Thermal permafrost degradation and coastal erosion in the Arctic remobilize
substantial amounts of organic carbon (OC) and nutrients which have
accumulated in late Pleistocene and Holocene unconsolidated deposits.
Permafrost vulnerability to thaw subsidence, collapsing coastlines and
irreversible landscape change are largely due to the presence of large amounts
of massive ground ice such as ice wedges. However, ground ice has not, until
now, been considered to be a source of dissolved organic carbon (DOC),
dissolved inorganic carbon (DIC) and other elements which are important for
ecosystems and carbon cycling. Here we show, using biogeochemical data from a
large number of different ice bodies throughout the Arctic, that ice wedges
have the greatest potential for DOC storage, with a maximum of
28.6 mg L
Vast parts of the coastal lowlands of Siberia, Alaska and Canada consist of
unconsolidated organic-rich, fine-grained deposits. These sediments, which
occur as glacigenic and Yedoma-type sediments (including their degradation
forms as thermokarst), are characterized by high ground-ice contents, both on
a volumetric (vol %) and gravimetric (wt %) basis (Brown et al.,
1997; Zhang et al., 1999; Grosse et al., 2013; Schirrmeister et al., 2013).
Yedoma deposits, which formed during the late Pleistocene cold stages in
unglaciated Beringia (Schirrmeister et al., 2013), for instance, are
characterized by absolute ground-ice contents, excluding ice wedges, of
40–60 wt % (Schirrmeister et al., 2011c). Ice wedges are one of the
most common types of ground ice in permafrost. They form when thermal
contraction cracks open in winter, which are periodically filled with snow
meltwater in spring that quickly (re)freezes at negative ground temperatures
to form ice veins and finally vertically foliated ice wedges. The ice wedges
are themselves characterized by volumetric ice contents
approaching 100 vol % and make up
much of the subsurface in these Yedoma deposits. Recent calculations of
ice-wedge volumes in east Siberian Pleistocene Yedoma and Holocene
thermokarst deposits show contents of 48 and 7 vol %, respectively
(Strauss et al., 2013). Combining ice wedges and other ice types in Yedoma
deposits gives a mean volumetric ground-ice content for those regions between
60 and 82 vol % (Zimov et al., 2006a, b; Schirrmeister et al., 2011b, c;
Strauss et al., 2013). High ground-ice contents are also typical for coastal
Alaska (43–89 vol %; Kanevskiy et al., 2011, 2013) and the western
Canadian Arctic (50–60 vol %; French, 1998). The presence of massive
ice (i.e., gravimetric ice content
Permafrost soils hold approximately 50 % of the global soil carbon pool (Tarnocai et al., 2009; Hugelius et al., 2014), mostly as particulate organic carbon (POC). These calculations of permafrost OC stocks, however, subtract the ground-ice content (Zimov et al., 2006a, b; Tarnocai et al., 2009; Strauss et al., 2013; Hugelius et al., 2013, 2014) and therefore disregard the OC, especially the amount of dissolved organic carbon (DOC), contained in large ground-ice bodies such as ice wedges and other types of massive ice. Although these numbers might be small compared to the POC stocks in peat and mineral soils, DOC from permafrost is chemically labile (Dou et al., 2008; Vonk et al., 2013a, b) and may directly enter local food webs. Due to its lability, DOC can become quickly mineralized by microbial communities and photochemical reactions (Battin et al., 2008; Vonk et al., 2013a, b; Cory et al., 2014) and returned to the atmosphere when released due to permafrost degradation (Schuur et al., 2009; Schuur and Abbot, 2011).
