MERCURY ANALYSES OF (FOSSIL) PLANT SUBSTRATES AS INDICATORS OF EARLY JURASSIC ATMOSPHERIC HG LOADING AND LIP VOLCANISM

Large Igneous Province (LIP) volcanism emitted large quantities of greenhouse and (toxic) gasses and compounds into the global atmosphere and oceans, often instigating significant climatic and environmental disturbance and mass extinctions. However, constraining temporal and causal links between (continental) LIP emplacement and events recorded in marine stratigraphic archives is complex. Elevated sedimentary mercury (Hg) levels within stratigraphic archives are therefore increasingly used as indicators of past LIP volcanic activity, because volcanism is the largest source of naturally occurring Hg emissions in the present-day environment. Stratigraphic variations in sedimentary Hg concentrations can however be strongly overprinted by temporal-spatial variations in mercury sequestration pathways, therefore questioning the usefulness of this proxy.

Here, we present a new approach to reconstruct changes in past Hg fluxes, more specifically temporal variations in atmospheric Hg levels, and by inference global environmental Hg loading, by analysing Hg levels in modern and fossil leaf tissue. Terrestrial flora acts as an intermediate sink in the natural mercury cycle and importantly, leaves obtain and sequester Hg predominantly through direct gaseous atmospheric uptake.

We here present new results on (i) the natural global variability of Hg concentrations in modern Gingko biloba leaves, and (ii) the potential impact of changes in atmospheric pCO₂ conditions on leaf-Hg uptake (based on growth chamber experiments). Analyses of (iii) Hg concentrations in Early Jurassic fossil leaf-cuticle and woody fragments from stratigraphic successions, spanning the Triassic–Jurassic transition and the Early Toarcian Oceanic Anoxic Event show 2–3 orders of magnitude variability in plant Hg levels, significantly larger than the observed modern natural variability. These findings suggest that plant tissue may be used to examine geochemical (Hg) variations in past atmospheres, and thus may trace temporal variations in atmospheric Hg loading and by inference the stratigraphic occurrence of past LIP magmatism.