Observational and modeling constraints on enrichment and the implications for the future Helen M. Amos, Jeroen E. Sonke, Daniel Obrist, Nicole Hagan, Hannah M. Horowitz, Robert P. Mason, Melanie Witt, Ian Hedgecock, E. S. Corbitt, Elsie M. Sunderland ICMGP – Jeju, South Korea 15 June 2015

LITHOSPHERE SOILOCEAN Humans have perturbed the global Hg cycle – But how much? Implications for the future?

First-order exchange between reservoirs Flux = k x (Mass of Hg) 7-box model of the global Hg cycle (Amos et al., 2013, GBC; 2014 ES&T; 2015 ES&T) Track the fate of Hg atmosphere fast terrestrial slow soil armored soil surface ocean subsurface ocean deep ocean ocean margin sediment deep ocean sediment Fate of a pulse to the atmosphere External forcing from anthropogenic releases

Use model to explore impacts of uncertainty in cycling and emissions Scenarios 1 & 2 Published emissions Scenario 3 Larger geogenic source Scenario 4 Greater ocean evasion Scenario 5 Greater retention in soils Scenario 6 More sediment burial

OCEAN Use multiple lines of evidence to evaluate plausibility of scenarios Aircraft Surface air Soil Seawater Historical documents

Observations from natural archives Median enrichment factor relative to “pre-industrial” Peat bogLake sediment 2.9 ( 95% CI, 1.6 to 6.3 ) 4.3 ( 95% CI, 2.3 to 14 ) Tomorrow 9:00-9:15 in G05-III Atmospheric Mercury Sonke et al., “Reconciling Hg deposition rates from lake sediment and peat bog archives” time pre-large-scale miningpre-industrial20C max 3000 BC s

Increasing time scale months to a year years to a decade decades Peat bog Watershed & lake Forced by Horowitz inventory Forced by Zhang inventory EF = 4.0 EF = 4.4 EF = 2.9

Pre-industrial Hg accumulate rates 5x higher than pre-colonial Enrichment factor relative to “pre-colonial” Peat bogLake sediment 17 ± 17 (n=7) 27 ± 14 (n=14) Tomorrow 9:00-9:15 in G05-III Atmospheric Mercury Sonke et al., “Reconciling Hg deposition rates from lake sediment and peat bog archives” time pre-large-scale miningpre-industrial20C max 3000 BC s

Silver refining in Colonial Spanish America Natural archives point to higher emission factor Kiln by J. M. Wolfe; Cooke et al. (2013) Pre-colonial model kiln Atmospheric emission factor for historical large-scale mining 7% to 85% Robins (2011); Hagan & Robins (2011); Guerrero (2012); Robins & Hagan (2012)

Atmosphere (Mg) Upper Ocean (pM) Deep Ocean (pM) Soil (Mg) Ocean Evasion (ng m -2 hr -1 ) Terrestrial Re-emission (ng m -2 hr -1 ) Pre-industrial Enrichment Factor (unitless) All-time Enrichment Factor (unitless) peat Amos et al. (2014) Mining emissions 3x Zero pre-1850 emissions Greater geogenic emissions Greater soil retention Greater burial Increased ocean evasion Amos et al. (2015) sediment Using observations to evaluate plausibility of different scenarios of cycling and emissions

Atmosphere (Mg) Upper Ocean (pM) Deep Ocean (pM) Soil (Mg) Ocean Evasion (ng m -2 hr -1 ) Terrestrial Re-emission (ng m -2 hr -1 ) Pre-industrial Enrichment Factor (unitless) All-time Enrichment Factor (unitless) peat sediment Base case Zhang emissions: Cut mining 3x Engstrom emissions: Cut 2x, zero pre-1850 Greater geogenic emissions Greater soil retention Greater burial Increased ocean evasion 1. Early mining could contribute significantly 2. Geogenic emissions less than 300 Mg/yr

Impact of biogeochemical processes on future trajectories can be as large as uncertainty in emissions Atmosphere Ocean, 0 to 1000 m Modeled response to terminating anthropogenic emissions (normalized to 2015) (%) Year 2015 levels Emissions increasing since 1950s Base case: Emissions peak 1970s Greater burial Amos et al. (2015), ES&T

Concluding remarks Signal of enrichment diminishes with increasing time scale of accumulation  time scale of decades and longer can completely obscure peaks in emissions. Observational and model evidence for historical mining emissions. Need for aggressive reductions to stabilize ocean concentrations is robust to uncertainty in emissions and Hg cycling. Model publicly available

Thanks to our funders US Environmental Protection Agency Contract No. EP-11-H European Research Council Grant ERC-2010-StG_