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Cenozoic climate has changed markedly from a warm, wet climate to the present cool, dry glacial-interglacial cycles. The largest of these cooling steps is thought to have taken place across the Eocene-Oligocene transition. Terrestrial airborne eolian loess is a direct result of land aridification; therefore, the prolonged formation of thick eolian loess deposits provides a key to aridification history in the geological past. However, the oldest eolian loess in the Asian interior has been dated to only 25–22 Ma. Here we present new sedimentological, geochemical, and magnetostratigraphic data from Cenozoic sections in southwestern Mongolia to show that the earliest eolian dust accumulation occurred by 34 Ma, ∼10 m.y. earlier than currently believed. We suggest that this oldest eolian dust accumulation provides direct evidence for enhanced aridification across the Eocene-Oligocene transition in Central Asia; this is explicable by reduced moisture transport caused by prevailing westerlies during retreat of the Neotethys Ocean. These climatic changes were related to the changing regional plate configuration as well as to global cooling and ice-sheet growth in Antarctica.

The Ordos Basin of China encompasses the Mu Us Desert in the northwest and the Chinese Loess Plateau to the south and east. The boundary between the mostly internally drained Mu Us Desert and fluvially incised Loess Plateau is an erosional escarpment, up to 400 m in relief, composed of Quaternary loess. Linear ridges, with lengths of ∼10 2 –10 3 m, are formed in Cretaceous- Quaternary strata throughout the basin. Ridge orientations are generally parallel to near-surface wind vectors in the Ordos Basin during modern winter and spring dust storms. Our observations suggest that the Loess Plateau previously extended farther to the north and west of its modern windward escarpment margin and has been partially reworked by eolian processes. The linear topography, Mu Us Desert internal drainage, and escarpment retreat are all attributed to wind erosion, the aerial extent of which expanded southeastward in China in response to Quaternary amplification of Northern Hemisphere glaciation.

The Chinese Loess Plateau (CLP) is a large, spatially well defined and persistent zone of loess accumulation developed near the fluctuating northwest margin of the East Asian monsoon. Many studies have analyzed its loess sediments to provide insights into paleoclimatic conditions. Although spatial and temporal variations in the grain sizes of CLP sediments are fundamental to this effort, controversy over the origin of the dominant coarse quartz silt has limited interpretations. Reexamination of the spatial pattern of grain-size distribution across the CLP and a field-scale experiment conducted in the Gobi Desert revealed a genetic association between the coarse silt fraction of the loess and primary production of coarse silt through eolian abrasion of sand in the proximal Mu-Us, Tengger, and Badain Jaran sandy deserts. Our results demonstrate the effectiveness of eolian abrasion of quartz sand in primary coarse silt production in Central Asia and identify this process as the most consistent with the wellrecognized systematic northwest-southeast depositional pattern of the CLP. We suggest that only abraded coarse quartz grains transported short distances by long-term persistent eolian activity can build up thick loess sequences to form a massive and spatially well defined loess plateau. These results decouple the production and transport of coarse silt and finer silt and clay particles, which have a more distant and wider provenance, changing the constraints on previous paleoclimatic reconstructions. © 2013 Geological Society of America.

The Chinese Loess Plateau of central Asia is composed of interbedded loess and paleosol layers, deposited during glacial and interglacial cycles, respectively, during the past ∼2.5 m.y. Understanding the provenance of loess is fundamental to reconstructing wind patterns during Quaternary glacial periods. We determined and compared U-Pb ages on zircon crystals from Loess Plateau strata and potential source areas. The results indicate that the loess was largely derived from the Qaidam Basin and the northern Tibetan Plateau to the west, both of which exhibit spatially extensive geomorphic landforms indicative of past (interpreted as pre-Holocene) wind erosion and/or deflation by westerly winds. This challenges the current paradigm that the loess of the Chinese Loess Plateau was largely sourced from deserts located to the northwest, as observed in the modern interglacial climate. We propose that during glacial periods, the mean annual positions of the polar jet streams were shifted equatorward, resulting in more southerly tracks for dust-generating storms and suppression of the East Asian monsoon by inhibiting the subtropical jet from shifting northward across the Tibetan Plateau.

Gravel-bed rivers organize to a threshold state in which the fluid stress during dominant flow conditions slightly exceeds the critical value. These channels change fairly abruptly to sand-dominated beds, facilitated by a paucity of sediment in the range of 1–10 mm. Eolian sediments are also bimodal; sand seas often transition rapidly downwind to silty loess deposits. In this paper we hypothesize that both transitions result from the same processes: (1) production of fine sediment by spallation from saltation abrasion of coarse grains, and (2) segregation of fine and coarse modes by suspension and saltation transport, respectively. We demonstrate that gravel rivers and eolian dune fields are dynamically equivalent in terms of threshold transport (Shields stress) and collision dynamics (Stokes number). While abrasion may not be the dominant cause of downstream fining of fluvial gravels, the reduction of collision efficiency with grain diameter sets a lower limit on gravel size, leading to a grain-size gap. The collision efficiency hypothesis potentially reconciles conflicting studies on the importance of sorting and lithology in downstream fining. Shields and Stokes similitude in rivers and deserts may be used to design appropriately scaled experiments to test and refine our hypothesis.

