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Australia: The Land Where Time Began |
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Permafrost stores an Amount of Mercury that is Globally Significant
Permafrost is thawing in northern regions as the climate changes to
warmer conditions with major implications for the global mercury (Hg)
cycle. Schuster et al.
estimated, based on in situ
measurements of the sediment total mercury (STHg), soil carbon (SOC),
and mercury to carbon ration (RHgC) mercury in regions of
permafrost combined with maps of soil carbon. Schuster et
al. measured a median (STHg)
of 43 ± 30 ng Hg/g of soil and a median RHgC of 1.6 ± 0.9 μg
Hg/g organic carbon which is consistent with published results of STHg
for tundra soils and 11,000 measurements from 4,926 temperate,
nonpermafrost sites in North America and Eurasia. The permafrost regions
of the Northern Hemisphere were estimated by Schuster et
al. to contain 1,656 ± 962 Gg
Hg, 793 ± 461 Gg Hg of which is frozen in permafrost. Almost twice as
much Hg is stored in permafrost soils as all other soils, the ocean and
the atmosphere combined, and as the permafrost thaws this Hg is
vulnerable to release over the next century. The amount of mercury in
permafrost soils has been greatly underestimated in existing estimates,
which indicates there is a need to reevaluate the role in the global
mercury cycle of the Arctic regions.
Put simply it is estimated by researchers that the amount of natural
mercury stored in permafrost in the Northern Hemisphere is twice as much
mercury as the rest of all soils, the oceans and the atmosphere
combined.
Sedimentation has buried mercury (Hg) that is bound to organic material
over thousands of years which has then been frozen in the permafrost
(Obrist et al., 2017).
Permafrost is soil that has remained at or below 0oC for at
least 2 consecutive years. The surface soil layer overlying the
permafrost is the active layer that thaws in summer and refreezes every
winter. Mercury bonds with the organic matter in the active layer as it
is deposited from the atmosphere. When the organic matter is then
consumed by microbial decay it releases the mercury (Smith-Downey et
al., 2010). Simultaneously,
sedimentation slowly increases the depth of the soil such that organic
matter at the base of the active layer becomes frozen into permafrost.
The organic matter is composed almost entirely of plant roots, and once
it has frozen microbial decay practically stops, and the mercury is then
locked into the permafrost. The permafrost has, however, begun thawing
under the changing climatic conditions (Hinzman et
al., 2005; Romanovsky et al.,
2008; Smith et al., 2010).
Once the permafrost and the organic matter that is associated with it
thaws, microbial decay will resume and release mercury to the
environment, which will them potentially impact the mercury balance in
the Arctic, aquatic resources, as well as human health (Dunlap et
al., 2007; Jonsson et
al., 2017; Obrist et
al., 2017; USGS Fact Sheet,
https://www2.usgs.gov/themes/factsheet/146-00/).
Schuster et al. took
the novel approach of making the first ever estimate of the storage of
mercury in the permafrost soils of the Northern Hemisphere using
empirical relationships that were based on
in situ measurements of
sediment total mercury (STHg) combined with maps that had been published
of soil organic carbon (Hugelius, Tamocai et
al., 2013; Hugelius, Bockheim
et al., 2103).
There are several atmospheric Hg sources that are unique to the high
latitudes of the Northern Hemisphere which have significant spatial and
temporal variability to explain STHg variability within and between
cores (Fitzgerald & Lamborg, 2003). Mercury is released to the
atmosphere by fires in the northern boreal forests which lead to spatial
variability in the deposition of mercury (Homann et
al., 2015; Rothenberg et
al., 2010; Turetsky et
al., 2006). Microbial
respiration rates are changed by spatial variation in temperature and
moisture. During summer peaks in atmospheric mercury resulting from
atmospheric mixing with ozone enhance the deposition of mercury (Banic
et al., 2003; Sonke &
Heimbürger, 2012). Following the polar sunrise in springtime,
atmospheric depletion of mercury to high latitudes may elevate the
deposition of mercury (Berg et al.,
2008; Fitzgerald et al.,
2005; Lindberg et al., 2002).
According to Schuster et al.
they see no evidence of terrestrial geologic mercury sources within
Alaska (Eberl, 2004; Williams, 1962) though there are geologic deposits
of mercury in southeast Alaska (Gray et
al., 2000). Mercury is
released into the atmosphere by volcanic eruptions, which leads to
mercury deposition being variable (Pirrone et
al., 2010; Pyle & Mather,
2003; Schuster et al., 2002),
though according to Schuster et
al. they didn’t see much evidence of it in deposition of mercury
from volcanic ash. The 13 sites that were cored were not the only sites
where these depositional processes occur; rather it is inclusive to the
high latitudes of the Northern Hemisphere.
