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Rates of soil development provide critical information about the types and rates of geomorphic and landscape evolutionary processes. Soil development in arid and semiarid regions of the southwestern United States is predominantly controlled by influx of eolian dust, yet our ability to quantify the rates of morphologic development and of dust and carbonate incorporation is limited by available age control. We describe 10 pedons in the Socorro area of central New Mexico and analyze their silt, clay, and carbonate contents. These soils have well-established direct or indirect age control that we used to estimate average rates of dust and carbonate accumulation over the past ~0.5–800 ka. We also computed the profile development index (PDI) for these soils using 10 common morphologic properties and compare our resulting PDI chronofunction to those from northern and southern New Mexico. We find that the net silt-and-clay content increases in progressively older soils at rates similar to the profile-mass carbonate contents, presumably sourced primarily from eolian dust. Our chronofunction comparison indicates that soil development occurs more rapidly in higher latitude regions of New Mexico than in the drier warmer climates of the Socorro area and southern New Mexico. We interpret the N–S regional trend of soil development considering a regional climate gradient. We conclude that greater mean annual precipitation and cooler mean annual temperatures and/or slower rates of eolian dust accumulation into the soil profile at higher latitude northern sites cause the observed differences in regional soil development. This would promote greater mobility of available silt and clay, and also increase rates of soil formation, as indicated by the presence of argillans in late-Pleistocene soils of northern New Mexico.

A late Pleistocene to Holocene alluvial stratigraphy has been established for the basins of La Paz and San Jose del Cabo, in the southern tip of the Baja California peninsula, Mexico. Six discrete alluvial units (Qt1 through Qt6) were differentiated across the region using a combination of geomorphologic mapping, sedimentological analysis, and soil development. These criteria were supported using radiocarbon, optically stimulated luminescence and cosmogenic depth-profile geochronology. Major aggradation started shortly after ∼70 ka (Qt2), and buildup of the main depositional units ended at ∼10 ka (Qt4). After deposition of Qt4, increasing regional incision of older units and the progressive development of a channelized alluvial landscape coincide with deposition of Qt5 and Qt6 units in a second, incisional phase. All units consist of multiple 1–3 m thick alluvial packages deposited as upper-flow stage beds that represent individual storms. Main aggradational units (Qt2-Qt4) occurred across broad (>2 km) channels in the form of sheetflood deposition while incisional stage deposits are confined to channels of ∼0.5–2 km width. Continuous deposition inside the thicker (>10 m) pre-Qt5 units is demonstrated by closely spaced dates in vertical profiles. In a few places, disconformities between these major units are nevertheless evident and indicated by partly eroded buried soils. The described units feature sedimentological traits similar to historical deposits formed by large tropical cyclone events, but also include characteristics of upper-regime flow sedimentation not shown by historical sediments, like long (>10 m) wavelength antidunes and transverse ribs. We interpret the whole sequence as indicating discrete periods during the late Pleistocene and Holocene when climatic conditions allowed larger and more frequent tropical cyclone events than those observed historically. These discrete periods are associated with times when insolation at the tropics was higher than the present-day conditions, determined by precessional cycles, and modulated by the presence of El Nino-like conditions along the tropical and northeastern Pacific. The southern Baja California alluvial record is the first to document a precession-driven alluvial chronology for the region, and it constitutes a strong benchmark for discrimination of direct tropical influence on any other alluvial record in southwestern North America.

Single grains of K-feldspar from alluvial fan units are dated using a more time-stable signal, the post-infrared infrared stimulated luminescence, or, ‘post-IR IRSL’. A quick measurement protocol is discussed, ‘fast post-IR IRSL,’ that stimulates first with the IR diodes at the lower temperature and then measures grain-by grain at the higher temperature. A criterion is offered for rejecting outlying grains based on hierarchical clustering. Single-grain fading rates are found to diverge from single aliquot fading values, and the fading rates from the brightest subset of grains correspond well with an infinite age cobble and independent age control. Age comparison with a cosmogenic depth-profile age shows agreement at 1σ. The depositional chronology suggests that the climate responsible for regionally-extensive, upper-regime floods which aggraded the older units, transitioned into a climate producing weaker channelized floods around the Late Pleistocene–Holocene transition.

