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Home , Pedogenesis, Soil erosion, Soil morphology, Soil water (retention)

and -Geomorphic Relationships in an Arid Mountain Range, Mojave Desert, California

Mountains are impressive features of many desert landscapes because of their elevation, complex topography, and Daniel R. Hirmas* sheer extent. Soil genesis and landscape processes were studied in the southern Fry Mountains, Mojave Desert, Dep. of Geography California. Our aim was to better understand the processes responsible for the distribution of soil properties in Univ. of Kansas this landscape. Measured properties in 65 soil pits across the study site show that dust, soluble salt, NO −–N, and Lawrence, KS 66045-7613 3 carbonate distributions are correlated with the prevailing wind direction. Th is fi nding suggests that the mountain range eff ectively traps eolian . mantling these mountains have accumulated, on average, 41 kg m−2 Robert C. Graham −2 −2 − −2 Soil and Sciences Program dust, 172 g m soluble salts, 3.3 g m NO3 –N, and 79 kg m carbonate and reached maximum −2 −2 −2 −2 Dep. of Environmental Sciences concentrations of 156 kg m , 1800 g m , 43 g m , and 398 kg m , respectively, on windward sides of the range. Univ. of California Th e basin fl oor encompassing Soggy , an upwind playa, is the probable primary source of these materials. Riverside, CA 92521-0424 and land surface characteristics from four major mountain landforms were used to interpret the pedogenic and soil-geomorphic processes that have led to the distribution patterns of these accumulations. Our − study demonstrates that arid mountains accumulate and store appreciable quantities of dust, soluble salts, NO3 , and carbonate and are therefore important to the overall geomorphic evolution and biogeochemical cycling of the region. Th e previously unaccounted storage of pedogenic carbonate in similar mountain ranges could increase the global soil inorganic C pool estimate by as much as 15 to 174 Pg C.

Abbreviations: CCE, calcium carbonate equivalent; EC, electrical conductivity; SAR, sodium adsorption ratio; TDS, total dissolved solids.

ountains are distinct and conspicuous parts of the desert landscape. Th ey Mcomprise approximately 38% of desert lands of the southwestern United States (Clements et al., 1957) and are characterized by extreme topography, com- plex slopes with steep gradients, and distinct geomorphic junctions at the moun- tain–piedmont boundary (Cooke et al., 1993). Because of their extent and setting, mountains probably control or strongly infl uence surfi cial processes in the desert landscape, yet few studies have directly explored pedogenic and soil-geomorphic processes of these landforms. Previous work has shown that, for desert soils generally, eolian dust accumulation and the amount and pattern of water infi ltration are key determinants of pedogenesis (Brown and Dunkerley, 1996; Shafer et al., 2007). In turn, dust trapping and infi ltration are controlled by land surface characteristics and near- surface soil horizons. For example, desert pavements and vesicular horizons are the products of those surface characteristics that are able to trap and store considerable quantities of dust (Wells et al., 1985). Th us, the process of dust fl ux across and into the surface is immensely important to the landscape and in these arid systems. Although dust is a major contributor to soils of desert piedmonts (Simonson, 1995), several studies have shown that dust also accumulates in arid mountains. Soils under steep bouldery talus slopes of the Buckskin Range, Nevada, are deep and well developed (Blank et al., 1996). Th ese soils have resulted from a combination

Soil Sci. Soc. Am. J. 75:192–206 Posted online 17 Nov. 2010 doi:10.2136/sssaj2010.0152 Received 29 Mar. 2010. *Corresponding author ([email protected]). © Society of America, 5585 Guilford Rd., Madison WI 53711 USA 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.

192 SSSAJ: Volume 75: Number 1 • January–February 2011 of eolian , subsequent transformation to of Los Angeles. Th e study area is a bolson (i.e., an internally drained smectites, and the upward raft ing of talus. Dust-fi lled pockets intermontane basin; Peterson, 1981) and includes Soggy Lake playa and crevices in the bedrock of mountain peaks and isolated ridges (Fig. 1). Bedrock exposures in the Fry Mountains include Mesozoic occur across the Mojave Desert (Reynolds et al., 2006). Th e diorites, monzonites, and interspersed high-grade metamorphic rocks propensity for dust accumulation in the soils of arid mountains (Nash, 1988). Soil ages on the bajada, estimated from luminescence has been linked to the distinctive land surface characteristics of dates at nearby trenches, range from 4400 to 28,000 yr (Rockwell et the uplands (Hirmas, 2008), yet few studies have investigated al., 2000). Annual rainfall commonly varies between 76 and 127 mm this connection. and occurs predominantly during the winter months (Howell et al., In this study, we examined the relationships between soil 2007), which yields an aridic regime that borders on genesis and geomorphologically controlled dust fl ux in an arid xeric. Creosote-bush [Larrea tridentata (DC.) Coville] and burro-weed mountain range by integrating multiple techniques. Th e ultimate [Ambrosia dumosa (A. Gray) W.W. Payne] dominate the , goal of this work was to understand and isolate the relevant with brittlebush (Encelia farinosa A. Gray ex Torr.) locally prominent in soil-geomorphic processes responsible for the distribution of soil the mountains and saltbush ( spp.) common at the playa margin morphological, textural, and chemical properties in this landscape. (Hirmas et al., 2010). Elevations within the ?430-ha study area range from 875 m in the playa to 1200 m at the mountaintop (Fig. 1). MATERIALS AND METHODS Soggy Lake playa is intermittently fl ooded by runoff from the Study Area surrounding piedmont and uplands during fl ash fl oods (French, Th is study was conducted in the southern Fry Mountains, 1977). Studies on paleolake levels and alluvial fans in the Mojave Mojave Desert, California (Fig. 1), approximately 150 km northeast Desert suggest that the region experienced several periods of increased moisture since the last glacial maximum (e.g., Enzel et al., 1989, 2003; Ely et al., 1993; Wells et al., 2003). Th ese wetter periods are known to be associated with intermittent lake stands of the nearby Mojave basin. We are, however, unaware of any studies documenting the paleohydrology of Soggy Lake. Th e site included six watersheds of the southern Fry Mountains, the associated piedmont, and the basin fl oor. Th e mountains were subdivided using the taxonomic logic of Peterson (1981) and terms consistent with Wysocki et al. (2000) where applicable. Previous work at the site revealed four major landforms within the southern Fry Mountains: mountaintop, mountainfl at, mountainfl ank, and mountainbase (Hirmas, 2008). Briefl y, mountaintops are defi ned as crest or ridgeline positions on a mountain and are characterized by gentle ridge slopes oft en interrupted by crags or knolls, which may form small isolated peaks. Mountainfl ats are broad, open expanses of subdued, low-relief topography containing hills, colluvial aprons, and pediments that are located within the mountain range and are elevated above the surrounding piedmont by at least 50 to 100 m. Mountainfl anks are long Fig. 1. Location and soil classifi cation of sampling sites in the southern Fry Mountain bolson study complex sideslopes of a mountain. area. Location of the Lenwood fault is approximate; H = Haplo, L = Lithic, P = Petro, N = nodic, S = Mountainbase landforms occur at Sodic, X = Xeric, THS = Typic Haplosalid, CA = Calciargid, HA = Haplargid, TO = Torriorthent, TPSA = the base of mountain slopes and are Torripsamment, CAL = calcid, and CAM = cambid. Elevation contours are given in 5-m intervals.

