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All exposed rocks on Earth's surface experience erosion; the fastest rates are documented in rapidly uplifted monsoonal mountain ranges, and the slowest occur in extreme cold or warm deserts-millennial submeterscale erosion may be approached only in the latter. The oldest previously reported exposure ages are from boulders and clasts of resistant lithologies lying at the surface, and the slowest reported erosion rates are derived from bedrock outcrops or boulders that erode more slowly than their surroundings; thus, these oldest reported ages and slowest erosion rates relate to outstanding features in the landscape, while the surrounding landscape may erode faster and be younger. We present erosion rate and exposure age data from the Paran Plains, a typical environment in the Near East where vast abandoned alluvial sur-faces (10 2 -10 4 km 2 ) are covered by well-developed desert pavements. These surfaces may experience erosion rates that are slower than those documented elsewhere on our planet and can retain their original geometry for more than 2 m.y. Major factors that reduce erosion converge in these regions: extreme hyperaridity, tectonic stability, fl at and horizontal surfaces (i.e., no relief), and effective surface armoring by a clast mosaic of highly resistant lithology. The 10 Be concentrations in amalgamated desert pavement chert clasts collected from abandoned alluvial surfaces in the southern Negev, Israel (representing the Sahara-Arabia Deserts), indicate simple exposure ages of 1.5-1.8 Ma or correspond to maximum erosion rates of 0.25-0.3 m m.y. -1 . The 36 Cl in carbonate clasts, from the same pavement, weathers faster than the chert and yields simple exposure ages of 430-490 ka or maximum erosion rates of 0.7-0.8 m m.y. -1 . These ages and rates are exceptional because they represent an extensive landform. The 10 Be concentrations from samples collected at depth and optically stimulated luminescence (OSL) dating reveal a two-stage colluvial deposition history followed by eolian addition of 40 cm of silt during the past 170 k.y. Our results highlight the effi ciency of desert pavement armor in protecting rocks from erosion and preserving such geomorphic surfaces for millions of years.

Alluvial fan deposits along the Providence Mountains piedmont in the eastern Mojave Desert that (1) are derived from diverse rock types, (2) are dated with luminescence techniques and soil-stratigraphic correlations to other relatively well dated fan, eolian, and lacustrine deposits, and (3) have some of the highest peaks in the Mojave Desert, provide a unique opportunity to study the influence of Pleistocene-Holocene climatic transition on regional fan deposition across diverse geomorphic settings. Geomorphic and age relations among alluvial and eolian units along the Providence Mountains and Soda Mountains piedmonts indicate that most of the late Quaternary eolian and alluvial fan units were deposited during similar time intervals and represent region-wide changes in geomorphic factors controlling sediment supply, storage, and transport. Deposition of alluvial fans in the desert southwestern United States during the latest Pleistocene has been largely attributed to (1) a more humid climate and greater channel discharge and (2) time-transgressive changes in climate and an increase in sediment yield. Stratigraphic and age relations among depositional units demonstrate that a regional period of major alluvial fan deposition occurred between ca. 9.4 and 14 ka, corresponding with the timing of the Pleistocene-Holocene climatic transition. This age range indicates that deposition of these fans is not simply a result of greater effective moisture and channel discharge during the last glacial maximum. Increases in sediment yield during the Pleistocene-Holocene transition have been largely attributed to a timetransgressive decrease in vegetative cover with an increase in hillslope erosion. Geomorphic relations along the Providence Mountains, however, suggest that that changes in vegetation cover during the Pleistocene-Holocene climatic transition may have had a limited impact on hillslope instability and sediment yield because of (1) the inherently high infiltration capacity of coarse-textured soils and colluvium, (2) possible strong spatial variations in soil cover across hillslopes, and (3) modern vegetation cover appears to provide enough stability for the buildup of soils and colluvium. An increase in sediment yield may instead be largely due to an increase in extreme storm events, possibly an increase in tropical cyclones. Extreme storms would provide the rainfall intensity and duration to mobilize permeable sediments from mountain catchments and into distal fan areas.