Climate effects on landscape evolution during the Late Pleistocene–Holocene transition (∼14.6–8 ka) in southwestern North America traditionally are linked to the activity of the North American Monsoon and to vegetation change related to a decrease in winter precipitation acting in response to orbital cyclicity. We performed an integrated analysis of regional alluvial fan, lacustrine and paleobotanical records for the area comparing them with hemispheric and regional paleoclimate proxies. Our focus was on the potential role the Tropical Pacific has as a synoptic pattern modulator and moisture source for hydrogeomorphic activity in the region. Our analysis indicates that the onset of alluvial fan aggradation in most of the region at ∼13.5 ka could have been a response to semi-permanent El Nino-like conditions in the Tropical Pacific, which enhanced the frequency of winter frontal storms as well as increased penetration of tropical cyclones in the region. The North American Monsoon was restricted in extent and intensity until ∼7 ka and probably was not a major factor in alluvial fan aggradation. A second stage of alluvial fan aggradation from 11.5 to ∼9 ka was dominated by hyper-concentrated flows and sheet-flood sedimentation, along with deposition in fluvial settings. Storms were probably were linked to landfall of enhanced water vapor bands in the leading edge of winter extra-tropical cyclones with moisture advected directly from the Tropical Pacific. At ∼8 ka, favorable conditions for the occurrence of these storms waned and storm tracks shifted northward. Analysis of modern analogs for storm types described above as prevalent during this period indicates that changes in circulation patterns across the Tropical Pacific can affect storm properties enough to explain the observed geomorphic effects, regardless of other factors traditionally considered of large impact like vegetation change. Our results suggest that the Tropical Pacific plays a larger role than currently thought in landscape evolution of the region.

Stratigraphy and age control of late Pleistocene loess, associated glacial outburst flood deposits and flood-cut unconformities in the Channeled Scabland, Washington State, United States, indicate a significant Cordilleran ice sheet advance during marine Oxygen Isotope Stage 4. Glacial outburst flood deposits from stage 2 (classic Bretz flood deposits, ca 21 to 13 ka) and related features in the Channeled Scabland overlie a widespread layer of loess that contains buried soils and the Mount St. Helens set C tephra (ca 46 ka). This loess in turn overlies deposits of the penultimate episode of giant outburst floods and an unconformity cut by those floods. Regional trends in the thickness, texture, and overall composition of the older loess are strikingly similar to those from the youngest loess, known to be derived from stage 2 flood deposits. We conclude that the older loess also is derived from fine-grained flood deposits. Luminescence ages, tephrochronology, and soil development rates indicate that the bulk of deposition of the older loess occurred during stage 3, following glacial outburst flooding marked by a regional flood-cut unconformity. The apparent cyclical pattern of cold-climate buried soils, flood deposits, and thick loess accumulations demonstrate that sediment supply renewed by flood episodes is a major control on accumulation of loess on glacial timescales.

The measurement of natural dust emissions from desert landforms is crucial in environmental hazard assessment and field checking the accuracy of global dust models. More than 500 individual dust measurements from eight common desert landforms in southern California were collected using the PI-SWERL (Portable In Situ Wind Erosion Lab). The largest emitters of dust are dry washes (13.787 to 0.007 mg m − 2 s − 1 ), dunes, playa margins, distal alluvial fans, and lacustrine beaches. Low emitters include salt-crusted playas (0.692 to 0.002 mg m − 2 s − 1 ), silt–clay-crusted playas, and desert pavements. High emissions are a function of saltating sand that bombards the surface, liberating dust-sized particles for entrainment. Low dust emissions are primarily a function of surface crusting, gravel armoring, and vegetation density. PI-SWERL measurements reveal that emission rates can vary by at least three orders of magnitude, reflecting local variability in soil texture and continuity of surface crusts. Shear-stress partitioning models can be applied to dust data measured by the PI-SWERL to account for large surface roughness features, such as vegetation. The results presented here give an approximation of the contributions to atmospheric dust loading by landforms in the Mojave Desert, and can potentially be used to improve atmospheric dust models.

The time at which deserts established their current arid or hyper-arid conditions remains a fundamental question regarding the history of Earth. Cosmogenic isotope exposure ages of desert pavement and welded, calcic–gypsic–salic Reg soils that developed on relatively flat alluvial surfaces ∼2 Ma ago in the Negev Desert indicate long geomorphic stability under extremely dry conditions. Over a short interval during their initial stage of development between 1–2 Ma, these cumulative soils are characterized by calcic soils reaching maximum stage III of carbonate morphology. This interval is the only period when calcic soil horizons formed on stable abandoned alluvial surfaces in the southern Negev Desert. Since ∼1 Ma pedogenesis changed toward more arid soil environment and the formation of gypsic–salic soil horizons that were later followed by dust accumulation. The dichotomy of only moderately-developed calcic soil (stages II–III) during a relatively long time interval (105–106 years) indicates an arid environment that does not support continuous development but only occasional calcic soil formation. The very low δ18O and relatively high δ13C values of these early pedogenic carbonates support soil formation under arid climatic conditions. Such an environment was probably characterized by rare and relatively longer duration rainstorms which occasionally allowed deeper infiltration of rainwater and longer retention of soil moisture. This, in turn enabled the growth of sparse vegetation that enhanced deposition of pedogenic carbonate. At ∼1 Ma these rare events of slightly wetter conditions ceased and less atmospheric moisture reached the southern Negev Desert leading to deposition of soluble salts and dust deposited in the soils. The combination of long-term hyperaridity, scarcity of vegetation and lack of bioturbation, salts cementation, dust accumulation and tight desert pavement cover, has protected the surfaces from erosion forming one of the most remarkably stable landscapes on Earth, a landscape that essentially has not eroded, but accumulated salt and dust for more than 106 yr.

