Characterisation of Three Regimes of Collapsing Arctic Ice Complex ...

PERMAFROST AND PERIGLACIAL PROCESSES Permafrost and Periglac. Process. 25: 172–183 (2014-07) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/ppp.1815

Characterisation of Three Regimes of Collapsing Arctic Ice Complex Deposits on the SE Laptev Sea Coast using Biomarkers and Dual Carbon Isotopes L. Sánchez-García,1*† J. E. Vonk,1‡ A. N. Charkin,2,4 D. Kosmach,2,4 O. V. Dudarev,2,4 I. P. Semiletov2,3,4 and Ö. Gustafsson1 1 2 3 4

Department of Applied Environmental Science (ITM), Stockholm University, Stockholm, Sweden Pacific Oceanological Institute, Russian Academy of Sciences, Vladivostok, Russia International Arctic Research Center (IARC), University of Alaska Fairbanks, Fairbanks, AK USA National Research Tomsk Polytechnic University, Tomsk, Russia

ABSTRACT Arctic amplification of climate warming is intensifying the thaw and coastal erosion of the widespread and carbon-rich Siberian Ice Complex Deposits (ICD). Despite the potential for altering long-term carbon dynamics in the Arctic, the susceptibility of organic carbon (OC) to degradation as the ICD thaw is poorly characterised. This study identifies signs of OC degradation in three Siberian ICD regimes of coastal erosion through elemental, isotopic and molecular analyses. The degree of erosion appears to determine the extent of degradation. The moisture-limited and beach-protected ICD bluff near Buor-Khaya Cape, characterised by thermokarst mounds (baydzherakhs), represents a dormant regime with limited ongoing degradation. Conversely, the more exposed ICD scarps on eroding riverbanks (Olenek Channel, Lena Delta) and coastal slopes (Muostakh Island) showed more pronounced signs of ongoing OC decay. Different parameters suggest that degradation can partially explain the shift of the OC signature with 14C age in the thawing ICD. Exposure time, degree of erosion, slope gradient and moisture conditions appear to be key factors determining the degradation propensity of OC in exposed ICD. These field results document the lability of OC in ICD upon thaw and illustrate the potential for transferring old OC into the rapidly cycling atmosphere-biosphere carbon pools. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS:

Ice Complex Deposits; thermal degradation; isotopic-molecular markers

INTRODUCTION Arctic soils are important reservoirs of organic carbon (OC). Northern soils contain c. 50 per cent (c. 1700 Pg) of the global belowground soil carbon pool (Kuhry et al., 2013; Tarnocai et al., 2009) frozen in permafrost. Approximately 25 per cent of this vast stock of soil-permafrost carbon exists as Ice Complex Deposits (ICD; also termed Yedoma) of Pleistocene age (Romanovsky, 1993; Schirrmeister et al., 2011; Wetterich et al., 2008; Zimov et al., 2006a). The ICD extend inland from Arctic coasts, predominantly in *Correspondence to: L. Sánchez-García, Department of Applied Environmental Science (ITM), Stockholm University, SE-106 91 Stockholm, Sweden. E-mail: [email protected]; [email protected] †Present address: University of Zaragoza, Institute of Environmental Science of Aragon (IUCA), C/Pedro Cerbuna 12, 50009 Zaragoza, Spain ‡ Present address: Utrecht University, Department of Earth Sciences, Utrecht, The Netherlands, and also University of Groningen, Arctic Centre, Groningen, The Netherlands. Copyright © 2014 John Wiley & Sons, Ltd.

northeastern Siberia. Its frozen status has largely kept this massive OC deposit from extensive exchanges with more active carbon pools such as the atmosphere. Amplified climate warming in the Arctic (Richter-Menge and Overland, 2010; Zwiers, 2002) may thaw carbon-rich permafrost soils and release large amounts of relict carbon (Khvorostyanov et al., 2008a; Schuur et al., 2008). The ICD can be destabilised through thermal collapse, sea-level rise and enhanced wave fetch from loss of (coastal) sea ice cover. Destabilisation and thaw of ICD transform thermokarst lakes into lagoons and peninsulas into islands, for example, on the Bykovsky Peninsula (Dudarev et al., 2006; Nicolsky et al., 2012; Overduin et al., 2007). In Siberia, coastal erosion is particularly intense and coasts retreat at rates of up to 20 m/yr (Grigoriev et al., 2000; Overduin et al., 2007; Rachold et al., 2000, 2004; Semiletov, 1999), accelerating the complete disappearance of islands such as Muostakh Island (Dudarev et al., 2006; Overduin et al., 2007; Günther et al., 2013a). Received 3 March 2014 Revised 21 May 2014 Accepted 11 July 2014