The STHg measurements of Schuster et
al. appear to be consistent
with similar measurements from permafrost regions, in spite of these
processes leading to depositional environments that are highly variable.
STHg values and vertical profiles and deposits of peat in Tomsk Oblast,
west Siberia (Lyapina et al.,
2009) were found to be similar to those of Schuster et
al. STHg of 40 (Rydberg et
al., 2010) ng of mercury in
soil below 25 cm depth, which is close to the median value of 43 ng of
mercury of soil mercury found by Schuster et
al.. In soils of the active
layer along an approximately 970 kl north-south transect of Alaska
ranged from 100 ng of mercury per gram of soil in the O horizon to 50 ng
mercury per gram of soil in the A horizon (Wang et
al. 2010). At a subantarctic
site in Tiera del Fuego (Peña & Rodriguez et
al., 2014), STHg ranged
between 12 and 375 ng mercury per gram of soil, and RHgC
varied from 1 to 11.3 μg mercury per gram of soil.
The STHg measurements appear, moreover, to be consistent with data from
nonpermafrost soils that have been published. Of the 11,000 data that
have been published most fall in the temperate midlatitudes, with 2,088
being from boreal forests, and only 67 points in permafrost regions. It
was shown in this study that a curve fit of the median STHg as a
function of SOC, with the 90% envelope being defined as the 5th
to 95th percentiles. When the data from this study are
superimposed it indicates that 90% of the STHg measurements from this
study fall within the 90% envelope of the published data, with a slight
shift towards higher STHg.
STHg appears to shift from a regime that is receptor-limited to one that
is flux-limited. Mercury enters plants through the roots or by dry
deposition from the atmosphere onto the leaves (Obrist et
al., 2017; Windham-Myers et
al., 2009), where it attaches
to appropriate receptors in organic molecules which would normally be
attachment sites for nutrients such as iron or magnesium. The number of
receptor sites limits the mercury that can be retained for mineral soils
that have SOC less than 10%, and with SOC (R2
= 0.98) the STHg increases strongly. Receptor sites appear unlimited for
organic soils with SOC greater than 10%, and the flux of mercury from
the atmosphere limits the STHg. The data become noisier and SOC appears
almost independent of SOC, as the atmospheric deposition if highly
variable over space and time. Schuster et
al. suggest the 2 regimes
might explain why some sites display correlations between STHg and SOC
that are statistically significant, like data that has been published
previously (Bargagli et al.,
2007; Erikson, 2014), whereas others do not. Sites with low SOC in the
regime that is that is receptor-limited tended to have correlations that
are statistically significant while sites with organic soils in the
flux-limited regime did not.
Schuster et al. estimated
that in permafrost regions soils contain an estimated 1,656 ± 962 Gg of
mercury, half or 793 ± 462 Gg of mercury is frozen in permafrost. They
estimated the total mass of mercury in each layer by multiplying each
individual pixel of by the grid cell area and summing across the
permafrost domain. The average of 9 estimates of global soil mercury
that been published previously is 454 ± 321, ranging from 325 to 1,000
Gg of mercury. These estimates, however, generally limit soil depth of
30 cm, which indicates that the 0-30 cm of soil mercury of Schuster et
al. for permafrost regions
(347 ± 196 Gg of mercury rivals the global soil estimates in previous
estimates. The known link between microbial decay and the release of
mercury are leveraged by these studies, often relying on biogeochemical
models that tend to underestimate soil mercury in permafrost regions. In
order to improve mercury estimates these models should account for large
drops in microbial activity under freezing conditions, and sediment
processes that bury and freeze and freeze organic matter into
permafrost. The results of this study indicate that the largest
reservoir of mercury on Earth is represented by the active layer alone.
There is almost twice as much mercury in the active layer and the
permafrost combined as all other soils, the atmosphere and the ocean
combined. Mercury in permafrost soils represents an environmental risk
as permafrost continues to thaw in the future. The mercury locked in
permafrost is effectively stable on human rime scales as the turnover
time associated with microbial decay of frozen organic matter is about
14,000 years. It is indicated by projections that there will be a 30-99%
reduction, however, of near surface permafrost by 2100, and once it has
thawed, the turnover for microbial decay drops to about 70 years (Koven
et al., 2013; Schaefer et
al., 2011). This makes the
reservoir of mercury in permafrost soils vulnerable to release over the
next century, with unknown environmental consequences.
In this study Schuster et al.
measured a median STHg 43 ± 30 ng of Hg in the soil and median RHgC
of 1.6 ± 0.9 μg Hg per g of carbon based on 588 samples from 13 soil
permafrost cores from the interior and the North Slope of Alaska. It
appears these values are consistent with results of Hg concentrations
for tundra soils and 11,000 nonpermafrost soil measurements from 4,296
different sites in North America and Eurasia that have been published.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |