Natural and Anthropogenic Mercury Sources
31
lived while the system reequilibrated with ambient Hg( 0), and the overall effect
of these pulses on annual fluxes has not been assessed.
Both elemental Hg and DMM are found in oceanic waters in the DGM fraction.
A number of studies (Fitzgerald et a1. 1994; Mason et a1. 1994a) suggest that direct
reduction of ionic mercury is predominant in low oxygen waters (Mason and
Fitzgerald 1993). Generally, both volatile species are found in comparable
concentration ranges, although there always seems to be more Hg(o) than DMM.
In contrast to terrestrial systems, dry deposition of mercury does not seem to be
an important process in marine ecosystems although no attempts have been made
to measure it. Thus, deposition estimates are derived from precipitation
measurements only. Reported results range from 0.04 to over 800 ng m -2 h -1.
The consequence of this enormous range seems to be that while on a global basis
emission and deposition over oceans seem to equalize approximately, there are
large regional differences depending on both the regional emission activity and
the regional deposition as a result of precipitation rates and degree of local
contamination. In the Arctic, for example, deposition was estimated to be five
times larger than emission, while the ratio is exactly opposite in the Equatorial
Pacific, leading to the conclusion that air-sea exchange of mercury actively
contributes to the global Hg transport from midlatitude to Arctic shelves. On the
basis of the existing data, it would, however, be more than daring to characterize
either the oceans or the oceanic margins as either absolute sinks or sources of
atmospheric mercury until more detailed and larger-scale studies have been done.
Studies in the northern Pacific Ocean show that Hg volatilization and
deposition in the tropical Pacific Ocean are pretty much equal, but while the
evasional flux was fairly constant at different latitudes, deposition varied
significantly, thus making the equatorial Pacific Ocean a source of atmospheric
Hg while the higher latitude regions turn out to be a sink (Mason et a1. 1994b).
Since the equatorial part of the Pacific Ocean is an upwelling area, it was
suggested on the base of model calculations that reactive Hg(II) supplied by the
upwelling process is the source of the observed increased Hg( 0) concentrations
and thus leads to the inferred atmospheric emission flux. Studies in the northern
Atlantic Ocean also identify this region as an atmospheric Hg source, with
emission fluxes averaging 139 ± 95 pg m- 2 year-I (Mason and Fitzgerald 1996).
The same trend has also been observed for coastal regions. While Hg evasion in
remote arctic shelf areas was very small (3.7-5.8 pg m2
year-I), it was shown to
increase in more polluted oceanic boundary regions (7.3-22 pg m- 2 year-I). The
authors conclude from these data that net atmospheric transfer of Hg from lower
latitudes to arctic regions is going on despite a net global balance for emissions
and depositions. Somewhat higher atmospheric mercury evasion fluxes have
been reported for estuaries (3.7-51 ~Lg m2
year-I), and the emission seems to be
correlated to the rivers' Hg content (Cossa et a1. 1996). In a very detailed review
of studies on Hg biogeochemistry in coastal regions, Cossa et a1. (1996) propose
the following semiquantitative budget for coastal areas. Overall, imports and
exports of Hg to ocean margins seem to neutralize within the range of error. The
major source of Hg is particulate matter transported in rivers. However, this
amount seems to be deposited in the estuaries via sedimentation under the
influence of the salinity gradient. Exchange between ocean margins and their
31
lived while the system reequilibrated with ambient Hg( 0), and the overall effect
of these pulses on annual fluxes has not been assessed.
Both elemental Hg and DMM are found in oceanic waters in the DGM fraction.
A number of studies (Fitzgerald et a1. 1994; Mason et a1. 1994a) suggest that direct
reduction of ionic mercury is predominant in low oxygen waters (Mason and
Fitzgerald 1993). Generally, both volatile species are found in comparable
concentration ranges, although there always seems to be more Hg(o) than DMM.
In contrast to terrestrial systems, dry deposition of mercury does not seem to be
an important process in marine ecosystems although no attempts have been made
to measure it. Thus, deposition estimates are derived from precipitation
measurements only. Reported results range from 0.04 to over 800 ng m -2 h -1.
The consequence of this enormous range seems to be that while on a global basis
emission and deposition over oceans seem to equalize approximately, there are
large regional differences depending on both the regional emission activity and
the regional deposition as a result of precipitation rates and degree of local
contamination. In the Arctic, for example, deposition was estimated to be five
times larger than emission, while the ratio is exactly opposite in the Equatorial
Pacific, leading to the conclusion that air-sea exchange of mercury actively
contributes to the global Hg transport from midlatitude to Arctic shelves. On the
basis of the existing data, it would, however, be more than daring to characterize
either the oceans or the oceanic margins as either absolute sinks or sources of
atmospheric mercury until more detailed and larger-scale studies have been done.
Studies in the northern Pacific Ocean show that Hg volatilization and
deposition in the tropical Pacific Ocean are pretty much equal, but while the
evasional flux was fairly constant at different latitudes, deposition varied
significantly, thus making the equatorial Pacific Ocean a source of atmospheric
Hg while the higher latitude regions turn out to be a sink (Mason et a1. 1994b).
Since the equatorial part of the Pacific Ocean is an upwelling area, it was
suggested on the base of model calculations that reactive Hg(II) supplied by the
upwelling process is the source of the observed increased Hg( 0) concentrations
and thus leads to the inferred atmospheric emission flux. Studies in the northern
Atlantic Ocean also identify this region as an atmospheric Hg source, with
emission fluxes averaging 139 ± 95 pg m- 2 year-I (Mason and Fitzgerald 1996).
The same trend has also been observed for coastal regions. While Hg evasion in
remote arctic shelf areas was very small (3.7-5.8 pg m2
year-I), it was shown to
increase in more polluted oceanic boundary regions (7.3-22 pg m- 2 year-I). The
authors conclude from these data that net atmospheric transfer of Hg from lower
latitudes to arctic regions is going on despite a net global balance for emissions
and depositions. Somewhat higher atmospheric mercury evasion fluxes have
been reported for estuaries (3.7-51 ~Lg m2
year-I), and the emission seems to be
correlated to the rivers' Hg content (Cossa et a1. 1996). In a very detailed review
of studies on Hg biogeochemistry in coastal regions, Cossa et a1. (1996) propose
the following semiquantitative budget for coastal areas. Overall, imports and
exports of Hg to ocean margins seem to neutralize within the range of error. The
major source of Hg is particulate matter transported in rivers. However, this
amount seems to be deposited in the estuaries via sedimentation under the
influence of the salinity gradient. Exchange between ocean margins and their