SSSAJ: Volume 75: Number 1 • January–February 2011 193 composed largely of thick wedges of . Bench landforms, which was collected from each location. Th e size distribution was occurred within the mountainfl anks, were grouped with mountaintops determined on suspensions of soil, dust trap, and natural trap samples on the basis of similar land surface characteristics, such as the frequency, using a Horiba LA 930 laser scattering particle-size and distribution size, and sorting of surface clasts. Th e locations of these landforms analyzer (Horiba Instruments, Irvine, CA), which yielded data in 73 within the study area are shown in Fig. 1. bins between 0.115 and 2000 μm (Hirmas et al., 2010). Saturated soil pastes were extracted with the procedure outlined Soil Sampling and Analyses in Laboratory Staff (1996). Soil weights and water volumes Th e number of sampling sites was approximately proportional to for each paste were recorded. Th ese values were combined with bulk the map area of each landform (Fig. 1) and sampling locations were density to convert concentration data from each horizon to a mass-per- randomly distributed within each landform. At each sampling site, the volume basis according to following land surface characteristics were measured: percentage of land CC f f surface covered by clasts, mean clast width, clast frequency, clast sorting, spRR sp 11 ad gr S b 1! 76 [1] percentage of land surface covered by overlapping clasts, and land C −3 C surface roughness. Mean values and procedures used to measure these where is concentration (g m ), sp is concentration of the saturation −1 variables are given by Hirmas (2008). Briefl y, a 1-m tape transect was paste extract (mg L ), θsp is the gravimetric of the placed on the ground in a random orientation. (To ensure randomness, saturation paste (i.e., water/air-dried soil ratio), θad is the gravimetric f −1 we tossed a fi eld knife into the air and chose the orientation of the blade water content of the air-dried soil, gr is the gravel fraction (kg kg ) when it came to rest on the ground as the orientation for the tape.) Every determined by sieving, ρb is the of the soil, including gravel −3 f surface rock touching the edge of the tape was counted (clast frequency) (g cm ), and >76 is the volumetric fraction of cobbles and stones and its b-axis (width) measured. Applying the line-transect method estimated by percentage diagrams in the fi eld. Within the upper 1 m of to this tape, the percentage of the land surface covered by clasts was each pedon, horizon concentrations were combined and converted to a measured as well as the overlap coverage (defi ned as the aerial coverage mass-per-unit-area basis with of overlapping clasts, >2 mm, not embedded in the soil surface). Land n MTC surface roughness was measured by the chain method (Saleh, 1993) and ¦ ii [2] i converted to standard deviation of elevations following Jester and Klik 1 (2005). In addition to land surface characteristics, soils were excavated where M is the upper 1-m concentration (g m−2), n is the lowest horizon by hand (45 pits in the mountains) and backhoe (20 pits in the within the upper 1 m, Ti is the thickness of the ith horizon (m), Ci is the −3 piedmont and basin fl oor), described and classifi ed (Soil Survey Staff , concentration of the ith horizon (g m ), and Tn is the thickness from 2010), and sampled by morphologic horizon for analyses. Bulk density the upper depth of the lowest horizon to either the bottom boundary of was determined by the paraffi n-coated clod method (Blake and Hartge, that horizon or 1 m, whichever is shallower. 1986) on triplicate samples from each horizon; these measurements Electrical conductivity (EC) and pH were determined on included the gravel fraction. saturation extracts immediately aft er extraction; EC was also measured To assess the particle size distribution of silicate dust at the site, in the supernatant of the dust trap samples on returning to the laboratory. six dust traps were constructed and placed across the study area in April Total dissolved solids (mg L−1) were calculated by multiplying the EC 2006 (Fig. 1). Th e dust traps were similar to those used by Reheis and (dS m−1) by a factor of 640 for low values (EC < 5.0 dS m−1) and 800 Kihl (1995) with the exception that they were placed on the ground for high values (EC ≥ 5.0 dS m−1) (U.S. Laboratory Staff , and secured with cobbles around the base instead of mounted 2 m 1954; Sparks, 2003). above the ground on steel fence posts. Hardware-cloth baskets were Base cations were determined on saturation extracts by inductively constructed to fi t within the traps (Reheis and Kihl, 1995) and held coupled plasma–optical emission spectroscopy (PerkinElmer Corp., approximately 500, 4.3-mm-diameter, glass marbles. Th e baskets were Waltham, MA) and used to calculate Na adsorption ratios (SARs; U.S. removed from the traps in April 2007, placed on a large plastic funnel, Salinity Laboratory Staff , 1954) for classifi cation purposes. Saturation − and rinsed with deionized water into 1-L plastic bottles. Th e remainder extracts were analyzed for NO3 –N by ion chromatography (Dionex of the trap was likewise rinsed into the bottles. Dust was separated from Corp., Sunnyvale, CA) with ion suppression EC detection. Th e CaCO3 suspensions in the laboratory by centrifugation and subsequently air equivalent (CCE) of the soil, gravel fractions, and dust trap samples was dried to pellet form. Th e total mass of dust was calculated by adding determined using a manometric method following Hirmas et al. (2010). the moisture-corrected pellet mass to the total dissolved solids (TDS) Th e spatial distributions of the various results were interpolated in the supernatant. by ordinary kriging using a spherical variogram model with nugget in In addition to dust traps, 11 “natural traps” were located and the Geostatistical Analyst extension of ArcGIS 9.2 (ESRI, Redlands, sampled (Fig. 1). Th ese were pockets, crevices, and spalls CA). Chemical data and land surface characteristics were interpolated of granitic boulders and outcrops on mountaintops and ridgelines. within each landform. Th ese were combined and smoothed across the Th e geomorphic position and orientation toward the prevailing wind study area to create a realistic picture of the spatial distributions. All direction made these natural traps logical places for the accumulation other statistics and graphics were analyzed and prepared with R 2.6.1 (R of dust. Th e natural traps were sampled similarly to the techniques used Development Core Team, Vienna). by Reynolds et al. (2006); the fi ne- fraction in the upper <2 cm