Characterisation of Ice Complex Deposits in Three Siberian Regimes

Coastal erosion generates large terrestrial inputs of OC from the ICD (OCICD) to the ocean. OCICD makes up the largest fraction of the total carbon in the surface sediments in the East Siberian Arctic Shelf, the world’s largest shelf sea system (Vonk et al., 2012). Early studies assumed that all thawed material (including OC) was entirely transferred, unaltered, from land to sea (Rachold et al., 2000; Semiletov, 1999; Stein and Fahl, 2000; Stein and Macdonald, 2004). In contrast, laboratory-based microbial incubations (Dutta et al., 2006; Gilichinsky and Wagener, 1995; Vonk et al., 2013) have demonstrated a substantial propensity for degradation of exposed OCICD before and after dispersal into the sea. Microbial decomposition of thawing permafrost is considered to be one of the most likely short-timescale positive climate feedbacks from terrestrial ecosystems to the atmosphere in a warmer world (Gruber et al., 2004; Schaefer et al., 2011; Schuur et al., 2009). Heat internally generated from the decomposition of OC in thawing ICD may trigger further thaw of the ICD, providing a positive feedback through self-sustaining transformation of frozen OCICD into carbon dioxide (CO2) and methane (CH4) (Khvorostyanov et al., 2008b; Koven et al., 2011). Yet, the degradability of OC from old ICD, representing about one-fourth of the global permafrost OC pool (Kuhry et al., 2013; Tarnocai et al., 2009; Zimov et al., 2006a), is severely understudied. Most carbon studies on ICD and other types of permafrost consist of laboratory experiments of the respiration activity of bacterial communities upon thaw (Dutta et al., 2006; Gilichinsky and Wagener, 1995; Rivkina et al., 1998; Sawicka et al., 2010; Zimov et al., 2006b), with very few in-situ measurements of respiration (Dorrepaal et al., 2009; Schuur et al., 2009; Vonk et al., 2012; Zimov et al., 2006b). The soil respiration rates observed increase with temperature in the few in-situ studies on the Kolyma riverbank (10–40 gC/m3/day; Zimov et al., 2006b), subarctic Swedish peatlands (increase of 50–60%; Dorrepaal et al., 2009), Alaskan tundra (increase up to 78%; Schuur et al., 2009) and Muostakh Island (3.2–440 mmol/m2/day; Vonk et al., 2012), suggesting a rapid decomposition of OCICD upon thaw, which continued at low rates for several years (Zimov et al., 2006b). Exploring the lability and degradation dynamics of OCICD in the field is essential to understand the behaviour of the climate-sensitive Arctic deposits of carbon upon climate warming. Here, we explore the degradability of OC that has thawed under three regimes of collapsing (coastal) ICD: (i) the southern end of Buor-Khaya Cape, a dormant site of coastal erosion; (ii) the Olenek Channel of the Lena Delta, an active site of strongly seasonal riverbank erosion; and (iii) Muostakh Island, an active site of intensive coastal erosion, partially discussed elsewhere (Vonk et al., 2012) and further elaborated here. We employ elemental, isotopic and molecular tools to examine the OCICD composition and investigate the influence of erosion regime upon the susceptibility of OCICD to degradation. Copyright © 2014 John Wiley & Sons, Ltd.

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MATERIALS AND METHODS Study Area The study area is situated in a continuous permafrost region (Brown et al., 1998) in northeastern Siberia (Figure 1). The composition of the ICD detrital sediments mainly consists of silt, organic-rich silt and organic-rich silty sand (Rachold et al., 2000), penetrated by large syngenetic ice wedges (Schirrmeister et al., 2003, 2011, and references therein). In the Laptev Sea, coastal erosion is considered to supply at least as much sediment as rivers (Are, 1999; Rachold et al., 2000). High cliffs and seasonal ice wedge melting result here in higher erosion rates than for other Arctic coasts (Forbes, 2010). The average shoreline and cliff top retreat rate along the ice-rich Laptev Sea coasts has been estimated at 2–2.5 m/yr (Lantuit et al., 2011), with the small islands showing retreat rates almost twice as high as those of continental sites or larger islands (Grigoriev et al., 2000). The Laptev Sea has the highest net sea ice production rates in the Arctic Ocean that impact ICD through scouring and erosion along shorelines of ~ 2400 km length (Rachold et al., 2000). The three study sites represent three distinct ICD regimes, where different geographical conditions and erosional forcing have resulted in variable exposure and preservation of the OCICD. Muostakh Island (c. 2 × 13 km and terrace height of ~ 25 m asl) is a remnant of the Siberian ICD lowland located on the western side of Buor-Khaya Bay (Figure 2a). A detailed description of the island is provided by Dudarev et al. (2006) and Schirrmeister et al. (2011). The combined effect of thermo-denudation (i.e. destabilisation due to temperatures above 0ºC) and thermo-abrasion (i.e. destabilisation due to mechanical impact and thermal energy of sea water, water level, storms, waves, wind and ice) has caused a rapid retreat of the island shoreline (averaging ~ 1–20 m/yr) (Are, 1980; Grigoriev et al., 2000; Günther et al., 2013a; Overduin et al., 2007; Rachold et al., 2000; Semiletov et al., 2011). The rapidly retreating coasts evolve into steep scarps of frozen/freshly thawed sediment interfaces, where aerial degradation is likely to occur due to the labile nature of OC in recently exposed ICD. The Olenek Channel is the westernmost outlet of the Lena River Delta (Figure 2b). It is part of an erosional remnant of Late Pleistocene sediments incorporated into the Holocene river delta (Grosse et al., 2008). The permafrost deposits here consist of ice-poor fluvial sands overlain by ICD (Schirrmeister et al., 2003). The topography is dominated by a flat plain with abundant water bodies, and some thermo-erosional valleys and thermokarst basins. Riverbank erosion is mostly limited to thermo-abrasion and shore washout during the violent ice break-up of the Lena River in spring, as well as thermo-denudation during summer. Retreat rates along the Olenek River channel are about one order of magnitude lower than at Muostakh Island shorelines but have not yet been accurately determined (Grigoriev et al., 2000). Permafrost and Periglac. Process., 25: 172–183 (2014-07)

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Figure 1 Map showing the location of the three erosion study sites in the southeastern Laptev Sea. This figure is available in colour online at wileyonlinelibrary.com/journal/ppp

Figure 2 Illustration of the three study sites on Ice Complex Deposits in (a) Muostakh Island, (b) Olenek Channel and (c) Buor-Khaya Cape. The red arrows in the slopes indicate the sampling elevations in each regime (see Tables 1–3 for sampling codes). The photograph of Muostakh Island corresponds to one of the four slopes sampled in the northern part of the island. This figure is available in colour online at wileyonlinelibrary.com/journal/ppp

Copyright © 2014 John Wiley & Sons, Ltd.