The Rio Diamante is located in a segment of the southern Central Andes that is transitional in terms of morphology and foreland tectonics (33–37°S). The piedmont reach of the river flows eastwards between the main mountain front and the San Rafael Block (the southernmost foreland uplift to the east of the southern Central Andes). Unlike adjacent rivers, the Rio Diamante has incised into pre-Quaternary bedrock to form a deep canyon across the piedmont. Five fill and strath terraces were mapped, correlated, and surveyed along the ∼35 km piedmont reach to determine their paleo-long profiles. Chronological data for the terraces were provided by geochemical correlation of interbedded tephras to a previously dated ignimbrite, as well as by eight cosmogenic 10Be ages, and suggest a synchronous relationship between fill terrace deposition and glaciation upstream. Terraces are tentatively correlated with oxygen isotope stages 16 (Qt1), 12 (Qt2), 4 (Qt3) and 2 (Qt4 and Qt5). Minor spatial variation in incision along the reach is apparent from the long profile of the Qt2 strath (∼450 ka), which shows low-amplitude folding. The long-term reach average rate of bedrock incision is estimated to be ∼0.1–0.5 m/kyr and the recent shorter term (post-OIS 2) rate is estimated at ∼2 m/kyr. Mid and late Quaternary uplift of the piedmont area is a likely cause for the incision; however, other possible explanations include a delayed response to pre-mid Quaternary uplift, or a response to changes in climate and sediment supply on a tectonically stable portion of the piedmont flanked by subsiding basins. Base-level fall due to subsidence on the eastern side of the San Rafael Block creating a westward migrating knickzone may have contributed to the incision, but the depth of incision is greater than the observed subsidence and no major knickzone is present in the modern river profile of the study reach. Studies such as this on piedmont geomorphic processes are important for understanding the topographic and tectonic transitions that characterise the southern Central Andes.

We correlate Upper Pleistocene glacial and fluvial deposits of the Cinca and Gallego River valleys (south central Pyrenees and Ebro basin, Spain) using geomorphic position, luminescence dates, and time-related trends in soil development. The ages obtained from glacial deposits indicate glacial periods at 85 ± 5 ka, 64 ± 11 ka, and 36 ± 3 ka (from glacial till) and 20 ± 3 ka (from loess). The fluvial drainage system, fed by glaciers in the headwaters, developed extensive terrace systems in the Cinca River valley at 178 ± 21 ka, 97 ± 16 ka, 61 ± 4 ka, 47 ± 4 ka, and 11 ± 1 ka, and in the Gallego River valley at 151 ± 11 ka, 68 ± 7 ka, and 45 ± 3 ka. The times of maximum geomorphic activity related to cold phases coincide with Late Pleistocene marine isotope stages and Heinrich events. The maximum extent of glaciers during the last glacial occurred at 64 ± 11 ka, and the terraces correlated with this glacial phase are the most extensive in both the Cinca (61 ± 4 ka) and Gallego (68 ± 7 ka) valleys, indicating a strong increase in fluvial discharge and availability of sediments related to the transition to deglaciation. The global Last Glacial Maximum is scarcely represented in the south central Pyrenees owing to dominantly dry conditions at that time. Precipitation must be controlled by the position of the Iberian Peninsula with respect to the North Atlantic atmospheric circulation system. The glacial systems and the associated fluvial dynamic seem sensitive to 1) global climate changes controlled by insolation, 2) North Atlantic thermohaline circulation influenced by freshwater pulses into the North Atlantic, and 3) anomalies in atmospheric circulation in the North Atlantic controlling precipitation on the Iberian Peninsula. Our scenario of glacial and fluvial evolution during the Late Pleistocene in northern Spain could be extrapolated to other glaciated mountainous areas in southern Europe.

One of the greatest challenges to the U.S. Department of Defense is maintaining the sustainability of military lands while maximizing their use for training activities. Restoration of disturbed lands in the Mojave Desert has proven to be a challenge, particularly revegetation from seed. This study presents soil moisture dynamics from three restoration sites at the National Training Center, Ft. Irwin, CA. The soil microclimate (matric potential, water balance, and temperature) was monitored beneath various surface treatments to better assess their effects on seed germination and establishment. Results show that under the typically hot and dry climatic conditions that prevail, the soil microclimate is a hostile environment. Irrigation had the most positive effect on the seedbed. Surface mulches including gravel, straw, emulsion, and bark had limited benefits often detrimental. Although seedlings germinated, survival after three months was negligible. Results presented here suggest that successful seeding with broadcast sprinkler irrigation in the central Mojave Desert is not possible unless the seedbed area is limited or advantageous environmental conditions prevail. Improved knowledge of the germination requirements and applied numerical modeling would result in more successful seedbed management.

Dust has long been recognized as an important parameter for testing military vehicles and equipment due to the potential for adversely affecting performance and durability. Vehicle dust test courses were originally established in areas of heavy underlying fine particulate layers (silt and clay) and used for cyclic tests. These soils, known as desert pavements, are geomorphically old and slowly develop through the trapping of atmospheric dust over tens of thousands of years. As such, these soils are both a sink and source for eolian material. However, this dust is a finite resource for military testing at the Yuma Proving Ground (YPG), the Army’s primary environmental hot weather test center. There is concern that the continuous utilization of YPG dust test courses over the past half century has eroded through the overlying dust-rich soil to the extent that the true natural environmental conditions are no longer adequately represented. The objectives of this study were (1) to characterize current dust courses at YPG in order to (2) gain a better understanding of the current state of dust availability and (3) assess the overall sustainability of high dust-potential soils needed for proper military testing. Soil samples collected from dust test courses and adjacent undisturbed soils, as well as dust collected on tactical test vehicles, were analyzed for mineralogy, geochemistry, and physical properties. Results indicate adjacent undisturbed soils exhibit a higher dust potential (higher silt and clay) and elevated reactive salts and carbonates indicating a significant loss of fines from the dust test courses. Routine vehicle traffic and surface preparation has eroded and mixed the dust layer with the underlying sand and gravel-rich horizons, thus depleting the dust test courses of their dust producing potential. Desert pavements are stable geomorphic landforms but the soils have finite dust content when disturbed and therefore are unsustainable for long-term repeated dust testing. Disturbance of these soils may potentially affect ecologically sensitive areas. As such, more geomorphically active areas such as floodplains may provide greater sustainability for long-term testing.

The U.S. Army Yuma Proving Ground is the Department of Defense desert environment test center within the Sonoran Desert of Arizona. The Yuma Proving Ground has ∼320 km of unpaved vehicle test courses that cross a variety of landforms of diverse geologic age and characteristics. The surface materials of the courses ranges from bedrock to silt and their topography varies from steep and rolling to flat. Research presented here aims to provide a systematic characterization of the terrain of eight vehicle endurance and three dust courses so that their comparability with other desert areas of the World may be assessed. Landform and surface cover (upper 1 m) characterization was accomplished by geomorphic mapping based on 1-m resolution IKONOS satellite imagery, 10-m digital elevation models, field verification, and by assimilating pre-existing soil surveys and geologic maps, as well as site-specific investigations. Results provide an assessment of each test course, including information on the landform, geology, surface materials, soil type, degree of desert pavement development, dust content, and percent slope. Data is presented both on individual terrain property maps for each course and in the form of tabulated data for each official milepost marker along the courses. The results for one course area and an example of how they may be used to assess comparability with another desert of interest are presented here with the objective to improve the fidelity of desert testing during material research, development, testing and evaluation prior to deployment in the field.

Salt flux through soils can significantly influence local and global processes. For example, desert soils can atypically concentrate NO 3 - at depth in soil profiles. CaCO 3 precipitation/dissolution can play significant roles as either sinks or sources of global carbon. The objectives of this work were to develop a salt-flux model for long-term (>1000 years) simulations of desert soils and examine the consequences of climate, soils, system inputs, and land-use change on salt movement in arid soils. The field study was conducted at the Nevada Test Site in the northern Mojave Desert. New additions to the CALGYP model allowing for site-specific parameterization included stochastic rainfall model, salt inputs, soil water-holding capacities, and soil CO 2 profiles. New ions added to the model included Na + , K + , Mg 2+ , Cl − , and NO 3 - . About 81% of Ca 2+ input remained within the surface 1.0 m of soil as CaCO 3 , which argues in favor of soil CaCO 3 serving as a recalcitrant sink for global carbon. In contrast, ≈99.96% of Na + , K + , Mg 2+ , Cl − , NO 3 - , and SO 4 2 - ions leached to soil depths >1.0 m and 94.3% leached to soil depths >2.0 m. This is true despite only 1.64% of the rainfall leached beyond 1.0 m and 0.020% of the rainfall leached beyond 2.0 m. The leachability of NO 3 - and Cl − to soil depths > 2.0 m agrees with NO 3 - and Cl − accumulations at depth in Mojave Desert soils (1.3–2.7 m). Simulation of extreme events and years with a stochastic rainfall model and accurate soil water-holding capacities are critical for modeling water and salt flux through soils.

