4. DISCUSSION
Permafrost regions have experienced dynamic environmental changes
induced by climate change, especially in the north-eastern Siberia
(Nitzbon et al. , 2020). Recent climate-induced increases in thaw
propagation have triggered changes in local relief in the Yedoma
uplands, including soil subsidence (Günther et al. , 2015),
activation of thermokarst and thermoerosion processes (Grigorievet al. , 2009; Morgenstern et al. , 2021), and the expansion
of pond and thermokarst lake areas (Nitze et al. , 2017; Veremeevaet al. , 2021). In the large rivers, an increase in runoff is both
expected and already observed: in the Kolyma, mean annual discharge has
increased over the 2010–2020 by 27.7% (94.6 to 120.7
km3 year-1) compared to a baseline
period of 1971–2000 (Mann et al. , 2022). The same authors
estimated an increase of 50% (±25%) in Kolyma’s mean annual runoff by
2100, in line with climate projections. Climate warming resulted also in
an increase in Kolyma’s sediment transport rates from 20 to 100 t
km-2 year-1 in 1941-2000 (Chalovet al. , 2018), which can be also attributed to mining impact
(Walling & Fang, 2003) or changes due to permafrost thaw (Chalovet al. , 2021).
The data presented here shows the hydrochemical patterns of different
waters in an environment shaped by permafrost seasonal and long-term
changes. The ionic composition of all sampled waters is likely impacted
by both suprapermafrost waters and an inflow of deeper, intra- and
subpermafrost waters; especially the permafrost creeks P1 and P2 are
likely connected to unfrozen groundwater (Cochand et al. , 2019).
Among the studied samples, the permafrost creeks differ from river
waters by the high content of DOC, major cations and anions, and
selected metals and metalloids, including heavy metals. The pathways of
OC transport and transformation in Siberian freshwater have been
thoroughly analysed recently, e.g. by Wild et al. (2019), Mannet al. (2022), or Jong et al. , (2022). Mann et al.(2022) stated that permafrost-derived OC additions will significantly
enhance inland OC turnover over the upcoming decades, however the fate
of OC is uncertain due to additional transformation factors (Keskitaloet al. , 2022). In the large rivers, OC occurs mainly as DOC: for
Kolyma, the dissolved share has been calculated by Mann et al.(2022) as 80%. Similar values are reported by others (Jong et
al. , 2022; Keskitalo et al. , 2022). Keskitalo et al.(2022) have concluded, based on isotopic evidence, that the smaller
fraction by mass, the POC, is more affected by the legacy Yedoma-derived
OC, than is DOC. Similarly, Wild et al. (2019) claim that POC in
the large Siberian rivers may originate from the direct erosion of the
old Yedoma layers, although they also estimate the legacy OC share in
the DOC and POC fractions in the Kolyma to be approximately equal.
Bröder et al. (2020) remarks that Kolyma transports more
suspended POC at higher discharges, and both the total POC and legacy
POC concentrations are higher there than in a small stream (the average
POC ages in Kolyma and a small stream were estimated at
~2,840 and ~590 years, respectively).
These claims are supported by Jong et al. (2022), who observed an
increased contribution of POC in the TOC transport in large rivers and
connected it to the increased direct erosion of the river banks, while a
smaller stream exhibited a higher share of DOC. Therefore, the POC in
Kolyma likely originates from recent vegetation and old permafrost
erosion combined. Mann et al. (2022) suggest also that the OC
flux and total export will rapidly increase due to climate change and
permafrost degradation.
