From air to leaves and soils: How atmospheric mercury released from informal gold mines moves through the ecosystems and degrades plant health.

Abstract

Since the 1990s, informal gold mining has spread across the Amazon. Due to its propensity to amalgamate with gold, mercury is added to fine sands, enabling the extraction of gold held within. This mercury is ultimately vaporized off gold amalgam, spreading downwind. Informal gold mining is now considered the leading global emitter of mercury, with 40% of gold mining emissions released in the Amazon. However, existing research has only begun to characterize the downwind fate and regional-to-global implications of Amazon mercury emissions into the atmosphere. In Chapter 2, we predict at a high resolution where emissions occur and how they move downwind. We use remote sensing, Lagrangian transport, and on-the-ground mercury data to jointly model Amazon gold-mining emissions and their downwind concentration footprints at a 1-kilometer scale, annually between 2001 and 2024. We estimate a 5-fold Amazon-wide mercury emission increase between 2001 and 2017; emissions rose with gold price and livelihood insecurity during the 2015-2016 El Niño drought but plateaued following the 2017 Minamata Convention. Our model predicts measured air-mercury concentrations with a log concordance correlation of 0.79 compared to near-zero correlations when atmospheric transport is applied to existing inventories. Extended across the Amazon, our model finds that 100,000 km^2 of land area experience more than double background mercury levels with disproportionate effects in urban centers, where gold shops burn mercury amalgam. As a result, between 8,000 and 220,000 urban residents are exposed to toxic levels, depending on the applied toxicity threshold. This model offers scientists, environmental nonprofits, and community organizations a tool to characterize mercury emissions, their drivers, and downwind exposure. In Chapter 3, we consider the impacts of mercury emissions from Amazon gold mining on forest productivity. Across a mercury contamination gradient, field sampling revealed significant cross-species declines in oxygen-evolving complex performance, leaf size, and diameter at breast height, with species-dependent declines in photosystem I performance and chlorophyll content. Combining satellite remote sensing of forest productivity and Chapter 2’s mercury concentration footprint maps, we estimate that long-term low mercury doses result in a 3.7% per ng m^(-3) exposure reduction in plant productivity with forests recovering 10-15 years after mercury exposure stops. Across the Amazon, mercury toxicity results in a 23 MtC annual loss in Amazon rainforest gross primary productivity, which exceeds the losses associated with direct deforestation. The combined deforestation and mercury-induced productivity declines result in a 9% reduction in annual net carbon storage across the Amazon. These productivity declines may propagate up the food chain and reduce crop productivity; may reduce transpiration, worsening Amazon drought and pushing an additional 1,500 km^2 of vulnerable forests towards a die-off tipping point; and may extend to global forest productivity declines from an enriched global atmospheric cycle. This chapter highlights hidden social and ecological impacts from gold mining due to mercury’s impact on plant productivity, whose social cost rivals the value of gold produced. Chapter 4 asks what happens to mercury once it has spread across the regional atmosphere. Using a gradient flux setup, we found that dry deposition of mercury directly from air into soil is the leading flux, outpacing leaf uptake. We estimated that approximately 40% of Amazon mercury emissions ultimately deposit regionally rather than entering the global mercury cycle. Once in soils, this mercury is subject to a couple of loss pathways. We found that 26-59% of terrestrial mercury soil deposition—primarily at low relative elevations—erodes into waterways, potentially explaining 76 ± 15% of observed river mercury in the heavily mined Madre de Dios region. Eroded mercury threatens aquatic ecosystems, whereas the remaining 41-74% of soil mercury likely evades back into the atmosphere, where it can travel globally. We found that Amazon deforestation rapidly accelerates evasion, emitting 1 to 2 times the mercury released from all Amazon alluvial gold mining. Together the atmospheric loading, erosion, and deforestation explain 74% of surficial soil-mercury variation, which allowed us to build 1-km resolution soil-mercury prediction maps that may support future research or community risk assessment. Together, this dissertation tracks the movement of mercury from emission sources, the downwind impacts on forest productivity, and this mercury’s long-term fate in regional ecosystems. These findings highlight the regional-to-global drivers and impacts of Amazon informal gold mining and may support regional mercury exposure mitigation and global mercury budgeting.