Helen M. Roberts

Last Glacial loess in the conterminous USA

The conterminous United States contains an extensive and generally well-studied record of Last Glacial loess. The loess occurs in diverse physiographic provinces, and under a wide range of climatic and ecological conditions. Both glacial and non-glacial loess sources are present, and many properties of the loess vary systematically with distance from loess sources. United States’ midcontinent Last Glacial loess is probably the thickest in the world, and our calculated mass accumulation rates (MARs) are as high as 17,500 g/m 2 /yr at the Bignell Hill locality in Nebraska, and many near-source localities have MARs greater than 1500 g/m 2 /yr. These MARs are high relative to rates calculated in other loess provinces around the world. Recent models of Last Glacial dust sources fail to predict the extent and magnitude of dust flux from the mid-continent of the United States. A better understanding of linkages between climate, ice sheet behaviour, routing of glacial meltwater, land surface processes beyond the ice margin, and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region. r 2003 Elsevier Ltd. All rights reserved.

Unprecedented last-glacial mass accumulation rates determined by luminescence dating of loess from western Nebraska

Abstract A high-resolution chronology for Peoria (last glacial period) Loess from three sites in Nebraska, midcontinental North America, is determined by applying optically stimulated luminescence (OSL) dating to 35–50 μm quartz. At Bignell Hill, Nebraska, an OSL age of 25,000 yr near the contact of Peoria Loess with the underlying Gilman Canyon Formation shows that dust accumulation occurred early during the last glacial maximum (LGM), whereas at Devil’s Den and Eustis, Nebraska, basal OSL ages are significantly younger (18,000 and 21,000 yr, respectively). At all three localities, dust accumulation ended at some time after 14,000 yr ago. Mass accumulation rates (MARs) for western Nebraska, calculated using the OSL ages, are extremely high from 18,000 to 14,000 yr—much higher than those calculated for any other pre-Holocene location worldwide. These unprecedented MARs coincide with the timing of a mismatch between paleoenvironmental evidence from central North America, and the paleoclimate simulations from atmospheric global circulation models (AGCMs). We infer that the high atmospheric dust loading implied by these MARs may have played an important role, through radiative forcing, in maintaining a colder-than-present climate over central North America for several thousand years after summer insolation exceeded present-day values.

Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry

Muhs, D. R., Bettis III, E. A., Aleinikoff, J. N., McGeehin, J. P., Beann, J., Skipp, G., Marshall, B. D., Roberts, H. M., Johnson, W. C., Benton, R. (2008). Origin and paleoclimatic significance of late Quaternary loess in Nebraska: Evidence from stratigraphy, chronology, sedimentology, and geochemistry. GSA Bulletin, 120(11-12), 1378-1407.

Holocene loess accumulation and soil development at the western edge of the Chinese Loess Plateau: implications for magnetic proxies of palaeorainfall

A high-resolution Holocene sequence of loess, palaeosols and incipient soils at the western edge of the Chinese Loess Plateau, dated usingoptically stimulated luminescence techniques, is used to identify the respective influence of dust accumulation rate, time and climate on soil magnetic properties. Dust deposition and soil formation have both been quasi-continuous through the Holocene at this site; the soils are thus accretionary in nature. The degree of soil development (as indicated both by geochemical and magnetic properties, which correlate strongly) varied through the Holocene. Compared with the less-weathered loess units, each palaeosol is enriched in nitrogen and organic carbon, depleted of carbonate due to leaching, and displays higher values of pedogenic magnetic susceptibility, frequency-dependent susceptibility (%), remanences and magnetisation. Magnetic grain size indicators show that the pedogenic ferrimagnets are ultrafine, of single domain and superparamagnetic dimensions. Sediment accumulation rates were lowest from B12 to 2.5 ka, providing ample time (100 s of years) for weathering and soil formation to proceed, yet pedogenesis through this interval was relatively weak. Conversely, soil development was stronger during later intervals—when loess accumulation rates were higher, and soil-forming intervals were correspondingly shorter. In terms of Jenny’s (1941) soil-forming equation, accumulation rate (and thus time), sediment source (parent material) and topography show no systematic variation between the loess and the palaeosols. Therefore, climate seems to be the key soil-forming factor which has controlled the geochemical and magnetic properties of these Holocene palaeosols. r 2002 Elsevier Science Ltd. All rights reserved.

Holocene sediment-accumulation rates in the western Loess Plateau, China, and a 2500-year record of agricultural activity, revealed by OSL dating

High-resolution optically stimulated luminescence (OSL) dating of a terraced loess section in the western margins of the Chinese Loess Plateau provides evidence of continuous and varying accumulation of dust throughout the Holocene. From 12030 to 2500 years ago, the sediment-accumulation rate was approxi mately 0.2 mm/year. After this time, it increased to approximately 0.8 mm/year, during a historically docu mented period of agricultural expansion in adjacent areas. From 680 years ago, a further increase in accumu lation rate, to approximately 3.4 mm/year, is evident. Particle-size analysis indicates that this increase in accumulation rate was associated with anthropogenic addition of sandy sediment, probably for soil improve ment. The OSL dating also identifies the period when the terrace was first cut for agricultural use, between 2500 and 2070 years ago.

Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements

Roberts, H. M., Wintle, A. G. (2003). Luminescence sensitivity changes of polymineral fine grains during IRSL and [post-IR] OSL measurements. Radiation Measurements, 37 (6), 661-671.

