Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian Yedoma region

SEDGEO-04948; No of Pages 11 Sedimentary Geology xxx (2015) xxx–xxx

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Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian Yedoma region Niels Weiss a,⁎, Daan Blok b, Bo Elberling b, Gustaf Hugelius a, Christian Juncher Jørgensen b, Matthias Benjamin Siewert a, Peter Kuhry a a b

Stockholm University, Department of Physical Geography, SE-106 91 Stockholm, Sweden University of Copenhagen, Center for Permafrost (CENPERM), DK-1350 Copenhagen, Denmark

a r t i c l e

i n f o

Article history: Received 18 June 2015 Received in revised form 4 November 2015 Accepted 10 December 2015 Available online xxxx Keywords: Permafrost Yedoma Thermokarst Carbon SOM decomposition Soil incubation

a b s t r a c t This study relates soil organic matter (SOM) characteristics to initial soil incubation carbon release from upper permafrost samples in Yedoma region soils of northeastern Siberia, Russia. Carbon (C) and nitrogen (N) content, carbon to nitrogen ratios (C:N), δ13C and δ15N values show clear trends that correspond with SOM age and degree of decomposition. Incubation results indicate that older and more decomposed soil material shows higher C respiration rates per unit incubated C than younger and less decomposed samples with higher C content. This is important as undecomposed material is often assumed to be more reactive upon thawing. Large stocks of SOM and their potential decomposability, in combination with complex landscape dynamics that include one or more events of Holocene thaw in most of the landscape, are of consequence for potential greenhouse gas release from permafrost soils in the Yedoma region. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Northern high latitude soils affected by permafrost (ground that remains at or below 0 °C for at least two consecutive years) (van Everdingen, 1998) contain vast amounts of carbon (C) stored in soil organic matter (SOM) (Tarnocai et al., 2009; Hugelius et al., 2014). Burial of SOM and sub-zero temperatures in combination reduce decomposition and result in accumulation of soil organic carbon (SOC). A warming climate could increase temperatures in permafrost soils and lead to permafrost thaw. Climate warming and permafrost thaw in turn could result in increased decomposition of SOM and greenhouse gas (GHG) release. This creates a potential positive feedback loop, triggering further atmospheric warming (Davidson and Janssens, 2006; Schuur et al., 2015). In order to know more about the potential of this positive feedback scenario, it is highly relevant to 1) accurately determine the amount of C stored in permafrost soils, 2) assess the potential decomposability (quality) of SOM in permafrost soils, and 3) determine potential C release as carbon dioxide (CO2) or methane (CH4) emissions to the atmosphere. The first of these knowledge gaps is addressed by ongoing efforts to improve and update circumpolar datasets, such as the Northern ⁎ Corresponding author. E-mail address: [email protected] (N. Weiss).

Circumpolar Soil Carbon Database (Hugelius et al., 2013), that combine results from individual field studies and regional assessments. There is a need to reduce uncertainties in SOC stock estimates, especially for specific C pools such as deep sedimentary deposits (Kuhry et al., 2010). Uncertainties remain large but overall estimates of the circum-arctic permafrost carbon pools are 1300 Pg (with an uncertainty range of 1100–1500 Pg C) for the whole northern permafrost region (17.8 × 106 km2) (Hugelius et al., 2014). The third point mentioned above is the topic of many studies throughout the Arctic. Numerous research projects are dedicated to measuring GHG release from permafrost soils in northern ecosystems using, for example, eddy covariance measurements (Wille et al., 2008; Parmentier et al., 2011) and in situ chamber measurements (van Huissteden et al., 2005; Schuur et al., 2009). Point two, however, which can be seen as the link between 1 and 3, is not as well studied and is recognized as needing further investigation (Kuhry et al., 2013). Highly specialized analyses can give insight into the decomposability of SOM, but are generally performed on a limited amount of samples because of the high cost and labor intensity of sample preparation and interpretation of the results (Routh et al., 2014; Strauss et al., 2015). The potential for decomposability cannot be predicted exactly, although Hugelius et al. (2012) showed that in discontinuous permafrost terrain, trends in C%, carbon to nitrogen (C:N) ratios, δ13C, and δ15N were consistent with SOM age, depth, and degree of decomposition.

http://dx.doi.org/10.1016/j.sedgeo.2015.12.004 0037-0738/© 2015 Elsevier B.V. All rights reserved.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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sediment (Schirrmeister et al., 2013). The greater part of the region however, is not undisturbed ice rich permafrost, but has undergone thaw events throughout the Holocene, that are likely to have influenced accumulation and decomposition of SOM. The cyclic, or repetitive nature of thermokarst processes is recognized in the literature (Burn and Smith, 1990) and is often linked to enhanced thaw because of recent climate change. Strauss et al. (2013) found that on average in Yedoma regions, 70% of the land surface has undergone some form of degradation (56% refrozen thermokarst, 4% deltaic and fluvial deposits and 10% lakes and rivers), leaving only 30% as intact Yedoma upland. Walter et al. (2007) and Walter Anthony et al. (2014) show that Yedoma sediments that have undergone a thermokarst cycle tend to have lower SOC content (~33% less) than intact Yedoma. This indicates a substantial loss of C to the atmosphere upon thaw, which results in particularly high and globally significant CH4 release from thawing thermokarst lakes in Yedoma regions (Walter et al., 2006, 2007). Near surface sediments in the Yedoma uplands have undergone soil formation throughout the Holocene, but without the marked permafrost degradation seen in the alas areas. Studies that compare SOM characteristics and GHG production potential of upland and thermokarst-affected Yedoma sediments are lacking. This study aims to investigate the influence of Holocene thaw dynamics on the quality and decomposability of Yedoma deposits by comparing soil characteristics with initial C release of thawed permafrost soils. SOM characteristics of the top 1 m, comprising organic layer, active layer and upper permafrost layer, give insight into how landscape dynamics have influenced near surface SOM properties and partitioning. Our incubation studies have been performed solely on top permafrost samples as these are the sediments that would be the first to thaw out following potential climate warming. The comparison between respiration results from upland Yedoma and alas upper permafrost samples will shed light on the influence of thermokarst on potential GHG release from the Yedoma region. We test the hypothesis that less decomposed permafrost SOM is more decomposable and therefore generate higher initial CO2 and CH4 production rates.