In our research, the DOC concentration in a permafrost ice sample
amounted to 9.13 mg L-1 and it was approximately 11
times less than the concentrations in the creeks draining the permafrost
cliff. Similar DOC values of 154-336 mg L-1 and 103.4
mg L-1, for creeks draining the permafrost cliff at
Duvanny Yar, were obtained by Vonk et al. (2013) and Jonget al. (2022), respectively, and the concentrations of POC in the
same samples amounted to 6700-9240 mg L-1, eclipsing
the DOC contribution. (This is also consistent with the additionally
sampled POC in P1 and P2 samples, amounting to 8500 and 12000 mg
L-1, respectively – methods are reported in theSupp. Mat. 1. ) Conversely, Jong et al. (2022) obtained DOC
concentrations (2.76 to 4.97 mg L-1) at approximately
double the POC concentrations (1.49 to 2.73 mg L-1)
for Kolyma below Duvanny Yar, and similar results for Maly and Bolshoy
Anyui (3.16-4.43 mg L-1 and 1.29-1.7 mg
L-1, respectively). In our research, thermokarst lakes
were studied also, with DOC concentrations similar to a small stream
studied by Jong et al. (2022), representing soil leaching and
active layer drainage ([DOC] = 21.5 mg L‑1). The
legacy DOC from permafrost can be approximated by the DOC concentration
in permafrost ice (9.13 mg L-1), yet the extra 90% of
DOC in permafrost creeks may originate both from permafrost active layer
(i.e. relatively recent atmospheric deposition and microbial processes)
and the exclusion of various organic compounds during groundwater
freeze. POC from the ice may also be a source of DOC in thaw waters. The
cluster analysis of our data has shown similarities in the water
chemistry between the permafrost ice sample (P3), the bottom layer of
water in Lake 2 and a sample from Kolyma below Duvanny Yar (K31, sampled
mid-depth away from the cliff side, where the main river current flows),
which suggests the influence of legacy permafrost thaw upon the Kolyma
river in this cross section. At the same time, the connection between
the other lake samples and the samples from the permafrost creeks
suggests a general permafrost waters influence over the lake waters,
although it is probably a combined influence of the active layer
drainage and deeper talik-type waters.
In the collected permafrost ice, a contrastingly high level of aluminium
concentration has been noted as compared to the other samples; a
relatively high concentration of Al was also found in the permafrost
creeks and the sampled lakes. Aluminium co-occurs with
K+, Be, but also with Co and As, frequently classified
as heavy metals. Arsenic occurred at elevated (against other samples)
concentrations in the lake bottom waters of this study. In the lake
waters, permafrost creeks and permafrost ice, we found also elevated
concentrations of other heavy metals (Co, Mn, Ni, Zn, Cu and Sb). Zinc,
copper and cadmium showed elevated values also in the Kolyma, while the
bottom layer waters of the thermokarst Lake 2 exhibited also a very high
concentration of Pb. Monhonval et al. (2021) characterised the
chemical composition of Yedoma waters, by exploring the mineral element
stock in sediments of the Yedoma. They found a relatively high
concentration for Si, followed by Al, Fe, K, Ca, Ti, Mn, Zr, Sr, and Zn.
Moreover, the stock of Al and Fe (598 ± 213 and 288 ±104 Gt) was at the
same order of magnitude as the OC stock (327–466 Gt). Our surface water
and ice samples from the Kolyma watershed exhibited similarly high
concentrations of Si and Fe, as well as Al. The cluster analysis of all
the collected samples shows that in cluster C3, the elements Si and Fe
were grouped together with Ni and Mn (Figure 6a). However, in river
waters they correlated with Zn, Sb, Cu, V, Co, As, and DOC (cluster c1,
Figure 7). Both clusters may be connected to the suprapermafrost waters
of the active layer, which are flowing intensely into the thermokarst
lakes and rivers. Ji et al. (2021) studied elements in permafrost
soils and concluded that Mn, Ca, Mg, Al, and Ti showed the highest
mobility from soil to suprapermafrost water and further into the ponds
and flowing waters. Furthermore, they considered Mn a proxy for
predicting the processes occurring in the active layer during
summer-autumn thaw. Conversely, the long-term research in the Yukon
River basin (1982-2014), underlain by discontinuous permafrost, shows a
relationship between the deepening of permafrost active layer and
significant increases in the Ca, Mg, and Na annual flux in the Yukon
River and its tributary Tanana, and in the SO4 and P
annual flux in the Yukon (Toohey et al. , 2016). The authors
described increased active layer expansion, weathering, and sulfide
oxidation due to permafrost degradation throughout the studied basin.
Frey & Mcclelland (2009) forecast also a significant increase in the
concentrations of major ions due to permafrost degradation and lowering
water tables (except for nitrate in Siberia). In the studied samples
from the Kolyma watershed, clusters C5 and c3 (in the all sample and
river sample analysis, respectively; Figure 6a, Figure 7) indicate the
likely influence of rock weathering on water composition. The visible
division within the c3 cluster into c3a (Sr, Se, K+,
Mg2+, Ca2+ and
SO42-) and c3b (Cl-,
Na+ and F-) may represent two
separate water supply factors, namely the shallow and deep groundwater
flow (Douglas et al. , 2013). An extra contribution to the total
sulphate load in the rivers may also result from the wet deposition
connected to wildfires, which have influenced considerably the
deposition of sulphur (and nitrogen) compounds in the regions of Siberia
and the Russian Far East (Berezin et al. , 2013). On the other
hand, the lower contribution of Ca2+ in thermokarst
lake waters may result, according to Monhonval et al. (2021),
from the leaching of soluble elements such as Ca during former thaw
periods.
