Gobbledygook Research Articles

Landscape evolution and development of eolian-modified unconsolidated gravel surfaces and yardangs in the Hami Basin, China

Unconsolidated gravel surfaces, alluvial terraces, and yardangs are widespread in the Turpan-Hami topographical depression of NW China. These landscape features provide an opportunity to investigate the wind erosion history and landscape evolution of the Hami Basin. Along with anomalously high near-surface wind speeds, these features suggest that the Hami Basin was an important source of mineral dust during the Pleistocene. We present the results of an integrated geomorphologic, stratigraphic, and chronologic study on the genesis of unconsolidated gravel surfaces and yardang fields in the Hami Basin. Mobilization of silt- to sand-sized fractions from the surfaces of the Quaternary alluvial sediments occurred under generally dry conditions, leaving unconsolidated gravels to organize into inflationary-type desert pavement surfaces (Type-1) and deflationary-type desert pavements surfaces (Type-2). We propose that the Pleistocene alluvial terraces of the Hami Basin formed principally during wetter intervals, which are assumed to correspond with interglacial periods. The coarse-grained alluvium atop terraces protected underlying strata from wind erosion. The bedrock then began to be sculpted into yardangs after the removal of the coarse-grained armoring surfaces. With mostly sub-horizontal bedding, the geometries of yardangs was controlled by lithology, wind and water erosion. Based on UAV-3D models, optically-stimulated luminescence (OSL) and electron spin resonance (ESR) ages, we find an erosion rate of ~0.23 mm/yr since ~200 ka, This, combined with widespread uneroded strata armored by unconsolidated gravel surfaces, indicates that the Hami Basin had considerable dust emission potential in the past.

Processes of Paleoindian site and desert pavement formation in the Atacama Desert, Chile

AbstractA distinct feature of many of the earliest archaeological sites (13,000-11,200 cal yr BP) at the core of the Atacama Desert is that they lie at or just below the surface, often encased in desert pavements. In this study, we compare these sites and undisturbed desert pavements to understand archaeological site formation and pavement development and recovery. Our results indicate these pavements and their soils are poorly developed regardless of their age. We propose that this is because of sustained lack of rain and extreme physical breakdown of clasts by salt expansion. Thus, the core of the Atacama provides an example of the lower limits of rainfall (<50 mm/yr) needed to form desert pavements. At site Quebrada Maní 12 (QM12), humans destroyed the pavement. After abandonment, human-made depressions were filled with eolian sands, incorporating artifacts in shallow deposits. Small and medium-sized artifacts preferentially migrated upwards, perhaps due to earthquakes and the action of salts. These artifacts, which now form palimpsests at the surface, helped – along with older clasts - to restore surface clast cover. Larger archaeological features remained undisturbed on top of a deeper Byzm horizon. The vesicular A horizons (Av horizons) have not regenerated on the archaeological sites due to extreme scarcity of rainfall during the Holocene.

Origin, structure and geochemistry of a rock glacier near Don Juan Pond, Wright Valley, Antarctica

AbstractThe South Fork of Wright Valley contains one of the largest rock glaciers in the McMurdo Dry Valleys, Antarctica, stretching 7 km from the eastern boundary of the Labyrinth and terminating at Don Juan Pond (DJP). Here, we use results from ground-penetrating radar (GPR), qualitative field observations, soil leaching analyses and X-ray diffraction analyses to investigate rock glacier development. The absence of significant clean ice in GPR data, paired with observations of talus and interstitial ice influx from the valley walls, support rock glacier formation via talus accumulation. A quartz-dominated subsurface composition and discontinuous, well-developed desert pavements suggest initial rock glacier formation occurred before the late Quaternary. Major ion data from soil leaching analyses show higher salt concentrations in the rock glacier and talus samples that are close to hypersaline DJP. These observations suggest that DJP acts as a local salt source to the rock glacier, as well as the surrounding talus slopes that host water track systems that deliver solutes back into the lake, suggesting a local feedback system. Finally, the lack of lacustrine sedimentation on the rock glacier is inconsistent with the advance of a glacially dammed lake into South Fork during the Last Glacial Maximum.

