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

Lower Cretaceous mudstones exposed at Speeton in North Yorkshire, UK, conta in lobsters and burrows preserved in diagenetic phosphate concretions. Isot opic compositions of sulphur in both diagenetic sulphide and structural sul phate in diagenetic phosphate have been measured in an attempt to constrain diagenetic porewater chemistry. The occurrence of phosphatized and pyritiz ed lobsters and similarly preserved burrows, allows a detailed comparison o f these microenvironments with the host sediments. Host sediments are exten sively bioturbated and characterized by very light sulphide isotopic compos itions (mean sulphide delta(34)S = -48.3 +/- 3 parts per thousand (1 sigma, n = 19)) and sulphate isotopic compositions that are lighter than Lower Cr etaceous seawater sulphate (mean sulphate delta(34)S = +8.7 +/- 3.2 parts p er thousand (1 sigma, n = 19)). These isotopic values can be explained by t he action of bioturbating macrofauna; the introduction of oxygen in ventila ting seawater results in the oxidation of early formed isotopically light p yrite, resulting in porewater sulphate that is enriched in S-32. Subsequent pyrite formation via bacteriogenic reduction of isotopically light porewat er sulphate leads to the formation of isotopically light pyrite, whilst res idual porewater sulphate apparently remains relatively enriched in the 32S isotope. Sulphur isotopic values for the burrows are very different; sulphi de isotopic compositions average -34.4 +/- 0.4 parts per thousand (1 sigma, n = 3) and sulphate isotopic compositions average -14.4 +/- 6.6 parts per thousand (1 sigma, n = 4). These isotopic compositions are the result of ra pid development of closed system conditions in burrows, resulting in the bu ild-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, som e lobsters appear to have died close to the sediment-water interface, whils t others appear to have been buried (in burrows) prior to death.