Field relationships and stable isotope geochemistry of concretions from James Ross Island, Antarctica

Early diagenetic (precompactional) concretions are abundant throughout the Cretaceous-Tertiary Marambio Group Larsen Basin, Antarctica. Four distinct concretion types are recognised: (1) spherical-subspherical concretions; (2) sheet concretions; (3) fossil-nucleated concretions; and (4) concretionary burrow networks. All concretion types have a micritic to microsparry variably non-ferroan to ferroan calcite cement. Stable isotope analyses show a wide spread in both delta-O-18 and delta-C-13. Delta-C-13 values are typically negative, ranging between -3.38 and -39.15 parts per thousand (PDB) (usually -16 to -30 parts per thousand). Delta-O-18 ranges between -1.28 and -13.81 parts per thousand (PDB) with most of the values between -5 and -10 parts per thousand.

The delta-C-13 signature is interpreted to represent carbon sourced from sulphate reduction and/or methane oxidation, with minor input from shell dissolution, and is consistent with a shallow burial, early diagenetic origin. A single mudstone hosted concretion has a delta-O-18 composition indicative of precipitation of carbonate from seawater. The low delta-O-18 signatures in the sandstone- and siltstone-hosted concretions are possibly due to early diagenetic modification of the pore water composition through volcaniclastic mineral dissolution/reprecipitation reactions and perhaps through input of meteoric water. Concretion distribution is related to (a) changes in sedimentation rate and (b) the dominance of diffusion on concretion cementation.