Runoff generation, rill erosion and time-scales for hyper-arid abandoned alluvial surfaces, the Negev desert, Israel

Sequences of abandoned alluvial surfaces are ubiquitous desert landscapes considered as responding to Quaternary-timescale climatic fluctuations and tectonic activity. The post-depositional evolution surficial properties of these alluvial surfaces change with time: (a) surface clast breakdown and development of desert pavement, (b) dust and salts accumulation (c) Reg (gravelly) desert soil evolution.In the present study, we focus on the hydrological response and the resulting erosional processes, to these changes in surface properties over a sequence of late-Pleistocene-Holocene, abandoned alluvial surfaces in Nahal Evrona catchment at the hyperarid southern Arava Valley north of the towns of Eilat (Israel) and Aqaba (Jordan).The methods used in the study are: (a) simulated rainfall experiments to determine hydrological characteristics, (b) Terrestrial Laser Scanning (TLS) to determine surface roughness, (c) OSL dating for the chronology, (d) surface and soil descriptions and measurements and grain-size analyses to determine texture of the uppermost Av horizon of the Reg soil (e) GIS analysis of the new drainage network developing over the older abandoned alluvial surfaces.The results show that with time, roughness decreases, infiltration decreases to about 8–9% relative to present rates, point runoff generation increases >5 fold, and as a result, rill and gully erosion initiated, finally leading to the destruction of the alluvial surface. Under the current hyper-arid climate in the southern Negev desert, between ~30 and 50 ka of surface weathering and soil development are required before significant runoff is generated and >100 ka until effective runoff initiates rill and gully erosion over these abandoned alluvial surfaces. This local-source runoff and gully erosion essentially destroys the alluvial surfaces in the southern Arava valley within a period of 400–500 ka. Roughness, extracted from spaceborne Lidar scanning, which is inversely related to surface age, directly reduces friction and resistance to runoff, can provide an estimate for the hydrological characteristics of the surfaces. This can support applications such as rainfall-runoff models, risk assessment, etc.

Spatial heterogeneity of surface sediment grain size and aeolian activity in the gobi desert region of northwest China

Studies have previously discussed characteristics of aeolian activity for different gravel densities on gobi surfaces. However, the significance of the spatial heterogeneity of its surface to sand flow over its surface and to dust emissions from the region has not been clearly demonstrated. This is also the case for spatial variation in the aeolian environment. Accordingly, this study established two observation transects, both through the desert pavement area and the dune field in the gobi desert region of northwest China, to measure surface sediment grain size, wind speed and sand transport along these transects. Results showed that in areas where surface gravel content decreased and mobile sand dunes increased, surface sediments were better sorted and grain-size distribution changed from multimodal to roughly unimodal. Furthermore, sand transport increased significantly and properties of aeolian activity varied markedly within a short distance. This extreme variation in both grain-size parameters and in sand transport indicated that strong spatial heterogeneity exists in the aeolian environment of the study area. Furthermore, dune fields within gobi area are likely the main cause of sand flow over its surface as well as dust emissions from the region itself. The findings in this study have universal applicability in gobi regions with uneven surface properties in other arid areas.

Population dynamics of dasyurid marsupials in dryland Australia: Variation across habitat and time

