Sedimentary and foliar mercury sequestrations in contemporary and palaeo-environments, and the establishment of a new leaf-based atmospheric mercury proxy

Abstract

Gaseous emissions from Phanerozoic large igneous province (LIP) emplacements have been implicated as instigators of global carbon cycle perturbations and associated global change and mass extinction events. However, sedimentary archives which record the extinction events are commonly void of magmatic deposits or ash layers, associated with LIP emplacement. Therefore, stratigraphic variation in sedimentary mercury (Hg) concentration has been used to identify major volcanic events, as volcanic emissions are the dominant natural source of atmospheric mercury and its ~1.5 yr residence time allows global stratospheric transport. However, environmental Hg loading as well as sedimentary sequestration pathways are highly debated. Furthermore, the role of the terrestrial biomass, which acts as an intermediate Hg reservoir, during major Hg cycle perturbation is largely unexplored. Firstly, three Pliensbachian-Toarcian successions, which were deposited under dominantly different redox regimes, representing an oxygenation gradient, were compared and demonstrated both spatial and temporal differences in dominant Hg sequestration pathways. Before the onset of the Torcian Oceanic Anoxic Event (T-OAE), the sedimentary mercury signal was controlled by detrital-sediment dominant sequestration in all three depositional basins. However, during the T-OAE, the sedimentary mercury signal was controlled by a redox-sensitive deposition in the case of both semi-restricted Mochras and fully oxygenated La Cerradura and organic-sulphide sequestration in the restricted Cleveland Basin. Secondly, utilizing new carbon isotope and Hg data from the marine Mecsek Basin (Reka Valley section, SW Hungary), a concordance is shown between elevated sedimentary Hg accumulation and the negative shifts in d13C, suggesting astronomical modulation of secondary environmental Hg fluxes superimposed on globally elevated primary volcanogenic Hg fluxes. Thus, I hypothesize that eccentricity-modulated precessional changes in seasonality affected the temporary retainment of Hg in intermediate (terrestrial biomass) Hg reservoirs, which acted as a global Hg capacitor. Thirdly, the baseline natural variation of modern Gingko biloba leaf Hg concentration, as well as the quantitative relationship between the atmospheric and the responding leaf Hg concentrations were defined, thus enabling the use of Ginkgo biloba leaves as a direct proxy for atmospheric mercury levels. Subsequently, the temporal variation in fossil Ginkgoales leaf Hg content, thus palaeoatmospheric Hg concentration in three distinct intervals, from the Triassic-Jurassic boundary to the early Middle Jurassic, demonstrates a clear linear relationship between atmospheric Hg concentration and pCO2, as well as with LIP volume. The latter relationship attests that large-scale changes in atmospheric Hg are ultimately governed by volcanic emissions in the natural mercury cycle. Furthermore, the elevated Hg content of fossil leaf fragments also attests that terrestrial biomass responds to changes in atmospheric Hg loading and is likely to have sequestered more Hg during LIP volcanism, therefore, terrestrial run-off was inherently enriched in Hg in those time intervals. Changes in terrestrial OM fluxes are thus not the main driver of stratigraphic variation in mercury concentration. Rather the sedimentary mercury signal is dependent on primary volcanic Hg emission rates, and astronomically paced climatic variability, by controlling secondary (terrestrial) mercury release, and depositional basin characteristics. Furthermore, my research highlights the importance of broader trends over multiple proxies across different depositional environments rather than individual peaks in a single proxy under time constraints to fully understand environmental mercury loading in the sedimentary basins during time intervals of LIP volcanism.