Loess Units Research Articles

Aeolian dust supply from the Yellow River floodplain to the Pleistocene loess deposits of the Mangshan Plateau, central China: Evidence from zircon U-Pb age spectra

The thick loess-palaeosol sequences in the Mangshan Loess Plateau (MLP; central China) along the south bank of the lower reach of the Yellow River provide high-resolution records of Quaternary climate change. In addition, substantial increases in grain-size and accumulation rate have been inferred in the upper part of the loess sequence, above palaeosol layer S2. This study investigates the sources of the long-term dust supply to the MLP and explores the mechanism behind the sudden increase in sediment delivery and coarsening of the loess deposits since S2 (∼240 ka) by using end member modelling of the loess grain-size dataset and single-grain zircon U-Pb dating. Our results indicate that the lower Yellow River floodplain, directly north of the MLP, served as a major dust supply for the plateau at least since the deposition of loess unit L9 and indirectly suggest that the integration of the Yellow River and the disappearance of the Sanmen palaeolake took place before L9 (∼900 ka). The sudden change in sedimentology of the Mangshan sequence above palaeosol unit S2 may result from an increased fluvial sediment flux being transported to the lower reaches of the Yellow River because of tectonic movements (initiated) in the Weihe Basin around 240 ka. Furthermore, sediment coarsening can be explained by the gradual southward migration of the lower Yellow River floodplain towards the MLP since the deposition of palaeosol S2. The migration is evidenced by the formation of an impressive scarp, and is likely caused by tectonic tilting of the floodplain area.

Paleoseismological study of the southern Zongwulong Shan fault, Qilian Mountains, western China

The Tibetan plateau is prone to destructive earthquakes. A number of strong thrust events occurred in the recent past, seeming to migrate from south to north, to the Qilian Mountains area. Many active thrust faults are known in the Qilian Mountains, and most of them are capable of producing large earthquakes. Here we study the southern Zongwulong Shan (shan means mountain) fault that lacks any paleoseismology studies so far. This fault is the eastern boundary of the Qaidam and Qilian blocks, posing a threat to the populated area of Delingha in western China. In this paper, we document its Holocene rupture history based on paleoseismic trenches, and we assess the seismic risk in this area. We excavated two trenches across 4.5±0.5m high fault scarps in alluvial fans. We found ruptures that affect young alluvial units and almost penetrate the top thick loess unit. We show that four paleoearthquakes occurred in the Holocene with an average recurrence interval of 2300±500years. The most recent event happened between 272BCE and 572CE. Three older events probably occurred in the past 9.5±0.8ka.

Aeolian silt transport processes as fingerprinted by dynamic image analysis of the grain size and shape characteristics of Chinese loess and Red Clay deposits

This study applied dynamic image analysis (DIA; Sympatec Qicpic) to characterize the grain size and shape of Chinese aeolian sediments in order to fingerprint their transportation processes. This is the first time this technique has been applied to late Neogene and Quaternary silt particles (2–63μm) from the Chinese Loess Plateau (CLP). We selected four well-studied Quaternary loess-palaeosol sequences along a north-south transect across the CLP and compared their grain size distribution obtained by DIA with that yielded by laser diffraction particle size analysis (LD; Fritsch Analysette 22 and Sympatec HELOS KR). This comparison demonstrated that DIA is successfully able to differentiate loess units from palaeosols, and to characterize clearly spatiotemporal variations in the grain size records of loess-palaeosol sequences formed during the last two glacial and interglacial periods. This is consistent with grain size results obtained using LD. DIA is also able to characterize spatial variations in the more fine-grained aeolian Red Clay deposits underlying the Quaternary loess, and allows the quantification of the fluvial contribution to Red Clay sequences. DIA of the characteristics of grain shapes in loess-palaeosol sequences and Red Clay deposits revealed a systematic pattern, whereby the aspect ratio decreased with increasing grain size, indicating that systematic shape sorting occurred during the aeolian transportation of these dust particles. It could be inferred from our study that particles in a certain grain size range correspond to a specific aspect ratio range, and may in turn be aerodynamically distinguishable from each other and further correlated with the wind velocity/strength. Also evident from analysis of our DIA data was a subtle but systematic downwind decrease in the aspect ratio of the particles in the loess units. This observation suggests that elongated and/or flat particles (with a low aspect ratio) were transported further downwind than more symmetrically shaped particles (with a high aspect ratio). This study indicates that DIA of grain size and shape characteristics can be an additional powerful tool for identifying grain size and shape sorting trends, determining the dominant mode of transport, and reconstructing transport pathways of silt-sized aeolian sediments.

