Joseph A. Mason

A 10,000 year record of dune activity；dust storms；and severe drought in the central Great Plains

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.

Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing

Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleoclimatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8–3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.

Dune mobility and aridity at the desert margin of northern China at a time of peak monsoon strength

Wind-blown sands were mobile at many sites along the desert margin in northern China during the early Holocene (11.5–8 ka ago), based on extensive new numerical dating. This mobility implies low effective moisture at the desert margin, in contrast to growing evidence for greater than modern monsoon precipitation at the same time in central and southern China. Dry conditions in the early Holocene at the desert margin can be explained through a dynamic link between enhanced diabatic heating in the core region of the strengthened monsoon and increased subsidence in drylands to the north, combined with high evapotranspiration rates due to high summer temperatures. After 8 ka ago, as the monsoon weakened and lower temperatures reduced evapotranspiration, eolian sands were stabilized by vegetation. Aridity and dune mobility at the desert margin and a strengthened monsoon can both be explained as responses to high summer insolation in the early Holocene.

Holocene climatic changes revealed by aeolian deposits from the Qinghai Lake area (northeastern Qinghai-Tibetan Plateau) and possible forcing mechanisms

Previous palaeoclimatic studies in the northeastern Qinghai-Tibetan Plateau (NETP) during the Holocene, mainly using lake sediments, have deepened our understanding of the climatic system in this remote region. The timing and forcing mechanisms of climatic change in this region are still controversial, however. Aeolian sand and silt deposits, which are widely distributed in the NETP, can be readily dated by optically stimulated luminescence (OSL) techniques and reveal the sensitive response of the landscape to climatic change. In this study, aeolian sand and loess at six sites around Qinghai Lake were studied to reconstruct millennial-scale climatic changes during the Holocene. Multiproxy data along with 24 OSL age determinations show that low effective moisture and aeolian activity occurred at c. 13 ka, 10—9.1 ka, and 8.9—7.8 ka. Periods of greater effective moisture may have occurred at ~11 ka and 9 ka, and there is evidence for a previously undocumented wet climate at 4—3 ka. These results show that millennial-scale Holocene palaeoclimatic changes in the NETP cannot be explained simply as direct responses to changes in monsoon precipitation forced by summer insolation. We suggest that changes in effective moisture were determined by the balance between monsoon-induced rainfall and evaporation loss (mainly controlled by temperature). Thus, climatic change in the NETP may have been influenced by complex interactions between the monsoon circulation and local convection/evaporation effects, in addition to large-scale change in the Asian monsoon and the westerlies.

A new model of topographic effects on the distribution of loess

A model of topographic influence on the regional transport and accumulation of loess in sparsely vegetated landscapes is proposed, based on the critical role played by saltating eolian sand in the entrainment and long-distance transport of finer-grained suspended dust. In this model, long-term accumulation of loess occurs mainly downwind of topographic obstacles that limit eolian transport of sand, and associated re-entrainment of dust. Regional and local patterns of distribution of late Wisconsinan loess in parts of the Upper Mississippi River basin are closely related to the distribution of topographic obstacles to sand transport. These cases of topographic influence occur in a region where evidence exists for sparse tundra-like vegetation during the deposition of loess in the late Wisconsinan. Similar topographic effects may not be evident further south in the Missouri and Mississippi River basins because vegetation density was generally sufficient to prevent the movement of eolian sand out of source valleys. Thus, topographic obstacles and dense vegetation may have similar effects on loess distribution under different environmental conditions.

Lethal Sandslides from Eolian Dunes

Fossil vertebrates entombed within the Upper Cretaceous Djadokhta Formation of southern Mongolia bear testimony to a heretofore unknown geologic phenomenon: mass wasting of eolian dunes during heavy rainstorms. Evaporation of shallow‐penetrating rainwater led to progressive calcite accumulation in a thin layer of sand about 0.5 m below the surface of dune lee slopes. During rare heavy rainstorms, a perched water table developed at the top of calcitic zones. Positive pore water pressure led to translational slides and fast‐moving sediment gravity flows that overwhelmed animals on the lee slopes of large dunes and in interdune areas.

Chemical and particle-size evidence for addition of fine dust to soils of the midwestern United States

Significant long-term atmospheric dust additions to soils are well documented in many parts of the world, but not in the midwestern United States. We investigated elemental mass fluxes associated with soil development in late Wisconsinan loess in Illinois and Minnesota, using Zr as a stable index element. Positive mass fluxes of Al, Fe, and Ti can most plausibly be explained by additions to these soils of fine far-traveled dust, with higher Al/Zr, Fe/Zr, and Ti/Zr ratios than the coarser locally derived loess. High-resolution particle-size analyses support this explanation. The proposed dust influx will complicate efforts to quantify weathering processes in these soils. Far-traveled dust influx could have occurred simultaneously with the final phase of local loess deposition, and/or later, in the Holocene. Depending on the timing of dust influx, many other soils of the region may have been affected by it.

