Contact metamorphism and sulfur release during Large Igneous Provence emplacement

The emplacement of Large Igneous Provinces (LIPs) and subsequent volatile release are associated with catastrophic changes to the earth system and mass extinctions. LIP volatiles can be directly released through igneous degassing and/or indirectly released through metamorphic processes as carbon and sulfur bearing sediments are heated by intrusions and lava flows. Sediment derived carbon emission has been given consideration for its impact on warming (Heimdal et al., 2018; Svensen et al., 2018). Svensen et al (2018) found through modeling that Siberian Trap sill emplacement was predicted to have released 2.3 × 10¹⁶ moles of sedimentary C in just 0.7–1.2% of the Tunguska Basin, and modeling by Heimdal et al (2018) proposed that Central Atlantic Magmatic Province (CAMP) sills could cause 2.0 × 10¹⁸ moles C to be degassed from sediment through contact metamorphism in just two CAMP basins. While the metamorphic carbon production during LIP emplacement has been given attention metasedimentary sulfur emission has been largely ignored. One study, Yallup et al (2013), looks at both metamorphic carbon and metamorphic sulfur emission during LIP emplacement finding evidence of decarbonation and desulfurization substantially increasing the sulfur yield to the surface.

Aside from Yallup et al., (2013) metamorphic sulfur degassing is largely disregarded partly due to the broad assumption that sulfur must reach the stratosphere to drive sustained cooling. However, if the input of sulfur into the troposphere itself is sustained, this can extend the climatic cooling. Metamorphic sulfur degassing during LIP emplacement offers a mechanism for this type of prolonged cooling. We will begin by presenting thermodynamic modeling of sediment metamorphism in tangent with a simple carbon cycle and planetary energy balance model. Together these models show carbon and sulfur emissions from contact metamorphism could be sustained long enough to cause centennial scale sulfate aerosol cooling spikes of several kelvin superimposed on millennial scale warming from carbon dioxide emission. This suggests that metamorphic sulfur should be considered as a plausible driver of sustained cooling.

Further, we present sulfur and carbon geochemical data from a field test of metamorphic volatile emissions to verify modeled mechanisms using an observational approach.  We use samples from the Sugar Grove dike, an Eocene basalt intruded into the Devonian Millboro black shale in West Virginia, as a well-exposed and accessible proxy for basaltic LIP intrusion. We find evidence of decreasing pyrite and increasing pyrrhotite concentrations in the shale approaching the dike as a potential indicator of sulfur release. We will also present isotopic data for pyrite and pyrrhotite sulfur, organic carbon, carbonate carbon, and carbonate oxygen. Together these results will constrain the magnitude of metamorphic sulfur release and test its viability as a mechanism for cooling before warming during LIP emplacement.