SR ISOTOPE RECORDS FROM LACUSTRINE CARBONATES; FIDELITY FROM MASS BALANCE MODELS

Many large modern lakes integrate drainage from across a geographically extensive watershed, and their character is often strongly influenced by climatic, geologic, and geomorphic processes occurring far from the lake itself. However, the depositional records of ancient lakes are nearly always interpreted solely in terms of local tectonic and climatic factors. Strontium isotopic ratios preserved in lacustrine carbonate offer a unique tool for deducing lakewater provenance, and therefore can facilitate reconstruction of the relationship between ancient lakes to their associated watershed. However, the ⁸⁷Sr/⁸⁶Sr ratios incorporated into lacustrine carbonates reflect not only the ⁸⁷Sr/⁸⁶Sr ratios of surrounding rocks, but the concentrations of Sr in the source terranes and their susceptibility to both physical and chemical weathering. Sr behavior should not be expected to be the same in every lake due to varying hydrology, geologic settings, and climate in lacustrine systems. In particular, the magnitude and rate of change expected in ⁸⁷Sr/⁸⁶Sr ratios and Sr concentrations of lacustrine waters needs to be considered in evaluation of ⁸⁷Sr/⁸⁶Sr records from lacustrine carbonates. In order to test controls on driving mechanisms associated with strontium isotope variability in lacustrine systems, we use a variable-volume reservoir model to investigate lacustrine evolution in response to changing concentration, composition, and size of influx in a variable-size Sr reservoir (lake), a model that best applies to lacustrine basins with closed hydrology.

Application of this model to the Green River Formation of Wyoming shows that rates of Sr sequestration can exert a strong influence on lacustrine Sr concentration, and consequently on rates of ⁸⁷Sr/⁸⁶Sr evolution in lacustrine systems. Overall, however, modeling suggests a short residence time for Sr (~10³-10⁴ years) in the Green River Formation lacustrine system. This in turn suggests that Sr isotopes from lacustrine carbonates can provide high-resolution (~10² years) records of water provenance in lacustrine systems if a sufficient range of ⁸⁷Sr/⁸⁶Sr ratios are present in the lacustrine catchment area.