Strontium Isotope Composition of Triassic Coastal and Shallow Marine Carbonates in the Western Balkanides, NW Bulgaria

Whole-rock samples of Triassic coastal and shallow marine carbonates were analyzed for their Sr isotope signatures and trace element concentrations were measured in order to assess the possible impact of diagenetic alteration. The 87Sr/86Sr ratios of Anisian to lower Carnian calcite micrites reproduce the global Triassic seawater variations fairly well. The recorded higher values can be explained by an influence from detrital aluminosilicate phases and a minor postdepositional alteration. The samples of tidal-flat dolomicrites from the base of the Anisian show no significant diagenetic overprint but have considerably more radiogenic 87Sr/86Sr signatures compared to the global Sr isotope curve that corresponds to the Triassic (Mesozoic) maximum. These anomalously high values may have resulted from some interaction between the dolomitizing brines and the underlying fluvial siliciclastics (Buntsandstein facies), but more likely from the high 87Sr/86Sr river waters discharged from a silicate-dominated drainage basin to the coastal area at the beginning of the Middle Triassic. This study demonstrates that bulk micrite can be used as a proxy for seawater 87Sr/86Sr after proper sample selection and diagenetic screening. It also suggests that the 87Sr/86Sr ratios of ancient carbonates precipitated in coastal settings should be interpreted with caution, considering the possible influence of continentally-derived fluids.