Abstract
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 CO 2 exchange in the atmosphere-ocean system during this interval of rapid biogeochemical change.
Highlights
The sulfate content of the ocean plays an important role in regulating the global carbon cycle and the chemical composition of the ocean-atmosphere system
Emplacement and evaporite deposition during the Early Cretaceous, demonstrate that coupled increases in hydrothermal and weathering inputs, followed by a period of massive evaporite deposition, best explain the negative S and Sr isotope shifts recorded in rocks drilled at Resolution Guyot
This indicates that marine sulfate concentrations likely increased through most of the Aptian interval before dropping to lower levels via late Aptian evaporite deposition
Summary
The sulfate content of the ocean plays an important role in regulating the global carbon cycle and the chemical composition of the ocean-atmosphere system. Marine sulfate concentrations have varied considerably over geologic time (Lowenstein et al., 2001), affecting the marine carbon cycle, the evolution of Earth’s climate system, and the long-term redox balance of the ocean-atmosphere system. The relative importance of marine sulfur inputs and outputs through time can be tracked in part by reconstructing the S isotope composition of sulfate phases (i.e., barite, calcium sulfate, and carbonate-associated sulfate) preserved in marine sedimentary rocks. The S isotope composition of marine sulfate (δ34Ssulfate) represents a balance between the input of 34S-depleted S from riverine (continental weathering) and hydrothermal sources and the removal of 34S-depleted S via MSR and associated pyrite burial (Canfield, 2001)
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