A Holocene alluvial morphostratigraphy, based on geomorphological mapping, sedimentological analysis, soil development and radiocarbon chronology is presented for the semiarid Blanca basin in the Bardenas Reales Natural Park (NE, Spain). Four morphopedosedimentary alluvial units (MU1, MU2, MU3 and MU4) were differentiated, which may correspond to two palaeogeographical stages. The first one includes the morphopedosedimentary unit MU1 and is the most extensive in the area. It is made up of a complex aggradation (fill up) sequence comprising a braided channel system, a sand–mud flat (9200–7900 cal BP) and a playa-lake environment related to a nearly closed depression. An intensive fluvial entrenchment period is recognized, leading the opening of the Blanca basin, before the second stage that includes the units MU2, MU3 and MU4. This set of units forms a sequence of cut-and-fill and nested fill terraces recording the alluvial activity in the Blanca basin during the last 6 ka. The MU2 unit, 5900–3300 cal BP in age, represents a period of high geomorphic instability with rapid alluvial sedimentation/channel incision oscillating changes. On the other hand, the MU3 unit has an age of 1050–500 cal BP and appears as a thin extensive alluvial cover overlying previous units. Finally, the MU4 unit comprises superposition of numerous flood depositional sequences, centimetric in scale, dated from 450–10 cal BP. The established soil chronosequence supports the numerical chronology of the four units. These accumulation periods (MU1, MU2, MU3, MU4) are associated with dry phases and/or high hydrological variability (floods and droughts) at a regional scale. Changes on the North Atlantic atmospheric circulation mode over Southern Europe may provide the key to explain the nature of alluvial activity during Holocene in Bardenas Reales., Projects BTE2003-01296 and CGL2006-08973/BTE of the Spanish Government and the European Regional Development Fund have supported this work. We thank Alejandro Urmeneta (Junta de Bardenas Reales de Navarra) for collaboration with permissions and technical support in the field and Sophie Baker for the assistance with editing this manuscript. This is a contribution by PALEOQ and Cuencas Sedimentarias Continentales groups (Aragón Regional Government).

The impact of large twentieth century floods on the riparian vegetation and channel morphology of the relatively wide anabranching and braided Nahal Arava, southern Israel, was documented as part of developing tools to (a) identify recent large floods, (b) determine these flood's respective magnitudes in alluvial ungauged streams, and (c) determine long-term upper bounds to flood stages and magnitudes. Along most of its course Nahal Paran, a major tributary that impacts the morphology, floods and sediments of Nahal Arava at the study reach, is a coarse-gravel, braided ephemeral stream. Downstream of the Arava–Paran confluence, aeolian and fluvial sand delivered from eastern Arava valley alters the channel morphology. The sand has accreted up to 2.5 m above the distinct current channels, facilitating the recording of large floods. This sand enhances the establishment of denser riparian vegetation (mainly Tamarix nilotica and Haloxylon persicum ) that interacts with floods and affects stream morphology. A temporal association was found between specific floods recorded upstream and tree-ring ages of re-growth of flood-damaged tamarix trees (‘Sigafoos trees’) in the past 30 years. This association can be utilized for developing a twentieth century flood chronology in hyperarid ungauged basins in the region. The minimum magnitude of the largest flood that covered the entire channel width, estimated from flood deposits, is approximately 1700–1800 m 3 s − 1 . This is a larger magnitude than the largest gauged flood of 1150 m 3 s − 1 that occurred in 1970 about 30 km upstream in Nahal Paran. Our estimation agrees with flood magnitude estimated from the regional envelope curve of the largest floods. Based on Holocene alluvial stratigraphy and OSL dating in the study reach we also conclude that flood stages did not reach the late Holocene (∼ 2.2 ka) surface and therefore we estimate a non-exceedance upper bound of ∼ 2000 m 3 s − 1 flood magnitudes for Nahal Arava during that interval. This study indicates that in unfavorable areas the combination of hydrology, fluvial morphology and botanic evidence can increase our understanding of ungauged basins and give information crucial for hydrology planning.