More recently Schädel et al. (2013) have combined 121 samples from several long term incubation studies thoughout the Arctic and found that C:N ratios appeared to be the strongest predictor of C loss over time. This finding strengthens earlier studies that found C:N ratio to be a useful indicator for degree of anaerobic SOM decomposition in peat deposits (Kuhry and Vitt, 1996) and aerobic decay in tundra soils (Ping et al., 1998), caused by preferential loss of C compared to N. δ13C and δ15N are both related to degree of SOM decomposition, as the fraction of the heavier isotope increases relatively over time as a result of preferential loss of the lighter isotopes 12C and 14N. Botanical composition however, has a strong control over C:N ratio's, δ13C, and δ15N. This poses a complicating factor when comparing SOM in samples from different locations and of differing ages. Permafrost soils are often C-enriched through burial of SOM and the process of cryoturbation (Ping et al., 1998), protecting SOM from decomposition due to sub-zero temperatures in the subsoil (Palmtag et al., 2015). SOM in C-enriched permafrost soils are therefore often presumed to exhibit a lower degree of decomposition (Kuhry et al., 2010) and as a result have a higher potential for decomposition, and thus GHG release. Within the permafrost zone, the Yedoma region is known to contain a substantial amount of frozen SOM. Yedoma deposits are Pleistocene syngenetic permafrost deposits of polygenetic origin (Schirrmeister et al., 2013), characterized by high ground ice content of up to 80% volume (Schirrmeister et al., 2011). Thick successions of loess-like ice complex deposits developed in parts of the Arctic that were not glaciated (Schirrmeister et al., 2011) and can reach a thickness of over 50 m (Schirrmeister et al., 2013). Syngenetic formation of Yedoma deposits is a result of continuous sedimentation of fine grained sediments on the vegetated tundra steppe surface, accompanied by upward movement of the permafrost table along with the soil surface. This process leads to rapid incorporation of fresh SOM into the permafrost, thus protecting it from decomposition. The Yedoma region is estimated to be ~ 1.4 × 106 km2, and to store 213 Pg C (with an uncertainty range of 164–267 Pg C) in frozen deposits (Strauss et al., 2013; Hugelius et al., 2014). Moreover, studies like Strauss et al. (2013) and Hugelius et al. (2014) emphasize the heterogeneity of the region. Intact Yedoma uplands are dissected by basins of former thermokarst lakes (the process of ground ice melt accompanied by local subsidence of the ground surface (Czudek and Demek, 1970)) with refrozen taliks (unfrozen ground surrounded by permafrost). Locally, refrozen taliks are also known as alases (Vereshchagin, 1974). Originally the term Yedoma was used in Russia to describe the hills, or uplands found among the extensive thermokarst basins. The term is now mostly used to describe this specific kind of frozen, ice rich

Beaufort Sea

East Siberian Sea

2. Study area description and methods 2.1. Field site The fieldwork area, throughout this study referred to as Kytalyk, is situated in the Kytalyk Wildlife Reserve (70.83 ∘N and 147.49 ∘E, 11 m a.s.l.), ~500 km north of the arctic circle in northeastern Siberia, Russia (Fig. 1a). Mean annual air temperature at the nearest weather station

Kytalyk

Laptev Sea

a

b

Fig. 1. (a) Location of Kytalyk and (b) the distribution of Yedoma upland and thermokarst-affected area in the region around the field station.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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(Chokurdakh, 70.62 ∘N, 147.88 ∘E, 61 m a.s.l. and ~ 30 km from the research site) is −13.8 °C with an average warmest month of 9.7 °C (July) and coldest month −34.2 °C (January) between 1939 and 2014 (WMO station 21946, Cokurdah, http://climexp.knmi.nl; last accessed Feb 18, 2015). Mean annual precipitation is 232 mm, most of which falls as rain during summer (van Huissteden et al., 2005). The field site is located on the north shore of the Berelekh river which connects ~35 km farther downstream (east) with the larger Indigirka River. The area around Kytalyk can be divided into three mayor landscape units. The (recent and sub-recent) floodplains and cut off meanders along the shores of Berelekh river form a fluvial zone (1), and the rest of the landscape which is mostly of aeolian origin (Schirrmeister et al., 2011, 2013) and shows a clear difference between intact Yedoma upland (2) and alases (3) (Fig. 1b). Yedoma uplands are ~ 30 m higher than the surrounding alas levels, and have a flat surface and a gentle, constant slope towards the alases (Fig. 2). South of the Berelekh river, intact Yedoma is more abundant, possibly caused by protection from erosion from underlying sandstone (Schirrmeister et al., 2012) or by shallower sediment cover caused by the closer proximity of bedrock to the surface. 2.2. Soil sampling Sampling sites have been selected semi-randomly using 150 m intervals along predetermined transects. Transects were designed to cover the main landscape units mentioned before, starting on the Yedoma upland and reaching down over the slope into the alas levels. A total of 19 sampling sites along two transects have been sampled, of which 14 have been selected for this study, being either located on Yedoma upland or in the alas levels (Fig. 3). Five sites were located on the slope between the alas and Yedoma upland and are therefore not used in comparisons between the two surface types, but are included in the incubation experiment. Additional samples have been collected from sites where natural exposures permitted collection from greater depths (KY EXP-1 and KY EXP-2). Opposed to the top permafrost samples taken from soil pits, with a depth of around 100 cm, the long exposure cores consist of older, intact, undisturbed Pleistocene Yedoma sediments, and can therefore be used to understand the influence of Holocene soil formation on top permafrost sediments. A total of 706 soil samples, with an average volume of 92 cm3, have been collected in 2012 from organic layer, active layer and permafrost down to ~ 100 cm from the soil surface. Fieldwork took place in early August so the frost table might not have represented the full active layer depth. The term active layer is used for the part of the active layer that was unfrozen during this fieldwork as a differentiation and comparison between sample groups, even though the maximum active layer depth is usually not reached until September. While collecting samples, the distinction is often quite clear between seasonally frozen ground and permafrost, as the hardness, wet bulk density and ice content of the latter is considerably greater. Where field observations allowed this distinction, seasonally frozen samples have been collected as active layer.