194 SSSAJ: Volume 75: Number 1 • January–February 2011 RESULTS correlation is low, the probability that there is no relationship Land Surface Characteristics between slope and width is extremely small (P < 0.004). Higher Land surface characteristics show a general decreasing trend clast frequencies (Fig. 2c) were observed on gently sloping from the mountains to the basin fl oor (Fig. 2). Th is trend is clearly landforms in the mountains, probably because of the inverse seen in clast cover (Fig. 2a), clast width (Fig. 2b), clast sorting relationship between clast frequency and width (Hirmas, (Fig. 2d), clast overlap (Fig. 2e), and land surface roughness (Fig. 2008). Th e mountainfl at, mountaintop, and bench landforms 2f ), all of which show a decrease with decreasing slope gradient. (all somewhat gently sloping) were mantled by a high density In addition, slope gradient appears to be linked to land of clasts. Th ese landforms had well-sorted (Fig. 2d; high values surface characteristics on major landforms within the mountains. represent poorly sorted surfaces), relatively smooth (Fig. 2f ) Clast width (Fig. 2b) was generally higher on surfaces with steep surfaces because they are blanketed with moderate to well- slopes (e.g., mountainfl anks) than on gently-sloping areas in the developed desert pavements (Table 1). mountains (e.g., mountaintops and mountainfl ats), probably because steep slopes involve larger gradients that can move Soil Morphology clasts of larger width. Figure 3 shows this positive relationship Th e distribution of soil morphological features in the between mean clast width and surface slope. Although the study area is related to mountain landforms (Table 1). Vesicular

Fig. 2. Spatial distribution of selected land surface characteristics in the southern Fry Mountain bolson study area showing (a) percentage of land surface covered by clasts, (b) mean clast width, (c) clast frequency, (d) clast sorting, (e) percentage of land surface covered by overlapping clasts, and (f) land surface roughness (standard deviation of elevations). North arrows shown. Vertical exaggeration = 2. Same scale for the x and y dimensions as in Fig. 1.

SSSAJ: Volume 75: Number 1 • January–February 2011 195

horizons (i.e., soil layers near the surface characterized by unconnected ovoid and spherical voids) were found in 42% of pedons in the mountains and showed the highest occurrence in mountainfl at (75%) and mountaintop and bench landforms (75%). Mountainfl ank soils did not generally exhibit vesicular horizons (only 10%). We found vesicular horizons in less than half of mountainbase soils. Argillic horizons were identifi ed in 13% of mountain soils (Table 1). Th ey were distributed evenly across mountain landforms, ranging in frequency from 10% in mountainfl ank soils to 20% in mountainbase soils. Calcic horizons occurred in nearly half of all mountain soils, with the highest frequency in soils of the mountaintop and bench landforms. Eight percent of the mountaintops and benches also contained soils with petrocalcic horizons. Lithic contacts were observed in 38% of pedons in the mountains, and paralithic contacts were observed in <10% (Table 1). Th e highest frequency of lithic contacts was observed in mountaintop and bench landforms (50%) while soils of Fig. 3. Linear regression of mean clast width as a function of slope gradient for mountain sampling locations (n = 45). mountainbase landforms had the lowest frequency of lithic contacts (20%). Soils of mountainfl at landforms showed the on mountainbase (40%) and mountainfl at landforms (38%). highest frequency of paralithic contacts at 38%. Much of the Saprock was seen in most pedons within mountainfl at (88%), bedrock at the site was penetrated by with horizontal mountainfl ank (75%), and mountaintop and bench landforms spacing <10 cm. Th is penetration frequency prevented more of (75%). occurred in only 7% of mountain soils and was the bedrock from being classifi ed as lithic or paralithic material. observed on all landforms except the mountainfl at. We classifi ed the weathered bedrock into three types: hard A detailed geologic map of the southern Fry Mountains is fractured bedrock, saprock, and saprolite. Because these terms not available; however, sampling pits revealed that much of the are used diff erently in diff erent contexts (Migoń and Th omas, basement rock in the study site is biotite-rich diorite. Observations 2002), we defi ne them here for this study. Hard fr actured bedrock of saprolite were restricted to this lithology, whereas hard was usually dense monzonite, granite, or diorite; breaking it fractured bedrock and saprock occurred in granite, monzonite, required extensive work with a pick axe and rock hammer. Th ese and diorite. Monzonite and diorite rock exposures appeared to be layers were described as R horizons in the fi eld and, although evenly scattered across the mountain landforms, whereas granite extremely hard, many did not meet lithic contact criteria because exposures were more common on ridgelines and mountaintops. of numerous fractures. Saprock was somewhat easily carved To the extent that the lithologies of the major landforms diff er, with a soil knife and crumbled to gravel and coarse on they confound the correlation between weathered bedrock type Saprolite excavation. was characterized Table 1. Distribution of soil morphological features within the southern Fry Mountains. by a saprock matrix with domains of All mountain Mountaintop Soil feature Mountainfl at Mountainfl ank Mountainbase well-weathered rock, especially along landforms and bench seams (i.e., soil-fi lled fractures) and ————————————————–%————————————————– joints. Well-weathered rock domains Desert pavement 44 83 75 15 20 of the saprolite were plastic when wet. Vesicular horizon 42 75 75 10 40 Th e mean bulk density for these three Argillic horizon 13 17 13 10 20 types of bedrock was approximately Calcic horizon 49 67 25 45 60 2.65 g cm−3 for hard fractured bedrock, Petrocalcic horizon 2 8 0 0 0 2.06 g cm−3 for saprock, and 1.57 g cm−3 Sodic conditions 16 25 13 10 20 for saprolite. Using the weathered rock Lithic contact† 38 50 38 35 20 classes of Clayton and Arnold (1972), Paralithic contact‡ 9 8 38 0 0 the hard fractured bedrock was less than Bedrock§ Hard fractured 58 58 38 70 40 Class 4, saprock was between Classes 5 Saprock 73 75 88 75 40 and 6, and saprolite contained domains Saprolite 7 8 0 5 20 of Classes 6 and 7. Hard fractured † Contacts of bedrock or hard fractured bedrock that have horizontal spacing of cracks that can be bedrock (Table 1) occurred most penetrated by roots >10 cm. frequently under soils of mountainfl ank ‡ Contacts of saprock and saprolite bedrock that have horizontal spacing of cracks that can be landforms (70%) and least frequently penetrated by roots >10 cm. § Multiple bedrock types often occurred in a single pit; hence, values do not sum to 100%.