Av horizons found in desert pavement environments are known to evolve pedogenically over geologic time. This study was conducted to determine whether increased pedogenic development of the Av (vesicular) horizon over relative time impacts the hydraulic properties of individual soil peds and the mechanism of infiltration as inferred by dye patterns. We examined peds from the Av horizons associated with desert pavements that mantled three different alluvial deposits with different relative surface ages (Qf5 (∼ 10 ka), Qf3 (∼ 50–100 ka), and Qf2 (∼ 10–50 ka)) and included an additional surface (Qf6 (∼ 4 ka)) for the dye studies. We hypothesized that increases in the development of the Av over time would lead to a more structured soil surface with greater potential flow between soil peds and lower hydraulic conductivity of the soil peds themselves. Results showed that average K s and α of the Qf5 peds were significantly greater than estimated for the Qf2 and Qf3 peds. Although K s was greater for the Qf5 peds, the steady-state infiltration rate was equal for the Qf3, Qf2, and Qf5 surfaces, perhaps indicating a reduction in matrix flow through soil peds and an increase in interped flow between soil peds. Studies were also conducted using dyed water and a tension disc infiltrometer, set to saturation. Following the tests the soil was excavated in 2-cm increments and photographed. The dye patterns for the Qf6 indicated that the water moved rapidly through the soil matrix into deeper soil, and without any preferential flow around peds. The Qf5 exhibited more dye-stained soil at depth than seen in the Qf2 and Qf3, perhaps due to higher ped conductivity. Qualitative observations on the Qf2 and Qf3 suggest that water flowed primarily along the ped faces and then toward the ped interiors. The results suggest that infiltration through Av horizons evolves from a matrix-dominated process on the younger soils to a preferential flow-dominated process on older surfaces.

Interrelated, biotic (flora and fauna) and abiotic (pedogenesis and hydrology) processes were examined at four sites (30, and approximately 1000–3000, 7000–12 000, and 125 000 years before present) in the northern Mojave Desert. Data collected at each included floral and faunal surveys; soil texture, structure, and morphology; and soil hydraulic properties. Separate measurements were made in shrub undercanopy and intercanopy microsites. At all sites, shrubs made up greater than 86 percent of total perennial cover, being least on the youngest site (4 percent) and most on the 7000–12 000-year-old site (31 percent). In the intercanopy, winter annual density was highest on the 1000- to 3000-year-old site (249 plants/m2) and lowest on the oldest site (4 plants/m2). Faunal activity, measured by burrow density, was highest on the 1000–3000- and 7000–12 000-year-old sites (0.21 burrows/m2) and density was twice as high in the undercanopy versus the intercanopy. Burrow density was lower at the two oldest sites, although density was not statistically greater in the undercanopy than intercanopy. At the older sites, the soil water balance was increasingly controlled by Av horizons in intercanopy soils in which saturated hydraulic conductivity (Ksat) decreased 95 percent from the youngest to the oldest site. No significant reduction in Ksat in undercanopy soils was observed. Decreases in the intercanopy sites correlated with decreases in annual plant density and bioturbation, suggesting these processes are interrelated with surface age.

Military training exercises at the National Training Center (NTC), Ft. Irwin, California have led to the degradation of large areas of the Mojave Desert. Soil physical properties in areas subjected to low and high disturbance at interspace and plant (Larrea tridentata) mound microsites were quantitatively compared to undisturbed surfaces of both well-developed soils formed on Pleistocene age alluvial deposits and weakly developed soils formed on Holocene age alluvial deposits. Results indicate that geomorphically young and old soils differ in hydraulic and pedogenic properties with and without anthropogenic disturbance. Multi-tension infiltrometers showed that young coarser textured surfaces had saturated conductivities (Ksat) that were nearly twice those of older surfaces. Mound microsites were more sand-rich than interspace locations although Ksat was not significantly different. After both low and high disturbance by tracket vehicles, bulk density was significantly increased from 1.54 to 1.62 and 1.80 g cm−3, respectively. Similarly, penetration resistance increased following low and high disturbance from 1.32 to 1.47 and 3.90 kg cm−2, respectively. Although low disturbance increased Ksat, high disturbance resulted in significant reductions in both Ksat (2.4 to 1.3 cm h−1 (p

