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    Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone

    Poulton, S.W. and Bottrell, S.H. and Underwood, Charlie J. (1998) Porewater sulphur geochemistry and fossil preservation during phosphate diagenesis in a Lower Cretaceous shelf mudstone. Sedimentology 45 (5), pp. 875-888. ISSN 0037-0746.

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    Abstract

    Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, contain lobsters and burrows preserved in diagenetic phosphate concretions. Isotopic compositions of sulphur in both diagenetic sulphide and structural sulphate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritized lobsters and similarly preserved burrows, allows a detailed comparison of these microenvironments with the host sediments. Host sediments are extensively bioturbated and characterized by very light sulphide isotopic compositions (mean sulphide δ34S = –48·3 ± 3‰ (1σ, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cretaceous seawater sulphate (mean sulphate δ34S = +8·7 ± 3·2‰ (1σ, n = 19)). These isotopic values can be explained by the action of bioturbating macrofauna; the introduction of oxygen in ventilating seawater results in the oxidation of early formed isotopically light pyrite, resulting in porewater sulphate that is enriched in 32S. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewater sulphate leads to the formation of isotopically light pyrite, whilst residual porewater sulphate apparently remains relatively enriched in the 32S isotope. Sulphur isotopic values for the burrows are very different; sulphide isotopic compositions average –34·4 ± 0·4‰ (1σ, n = 3) and sulphate isotopic compositions average –14·4 ± 6·6‰ (1σ, n = 4). These isotopic compositions are the result of rapid development of closed system conditions in burrows, resulting in the build‐up of acidity necessary for phosphate precipitation and coprecipitation of isotopically light sulphate (formed by oxidation of isotopically light sulphide surrounding the burrows). Lobster shell isotopic compositions fall between these two groups. On the basis of their isotopic compositions, some lobsters appear to have died close to the sediment–water interface, whilst others appear to have been buried (in burrows) prior to death.

    Metadata

    Item Type: Article
    School: Birkbeck Schools and Departments > School of Science > Earth and Planetary Sciences
    Depositing User: Sarah Hall
    Date Deposited: 21 May 2019 16:21
    Last Modified: 06 Aug 2019 11:39
    URI: http://eprints.bbk.ac.uk/id/eprint/27619

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