Sedimentary aggregates in the Peoria Loess of Nebraska, USA

Loess grain size data used to infer transport direction or wind strength are generally derived from vigorously disaggregated samples. However, these data may not adequately represent the effective particle size distribution during loess transport, if the transported dust contained aggregates of finegrained material. Thin sections of minimally altered C and BC horizons in the late Pleistocene Peoria Loess of Nebraska, USA, indicate the presence of aggregates with diameters of 30–1000 Am. The larger aggregates (>250 Am) are unlikely to have been transported, and are interpreted as the result of soil faunal activity and other pedogenic processes after deposition. Aggregates smaller than 250 Am could have a similar origin, but laser diffraction particle size analysis suggests that many are sedimentary particles. Comparison of minimally and fully dispersed particle size distributions from each sampling site was used to estimate the modal diameter of aggregates. The aggregate modal diameter becomes finer with decreasing loess thickness, representing increasing distance from the source. A similar trend was observed in the modal diameter of fully dispersed particle size distributions, which represents the mode of sand and silt transported as individual grains. We interpret both trends as the result of sorting during transport, supporting the interpretation that many of the aggregates were transported rather than formed in place. Aggregate content appears to increase with distance from the source, explaining a much more rapid downwind increase in clay content than would be expected if clay were transported as particles smaller than 2 Am diameter. Although the Peoria Loess of Nebraska contains sedimentary aggregates, many of the coarse silt and sand grains in

Late Quaternary climate change, loess sedimentation, and soil profile development in the central Great Plains: A pedosedimentary model

Research on soil genesis often assumes a “top-down” model, in which the soil profi le develops downward from a stable land surface. This model is inapplicable to upland landscapes affected by frequent dust deposition, where soils grow upward as they develop. On the central Great Plains, late Quaternary loess sections proximal to immediate source areas contain the Brady Soil, a prominent marker separating late Pleistocene Peoria Loess from Holocene Bignell Loess. Farther from immediate dust source areas, the Brady Soil and Bignell Loess are not recognizable in the fi eld. On loess tablelands in these distal regions, surface soils typically contain a prominent, clay-rich B horizon below a thick silty A horizon. Assuming top-down pedogenesis, this could be interpreted as a postglacial soil profi le formed in Peoria Loess, with the B horizon produced by weathering and clay illuviation. We propose a strikingly different interpretation, in which the upper B horizon at distal sites is the Brady Soil A horizon that has been transformed by burial, organic matter loss, and modern subsoil structure formation processes. The overlying modern A horizon represents Bignell Loess. Properties of the Brady Soil at proximal sites (a distinctive burrowed zone, high clay content and TiO 2 /ZrO 2 , and low volcanic glass content) can be traced to the B horizon in distal soils. A decrease in smectite abundance above the Brady Soil at proximal sites is identifi able at the top of the clay-rich B horizon in distal soils. The spatial variation of clay content in loess and soil horizons is best explained by eolian sedimentation patterns. The higher clay content in the Brady Soil and distal B horizons defi nes a fi ne-grained zone that represents a late phase of Peoria Loess accumulation. Evidence of chemical weathering is minimal, and illuvial clay is rare to absent in the clay-rich B horizons. Illuvial clay does often occur deep in the solum and is related to the depth below the top of the Brady Soil. The depth of occurrence of illuvial clay is not related to modern climate parameters, although depth to secondary carbonate appears to be in equilibrium with modern climate. Upland soils in the central Great Plains are composite soils; their properties are the result of a pedosedimentary history linked to regional climate change that has infl uenced sedimentation and pedogenesis since the late Pleistocene.

The evolution of coupling of Asian winter monsoon and high latitude climate of Northern Hemisphere

The evolution and driving mechanism of the Asian winter monsoon system are of great importance to understanding the present-day climate. Through high-resolution particle size analysis of the oldest loess-red clay sequence known so far (with a basal age of about 8 Ma) and comparison of the results with oxygen isotope curves from North Atlantic marine sediments, 4 stages of the evolution of the Asian winter monsoon were clearly demonstrated. During the first stage, between about 8.1 and 4.3 Ma, there was no relation between Asian winter monsoon and Northern Hemisphere ice volume and high latitude climate inferred from marine sediments. A weak relation developed during the second stage, about 4.3 to 3.5 Ma. During the third stage (3.5 to 2.6 Ma) an Asian winter monsoon system similar to the present formed, initiating a stronger relation between the winter monsoon and Northern Hemisphere ice volume and high latitude climate. In the final stage (2.6 to 0 Ma) the present Asian winter monsoon system was fortified and stabilized and changes in the winter monsoon system were almost in phase with Northern Hemisphere ice volume and climate. The staggered uplift of Tibetan Plateau at ≈8, 3.6, 2.6 Ma and later might be the driving force for the evolution of the Asian winter monsoon.