Several studies have shed light on the POC stocks contained in permafrost (e.g., Zimov et al., 2006a; Tarnocai et al., 2009; Schirrmeister et al., 2011b; Strauss et al., 2013; Hugelius et al., 2013, 2014; Walter Anthony et al., 2014) and how much of these stocks is potentially mobilized due to thermal permafrost degradation and coastal erosion (Rachold et al., 2004; Jorgenson and Brown, 2005; Lantuit et al., 2009; McGuire et al., 2009; Ping et al., 2011; Schneider von Deimling et al., 2012; Vonk et al., 2012; Günther et al., 2013, 2015; Wegner et al., 2015). DOC fluxes have also been quantified in western Siberian catchments (Frey and Smith, 2005), and monitoring efforts of the large rivers draining permafrost areas and entering into the Arctic Ocean have provided robust estimations of the riverine DOC export (Raymond et al., 2007; McGuire et al., 2009). However, DOC stocks in permafrost ground ice and the resulting potential DOC fluxes in response to coastal erosion and thermal degradation are still unknown (Guo et al., 2007; Duo et al., 2008). At this moment, any inference about DOC stocks in permafrost and fluxes from permafrost is derived from measurements in secondary systems such as lake (e.g., Kling et al., 1991; Walter Anthony et al., 2014), river (e.g., Benner et al., 2004; Finlay et al., 2006; Guo et al., 2007; Raymond et al., 2007; Holmes et al., 2012) and ocean waters (e.g., Opsahl and Benner, 1997; Dittmar and Kattner; 2003; Cooper et al., 2005) or from laboratory experiments (Dou et al., 2008). In contrast, the purpose of this study was to sample and measure DOC at the source (i.e., ground ice in permafrost) directly, before it gets altered by natural processes such as exposure to the atmosphere, lithosphere and hydrosphere.
Study area and study sites (dots) for massive ground-ice sampling in the Arctic lowlands of Siberia and North America. All study sites are located within the zone of continuous permafrost (dark purple), except for the Fairbanks area, which is the zone of discontinuous permafrost (light purple). Blue line in the Arctic Ocean marks the northerly extent of submarine permafrost according to Brown et al. (1997).
Here, we present an Arctic-wide study on DOC stocks in ground ice, aiming at
incorporating massive ground ice into the Arctic permafrost carbon budget.
The specific objectives of our study are
to quantify DOC contents in different massive ground-ice types; to calculate DOC stocks in massive ground ice at the Arctic
level; to put ground-ice-related DOC stocks into the context of the terrestrial Arctic OC pools and
fluxes; to introduce relationships between organic and inorganic geochemical parameters, stable water isotopes, stratigraphy,
and genetic and spatial characteristics to shed light on the origin of DOC and the processes of carbon sequestration in ground ice.
Summary of study areas, study sites, stratigraphy of the host sediments, ground-ice inventory and the studied ice types.
Ground-ice conditions and examples of studied ground-ice types in the Siberian and North American Arctic. Place names are plotted on Fig. 1.
This study was carried out along the coastal lowlands of east Siberia, Alaska and northwest Canada (Fig. 1). All study sites, except for the Fairbanks area, are located within the zone of continuous permafrost. The sites cover a wide and representative range of geomorphological settings, terrain units and ground-ice conditions (Table 1). All studied ground-ice bodies were found in ice-rich unconsolidated Holocene and late Pleistocene (Marine Isotope Stages 2–5) deposits. Outcrops in permafrost either were accessible due to strong rates of coastal erosion along the ice-rich coasts forming steep exposures (Forbes, 2011) or were technically constructed for research purposes, such as the CRREL (Cold Regions Research and Engineering Laboratory) Permafrost Tunnel in Barrow, or for mining, such as the Vault Creek Tunnel near Fairbanks, Alaska.
Coastal outcrops in Siberia were dominated by large late Pleistocene ice
wedges reaching up to 20 m in depth and up to 6 m in width (Schirrmeister
et al., 2011c). They formed syngenetically during periods of rapid
sedimentation of Ice Complex deposits, also known as Yedoma (Schirrmeister et
al., 2013). Holocene epigenetic and syngenetic ice wedges of 1–6 m in depth
and We refer to the Lateglacial as a
stratigraphic and geochronological period at the transition between the
Pleistocene and the Holocene. The Lateglacial spans the latest part of the
Late Weichselian/Late Wisconsin glacial period. It includes the Bølling,
the Older Dryas, the Allerød and the Younger Dryas, from ca. 14 700 to
11 600 years before present (cf. de Klerk, 2004).
