Dimethylmercury in Seawater:
A Potential Source of Monomethylmercury in Fog
Kenneth Coale1, Wesley Heim1, Alex Olson1, Holly Chiswell1, Amy Byington1, Adam Newman1, Autumn Bonnema1,
Mechelle Johnson2, Daniel Fernandez3, Peter Weiss-Penzias4, Claire Parker4
1. Moss Landing Marine Laboratories, Moss Landing, CA 2. Kirkwood Community College, Cedar Rapids, IA
3. California State University at Monterey Bay, Seaside, CA 4. University of California at Santa Cruz, Santa Cruz, Ca
Fog: 17,000 - 54,000 fM
Rain: 200 – 9000 fM
On the average Fog water
Contains 20 x the MMHg
Than does rain water
Biota from foggy regions may contain
as much as 10 times more mercury
than biota from non-foggy regions
It happens in the ocean, let’s find out why
Satellite Altimetry Spots Upwelling and Downwelling
Counter-Clockwise
Upwelling Eddy
Clockwise
Dowwelling Eddy
DMHg in upwelling and downwelling eddies
Flux = Kz d[DMHg]/dz
Kz = 0.5-1 x 10-4 m2 s-1
d[DMHg] = 250-13 = 237 pmol m-3
dz = 186 m
Ocean Flux = 11 pmol m-2 d-1
Literature: 0.2 to 0.4 pmol m-2 d-1
(Hammerschmidt & Bowman, 2012)
Fogwater Flux to Land = 1 pmol m-2 d-1
So, where does all this dimethyl mercury go?
Deckboard incubation experiments show :
1) Dimethyl Mercury is stable at natural pH
2) Dimethyl Mercury converts to Monomethyl
Mercury at low pH.
Fog is acidic and the hydrous coating
on marine aersosols can be very acidic
Low pH in Fog or on the acidic hydrous coating
on marine aersosols can turn Dimethyl
Mercury into Monomethyl Mercury
The new model:
SO4
Acidic Fog or Marine Aerosol Particle
Cl-
H+
Hg
CH3
+
CH + H
3
CH3Hg+ + ClCH3Hg+ + CH4
Na+
Hg
CH3 CH3
2-
CH3HgClo
H+
Wet or Dry Deposition
CH3HgClo
Dimethyl Mercury in Seawater: A Potential Source of Monomethyl Mercury in Fog
K. Coale1 ([email protected]), W. Heim1, A. Olson1, H. Chiswell1, A. Byington1, A. Newman1, A. Bonnema1, M. Johnson2, D. Fernandez3, P. WeissPenzias4, C. Parker4
1) Moss Landing Marine Laboratories, 2) Kirkwood Community College, 3) California State University, Monterey Bay, 4) University of California, Santa Cruz
Observations:
1) We have observed twenty times the monomethyl mercury (MMHg) concentrations of in fog, compared to rain.
2) Because MMHg is bioaccumulated, biota from foggy watersheds contain up to 100 times more mercury than biota from non-foggy watersheds.
3) Fog originates over the ocean, implicating a marine source, and fog is the vector to land.
Question:
What processes/mechanism may act to account for this pathway?
Approach:
We carried out several cruises to quantify mercury species in several different reservoirs including fog, plankton, marine snow, sediments, water, etc… from
nearshore to 200 miles offshore and in cyclonic and anticyclonic mesoscale eddies.
Findings:
We found that
Cyclonic mesoscale eddies are major features of the California Current and major regions of DMHg evasion, decoupled from normal coastal upwelling.
This flux, derived by DMHg gradients and vertical eddy diffusion, far exceeds previous estimates based on physical gas evasion calculations.
Although DMHg seems to be stable at high pH (8), it can demethylate to MMHg rapidly under acidic conditions such as those found in fog and on marine
aerosols.
Only about 10% of the daily DMHg flux and its demethylation is necessary to explain the MMHg found in fog and its subsequent deposition to land on an
equal area basis.
A New Model:
We propose that
DMHg evades from the surface waters, especially in regions of upwelling and cyclonic eddies.
Demethylation to MMHg occurs in acidic fog water, or within the acidic hydrous coating of aerosol particles.
As fog drifts ashore, fog water rich in MMHg collects in the watershed on leaves and stems (wet deposition). As fog dissipates, MMHg, primarily in the >0.45
μ fraction, falls out and accumulates on the watershed where it can be trophically biomagnified.
Fog is enriched in MMHg (relative to rain) because it forms at the surface of the ocean, in direct proximity to the source of DMHg, and to the production of
marine aerosols.
The Movie
https://www.youtube.com/watch?v=C_mviqitVLw