Plant cuticle as a possible palaeo-Hg proxy: Implications from Hg concentration data of extant Ginkgo L. and extinct ginkgoaleans

Abstract

Vegetative leaves have long been considered a significant receiver of gaseous Hg in the atmosphere, offering the potential to passively monitor palaeo-atmospheric Hg concentrations; however, few constraints on this exist. In this study, we conduct Hg measurements on three Ginkgo leaf collections: i) modern leaves from ten sampling sites across China, ii) modern leaves collected monthly across one growing season in Nanjing (China), iii) fossil ginkgoaleans leaves from the Middle Jurassic (China). The results from this study reveal that the foliar Hg concentrations (with an average concentration of 61 ng·g−1, N = 272) were higher than those observed in Ginkgo leaf samples previously studied from Ireland and the USA. Additionally, the leaf age and atmospheric Hg concentrations represent two primary factors impacting foliar Hg contents in Ginkgo. Hg concentrations in fossil cuticular samples (with an average concentration of 585.5 ng·g−1) were observed notably higher than those in modern Ginkgo leaves (avg. 61 ng·g−1) and sediments from the same layers (avg. 113 ng·g−1). Considering possible Hg migration during fossilization, we suggested that the elevated Hg concentrations in fossil cuticles were attributed to both the retention of Hg in leaves and the loss of leaf content during fossilization. Based on 23 fossil ginkgoalean samples from 6 beds of the Dameigou section (spanning from the Early to the Middle Jurassic), Qaidam Basin, China, we detected a Hg anomaly through Hg concentrations in fossil cuticles during the presumed palaeo-volcanic event (the Karoo-Ferrar Large Igneous Province (LIP)). This preliminary test supports the notion that variations in Hg concentrations in fossil cuticle may potentially reflect the gaseous Hg changes in the Jurassic palaeo-atmosphere, triggered by LIP volcanism at this time. This finding highlights the possibility of using fossil plant cuticle as a Hg proxy of palaeo-atmospheric Hg loading.