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packed gravel that overlies a thin (3-10 cm), fine-grained, gravel-poor, vesicular A (Av) soil horizon. 1 Desert pave-Desert pavements are prominent features in arid environments ments are prominent features in arid environments and and consist of a surface layer of closely packed gravel that overlies a thin, gravel-poor, vesicular A (Av) soil horizon. Well-developed Av can be found on a variety of landforms of significantly horizons form distinct and highly structured columnar peds. These diverse ages ranging from Holocene to Tertiary (Bull, structures, along with their silt-and clay-rich texture, are hypothesized 1991; Cooke et al., 1993). Pavements have been used in as controlling infiltration and hence the overall hydrologic conditions subdividing and correlating Quaternary alluvial fans for in the soil profile. The objectives of this study were to (i) evaluate studying neotectonics and Quaternary climate change how pedological development in near-surface soil horizons in an arid (McFadden et al., 1998; Bull, 1991; McDonald et al., alluvial fan complex affects the soil hydraulic characteristics, and (ii) 2003). Other research has shown that age is an important to compare the use of Wooding's equation and inverse modeling for consideration in development of desert pavements, esevaluating hydraulic conductivity in highly layered, near-surface soils. pecially in areas downwind of source zones for aeolian These objectives were approached through field tension infiltrometer deposited material (McDonald, 1994; McDonald et al., studies, soil sampling, and laboratory analyses of soil texture, water content, and soluble salt concentrations. Soils at five sites were studied 1996; McFadden et al., 1998). Pavements tend to be more at the Mojave National Preserve, California, representing a soil chro-prevalent and more strongly developed on older surnosequence (50-100 000 yr) with varying degrees of desert pavement faces where aerosolic clay-and silt-sized particles are development. Results indicated 100-fold and threefold declines, redeposited on the surface and are subsequently transspectively, in saturated hydraulic conductivity (K s ) with both analytical located downward into the soil profile. Increasing accumethods, and ␣ w using Wooding's method, as the soils aged. No clear mulation of aerosolic fines with time enhances the develtrends in K s or ␣ w were detected in the underlying horizon, indicating opment of a highly structured Av horizon consisting of that the controlling feature at these sites, in terms of water entry, was distinct columnar-shaped peds, ranging in diameter from the K s of the surface (Av) horizon. Soluble salt concentrations within about 3 to 8 cm, that part to platy peds, ranging in thickthe profile indicated reduced infiltration with increased pavement ness from 0.2 to 1 cm. The formation of desert pavement development. Results showed that surface age can be used as an excellent predictor of saturated hydraulic conductivity (r 2 ϭ 0.9254).and the Av horizon is extremely slow and can take from...

In the arid southwestern United States, subtle differences in soil horizon development affect seasonal soil hydrology and consequently influence plant performance and community structure. We measured canopy development, population structure, and seasonal ecophysiology (predawn water potential, pd , and midday net photosynthetic assimilation, A net ) of two co-dominant warm-desert shrubs, the evergreen Larrea tridentata and drought-deciduous Ambrosia dumosa, in five Mojave Desert soils varying in surface and sub-surface soil development, and we used process-based soil hydrology modeling output to determine longer-term soil water dynamics underlying soil/plant responses. We hypothesized that ecophysiological performance would covary with plant development, which would reflect soil hydrological characteristics.Among three sites on alluvial fan deposits of different geological ages (Young Alluvial, Ͻ4000 yr BP; Intermediate Alluvial, ϳ12 000 yr BP; Old Alluvial, ϳ40 000 yr BP), total canopy volume of Larrea (cubic meters per 100 m 2 ground area) was highest on the Young Alluvial site, in close agreement with soil modeling results showing that these coarsetextured, weakly developed soils permit deeper water infiltration. In older, stronger developed soils, infiltration and persistence of soil water was sharply reduced, which was reflected by lower individual Larrea plant volumes. However, during peak spring conditions, pd and A net were highest in Larrea at the Intermediate Alluvial site (Ϫ4.2 Ϯ 0.32 MPa and 3.2 Ϯ 0.91 mol·m Ϫ2 ·s Ϫ1 ), where soils had substantial surface and subsurface horizons, and at the Pavement site, where soils had strong surface layers but little subsurface development. Concurrent plant performance at the Young Alluvial site was unexpectedly low (Ϫ4.8 Ϯ 0.49 MPa and 1.7 Ϯ 0.56 mol·m Ϫ2 ·s Ϫ1 , respectively). During summer drought pd and A net remained high in Intermediate Alluvial plants, but were extremely low in Pavement site Larrea (Ϫ8.17 MPa and Ϫ0.04 mol·m Ϫ2 ·s Ϫ1 , respectively), due to curtailed infiltration of summer precipitation. These findings suggest that Larrea growing in older soils experience greater mortality and reduced growth but are not subject to strong intra-specific competition resulting from the persistence of large individuals apparent in younger, coarser textured soils.In contrast to Larrea, density of Ambrosia increased with soil horizon development, but smaller plant sizes resulted in similar canopy volume per area, and identical pd and A net across all soils where it occurred, suggesting greater plasticity to the transmittal of precipitation. These findings show that a strong geomorphology and soils context is essential for understanding the variation in plant responses and vegetation structure in desert environments.