Quantitative Reconstructions of Large Igneous Province Gas Emissions Using Mercury Chemostratigraphy

Large Igneous Provinces (LIP) emplacement is commonly associated with severe environmental change. A primary way LIPs affect the environment is via the emission of climatically active gases, such as carbon (CO₂, CH₄) and sulfur (SO₂, SO₄ aerosol). The flux and tempo of these gas emissions control the effect they have on the environment, with different feedback effects dominating depending on emission tempos. Hence, estimates of LIP gas emissions at high temporal resolution are required to constrain the potential environmental impacts of a specific LIP. However, complex LIP chronostratigraphy and non-eruptive degassing make these estimates challenging.

Volcanic gas emissions are the main natural source of mercury to the environment. Increases in mercury concentration in sedimentary archives have thus been commonly used as a qualitative indicator of LIP activity. Our recent work has expanded this tool to quantitative reconstruction of volcanic gas fluxes. This technique requires understanding the size and rate of mercury emissions that correspond to an observed change in sedimentary records. However, a critical issue is that mercury records sometimes exhibit different patterns within the same time interval, complicating interpretation.  

We use our understanding of the mercury cycle as represented by environmental mercury box models to evaluate several questions: A) What size/duration of eruptions are resolvable in sedimentary mercury records? Modern large explosive eruptions are rarely observed, whereas LIPs are. What are the limits? B) How do mercury records vary between different environments (e.g., terrestrial, coastal marine, deep marine settings)? C) Can we understand spatial and temporal changes in mercury deposition as a function of environmental conditions (e.g., regional riverine flux and long-term trends in volcanic activity)?

To answer these questions, we have developed several new tools. First, we adapt an existing environmental mercury box model to paleoenvironmental conditions, using parameters from continental hydrological models and background mid-ocean ridge and subduction zone volcanic activity. This model is used to simulate mercury deposition in different environmental settings for a variety of eruption (Hg emission event) rates and durations.

Then, we use a novel Bayesian inversion framework to analyze these results with published Hg records across multiple time periods and depositional environments, to test whether different coeval records are consistent with the same underlying forcing. We find that our model results, accounting for sediment accumulation rate and sampling resolution, effectively predict enrichment patterns across environmental settings, supporting the use of mercury records as a quantitative proxy. Additionally, the geologically short lifetime of mercury in the surface environment makes results highly sensitive to sediment accumulation rate and to volcanic pulse duration - e.g., short (<~100 year) pulses are not likely to be distinguishable from background variability in many sedimentary environments.