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Fig. 1.
Scheme of Hg cycle in the Arctic in permafrost thaw systems (process from 1 to 11). In the atmosphere, the main form of Hg is Hg(0) but it can also be
oxidized to Hg(II) (process 1). The Hg(0) can be deposited through foliar uptake (process 2). During snow fall, Hg(II) can also be deposited through wet deposition
and can be converted into Hg(0) and re-emitted back to the atmosphere (process 3). A portion of Hg can be transferred to soil or aquatic systems during snowmelt
(process 3). Wet deposition of Hg(II) also occurs during rainfall (process 4). Re-emission of accumulated Hg occurs during forest fires (process 5). In soils occurs the
deposition of Hg and conversion between Hg(0) and Hg(II), which forms complexes with organic matter (OM) and can be methylated by bacteria forming MMHg
(process 6). This Hg form can also form complexes with OM. In permafrost areas, there has been an increase in the distribution of thermokarst lakes. In these, occurs
the deposition of Hg and conversion between Hg(0) and Hg(II), but there is also the formation of MMHg, a process that can be intensified due to the high OM content
(process 7). The concentration of Hg in these lakes is also affected by the transport of previously accumulated Hg in permafrost (process 8). The degradation of
permafrost also increases the hydrological connections between aquatic systems and leads to the transport of Hg(II) and MMHg from lakes to rivers (process 9). In
these, a fraction of Hg is sedimented (process 10). Retrogressive thaw slumps mobilize high volumes of sediments into rivers and transport particle-bound Hg
(process 11).
Fig. 2.
Permafrost distribution in the Northern Hemisphere,
Fig. 3.
Scheme of permafrost layers. The active layer that thaws seasonally
controls the heat transferred to the permafrost and the top vegetation can
significantly influence this process. In permafrost, discontinuous pieces of
frozen water create ice lenses or larger ice wedges. The red line represents the
temperature (T) variation with depth (z) in the layers of soil and the seasonal
differences.
Fig. 4.
Map of estimation of soil organic carbon stock (kg m
-2
) in permafrost
from 0 to 3 m in the Northern Hemisphere by (
Hugelius et al., 2014
).
Fig. 5.
Map of estimation of Hg stock (mg Hg m
-2
) in permafrost in the Northern Hemisphere by (
Schuster et al., 2018
). The maps are divided into four soil layers:
0
–
30 cm, 0
–
100 cm, 0
–
300 cm, and permafrost.
Fig. 6.
Thermokarst lakes formation from ice-rich permafrost in continuous and discontinuous areas (retrieved and adapted from
Bouchard et al. (2017)
)
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