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
SOIL OCEAN
The global biogeochemical cycle of mercury
atmosphere
fast terre
strial 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
10,0001,000100101
Time (years)
1
0
0.5
Fra
ctio
n
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 oceanland
EnrichmentAtmospheric 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Use observations to evaluate plausibility of modeled scenarios.• Archival records• Historical documents• Present-day air, soil, seawater data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 bog Lake sediment
340
2800 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 bog Lake 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 bog Lake sediment
1760 to 1880 AD 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 bog Lake sediment
17 ± 17 27 ± 14
14C-dated or varved included. 210Pb extrapolation excluded.
Peat / sediment analysis by Jeroen Sonke
Amos et al. (2015), ES&T
Silver refining in Colonial Spanish AmericaNatural 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)
OceanEvasion
(ng m-2 hr-1)
TerrestrialRe-emission
(ng m-2 hr-1)
Pre-industrialEnrichment
Factor(unitless)
All-timeEnrichment
Factor(unitless)
peat
Amos et al. (2014)Mining emissions 3xZero pre-1850 emissions
Greater geogenic emissionsGreater soil retentionGreater 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)
OceanEvasion
(ng m-2 hr-1)
TerrestrialRe-emission
(ng m-2 hr-1)
Pre-industrialEnrichment
Factor(unitless)
All-timeEnrichment
Factor(unitless)
peat
sediment
Amos et al. (2014)Mining emissions 3xZero pre-1850 emissions
Greater geogenic emissionsGreater soil retentionGreater 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)
(%) (%)
2015 20152050 2050Year Year
2015 levelsDecrease 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
evasionWilson’10
Streets’11
observed
ship cruiseland-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
LandWater
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
LandWater
Landfill
Flatter recent trajectory• ASGM: Muntean’14• China: Zhang’15
Credit: Yanxu Zhang
Atmospheric Hg0 trend, 1990-2010
US wet deposition trend, 1990-2010US utilities HgII emissions
Slide courtesy of Yanxu Zhang, project lead, [email protected]
Decreased Hg use in products and co-benefits from SO2 control explain Hg trends
Horowitz’14 inventory with products
Hg
emis
sion
s (M
g a1 )
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 availablehttp://bgc.seas.harvard.edu/models.html