SPATIAL AND TEMPORAL ATTRIBUTES OF THE ACTIVE-LAYER THICKNESS RECORD, BARROW, ALASKA, U.S.A.

F.E. Nelson1,4, S.I. Outcalt2, J. Brown3, N.I. Shiklomanov1, K.M. Hinkel2

1. Department of Geography, University of Delaware, Newark, DE, U.S.A. 19716

2. Department of Geography, University of Cincinnati, Cincinnati, OH, U.S.A. 45221 3. International Association, P.O. Box 7, Woods Hole, MA, U.S.A. 02543

Abstract

Active-layer thickness was measured during the periods 1962-1968 and 1991-1997 in a series of 19 plots repre- senting different landcover and terrain characteristics near Barrow, Alaska. Mean values in the 1960Õs were gen- erally higher than in the 1990Õs. Matrix Analysis (CMA) reveals strong internal consistency in the active-layer data. Plotted for the entire record, considerable scatter exists in the relation between active-layer thickness and the square root of the thawing index. Partitioning of the data into interdecadal segments, howev- er, reveals very strong relations between these variables. This discrepancy may be an artifact of interannual or interdecadal variations in moisture, ice content in the uppermost permafrost, ground cover characteristics, and/or nonconductive heat-transfer processes. The Barrow active-layer record illustrates the value of systemat- ic, long-term observations of , currently being performed under the Circumpolar Monitoring (CALM) program.

Introduction and terrain characteristics near Barrow (Brown and Johnson, 1965; Brown, 1969). That programme ended in Barrow, Alaska (71¡ 18Õ N, 156¡ 47Õ W) has been a cen- 1968 and measurements at the CRREL plots were made ter of permafrost-related research since the inception of in only the first year of the 1970-74 IBP the Naval Arctic Research Laboratory in the late 1940Õs project at Barrow (Brown et al., 1980). At the conclusion (e.g., Brewer, 1958; Drew et al., 1958; Lachenbruch, of the 1960s observations, it was assumed that they con- 1959; Kelley and Weaver, 1969). Situated at the conflu- tained the minimum and maximum mean thaw depth ence of the Beaufort and Chukchi Seas, the absence of for the period of climate record at Barrow (1921 to pre- erect shrubs at mesic sites in the Barrow Peninsula indi- sent). However, resumption of the observation pro- cates its proximity to the southern boundary of the gramme in the 1990Õs refuted this supposition. Annual High Arctic Tundra subzone. Wet sedge meadows cover observations at the CRREL sites were resumed in 1991 a landscape dominated by ice-wedge polygons and as part of the Arctic Systems Science/Land- revegetated, oriented thaw lake basins of varying age Atmosphere-Ice Interactions (ARCSS/LAII) program, (Britton, 1957; Carson and Hussey, 1962; Brown, 1967). supported by the U.S. National Science Foundation. Mean annual temperature is -12.6¡C and measured pre- The area in which measurements are made is part of the cipitation averages 12.4 cm/yr. A steep climatic gradi- recently established Barrow Environmental ent exists near the coast (Clebsch and Shanks, 1968). Observatory (BEO), an area set aside and protected for Extrapolation of borehole temperatures indicates that research by the Ukpeagvik Inupiat Corporation (inset, permafrost thickness in the area exceeds 400 m Figure 1). The observations reported here are a contri- (Lachenbruch et al., 1988). A recent study by bution to the Circumpolar Active Layer Monitoring Waelbroeck et al. (1997) used geocryological and eco- (CALM) program, a coordinated effort operated on a logical data from Barrow to model relations between voluntary basis by permafrost researchers at some 70 active-layer thickness and carbon dynamics over time. locations in ten countries (Brown, 1997). The set of stan- dardised monitoring procedures employed at many of In 1962, the U.S. ArmyÕs Cold Regions Research and these sites is detailed in the ITEX (International Tundra Engineering Laboratory (CRREL) initiated a research Experiment) Manual (Nelson et al., 1996). programme that included sampling thaw depth at a series of plots chosen as representative of landcover

