Dendrochronological Reconstructions of Atmospheric Mercury in South-Central Chile: Pre-Industrial Baselines, Industrialization and Volcanic Inputs from Araucaria araucana tree rings

Abstract

Mercury (Hg) in the Southern Hemisphere remains poorly understood, particularly in remote forest ecosystems lacking long-term instrumental records. Tree rings offer a unique opportunity to reconstruct historical Hg deposition, yet few studies have applied this approach in southern latitudes or in volcanically influenced environments. In this thesis, we investigate total Hg concentrations (THg) in the tree rings of Araucaria araucana from three sites in south-central Chile: Nahuelbuta National Park (NNP) in Chile's Coastal Mountain Range, and two Andean sites (Lonquimay and La Fusta) located near historically active stratovolcanoes. Crossdated ring segments from mature trees were analyzed for THg, averaged over decadal intervals. Results from Nahuelbuta reveal low, stable THg concentrations from the 13th to 15th centuries, followed by a pronounced increase between 1550 and 1600 CE coinciding with the early stages of Spanish colonization, likely driven by widespread biomass burning during colonial conflict, and a sustained rise after 1850 CE aligning with Chile’s industrialization. Volcanic sites displayed THg concentrations approximately 2–3 times higher than Nahuelbuta, consistent with long-term exposure to passive volcanic degassing and regional anthropogenic sources, but no consistent peaks associated with documented eruptions, suggesting that plume dynamics and elevation strongly influence deposition. Stable isotope values at Nahuelbuta were unusually negative (Δ¹⁹⁹Hg: −0.47‰ to −0.79‰), indicating enhanced photochemical cycling at high elevation. Also, a cross-site analysis found no evidence supporting the proposed “young tree effect,” challenging the assumption that juvenile trees inherently accumulate more Hg due to physiological traits. These findings highlight the capacity of dendrochronological archives to disentangle anthropogenic and volcanic contributions to Hg deposition, provide rare long-term baselines for the Southern Hemisphere, and improve understanding of Hg cycling in high-elevation forest ecosystems.