Permafrost and Periglac. Process., 25: 172–183 (2014-07)


Buor-Khaya Cape is a headland in the Laptev Sea (Figure 2c), between Buor-Khaya Gulf to the west and Yana Bay to the east. The sea surrounding the cape is frozen for about nine months per year and often clogged with ice floes. Thermo-abrasion is extensive along all the Buor-Khaya Bay coasts, which are retreating at average rates of ~ 1–7 m/yr (Günther et al., 2013b; Lantuit et al., 2011; Rachold et al., 2004; Semiletov, 1999). However, at our site, a wide beach has developed (Figure 2c) that now minimises the impact of waves, storms and ice on the ICD (i.e. limiting thermoabrasion) and restricts erosion dynamics mostly to wind, high tides and thermo-denudation. Here, the ice wedges in the permafrost bluff have melted and washed out, resulting in conical thermokarst mounds (baydzherakhs) with a denser soil of lower ice content and a drier microenvironment than the original ICD. Sample Collection and Approach In July 2006, soil samples (0.3–40 g) were collected from the ICD bluffs along exposed scarps in the three systems. To investigate the alteration of OCICD after thaw, we collected samples of recently thawed surface ICD material along slope transects, where the OCICD has been atmospherically exposed for longer in this natural ‘microbial reactor’ than frozen soil, likely changing the OCICD signature. We did not sample frozen soil as our aim was to identify in-situ post-thaw changes in OCICD. In the coastal scarps, thermally destabilised, altered and disaggregated ICD material is intermittently transported by erosion, solifluction and surface sliding down the slope. These processes mix the material on the slopes, masking the original age-depth patterns visible in frozen ICD. In total, 17 samples were collected from one slope in the Olenek Channel (n = 5), one slope in Buor-Khaya Cape (n = 4) and four slopes on Muostakh Island (n = 8). The samples were frozen after collection, and then completely dried (to prevent moistureinduced degradation) until analysis. For the ICD compositional and degradation characterisation, the samples from all three sites were analysed for elemental (C and N), isotopic (13C and 14C) and molecular (lipid biomarkers) composition. Elemental and Isotopic Carbon Analyses Subsamples of dried and finely ground ICD soil were decarbonated and analysed for their OC content, total nitrogen content, stable isotopic carbon signature (δ13C) (Carlo Erba NC2500 elemental analyser (Stockholm, Sweden) - Finnigan MAT Delta Plus (Huddinge, Sweden) mass spectrometer, Stockholm University (Svante Arrhenius, Stockholm)) and radiocarbon content (at the National Ocean Sciences Accelerator Mass Spectrometry Facility at Woods Hole Oceanographic Institution (Massachussets, US)). All δ13C and Δ14C (‰) data are reported relative to Vienna Pee Dee Belemnite (VPDB) and oxalic acid I standards, respectively. Details of these analytical methods are provided by Karlsson et al. (2011), Sánchez-García et al. (2011) and Vonk et al. (2010). Copyright © 2014 John Wiley & Sons, Ltd.

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Lipid Extraction, Fractionation and Analysis Freeze-dried soil subsamples (0.1–5 g) from all three sites were solvent extracted using Accelerated Solvent Extraction (ASE) for molecular biomarkers analysis, and the isolated lipid biomarkers were quantified by gas chromatography/mass spectrometry. Cleaning and derivatisation steps are described in van Dongen et al. (2008) and Vonk et al. (2008), and chromatographic identification and quantification details in Vonk et al. (2010). Recoveries of the internal standards averaged 73 ± 38 per cent.

RESULTS Elemental and Isotopic Signatures of OC on the ICD Scarps In the three study sites, the ICD samples showed 14C signals (Δ14C) ranging from 998‰ to 412‰, which correspond to radiocarbon ages from > 48 kyr BP to ~ 4 kyr BP, respectively. Within each ICD regime, Δ14C varied similarly (Figure 2), with values from 988‰ to 474‰ in Muostakh Island (Table 1), from 995‰ to 412‰ in the Olenek Channel (Table 2) and from 998‰ to 631‰ in Buor-Khaya Cape (Table 3). Four samples showed OC ages younger than 11 kyr BP (two in Muostakh, one in Olenek and one in Buor-Khaya) that could be the result of limited (downslope) mixing of shallow Holocene topsoil with ICD. The ICD samples contained similar OC content amongst the three regimes (Figure 3a), with 0.7–3.5 per cent of soil dry weight in Muostakh Island (Table 1), 0.3–3.3 per cent in the Olenek Channel (Table 2) and 0.9–2.9 per cent in Buor-Khaya Cape (Table 3). Younger samples generally had higher OC contents than older samples (Figure 3a). The OC to total nitrogen ratio (OC/N) ranged between 7.5 and 20 in the three sites. Muostakh Island showed the widest range of OC/N (7.5 to 17.9; Table 1), with somewhat lower values than typical terrestrial ratios (≥20; Meyers, 1994). In the Olenek Channel and Buor-Khaya Cape, OC/N ratios were generally higher, ranging from 13.6 to 18.9 (Table 2) and from 13.6 to 19.6 (Table 3), respectively. The OC/N differences between Muostakh Island and the other two regimes are significant at a confidence level (CL) of 95 per cent, while the latter two regimes were not significantly different (Table 4). The presence of inorganic nitrogen in soils or sediments may lower typical terrestrial OC/N ratios (Meyers, 1997). This may be the case for the Muostakh ICD, where some inorganic nitrogen (~0.09%, as derived from the Y-axis intercept, i.e. OC % = 0 in a % of total nitrogen against OC %) may have lowered OC/N ratios relative to those in Olenek and Buor-Khaya (Figure 3b), both of which have lower concentrations of inorganic nitrogen (0.003% and 0.01%, respectively). Irrespective of the presence of inorganic nitrogen, the OC/N ratios revealed an overall decreasing trend with 14C age (Figure 3b), similar to that observed for the %OC (Figure 3a). Both trends are less pronounced in Buor-Khaya Cape. Permafrost and Periglac. Process., 25: 172–183 (2014-07)