Nd-Sr isotopic signatures of loess, modern dust, and their potential source materials are systematically investigated to discriminate the natural and anthropogenic sources of eastern Asian dust. Chinese loess is an eolian deposit that has accumulated over the past few million years, and is characterized by a negative Nd-Sr isotopic correlation that implies binary sources for the natural background of eastern Asian dust. Loess in northeastern China shows the highest eNd(0) value, while loess in western China has the lowest eNd(0). Considering the Nd-Sr isotopic signatures of the potential sources, we propose that the high eNd(0) end member is derived from the arid lands around the northern boundary of China while the low eNd(0) end member originates from the deserts on the northern margin of the Tibetan Plateau. Chinese Loess Plateau deposits are dominated by northern margin Tibetan Plateau dust, suggesting a strong link between the thick eolian deposit in this region and the evolution of the Tibetan Plateau. The long-range transported spring dust in both Nanjing (south China) and Japan has an Nd-Sr isotopic composition similar to that of loess. However, spring dust in Beijing (north China) has a much lower eNd(0) value, indicating an additional anthropogenic contribution of low eNd(0) material, possibly from the sandy lands in the adjacent north and west.

Controversy exists regarding the sources of the eolian dust for the immense Loess Plateau of north-central China, the largest accumulation of windblown loess in the world. Because the loess accumulation rate, unit thickness and particle size all decrease from northwest to southeast, it has long been thought that the northern deserts, especially the Mongolian Gobi, are the major loess source, a view supported by newly applied mineralogical (electron spin resonance, ESR) provenance techniques. Here, we examine surface samples from the Gobi Desert, and compare their magnetic and geochemical properties with those of last glacial loess samples from across the Loess Plateau region. The mineralogy, geochemistry and magnetic properties of Gobi Desert samples are variable (most likely reflecting local lithological complexity), distinctive and, critically, non-overlapping with the notably homogenous characteristics of the last glacial loesses spanning the Loess Plateau. It is likely that the source areas for the Plateau encompass a much larger area than any one proximal desert region, in order to account for a) the extreme degree of mixing, b) the volume of loess generated and transported, and c) the mineralogical and magnetic mismatch evident here between the Mongolian Gobi samples and the last glacial loess.

Development of continental aridity has been linked to late Cenozoic global cooling, but the evidence is indirect, based on terrestrial loess deposits and eolian silt in marine sediments, whereas direct dating of the inception of arid landforms has been frustrated by a lack of suitable methods. Here we report the first age determination of a major arid-zone dune field, based on cosmogenic 10 Be and 26 Al measurements of drill cores from dunes in the Simpson Desert, central Australia. Results show that the dune field began to form ca. 1 Ma, whereas dating using quartz optically stimulated luminescence indicates episodic dune building during late Quaternary ice ages. Less intense desertification began earlier; the previous cosmogenic exposure dating showed that neighboring stony deserts began to form at the onset of Quaternary ice ages 2–4 Ma. Aridity deepened and the dune field formed when ice age cycles increased their amplitude and switched their periods from 40 k.y. to 100 k.y. ca. 1 Ma.

The biogeochemical impacts of iron-rich dust to the oceans are known for Earth’s recent record but unexplored for deep time, despite recognition of large ancient dust fluxes, particularly during the late Paleozoic. We report a unique Fe relationship for Upper Pennsylvanian mudrock of eolian origin that records lowstand (glacial) conditions within a carbonate buildup of western equatorial Pangaea (western United States) well removed from other detrital inputs. Here, reactive Fe unambiguously linked to dust is enriched without a corresponding increase in total Fe. More broadly, data from thick coeval loess deposits of western equatorial Pangaea show the same marked enrichment in reactive Fe. This enrichment—atypical compared to modern marine, fluvial, glacial, loess, and soil sediments—suggests an enhancement of the reactivity of the internal Fe pool that increased the bioavailability of the Fe for marine primary production. Regardless of the mechanism behind this enhancement, our data in combination with other evidence for high dust fluxes imply delivery of extraordinarily large amounts of biogeochemically reactive Fe to glacial-stage late Paleozoic seas, and modeling of this indicates major impacts on carbon cycling and attendant climatic feedbacks.

A comparison between particle size distributions determined by laser diffraction and the pipette method confi rms previous studies that point to a variable underestimate of the clay fraction using laser diffraction, relative to the values obtained by pipette analysis. In order to yield four particle-sized fractions ( 8 μm) for further analysis, the pipette method was used on loess and paleosol samples from sites on the Chinese Loess Plateau spanning the past 22 m.y. In all cases, a subsidiary mode in the 8 μm fraction provides a basis for evaluating the relative contributions of pedogenic and detrital components. Pretreatment using acetic rather than hydrochloric acid for carbonate removal results in no signifi cant loss of ferrimagnetic minerals and allows magnetic characterization of the separate fractions. This confi rms the isolation of almost all the fi nest, pedogenic ferrimagnetic grains in the clay fraction and their negligible contribution to the magnetic properties of the coarse fractions in all the samples from early Miocene time onward. Because similar conclusions may be drawn from previous studies of late Pleistocene samples from the east-central and extreme western parts of the Chinese Loess Plateau, we conclude that this approach provides a basis for separately characterizing eolian deposition and pedogenesis throughout the region and for the whole time span of loess accumulation.

