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 δ18O and δ13C. δ13C values are typically negative, ranging between –3.38 and –39.15%o (PDB) (usually –16 to 230%). δ18O ranges between –1.28 and –13.81% (PDB) with most of the values between –5 and –10%. The δ13C 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 δ18O composition indicative of precipitation of carbonate from seawater. The low δ18O 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.