Early cretaceous Sulfur cycle


The global sulfur (S) cycle is coupled to the marine carbon (C) cycle via microbial sulfate reduction (MSR) and plays an important role in regulating the redox state of the oceans, mineralogical characteristics of seawater precipitates, nutrient availability in seawater, and climate.  Understanding the natural variability in the behavior of the global S cycle under different climatic states and during major perturbations is critical to improve our comprehension of the couplings and feedbacks amongst the global biogeochemical cycles that regulate the ocean-atmosphere-climate system.  The Early Cretaceous (145-100 Ma) provides an excellent time to examine perturbed biogeochemical cycles as it is marked by a concentration of volcanic and tectonic activity with elevated mid-ocean ridge spreading rates from the continued break up of Pangea, the eruption of several LIPs, and the formation of a massive evaporitic basin in the South Atlantic.  In addition, the Cretaceous is marked by periodic increased burial of organic matter producing oceanic anoxia and a fluctuating greenhouse climate state.  My research is focused on generating a paired S isotope record of marine sulfate and sedimentary pyrite during the Early Cretaceous to illuminate the nature of and identify the mechanisms involved in perturbing the S cycle and their impact on the climate state.

Figure 1. Schematic illustration of the global marine sulfur cycle indicating the key input and output fluxes to the marine sulfur reservoir along with its coupling to other key biogeochemical cycles (carbon, oxygen, phosphorus, and iron).  The sulfur and carbon cycles are linked through microbial sulfate reduction (MSR) in anaerobic environments where organic matter (CH2O) from primary productivity is oxidized and sulfate is reduced to hydrogen sulfide.  While the degradation of organic matter via MSR can release key nutrients to feed primary producers the resultant hydrogen sulfide readily reacts with iron-oxyhydroxides to form pyrite and release adsorbed ions such as phosphate that further supply photosynthetic activity.  Over long periods of time (100s kyrs -1000s kyrs) the increased nutrient supply for primary producers may enable elevated rates of organic matter and pyrite burial resulting in a net production of free oxygen and a drawdown of atmospheric CO2.  The major outputs of the marine sulfur cycle are deposition of sulfate bearing evaporites and MSR induced sedimentary pyrite.  The major inputs of sulfur to the marine reservoir are weathering of evaporites and sulfides, subaerial volcanic gases, mid-ocean ridge hydrothermal fluids, and large igneous province (LIP) gas and/or hydrothermal fluid release.