Dune fi elds and loess deposits of the Great Plains of North America contain stratigraphic records of eolian activity that can be used to extend the short observational record of drought. We present a 10,000 yr reconstruction of dune activity and dust production in the central Great Plains region, based on 95 optically stimulated luminescence ages. The integration of data from both eolian sand and loess is an important new aspect of this record. Clusters of ages defi ne episodes of extensive eolian activity, which we interpret as a response to frequent severe drought, at 1.0‐0.7 ka and 2.3‐4.5 ka (with peaks centered on 2.5 and 3.8 ka); sustained eolian activity occurred from 9.6 to 6.5 ka. Parts of this record may be consistent with hypotheses linking Holocene drought to sea surface temperature anomalies in the Pacifi c or Atlantic oceans, or to the El Nino-Southern Oscillation phenomenon, but the record as a whole is diffi cult to reconcile with any of these hypotheses.

We present the first detailed record of Holocene climate variation from Icelandic eolian soil deposits. Seven cold and windy episodes occurred in Iceland during the past 10 k.y., including the well-documented Little Ice Age (0.6‐0.1 ka) and the 8.2 ka event. These windy events are associated with enhanced drift-ice discharge into the North Atlantic, and several are associated with evidence for cold and windy climate in central Greenland and diminution of deep-water formation in the North Atlantic. Although the Arctic Oscillation‐North Atlantic Oscillation (AO‐NAO) has been invoked to explain other climate teleconnections in the North Atlantic, our paleorecord of windiness in Iceland is not consistent with a persistent negative phase of the AO‐NAO.

New environmental magnetic data from loess and paleosol successions in outcrops in the upper reaches of the Ob River drainage, southern Siberia, track the major climatic variations over the last glacial-interglacial cycle. Profiles of magnetic susceptibility and alternating deposition of loess and soil-formation events correspond to oxygen isotope stages 1–5. The magnetic-susceptibility data, in association with the stratigraphic succession, confirm that the wind-vigor magnetoclimatological model is a viable alternative to the classic pedogenic model. Interpretation of magnetic-susceptibility data from loess- paleosol successions must therefore consider eolian dynamics, available source materials, and transport directions, in addition to pedogenic processes. Rapid magnetic fluctuations are also observed. These are identified—for the first time in Siberian records—as the signature of the abrupt cold pulses responsible for the Heinrich layers in North Atlantic marine sediments. The data thus form a component of climatic teleconnections across the Northern Hemisphere, allowing correlations to be made among (1) Siberian magnetic susceptibility stratigraphy, (2) data recorded from other loess-paleosol successions in China, European Russia, Europe, and North America, (3) North Atlantic ice-rafted detritus, and (4) sea-surface temperatures derived from molecular stratigraphy of marine sediments off the northwest coast of Africa.

The upper Couch Formation is part of the lower of two formations composing the Ogallala Group in Blanco and Yellowhouse canyons in northwestern Texas. An eolian origin for the upper Couch Formation is indicated by its mean grain size, pedogenic carbonate nodules, massive bedding, and blanketlike morphology. The unit conforms poorly to the usual eolian depositional models; it resulted from a combination of the processes involved in loess and sand-sheet formation. Grassland or savanna vegetation probably existed over the area and aided in sediment trapping. Vertebrates are unusual in eolian units, but the adaptations and mode of preservation of those in the upper Couch Formation also support an eolian interpretation. This and other widespread silty sand sheets in the Ogallala indicate major fluctuations in depositional style, possibly climatically controlled. Lateral continuity and preservation of vertebrates give silty sand sheets great potential as correlation tools.

Deposition of the basal fluvial sediments of the Miocene-Pliocene Ogallala Formation in western Texas and eastern New Mexico was controlled by topography on the underlying erosional surface. Paleovalley-fill facies consist of gravelly and sandy braided-stream deposits interbedded with and overlain by eolian sediments deposited as sand sheets and loess. Uplands on the pre-Ogallala erosional surface are overlain primarily by similar eolian sediments. Calcic paleosols, consisting mostly of glaebules and rhizoconcretions of CaCO/sub 3/, occur throughout the eolian facies. Massive to laminated and locally pisolitic, brecciated, and recemented pedogenic calcretes occur primarily near or at the top of the Ogallala Formation. Eolian facies preserve numerous superposed calcretes and calcic paleosols, reflecting slow episodic aggradation on a savannah or grassland under arid to subhumid climatic conditions. The change from fluvial to mostly eolian sedimentation probably resulted from diversion of streams that deposited fluvial sediments of the Ogallala Formation to form the Pecos and Canadian rivers. Source areas for eolian sediments may initially have been flood plains of Ogallala braided streams and later the flood plains of the newly formed Pecos and Canadian rivers. 38 references.