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Samples for incubation have been taken from well below (at least 10 cm) the permafrost table to assure the material was from the upper permafrost and not from seasonally frozen soil. It is important to emphasize that samples from transect based Yedoma upland sites do not consist of pristine Yedoma sediment, as they are influenced by Holocene soil processes. For sample identification the term ‘Yedoma’ refers to the location where the sample has been taken (Fig. 3), whereas for geomorphological analysis the term ‘Yedoma’ is used to describe the origin of the parent material. Organic layer samples were taken as undisturbed blocks of known dimension and volume. Because of strong heterogeneity of the soil surface and organic layer composition and depth, two additional organic layer replicates were taken randomly within a 1–2 m radius around the actual sampling site. A soil pit was then dug down to the permafrost table after which a general description was made and (when present) additional features (e.g., buried organic layers or cryoturbation) were identified and documented. Samples for radiocarbon dating were taken separately from specific soil layers or other material of interest (e.g., bone fragments). Using a fixed volume cylinder, active layer samples were taken from the wall of the soil pit at 5 cm depth increments. From the permafrost table down, a steel pipe was hammered into the soil with 5–10 cm depth increments after which the material was carefully extruded from the pipe before sampling the next 5–10 cm. Because of logistical constraints, homogeneous soil material and extremely ice rich material (N95% ice) has not been collected continuously. The omitted soil samples were assumed to be similar to under- and overlying samples and parameter values have been interpolated. 2.3. Laboratory analyses Sample volume, wet and dry weight, and weight loss upon drying were determined in the laboratory for dry bulk density and gravimetric ice/water content calculations. All samples have undergone loss on ignition (LOI) (Heiri et al., 2001) at 550 °C and 950 °C to obtain organic C, and carbonate content estimates respectively. Stable isotope analyses for C and N were performed on about a third of the samples using a Carlo Erba NC2500 analyzer connected to a Thermo Delta V advantage mass spectrometer with a split interface to reduce gas volume. From these measurements reproducibility was calculated to be better than 0.15 for δ13C and δ15N. C and N values were determined simultaneously with isotope ratios. The relative error was b 1% for both measurements. LOI at 950 °C after 550 °C (indicating loss of carbonate) was very low for all samples (b 1%) so total measured C content from elemental analysis was assumed to equal SOC content. The estimation of SOC content for samples where only LOI550 values were obtained is based on the site specific polynomial regression model for total C content based on LOI at 550 °C (LOI550) and results (SOCC = 0.0000011058 × LOI5504 − 0.0002169436 × LOI5503 + 0.0122945983 × LOI5502 + 0.3225222376 × LOI550 ; R2 = 0.99 (Siewert et al., 2015) that closely resembles the 0.5 factor seen in

Fig. 2. Slope from the first alas level (left) to the Yedoma upland (right).

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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2.5. Statistics

KY T1-12 KY T1-11 KY T1-9 KY T1-8 KY T1-4 KY T1-3 KY T1-2 KY T1-1 KY EXP-2 KY EXP-1

KY T2-3

KY T2-1 KY T2-2

KY T2-6 KY T2-7

Fig. 3. Detailed map of the fieldwork area with individual sampling sites. The crater-like shapes indicate different stages of relict thermokarst features.

other studies (Dean, 1974), meaning that ~50% of the material burnt at 550 °C is SOC. In this study the 4th order polynomial function was preferred over the simple 0.5 factor, to obtain a more accurate correlation at very low SOC concentrations. The total organic carbon (TOC) fraction of a selection of bulk samples representing basal peat, buried peat, C-enriched deeper cryoturbated pockets and Pleistocene sediments has been analyzed at the Poznań Radiocarbon Laboratory, Poland, for AMS 14C dating. Radiocarbon ages (Age 14C BP) were calibrated using OxCal v4.2.4 (Bronk Ramsey, 2009) with the IntCal 13 atmospheric curve, and are presented in calibrated years BP (cal BP).

