MASSIVE VOLCANISM, EVAPORITE DEPOSITION AND THE CHEMICAL EVOLUTION OF THE CRETACEOUS OCEAN

The continued break up of Pangea during the Early Cretaceous was accompanied by extensive volcanism, massive evaporite deposition, and presumably significant changes in ocean chemistry. The largest recorded shift in the S isotope composition of marine sulfate in the past 135 million years occurs in the midst of these environmental perturbations. Two hypotheses have been proposed to explain this excursion: (1) massive evaporite deposition associated with rifting during the opening of the South Atlantic and a corresponding decrease in pyrite burial rates and (2) increased inputs of volcanic-derived S due to the emplacement of large igneous provinces (LIPs). While important caveats exist, either of these processes operating in isolation could account for the large negative S isotope excursion. However, each process has a very different impact on marine sulfate concentrations: increased volcanism and continental weathering rates increases sulfate delivery to the ocean whereas evaporite deposition removes it during calcium sulfate precipitation. Resolving this question is critically important because in the modern sulfate-rich ocean, sulfate is the primary oxidant for most of the organic matter and nearly all of the methane in marine sediments. A substantial drop in marine sulfate concentrations would influence the rates and reaction pathways utilized during organic carbon remineralization in marine water columns and sediments, affect nutrient recycling efficiency and therefore the exogenic carbon and oxygen cycles, and ultimately impact climate.

The Sr isotope record of seawater provides a tool for distinguishing between these influences. Here, we present S isotope data from Resolution Guyot, Mid-Pacific Mountains and utilize previously published Sr isotope records in order to identify the mechanism/s driving the S isotope evolution of the Cretaceous ocean. Our results indicate that the S and Sr cycles were tightly coupled through much of the Early Cretaceous, which suggests that changes in hydrothermal and weathering fluxes played a dominant role in driving changes in the S cycle and that the large negative S isotope shift was not accompanied by a significant reduction in marine sulfate levels.