3 research outputs found
Seasonal dynamics of organic carbon and metals in thermokarst lakes from the discontinuous permafrost zone of western Siberia
Despite relatively good knowledge of the biogeochemistry of Siberian
thermokarst lakes during summer base flow, their seasonal dynamics remains
almost unexplored. This work describes the chemical composition of
~130 thermokarst lakes ranging in size from a few m2 to several
km2, located in the discontinuous permafrost zone. Lakes were sampled
during spring flood, just after the ice break (early June), the end of summer
(August), the beginning of ice formation (October) and during the full
freezing season in winter (February). The lakes larger than 1000 m2 did
not exhibit any statistically significant control of the lake size on
dissolved organic carbon (DOC), the major and trace element concentrations
over three major open water seasons. On the annual scale, the majority of
dissolved elements including organic carbon increased their concentration
from 30 to 500%, with a statistically significant (p < 0.05) trend
from spring to winter. The concentrations of most trace elements (TEs)
increased in the order spring > summer > autumn > winter. The ice
formation in October included several stages: first, surface layer freezing
followed by crack (fissure) formation with unfrozen water from the deeper
layers spreading over the ice surface. This water was subsequently frozen and
formed layered ice rich in organic matter. As a result, the DOC and metal
(Mn, Fe, Ni, Cu, Zn, As, Ba and Pb) concentrations were highest near the
surface of the ice column (0 to 20 cm) and decreased by a factor of 2
towards the bottom. The main implications of discovered freeze-driven solute
concentrations in thermokarst lake waters are enhanced colloidal coagulation
and removal of dissolved organic matter and associated insoluble metals from
the water column to the sediments. The measured distribution coefficients of
a TE between amorphous organo-ferric coagulates and lake water
(<0.45 ΞΌm) were similar to those reported earlier for Fe-rich
colloids and low molecular weight (<1 kDa, or <1β2 nm) fractions
of thermokarst lake waters, suggesting massive co-precipitation of TE with
amorphous Fe oxyhydroxide stabilized by organic matter. Although the
concentration of most elements was lowest in spring, this period of maximal
water coverage of land created a significant reservoir of DOC and soluble
metals in the water column that can be easily mobilized to the hydrological
network. The highest DOC concentration observed in the smallest
(2) water bodies in spring suggests their strongly
heterotrophic status and, therefore, a potentially elevated CO2 flux
from the lake surface to the atmosphere
Permafrost coverage, watershed area and season control of dissolved carbon and major elements in western Siberian rivers
Analysis of organic and inorganic carbon (DOC and DIC, respectively), pH, Na,
K, Ca, Mg, Cl, SO4 and Si in ~ 100 large and small rivers
(2) of western Siberia sampled
in winter, spring, and summer over a more than 1500 km latitudinal gradient
allowed establishing main environmental factors controlling the transport of
river dissolved components in this environmentally important region,
comprising continuous, discontinuous, sporadic and permafrost-free zones.
There was a significant latitudinal trend consisting in a general decrease in
DOC, DIC, SO4, and major cation (Ca, Mg, Na, K) concentration northward,
reflecting the interplay between groundwater feeding (detectable mostly in
the permafrost-free zone, south of 60Β° N) and surface flux (in the
permafrost-bearing zone). The northward decrease in concentration of
inorganic components was strongly pronounced both in winter and spring,
whereas for DOC, the trend of concentration decrease with latitude was absent
in winter, and less pronounced in spring flood than in summer baseflow. The
most significant decrease in K concentration from the southern
(< 59Β° N) to the northern (61β67Β° N) watersheds occurs
in spring, during intense plant litter leaching. The latitudinal trends
persisted for all river watershed size, from < 100 to
> 10 000 km2. Environmental factors are ranked by their increasing
effect on DOC, DIC, Ξ΄13CDIC, and major elements in
western Siberian rivers as follows: watershed area < season < latitude. Because the degree of the groundwater feeding is different
between large and small rivers, we hypothesize that, in addition to
groundwater feeding of the river, there was a significant role of surface and
shallow subsurface flow linked to plant litter degradation and peat leaching.
We suggest that plant-litter- and topsoil-derived DOC adsorbs on clay mineral
horizons in the southern, permafrost-free and discontinuous/sporadic
permafrost zone but lacks the interaction with minerals in the continuous
permafrost zone. It can be anticipated that, under climate warming in western
Siberia, the maximal change will occur in small (2
watershed) rivers DOC, DIC and ionic composition and this change will be
mostly pronounced in summer