Parameters from Yedoma and alas soils and samples have been compared using Student's t-test (t-ratio and p value provided, p b 0.05 indicated with *), provided that normality was not rejected using the Shapiro-Wilk test. Incubation sample size (n = 19) and replicates were insufficient to predict significance of correlation so trends in respiration results were tested for direction using the non-parametric Spearman ρ test (significance at p b 0.05 is indicated with *, near significance was considered when pb 0.1). All statistical tests were performed using JMP software (version 11, SAS Institute Inc. Cary, NC, US). Mean values are presented as μ ±one standard deviation, with relevant population size (n). Regression analysis was carried out to correlate CO2 production and C content, where the best fit was a logarithmic function (R2 and p values provided). 3. Results 3.1. Typical soil profiles Soil characteristics of one representative Yedoma and one alas site are presented in Fig. 4. A clear difference is the organic layer thickness: 9 cm for the Yedoma and 26 cm for the alas site. Active layer depths are similar for the Yedoma and alas profiles. Deeper in the profile, both soils show signs of organic matter that has been incorporated in the permafrost. The alas site shows SOM enriched material buried through cryoturbation at ~ 55 cm and ~ 75 cm. From 100 to 110 cm a buried peat layer was found. This buried peat layer has a radiocarbon age of 3251 cal BP, and the lower cryoturbation feature at 75 cm is 2885 cal BP (Table 2). The Yedoma site does not contain buried peat but cryoturbation has incorporated SOM from the organic layer deeper into the soil profile. At 90 cm, cryoturbated SOM was dated to 11449 cal BP (Table 2). The Yedoma upland soil shows lower SOC content (t = −2.29, p = 0.04*), lower C:N ratios (t = −3.82, p b 0.01*) and higher δ13C values (t = 4.35, p b 0.01*) in the mineral part of the profile (top organic layer excluded), compared to the alas soil. SOC content has a similar distinct signature for the peaty top organic layer, and for the organically enriched layers deeper in the profile. From the surface down, δ13C values rapidly increase in the upper part of the mineral Yedoma permafrost, and then stay relatively constant, except for a clear dip in the lowermost cryoturbated layer.

2.4. Field incubation experiments 3.2. General Yedoma and alas characteristics Incubation experiments were made under field-native (oxic) moisture conditions, during the field campaign in August 2012 and included permafrost cores taken from about 10 cm below the thaw table to avoid the transient layer or samples influenced by seasonal thaw. Samples were kept frozen until subsamples (one replicate per sample) of about 40 g were pre-incubated under a headspace of ambient atmospheric air for about 72 h at 2 °C in glass vials with a total volume of 133.6 ml. After preincubation the vials were flushed with ambient air and resealed, ensuring ambient levels of CO2, CH4 and O2 concentrations in the headspace of the vials before incubation. During the following 20– 30 h of incubation at 2 °C in an insulated box on the top of the permafrost (temperature inside the box was logged), three gas samples of 6 ml were taken using a syringe and injected into evacuated vials (Exetainers, Labco Inc., UK), after which an equal amount of ambient air was re-injected. Concentrations of CO2 and CH4 in the vials were measured in parallel by gas chromatography (HP7890A, Agilent, Wilmington, USA) with flame ionization detection (FID for CH4) at DTU, Risø, Denmark, 2–3 months after collection. Earlier studies in the same facility have shown no significant loss within 6 months of storage. Potential CO2 and CH4 production rates were calculated based on the linear increase in gas concentration over time (p b 0.05). Final rates were normalized to gram of dry weight (g DW) and g C in each sample.

In the ~1500 km2 area around Kytalyk (Fig. 1b), only 12% of the area is covered by intact Yedoma upland. A quarter of the landscape is covered by lakes and rivers, and another 17% is other deposits such as fluvial sediments on the Berelekh and Indigirka floodplains. The remaining 46% is alas surface, which were originally Yedoma uplands, but have undergone one or more thermokarst events. Fig. 5 shows data for all Yedoma upland and alas sites combined. The thicker organic layer as shown for the individual alas profile (Fig. 4) is visible for nearly all alas sites (Table 1), although there is no significant difference between active layer depths. Both active layer and permafrost show a higher gravimetric water content for alas sites compared to Yedoma upland sites (t = −1.44, p = 0.08 and t = −3.67, p b 0.01* respectively, top organic layer not included), and thus a lower bulk density. Gravimetric ice content of all collected permafrost samples in Yedoma is significantly lower than in alas permafrost (μ = 49.1±11.5%; n = 29 and μ = 60.1±15.5%; n = 58, t = −3.73, p b 0.01; samples from deep Yedoma cores are excluded, and samples that were not collected because of N 90% visual ice have been included as containing 90% ice). Alas sites show more variability in SOC content caused by more abundant SOM enrichment in the form of cryoturbation and buried peat layers. The greater variability for alas sites is less visible for C:N

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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Fig. 4. Comparison of one alas site and one Yedoma site. Organic layer (OL), active layer (AL), and permafrost (PF), as well as cryoturbated or peat layers that were identified in the field, are indicated.

are very similar and show no significant difference, on average 2.9 × 10−2 ± 2.2 × 10−2 g C cm−3 (n = 24) for Yedoma samples and 2.8 × 10−2 ± 1.8 × 10−2 g C cm−3 (n = 46) for alas samples (t = 0.18, p = 0.42). Deeper Yedoma samples from a longer exposure (170–560 cm depth) show lower carbon contents, 1.8 × 10−2 ± 3.3 × 10−3 g C cm−3 (n = 31), that are relatively constant throughout the profile. There is a slight negative trend in SOC content (both in g C cm−3 and in %) as well as in C:N ratio, and a positive trend for δ13C and δ15N (Fig. 6) with depth, corresponding with increasing age and degree of SOM decomposition with depth.

