Mercury in Fossil Leaves as a Proxy for Tracking Large Igneous Province Volcanism

Abstract: During some mass extinctions, usually minor, world oceans also underwent anoxic to euxinic conditions, events called Oceanic anoxic events (OAEs). Such an event unfolded during the Toarcian (∼183 Ma), called the T-OAE which is suggested to have been caused by Karoo-Ferrar Large Igneous Province (LIP) volcanism. In the present day, volcanoes release extensive amounts of Hg, it has therefore been hypothesized that elevated mercury (Hg) levels in sediments can be used as a proxy to correlate the two. During the T-OAE, increased Hg-levels have been observed in marine records from numerous basins across multiple continents suggesting atmospheric Hg0 to have been significantly raised. Leaves have a substantial uptake of Hg0, therefore this study investigated the possibility of using fossil leaves as a proxy to detect raised levels of Hg0. Mercury analysis was conducted on leaves from the Toudahoe section, from Hubei Province (Southern China), where the T-OAE already has been located stratigraphically, in a section of Toarcian age, through paleoflora, elevated atmospheric PCO2, negative carbon isotope excursion (CIE) (characteristic negative shift of ∼3–9 ‰ interrupting a positive excursion), and raised global temperatures by ∼6°C. Raised levels of Hg were observed in leaves in, and leading up to, the beds suggested to represent the T-OAE. The observed elevated fossil-leaf Hg values directly coincide with elevated Hg-concentrations in marine sedimentary records from other basins, such as the Mochras borehole sedimentary archive from the Cardigan Bay Basin (UK),inferring elevated Hg in fossil leaves to be useful as a proxy, validating the stratigraphy and concluding that atmospheric Hg0 was significantly raised during, and leading up to, the T-OAE. It also suggests that episodes of volcanic activity happened before the onset of the T-OAE, while the main phase of Karoo-Ferrar volcanism took place during the T-OAE. Analysis on terrestrial sediments showed high Hg/TOC results in samples with low/ or high TOC-values, deducing that factors such as depositional environment, type of organic matter (OM) and clay- and sulfur content may affect the ratio, which needs to be analysed in order to produce a valid terrestrial Hg/TOC- record. Further, samples with TOC <1% needs to be analysed for TOC multiple times, because of large analytical errors when carbon is very low.