Geochemistry of Metastable Carbonate Mineral in Coral Skeleton and Reef Sediments, Gulf of Mannar, Tamilnadu

ECO-CHRONICLE 59 ECO-CHRONICLE VOL. 1, No. 2. JUNE 2006, PP 59 - 66. GEOCHEMISTRY OF METASTABLE CARBONATE MINERAL IN CORAL SKELETON AND REEF SEDIMENTS, GULF OF MANNAR, TAMILNADU M. V. Mukesh,* N. Ramanujam,** S. R. Singarasubramainan,* H. M. Sabeen,*** B. Gurugnanam* and K. Manoharan*. * Department of Earth Sciences, Annamalai University, Annamalainagar, Tamilnadu ** Department of Geology and Research Center, V.O.Chidambaram College, Thoothukudi, Tamilnadu *** Department of Mining and Geology, District Office, Kollam, Kerala ABSTRACT Carbonate sediments are born, not made. The catalytic activities of the coral transform the dissolved calcium and carbonate ions from seawater into body skeletons of corals, made up of CaCO3. Skeletons contain minerals like Aragonite, Calcite and Dolomite in considerable amount and rare minerals like Vaterite. Absence of the rare minerals like vaterite in the ancient carbonate sediments, the analog of the modern carbonate sediments, needs further study. To identify the carbonate mineral group in the coral skeletons and reef sediments in the vicinity of Van, Koswari and Kariyashuli islands in the Gulf of Mannar, the samples were subjected to XRD analysis. XRD analyses of the powdered coral skeletons reveal 70% as Aragonite and 17% as Vaterite. The remaining 13% is represented by Calcite, Dolomite, Magnesium and Strontium. Similar analyses of reef sediments indicate the reduction of Aragonite percentage to 45% and increase of vaterite percentage to 36%. No appreciable variation in the percentage for calcite, magnesium and dolomite for both coral and reef sediments has been reported. But earlier studies of ancient carbonate rocks reported the presence of calcite and dolomite only, not aragonite and rare mineral like Vaterite. The dissolution and inversion processes had inverted the Aragonite and Vaterite from coral skeletons and reef sediments into stable mineral form, such as calcite and dolomite. The causes of the inversion of mineral phase evolved from the incipient stage to the final stage in the evolutionary trends of the carbonate minerals can be easily understood by studying the sequential changes of mineral phases from Gel Vaterite Aragonite Calcite. The calcite and aragonite are more stable, when compared to the earlier mineral phases of gel and vaterite. The elimination of mineral phases of gel, vaterite and aragonite and ultimate enrichment of calcite and dolomite are aided with the removal of ion of Sr2+ from coral skeletons and transformation of gel (Semi-Solid) into lattice of solid crystalline material with same chemical composition. Key words: Metastable carbonate, scleractinian coral, reef sediments, Gulf of Mannar contributing enough sediments to form a good INTRODUCTION proportion as carbonate sediments. The high Corals are exciting biochemical factories magnesium rich spicules (12% to 15% mol) which catalyze the large scale transfer of constitute more than one percent of unconsolidated dissolved calcium and carbonate from sea sediments counted in the carbonate sediments were water in tropical and subtropical and derived from the decaying tissues of soft corals temperate climate zones. The mineral (Mukesh, 1997). It has been reported that a rare composition of the reef sediments depends mineral has been found in the skeletons of living mainly on biological constituents of the corals, corals, molluses and gastropods (Friedman and and other marine organisms. Sander, 1978). But the analysis of the recent biogenic and as well as the ancient counter part of the Unconsolidated, uncemented carbonate carbonate sediments reveals only the presence of sediments found in Florida bay, Bahama bank, Calcite, Aragonites and Dolomite; not any other rare Persian Gulf, Great Barrier Reef and Gulf of minerals. Mannar are believed to be the recent analogs LOCATION AND CORAL DISTRIBUTION of the most common type of ancient rocks (Bricker, 1971). Soon after the death of the To identify the uncommon mineral found in the organism or even during the lifetime, they are biogenic constituents before its inversion and 60 ECO-CHRONICLE recrystallization into stable form, living Institute, Karaikudi. The XRD pattern and XRD Scleractinian corals were collected in the vicinity analysis were carried out using computer of Van, Koswari and Kariyashuli islands, in the controlled X-ray powder system, JEOL, JDX- Gulf of Mannar, India (Fig.1). 