In this study, mercury was found only in permafrost creeks, permafrost
ice and in the Maly Anyui. Recent research has shown that Hg may be
released from continental permafrost with climate-change-induced thaw,
and there are very different quantitative estimates of Hg fluxes
connected to various local sources (permafrost is considered one source
among the many; Campeau et al. , 2022; Ci et al. , 2020; Muet al. , 2020; Schuster et al. , 2018). Schuster et
al. (2018) estimated that the entire Northern Hemisphere permafrost
area contains 1,656 ± 962 Gg Hg, of which 793 ± 461 Gg Hg
(~47%) is frozen in permafrost. Except in permafrost
ice, our samples may contain mercury of various origin, both from
permafrost thaw and from atmospheric deposition (Schuster et al. ,
2018), and in the case of Maly Anyui it may also be connected to mining.
(Gold mining occurs in parts of both the Maly and Bolshoy Anyui
catchments, yet higher production comes from around the Maly Anyui
tributary Karalveem River). However, the low Hg concentrations
(<LOD) in the other collected samples (from Kolyma, Omolon and
the lakes) indicate that any atmospheric sources of Hg would likely be
only local and periodic (e.g., forest fires) (Francisco López et
al. , 2022). Furthermore, it cannot be excluded that most Hg in the
analysed samples occurred in the particulate form, which has not been
investigated here. Lim et al. (2019), who have analysed the POC
and Hg fluxes in the rivers of various permafrost zones, indicate
minimal particulate Hg export at the sporadic to discontinuous
permafrost zone. According to these authors, the current climate
warming, the northward shift of permafrost boundaries and the increasing
active layer depth in Western Siberia, may result in an enhanced
particulate Hg export by small rivers to the Arctic Ocean by a factor of
two over the next 10-50 years. The possible origin of the Hg may be also
connected to mining areas (both abandoned and active) distributed over
Maly Anyui river.
Besides mercury, also As and Cd are heavy metals sometimes mentioned as
posing a new hazard due to permafrost thaw (Zhang et al. , 2021).
In this study, a potential source of those is the erosion of a
permafrost cliff, given the significant enrichment of As and Cd in
permafrost ice, the extremely high EF in permafrost creeks, and very
high to extremely high EF in the deep waters of thermokarst lakes (Table
S4, Suppl. Mat. 1 ). The supply of Cd may be connected to the
drainage of mixed suprapermafrost water, which may include atmospheric
deposition, and deeper water both. With the active layer deepening,
atmospheric pollutants deposited in the past, which have been excluded
from the freeze-thaw cycle for a time (Ji et al. , 2021), may be
remobilised (Edwards et al. , 2021; Li et al. , 2020; Limet al. , 2019; Rubino et al. , 2016). The notable increase
in Cu, Ni, Pb and Zn concentrations in deep lake waters indicate the
likely source of these elements in permafrost thaw, talik water, or
enhanced mobility of suprapermafrost water eluting them from soil.
Importantly, it is not only in the Lake 1 near the Cherskii research
station and the settlement which experienced the elevated concentrations
of these metals in the bottom waters, but also in the further from any
human activity Lake 2, showing that such contamination is probably
irrelevant of local infrastructure.
In the studied freshwaters, it was evident that thermokarst lakes form
at least temporary traps for heavy metals connected to one or several
types of permafrost influence. Thermokarst lakes form one of the more
dynamic elements of the continental permafrost landscape, and their mass
emergence is among the more frequently observed impacts of climate
change in the Northern Hemisphere (in’t Zandt et al. , 2020;
Karlsson et al. , 2012; Plug et al. , 2008), including the
Kolyma region (Veremeeva et al. , 2021). Due to the dynamically
increasing number of thermokarst lakes, and their predicted drainage
with the advancing climate change and deepening active layer, the
sedimentation in these lakes and the biogeochemical processes happening
in them (in’t Zandt et al. , 2020) will likely be of high
importance in studies of continental permafrost areas.