Anthropogenic perturbations to the biogeochemical cycle of mercury

Humans have been releasing mercury (Hg) to the environment for millennia. Hg is a naturally occurring element that cycles continuously among the ocean, atmosphere, and land, but humans have greatly perturbed the natural biogeochemical cycling of Hg. As a direct result of anthropogenic Hg releases, the atmospheric, oceanic, and terrestrial reservoirs have become enriched relative to their natural background levels. Through pursuits such as gold mining and fossil fuel combustion, humans have increased the flux of Hg from the lithosphere to the atmosphere. Analogous to the global carbon cycle, only a fraction of the anthropogenic Hg liberated from the lithosphere to the atmosphere is sequestered in more recalcitrant reservoirs, while the majority continues to cycle among the land, ocean, and atmosphere. The timescales required for Hg liberated from the lithosphere to return to the deep, stable mineral reservoirs in the ocean and on land are thought to be many thousands of years. The extensive recycling of Hg over long timescales raises the concern that our ability to decrease atmospheric Hg through international treaties and regulation is diminished over time as more anthropogenic Hg is released from deep mineral reservoirs. Here we use an eight-compartment global model that includes dynamic coupling between the atmosphere, ocean, and terrestrial reservoirs to the investigate anthropogenic perturbations to the global biogeochemical cycle of Hg. We force the model with all-time (2000 B.C. – 2008 A.D.) historical anthropogenic emissions estimates and use the results to quantify the extent of anthropogenic enrichment of these eight biogeochemical reservoirs and characterize the timescales required for anthropogenic Hg to be removed from actively cycling reservoirs. We investigate the uncertainty in simulated anthropogenic enrichment as a function of the uncertainty in global Hg budget estimates reported across the literature. We also use a model adjoint to compute the time-dependent sensitivity of each reservoir to anthropogenic emissions. Lastly, we explore whether our understanding of the current biogeochemical Hg cycle and the trajectory of modern anthropogenic emissions can be reconciled with observed declines in atmospheric Hg over the last 15 years.