Legacy mercury and trends Helen M. Amos* and Elsie M. Sunderland AGU Joint Assembly - Montreal, Canada 4 May 2015 Funding: EPA, EPRI, NSF

We’ve been using mercury since antiquity 2000 B.C. Today

Human exposure motivates environmental Hg research – ocean concentrations are a key endpoint U.S. Population Methylmercury Intake Sunderland (2007), EHP

LITHOSPHERE SOILOCEAN The global biogeochemical cycle of mercury

atmosphere fast terrestrial slow soil armored soil surface ocean subsurface ocean deep ocean ocean margin sediment deep ocean sediment Fate of unit pulse to the atmosphere Amos et al. (2013), GBC; Amos et al. (2014), ES&T Characterizing the intrinsic timescales of mercury cycling, independent of anthropogenic perturbation Fate of a unit pulse to the atmosphere 10,000 1, Time (years) Fraction

Balance of land vs. ocean Hg storage: Better data coverage is transforming our understanding 240,000 Mg >300,000 Mg Smith-Downey et al. (2010) Hararuk et al. (2013) Data: USGS, Figure: Dave Krabbenhoft >500,000 Mg ??

The previous focus Focused on present day Isolated systems atmosphere ocean land Enrichment Atmospheric deposition increased 3x (2-5x) since 1850

Historical emission inventory provides external forcing -- enables independent estimation of enrichment Amos et al. (2015), ES&T

Objective Approach Bracket plausible scenarios for global enrichment based on current estimates of primary emissions and rates of exchange between environmental reservoirs. Use a model to examine: Sensitivity in enrichment Implications for future trajectories Use observations to evaluate plausibility of modeled scenarios. Archival records Historical documents Present-day air, soil, seawater data

Sediment-peat difference an order of magnitude smaller than previously reported Enrichment factor relative to “pre-industrial” Peat bogLake sediment BC to 1350 AD 1760 to 1880 AD Biester et al. (2007) Peat / sediment analysis revisited by Jeroen Sonke

Sediment-peat difference an order of magnitude smaller than previously reported Enrichment factor relative to “pre-industrial” (1760 to 1880 AD) Peat bogLake sediment 2.9 ( 95% CI, 1.6 to 6.3 ) 4.3 ( 95% CI, 2.3 to 14 ) Peat / sediment analysis by Jeroen Sonke Amos et al. (2015), ES&T

Sediment-peat difference an order of magnitude smaller than previously reported Enrichment factor relative to “pre-industrial” (1760 to 1880 AD) Peat bogLake sediment 1760 to 1880 AD Peat / sediment analysis by Jeroen Sonke Amos et al. (2015), ES&T Differences in enrichment may be reasonably explained by different time scales of accumulation 4.3 ( 95% CI, 2.3 to 14 ) 2.9 ( 95% CI, 1.6 to 6.3 )

EF = 4.0 EF = 4.4 EF = 2.9 Increasing time scale months to a year years to a decade decades

Pre-industrial Hg accumulate rates 5x higher than pre-colonial Enrichment factor relative to “pre-colonial” (3000 BC to 1500 AD) Peat bogLake sediment 17 ± ± C-dated or varved included. 210 Pb extrapolation excluded. Peat / sediment analysis by Jeroen Sonke Amos et al. (2015), ES&T

Silver refining in Colonial Spanish America Natural archives point to higher emission factor Cooke et al. (2013) Model kiln by J. M. Wolfe 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 Weight of evidence suggests early Hg releases contribute to significant enrichment

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 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) Weight of evidence suggests early Hg releases contribute to significant enrichment

The need for aggressive emission reductions to stabilize ocean Hg is robust to uncertainty Atmosphere Ocean, 0 to 1000 m Modeled response to terminating anthropogenic emissions (normalized to 2015) (%) Year 2015 levels Decrease mining 3x Amos et al. (2014) Faster ocean evasion Greater burial Amos et al. (2015), ES&T

Explaining measured trends has challenged our understanding of Hg cycling and emissions Soeresen et al. (2012); Ebinghaus et al. (2011); Slemr et al. (2011) Marine Boundary Layer North Atlantic South Atlantic evasion Wilson’10 Streets’11 observed ship cruise land-based

Emissions from China and artisanal gold mining drive global trajectory in recent decades Horowitz et al. (2014), ES&T Global Anthropogenic Releases of Hg Streets’11 mining Streets’11 other Additional air Land Water Landfill

Emissions from China and artisanal gold mining drive global trajectory in recent decades Horowitz et al. (2014), ES&T Global Anthropogenic Releases of Hg Streets’11 mining Streets’11 other Additional air Land Water Landfill Flatter recent trajectory ASGM: Muntean’14 China: Zhang’15 Credit: Yanxu Zhang Flatter recent trajectory ASGM: Muntean’14 China: Zhang’15 Credit: Yanxu Zhang

Atmospheric Hg 0 trend, US wet deposition trend, US utilities Hg II emissions Slide courtesy of Yanxu Zhang, project lead, Decreased Hg use in products and co-benefits from SO 2 control explain Hg trends Horowitz’14 inventory with products Hg emissions (Mg a 1 )

Concluding remarks Important research fronts for global enrichment and future trajectories: Inventories of primary anthropogenic releases Retention in soil Stability of coastal sediment as a sink Model publically available