8030 model. The copper K alpha radiation =1.54180 with nickel filter is used at a rating of 40 KV, 20mV. The 0Ø at a step of 0.12Ø with measuring time of 1 second per step. The observed XRD data were compared with standard XRD data files of the geological samples. Peak search and search match software available with the systems were used to tabulate the XRD data and it’s matching with the standard value. The phase identification and phase composition were carried out with usual XRD analytical procedure. By measuring the 2Ø Fig. 1. Location map of the study area values and the corresponding peak values, the minerals were identified from the X-ray diffraction In the Gulf of Mannar 96 genera of corals (in graph. From the analysis of the diffraction graphs that 86 genera are hermatypic and 10 genera are dominant minerals in each sample were identified ahermatypic) were reported between and tabulated (Tables 3 & 4). Rameshwaram and Thuthookudi, of which stony corals like Acropora, Pocillopora, Porite, MINERALOGY OF SCLERACTINIAN CORALS Agariciidae and Favidae, predominate in shallow AND REEF SEDIMENTS water (Pillai, 1977). Based on the diagnosis of the important features such as form and size, Approximately sixty minerals occur in nature calice arrangement, coenostem, corallite wall, that have CO group in common. Large scale septa, skeleton colour, and ecological conditions 3 transformations of ions of calcium, magnesium, different families were identified (Wood, 1983 and strontium, and carbonates takes place as a result Pillai, 1983) (Table 1). Scleractinian corals like of the the catalytic activities of coral species to Montipora digitata, Montipora foliasa, Montastrea form a solid. The important carbonate minerals annularis, Acropora nobilis, Pocillopora found in the Scleractinian corals are: damicornis and Agaricia fragilis and very recent Calcite - CaCO sediment samples in the reef areas of Van, 3 Aragonite - CaCO Koswari and Kariyashuli islands were subjected 3 Vaterite - CaCO to X-ray diffraction analysis to determine the 3 Dolomite - CaMg (CO ) pattern of minerals (Table 2). 3 2 Calcium carbonate is a polymorphic form commonly found in nature as Calcite and The petrographic study of sieved fractions of Aragonite. Analysis of the X-ray diffraction graphs the reef sediment, from the reef area exhibit of the powder samples of Monipora digita, peculiar spicules with straight or curved shape. Montipora foliosa, Montastrea annularis, The size of the spicules found in the sieve Acropora hyacinthus, Acropora nobilis, Agrilia fractions varied from 1Øto 2.5Ø. These spicules fragilis and Pocillopora damicornis exhibit that were found as derivations of the decaying tissues the Aragonite is a dominating mineral with 59% of soft coral Octa corallia. These spicules were to 81% (average 71%) of the total carbonate segregated for XRD analysis. The collected percentage in the coral skeleton. Vaterite is the corals, sediments and spicules were crushed to next mineral with a wider variation from 2.5% to powder by using pestle and mortar very slowly to 25% (average 18%) in the coral skeleton. avoid the excessive heat generation. The Though, the calcite appears as dominating powdered samples were sieved through 80 mesh mineral in the ancient and recent consolidated and packed for analysis. origin, its average percentage is found as low as RESULTS 9% in the coral skeleton, when compared to Aragonite and Vaterite in same species. X-RAY- DIFFRACTION METHODS Magnesium, Strontium and Dolomite were not X-ray diffraction analysis of the samples was detected in all coral samples. X-ray diffraction carried out at Central Electrochemical Research pattern of powdered samples of Octo coral (soft Table 1. Features that assist in the diagnosis of corals from families of Agariciidae, Pocilloporidae, Acroporidae, Facviidae, Agariciidae, Pocilloporidae, Acroporidae and Faviidae. Some of the genera were easily confused under water and careful examination of the skeleton were carried out for positive identification (References: Wood, 1993; Pillai, C.S.G. 1971., 1983; Mukesh, M.V. 1997) ECO-CHRONICLE 61 Table 1. (...contd.) Features that assist in the diagnosis of corals from families of Agariciidae, Pocilloporidae, Acroporidae, Facviidae, 62 Agariciidae, Pocilloporidae, Acroporidae and Faviidae. Some of the genera were easily confused under water and careful examination of the ECO-CHRONICLE skeleton were carried out for positive identification (References: Wood, 1993; Pillai, C.S.G. 1971., 1983; Mukesh, M.V. 1997) ECO-CHRONICLE 63 Species Samples Calcite Aragonite Vaterite Dolomite Others Acroporidae Montipora digitata X X Acroporidae Montipora foliosa X X X Faviidae Montastrea annularis X X Acroporidae Acropora hyacinthus X X X Acroporidae Acropora nobils X X Agariciidae Agaricia fragilis X X Pocilloporidae Pocillopora damicornis X X Sediments Location Calcite Aragonite Vaterite Dolomite Others Van island X X Koswari island X X Kariyashuli island X X Table 2 List of species and sediments used for X-ray diffraction and the mineral identification coral) species revealed that Magnesium is found Though, Calcite

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