AbstractThe irruptive population dynamics of rodents are a globally renowned wildlife phenomenon; however, the dynamics of other small mammals with which rodents are sympatric are poorly understood. Dryland Australia supports a high diversity of small (<200 g) arthropod‐eating marsupials (Dasyuridae). Here, we test the hypothesis that dasyurid marsupials do not exhibit the same degree of irruptive population dynamics that are shown by rodents. We addressed this question by sampling small mammal assemblages on 20 permanent trapping sites in the Simpson Desert on 20 occasions from 2007 to 2017. Sampling was stratified across three broad habitat types: sandridge, gibber plain and clay plain. We captured 478 dasyurid marsupials of nine species, ranging in mean body mass from 5.75 to 93.50 g, at a capture rate of 1.71 per 100 trap‐nights. Capture rate varied across habitat and over time and the interaction between these two effects was also significant. Capture rate was highest on clay plain (3.35 captures/100 trap‐nights), followed by gibber plain (2.16 captures/100 trap‐nights) and lowest on sand habitat (0.54 captures/100 trap‐nights). Each species had a clear preference for one of the dominant habitat types. Dasyurid assemblages responded to high rainfall pulses in November–December 2008 and January 2015; however, the largest rainfall period in 2010–2011 resulted in very low captures. Likewise, a peak in abundance occurred in April 2008 although it was not preceded by high rainfall. We conclude that, although dasyurid marsupial capture rates varied up to 34 fold during the study period, population changes are not strongly tied to rainfall. Heterothermic physiology in this family, in particular the ability to use daily torpor to save energy, may be central to the decoupling of population dynamics from rainfall‐driven primary productivity.

Sand circles in stony landscapes of Namibia are caused by large Euphorbia shrubs

Thousands of nearly stone-free, sandy, circular patches that lack vegetation (sand circles) are found in stony landscapes (including desert pavement areas) of Namibia and adjacent northwestern South Africa. Their distribution is very similar to that of Euphorbia gregaria, E. gummifera and E. damarana. The aim of this study was to determine if these sand circles represent sites of former occupancy by shrubby euphorbia species in the stony landscapes. Extracts of the soil from within and between sand circles (matrix), as well as from beneath dead E. gregaria plants were compared to an extract of a living E. gregaria with 1H-NMR and PCA multivariate statistical analysis. The results clearly showed that compounds characteristic of E. gregaria were present inside the sand circles and soil collected from beneath the dead plants but were not in extracts from the matrix soil. Diameters of living aerial parts of E. gregaria plants and sand circles measured in Google Earth™ satellite imagery showed that the average diameter of the sand circles and euphorbias were not statistically different.The aerial parts of Euphorbia gregaria are very dense and accumulate wind-blown sand particles. The accumulated sand buries the original stony surface beneath the aerial parts. When the plants die, a nearly circular sandy patch remains in these otherwise rocky areas. Many dying euphorbias have been observed in these areas and the healthy population seems to be declining. Previous reports on declining numbers of quiver trees (Aloe dichotoma) in Namibia were linked to climate change and the same might apply to succulent euphorbias. Since the shrubby euphorbias leave a sandy circle ‘footprint’ in the stony landscapes after they die, these features could potentially be used to estimate former population sizes and distributions and serve as a possible indicator of climate change.

Intermittent Aeolian Saltation Over a Gobi Surface: Threshold, Saltation Layer Height, and High‐Frequency Variability

AbstractGobi is a type of desert pavement in Asia over which aeolian saltation generates atmospheric dust. However, the high‐frequency behavior of aeolian saltation over gobi is still poorly understood. In this study, coupled high‐frequency observations of three‐dimensional (3‐D) wind speed (U) and saltation particle count rate in a vertical array of Sensit sensors were performed in a gobi region of Milan County, southern Xinjiang, China. The results revealed that aeolian saltation over a gobi surface exhibited an intermittent phenomenon, and the level of the intermittency was governed by the relative wind strength (s), which is consistent with that of sand surfaces. The threshold wind speed (Ut) decreased exponentially with increasing transport activity parameter (AP), and a prominent decrease in Ut was examined when the AP values reached 0.7; comparatively, for sand surfaces, a linearly declining trend occurred over the whole AP range of 0–1. The saltating particles over the gobi surface were distributed in layers of 0.08–0.30 m with an average value of 0.20 ± 0.04 m, which was significantly larger than that of sand surfaces. The relationship between the instantaneous sand transport rate (Q) and U over the gobi surface was also relatively poor and could be improved by lagging Q by 2 s. The mechanism for the collision between saltating particles and the gobi bed, which is rigid and complex in texture, was significantly different from that of the soft and deforming sand bed, causing the difference in aeolian saltation between gobi and sand surfaces.

