James G. Bockheim

Vulnerability of permafrost carbon to climate change: Implications for the global carbon cycle

ABSTRACT Thawing permafrost and the resulting microbial decomposition of previously frozen organic carbon (C) is one of the most significant potential feedbacks from terrestrial ecosystems to the atmosphere in a changing climate. In this article we present an overview of the global permafrost C pool and of the processes that might transfer this C into the atmosphere, as well as the associated ecosystem changes that occur with thawing. We show that accounting for C stored deep in the permafrost more than doubles previous high-latitude inventory estimates, with this new estimate equivalent to twice the atmospheric C pool. The thawing of permafrost with warming occurs both gradually and catastrophically, exposing organic C to microbial decomposition. Other aspects of ecosystem dynamics can be altered by climate change along with thawing permafrost, such as growing season length, plant growth rates and species composition, and ecosystem energy exchange. However, these processes do not appear to be able to compensate for C release from thawing permafrost, making it likely that the net effect of widespread permafrost thawing will be a positive feedback to a warming climate.

Late Wisconsin and early Holocene glacial history, inner Ross Embayment, Antarctica

Abstract Lateral drift sheets of outlet glaciers that pass through the Transantarctic Mountains constrain past changes of the huge Ross ice drainage system of the Antarctic Ice Sheet. Drift stratigraphy suggests correlation of Reedy III (Reedy Glacier), Beardmore (Beardmore Glacier), Britannia (Hatherton/Darwin Glaciers), Ross Sea (McMurdo Sound), and “younger” (Terra Nova Bay) drifts; radiocarbon dates place the outer limits of Ross Sea drift in late Wisconsin time at 24,000–13,000 yr B.P. Outlet-glacier profiles from these drifts constrain late Wisconsin ice-sheet surface elevations. Within these constraints, we give two extreme late Wisconsin reconstructions of the Ross ice drainage system. Both show little elevation change of the polar plateau coincident with extensive ice-shelf grounding along the inner Ross Embayment. However, in the central Ross Embayment one reconstruction shows floating shelf ice, whereas the other shows a grounded ice sheet. Massive late Wisconsin/Holocene recession of grounded ice from the western Ross Embayment, which was underway at 13,040 yr B.P. and completed by 6600-6020 yr B.P., was accompanied by little change in plateau ice levels inland of the Transantarctic Mountains. Sea level and basal melting probably controlled the extent of grounded ice in the Ross Embayment. The interplay between the precipitation (low late Wisconsin and high Holocene values) and the influence of grounding on outlet glaciers (late Wisconsin thickening and late Wisconsin/Holocene thinning, with effects dying out inland) probably controlled minor elevation changes of the polar plateau.

Solution and use of chronofunctions in studying soil development

Thirty-two chronosequences from 27 areas were selected from the literature for constructing chronofunctions and for correlating rates of change in soil properties with variables representing climate and parent material. The chronosequences originate from areas situated between 66°N and 78°S latitude and represent seven climatic regions, ranging from tropical rainy to cold desert, and seven types of parent materials, including till, aeolian sand, alluvium, mine spoil, volcanic ash, raised beach deposits, and mudflows. Fourteen of the chronosequences contain soils which range in age from 0 to 500 yr; seven span a 12,000-yr period, three a 100,000-yr period, and eight a period of greater than one million yr. Three linear and non-linear models were tested on 15 soil properties. The single-logarithmic (Y = a + b log X) model yielded the best correlation coefficients, when soil property (Y) was correlated with time (X), using linear regression techniques. The dates and equations allow for the following conclusions: 1. (1) The rates of decrease in pH and in base saturation are similar, regardless of the nature of the parent material or climate. 2. (2) The rates of increase in clay content of the B horizon and solum thickness are positively correlated with clay content of the parent material. 3. (3) The rates of increase in solum thickness, oxidation depth, soluble salt content of the salt-enriched horizon, and clay content of the B horizon are positively correlated with mean annual temperature whereas the rate of increase in total N in the surface mineral soil is negatively correlated with present-day mean annual temperature. 4. (4) The increase in bulk density of the surface soil is positively correlated with present mean annual precipitation. 5. (5) The rate of change in C:N is not correlated with variables representing climate and parent material.