A total number of 101 ice samples from 29 ice bodies and 3 surface water
samples from 3 thermokarst lakes were studied. Ice blocks were cut with a
chain saw in the field and kept frozen until further processing with a band
saw in a cold lab at
Further analyses for hydrochemical characterization included pH, electrical
conductivity, major anions and cations, and stable water isotopes (
Principal component analysis (PCA) was used to summarize the variation in a
biplot by reducing dimensionality of the data while retaining most of the
variation in the data set (Jolliffe, 2002). Ordinally scaled variables (i.e.,
chemical data set) were log-transformed, centered and standardized, except for
pH,
A powerful tool to explore the relationship between a single continuous response variable (DOC concentration) and multiple explanatory variables is a regression tree (Zuur et al., 2007). Tree models perform well with nonlinearity and interaction between explanatory variables. UTMs are used to find interactions missed by other methods and also indicate the relative importance of different explanatory variables. Univariate tree modeling was performed using the computing environment R and Brodgar 2.6.5 software for Windows (ter Braak and Šmilauer, 2002; R Core Team, 2014).
Summarized DOC and DIC concentrations of different massive ground-ice types. For individual sample values see Table S1.
Table 2 provides an overview of mean DOC and DIC concentrations and range for
each ground-ice type. We found strong variations of DOC concentrations within
and across individual ground-ice types. The highest DOC concentrations were
found in ice wedges with a mean of 9.6 mg L
The highest DIC concentrations were found in modern surface water with on
average 22.6 mg L
Boxplots of
Correlation matrix. Correlations mentioned in the text are printed in bold. Strong positive correlations of paired variables are indicated by dark bluish colors, while strong anti-correlations are depicted in red. Hatching from the upper right to the lower left depicts positive correlations, whereas negative correlations are reversely hatched for better perceptibility in a black-and-white print. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
PCA biplot for ground-ice data. Inactive supplementary parameters (i.e., ice wedge, buried lake ice, basal glacier ice, snowpack ice, surface water, Pleistocene, Holocene, recent) are shown in grey italic. For individual sample values see Table S1.
With the help of a correlation matrix (Corrgram Package v1.6 in R version
3.1.2, R Core Team, 2014) , environmental processes and chemical
relationships can be visualized that may help to explain the sequestration of
DOC into ground ice. Pearson's correlation coefficients were calculated and
plotted in a correlation matrix in order to assess the degree of association
between DOC, chemical properties, stable water isotopes and spatial variables
(Fig. 4). A strong positive correlation suggests a mutual driving mechanism,
whereas negative values imply an inverse association. Most importantly, DOC
is positively related to the relative proportion of Mg
The first two axes of the PCA explain 43.9 % of the variation in the data
(Fig. 5). Cl
Na
The UTM (Fig. 6a) shows that differences in DOC concentrations can be explained
according to inorganic geochemical properties. The first two nodes split on
Mg
DOC stocks and pools in late Pleistocene and Holocene permafrost containing ice wedges (IW) based on calculated wedge-ice volumes (WIVs) in Yedoma and thermokarst basin deposits. All other ground-ice types, especially non-massive intrasedimental ice, are not included.
Univariate tree model
(UTM) explains variability pattern in DOC concentration.
While the riverine DOC export to the Arctic Ocean has been estimated as
33–34 Tg a
Ulrich et al. (2014) have calculated maximum wedge-ice volumes (WIVs), which
range from 31.4 to 63.2 vol % for late Pleistocene Yedoma deposits and
from 6.6 to 13.2 vol % for Holocene thermokarst deposits in east Siberia
and Alaska. Strauss et al. (2013) have shown similar averages for WIVs of
48 vol % in late Pleistocene Yedoma and 7.0 vol % for Holocene
thermokarst deposits. Together with average DOC concentrations of
11.1 mg L
However, all types of non-massive intrasedimental ice, raising the total
ground-ice volume to
The origin and sequestration process into ground ice seems to play an important role in the magnitude and bioavailability of DOC. Sequestration of OC into ground ice is a complex process that is dependent on water source, freezing process, organic matter origin and inorganic geochemical signature of the ambient water to form ground ice.