2-3


1.1 14 0

1.6 4.1 1.8

3.0 6.7 3.9 3.2 15 2.2

0.86 7.0 0.71

13 0.7 ± 0.02 7.5 23.9 17300 ± 90 885 ± 9.9

2-1

0.77 6.7 0.70

16 1.0 ± 0.01 9.2 24.2 15750 ± 75 860 ± 7.9

2-2

MI Slope 2

0.31 6.3 0

2.8 4.7 3.0

0.91 3.0 0.39

24 1.4 ± 0.13 10.3 24.3 15400 ± 70 854 ± 7.6

5-2

5-1

0.31 9.3 0

0.76 4.0 1.1

0.69 3.2 0.51

1 0.7 ± 0.02 7.7 23.5 25500 ± 130 959 ± 16.1

MI Slope 5 6-1

7-5

MI Slope 7 Mean ± stdh

6.6 27 2.8

7.0 4.4 2.6

2.7 7.1 0.025

1.4 18 1.4

11 7.2 18

0.6 10.8 0.27

11 62 4.1

6.3 3.0 0.45

14 4.0 0.51

6.9 ± 10 19.8 ± 18.3 1.6 ± 1.5

7.0 ± 7.5 5.0 ± 1.5 4.7 ± 5.6

3.0 ± 4.5 5.7 ± 2.7 0.40 ± 0.25

23 2 23 3.3 ± 0.20 1.1 ± 0.03 2.5 ± 0.36 1.8 ± 1.1 16.4 9.1 12.9 11.4 ± 4.0 27.0 24.0 27.2 25.4 ± 2.0 9380 ± 45 19950 ± 95 35200 ± 300 17949 ± 9325 691 ± 4.0 917 ± 11.0 988 ± 39.8 828 ± 169

6-2

MI Slope 6

b

See Figure 1 for identification of samples according to their sampling elevation. The uncertainty of OC % is expressed as a standard deviation of analytical replicates (n = 2–6). c The uncertainty of 14C age corresponds to the analytical error reported by National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS). We propagate this error to estimate the Δ14C uncertainty. d HMW, high-molecular weight, is the sum of C20–C34 for n-alkanes, the sum of C20–C30 for n-alkanoic acids and the sum of C20–C30 for n-alkanols. e Calculated CPIi-n, carbon preference index, = ½ Σ(Xi+Xi+2+…+Xn)/ Σ (Xi-1+Xi+1+…+Xn-1) + ½ Σ(Xi+Xi+2+…+Xn)/ Σ(Xi+1+Xi+3+…+Xn+1), where X is concentration. f Even and low-molecular weight n-alkanes (C16–C20) to odd and high-molecular weight n-alkanes (C27–C31). g 24-Ethylcholest-5-en-3β-ol. h The standard deviation of all parameter means results from averaging all sample values.

a

Bulk properties Profile height (m) 18 3.5 ± 0.83 OC (%)b OC/N 17.9 28.8 δ13C (‰) 14 C age (yrs)c 5110 ± 35 474 ± 2.0 Δ14C OC (‰)c n-Alkanes HMW n-alkanesd 3.29 3.9 CPI 21-31e Even ≤ C20/Odd ≥ C27f 0.12 n-Alkanoic acids 23 HMW n-alkanoic acidsd CPI 20-30e 5.9 7.2 HMWn-alkanoic acids/HMWn-alkanes n-Alkanols HMWn-alkanolsd 31 6.6 CPI 20-30e β-Sitosterolg/HMWn-alkanes 2.0

Sampling code (top-bottom in each slope)a

Table 1 Elemental, isotopic and molecular composition of the Ice Complex Deposits on Muostakh Island (MI). The concentration of terrestrial lipid biomarkers is expressed as mg/g organic carbon (OC).

176 L. Sánchez-García et al.



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Table 2 Elemental, isotopic and molecular composition of the Ice Complex Deposits in the Lena Delta-Olenek Channel (OCh). The concentration of terrestrial lipid biomarkers is expressed as mg/g organic carbon (OC). Sampling code (top-bottom)a Bulk properties Profile height (m) OC (%)b OC/N δ13C (‰)b 14 C age (yrs)c 14 Δ C OC (‰)c n-Alkanes HMWn-alkanesd CPI 21-31e Even ≤ C20/Odd ≥ C27f n-Alkanoic acids HMWn-alkanoic acidsd CPI 20-30e HMWn-alkanoic acids/HMWn-alkanes n-Alkanols HMWn-alkanolsd CPI 20-30e β-Sitosterolg/HMWn-alkanes

OCh-5

OCh-6

OCh-7

OCh-8

OCh-9

Mean ± stdh

17 3.3 ± 0.37 13.6 25.6 ± 0.16 42500 ± 370 995 ± 40

8 3.2 ± 0.20 18.9 28.1 ± 0.16 4210 ± 30 412 ± 1.6

2 0.3 ± 0.01 15.2 25.7 ± 0.07 16500 ± 70 873 ± 7.5

1 1.3 ± 0.16 17.7 26.4 ± 0.24 11450 ± 760 ± 4.7

0.5 1.1 ± 0.23 15.8 26.1 ± 0.30 15800 ± 50 861 ± 8.6

1.9 ± 1.3 16.2 ± 2.08 26.4 ± 1.01 18092 ± 14495 780 ± 222

0.26 3.7 0.50

0.40 7.3 0.08

0.21 4.1 0.43

0.15 4.3 0.63

0.17 3.4 1.32

0.24 ± 0.10 4.6 ± 1.6 0.59 ± 0.46

2.2 5.2 8.2

76.3 4.3 192

2.99 27.6 18.9

1.40 22.0 5.0

4.9 5.1 23

7.4 8.0 50

9.1 6.1 53

20 ± 32 5.7 ± 1.4 65 ± 73

0.80 26.7 7.5

0.41 27.1 20.7

0.40 19.7 13.6

1.20 ± 1.1 24.6 ± 3.6 13.1 ± 6.9

a

See Figure 1 for identification of samples according to their sampling elevation. The uncertainty of OC % and δ13C is expressed as a standard deviation of analytical replicates (n = 2–6). c–h See Table 1 for ratio acronyms and uncertainties. b