Isotopic signatures of soil components are commonly used to infer past ecologic and climatic shifts in the soil record. The theory behind the fractionation of isotopes that occurs during ecosystem processes is well understood; however, few isotopic studies have explored ecosystem relationships in modern soils. We discuss relationships of stable carbon isotopic signatures in plant tissue, soil organic carbon (SOC), laboratory-respired CO2, and modern carbonates at 10 sites (seven containing pedogenic carbonates) along a C3-dominated climatic gradient (mean annual precipitation (MAP) ranging from 200 to 1000 mm) in the Palouse region of eastern Washington state. A horizon soil organic carbon (SOC) d 13 C values varied from � 24.3xto � 25.9xPDB. Values in the arid portion of the gradient (200 to approximately 500 mm MAP) generally decreased and linear regression of SOM 13 C vs. MAP was significant (r 2 =0.71,p=0.02). Trends in plant- 13 C of two grass species (Agropyron spicatum andFestuca idahoensis) found throughout this portion of the gradient were similar to that of SOC. Mean pedogenic carbonate d 13 C values varied from � 4.1xto � 10.8x PDB. Linear regression was significant for carbonate 13 C vs. MAP (r 2 =0.79, p=0.007), estimated above-ground productivity (r 2 =0.88, p=0.002) and soil carbon content (r 2 =0.83, p=0.004). Carbonate d 13 C values at the most arid site exhibited higher variability than other sites (presumably due to greater spatial variation in plant respiration vs. atmospheric diffusion). Our data suggest that carbon isotopic relationships among ecosystem components may prove useful in determining ecosystem level properties in modern systems, and potentially in ancient systems as well. D 2004 Elsevier B.V. All rights reserved.

The vesicular, fine-grained A horizon (Av) is the widespread, ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages. Now known to form mostly through accumulation of eolian dust, recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation. Changes in the magnitude of eolian activity and effective leaching related to Quaternary climatic changes are also hypothesized to have influenced the evolution of the Av horizon. Numerical modeling, geochronologic, and field/laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of Av, how the changing Av horizon increasingly influences soil infiltration and carbonate translocation and accumulation, and the control that clasts of the evolving pavement exert on pedogenic processes. Results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived (eolian) material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by Av horizon formation. The early development of a weakly to moderately developed Av horizon directly over gravelly parent material in late and middle Holocene soils moderately influences soil infiltration, but significant leaching of very soluble materials and some carbonate in dust are permitted. In older, Pleistocene soils, however, the texturally more mature Av and underlying, cumulic nongravelly horizons more strongly limit the rate and depth of leaching, and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material. Other aspects of Av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate, referred to as carbonate collars. Development of a numerical model that integrates soil hydrology, a CO2 production–diffusion model, calcite kinetics and thermodynamic considerations, composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation. Model results, combined with results of δ13C and δ18O analyses of collar carbonate, demonstrate how precipitation of calcite on pavement clasts and within the Av is favored at a depth much shallower than that indicated by the classic carbonate depth–climate relationship of Jenny and Leonard [Jenny, H.J., Leonard, C.D., 1935. Functional relationships between soil properties and rainfall. Soil Science 38, 363–381] and Arkley [Arkley, R.J., 1963. Calculations of carbonate and water movement in soil from climatic data. Soil Science 96, 239–248], or simulated by numerical models of carbonate accumulation. Simultaneous development of thick carbonate collars and the Av horizon requires the sustained pavement clast–Av horizon coupling for at least centuries to possibly millennia. New thermoluminescence ages also indicate that much of the Av horizon formed in the Holocene, and that it is certainly much younger than the older Pleistocene pavements. This supports the previously proposed hypothesis that increased dust flux during the Pleistocene-to-Holocene transition triggered and/or greatly accelerated Av horizon development. An understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments, but it also provides important clues into processes of pavement evolution and preservation of Av horizons during long glacial periods. The Av horizon is not merely an insignificant surficial zone of recent dust accretion; instead, its development profoundly influences the genesis of desert soils and pavements.

21 Ne depth profiles measured from amalgamated samples of sand-sized quartz from the Quaternary Bandelier Tuff were used to estimate inheritance-corrected exposure ages in stream terraces and an alluvial fan on the Pajarito Plateau. Stable alluvium and associated soils display a downward decreasing (Type I) nuclide profile pattern. Bioturbated colluvium with uniform (Type III) nuclide concentrations has buried these alluvial surfaces. Another possible profile type with downward increasing nuclide concentrations (Type II) is described but was not found on the Pajarito Plateau. We present mathematical models for each of the profile patterns relating nuclide concentration to inheritance, time-averaged bulk density, and either exposure time or burial rate. Curve fits of Type I profiles are used to estimate the age of alluvial landforms. A burial correction technique for deposits buried by younger colluvium is also presented. Inheritance- and burial-corrected 21 Ne surface exposure ages of 144±17 ka and 63±8 ka for two Pajarito Plateau stream terraces agree within error with independent soil profile development index dates of 160 ka and 68–78 ka. A soil overlying the 50–60 ka El Cajete pumice yields an inheritance-corrected 21 Ne exposure age of 44±11 ka that also agrees closely with a 14 C age on soil charcoal of 41 ka. These results demonstrate that reliable cosmogenic 21 Ne exposure ages can be obtained for fine-grained alluvial surfaces with the depth profile technique.

