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
Humans have perturbed the global Hg cycle –
But how much? Implications for the future?
SOIL
OCEAN
LITHOSPHERE
7-box model of the global Hg cycle
(Amos et al., 2013, GBC; 2014 ES&T; 2015 ES&T)
First-order exchange between reservoirs
External forcing from anthropogenic
Flux = k x (Mass of Hg)
releases
Track the fate of Hg
Fate of a pulse to the atmosphere
deep ocean
sediment
Use model to explore impacts of uncertainty
in cycling and emissions
Scenario 3
Larger geogenic source
Scenarios 1 & 2
Published emissions
Scenario 4
Greater ocean evasion
Scenario 5
Greater retention in soils
Scenario 6
More sediment burial
Use multiple lines of evidence to evaluate
plausibility of scenarios
Aircraft
Surface air
Historical documents
Seawater
Soil
OCEAN
Observations from natural archives
Median enrichment factor relative to “pre-industrial”
Peat bog
Lake sediment
4.3
2.9
(95% CI, 2.3 to 14)
(95% CI, 1.6 to 6.3)
pre-large-scale mining
time
3000 BC
1550
pre-industrial
1760
1880
20C max
1960s
Tomorrow 9:00-9:15 in G05-III Atmospheric Mercury
Sonke et al., “Reconciling Hg deposition rates from lake sediment and peat bog archives”
Peat bog
Forced by Horowitz
inventory
EF = 4.4
Forced by Zhang
inventory
EF = 4.0
EF = 2.9
Watershed & lake
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”
Peat bog
Lake sediment
27 ± 14 (n=14)
pre-large-scale mining
time
3000 BC
1550
17 ± 17 (n=7)
pre-industrial
1760
1880
20C max
1960s
Tomorrow 9:00-9:15 in G05-III Atmospheric Mercury
Sonke et al., “Reconciling Hg deposition rates from lake sediment and peat bog archives”
Silver refining in Colonial Spanish America
Natural archives point to higher emission factor
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)
Kiln by J. M. Wolfe; Cooke et al. (2013)
Using observations to evaluate plausibility of
different scenarios of cycling and emissions
Pre-industrial
Enrichment
Factor
Atmosphere
(Mg)
(unitless)
All-time
Enrichment
Factor
(unitless)
Upper
Ocean
Deep
Ocean
(pM)
(pM)
Soil
Ocean
Evasion
Terrestrial
Re-emission
(Mg)
(ng m-2 hr-1)
(ng m-2 hr-1)
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)
1. Early mining could contribute significantly
2. Geogenic emissions less than 300 Mg/yr
Pre-industrial
Enrichment
Factor
Atmosphere
(Mg)
(unitless)
All-time
Enrichment
Factor
(unitless)
Upper
Ocean
Deep
Ocean
(pM)
(pM)
Soil
Ocean
Evasion
Terrestrial
Re-emission
(Mg)
(ng m-2 hr-1)
(ng m-2 hr-1)
peat
sediment
Base case
Zhang emissions: Cut mining 3x
Engstrom emissions: Cut 2x, zero pre-1850
Greater geogenic emissions
Greater soil retention
Increased ocean evasion
Greater burial
Impact of biogeochemical processes on future
trajectories can be as large as uncertainty in emissions
Modeled response to terminating anthropogenic emissions
(normalized to 2015)
Atmosphere
Ocean, 0 to 1000 m
Emissions increasing since 1950s
2015 levels
(%)
(%)
Base case:
Emissions peak 1970s
Greater burial
2015
Year
Amos et al. (2015), ES&T
2050
2015
Year
2050
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
http://bgc.seas.harvard.edu/models.html
Thanks to our funders
US Environmental Protection Agency
Contract No. EP-11-H-0013646
European Research Council
Grant ERC-2010-StG_20091028