Table 3 Elemental, isotopic and molecular composition of the Ice Complex Deposits at Buor-Khaya Cape (BK). The concentration of terrestrial lipid biomarkers is expressed as mg/g organic carbon (OC). Sampling code (top-bottom)a Bulk properties Profile height (m) OC (%)b OC/N δ13C (‰)b 14 C age (yrs)c Δ14C OC (‰)c n-Alkanes HMWn-alkanesd CPI 21-31e Even ≤ C20/Odd ≥ C27f n-Alkanoic acids HMWn-alkanoic acidsd CPI 20-30e HMWn-alkanoic acids/HMWn-alkanes n-Alkanols HMWn-alkanolsd CPI 20-30e β-Sitosterolg/HMWn-alkanes

BK-2

BK-3

BK-4

Mean ± stdh

6 2.7 ± 0.14 19.6 26.9 ± 1.17 > 48000 998 ± 117

3 2.9 ± 0.98 14.3 27.6 ± 0.48 7960 ± 65 631 ± 4.9

1 1.6 ± 0.32 15.5 27.1 ± 0.85 19450 ± 75 912 ± 8.2

1 0.93 ± 13 13.6 27.0 ± 0.46 14300 ± 65 833 ± 6.9

2.0 ± 0.9 15.8 ± 2.7 27.1 ± 0.32 13903 ± 5755 844 ± 157

0.10 3.9 0.38

0.23 4.2 0.51

0.15 4.4 0.50

0.22 4.0 0.45

0.17 ± 0.06 4.1 ± 0.19 0.46 ± 0.06

BK-1

16.4 12.0 164 0.35 21.7 3.0

16.1 9.4 71

8.2 10.0 56

11.6 11.9 54

13.1 ± 3.9 10.8 ± 1.3 86 ± 52

1.52 43.8 4.3

1.24 37.7 12.4

1.25 25.4 3.8

1.09 ± 0.51 32.2 ± 10.4 5.9 ± 4.4

a

See Figure 1 for identification of samples according to their sampling elevation. The uncertainty of OC % and δ13C is expressed as a standard deviation of analytical replicates (n = 2–6). c–h See Table 1 for ratio acronyms and uncertainties. b

The OCICD in the three studied sites showed δ13C ratios ( 28.8 to 23.5‰) in the range of those of terrestrial C3 plants, which typically vary from 23 to 34‰ (Meyers, 1997). In the three regimes, the δ13C ratio showed a significant (p-value of 0.1) enrichment with increasing Δ14C Copyright © 2014 John Wiley & Sons, Ltd.

(r2 = 0.50 in Muostakh, r2 = 0.98 in Olenek and r2 = 0.87 in Buor-Khaya), similar to that observed for %OC and OC/N (Figure 3). The δ13C enrichment was greatest in Muostakh Island, with a mean value of 25.4‰ and a broad range of 28.8 to 23.5‰ (Table 1). In comparison, the Olenek Permafrost and Periglac. Process., 25: 172–183 (2014-07)

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Figure 3 Elemental and isotopic composition of the Ice Complex Deposits (ICD) in Muostakh Island, Olenek Channel and Buor-Khaya Cape: (a) organic 13 2 carbon content (OC %); (b) OC to total nitrogen ratios (OC/N); (c) δ C of OC (‰). Regression coefficients (r ) show the linear fit with the radiocarbon age for each ICD regime, with asterisks in brackets indicating the significant results at p-values of 0.05 (*) and 0.1 (**). This figure is available in colour online at wileyonlinelibrary.com/journal/ppp

Channel and Buor-Khaya Cape showed more depleted values, with means of 26.4‰ and 27.1‰, respectively, and ranges of 28.1 to 25.6‰ and 27.6 to 26.9‰, respectively (Tables 2 and 3). The differences between the regimes were significant at CLs of 74–88 per cent (Table 4). Molecular Signatures of OC on the ICD Scarps On a molecular level, the decay of organic matter involves, for example, the loss of functional groups such as carboxylic acids and alcohol moieties (e.g. Brassell et al., 1984; Meyers and Ishiwatari, 1993; van Dongen et al., 2008). Since the abundance of functional groups tends to decrease upon degradation, in contrast to more stable compounds without functional groups (i.e. n-alkanes), the use of ratios between these compounds provides qualitative information on OC degradation (e.g. Killops and Killops, 2005; Meyers, 1994, 1997). For instance, the ratio of high-molecular weight (HMW) n-alkanoic acids to HMW n-alkanes (HMWn-alkanoic acids/HMWn-alkanes) is expected to decrease as OC decomposition advances (e.g. Van Dongen et al., 2008; Vonk et al., 2008, 2010), similar to the ratio of β-sitosterol/HMWn-alkanes (e.g. Van Dongen et al., 2008). The carbon preference index (CPI) of HMW n-alkanes is another molecular tool to assess the decay extent of OC, where values > 5 are typical of extant plants and values approaching ~ 1 indicate an increase in maturity/degradation (e. g. Hedges and Prahl, 1993). Another molecular alternative is tracing microbial activity in soils using the ratio of even low-molecular weight (LMW) n-alkanes (C16, C18 and C20) to odd HMW n-alkanes (C27, C29 and C31), since the presence of LMW n-alkanes with a strong even carbon number preference is indicative of microbial degradation (Grimalt and Albaigés, 1987). The HMWn-alkanoic acids/HMWn-alkanes ratio ranged from 0.5 to 164. The lowest ratios were observed in the Muostakh ICD (Figure 4a), where HMWn-alkanoic acids/ HMWn-alkanes were < 18 and averaged ~ 5 (Table 1). The ratio was higher in the Olenek Channel (8–192, mean of 65; Table 2) and highest in Buor-Khaya Cape (54–164, mean of 86; Table 3). The differences in the HMWn-alkanoic acids/HMWn-alkanes ratio were significant for Muostakh relative to Olenek and Buor-Khaya at CLs of 86 per cent and 90 per cent, respectively (Table 4). Similarly, β-sitosterol/HMWn-alkanes ratios were Copyright © 2014 John Wiley & Sons, Ltd.