F.E. Nelson, et al. 797 Figure 1. Map of research area near Barrow. Labeled dots indicate locations of the 20 CRREL experimental plots. Numbers correspond to those shown in Table 1. Crosses mark locations of nodes on the 1*1 km ARCSS grid (Nelson et al.,1998). Boundary of the Barrow Environmental Observatory is shown on inset. Coordinates in corners of map area show position (feet) within the Alaska Coordinate System (Zone 6). Numbers along bottom and left side are eastings and nor- things (metres), respectively, in the Universal Transverse Mercator system, Zone 4. From Hinkel et al. (1996). Sampling design summer observations. Data summaries for 1962-68 and 1991-97 are available on the IPAÕs Circumpolar Active- Beginning in 1962, the thickness of the active layer layer Permafrost System (CAPS) compact disc, com- was determined in the CRREL plots, which are distri- piled and distributed by the National Snow and Ice buted along a transect between Central Marsh and Data Center in Boulder, Colorado. The combined end- Elson Lagoon (Figure 1). The sampling design was dis- of-season average value for all of the CRREL plots is in cussed in detail by Brown and Johnson (1965). Each close agreement with the average value for 121 sam- 10 x 10 m plot was marked with a permanent stake, and pling points on the 1 km2 ARCSS/LAII grid at Barrow, 36 sampling points were delimited with a series of sec- which partially overlaps the CRREL transect (Figure 1; ondary markers. Measurements were made with a Nelson et al., 1998). 1.0 cm diameter steel rod pushed into the soil to the point of refusal. Comparison of paired observations Analysis obtained by this method with thermal determinations made at nearby (~10 cm away) points indicate no sig- Haugen et al. (1983) employed data from a sparse and nificant differences in this vicinity (Nelson, irregularly distributed climate observation network to unpublished data). estimate temperatures in a remote region of northern Alaska. The technique used in that study, Climate To minimise disturbance to the plots, measurements Matrix Analysis (CMA), is easily adapted to problems were initially made at weekly intervals near the outer- involving active-layer thickness. A matrix consisting of most 20 points, with end-of-season determinations data elements with temporal (climatic time series) and also made near the interior 16 markers. Observations geographic (location) coordinates is constructed, and were made at small staggered distances from the mar- operations are performed for either or both of two pur- kers to avoid a cumulative impact at the sampling loca- poses: (1) to produce estimates of missing elements in tions. Although measurements were made at weekly the matrix; and (2) to assess the degree of the internal intervals during several years in both decades, the consistency of data values in the matrix with respect to record for most years contains only a single set of late- the set of spatial/temporal coordinates. Because

798 The 7th International Permafrost Conference Table 1. Average active-layer thickness (cm) at 19 CRREL plots in 13 years. Temporal vector C and geographic vector G, given by equations (1) and (2), form right-hand and bottom-most column and row respectively

active-layer data from the CRREL plots were not col- The ÒunexplainedÓ variance Vu resulting from the lected for 20 consecutive years (1971-1990), only the CMA, expressed in per cent, is defined as second CMA function is pursued in this study, and esti- mation of values for missing cells was not attempted. [4] VRXu =()var / var· 100 In the present analysis, the ÒclimateÓ matrix X (Table 1) consists of an m x n array of (late summer) active- The ÒexplainedÓ variance Ve , given by layer thickness observations xi,j , where i is a temporal index for the m = 13 years covered in the data set, and j VV=-100 [5] is a ÒgeographicÓ index referencing the n = 19 plots cu included in the study. Stated alternatively, xi,j repre- sents the mean value of active-layer thickness in year provides a goodness-of-fit type index indicating the i at CRREL plot j. degree of consistency in the spatial/temporal array. Values of Ve close to unity indicate a high level of An m-element Òtemporal vectorÓ C, with elements ci covariation between the elements of X, and that indi- defined by vidual xi,j can be predicted accurately from others. This result has important implications for efficient sampling. n [1] If V is reasonably large (e.g., ³80%), missing elements cXn= / e iijå , can be replaced by their estimated values (c + g ); this j =1 i j is the approach used by Haugen et al. (1983). Climate is depicted in the right-hand column of Table 1. An n- Matrix Analysis can also be used as a check on the element Ògeographic vectorÓ G, with elements g internal consistency of complete matrices. Large resi- j dual elements may indicate measurement or transcrip- obtained by tion errors, or a physical effect such as station reloca- tion. Large values (>20%) of V indicate that the m [2] u gXcm/ method is not suitable for a particular data set. This jiji=-å(), ij= situation can arise through application of the technique to climatically heterogeneous data sets, such as those appears as the lowermost row in Table 1. arising in complex terrain.