196 SSSAJ: Volume 75: Number 1 • January–February 2011 Table 2. Distribution of soil taxonomic classes across the southern were taxonomically shallow (Table 2). Most of the soils in the Fry Mountain bolson study area. mountains (80%) were not in lithic subgroups. Soil classifi cation Total study area Basin fl oor Piedmont Mountain ——————————–%——————————– Particle Size Distributions 72 100 75 69 Th e basin fl oor, piedmont, and mountain soils showed Salids 5 75 0 0 diff erences in particle size distributions. Median percentages Argids 9 0 0 13 of particle size fractions and 95% confi dence intervals are Calcids 43 25 44 45 Cambids 15 0 31 11 given as center bars and notches, respectively, within the box 28 0 25 31 plots of Fig. 4. Sand percentages were signifi cantly diff erent 2 0 6 0 (P < 0.05) from one another and ranged from 11% in the 26 0 19 31 basin fl oor to 66% in the piedmont. Mountain soils contained Lithic subgroups 14 0 0 20 signifi cantly (P < 0.05) more silt (43%) than piedmont soils Nonlithic subgroups 86 100 100 80 (27%). Basin fl oor soils showed the highest median value of at 63%. percentages for the piedmont and mountain and landform. Th is fi nding may limit the extrapolation of these soils were similarly low at 6 and 7%, respectively, compared with results beyond the southern Fry Mountains. those of the basin fl oor (24%). Th us, the basin fl oor was relatively Th e spatial distribution and taxonomic classifi cation of soils low in sand but high in silt and clay, the piedmont was high in across the study area are related to mountain landforms (Fig. sand but lower in silt and clay, and the mountain was high in sand 1). Th ese data for the mountains, piedmont, and basin fl oor are and silt but low in clay. Figure 4 also shows a trend of increasing synthesized in Table 2. Remarkably, piedmont and mountain spread in the data from basin fl oor to mountain landforms; this soils showed similar diversity at the order level: 69% of mountain is represented by the separation between the upper and lower box pedons were classifi ed as Aridisols and 31% as Entisols, compared hinges and length of the whiskers. Th e increase in variation was with 75% Aridisols and 25% Entisols for piedmont pedons. All probably caused by the increased complexity of geomorphic and basin fl oor pedons were classifi ed as Aridisols (75% Salids and pedogenic processes from basin fl oor to mountain landforms. 25% Calcids). At the suborder level, Argids occurred only in the Ternary particle size plots for soil horizons from each mountains and Calcids were found in the piedmont (44%) and mountain landform fall into one of six textural classes (Fig. 5): mountains (45%). Cambids were observed more frequently in sand, loamy sand, sandy , loam, silt loam, and silty clay loam. the piedmont (31%) than in the mountains (11%). Psamments All soil horizons in the mountains contained <35% clay. Th e A occurred only in the piedmont (6%); other Entisols were classifi ed horizons showed a wide spread across the six soil textural classes as Orthents. Although lithic contacts were observed in 38% of (Fig. 5b). Th e fi ne-earth fraction of vesicular (Fig. 5c) and B (Fig. all mountain pedons (Table 1), only a fraction of those (20%) 5d) horizons, however, was concentrated toward the silt region in loam, silt loam, or silty clay loam textural classes. Particle size distributions of Cr horizons (Fig. 5e) concentrated toward the silt-poor, sand-rich corner of the textural triangle in sand, loamy sand, and sandy loam textural classes. Th e fi ne-earth fraction from R horizon fractures (Fig. 5f ) showed a bimodal pattern, where samples tended more toward the sand and silt regions of the diagram than the center. Particle-size histograms for the dust sampling locations (Fig. 1) show a consistently well-defi ned peak in the coarse silt to fi ne sand range (Fig. 6). Mean values for these peaks are 119 μm (dust traps) and 106 μm (natural traps). Two anomalous peaks in the coarse and very coarse sand range (?1000 μm) probably refl ect localized eff ects. For example, at the of sampling, coarse-textured debris at the entrance of a fresh rodent was found approximately 2 m upwind from the anomalous dust trap. Th is burrow debris may have contributed to the bimodal distribution of that sample. Fig. 4. Box plots of sand (s), silt (si), and clay (c) percentages Given that these samples were taken at widely spaced comparing the three major physiographic parts of the landscape: locations and at varying elevations in the southern Fry basin fl oor, piedmont, and mountain. Boxes show the upper and Mountains (Fig. 1), it is noteworthy that they should have such lower quartiles, center bars show median values, whiskers extend to extreme data points (with a distance from the box of no more well-defi ned particle size peaks, which occurred at approximately than 1.5 the interquartile range), points show very extreme 110 μm. Th at is, the data do not show a particle-size sorting values, and notches around center bars are roughly 95% confi dence eff ect that might be expected across the range of elevations intervals around the median values.

SSSAJ: Volume 75: Number 1 • January–February 2011 197 and distances represented in this sampling scheme. Th erefore, fraction of soil attributable to dust (Fig. 6). Th at is, although we defi ned a “target dust size” between 32 and 373 μm that the dust contained <32 μm, these particle sizes were consistently contained these peaks to conservatively estimate the not used to estimate the dust fraction in soil samples because

Fig. 5. Particle size ternary plots and textural classes for the fi ne-earth fraction (<2 mm) of horizons across mountain landforms showing (a) all horizons, (b) A horizons, (c) vesicular horizons, (d) B horizons, (e) Cr horizons, and (f) sampled R horizons.

198 SSSAJ: Volume 75: Number 1 • January–February 2011 Table 3. Total quantity of dust, total dissolved solids (TDS), NO3–N, and CaCO3 equivalent (CCE) per area for landforms of the southern Fry Mountain bolson study area. − Landform Dust TDS NO3 –N CCE kg m−2 ——— g m−2 ——— kg m−2 Basin fl oor 53 10568 565 208 Piedmont 136 2311 1.4 72 Mountain 41 172 3.3 79 Total study area 76 2203 72 92