Figures 4 and 6a show that the electrical conductivity (i.e., total ion
content) of ground ice is unrelated to DOC but that the ion composition and
therefore the ion source seem to be relevant. Mg
Ice wedges are fed by meltwater from atmospheric sources that have been in contact with vegetation and sediments of the tundra surface before meltwater infiltrated the frost cracks in spring. By contrast, glacier ice, snowbank ice and lake ice are primarily fed by atmospheric waters having less interaction with carbon and ion sources. Yet, the yellowish brown to gray late Pleistocene and the milky-white Holocene ice wedges have incorporated sediments and organic matter that originate from surface soils and vegetation debris that was carried along with the meltwater into the frost crack (e.g., Opel et al., 2011). Spring snow meltwater interacts with the soil material leaching out carbon as it trickles downward toward the ice wedges. Also, since wedges may take thousands of years to form and the location of their upper surface changes with time, there are numerous spatial and temporal ways that deeper soil pore waters can get incorporated into the wedge ice. Leaching of DOC from relatively young surface organic matter takes place (Guo et al., 2007; Lachniet et al., 2012) as well as dissolution of ions from sediment particles. Snowmelt feeding ice wedges strongly attracts leachable components because of its initial purity. This might be the reason why especially ice wedges contain relatively high amounts of bioavailable DOC with low-molecular-weight compounds that may be old but remained fresh over millennia (Vonk et al., 2013b).
Principal component analysis clusters ice wedges into two main groups along
the first axis based on Na
So far we have shown that coastal/maritime and terrestrial environmental
conditions can be differentiated based on inorganic hydrochemistry and that
terrestrial surface OC sources feed the DOC signal in ice wedges. DOC
sequestration into ground ice is also dependent on active-layer properties,
vegetation cover, vegetation communities and deposition rates. Long-term
stable surfaces and relatively constant active-layer depths will lead to
substantially leached soil layers in terms of DOC (Guo and Macdonld, 2006)
and inorganic solutes (Kokelj et al., 2002). Based on
The absolute numbers of DOC in permafrost might still be small compared to the POC. However, POC from both peat and mineral soil has a relatively slow decomposition rate after thaw compared to DOC (Schuur et al., 2008). Organic matter from melting ground ice was shown to be highly bioavailable and can even enhance organic matter degradation of the host material by increased enzyme activity in ice wedge meltwater (Vonk et al., 2013b). Bioavailability experiments with Yedoma DOC from thaw streams fed by ice wedge meltwater in NE Siberia illustrated the rapid decomposability of Yedoma OC, with OC losses of up to 33 % in 14 days (Vonk et al., 2013a). Incubations with increasing amounts of ice wedge water in the Yedoma-water suspension enhanced DOC loss over time. Vonk et al. (2013b) concluded that ice wedges contain a DOM pool of reduced aromaticity and can therefore be regarded as an old but readily available carbon source with a high content of low-molecular-weight compounds. Additionally, a co-metabolizing effect through high potential enzyme activity in ice wedges upon thaw leads to enhanced degradation rates of organic matter of the host material. When studying organic matter cycling in permafrost areas, we have to abandon the paradigm, which holds true for temperate regions and Arctic oceanography, that old OC is refractory and that only young OC is fresh, bioavailable and therefore relevant for foods webs and greenhouse gas considerations.
We suggest that reduced organic matter degradation during cold periods is the main reason why late Pleistocene syngenetic ice wedges have incorporated more DOC on average than Holocene ice wedges. Incorporation of soluble organic matter into ground ice might have been more effective than today for various reasons. Ice Complex deposits in the coastal lowlands formed during the late Pleistocene cold period, when high accumulation rates of fine-grained sediments and organic matter were accompanied by rapidly aggrading permafrost (Hubberten et al., 2004). This means that organic matter is less decomposed because it was rapidly incorporated into perennially frozen ground and into the surrounding syngenetic ice wedges as the permafrost table rose together with the rising surface during deposition (Schirrmeister et al., 2011b). Also, colder annual air temperatures led to reduced decomposition rates of organic matter which originated from vegetation communities dominated by easily decomposable forbs (Willerslev et al., 2014) in contrast to resistant sedge–moss–shrub tundra vegetation since postglacial times (Andreev et al., 2011). Additionally, low precipitation and reduced runoff presumably retained more DOC in the landscape, ready to be transported into frost cracks.