To assess the internal consistency (degree of Discussion spatial/temporal covariation) in X, the elements ri,j of a residual matrix R are defined as For the Barrow data, CMA yields Ve = 90.8%, indica- ting a high degree of internal consistency in the data [3] matrix. Large residuals arise in most years for Plots 16 rXij,,=-+ ij() cg i j and 19 because these locations occupy a small drainage line in which soil moisture conditions are variable.

F.E. Nelson, et al. 799 (1) occurs preferentially at the bottom of the active layer in situations where two-sided free- zing takes place (Williams and Smith, 1989, pp. 235- 236); repeated on a year-to-year basis, an ice-rich zone develops, and is readily apparent in the stratigraphy of the active layer and upper permafrost (Brown and Sellmann, 1973; Estabrook and Outcalt, 1984; Shur et al., 1995). The large latent heat necessary to thaw this ÒbufferÓ layer inhibits rapid thaw, even in warm years, and tends to govern the thickness of the active layer. However, in a climatically extreme year the active layer may not reach the ice-rich zone, and ice segregation will occur at a new level that tends to govern the thickness of the active layer in subsequent years. Similarly, a series of relatively warm years could thaw the ice-rich layer and reset the active layer at a position deeper in the soil.

(2) The shallow annual depths of thaw in the early 1990Õs could be related to changes in the thermal pro- perties of near-surface, organic-rich . Miller et al. Figure 2. Plot of thawing degree-day accumulation vs. active-layer thickness (1998) and Nelson et al. (1998) found that the largest in Barrow CRREL plots during the periods 1962-68 and 1991-97. ÒBest-fitÓ lines were computed separately for 1960s and 1990s data by linear regression relative interannual changes in active-layer thickness (Eq.(6)). Data points are labeled by year. on the coastal plain occurred in areas of low-centred Annual values of active-layer thickness and the thaw- ice-wedge polygons that experience substantial varia- ing index (degree-day accumulation above 0¡C) are tions in soil moisture content. Oechel et al. (1995) shown as a scattergram in Figure 2. There is conside- reported a decrease in soil moisture, based on data from rable deviation from the close correspondence that has the nearby IBP Tundra Biome study area (Brown et al., often been found between empirical data and simpli- 1980) between the 1970Õs and 1990Õs. In a related study fied versions of the Stefan equation, one of which is (Hinkel et al., 1996), comparison of the ice content in given by cores from the CRREL plots in the 1960Õs and 1990Õs indicated an increase in the ice content of the upper 12/ [6] permafrost. Zal =+b DDT a (3) Evidence exists that fusion and conductive heat a regression-type equation in which Z is active-layer al transfer are not sufficient to explain interannual varia- thickness (m), DDT is the seasonal thawing index (¡C tions in active-layer thickness at Barrow (Outcalt et al., days), b represents the rate of thaw, and a is an inter- 1998). Field experiments and modelling have demon- cept (Hinkel and Nicholas, 1995). The large degree of strated that internal evaporation can retard the advance dispersion in Figure 2 plot indicates that the record of of the thaw front at midday (Outcalt and Nelson, 1985). thaw depth at Barrow cannot be explained effectively There is significant interannual variability in the evapo- by an examination of only the above-ground transpiration component of the balance in the temperature record. Alaskan tundra (Dingman et al., 1980). Analysis of a conduction/fusion model of mean annual temperatures When data sets from the 1960Õs and 1990Õs are consi- compared with field observations indicates that internal dered independently, as shown by the solid lines evaporation can significantly cool the active layer adja- (obtained from Eq. (6)) in Figure 2, they are consistent cent to the frost table (Outcalt et al., 1998). with the Stefan relation. This artifact indicates that one or more of the parameters represented by , which b (4) Schultz (1964) suggested an ecological hypothesis include soil thermal properties, moisture conditions, for quasi-periodic behaviour in active-layer thickness. latent heat, and above-ground biomass may ÒresetÓ the Schultz observed that the active layer at Barrow active layer to a new regime that is continued for sever- remained relatively thin over the course of several sum- al years. Stated alternatively, the active layer may mers in the late 1950Õs following a deep thaw in 1956 exhibit Markovian behaviour. that coincided with a peak in the local lemming popula- Several possibly complementary hypotheses may tion. The proposed association, part of the Ònutrient- account for the differences between the 1962-68 and recovery hypothesis,Ó involved intense grazing, reduc- 1991-97 series: tion of insulation, and a thick active layer during years

