores through modern reefs of Mauritius, Re´union Island and Mayotte allows the reconstruction of sea level changes and reef growth patterns during the Holocene. The branching-coral facies systematically predominates over coral head facies throughout the Holocene reef sequences, and Acropora is the main frame builder among the branching forms. The reconstructed sea level curves, based both on identification of coral assemblages and on radiometric U/Th ages, are characterized by a rapid rise between 10 and 7.5 ky BP, followed by a clear inflection between 7.5 and 7 ky BP. The stabilization of sea level at its present level occurred between 2000 and 3000 years ago, probably without a higher sea level stand. Rates of vertical reef accretion range between 0.9 and 7 mm. y\.
New sea-level and δ18O curves for the past 34,000 yr, based on uranium–thorium chronology, are proposed for the southwestern part of the Indian Ocean. The archives include cores drilled from onshore coral reefs and submersed samples from foreslope corals of Mayotte in the Comoro Islands. The Mayotte sea-level curve shows a lowstand of 145 ± 5 m below the present level during the last glacial maximum dated at 18,400 yr. This lowstand is supported by the maximum18O enrichment in the coral colonies. The residual signal (Δδ18O), controlled by sea-surface temperature changes, indicates that surface waters 18,400 yr ago were approximately 5°C cooler than present. The deglacial sea-level rise is clearly recorded, with a mean rate of about 1.7 cm yr−1between 18,400 and 10,000 yr ago. The deglaciation phase is characterized by a strong18O depletion marked by two pulses related to meltwater discharges into the North Atlantic Ocean but also characterized by responses specific to the tropical Indian Ocean.
The inner perimeter ridge of Wodejebato Guyot (Marshall Islands) is the subject of a detailed sedimentologic, seismic, and geochemical study. The deposition of the shallow-water carbonate sequence, 177.7-190.2 m thick, is related to a two-stage transgression leading to the development of fringing sand shoal facies overlain by early cemented rudist-coralgal "reef" units that constituted barrier-like features. Wodejebato Guyot exhibited, at least temporarily, an atoll-like morphology. The irregularities of the upper surface of platform carbonates, the occurrence of a trough separating the inner perimeter ridge from the outer one, and the formation of deep solution cavities in platform carbonates suggest karstification processes before the final drowning of the carbonate platform during Maastrichtian time. Drowning events were probably related to the combination of a rapid sea-level rise and climatic changes that characterize Maastrichtian time. Following the demise of the carbonate platform, subsidence possibly combined with eustatic sea-level changes carried rapidly the platform carbonates in a basinal setting below the aragonite saturation depth, resulting in dissolution of aragonite and subsequent in place precipitation of calcite cements possibly throughout the Cenozoic. Post-drowning sediments correspond to a phosphate-manganese crust that formed from the late Paleocene to the middle Eocene.
Cores from five sites on Wodejebato Guyot show that diagenesis played a major role in forming the platform limestones. Five morphologically distinct cement types representing several diagenetic environments, extensive dissolution porosity, and widespread neomorphism of metastable components to calcite with mimic or coarsely crystalline textures are observed. Radiaxial cement (RC) and a fibrous variant (RC-FIC) occur almost exclusively in primary pores in the upper 70 m of the inner perimeter ridge at the platform edge. These cements precipitated from shallow, warm marine waters. Prismatic limpid cement with uniform extinction (PLUC) occurs throughout the platform limestones in both primary and secondary porosity. Geologic, petrographic, and isotopic data indicate formation in deeper, cool marine waters, after drowning of the platform. Columnar, bladed, magnesian-calcite (7 mole % MgCO 3) cement with sweeping extinction (CC) is confined to a single core of a reef interval on the inner perimeter ridge. It occupies primary porosity as the first cement or overlies RC. This is an early, shallow-marine cement. Many caprinid rudist shells in this interval maintain their primary aragonite composition. Syntaxial overgrowth cement (SOC) is widely distributed, in small quantities, mainly in grainstones. It appears to have precipitated concurrently with or somewhat later than PLUC, presumably in the same deeper, cool, marine water. Dissolution formed extensive molds, some vugs, and some small caverns, particularly in grainstones. Dissolution was most intense in the outer perimeter ridge at the platform edge. Molds also occur in muddier rocks and in reworked cobbles of wackestone. Geologic and petrographic evidence suggests that dissolution occurred both in meteoric waters during early subaerial exposure of Wodejebato Atoll and in cold marine waters after drowning, perhaps continuing to the present. Neomorphism is most apparent in skeletons that were originally aragonitic, rudists and other mollusks in particular. Where the neomorphic calcite closely mimics the original wall structure, the original isotopic composition is only slightly altered. Coarsely crystalline neomorphic calcite has lower δ 18 θ and δ 13 C values than the original shells, indicating meteoric alteration. Burial diagenesis is not an alternative on this guyot. Matrix and magnesian calcite shells, such as benthic foraminifers and red algae, rarely show petrographic evidence of neomorphism, but some samples show dramatic isotopic shifts to lower 18 O and C, suggesting meteoric alteration. Isotopic shifts to lower values suggest two incursions of meteoric water at the lagoon site and on the inner perimeter ridge. Traces of root structures indicate a third exposure episode within the lagoon. The clearest signal of meteoric water is in a shift to lower isotopic values, primarily in 18 O, at the top of platform carbonates. Isotopic data are available only from the lagoon and inner perimeter ridge sites, but a probable microkarst surface truncates platform limesto...
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