Guo et al. (2007) concluded that most of the DOC in Arctic rivers is derived
from young and fresh plant litter and upper soil horizons. Leaching of deeper
seasonally frozen soil horizons is accompanied by much lower DOC
concentrations due to the refractory and insoluble character of the remaining
organic matter compounds (Guo et al., 2007). DOC impoverishment in the active
layer is logical as it is leached each season over a long time under modern
climate conditions, where permafrost aggradation is much slower than during
cold stages – if it happens at all. The quantity and quality of DOC pools in
deeper permafrost is probably much higher because of – so far – suppressed
remobilization. Dou et al. (2008) studied the production of DOC as
water-extractable organic carbon yields from organic-rich soil horizons in
the active layer and permafrost from a coastal bluff near Barrow (Alaska)
facing the Beaufort Sea. Besides high DOC yields in the uppermost horizon
(0–5 cm below surface) the second-highest DOC yields derived from
permafrost although the sampled horizon showed lower soil OC contents than
others (Dou et al., 2008). Interestingly, higher fractions of
low-molecular-weight DOC, which is regarded to be more bioavailable, were
generally found at greater depths. This supports the view that permafrost
deposits hold a great potential for mobilizing large quantities of highly
bioavailable organic matter upon degradation. Coastal erosion and thermokarst
often expose old and deep permafrost strata. Contained organic matter is
directly exposed to the atmosphere and transferred into coastal and
freshwater ecosystems without degradation because of short travel and
residence times. Therefore, Arctic coastal zones are supposed to receive high
loads of bioavailable dissolved and particulate organic matter. Dou et
al. (2008) used pure water (presumably MilliQ) and natural sea water as a
solvent for studying the production of DOC. It turned out that seawater
extraction significantly reduced DOC yields which were attributed mainly to
reduced solubility of humic substances due to the presence of polyvalent
cations such as Ca
An open question remains as to how much DOC can be found in intrasedimental ice and how much DOC is produced upon degradation of old permafrost (e.g., late Pleistocene Yedoma type), for example as a result of coastal erosion. To answer this question, it is crucial to follow the fate of permafrost organic matter upon remobilization. Additionally, robust estimations of carbon release are crucial for predicting the strength and timing of carbon-cycle feedback effects, and thus how important permafrost thaw will be for climate change this century and beyond.
Ground ice in ice-rich permafrost deposits contains DOC, DIC and other nutrients which are relevant to the global carbon cycle, Arctic freshwater habitats and marine food webs upon release.
The following conclusions can be drawn from this study:
Ice wedges represent a significant DOC (45.2 Tg) and DIC (33.6 Tg) pool in the studied
permafrost areas and a considerable freshwater reservoir of 4200 km Syngenetic late Pleistocene ice wedges have the greatest potential to host a large pool of
presumably bioavailable DOC because of (i) highest measured average DOC concentrations in combination
with (ii) their wide spatial (lateral, vertical) distribution in ice-rich permafrost areas and (iii)
the sequestration of fresh and easily leachable OC compounds. Increased incorporation of DOC into ground ice is linked to relatively high proportions of
terrestrial cations, especially Mg
Based on our results about the stocks and chemical behavior of DOC in
massive ground-ice bodies we propose that further studies shall strive to
quantify DOC fluxes in the Arctic from thawing permafrost, melting ground ice and coastal
erosion; differentiate between DOC and POC in permafrost including non-massive intrasedimental
ice; quantify DOC production from permafrost in different stratigraphic settings and with different natural solvents to answer the question of what fraction of soil OC will be leached as
DOC; assess the age and lability of DOC versus POC in permafrost and the potential impact on coastal food webs and freshwater ecosystems.
We thank the Yukon territorial government; the Herschel Island Qiqiktaruk Territorial Park; the Parks Canada office; and the Aurora Research Institute – Aurora College (ARI) in Inuvik, NWT, for administrative and logistical support. This study was partly funded by the International Bureau of the German Federal Ministry of Education and Research (grant no. CAN 09/001, 01DM12002 to H. Lantuit), the Helmholtz Association (grant no. VH-NG-801 to H. Lantuit), the German Research Foundation (grant no. OP217/2-1 to T. Opel) and a fellowship to M. Fritz by the German Federal Environmental Foundation (DBU). Analytical work at AWI received great help from Ute Kuschel. Sebastian Wetterich, Dave Fox and Stefanie Weege assisted in the field. We acknowledge two anonymous reviewers and the editor Stephan Gruber for their helpful comments and suggestions. Edited by: S. Gruber