Reconnaissance geology and geophysics of the Mercurio structural dome, Chihuahua, Mexico

The Mercurio structural dome is a poorly exposed and complex structure located in the transitional region between the Coahuila Calcareous Platform and the San Pedro El Gallo sector of the Sierra Madre Oriental, Mexico. It is located in the State of Chihuahua, close to the limits with Coahuila and Durango, Mexico. The dome is a circular structure, ~16 km in diameter, that can be seen in air-photos, satellite images, and shaded relief maps, but that has a subtle topographic expression on the ground. As seen in the field, the most conspicuous topographic features in the area are several hills with the morphology of volcanic necks that rise up to 250 m above the surrounding terrain. The deformation fringe of the dome is a series of cuesta-like low hills, less than 30 m high, where a poorly lithified volcano-sedimentary succession (litharenites, polymictic conglomerates, and ignimbrites) is almost completely masked by desert pavement, which is mainly constituted by well-rounded calcareous clasts derived from the Mesozoic sedimentary marine rocks and by less abundant Paleogene volcanic rocks exposed in the region. Inside the dome the following units are exposed: 1) the pre-volcanic basement in a NW-trending, upright, open anticline developed in limestone of the Aurora Formation, 2) a series of hills where is exposed a succession of epiclastic and volcanic rocks, which are similar, in age and lithology, to some facies of the Ahuichila Formation, and 3) a NW-trending dike, exposed at Cerro Dinamita, which is interpreted as an offshoot of the buried subvolcanic body that created the dome. The deformation fringe around the buried intrusive has a quaquaversal array in the bedding and forms a simple monocline-like structure in the NE part of the dome. A set of SE- and NW-trending plunging folds forms the SE and SW portions of the dome, respectively. The NW part of the fringe is nearly completely masked by volcanic rocks, but there is a ~W plunging syncline in the area. Geophysical data show a broad gravimetric high in the region, and there is a distinct aeromagnetic anomaly inside the dome. The morphological expression of the dome lies just east of a NW-trending lineament of gravity and magnetic anomalies, which may be the buried portion of a normal fault shown in geologic maps of the region northwest of the studied area. Another possible cause is an alignment of buried intrusive bodies suggested by the regional aeromagnetic data, a small diorite outcrop south of Sierra El Diablo, and presence of volcanic necks in the northern portion of Sierra Los Alamos. Available geological and geophysical information was used to model a near-surface, irregular intrusive body with variable magnetic susceptibilites. This variation in susceptibilities is consistent with observed differences in rock composition in the exposed volcanic rocks and with evidence that the structure was formed by a bimodal (andesite-rhyolite) magmatic system where mixing/mingling occurred. As a whole, the set of structures is interpreted as a dome formed by forceful magma injection into a previously folded Paleogene volcano-sedimentary succession. U-Pb zircon ages were used to bracket the age of the deformation pulses registered in the rocks. Litharenites from the deformed volcano-sedimentary succession yielded an Ypresian zircon age of ~51 Ma. A tilted, lithic-rich ignimbrite collected near the top of the exposed volcano-sedimentary succession has mean age of 46.4 +0.8/-1.6 Ma, and the Cerro Dinamita dike has a mean age of 29.37 ± 0.24 Ma. Thus, the youngest pulse of Laramide deformation in the area is younger than ~46 Ma and the re-folding, associated with emplacement of the dome occurred at ~29 Ma. Detrital zircon U-Pb ages from Mercurio sandstones suggest dominant sediment sources from plutonic and/or volcanic rocks exposed along western Mexico. Likely subordinate sources are Mesozoic sedimentary rocks in northern and central Mexico. Distribution of detrital zircon U-Pb ages in the studied samples is similar to that documented in sandstones of the Difunta Group at the Parras and La Popa basins, except that older grains (>1.0 Ga), documented in the clastic rocks of these basins, are scarce in the sandstones of the Mercurio area.

