Massive volcanism, evaporite deposition, and the chemical evolution of the Early Cretaceous ocean

Early Cretaceous (145-100 Ma) rocks record a ~5‰ negative shift in the sulfur isotope composition of marine sulfate, the largest shift observed over the past 130 m.y. Two hypotheses have been proposed to explain this shift: (1) massive evaporite deposition associated with rifting during opening of the South Atlantic and (2) increased inputs of volcanically derived sulfur due to eruption of large igneous provinces. Each process produces a very different impact on marine sulfate concentrations, which in turn affects several biogeochemical phenomena that regulate the global carbon cycle and climate. Here we present sulfur isotope data from Resolution Guyot, Mid-Pacific Mountains (North Pacific Ocean), that track sympathetically with strontium isotope records through the ~5‰ negative sulfur isotope shift. We employ a linked sulfur-strontium isotope mass-balance model to identify the mechanisms driving the sulfur isotope evolution of the Cretaceous ocean. The model only reproduces the coupled negative sulfur and strontium isotope shifts when both hydrothermal and weathering fluxes increase. Our results indicate that marine sulfate concentrations increased significantly during the negative sulfur isotope shift and that enhanced hydrothermal and weathering input fluxes to the ocean played a dominant role in regulating the marine sulfur cycle and CO₂ exchange in the atmosphere-ocean system during this interval of rapid biogeochemical change.