histogram data within the target dust size range and integrated from the base of the curve to the peak. By integrating from the base of the curve rather than zero, we fi ltered out the contribution of in situ weathered particles within that size range (32–373 μm). Volumetric dust fractions from fi ne-earth soil samples were, in this way, conservatively estimated strictly from particle-size data. Dust fractions were converted to a mass per unit area basis with calculations similar to Eq. [1] and [2], correcting for gravel, cobble, and stone fractions. Dust concentrations within the upper 1 m were highest (approximately 380 kg m−2) on the western extent of the basin fl oor, where our sampling included coppice that dot the surface (Fig. 7a). Th e piedmont showed the highest quantities of dust in soils of inset fans at the eastern and western ends of the study area (Fig. 7a). Mountain soils (Fig. 7b) showed a general decreasing trend in dust content from southwest to northeast. In addition to this trend, major landforms accounted for some of the distribution patterns with respect to dust. For example, soils of mountaintop landforms had low dust contents compared with mountainfl ank soils (Fig. 7b). Dust concentrations were 53 kg m−2 in the basin fl oor, 136 kg m−2 in the piedmont, and 41 kg m−2 in the mountains (Table 3). Although mountain soils had smaller quantities of dust compared with piedmont soils Fig. 6. Particle size histograms for dust trap (n = 6) and natural trap (Table 3), their fi ne-earth fractions had higher concentrations of (n = 11) samples. Target dust size peaks are at approximately 110 μm. silt-size particles (Fig. 4). they were indistinguishable from fi nes that had weathered To t a l d u s t fl ux (including sand) measured in dust trap samples in situ. We determined the boundaries of the target dust size across the range (Fig. 1) had an average value of 77.8 g m−2 yr−1 window using the infl ection points that correspond to the base (Table 4), which is higher than most values reported for the above and below the consistent peaks in Fig. 6. For each soil Mojave Desert (e.g., Reheis and Kihl, 1995). Our study area, sample (horizon), a 10th-order polynomial was fi tted to the however, is part of the larger Bureau of

Fig. 7. Spatial distribution of dust in the upper 1 m (a) across the southern Fry Mountain bolson study area and (b) rescaled for the mountains. North arrows shown. Vertical exaggeration = 2. Same scale for the x and y dimensions as in Fig. 1.

SSSAJ: Volume 75: Number 1 • January–February 2011 199

Johnson Off - Vehicle Area. Table 4. Measured fl uxes of total dust (including sand), CaCO3 equivalent (CCE), and total dissolved solids (TDS) in dust trap samples from the southern Th is area contains patches of vehicular surface Fry Mountains determined for the period April 2006 to April 2007. disturbance on piedmonts and basin fl oors, Dust trap Total dust Latitude Longitude CCE fl ux TDS fl ux NO −–N fl ux especially a few kilometers south of the southern sample fl ux 3 Fry Mountains (McAuliff e et al., 2007). Such ——— g m−2 yr−1 ——— mg m−2 yr−1 areas of disturbance are more vulnerable to wind DT1 N 34°28.162′ W 116°42.169′ 147.4 3.5 1.4 134 (van Donk et al., 2003), which may DT2 N 34°28.516′ W 116°41.843′ 98.3 1.8 1.9 150 explain, in part, the higher values. Additionally, DT3 N 34°28.944′ W 116°41.456′ 35.0 1.5 3.6 374 sediment fl ux values increase with decreasing DT4 N 34°28.275′ W 116°41.763′ 12.6 0.6 5.5 226 height above the soil surface (van Donk et al., DT5 N 34°28.442′ W 116°41.561′ 80.9 2.4 2.5 136 2003). Because our dust traps were placed on the DT6 N 34°28.207′ W 116°41.428′ 92.6 3.7 2.4 173 ground, this may also explain the higher fl uxes. north compared with salinity. Th e landform distribution of − NO3 –N within the southwest area of the mountains showed Th e pattern of salinity (TDS) within the upper 1 m across a trend that was additionally similar to salinity. Mountainfl ank − −2 the study area is related to landform distribution (Fig. 8a soils contained lower concentrations of NO3 –N (?20 g m ) and 8b). Total dissolved salts were highest for the basin fl oor, compared with mountaintop soils (43 g m−2) in the west and averaging 10.6 kg m−2; this was approximately fi ve times greater southwest parts of the range. Th e reverse trend was observed in the than for the piedmont (2.3 kg m−2) and 60 times greater than southern tip of the range: mountainfl anks had higher concentrations −2 − −2 −2 for the mountains (0.17 kg m ; Table 3; Fig. 8a). Within the of NO3 –N (43 g m ) than mountaintops (<1 g m ) (Fig. 8d). − basin fl oor, depth-weighted EC determined on saturated-paste Th e average NO3 –N fl ux measured in dust trap samples was extracts ranged from 11.5 dS m−1 in the coppice area to 199 mg m−2 yr−1 (Table 4). 36.7 dS m−1 in Soggy Lake playa (locations shown in Fig. 7a). Calcium carbonate equivalent values were generally Th e distribution of salinity across the mountains follows a highest (nearly 400 kg m−2) in soils of Soggy Lake playa and conspicuous decreasing trend from southwest to northeast (Fig. mountainfl anks (Table 3; Fig. 8e). Average integrated CCE 8b). Two opposing landform eff ects were observed in addition to values for the upper 1 m were 208 kg m−2 in the basin fl oor, the general salinity trend. Soils on gently sloping mountaintops 72 kg m−2 in the piedmont, and 79 kg m−2 in the mountains toward the west and southwest showed higher total salinity (Table 3). Th e general distribution of CCE in the mountains (maximum TDS = 1.8 kg m−2) in the upper 1 m compared with loosely mimics salinity: values decreased from southwest to mountainfl anks downslope (maximum TDS = 0.8 kg m−2) (Fig. northeast. Geochemical analyses of the saturation extracts from 8b). Toward the south of the range, however, mountainfl anks each horizon revealed that >99% of the CCE is (CaCO3) −2 showed higher salinity (maximum TDS = 1.8 kg m ) than with only a minor contribution (<1%) from NaHCO3. Calcium mountaintops (maximum TDS < 0.1 kg m−2). Th e average fl ux carbonate equivalent fl ux measured in dust trap samples had an of TDS from dust trap samples was 2.9 g m−2 yr−1 (Table 4). average value of 2.3 g m−2 yr−1 (Table 4). In addition to salinity, 14% of the sampled pedons had subgroup classifi cations that indicated high SARs (Fig. 1). DISCUSSION Sixteen percent of mountain soils exhibited sodic conditions Links between Landforms, Soil Morphology, (SAR ≥ 13) within one or more horizons (Table 1). Th e and Dust Flux distribution of soils aff ected by a high SAR ranged from 10% in Dust Flux the mountainfl ank to 25% in mountaintop and bench landforms Th e quantity of soil material attributable to dust is linked (Table 1). Although not expressed at the subgroup level in Fig. to landforms at two scales. At the broad scale, there is a link 1, all pedons in the basin fl oor and 63% of soils in the piedmont between the steep topography of the mountains, the prevailing had horizons with sodic conditions. wind direction, and the distribution of dust (Pelletier and Cook, − Interpolated maps of NO3 –N concentrations in the 2005). blow sediment from the basin fl oor and − upper 1 m are depicted in Fig. 8c and 8d. Th e average NO3 –N piedmont to the mountains in an easterly direction; this fi nding concentration in the basin fl oor was remarkably high at 565 g m−2 was interpreted from the leeward edges of coppice dunes at (Table 3) and reached 930 g m−2 within Soggy Lake playa (Fig. the site (Fig. 7a) and observations of sand accumulating on the 8c). Th is concentration range was more than 100 times the western fl anks of ranges in the area (Nash, 1988). At this scale, concentration in the piedmont and mountain soils (Table 3). the mountains act as a wind baffl e, encouraging dust deposition − Mountain soils contained about twice as much NO3 –N on especially near the mouths of the two westernmost watersheds average (3.3 g m−2) as piedmont soils (1.4 g m−2; Table 3). Figure in the study area (Fig. 7b). Th e orthogonal orientation of the − 8d shows a trend in the mountains for NO3 –N distribution mountain front to the prevailing wind direction enhances − similar to that for salinity (Fig. 8b); NO3 –N concentrations dust deposition on these watersheds. Th is process gives rise did, however, decrease somewhat more strongly from south to to the general trend observed in Fig. 7b of decreasing dust