ratios although overall values are again slightly higher than for Yedoma. Likewise, the δ13C signature is not significantly different between Yedoma and alas, but Yedoma samples still appear to be slightly more enriched. 3.3. Carbon stocks Total SOC stocks for the top 1 m in alas sites (μ = 30.2± 7.0 kg C m−2; n = 8) are slightly higher than inYedoma upland sites (μ = 27.4 ± 9.3 kg C m−2; n = 6), although not significantly (t = −0.63, p = 0.27). SOC content for permafrost samples in the top of the permafrost

Gravimetric ice/ water content (%) 20 40 60 80

Bulk density (g cm-3)

C (%)

C:N

∂13C (‰ vs PDB)

0 0.4 0.8 1.2 1.6 0 10 20 30 40 10 15 20 25 30 -32 -30 -28 -26

∂15N (‰ vs air) 0

2

4

6

Alas combined

20 40 60

100

20

Yedoma combined

Depth (cm)

80

40 60 80 100

Peat Organic layer Active layer Permafrost

Fig. 5. Soil characteristics for alas and Yedoma upland sites combined.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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Table 1 Average depth of key soil layers.

Organic layer depth (cm) Active layer deptha (cm) n a

Yedoma

Alas

t-ratio

p

11.3±2.1 45.7±6.3 6

20.3±2.3 41±8.4 8

−7.50 1.19

b0.01* 0.12

Thaw depth measured beginning of August, 2012.

3.4. Radiocarbon dates Calibrated radiocarbon dates are presented in Table 2. Alas sites are mainly overlain with modern to century old, shallow peat. Deeper in the alas soils, buried peat layers are present with ages of several millennia. Yedoma samples give a clear indication of the Pleistocene origin of these sediments with ages of ~ 20–30 cal kyr BP at ~ 2 m depth. In the Holocene soils that formed in Yedoma sediments, cryogenic activity is visible in the form of cryoturbation with dates from ~ 2 to 10 cal kyr BP. 3.5. Field incubation experiments All incubated samples (n = 20) were taken from the top permafrost, but actual sample depths vary between sites because of different active layer depths and sampling intervals. Incubated samples were collected between 35 and 105 cm (μ = 58.4 ± 15.4) depth and there is no observed trend in either CO 2 or CH 4 production with sample depth. Samples had variable gravimetric ice content (28–79%, μ = 44%), but there is no significant relationship between ice content and respiration values (p = 0.06; Fig. 7, Table 3). Top permafrost soil material is mostly mineral but occasionally enriched with SOM of cryoturbated or buried peat origin: %C in the incubated top permafrost samples ranges between 1.5 and 36.4% (μ = 5.9 ± 8.7 %). Mean CO2 production is more than two orders of magnitude greater than CH4 production both per g C (μ = 362.9 μg CO2-C gC−1 day−1 and μ = 1.6 μg CH4-C g C−1 day−1) as well as per g dry weight (μ = 10.9 μg CO2-C g DW−1 day −1, μ = 5.9 × 10−2 μg CH4-C g DW−1 day−1). CH4 production in this study represents a very small methane component in aerobic incubation under field conditions. Per g C, CH4 production is on average 0.4% of the CO2 production, with a maximum of 4.2% and a minimum of 2.2 × 10−3%.

CO2 production during SOM incubation shows no significant difference between alas and Yedoma upland sites, neither per g incubated C (μ = 394.96±317.13 and μ = 361.65±415.21 μg CO 2 -C g C−1 day −1 respectively, n = 10; t = −0.15, p = 0.87) nor per g dry weight (μ = 9.47±2.64 and μ = 11.39±3.63 μg CO 2 -C g C−1 day−1 respectively, n = 5; t = 1.04, p = 0.33). CH4 production rates per g C however are significantly higher for samples from alas sites compared to Yedoma upland sites (μ = 2.77±2.13 and μ = 0.46± 0.66 μg CH4-C g C−1 day−1 respectively, t = −3.13; p b 0.01*). This significant difference is no longer visible for CH4 production per g dry weight. All SOM properties that are considered as indicators for degree of decomposition described before show a positive trend with initial respiration rates per unit incubated C, albeit not significant for most (Fig. 7). The number of samples with %C available is relatively high (n = 20), however the more limited sample size (n = 10) for other decomposition indicators makes it dubious to establish a (linear) regression. Even so, the direction and partial significance of Spearman ρ correlation is significant or near significant for 5 out of 7 parameters, in the case of CO 2 production per g dry weight (Table 3). Because of the significant Spearman ρ correlation and higher n (= 20) for %C from LOI, we have focused in more detail on the relationship between C respiration per g C and %C. The best fit regression for the data is a logarithmic function (CO2-C production rate per g C = 1106 × %C −1.1 ), which shows that samples with low %C have the highest respiration rates per g C (Fig. 8).

