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Use observations to evaluate plausibility of modeled scenarios.• Archival records• Historical documents• Present-day air, soil, seawater data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, yxzhang@seas.harvard.edu

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