Early diagenetic celestine replacement of demosponges in upper campanian-upper maastrichtian chalk
|Location||International Geological Congress,oslo 2008|
|Author||Madsen, Heine Buus۱; Stemmerik, Lars۲|
|Holding Date||27 September 2008|
Nodular celestine (SrSO4) is rare in deep-sea carbonates and has so far only been reported from DSDP Leg 90. Celestine nodules are found to be common in Upper Campanian-Upper Maastrichtian chalk in two cores drilled at Stevns Peninsula, eastern Denmark. Petrographic evidence indicates that the celestine nodules formed by replacement of Lithistida demosponges formed prior to compaction and flint formation in locally reduced environments. 87Sr/86Sr values of the celestine range from 0.70766 to 0.70787 and are similar to Campanian-Maastrichtian seawater values. The δ34S ranges from 26.5 to 35.0‰, indicating a positive shift of 9-16‰ compared to reported Campanian-Maastrichtian seawater values of 17.8 to 18.8‰.
The shift in δ34S in the nodules towards more positive values is linked to activity of sulphate reducing bacteria and the 87Sr/86Sr values indicate that the Sr is sourced from seawater or local dissolution of calcite.
Quick burial of the sponges, probably by contour-parallel bottom currents, was followed by decomposition of the organic tissue by sulphate reducing and sulphide oxidizing bacteria. This resulted in a locally reduced environment and can explain the heavy δ34S values. Sulphides generated by the decomposition were reacting with available iron to form pyrite. Any excessive sulphide was oxidized to sulphate by sulphide oxidizing bacteria or degassed as hydrogen sulphide resulting in decreasing pH and dissolution of carbonates. The reduced and slightly more acidic environment created by the decomposition of the sponges caused strontium from sea water to precipitate as sulphate in dysaerobic environment or at the anaerobic-dysaerobic interface.