4. Discussion 4.1. Typical soil profiles There are differences in C:N ratio, δ13C and δ15N values between alas and Yedoma organic top soils. This is likely caused by peat layers in the alas area, with typical botanical imprint and isotopic signatures, as opposed to the shrub tundra on the dryer Yedoma upland. As mentioned before, botanical composition has an influence on these characteristics, so comparisons between different sites and depositional environments should be considered with care. The buried peat layer at 100–110 cm depth in the alas soil (Fig. 4) indicates a prior stage of peat accumulation, before mineral sedimentation

Fig. 6. Trends in soil characteristics for a deep Yedoma core (KY EXP-2). Samples between 475 and 560 cm were not collected because of near 100% ice content.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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Table 2 Radiocarbon dating. Surface

Sample description

IDa

Depth (cm)

Sample type

Age 14C BP

Age cal BPb

Yedoma

Cryoturbation in permafrost in Holocene soil

KY T1-1 KY T2-6 KY EXP-1 KY EXP-2 KY T1–8 KY T1–12 KY T2–2 KY T2–3 KY T2–2 KY T 2–2 KY T 2–3

90–91 62–63 178 200 21–22 18 25–26 16 101–102 75–76 84

TOC TOC TOC Bone fragment TOC TOC TOC TOC TOC TOC TOC

9,970±60 1,765±30 27,920±210 17,270±80 250±25 570±30 470±30 1,180±30 3,040±35 2,785±30 7,190±50

11,449 1,674 31,745 20,829 272 588 516 1,109 3,251 2,884 8,016

Pleistocene sediment in deep exposure Alas

Basal peat

Buried peat Cryoturbation in permafrost a b

Location corresponding to map in Fig. 3. Estimated mean age of the highest probability interval using OxCal 4.2.4 in calendar years before 1950.

became more dominant. This supports the theory of different stages of thermokarst development in the alas areas. The lower SOC content, lower C:N ratios, and higher δ13C values found in the Yedoma soil indicate a more advanced degree of decomposition. This is in correspondence with the age of 11.5 cal kyr BP at 90 cm in the Yedoma soil compared to the organic matter in the alas soil that is dated to ~3 cal kyr BP. For the alas site, it is clear how the buried peat layer spikes the SOC content to values comparable to fresh peat in the top organic layer. Cryoturbated layers show a weaker, although still recognizable, SOC peak. A possible explanation is the relative dilution of cryoturbated SOM by mineral subsoil during the process of cryoturbation. A distinct buried peat layer is expected to be less mixed with surrounding mineral soil, and with sufficient thickness, the sample can possibly consist more purely of organic matter. C:N ratios show a similar trend as SOC content, although the buried peat has a less distinct peak, which might be explained by a higher degree of decomposition of old peat compared to young peat. In the Yedoma site δ13C increases with depth as the heavier C isotope increases in relative abundance. The alas site shows a more diffuse signal, possibly caused by the overall enrichment of SOM that masks influence by specific layers.

4.3. Carbon stocks Carbon content observed in deeper Yedoma (~ 2–5 m) in Kytalyk, appears to be comparable to intact Yedoma deposits throughout the Yedoma region (Schirrmeister et al., 2011). The higher SOC content in top permafrost samples from transect based sites opposed to the deeper samples from exposure sites highlight the Holocene pedogenetic influence and SOM enrichment in samples from the top of the permafrost. It once more emphasizes the difference between ‘intact Yedoma’ as a landscape unit and the geochemical term ‘intact Yedoma’, indicating unaltered Pleistocene sediments. 4.4. Landscape development Figure 9 shows an interpretation of the development of the different alas levels in the region. Because of the extremely high ground ice content in Yedoma deposits, subsidence caused by the initial thermokarst events (first generation) would have caused substantial ground subsidence and the formation of extensive lakes. Limited relief in the region creates spatially extensive water bodies that, due to shore erosion, are able to expand progressively (Burn and Smith, 1990). Broaching of lakes by fluvial systems or infilling with organic material causes terrestrialization of the surface and illustrates the cyclical nature of thermokarst lake formation, after the initial collapse (Mackay, 1988; Smith et al., 2005; Jones et al., 2011). Different ages of basal and buried peat in alas sites indicate periods of peat accumulation (Table 2). The oldest cryoturbated alas material corresponds with the Holocene thermal maximum (ca. 7 kyr BP), a period of extensive initiation of thermokarst throughout the Arctic (Burn and Smith, 1990; Walter et al., 2007) and extensive West Siberian peatland expansion (Smith et al., 2004). Refrozen thermokarst deposits (former taliks, also known as taberites), often overlain by an organic rich wetland, continued to

4.2. General Yedoma and alas characteristics

gC-1 day-1 gDW-1 day-1

CO2 production rate (µg CO2–C per

Although uplands have not undergone massive thaw and subsequent ground subsidence, near surface sediments have been affected by active layer dynamics and Holocene soil formation. The areal coverage of different landscape types appears to be similar to the averages calculated for the entire Yedoma region (Strauss et al., 2013), although considerably less surface is covered by intact Yedoma upland in the Kytalyk study area (12%) than in the Yedoma region as a whole (30%).

1000 800 600 400 200 0 20

ρ = -0.92 *

ρ = -0.93 *

ρ = -0.62

ρ = 0.37

ρ = -0.02

ρ = -0.31

ρ = -0.36

ρ = -0.29

ρ = 0.53

ρ = 0.08

ρ = 0.39

ρ = -0.43

ρ = -0.30

ρ = 0.21

15 10 5 0 0

10

20

C (%)

30 0.0 0.5 1.0 1.5 10 12 14 16 18

N (%)

C:N

-30

-28

-26

∂13C (‰ vs PDB)

1

2

3

∂15N (‰ vs air)

0.0 0.20.40.60.81.0 20 30 40 50 60 70

Bulk density (g cm-3)

Grav. ice cont. (%)

Fig. 7. CO2 production per g C and per g dry weight, plotted against SOM properties. Spearman ρ indicating significance (* at p b 0.05).