Late Quaternary Tectonics, Incision, and Landscape Evolution of the Calchaquí River Catchment, Eastern Cordillera, NW Argentina

AbstractUnraveling the relative impacts of climate, tectonics, and lithology on landscape evolution is complicated by the temporal and spatial scale over which observations are made. We use soil and desert pavement classification, longitudinal river profiles, 10Be‐derived catchment mean modern and paleo‐erosion rates, and vertical incision rates to test whether, if we restrict our analyses to a spatial scale over which climate is relatively invariant, tectonic and lithologic factors will dominate the late Quaternary landscape evolution of the Calchaquí River Catchment, NW Argentina. We find that the spatial distribution of erosion rates, normalized channel steepness indices, and concavity indices reflect active tectonics and lithologic resistance. Knickpoints are spatially coincident with tectonic and/or lithologic discontinuities, indicating local base‐level control by faulting. Catchment mean erosion rates, ranging from 22.5 ± 2.6 to 121.9 ± 13.7 mm/kyr, and paleo‐erosion rates, ranging from 56 +43/‐19 to 105 +60/‐33 mm/kyr, are similar, possibly suggesting that Quaternary climate changes have not had a strong enough influence on erosion rates to be detected using cosmogenic 10Be. However, punctuated abandonment of pediment and strath terraces at 43.6 +15.0/‐11.6, 91.2 +54.2/‐22.2, and 151 +92.7/‐34.1 ka and disparities between vertical incision rates and catchment mean erosion rates could suggest periods of landscape transience, possibly reflecting climate cyclicity. Our results emphasize the role of tectonic uplift and lithologic contrasts in shaping long‐term erosion rates and channel morphology at the relatively local scale of the Calchaqui River Catchment, in contrast to regional‐scale studies which find precipitation to exert the dominant control.

Abrasion rates of ventifacts

Although ventifacts are often utilized to determine past and present wind directions and sediment mobilities in arid, coastal, and periglacial environments, their morphodynamics have not been well understood. In this study, based upon the principles of classical mechanics, the abrasion rate of ventifact is analytically expressed by the elastic properties of rock or mineral components and the mass flux density of sediment transported by wind. The unknown abrasion coefficient can be estimated via wind tunnel experiments. The practicability of this novel formula is shown by the calculation of short-term abrasion depth of ventifacts over a desert pavement. It is expected that the current work will improve the quantitative reconstruction of paleo-environments and paleo-climates using ventifacts.

Soil horizon development and the tempo and modes of vegetation change during the Holocene in a Sonoran Desert basin, USA

Silt-rich and clay-rich vesicular horizons (V horizons) that develop beneath desert pavements limit infiltration and plant-available soil moisture. A radiocarbon date from buried charcoal constrains the timing of emplacement of a Sonoran Desert alluvial fan deposit to the very end of the Pleistocene (approximately 12.7 ka). The surface of the fan is dotted with 1.5–3 m light-colored plant scars that mark places once occupied by larger perennial plants. A few very large, ring-like clones of Larrea tridentata have persisted on the surface and represent plants that may have initially established as long ago as the early Holocene. The gravelly fan deposit initially would have possessed high infiltration capacities and fostered the establishment of vegetation. Two principal sets of factors have driven vegetation change over the course of the Holocene. During the first part of the Holocene, differences in life history attributes of various species (dispersal and colonization vs. competitive abilities) would have contributed to a succession of perennial plant occupants, culminating with the predominance of the slow-growing, long-lived, but competitively dominant L. tridentata. During the latter part of the Holocene and continuing to the present, continued development of the pavement and associated V horizon has increasingly diminished the capacity of the surface to absorb precipitation and is responsible for the more recent phase involving progressive elimination of vegetation. Development of soil features that markedly influence soil hydrology is a major component of vegetation change during the late Quaternary in the more arid portions of the Sonoran and Mojave deserts.