200 SSSAJ: Volume 75: Number 1 • January–February 2011 Fig. 8. Upper 1-m spatial distribution across the southern Fry Mountain bolson study area showing (a) total dissolved solids (TDS), − − (b) TDS rescaled for the mountains, (c) NO3 –N, (d) NO3 –N rescaled for the mountains, and (e) CaCO3 equivalent. North arrows shown. Vertical exaggeration = 2. A scale for the x and y dimensions is given in Fig. 1.

content from southwest to northeast and is consistent with content in soils near the mountainbase and low dust content experimental and modeling data that show dust preferentially in mountaintop soils (Fig. 7b) coincide with diff erences in deposits on windward slopes (Goossens, 1988). Reheis and Kihl slope shape. Concave slopes, which characterize zones near the (1995) reported that dust deposition rates were higher on north mountainbase, tend to have high dust accumulation, whereas and west (i.e., windward) sides of mountain ranges oriented convex slopes (e.g., mountaintops) are associated with low dust transverse to the prevailing wind direction in the Mojave Desert. accumulation (Goossens, 2000). Th is result, however, may be At fi ner scales, individual landforms infl uence the confounded by the increased distance from the source area. Th at distribution of dust. Mountaintop and bench landforms contain is, mountainbase landforms (concave slopes) are closer to dust- less dust than mountainfl ank or mountainbase landforms. Steep producing areas than mountaintops (convex slopes). landforms are characterized by rough surfaces (Fig. 2f ) with poorly sorted clasts (Fig. 2d) that, together with slope gradient, Desert Pavement and Near-Surface Horizons increase the potential for dust deposition and retention (Pye, Gently sloping mountain landforms (e.g., mountaintops) 1995). Th is is because microsites between surface clasts trap and have surfaces mantled by desert pavement (Table 1). Th e protect the dust from entrainment by the wind (Cooke et al., formation of these pavements requires a degree of geomorphic 1993). Mountaintop landforms, by contrast, have well-sorted stability and antiquity (McFadden et al., 1989), as shown by the and smooth surfaces on gentle slopes with fewer microsites presence of well-developed desert varnish, smaller surface clast that can trap and retain dust. In addition, the high dust widths (Fig. 2b and 3), increased surface clast frequency (Fig.

SSSAJ: Volume 75: Number 1 • January–February 2011 201

2c), and well-sorted surface clasts (Fig. 2d). Th ese last three lines (Table 1). Steep mountainfl anks are susceptible to gravitational of evidence indicate surfaces that have been stable long enough mass movement processes. Th ese processes move , which for clasts to comminute and form smooth, dense, and well-sorted thickens downslope, oft en deeply burying underlying bedrock. pavements (Wells et al., 1985). Vesicular horizons, formed by the Steep mountain landforms are mantled with deeper regolith deposition and incorporation of eolian material (Fig. 5c), are than gently sloping landforms (mountaintops and benches), common on these landforms (Table 1) and are evidence of past causing lithic contacts to occur deeply below the surface and dust accumulation (McFadden et al., 1986). Although gently probably beneath the sampling depths used in this study. In sloping landforms have a lower potential to trap dust than steep addition, the high fracture density of bedrock (which prevents landforms, they have stable surfaces that have retained dust it from meeting the requirements of a lithic contact) may be longer. Because desert pavement formation smoothes landform related to the activity of the Lenwood fault (Fig. 1), which may surfaces (Birkeland, 1999), mountaintops and benches may have shattered bedrock in the mountains. Paleoseismic studies have previously had rougher surfaces that trapped dust more have revealed that this part of the eastern California shear zone effi ciently than the current land surface characteristics suggest. has experienced at least three clusters of earthquake activity in the last 15,000 yr (Rockwell et al., 2000). Subsurface Horizons Hard fractured bedrock, which oft en occurred below Soil morphology indicates that the mountaintop and bench saprock, was common under mountainfl ank soils (70%). Such landforms are older and more stable than the steeply sloping bedrock on steep slopes was usually overlain by thicker regolith mountainfl anks. Soils on mountaintops and benches have argillic than on mountaintop and bench landforms. Although most of horizons more frequently than those on mountainfl anks (Table this regolith is colluvial, water and dust infl ux on steep landforms 1). Argillic horizons require thousands to tens of thousands of may promote bedrock weathering. Mountainfl anks have surfaces years to form in desert environments (Birkeland, 1999) and are that promote the infi ltration of water and the deposition and encouraged by pluvial suitable for the weathering and retention of dust. Th e dust contains relatively high quantities translocation of clay (Reheis, 1987; Chadwick and Davis, 1990). of soluble salts and these salts can be leached into the bedrock. Th e presence of argillic horizons, therefore, indicates geomorphic (We measured an average TDS fl ux of 2.9 g m–2 yr–1 in dust trap stability probably throughout the Holocene or longer. Petrocalcic samples [Table 4] and an average TDS fo 13.1 kg m–3 in surface horizons, also more abundant on mountaintops and benches, horizons of basin fl oor soils from which this dust probably take much longer to form (Machette, 1985) and thus imply even arises.) Soluble salts may encourage bedrock weathering if they greater stability. are carried in into rock pores where they crystallize by Although calcic horizons do occur on mountainfl anks, evapoconcentration (Smith, 2009). Th e susceptibility of igneous they oft en have abrupt upper boundaries to overlying colluvial bedrock at the study site to salt weathering depends on how easily material. Th ese abrupt boundaries suggest that the calcic horizons the percolating salt solution can enter the rock (encouraged by have had their upper portions stripped by mass movement and fi ssures and macropores) and the grain size (medium- to coarse- have been buried by subsequent colluvial accumulations. Because grained rocks are more susceptible to weathering), and pore carbonate in gravelly soils commonly precipitates fi rst as coats structure (Cardell et al., 2003) of the bedrock. Although we did on the bottom of clasts (i.e., pendants) (Gile et al., 1966), the not assess this susceptibility, the common fractures and seams common observations of carbonate coats randomly oriented on in addition to the occurrence of medium- and coarse-grained gravels in the upper mantle of these soils indicate colluvial additions granitic and dioritic rock present a physical environment in which from upslope carbonate-enriched horizons. salt weathering may be eff ective. In addition, the weathering of Th e presence of calcic horizons on mountaintop and bench biotite (much of the subsurface bedrock at the site was identifi ed landforms not only indicates that the soils have some stability, as biotite diorite) to vermiculite is a common mechanism of but that they are infl uenced by dust infl ux. Mojave Desert dust saprock formation (Nettleton et al., 1970; Graham et al., 2010) is known to contain carbonates (Reheis and Kihl, 1995; Reheis, and may have occurred in the biotite-rich diorites of the Fry 2006) and with few exceptions (e.g., Boettinger and Southard, Mountains during wetter climates. 1991) the formation of calcic horizons in the Mojave has been Carbonates are ubiquitous in seams and fractures of bedrock attributed to eolian inputs of CaCO3 (Reheis et al., 1995). In in the mountains. Th e various types of weathered bedrock (hard particular, the local basin fl oor upwind from the fractured, saprock, or saprolite) could provide long-term storage mountain are carbonate rich (Table 3) and an average CCE fl ux for inorganic C. Figure 9 illustrates the reason for this long-term value of 2.3 g m−2 yr−1 was measured in dust trap samples across storage. Rough, steep, windward landscape positions encourage the range (Table 4). carbonate-laden dust deposition (e.g., Fig. 7b and 9a) and water infi ltration in arid environments (Yair and Klein, 1973; Rock Goossens, 1988). Seams and fractures of the bedrock are calcifi ed Lithic contacts were more common under low-gradient as percolating meteoric water dissolves near-surface carbonate mountain landforms such as the mountaintop and bench and and concentrates it at depth (Fig. 9a) (Quade and Cerling, less so under mountainfl ank and mountainbase landforms 1990; Boettinger and Southard, 1991). During intense rainfall,