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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N. Weiss et al. / Sedimentary Geology xxx (2015) xxx–xxx

elevation, successive generations of alas levels are still clearly recognizable both in the field and from satellite imagery, because of their characteristic, circular contours (as outlined in Figs. 3, 9), and differences in vegetation.

Table 3 Direction and significance of respiration trends by soil characteristic as presented in Fig. 7. Variable

By

Spearman ρ

p

CO 2-C production per g C

%C %N C:N δ13C δ15N Dry bulk density Ice content %C %N C:N δ13C δ15N Dry bulk density Ice content

−0.92 −0.93 −0.62 0.37 −0.02 0.39 0.43 −0.31 −0.36 −0.39 0.53 0.19 −0.30 0.21

b0.01* b0.01* 0.05 0.29 0.96 0.09 0.06 0.38 0.31 0.26 0.12 0.60 0.20 0.37

CO 2-C production per g DW

4.5. Field incubation experiments As seen in Fig. 7, all presumed indicators of decomposability suggest a relationship with CO2 production. That is to say, the higher the degree of decomposition, the higher the observed respiration rates. As mentioned before however, only the correlation with %C and %N are significant and therefore the other trends should primarily be seen as indications for possible relationships, that should be explored with larger datasets. The highest respiration rates, mostly per g C but still visible per g dry weight, come from the most carbon poor samples with the lowest C:N ratios, the least negative δ13C and highest δ15N values. The fact that these trends seem to be similar for g C and g dry weight, even though less clear for the latter, is especially interesting from a landscape perspective, as it means that it describes not merely a theoretical relationship. The significant relationship shown in Fig. 8a (R2 = 0.72; p b 0.01) is maintained when the one high %C value (buried peat) is subjectively removed from the dataset (R2 = 0.60; p b 0.01*; Fig. 8b). This shows that the correlation is not mainly driven by the single data point with high %C. Similar relationships are furthermore present when splitting the data into Yedoma (R2 = 0.91; p = 0.01*), alas (R2 = 0.89; p b 0.01*) and slope samples (R2 = 0.13; p = 0.56), meaning that wetland and upland samples show similar trends in CO2 production per unit C. A recent meta-analysis of long-term aerobic incubation experiments of SOM from the northern permafrost region has shown a positive relationship between respiration C:N (and %C) of soil samples (Schädel et al., 2013). Our results are therefore very surprising and deserve further scrutiny. The best predictor for C loss in the study by Schädel et al. (2013) was C:N ratio. In our study, C:N ratios suggest the same opposite trend as observed for %C, although only near significant, arguably because of our limited dataset. The measured values are initial respiration after pre-incubation, which means that observed values cannot be ascribed to physical release of trapped gases. However, the observed trends might still be related to the short period of incubation, implying that long-term incubation could yield different results. Microbial assemblages, preadapted to an upper permafrost soil environment with low organic substrate availability, might not have (yet)

1000

R2 = 0.60 *

800

-1.1

y = 1106 x R2 = 0.72 *

600 500 400 300

b

200

1

10

100 80

R2=

60 50 40

R

R2

2

=

30

0. 91

a

20

1

=

2

3

4

5 6

8 10

0.1

3

88

20

Combined Alas Yedoma Slope

0.

*

CO 2 production rate (µg CO2-C gC -1 day -1)

accumulate peat or mineral deposits, causing renewed syngenetic permafrost growth and ground ice build up in the form of vein ice and segregated ice. Ice wedges in the younger alas are smaller than the ground ice in intact Yedoma deposits, as they are of Holocene origin and are thus less well developed (Fig. 10). The observed higher gravimetric ground ice content in alas permafrost versus Yedoma permafrost can be attributed to the fact that these are upper permafrost samples and therefore do not necessarily reflect properties of deeper sediments. Alas soils have high ground ice content in the top of the permafrost caused by seggregated ice at the frost table (transient layer) and/or seasonal freezing of ground water from the active layer in these lowest parts of the landscape, that are often waterlogged. At greater depth (KY EXP-2), Yedoma samples show similar ice content to the values for the upper permafrost, whereas deeper alas samples are lacking but are expected to have lower ice content as these sediments are refrozen, following previous thaw events. Ages of the deeper Yedoma cores support the Pleistocene age of the sediments (Table 2). Disturbances in the ground surface or vegetation cover can cause new thermokarst to develop (Nauta et al., 2015), however, the underlying taberites no longer contain the massive ice of the parent Yedoma deposits, making the ground subsidence caused by thermokarst in alases considerably less, and resulting in shallower successive generations (second, third, etc.) of lakes. Drainage of these younger generation lakes develops a series of alas levels with elevation differences of only ~ 1 m instead of the initial ~ 30 m drop from the Yedoma upland (Fig. 9). Despite the limited difference in

*

30 40 50 60

C (%) Fig. 8. CO2 production against %C. Trend lines and R2 values for the combined dataset as well as split up in surface groups: Yedoma (n = 5), alas (n = 10) and the slope transitioning from Yedoma into alases (n = 5). The top right inset graph is the same complete dataset with only the high %C sample (alas) removed, showing that the relationship is not driven by the point with the highest %C.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