Increasing effect of aeolian sediment transport on riverbed evolution under decreased flow conditions in the alluvial desert reach of the Upper Yellow River

Increasing effect of aeolian sediment transport on riverbed evolution under decreased flow conditions in the alluvial desert reach of the Upper Yellow River

Soil organic matter determines the distribution and abundance of nematodes on alluvial fans in Death Valley, California

AbstractDeath Valley, located within the Mojave Desert of the United States, is one of the most extreme terrestrial environments on Earth. We studied free‐living soil nematode abundance and community structure in soil samples collected from alluvial fans in Death Valley between 2013 and 2018. Our objective was to establish the distribution and abundance of soil nematodes on alluvial fans with respect to soil properties, vegetation, and channels that reflect past and future water flows. The study period spanned an extreme drought for the region but also included a 2016 superbloom of spring annual plants that followed a strong El Niño event. We found that nematode communities were dominated by bacterial‐feeding species and had higher densities under shrub canopies than in adjacent interplant soils. Nematode abundance was similar between areas on alluvial fans that were channelized by past water flows and adjacent desert pavement and does not appear to be linked to how water distributes on the fans following rainfall events. Nematode abundance increased during the superbloom in soils around annual plants. Across the study period, soil organic matter content was the only soil variable measured that was significantly and positively correlated to nematode abundance. Overall, nematodes in this extreme environment are adapted to drought, but they are highly dependent on the growth and distribution of both perennial and annual plants to supply the soil organic matter that their microbial food sources rely upon.

Rainfall Thresholds for Flow Generation in Desert Ephemeral Streams

AbstractRainfall thresholds for streamflow generation are commonly mentioned in the literature, but studies rarely include methods for quantifying and comparing thresholds. This paper quantifies thresholds in ephemeral streams and evaluates how they are affected by rainfall and watershed properties. The study sites are in southern Arizona, USA; one is hyperarid and the other is semiarid. At both sites rainfall and streamflow were monitored in watersheds ranging from 10−3 to 102 km2. Streams flowed an average of 0–5 times per year in hyperarid watersheds and 3–11 times per year in semiarid watersheds. Although hyperarid sites had fewer flow events, their flow frequency (fraction of rain events causing flow) was higher than in semiarid sites for small (<1 km2) watersheds. At both locations flow frequency decreased with drainage area, but the decrease was steeper in hyperarid watersheds. Watershed mean 60‐min intensity thresholds ranged from 3–13 mm/hr in hyperarid watersheds and 7–16 mm/hr in semiarid watersheds. Higher runoff thresholds and lower flow frequencies in small semiarid watersheds likely relate to greater ground cover and soil development compared to the desert pavement and bedrock surfaces in hyperarid sites. The choice of rain data strongly influenced threshold values; single rain gauges were only adequate for threshold prediction in <1‐km2 watersheds, and incomplete rainfall data led to increases in thresholds with drainage area. We recommend using mean rainfall intensity over the drainage area for threshold analysis because this reduces apparent scale dependence in thresholds caused by incomplete rainfall information.

Semi-arid soils from a topolithosequence at James Ross Island, Weddell Sea region, Antarctica: Chemistry, mineralogy, genesis and classification