We used a process-based soil-water balance model to simulate the downward flux of soil-water under varied conditions of climate, vegetation, and soil texture to determine the potential impact of episodic periods of wetter (pluvial) climate during the Holocene on calcic soils in the Mojave Desert that have a bimodal distribution of carbonate. Daily weather data associated with a relatively “wet” climate (years with extreme increases in annual rainfall, ∼ 33 cm/yr) and “dry” climate (historic average annual rainfall, ∼ 15 cm/yr) was used to simulate the affects of Pleistocene and Holocene climate change on soil-water balance. Linkages among atmospheric circulation patterns, regional increases in precipitation, and historic flooding in the Mojave Desert, California, suggest that historic wet years provide an analog for wetter climates that occurred during the last glacial period (latest Pleistocene) and episodically during Holocene periods of pluvial activity. Modeling results indicate that soil-water balance for dry and wet years strongly corresponds with the upper and lower zones of carbonate accumulation respectively. Soil-water only reached the lower zone of carbonate during a wet year when extreme increases in winter/spring storm activity resulted in a significant increase in precipitation and the downward flux of soil water. The linkage between increases in frontal storm activity and pluvial events suggests that the shallow zone of the bimodal distribution of carbonate is a result of periods of significant decreases in winter and spring rainfall and not primarily due to increases in Holocene temperature or the development of clay-rich horizons. Calculation of carbonate solubility and accumulation rates suggests that the bimodal distribution of carbonates in soils may have also been impacted by episodic periods of extreme increases in precipitation associated with perennial lakes during the Holocene. Results suggests that much of the carbonate in the upper 75 cm of Pleistocene soils may have accumulated during the late Holocene rather than throughout the entire Holocene.

A model is presented describing the factors and processes which determine the measured C-14 ages of soil calcium carbonate. Pedogenic carbonate forms in isotopic equilium with soil CO2. Carbon dioxide in soils is a mixture of CO2 derived from two biological sources: respiration by living plant roots and respiration of microorganisms decomposing soil humus. The relative proportion of these two CO2 sources can greatly affect the initial C-14 content of pedogenic carbonate: the greater the contribution of humus-derived CO2, the greater the initial C-14 age of the carbonate mineral. For any given mixture of CO2 sources, the steady-state (14)CO2 distribution vs. soil depth can be described by a production/diffusion model. As a soil ages, the C-14 age of soil humus increases, as does the steady-state C-14 age of soil CO2 and the initial C-14 age of any pedogenic carbonate which forms. The mean C-14 age of a complete pedogenic carbonate coating or nodule will underestimate the true age of the soil carbonate. This discrepancy increases the older a soil becomes. Partial removal of outer (and younger) carbonate coatings greatly improves the relationship between measured C-14 age and true age. Although the production/diffusion model qualitatively explains the C-14 age of pedogenic carbonate vs. soil depth in many soils, other factors, such as climate change, may contribute to the observed trends, particularily in soils older than the Holocene.

Variable rates of loess deposition contributed to dramatic regional variation in a soil-stratigraphic unit, the Washtucna Soil, in the Palouse loess deposits in the Channeled Scabland of eastern Washington state. Throughout most of the Channeled Scabland, the morphology of the Washtucna Soil is that of a single buried soil, but it bifurcates into two well-developed and pedologically distinct buried soils in areas immediately downwind of the major source of loessial sediment. Regional loess stratigraphy confirms that the two well-developed soils formed during the same interval of time during which only one soil formed in areas that are distal to loess source areas. The variable and perhaps rapid rates of soil formation suggested by the stratigraphy resulted from an interaction between variable rates of loess deposition and the formation of superimposed calcic soils. Petrocalcic horizons with weak Stage IV morphology formed as the zone of carbonate accumulation moved up into former A and cambic horizons that had been profusely burrowed by cicadas. The development of cicada burrows in one phase of soil development that were subsequently engulfed by pedogenic carbonate under a rising land surface seems to have greatly accelerated the development of the petrocalcic horizons. Accelerated rates of formation of the petrocalcic horizons occurred when extrinsic (pulses of loess deposition) and intrinsic (engulfment of burrowed horizons) thresholds were exceeded. Stratigraphic evidence suggests that the soil formation that accompanied the rise in the land surface due to additional loess deposition may have occurred during the late Wisconsin glaciation when giant glacial outburst floods in the channeled Scabland triggered a new cycle of loess deposition.