Spatial Extent, Age, and Carbon Stocks in Drained Thaw Lake Basins on the Barrow Peninsula, Alaska

Abstract Thaw lakes and drained thaw lake basins are ubiquitous on the Arctic Coastal Plain of Alaska. Basins are wet depositional environments, ideally suited for the accumulation and preservation of organic material. Much of this soil organic carbon (SOC) is currently sequestered in the near-surface permafrost but, under a warming scenario, could become mobilized. The relative age of 77 basins on the Barrow Peninsula was estimated using the degree of plant community succession and verified by radiocarbon-dating material collected from the base of the organic layer in 21 basins. Using Landsat-7+ imagery of the region, a neural network classifying algorithm was developed from basin age-dependent spectra and texture. About 22% of the region is covered by 592 lakes (>1 ha), and at least 50% of the land surface is covered by 558 drained lake basins. Analysis of cores collected from basins indicates that (1) organic layer thickness and the degree of organic matter decomposition generally increases with basin age, and (2) SOC in the surface organic layer tends to increase with basin age, but the relation for the upper 100 cm of soil becomes obscured due to cryoturbation, organic matter decomposition, and processes leading to ice enrichment in the upper permafrost.

Estimating active-layer thickness over a large region: Kuparuk River Basin, Alaska, U.S.A

Active-layer thickness was mapped over a 26,278-km2 area of northern Alaska containing complex and highly variable patterns of topography, vegetation, and soil properties. Procedures included frequ...

Patterns of soil temperature and moisture in the active layer and upper permafrost at Barrow, Alaska: 1993–1999

Soil temperature has been monitored continuously at hourly intervals to a depth of 1 m since 1993 at a site near Barrow, AK. Time series of soil moisture from the active layer and upper permafrost have been collected since 1996 at the same location. These records are supplemented by meteorological data from NOAAs Barrow Climate Monitoring and Diagnostics Laboratory facility and detailed description of depth-dependent soil properties at the site. Soil sensors are situated within a low-centered ice-wedge polygon characterized by meadow tundra vegetation. A thin (7 cm) organic layer grades into reworked marine silts at depth. The soil temperature and moisture are used in a site-specific, multiyear thermal analysis of the atmosphere/snow/active-layer/permafrost system. Fusion retards soil freezing during early winter as soil water is converted to ice. Soil heat transfer is dominated by conduction in winter. Infiltration of snow meltwater in spring produces a series of thermal pulses in the active layer, causing rapid warming of the upper several decimeters by about 1°C. The thermal impact is limited because the soil tends to be nearly saturated at the time of freezeback. Volumetric soil water content in summer is generally around 35–40% at a depth of 15 cm, while the base of the thawed zone remains saturated near 50%. The near-surface soil exhibits drying from evapotranspiration and rewetting from precipitation events. During the period of thaw, the apparent thermal diffusivity is around 2–3×10−7 m2 s−1 and increases with depth to reflect the greater soil water content. The maximum thaw depth at the site is typically around 35 cm. However, end-of-season thaw depth has been monitored near Barrow since 1994 and has increased between 1994 and 1998. This warming trend is also reflected in the thawing degree days calculated for the thawed soil volume. A strong correlation exists between maximum annual thaw depth and annual thawing degree days at this site over the period of record.

Characteristics of cryogenic soils along a latitudinal transect in arctic Alaska

The morphological, chemical, and physical properties of arctic tundra soils were examined along a 200-km latitudinal gradient in northern Alaska which includes two major physiographic provinces; the Arctic Coastal Plain and the Arctic Foothills. Annual air temperature and precipitation increase along the gradient from north to south. Soils on the Arctic Coastal Plain support wet, nonacidic tundra vegetation and have high carbonate contents. Soil on the Arctic Foothills support moist, nonacidic tundra in the northern part and moist acidic tundra in the southern part. Most arctic tundra soils are characterized by medium texture, poor drainage, and high organic matter content. From north to south along the transect, the base saturation of the active layer decreases and exchangeable aluminum increases from north to south. Most soils have strongly developed cryogenic features, including warped and broken horizons, ice lenses, thin platy structure, and organic matter frost-churned into the ice-rich upper permafrost horizons.