Climate and parent material have a significant impact on the development of Antarctic soils, and areas dominated by basaltic-andesitic volcanic rocks, sedimentary rocks, and metamorphic rocks have been widely studied. James Ross Island, located in the semi-arid transition between East Antarctic Peninsula/Weddell Sea sector, receives both continental and maritime climatic influences, resulting in distinct, transitional soil formation process. In Antarctica, there is little research on semi-arid soils formed on volcano-sedimentary rocks, as well as their pedogenetic processes. In this study, a topolithosequence of volcanic and sedimentary rocks in this semi-arid transitional climate was investigated. Nine profiles, three on marine terraces, four on sedimentary rocks, and two on volcanic rocks were described, sampled, and analyzed for morphological, physical, chemical, and mineralogical properties. Soils are predominantly Gelisols/Cryosols and present cryoturbation features despite the current dry climate. Vegetation is scarce and nesting birds are absent, so that most soils can be characterized as ahumic. All profiles have a skeletic character, with a high percentage of coarse materials. The soil pH varied from neutral to alkaline and the potential acidity values were null. The mineralogy of the clay fraction presented kaolinite, which was likely inherited from warmer and wetter paleoclimatic conditions during sedimentation. The soils have intergrade properties between maritime and continental Antarctic, such as cryoturbation and redoximorphism, side-by-side with desert pavements and ahumic character. Soil properties were more sensitive to the variations in parent material than to the topographic position in the landscape.

Enhanced zoogeomorphological processes in North Africa in thehuman-impacted landscapes of the Anthropocene

New zoogeomorphological features discovered in dryland landscapes of Northern Africa reflect human-animal agency since prehistory, and attest to complex, networked activities over great distances. We discuss the role of zoogeomorphology in shaping Earth's surface since the beginning of the Anthropocene, the timeframe when natural processes shifted and landscape evolution became more human-dominated. We focus on contexts in arid and semiarid lands of Northern Africa, which are metastable, sensitive ecosystems that are prone to modifications triggered by climatic and anthropogenically forced factors. Studying the geoarchaeological record in context of landscape impact and animal procurement by people throughout Antiquity is important for reconstructing domestication and husbandry of cattle, sheep, and goats in this region. Among the features we recognize in association with transhumance, pastoralism, and herding are trails, trackways, footholds, animal daybeds, stables, animal dwellings, rockshelters, game blinds, and monuments, to name a few. Related activities with landscape-scale impacts include herding, transport, corralling and browsing of cattle (Bos sp.), goats, and sheep (ovicaprines) as well as pasturage activities like cropping, fire-setting, and manuring. These activities were disturbances that affected surface processes like erosion and dust mobilization, as well as reduced vegetation and ecosystems productivity. In dryland Africa, and especially in the Sahara, intensive herding led to the alteration of the pristine aspects of bare rock surfaces and of the stone desert pavement (i.e., the hamada); many regions preserved evidence of middle-late Holocene animal daybeds, trampled areas, and barren tracks and trails. We suggest that human and herd animal activities affected geomorphic surfaces that affected slope stability, intensified erosion and dust mobilization, and enhanced dust export from the African continent offshore. We reinterpret the increased dust emission from North Africa during the mid-Holocene at the end of the African Humid Period, as has been interpreted from ocean cores; aridification of the Green Sahara followed the insolation-forced monsoonal maximum, and was exacerbated by human-animal activities across the Sahara and the Sahel. We argue that the spread of human activities and intensive husbandry of cattle, and caprines (goat and sheep) in this region significantly influenced the geomorphic stability, ecosystem and landscape sustainability in a comparable manner of overuse observed in present-day arid and marginal environments, where pastoral overgrazing pressure increases erosion processes and enhances dust mobilization. We suggest that human-animal activities have amplified dust generation from the North African continental interior since ~7 ka BP. This evidence of prehistoric human impacts on surface processes in North Africa supports arguments for an early beginning of the Anthropocene.

Last Glacial aeolian landforms and deposits in the Rhône Valley (SE France): Spatial distribution and grain-size characterization