800 The 7th International Permafrost Conference with large lemming populations. Nutrients supplied Nelson et al. (1998) from several locations on the North during periods of population peaks fertilize the tundra, Slope, although the temporal database was of much so that insulation increases markedly in subsequent shorter duration in the latter study. If substantiated fur- years, reducing the thickness of the active layer until ther, this tentative conclusion has very positive implica- the next major buildup in the lemming population. tions for ongoing monitoring studies on active-layer Similarly, Moen et al. (1993) observed substantial thickness. decreases in both graminoids and mosses during peri- ods of peak lemming activity. There were no studies of (2) Spatial analytic and process-oriented research at lemming populations in the 1990Õs at Barrow, but large Barrow (Miller et al., 1998; Nelson et al., 1998; Outcalt et quantities of litter (ÒmulchÓ) were observed at al., 1998) indicates that interannual variations in active- both the IBP and CRREL sites in 1991 and 1992, when layer thickness are controlled by several factors, inclu- active-layer thickness was at its minimum values. ding soil moisture, , and nonconductive heat- transfer processes. Additional effort should be given to Conclusions and recommendations evaluating internal evaporation effects using microme- terological investigations and the historical record of Consistent active-layer measurements at Barrow, cov- standard climatic variables (dew point, wind speed) ering the period 1962-68, yielded mean values between that limit the magnitude of evapotranspiration. 31 and 43 cm. Re-establishment of the programme in the early 1990Õs began with very low mean values (22- (3) Data obtained from well-designed sampling expe- 28 cm), and gradually increased to reach the lower part riments involving active-layer thickness, exemplified of the 1960Õs range. by the CRREL data-collection effort of the 1960Õs, have considerable value for ongoing process-oriented inves- Analysis of the active-layer record at Barrow points to tigations, as well as for climate-change studies. several tentative conclusions: Establishment of the Circumpolar Active Layer Monitoring (CALM) network has expanded the global (1) The close covariation between plots with dissimi- active layer database substantially and links it with lar ground cover, microtopography, and soil character- existing archives devoted to other components of the istics indicates that continuation of intensive sampling cryosphere. at all of the CRREL plots may not increase the yield of information on geographic variation substantially. For Acknowledgments future monitoring programmes elsewhere, intensive exploratory sampling is necessary at new observation This research was sponsored by the U.S. National sites to determine the scale(s) of maximum variability Science Foundation, ARCSS/LAII grants OPP-9318528 and an appropriate sampling design for ongoing moni- and OPP-9612647. C. Bay, J. Fagan, L.L. Miller, G.R. toring. In subsequent years, however, accurate results Mueller, and Y. Shur performed many of the field mea- can be obtained by monitoring a carefully chosen sub- surements under the NSF-supported projects. The set of the original observation locations. At the Barrow administration of the Ukpeagvik Inupiat CRREL site, the component of variance attributable to CorporationÕs (UIC) Barrow Environmental climate can be detected by confining measurements to Observatory facilitated these and continuing geocry- those plots containing the extreme and mean thaw ological investigations. The paper benefited from depths. This interpretation is in partial accord with reviews by C.R. Burn and an anonymous referee. results reported by Pavlov (1998) in West Siberia and by

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