Summary

Two case studies demonstrate the different influence of nearsurface meteoric diagenesis and contact metamorphism on the preservation of primary d¹⁸O signals in brachiopod shells. Most brachiopods from the shallow buried sequence of Gotland (Sweden) show no signs of diagenetic alteration, although the sequence is characterized by repeated subaerial exposition and infiltration of meteoric fluids. In contrast, almost all specimens from the Oslo Area (Southern Norway) reveal diagenetically altered d¹⁸O signatures. This alteration is not always reflected in trace element contents, microstructural preservation and cathodoluminescence of the shells. Scattered and low d¹⁸O values for most Norwegian brachiopod shells and very characteristic isotopic and trace elemental signatures of the associated calcitic matrix and calcite cements can be related to the infiltration of hydrothermal fluids and contact metamorphism during deeper burial. The sample set from the Oslo Graben shares several characteristics with other published case studies of unusual low d¹⁸O values of brachiopods and marine cements. The isotopic and elemental composition of the carbonate matrix and calcite cements should therefore be always included in the evaluation of brachiopod-based me isotope data. However, a general rejection of brachiopods as indicators of Paleozoic marine d¹⁸O signatures is not confirmed by the presented data.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah