Published online 29 June 2007
Published in Soil Sci Soc Am J 71:1335-1342 (2007)
DOI: 10.2136/sssaj2006.0414N
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
PEDOLOGY NOTE
Importance of Cryoturbation in Redistributing Organic Carbon in Permafrost-Affected Soils
J. G. Bockheim*
Dep. of Soil Science, Univ. of Wisconsin, 1525 Observatory Dr., Madison, WI 53706-1299
* Corresponding author (bockheim{at}wisc.edu).
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ABSTRACT
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This study examined the amount of soil organic carbon (SOC) incorporated by cryoturbation into the active layer and near-surface permafrost of Turbels from northern Alaska. An analysis of 21 pedons revealed that an average of 55% of the SOC density of the active layer and near-surface permafrost could be attributed to redistribution from cryoturbation. Cryoturbation occurs most strongly under conditions of poor drainage, where the parent materials are enriched in silt, and where frost boils are present. Based on published radiocarbon dates of buried SOC, cryoturbation was particularly important during periods of the mid-Holocene when the arctic underwent warming. These results suggest that continued warming of the arctic could accelerate cryoturbation and enable the soil to store more SOC than at present, thereby mitigating some of the loss of CO2 to the atmosphere from increased soil respiration.
Abbreviations: SOC, soil organic carbon
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INTRODUCTION
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CRYOTURBATION IS A DOMINANT PROCESS in permafrost regions and refers collectively to all soil movements due to frost action (French, 1976; Washburn, 1980). Although some cryoturbation may occur in areas of seasonal frost, it most commonly occurs in permafrost areas and often is accompanied by patterned ground. Cryoturbation is favored by, but not restricted to, conditions of imperfect drainage, silt-rich parent materials, frequent freeze–thaw cycles, and permafrost in the upper 200 cm of the profile (Washburn, 1980).
Cryoturbation is used at the highest level in classifying permafrost-affected soils in the USA (Soil Survey Staff, 1999), Canadian (Soil Classification Working Group, 1998), and Food and Agriculture Organization (1998) soil classification systems. Bockheim and Tarnocai (1998) illustrated how cryoturbation can be recognized for classifying permafrost-affected soils. Since their review, cryoturbation has received considerable attention, particularly with regard to frost-boil ecosystems and complex interactions among landforms, soils, vegetation, and climate (Peterson and Krantz, 2003; Ping et al., 2003; Walker et al., 2003, 2004).
Numerous studies have described relict cryoturbation features and their relation to Holocene climate change (e.g., Zoltai et al., 1978; Burn et al., 1986; van Vliet-Lanoë, 1998; van Vliet-Lanoë and Seppala, 2002; Hormes et al., 2004; Fortier and Allard, 2004). These studies and others suggest that cryoturbation was particularly active during mid-Holocene warming periods in the arctic. In view of arctic warming during the past three decades (Serreze et al., 2000), key questions are: what effect will sustained warming have on redistribution of soil organic carbon (SOC) and will this redistribution exacerbate or mitigate release of CO2 to the atmosphere?
The main objectives of this study were to determine the magnitude of incorporation of SOC into the active layer and near-surface permafrost by cryoturbation and to assess the likely response of this process to arctic warming.
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Study Areas
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The analysis includes 21 sites from northern Alaska, 10 of which are in the Arctic Coastal Plain and 11 are in the Arctic Foothills physiographic provinces (Table 1). All of the sites are located within the zone of continuous permafrost. The active layer for the study sites ranges from 30 to 90 cm and averages 47 cm in thickness. The mean annual air temperature ranges between –10 and –18°C.
The landforms include moraines (eight sites), drained thaw-lake basins (seven sites), river terraces (two sites), late Tertiary residual surfaces (two sites), a dissected marine terrace (one site), and a dune field (one site) (Table 1). The surfaces range from mid-Holocene to late Tertiary in age (Dinter et al., 1990; Hamilton, 2002; Hinkel et al., 2003). The vegetation includes moist acidic tundra (11 sites), moist nonacidic tundra (eight sites), and shrub (Betula nana L.)–tussock tundra (two sites) typical of the low arctic (Auerbach et al., 1997). The soils are primarily Turbels in Soil Taxonomy (Soil Survey Staff, 1999). Examples of Turbels with redistributed soil organic matter are shown in Fig. 1.
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Fig. 1. Examples of cryoturbated soil organic matter in northern Alaska: (A) buried and redistribution soil organic carbon (SOC) in sandy alluvium of the Meade River, Atqasuk; (B) patches of redistributed SOC in sandy alluvium of the Meade River, Atqasuk; (C) cryoturbation in Spodosol-like soil (Psammoturbel) in sandy alluvium near Okpikpak River, northern Alaska; and (D) SOC concentrated in near-surface permafrost of a drained thaw-lake basin, northern Alaska.
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MATERIALS AND METHODS
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Soil pits were dug by hand to the surface of the permafrost table in early August when the active layer was at its thickest and additionally excavated to a depth of 100 cm or more with a gasoline-powered Pico impact drill (Bockheim et al., 1998). Soils were described according to Soil Survey Division Staff (1993) and drawn to scale using the techniques of Kimble et al. (1993).
Bulk samples were collected from each horizon, and bulk density samples were collected using the pressed tin method for the active layer or the clod method for the near-surface permafrost (Blake, 1965). Oven-dried samples were ground to pass a 0.5-mm screen, and subsamples were analyzed at the University of Wisconsin using a Dohrmann DC-190 total organic C analyzer (Tekmar-Dohrmann, Mason, OH). None of the samples reacted with 1 M HCl and, therefore, were judged not to contain inorganic C.
Soil horizon C density was calculated using the equation
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where 
hor represents the horizon bulk density expressed on an oven-dry (60°C) basis and %Chor is the concentration of total C in the horizon. Adjustments were made for coarse fragments (>2 mm). Horizons influenced by cryoturbation were weighted from scaled drawings, and adjusted horizon depths were determined (Kimble et al., 1993). The proportion of SOC incorporated into the upper 100 cm by cryoturbation was determined by dividing the C density of all cryoturbated horizons by the total C density of the profile (Table 2).
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RESULTS
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Profile quantities of SOC to a 100-cm depth ranged from 17 kg m–3 in soils over ice wedges to 121 kg m–3 in a palsa in the White Hills and averaged 57 kg m–3 (Table 3). This average is comparable to the 53 kg m–3 average value for permafrost-affected soils in the Arctic Coastal Plain and Arctic Foothills of northern Alaska (Michaelson et al., 1996).
The proportion of the SOC contained in cryoturbated horizons ranged from a low of 8% in a Holocene-aged soil derived from alluvium to 100% in a frost boil near Barrow, AK, and averaged 55% (Table 3). The major factors related to soils with a large proportion of the SOC density contained in cryoturbated horizons were explored. Soils containing the greatest amount of cryoturbation tended to be those in frost boils and derived from loess and containing >60% silt (Table 3). All of the soils with 
35% of the SOC in cryoturbated horizons had imperfect drainage. Soils with a low proportion of SOC in cryoturbated horizons included those with a coarse texture and those underlain by ice wedges. Physiographic province and age of geomorphic surface do not appear to play major roles in cryoturbation and redistribution of SOC.
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DISCUSSION
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There are four commonly accepted models of cryoturbation: (i) the convection-cell equilibrium model (Mackay, 1980); (ii) the cryostatic model (Vandenberghe, 1992); (iii) the diapiric or differential collapse model (Swanson et al., 1999); and (iv) the differential frost-heave model (van Vliet-Lanoë, 1991; Peterson et al., 2003). In the convection-cell equilibrium model, heaving–subsidence processes at the top of the active layer produce a net downward and outward movement of material and those processes at the bottom of the active layer produce a net upward and inward movement (Mackay, 1980). The heave–subsidence processes combine to produce a resultant slow upward cell-type circulation, which has been invoked in explaining the origin of earth hummocks.
The cryostatic model involves two freezing fronts moving in opposite directions, downward from the surface and upward from the permafrost table; this causes pressure on the unfrozen materials between the fronts, resulting in cryoturbation (Vandenberghe, 1992). Swanson et al. (1999) proposed that the unstable bulk-density profiles and the viscous nature of wet soils with permafrost facilitate flow of soil by diapirism. Movement of diapers was viewed as an important mechanism in the formation of involutions and mud boils. The differential frost-heave model recognizes the nonuniformity of soil materials and uplifting of the ground surface due to ice lens formation within the active layer (van Vliet-Lanoë, 1991; Peterson et al., 2003). This model also has been used to explain the "self-organizing" process of frost boil development. All of the proposed mechanisms involved in cryoturbation are temperature mediated and tend to be predominant in areas with a large number of freeze–thaw cycles (Washburn, 1980).
One of the major concerns with continued warming in the arctic is recession of the permafrost table and release of CO2 due to increased soil warming and respiration (Waelbroeck et al., 1997; Zimov et al., 2006). In contrast, the effect of warming on cryoturbation and redistribution of SOC has received minimal attention in the literature (Bockheim et al., 1999). Published radiocarbon dates of redistributed organic matter from cryoturbation are related to warming episodes during the Holocene Thermal Maximum or Climate Optimum (ca. 9500–6500 yr BP) and the Medieval Warm Period (1000–700 yr BP) (Table 4). A key question, however, is whether the SOC redistributed to deeper depths will constitute a net ecosystem source or sink of CO2 to the atmosphere.
There are five lines of evidence suggesting that increased cryoturbation from arctic warming will result in increased storage of SOC. First, soil temperature is a key driver in SOC decomposition (Schulz and Körschens, 1998). Once cryoturbation has moved SOC to the cold, deeper soil layers, little or no biological decomposition will take place. Second, low-density SOC may be more susceptible to decomposition than high-density SOC (Yang et al., 2002). Based on data from Bockheim et al. (2003), major organic horizons that are cryoturbated, such as Oijj, Oejj, and Oajj horizons, are 10 to 50% more dense than the equivalent uncryoturbated horizons (Oi, Oe, and Oa). Third, the chemical form of SOC is important relative to its decomposability. Low-molecular-weight, neutrally charged organic compounds are more biodegradable than high-molecular fractions such as humic and fulvic acids (Dai et al., 2000). Fourth, Kaiser et al. (2007) reported lower decomposition rates of redistributed SOC in Siberian subsoils than in equivalent material collected from the surface. Finally, mechanistic models (Waelbroeck et al., 1997) predict that sustained arctic warming will result in permafrost thawing and a delayed long-lasting increase in SOC storage.
These data suggest that, whereas continued warming of the arctic will increase the thaw depth and expose C-rich materials in the near-surface permafrost, the cold temperatures at this depth may hinder microbial decomposition and release of CO2 to the atmosphere. Moreover, an increase in cryoturbation from climate warming may redistribute biologically active forms of C to the subsoil, enabling it to be preserved for thousands of years. These interpretations support those of Kaiser et al. (2007) that cryoturbation will lead to additional long-term storage of SOC by retarding decomposition and enabling the soil to restart SOC accumulation in surface soil layers. Moreover, mechanistic models, such as that developed by Waelbroeck et al. (1997), predict that sustained arctic warming will result in permafrost thawing and a delayed, long-term increase in SOC storage.
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CONCLUSIONS
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An analysis of 21 pedons revealed that 55% of the SOC density of the active layer and near-surface permafrost could be attributed to redistribution from cryoturbation. The magnitude of this process is dependent on local factors such as the occurrence of frost boils and ice-wedge polygons. Based on published radiocarbon dates of buried SOC, cryoturbation was particularly important during the mid-Holocene when the arctic underwent warming. These results suggest that continued warming of the arctic may accelerate cryoturbation. This, in turn, will increase the incorporation of dense, high-molecular-weight SOC at depth, thereby enabling the soil to store more SOC than at present and reducing the loss of CO2 to the atmosphere from soil respiration.
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ACKNOWLEDGMENTS
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The NCEAS Carbon Vulnerability in Permafrost working group encouraged this project. Funding for sample collection and analysis was provided by the National Science Foundation, Office of Polar Programs, Arctic Systems Science Program. K.M. Hinkel, W.R. Eisner, D.A. Walker, F.E. Nelson, L.R. Everett, and many students assisted in the field.
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NOTES
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All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.
Received for publication November 30, 2006.
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ABSTRACT
INTRODUCTION