202 SSSAJ: Volume 75: Number 1 • January–February 2011 and piedmont landforms (e.g., playas and alluvial fans); these have been shown, however, to act as variable sources of dust in the Mojave Desert (Reheis, 2006). Th e ability of an alluvial fan, for example, to emit dust depends on whether the fan is active or not. Four observations from this study suggest that mountain landforms, by contrast, act primarily as sinks, providing long-term storage for eolian dust. First, the quantity of dust decreases signifi cantly from southwest (>150 kg m−2) to northeast (<1 kg m−2; Fig. 7b). Th e mountain soils must be stable long enough to accumulate, store, and maintain this pattern of distribution and high gradient of dust, probably across time scales of thousands to tens of thousands of years. Second, the occurrence of dust at considerable depths (oft en to >1 m) in these soils suggests that they have been stable long enough for dust translocation between coarse clasts and into weathered and fractured bedrock. Th ird, although not quantifi ed in the study, faunal were noticed more frequently in the piedmont than in the mountains, possibly because burrowing is easier in piedmont soils, which have fewer rock fragments. Soil mounds from faunal burrowing are susceptible to defl ation and entrainment by wind, serving as a dust source on inset fans of the piedmont. Fewer burrow mounds in the mountains means dust is less likely to be derived by this process. Fourth, the complex topography of the mountains makes trapped dust unlikely to defl ate from the surface without being retrapped downwind on other parts Fig. 9. Diagram illustrating the process by which inorganic C (CaCO3) in weathered and of the range. Regolith on mountain landforms was fractured bedrock on mountainfl anks is protected from erosion and stored over large dominated by a clast-supported matrix, suggesting time scales. that although landforms are collecting dust, these erosional events may be triggered, causing soils and sediment landform surfaces have not appreciably aggraded by eolian above the bedrock to be eroded and transported downslope processes. Th e storage time of dust in the mountains is primarily (Fig. 9b), eventually reaching the piedmont. Subsequently, new controlled by the stability of the regolith with respect to alluvial colluvial debris is transported from upslope areas to mantle the and colluvial processes. bedrock (Fig. 9c) and the process is repeated (Fig. 9d). Similar Th e considerable concentrations of the dust size fraction hillslope sensitivity to more frequent intense in the young inset fans (Fig. 7a) may be largely reworked dust, accompanying climatic shift s in the southwestern United States translocated from the mountains to the piedmont by alluvial has been documented (e.g., Harvey et al., 1999; Pederson et processes. Th is fi nding is evident from the alluvial stratifi cation al., 2001; Hunt and Wu, 2004). Seams and fractures within preserved throughout most of the sampling depths. Vesicular the bedrock thus act as geomorphically stable sinks for eolian horizons, which are formed in near-surface accumulations of carbonate. Th e presence of carbonate in seams and fractures of dust, were absent on these landforms, suggesting that the dust bedrock may enhance weathering by its higher water retention size fraction was deposited by water rather than wind. Th ese fans, (Duniway et al., 2007). Th e distribution of these carbonates therefore, may be more signifi cant sources, rather than sinks, of (generally concentrated on windward slopes of the range) this dust size fraction. suggests that eolian deposition is their primary source (Fig. 8e). Th e concentration of salinity in soils of the southern Fry Mountains decreases from southwest to northeast. Th e source Eolian Dust Flux and Mountain for this salt is probably surface sediments just west of Soggy Th e southern Fry Mountains have a dust concentration of 41 Lake (across the Lenwood fault; Fig. 1). Using the classifi cation kg m−2 on average and a concentration of 156 kg m−2 on windward proposed by Reheis (2006), Soggy Lake is a “dry playa,” slopes (Table 3; Fig. 7). Higher concentrations occur on basin fl oor which means that it is characterized by a depth

