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

Early Cretaceous (145–100 Ma) rocks record a ∼5‰ negative shiftin the sulfur isotope composition of marine sulfate, the largest shiftobserved over the past 130 m.y. Two hypotheses have been proposedto explain this shift: (1) massive evaporite deposition associated withrifting during opening of the South Atlantic, and (2) increased inputsof volcanically derived sulfur due to eruption of large igneous provinces.Each process produces a very different impact on marine sulfateconcentrations, which in turn affects several biogeochemicalphenomena that regulate the global carbon cycle and climate. Herewe present sulfur isotope data from Resolution Guyot, Mid-PacificMountains (North Pacific Ocean), that track sympathetically withstrontium isotope records through the ∼5‰ negative sulfur isotopeshift. We employ a linked sulfur-strontium isotope mass-balancemodel to identify the mechanisms driving the sulfur isotope evolutionof the Cretaceous ocean. The model only reproduces the couplednegative sulfur and strontium isotope shifts when both hydrothermaland weathering fluxes increase. Our results indicate that marine sulfateconcentrations increased significantly during the negative sulfurisotope shift and that enhanced hydrothermal and weathering inputfluxes to the ocean played a dominant role in regulating the marinesulfur cycle and CO 2 exchange in the atmosphere-ocean system duringthis interval of rapid biogeochemical change.