Recognition of cryoturbation for classifying permafrost-affected soils

Cryoturbation is a dominant pedologic process in permafrost-affected soils and is used to delineate Gelisols in soil taxonomy and Cryosols in the Canadian and recently proposed World Reference Base for Soil Resources, and Cryozems in Russian systems of soil classification. In this paper we summarize evidence for cryoturbation that can be used for classifying soils containing permafrost. Based on a literature review and our own observations, cryoturbation in the soil profile is manifested by irregular and broken horizons and textural bands, involutions, organic matter accumulation on the permafrost table, oriented stones, silt caps and accumulations, and deformed soil material associated with movements due to ice- and sand-wedge growth.

Consequences of elevated carbon dioxide and ozone for foliar chemical composition and dynamics in trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera).

Atmospheric chemical composition affects foliar chemical composition, which in turn influences the dynamics of both herbivory and decomposition in ecosystems. We assessed the independent and interactive effects of CO2 and O3 fumigation on foliar chemistry of quaking aspen (Populus tremuloides) and paper birch (Betula papyrifera) at a Free-Air CO2 Enrichment (FACE) facility in northern Wisconsin. Leaf samples were collected at five time periods during a single growing season, and analyzed for nitrogen. starch and condensed tannin concentrations, nitrogen resorption efficiencies (NREs), and C:N ratios. Enriched CO2 reduced foliar nitrogen concentrations in aspen and birch; O3 only marginally reduced nitrogen concentrations. NREs were unaffected by pollution treatment in aspen, declined with 03 exposure in birch, and this decline was ameliorated by enriched CO2. C:N ratios of abscised leaves increased in response to enriched CO2 in both tree species. O3 did not significantly alter C:N ratios in aspen, although values tended to be higher in + CO2 + O3 leaves. For birch, O3 decreased C:N ratios under ambient CO2 and increased C:N ratios under elevated CO2. Thus, under the combined pollutants, the C:N ratios of both aspen and birch leaves were elevated above the averaged responses to the individual and independent trace gas treatments. Starch concentrations were largely unresponsive to CO2 and O3 treatments in aspen. but increased in response to elevated CO2 in birch. Levels of condensed tannins were negligibly affected by CO2 and O3 treatments in aspen, but increased in response to enriched CO2 in birch. Results from this work suggest that changes in foliar chemical composition elicited by enriched CO2 are likely to impact herbivory and decomposition, whereas the effects of O3 are likely to be minor, except in cases where they influence plant response to CO2.

The role of soil-forming processes in the definition of taxa in Soil Taxonomy and the World Soil Reference Base

Modern soil taxonomic systems, including Soil Taxonomy (ST) and the World Reference Base (WRB) for Soil Resources, classify soils using diagnostic horizons, properties, and materials. Although these systems are based on genetic principles, the approaches used have de-emphasized the role of soil processes in soil taxonomic systems. Meanwhile, a consideration of soil processes is important for understanding the genetic underpinnings of modern soil taxonomic systems and developing quantitative models of pedogenic systems. Seventeen generalized soil-forming processes are identified, briefly discussed, and linked to soil taxa and diagnostic horizons, properties, and materials in ST and the WRB. The processes are illustrated in simple diagrams and include: (1) argilluviation, (2) biological enrichment of base cations, (3) andisolization, (4) paludization, (5) gleization, (6) melanization, (7) ferrallitization, (8) podzolization, (9) base cation leaching, (10) vertization, (11) cryoturbation, (12) salinization, (13) calcification, (14), solonization, (15) solodization, (16) silicification, and (17) anthrosolization. The implications of soil-forming processes on present and future soil classification systems and pedogenic models are discussed.