QUANTITATIVE ESTIMATES OF DECCAN FLOOD BASALT ERUPTIVE FLUXES AND CLIMATE CONSEQUENCES

Large Igneous Provinces (LIPs) impact climate through the release of CO₂ and SO₂. The climate effects of these volatiles, particularly SO₂, are highly dependent on the rate of emission, and thus high-resolution LIP eruptive fluxes can predict potential climate impacts – assuming eruptive degassing is the primary form of volatile release. While main-phase LIP emplacement lasts ~ 1 million years, active eruptions may occur only 1% of the time, and the tempo of individual eruptive episodes is uncertain. We constrain Deccan Traps (DT) eruptive tempo and flux using high-resolution mercury chemostratigraphy from new terrestrial localities in Montana. To compare mercury records from different locations and depositional environments, one must account for differences in sedimentation rate, sampling resolution, and sedimentary conditions such as organic carbon and sulfur concentration. We have developed a new statistical framework which uses environmental mercury cycle box models to estimate eruptive rates and volumes by quantitatively evaluating the changes in global mercury budget indicated by varying concentrations in sedimentary records.

We find that each eruptive episode lasted up to a few centuries and erupted ~ 50-250 km³ of lava per year. Episodes were typically separated by ~ 5000 years, with no significant (>100 ka) hiatuses, and there were more frequent and longer eruptions in the latter part of the sequence. These estimates are compatible with our assessments of DT eruptive tempo from ⁴⁰Ar/³⁹Ar geochronology of DT lavas, a new analysis of the paleo-secular variation recorded in successive lava flows (directional groups), and detailed flow-by-flow DT stratigraphy. Earth system models predict that this estimated eruptive flux could cause significant cooling during eruptions, but only a few degrees of warming over the entire DT interval. However, the warming observed in high-resolution paleoclimate records is much larger than predicted, and peak warming occurs before the K-Pg boundary, whereas most of the DT volume erupts close to or after the boundary. These results suggest that either other factors drove this warming event, or that eruptive degassing is not the dominant mode of DT volatile input to the atmosphere, and thus eruptive rates do not sufficiently constrain its potential to cause climate warming.