SSSAJ: Volume 75: Number 1 • January–February 2011 203

of >10 m (French, 1977; nwis.waterdata.usgs.gov/usa/nwis/ (Boettinger and Southard, 1991) and calcite biomineralization gwlevels, verifi ed 17 Oct. 2010) and has a surface dominated by (Monger et al., 1991), the high concentrations on the windward hard-packed silt and clay. On the western side of the Lenwood sides of the range suggest that eolian deposition and of fault, however, a puff y, surface salt layer was observed aft er a carbonate is a dominant mechanism for the site. Arid mountains summer rainstorm event in June 2006; this is characteristic in the Mojave Desert and elsewhere should, therefore, be viewed of a “wet playa” or at least a transition between a dry and wet as potential signifi cant sinks for inorganic C (especially on playa (Reheis, 2006). Depth to the is approximately windward slopes). 20 m on the eastern side of the fault and 12 m on the western Current global estimates of soil inorganic C (SIC) exclude side, as estimated from widely spaced wells (French, 1977). the direct contribution from arid mountains. For example, the Water-table contours indicate that groundwater fl ows from 1:5,000,000 of the World (FAO-UNESCO, 1975), southwest to northeast near the playa and that the Lenwood which has been used in digitized form to estimate global SIC fault is a hydrologic barrier obstructing this fl ow (French, 1977). stocks (e.g., Batjes, 1996; Eswaran et al., 2000), shows for the Observations of wet playa surface conditions indicate that the Mojave Desert region that inorganic C values were estimated Lenwood fault is indeed obstructing groundwater fl ow and from alluvial soils only. Mountains comprise an estimated 38% causing a rise in the water-table level on its western side. Th e of the map area in this region and between 40 and 50% in other groundwater is probably within 10 m of the surface and able to desert regions of the world (Clements et al., 1957; Goudie, transport salts by capillary rise under high evaporative surface 2002), although mountainous areas have not been quantifi ed conditions, especially aft er summer precipitation events. Th e for all deserts (Cooke et al., 1993). Th erefore, this study used transport of salts by capillary rise is signifi cant because this area 38% as a conservative estimate. Using this value (38% of the can serve as a point source for dust enriched with soluble salts desert land surface area ? 18.9 million km2; relative to other parts of the basin fl oor. Th e process of surface Environment Programme, 1997) and the average CaCO3 of salt concentration aft er wetting–drying events and subsequent the southern Fry Mountains (79 kg m−2 CCE, Table 3), arid deposition downwind has been observed across the Mojave mountains may contain as much as 174 Pg of inorganic C, thus Desert (Reheis, 2006, Reynolds et al., 2007, 2009). increasing the global SIC estimate from 750 Pg (Batjes, 1996) to − High levels of NO3 –N were observed on some 924 Pg. If current estimates have neglected only the diff erence mountaintop and bench landforms within the mountains (Fig. between mountain and piedmont carbonate concentrations (i.e., 8d), in part because of the presence of desert pavement on these 7 kg m−2 CCE; Table 3), arid mountains add a more modest, but landforms (Graham et al., 2008). Soils on other mountaintop still signifi cant, amount (15 Pg) of SIC to the global inventory. and bench landforms (e.g., in the southern part of the range) Although it is uncertain that this alone accounts for the “missing − − contained relatively little NO3 –N. High levels of NO3 –N carbon sink” referred to in global and regional C studies (e.g., were also observed in mountainfl anks at the southern end of the Stephens et al., 2007; Tian et al., 2009), accounting for pedogenic − range (Fig. 8d). As with soluble salts, the NO3 –N distribution carbonate in arid mountains may signifi cantly contribute toward mirrors the relative distribution of the basin fl oor, thus suggesting balancing global C budgets. that the basin fl oor is the primary source for both soluble salts − and NO3 –N in the southern Fry Mountains. In addition, we CONCLUSIONS − −2 −1 measured an average NO3 –N fl ux of 199 mg m yr from Arid mountains are host to diverse geomorphic processes dust trap samples across the range (Table 4). Although the source in the Mojave Desert. Th e results of this study show that the − of the large pool of NO3 –N in the basin fl oor is unclear, desert southern Fry Mountains act as a baffl e against the prevailing winds − − sources of NO3 include precipitation, dry deposition of NO3 and trap eolian sediment from the basin fl oor and piedmont. salts, nitrifi cation of NH4, biological assimilation of atmospheric Th us, these mountains are a sink for signifi cant quantities of −2 −2 − N2 and subsequent mineralization, and N2 fi xation by lightning silicate dust (41 kg m ), soluble salts (172 g m ), NO3 –N (Kumar et al., 1995; Walvoord et al., 2003). (3.3 g m−2), and carbonate (79 kg m−2). Distributions of these constituents on windward slopes of the mountains are controlled Inorganic Carbon and Implications for the Global by landforms (through their land surface characteristics) and Carbon Inventory account for diff erences in soil morphology. Th ey also explain the Th e distribution of inorganic C diff ers somewhat from the diversity of soils across landforms. Where present, lithic contacts − distributions of dust, soluble salts, and NO3 –N (Fig. 7a, 8a, 8c, were deeper and the associated soils showed less morphological and 8e). Carbonate concentrations were as high in the mountains development on steep slopes (e.g., mountainfl anks) than in soils as in other parts of the landscape and, on average, the mountains of gently sloping landforms in the mountains. If the relationships contained about 10% more than the piedmont (Table 3). Values observed in this range hold for other desert regions, arid were especially high in the southern and western portions of mountains are storing massive amounts of carbonate that may the range, reaching 398 kg m−2 CCE (47.8 kg m−2 inorganic add as much as 174 Pg of inorganic C to the global C pool C). Although carbonate can accumulate in soils through the estimate. Th is C appears to be held in long-term storage within direct sequestration of atmospheric C via mineral weathering the soils and fractured bedrock of arid mountain ranges.

204 SSSAJ: Volume 75: Number 1 • January–February 2011 ACKNOWLEDGMENTS sequences of carbonate accumulation in desert soils. Soil Sci. 101:347–360. Th is research was supported in part by a graduate fellowship from Goossens, D. 1988. Th e eff ect of surface curvature on the deposition of : A the UC Kearney Foundation of Soil Science. Intermap Technologies physical model. 15:179–194. Goossens, D. 2000. Dry aeolian dust accumulation in rocky deserts: A medium- generously provided the digital elevation model used in this work. term fi eld experiment based on short-term wind tunnel simulations. Earth Special thanks are given to Brad Hewitt, Josh Barraza, Megan Harlow, Surf. Processes Landforms 25:41–57. Judy Turk, Héctor Estrada-Medina, Ann Rossi, Dr. Brian P. Black, Goudie, A.S. 2002. Great warm deserts of the world: Landscapes and evolution. Jorge Hirmas, Christopher Friesen, Wendell Icenogle, Dr. Rodrigo Oxford Univ. Press, Oxford, UK. 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