lowest in Muostakh Island (~0–4, mean of 2) and higher in Olenek (5–21, mean of 13) and Buor-Khaya (3–12, mean of 6) (Figure 4b). The differences were significant for Olenek relative to Buor-Khaya and Muostakh at CLs of 90 per cent and 95 per cent, respectively, and for Muostakh relative to Buor-Khaya at 74 per cent CL (Table 4). The CPI ranged from 3 to 11 in the three sites, with identical ranges in Muostakh Island (3–11; Table 1), ranges of 3–7 in the Olenek Channel (Table 2) and a virtually constant CPI of ~ 4 in Buor-Khaya Cape (Table 3). The CPI values were significantly different in Muostakh relative to Olenek and Buor-Khaya at CLs of 62 per cent and 79 per cent, respectively (Table 4). The even LMW n-alkanes/odd HMW n-alkanes ratio varied from 0.02 to 1.3 in all samples, with generally lower values in Muostakh Island (mean of 0.4; Table 1) and higher values at Olenek (mean of 0.59; Table 2) and Buor-Khaya Cape (mean of 0.46; Table 3). Site-specific differences for this ratio were only significant for Muostakh relative to Olenek at a CL of 64 per cent. DISCUSSION Elemental, Isotopic and Molecular Patterns of OCICD with C Age

14

Organic-geochemical signals may reveal the existence and relative extent of any in-situ degradation of OC from thawing surfaces of ICD along the Siberian-Arctic coasts. Interpreting trends along eroding slopes is challenging because of the irregular nature of the erosion. In addition, because the intrinsic heterogeneity of ICD during formation cannot be assessed, we cannot be certain that the observed organic-geochemical trends are due to differences prior to thaw (e.g. stratigraphy, cryoturbation, formation conditions) or post thaw (e.g. degradation, winnowing, physical sorting). However, previous stratigraphic studies from the Laptev Sea and New Siberian Island region (Wetterich et al., 2008, 2009; Schirrmeister et al., 2002, 2011) show that: (i) the age of frozen ICD generally increases with depth; and (ii) bulk geochemical patterns (e.g. C/N, δ13C) of frozen ICD with depth are somewhat variable and lack clear trends with depth. Permafrost and Periglac. Process., 25: 172–183 (2014-07)


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Table 4 Mean statistical differences for the elemental, isotopic and molecular proxies, based on the Student’s t-test. OC%

OC/N

δ13C

Δ14C

HMW alk.ac. / HMW alkanes

β-sitosterol/ HMW alkanes

evenLMW/ oddHMW

CPI

alkanes

Muostakh Island Buor-Khaya Cape Olenek Channel

* *

95% 95%

88% 74%

* *

90% 86%

74% 95%

* 64%

79% 62%

90%

*

*

Olenek Channel Buor-Khaya Cape

*

*

79%

*

*

The percentages indicate the confidence level (CL) at which mean values are significantly different. For CL < 50% (*), values are not considered significantly different. See Table 1 for ratio acronyms and uncertainties.

On an elemental-isotopic level, the OCICD patterns that we observed from thawed slope samples suggest greater OC degradation in older samples. The observed δ13C values are similar to those described by Schirrmeister et al. (2011) on (frozen) sample sequences on Muostakh Island ( 27.6‰ to 23.1‰), and Wetterich et al. (2008) on Kurungnakh Island (mostly between 25‰ and 27‰), close to our study site at the Olenek Channel. In ICD samples from the Kolyma basin, Dutta et al. (2006) and Vonk et al. (2013) also measured high δ13C ratios ( 26.0‰ to 22.0‰) in soils and suspended sediments in the same range as observed here for the older OC samples (Figure 3c). Decreasing values of %OC and OC/N were accompanied by δ13C enrichment as the Δ14C increased (Figure 3b, c), with more than 50 per cent of the variation in the OC/N and δ13C ratios being significantly explained (p-value 0.1) by the radiocarbon age (Figure 3). Declining OC/N values have been observed in soils (Hugelius and Kuhry, 2009; Sollins et al., 1984) in relation to the remineralisation and release of carbon accompanied by the addition of proteinaceous components from bacterial activity. Because frozen ICD lacked such geochemical patterns with 14C age in neighbouring ICD environments (Wetterich et al., 2008, 2009; Schirrmeister et al., 2002, 2011), we propose that the decrease in OC/N ratios with increasing 14C age observed here may reflect selective degradation of labile carbonaceous forms in the recently thaw-exposed OCICD. Likewise, the stable carbon isotope composition of soil organic matter can potentially be modified with time, by selective loss of isotopically light components (e.g. mostly lipids, or cellulose) and incorporation of isotopically heavier proteins (Meyers, 1997). In the three studied sites, alteration during degradation (post thaw) may have resulted in the compositional shift of the OCICD with time, with a relative enrichment of the isotopically heavier OC. This is consistent with 13 C enrichment observed in thawing ICD from the Dmitry Laptev Strait coasts (O. V. Dudarev et al., in preparation). Three different molecular ratios suggest that older OCICD is more degraded. The generally lower values of HMWn-alkanoic acids/HMWn-alkanes and β-sitosterol/HMWn-alkanes, as well as higher values of even ≤ C20/odd ≥ C27, in the 14C older ICD samples relative to the younger samples (Figure 4; Tables 1–3) consistently indicate the loss of functional groups such as Copyright © 2014 John Wiley & Sons, Ltd.