Reconstruction of Holocene foreland progradation using optically stimulated luminescence (OSL) dating: an example from Dungeness, UK:

Dungeness Foreland, southeast UK, is comprised of a series of distinctive gravel ridges overlying sub- and intertidal sands. The successful application of optically stimulated luminescence (OSL) to the Holocene sub- and intertidal sands at Dungeness provides a well-resolved chronology (precision ~5%) for exploring the sedimentary response of a gravel foreland to changes in sea level, storms, sediment supply and coastal dynamics. The nature of foreland progradation at Dungeness is revealed through a three-dimensional network of 35 new quartz OSL ages, and these data also constrain the timing of gravel emplacement. The OSL chronology places the early formation of the underlying shoreface in the west at ~5000 years ago, with ages decreasing progressively eastwards to ~2000 years ago beneath the main body of the present foreland, and 1000—600 years ago under the present-day ness in the east. The uppermost OSL ages for the subgravel sand unit provide maximum limiting ages for emplacement of the gravel, which together with 14C ages from organic deposits on the gravel surface provide bracketing ages for the deposition of the gravel. The short lag time between shoreface sand deposition and gravel ridge formation is indicative of a high degree of dependency of gravel foreland progradation on the pre-existence of an emergent sand substrate. Sigmoidal isochrons constructed using the subgravel sand OSL ages, and consideration of down-core OSL ages, demonstrates nonlinearity in coastal response; this is expressed in the form of changes in the direction of foreland progradation rather than temporal changes in sedimentation rate.

Late Quaternary eolian and alluvial response to paleoclimate, Canyonlands, southeastern Utah

began at ca. 6 ka and ended by ca. 3‐2 ka, followed by a shift to drier modern conditions; localized mobilization of dune sand has persisted to the present. These interpretations are similar to those of studies at the Chaco dune fi eld, New Mexico, and the Tusayan dune fi eld, Arizona, and are consistent with paleoclimate interpretations of pollen and packrat middens in the region. A period of rapid deposition and infi ltration of eolian dust derived from distant igneous source terranes occurred between ca. 12 and 8 ka. Before ca. 17 ka, and apparently back to at least 45 ka, paleosols contain little or no such infi ltrated dust. After ca. 8 ka, either the supply of dust was reduced or the more arid climate inhibited translocation of dust into the soils.

Evidence for dune reactivation from GPR profiles on the Maputaland coastal plain, South Africa

Abstract The Maputaland area of northeastern South Africa is characterized by extensive dunefields which developed during polyphase reworking of regional aeolian cover sand from the Mid-Pleistocene to the Holocene. Extended parabolic dunes, many preserved only as wind-rift trailing limbs, as well as areas of sinuous crested dunes, hummocky dune systems and the high, composite, accretionary coastal barrier dune cordon are the dominant dune forms. There are few natural sections exposing the stratigraphic succession and unequivocal relative age relationships between dune systems are uncommon. A ground penetrating radar (GPR) survey of dunes and representative aeolian sand stratigraphic units was undertaken in order to investigate the internal structure of the different dune forms and identify stratigraphic relationships between buried sedimentary units. The GPR profiles revealed that the trailing limbs of almost all the parabolic dunes that were surveyed comprise stacked sand units, separated by low-angle reflections interpreted as bounding surfaces, which accumulated through polyphase vertical accretion. Most extended parabolic dunes are aligned north-south and the upper parts of the dunes are characterized by inclined reflections in GPR profile interpreted as large-scale sets of cross-stratification with apparent dips toward the west. A hummocky dune revealed cross-stratified aeolian sand superimposed on a truncated dune form and probably formed through deflation of pre-existing dunes. Using 100 MHz and 200 MHz antennae, it is clear that GPR is capable of imaging very fine sedimentary structures and buried erosional surfaces in the homogeneous aeolian sand of Maputaland. At some of the sites investigated, the buried sand units identified were sampled by hand-augering for infrared-stimulated luminescence dating. The age determinations on these samples suggest that vertical accretion of up to 7 m of sand occurred intermittently over variable time scales up to 25 000 years on some parabolic dune limbs during the Late Pleistocene. In some complex dunefields, adjacent dunes were mobilized at different times, suggesting that remobilization was localized. The implications of the complex internal structure and vertical accretion of extended parabolic dunes are discussed in the context of changes in vegetation cover and water table due to seasonal and short-term cyclical climate variations as well as long-term climate change patterns during the last glacial cycle and the Holocene.

U- and Th-series disequilibria in coastal infill sediments from Praia da Rocha (Algarve Region, Portugal): a contribution to the study of late Quaternary weathering and erosion

U- and Th-series disequilibria observed in a sequence of infill sediments from Praia da Rocha, southern Portugal, were used in combination with geochemical and particle size data to investigate sediment provenance with a view to resolving the late Quaternary weathering and erosion history of the Algarve region, and the stratigraphy of coastal karstic exposures of the Faro/Quarteira (FQ) formation. The red infill units can be distinguished from the brown and buff units on the basis of their lower residual U- and Th-concentrations, their differing post-depositional histories (as revealed by U- and Th-series disequilibria in sequentially extracted sediment phases), and their greater degree of sediment processing. Hence, the buff and brown infill units appear to be derived locally from weathering of the Miocene limestone whilst the red infill may be linked to large-scale mass movement of the FQ formation from further inland, but south of Silves, during the Late Pleistocene. This sequence of events confirms a close association between the formation of karst topography and infilling by gravitational slumping, debris flow and fluvial activity, and, hence, accounts for the complex (three-stage) sediment provenance of the infill material.