In the Rhône Valley, a north-south oriented Cenozoic rift in southeast France, thick Pleistocene loess deposits have been recognized since the beginning of the last century. These loess records, which are disconnected from the North European Loess Belt (NELB), are of significant interest to document the evolution of perimediterranean landscapes and environments during the Last Glacial. To overcome the poor precision of available aeolian distribution maps, aeolian deposits were mapped using the topsoil textural database provided by the Land Use and Cover Area frame Statistical Survey project (LUCAS). The grain-size distribution of aeolian sand and loess was first determined using 116 samples taken from surveyed outcrops. Then, the areas showing a similar grain-size composition were extracted from the LUCAS rasters. The resulting map reproduces the conventional maps correctly but suggests a more significant extension of loess, in better agreement with the known distribution of outcrops. The map shows that the distinctive morphology of the valley dominantly controls the distribution of aeolian deposits. The deflation-related landforms, i.e., yardangs, closed depressions (pans), and desert pavements, are widespread south of narrowings of the Rhône Valley between latitudes 44°N and 45°N. They indicate palaeowinds blowing from the north/northwest. Aeolian sand, loessic sand, sandy loess, and loess deposits successively spread on both sides of the Rhône River. The loess is characterized by a coarse texture (main mode around 60 μm), strong local thickness (>5 m), limited extension, and abundant bioturbation. This preservation results from the persistence of a shrub vegetal cover during the coldest and driest phases of the Last Glacial that allowed for trapping the saltating and suspended particles close to the alluvial sources.

Tracking the <sup>10</sup>Be–<sup>26</sup>Al source-area signal in sediment-routing systems of arid central Australia

Abstract. Sediment-routing systems continuously transfer information and mass from eroding source areas to depositional sinks. Understanding how these systems alter environmental signals is critical when it comes to inferring source-area properties from the sedimentary record. We measure cosmogenic 10Be and 26Al along three large sediment-routing systems (∼ 100 000 km2) in central Australia with the aim of tracking downstream variations in 10Be–26Al inventories and identifying the factors responsible for these variations. By comparing 56 new cosmogenic 10Be and 26Al measurements in stream sediments with matching data (n= 55) from source areas, we show that 10Be–26Al inventories in hillslope bedrock and soils set the benchmark for relative downstream modifications. Lithology is the primary determinant of erosion-rate variations in source areas and despite sediment mixing over hundreds of kilometres downstream, a distinct lithological signal is retained. Post-orogenic ranges yield catchment erosion rates of ∼ 6–11 m Myr−1 and silcrete-dominant areas erode as slow as ∼ 0.2 m Myr−1. 10Be–26Al inventories in stream sediments indicate that cumulative-burial terms increase downstream to mostly ∼ 400–800 kyr and up to ∼ 1.1 Myr. The magnitude of the burial signal correlates with increasing sediment cover downstream and reflects assimilation from storages with long exposure histories, such as alluvial fans, desert pavements, alluvial plains, and aeolian dunes. We propose that the tendency for large alluvial rivers to mask their 10Be–26Al source-area signal differs according to geomorphic setting. Signal preservation is favoured by (i) high sediment supply rates, (ii) high mean runoff, and (iii) a thick sedimentary basin pile. Conversely, signal masking prevails in landscapes of (i) low sediment supply and (ii) juxtaposition of sediment storages with notably different exposure histories.

Formation of desert pavements and the interpretation of lithic-strewn landscapes of the central Sahara

This study focuses on two different but interlinked lines of evidence that critically examine land surface processes contributing to the formation of desert pavements in the central Sahara. (1) Soil pedostratigraphies from the Messak plateau (SW Libya) illustrate phases of land surface stability and instability that reflect humid and arid phases of Quaternary climate, respectively. (2) The density and morphologies of surface lithic scatters in the same region were re-examined, based on data previously published by Foley and Lahr (2015, PLoS ONE). This re-examination shows that many surface clasts previously interpreted as lithics are better interpreted as formed by in situ weathering and wind abrasion. Furthermore, weathering, abrasion and deflation are the major processes by which desert clasts are formed and concentrated on the land surface, not human agency. Erosional Quaternary periods allowed for the formation of condensed pedostratigraphies by which surface clasts and lithics were mixed and concentrated on the land surface. These two independent lines of evidence show that desert land surfaces do not reflect a single time period of formation, and that Quaternary desert pavements (including any lithics located thereon) evolved in response to subaerial weathering and erosion processes.