n-alkanoic acids or β-sitosterol, and indicate more pronounced signals of microbial activity with sample age. The CPIn-alkanes values showed signs of degradation (low CPIs) mixed with relatively intact terrestrial material (high CPIs) in all three regimes, and hence a degradation trend with 14C age was not observed. In sum, we find multiple elemental, isotopic and molecular proxies that suggest ongoing degradation in the surface ICD scarps, likely beyond the intrinsic heterogeneity of the OCICD samples. Bulk and molecular signatures generally indicate greater degradation in the 14C older ICD samples. This contrasts with the traditional view that older (soil) OC is typically more recalcitrant than younger, fresh, soil OC. Pleistocene ICD however, differs from normal soils (Zimov et al., 2006a) inasmuch as: (i) it formed by continuous sediment deposition mixed with a short seasonal OC build-up; and (ii) it has been preserved in a frozen state ever since formation. The relatively high degradability of old OCICD may be explained by the lack of pre-processing of the OC prior to thaw (Dutta et al., 2006), and the abrupt spike in the activity of ICD intrinsic bacteria upon thaw (Rivkina et al., 1998) is supported by ex-situ incubation studies (Vonk et al., 2013; Mann et al., 2014; G. D. Abbott et al., submitted). Longer exposure during either one or more thaw episodes, depending on slope steepness, winnowing and physical sorting patterns, tidal/coastal influence, or ice content, could result in more pronounced degradation signs in the older samples. Inter-Regime Comparison of the OCICD Degradation Signature The elemental, molecular and isotopic data of the ICD samples suggest different extents of OC degradation in the three ICD regimes. Although the range in %OC and 14C age was similar in samples from all sites (Table 4), the amount of functional groups, the remains of microbial activity and stable isotope signatures differed significantly between sites (Figures 3 and 4; Tables 4). On Muostakh Island, the most enriched δ13C signatures (Figure 3c) and lowest values of OC/N (Figure 3b), HMWn-alkanoic acids/HMWn-alkanes (Figure 4a) and β-sitosterol/HMWn-alkanes (Figure 4b) suggest that the greatest degradation of OCICD has occurred in this erosion regime. Similarly, the relative change in the Permafrost and Periglac. Process., 25: 172–183 (2014-07)

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Figure 4 Molecular ratios indicative of degradation in the studied Ice Complex Deposits (ICD) from Muostakh Island, Olenek Channel and Buor-Khaya Cape: (a) high-molecular weight (HMW) n-alkanoic acids (sum of C20–C30) over HMW n-alkanes (sum of C20–C34); b) β-sitosterol (24-Ethylcholest5-en-3β-ol) over HMW n-alkanes; and (c) even low-molecular weight (LMW) n-alkanes (C16, C18, C20) over odd HMW n-alkanes (C27, C29, C31). 2 Regression coefficients (r ) show the linear fit with the radiocarbon age for each ICD regime, with asterisks in brackets indicating the significant results at p-values of 0.05 (*) and 0.1 (**). This figure is available in colour online at wileyonlinelibrary.com/journal/ppp

analysed proxies with Δ14C was more pronounced in Muostakh Island than in the Olenek Channel and in the Buor-Khaya Cape (Figures 3 and 4). The least distinct pattern with age in the Buor-Khaya Cape (Figures 3 and 4) might suggest that the most labile organic forms have already been degraded and outgassed in this regime, where the baydzherakhs seem to be more stable as a result of limited erosion at the cape, which is surrounded by a wide beach (Figure 2c), preventing the direct effect of waves, ice and low tides and limiting slope erosion to storms and high tides. The drier conditions of the baydzherakhs relative to other types of ICD are less favourable for microbial activity, because soil microbes need water for growing and decomposing (Zimov et al., 1993). In contrast, ICD cliffs in Muostakh Island and the Olenek Channel still contain abundant ice wedges, pore ice, or small water bodies, maintaining higher moisture contents during summer thaw. The most prominent degradation trend with 14C age on Muostakh Island relative to the Olenek Channel and BuorKhaya Cape is attributed to more active erosion of Muostakh Island (mean annual erosion rate of 1.8 ± 2.0 m/y since 1951 to 2010; Günther et al., 2013a) relative to the other sites (0.5 ± 0.1 m/y for approximately the same ~ 43-yr period at Buor-Khaya Cape; Günther et al., 2013b). Additionally, the degree of physical forcing related with different types of erosion may impact the degradation signature differently in the three ICD regimes. Episodes of thermal collapse, slumping and destabilisation occur in all three ICD regimes, but on Muostakh Island blocks of ice-wedge polygon size are detached, storm waves erode the coast in autumn and ice scours it in winter and spring. As a result, much of the collapsed slope material is removed annually, in contrast to slower erosion at the Olenek Channel and Buor-Khaya Cape. High-resolution sampling on Muostakh Island allowed greater insight into the degradation patterns of OCICD. The biomarker-derived signs of degradation and microbial processing are supported by previously reported patterns of emission of CO2, reaching 440 mmol/m2/day at the bottom of the slope transects (Vonk et al., 2012). The CO2 flux was also reported in the shallow waters (1 m depth) off the northeastern side of the island, suggesting that Copyright © 2014 John Wiley & Sons, Ltd.

