Coupled strontium-sulfur cycle modeling and the Early Cretaceous sulfur isotope record

Coupled modeling of biogeochemical cycles leads to improved constraints on elemental fluxes and helps to identify mechanisms that drive secular variations in isotopic records. Here, we develop a coupled strontium-sulfur isotope mass-balance model to examine the Early Cretaceous marine sulfate (δ34Ssw) and strontium (87Sr/86Sr) isotope records. We also present an expanded marine barite S isotope record and pyrite S isotope record for the Early Cretaceous with updated age models in line with the Geologic Time Scale 2012. Collectively constraining the primary input fluxes that drive both Sr and S cycles - hydrothermal and weathering - helps to identify which parameters primarily control coherent, oppositional, and divergent isotopic perturbations. With the primary input fluxes constrained in the coupled model, the importance of additional factors responsible for secular changes in δ34Ssw can be examined. Model results indicate that emplacement of the Ontong Java-Manihiki-Hikurangi Plateau, along with elevated mid-ocean ridge spreading rates, increased continental arc volcanism, and eruption of additional large igneous provinces, all contributed to the Early Cretaceous ~5‰ negative δ34Ssw shift and maintained low δ34Ssw over long time scales. The coupled strontium-sulfur modeling was able to demonstrate that a complex interplay of changes in multiple S cycle parameters - not simply total weathering and hydrothermal fluxes - was required to produce this major shift and new alternate state within the S isotope record. In addition, coupled modeling highlights the differences in chemical behavior (volatility) between S and Sr. Review of S cycle modeling and examination of a variety of initial conditions indicates a distinctly different range of δ34S values for the total weathering flux, weathered evaporite-pyrite ratios, the pyrite burial fraction, and the global integrated fractionation factor for pyrite burial compared to those used in previous S cycle models. Coupling the Sr cycle to other biogeochemical cycles that also reflect a balance between hydrothermal and riverine inputs offers a powerful tool for interpreting marine isotopic records.