N. Weiss et al. / Sedimentary Geology xxx (2015) xxx–xxx

a

d

b

e

c

f

9

Fig. 9. Stages of primary and secondary thermokarst development in Yedoma sediments in Kytalyk. a) Initial situation in the early Holocene: extensive ground ice (up to 80%) present throughout the landscape in Yedoma deposits. b) Thermokarst lake formation towards Holocene thermal maximum. Vast subsidence caused by ground ice melt. Lakes are shallow but extensive because of absence of significant relief in the region. Unfrozen sediments (taliks) underlie lakes. c) Due to organic and lacustrine infilling and/or drainage of lakes due to breaching by rivers or streams, lakes become terrestrialized basins (a.k.a. alas) with organic rich top soil, underlain by a layer with increased input of organic matter from the overlying organics, on top of taberites (organically depleted, refrozen talik). d) Permafrost aggrades in the alas and new ice wedges form, less ice volume than in the intact Yedoma area. e) Melt of newly formed ice wedges and ground ice results in second generation thermokarst. Size and/or depth are in correspondence with the volume of ice. f) Drainage, terrestrialization, stabilization and formation of new ice wedges in secondary alas level.

adapted to the new environmental conditions following permafrost thaw and the availability of more organic-rich and less decomposed materials. In that sense, it is interesting to highlight that alas samples produced significantly more CH4 per g C than the upland Yedoma samples. This could tentatively be ascribed to the fact that the microbial community developed under the generally wetter conditions prevailing in the alases is better predisposed to methanogenesis. The higher CH4 production might intuitively be ascribed to the generally higher ice content in alas top permafrost, however this is not reflected in a direct correlation between the two. It should be stressed however that strong conclusions for CH4 production should not be drawn based on this

study because of the limited period and aerobic procedure of the incubation experiment. High respiration values for samples with low %C might be the initial effect of a more labile dissolved organic C (DOC) pool. Mergelov and Targulian (2011) found higher concentrations of hydrophilic components in the transitional permafrost layer of NE Siberian soils, formed through the process of retinization or the accumulation of DOC at the permafrost table. This labile DOC pool might represent a larger proportion of the total C found in mineral subsoil samples with low %C. The effect of this labile pool on respiration diminishes when the remaining SOM pool becomes larger (i.e., the %C increases). Furthermore, once

Fig. 10. a) Holocene ice wedge polygons in peat-covered alas basin. b) Massive ground ice in Pleistocene Yedoma, exposed by lake erosion.

Please cite this article as: Weiss, N., et al., Thermokarst dynamics and soil organic matter characteristics controlling initial carbon release from permafrost soils in the Siberian..., Sedimentary Geology (2015), http://dx.doi.org/10.1016/j.sedgeo.2015.12.004

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this labile pool is exhausted, the decomposition of the remaining SOM may proceed at a much lower rate. On the other hand, the observed relationship between respiration rates per g C and %C might also be due to effects that could be maintained over a longer period of time. The role of physical protection through organo-mineral associations, suggested to be a key mechanism for SOM stabilization in temperate soils, remains unclear in permafrost soils (Höfle et al., 2013; Gentsch et al., 2015, e.g). Kaiser et al. (2007) found higher respiration rates per g C in mineral subsoil samples of the active layer compared to organic-rich top soil layers and cryoturbated pockets. We can hypothesize that the well decomposed SOM with low %C could be distributed more evenly over the mineral grain material and be more readily available for decomposition than clustered, more undecomposed organic material in higher %C soil samples. Another possible explanation could be that in samples with lower C:N ratios there is greater N availability for the microbial community promoting SOM decomposition, an effect that also could be maintained over a longer time period. Without further detailed study it is not possible to fully assess the short- and long-term controls on SOM decomposition in our soil samples, but it is clear that these results deserve careful analysis in future assessments of permafrost SOM decomposability. 5. Conclusions The Yedoma region can be defined by the origin of the parent material. In those areas where the original Pleistocene Yedoma landscape has remained intact, Holocene soils have formed in Yedoma parent material. Throughout the Holocene much of this dynamic landscape has undergone one or more thermokarst events, with lake formation and drainage, and the subsequent accumulation of thin peat layers. These developments greatly influence sediment characteristics by the initial loss of Pleistocene ground ice and thawing of organic rich sediments, and Holocene re-aggradation of permafrost in the form of segregated and vein ice under peaty substrates. Our analysis of profiles shows geochemical differences between the soils developed in Yedoma uplands and the more peaty substrates in thermokarst (alas) depressions. The generally lower SOC content, lower C:N ratios, and higher δ13C values found in Yedoma soils indicate a more advanced degree of decomposition. Our field incubation experiment of upper permafrost layer samples from the Kytalyk study area yielded surprising results. Simple geochemical indicators indicative of degree of decomposition (%C, C:N ratio, δ13C, and δ15N) all suggest that the SOM in organically enriched samples deeper in the soil profiles, such as cryoturbated pockets and buried peat layers, seem to be relatively undecomposed. Therefore, the SOM in these C-enriched layers could be expected to be more labile than SOM in the adjacent mineral subsoil samples (see Schädel et al. (2013)). However, our upper permafrost soil incubations suggest the opposite to be true, i.e. the mineral subsoil samples with the lower %C have significantly higher respiration rates per g C. The observed significant relationship with %C (and near significant relationships with other geochemical indicators) deserves careful further investigation, to assess short- and long-term controls on SOM decomposition. Our findings suggest a high lability of old SOM stored in Yedoma deposits, which can be of great relevance to assess past, present and future implications for the permafrost carbon feedback, caused by landscape dynamics and climate change. Acknowledgments This work has been supported by the PAGE21 project, grant agreement number 282700, funded by the EC Seventh Framework Programme Theme FP7-ENV-2011. Financial support for incubation

studies was provided by a grant from the Danish National Research Foundation (CENPERM DNRF 100).

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