decomposition of OC on the thaw-eroded ICD continues beyond the shoreline, in agreement with previous observations of significant CO2 supersaturation in areas of the Laptev and East Siberian Seas impacted by coastal erosion (Andersson et al., 2009; Semiletov et al., 2005, 2011, 2012). CO2 is the major metabolic gas emitted from soil under aerobic conditions, which is supported by field measurements of high CO2 (0.5–3.1%) and low CH4 (0.01%) content (as equilibrated gas concentration measured by thermo-conductivity) in ICD samples from Duvanny Yar (Semiletov et al., 1996) in the Lower Kolyma River (northern Yakutia). Its production in the unvegetated scarps of Muostakh Island, even beyond the shoreline, manifests the importance of the microbial processing of the island OCICD and combines with the isotopic and molecular indications of ongoing degradation in the current paper. This supports the evidence for the labile nature of the OCICD previously documented in ex-situ laboratory experiments of this type of material (Dutta et al., 2006; Zimov et al., 2006b; Vonk et al., 2013) and suggests that old OCICD thawing out may readily degrade. Dominant Factors Affecting the OCICD Degradation OC from ICD exposed to coastal erosion undergoes microbial decomposition to a different extent depending on a few key regulating factors. (i) Exposure time seems to be important in the extent of decomposition of the thermally susceptible OCICD. Temperatures above 0ºC promote the environmental conditions necessary for the microbial community to function: the longer the ICD scarps are exposed during summer, the further the organic components degrade. (ii) The amount and intensity of physical forcing factors (wind, waves, tides, storms, ice break-up, etc.) are crucial for the degree of exposure of the vulnerable OCICD. The frequency, duration and intensity of the erosion determine the accessibility of the microbial community to the OCICD on the scarps. This accessibility also depends on (iii) the landscape evolution (e.g. beach development) and slope gradient of the ICD regime, with the steepest ICD scarps being most susceptible to collapse and impact of wave fetch and pack ice. (iv) Moisture availability is essential to microbial activity on the eroding ICD. The presence/absence and Permafrost and Periglac. Process., 25: 172–183 (2014-07)


intensity of these regulating factors results in three Siberian ICD regimes with different extents of OC degradation. Our field observations support previous laboratory investigations and document the susceptibility of the Siberian ICD to sub-aerial degradation. The heating released during the decomposition of OC may cause additional frozen soil to thaw. This mechanism of microbial soil self-heating (‘composting’) might provide sufficient warming to stimulate the continuous activity of soil biota during winter (Zimov et al., 1993; Khvorostyanov et al., 2008b), allowing the development of taliks and sustaining year-round thaw of permafrost and triggering a potentially larger release of greenhouse gases CO2, CH4 or nitrous oxide (Koven et al., 2011; Shakhova et al., 2009). Moreover, some anaerobic bacteria are highly resilient and may survive repeated freeze-thaw cycles, enabling them to decompose OCICD even after long cold periods (Sawicka et al., 2010). Enhanced microbial decay combined with a future intensification of climate warming, storm events and wave fetch due to ice loss may significantly impact Arctic carbon cycling.

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contents decrease and ratios of OC/N, HMWn-alkanoic acids/ HMWn-alkanes and β-sitosterol/HMWn-alkanes decrease. These field observations support earlier laboratory assessments reporting the propensity for ex-situ microbial respiration of thawing ICD. The in-situ degradation signs indicated by multiple independent proxies/measurements in three different thawing-ICD regimes confirm the labile nature of the OCICD, where the combined effect of thermal destabilisation and coastal abrasion makes the old OC available to latent microorganisms. Exposure time, degree of physical forcing and landscape and slope evolution, in combination with moisture availability, seem to play key roles in the preservation of thawing ICD. The aerial degradation of OCICD and the resulting CO2 outgassing from the exposed scarps imply that relict OCICD is introduced into the rapidly cycling atmosphere-biosphere carbon pools. If coastal exposure and erosion continue to increase in a warming climate, this may affect long-term carbon cycling in this climate-susceptible region. ACKNOWLEDGEMENTS

CONCLUSIONS This study demonstrates the degradability of relict OCICD that is thawing and eroding along the NE Siberian coast by analysing elemental, isotopic and molecular proxies in three regime types of collapsing Arctic ICD. The decay intensity appears to depend on the ICD regime. Limited coastal erosion in the beach-protected site on Buor-Khaya Cape, where ice wedges have melted, has reduced OCICD degradation in this moisture-limited setting. In contrast, more active erosion by wind, storms, waves, tides and spring ice break-up have enhanced OCICD decay in the riverbank and island regimes. The most pronounced signs of degradation were detected in the most active erosion regime (Muostakh Island), where violent physical forcing factors affect the steepest coastal segments of the island. Evidence of increasing microbial decay in older OCICD is found in even LMWn-alkanes/odd HMWn-alkanes and δ13C ratios. Other geochemical parameters also indicate that ICD degradation increases upon exposure to thaw, as OC REFERENCES Andersson LG, Jutterström S, Hjalmarsson S, Wåhlström I, Semiletov IP. 2009. Outgassing of CO2 from Siberian Shelf seas by terrestrial organic matter decomposition. Geophysical Research Letters 36: L2060. Are FE. 1980. Thermal Abrasion of the Sea Coasts. Nauta Publ.: Moscow. Are FE. 1999. The role of coastal retreat for sedimentation in the Laptev Sea. In LandOcean Systems in the Siberian Arctic: Dynamics and History, Kassen H, Bauch H, Dmitrenko I, Eicken H, Hubberten HHW, Melles M, Thiede J, Timokhov L (eds). Springer: Berlin; 287–299. Copyright © 2014 John Wiley & Sons, Ltd.

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