Quaternary Stratigraphy and Stratigraphic Nomenclature Revisions in Kansas

This paper outlines Quaternary nomenclature changes to Zeller (1968) that have been adopted by the Kansas Geological Survey (KGS). The KGS formally recognizes two series/epochs for the Quaternary: the Holocene and Pleistocene. Pleistocene stage/age names Kansan, Aftonian, Nebraskan, and Yarmouthian are abandoned and replaced with the broader term "pre-Illinoian." Formation names Bignell, Peoria, Gilman Canyon, and Loveland are maintained for loess units. Formation names for the following alluvial lithostratigraphic units are abandoned: Crete, Sappa, Grand Island, Fullerton, and Holdrege. The Severance Formation is adopted as a new lithostratigraphic unit for thick packages of late Pleistocene alluvium and colluvium in Kansas. The DeForest Formation is accepted as a valid lithostratigraphic unit for deposits of fine-grained Holocene alluvium in Kansas. Formation names Iowa Point, Nickerson, and Cedar Bluffs for glacial tills and Atchison and David City for glaciofluvial deposits are abandoned. The Afton and Yarmouth Soils are abandoned as pedostratigraphic units, whereas the Sangamon Geosol and Brady Geosol are maintained.

Lithological indicators of loess sedimentation of SW Poland

Abstract High-resolution grain-size investigations were carried out in two SW Polish loess sections: Biały Kościół (Niemcza-Strzelin Hills) and Zaprężyn (Trzebnica Hills). Each sequence was sampled by using the same methodology and samples were taken at 5 centimeters intervals. The particle size distribution was obtained with a Mastersizer 2000 laser, used for diffraction methods. From the obtained results the basic parameters and grain size indicators were calculated: Mz, Grain Size Index ratio, U-ratio and the percentage content of clay (< 4μm) and sand (> 63 μm). Both loess-soil sequences are composed of interfluve and slope loess facies and consist of five litho-pedostratigraphic units developed during the Late Pleistocene and Holocene: two loess units L1LL1, L1LL2 and three polygenetic fossil soils sets S0, S1 and L1SS1. The distance between these two profiles is about 60 km. Zaprężyn, as a section located more to the north, has almost no lower younger loess and higher level of weathering which could be related to proximity of this site to the Ice Sheet margin. The climate here was more extreme and harsh. What is more, the difference in development of soil L1SS1 can be observed: while in Biały Kościół pedogenesis process was slower and less disturbed than in Zaprężyn. The upper part of L1SS1 in Biały Kościół was deformed by gelifluction, frost heave and other periglacial processes. Mz indicator by the grain-size distribution in these sediments reflects subtle variations in the climatic system. Moreover, in Zaprężyn the content of sand fraction is higher than in Biały Kościół what can be the evidence of short episodes of strong winds during cold period of sedimentation. The aim of this paper is to compare two loess profiles by their stratigraphical and lithological similarities which are result of climate conditions and features of surrounding environment.

Vegetation variation of loess deposits in the southeastern Inner Mongolia, NE China over the past ∼1.08 million years

The stable carbon isotopic composition of organic matter of aeolian silt deposits is regarded as an appropriate proxy index of paleovegetation, especially in the Chinese Loess Plateau in central China. In this study, a loess–paleosol sequence in the southeastern Inner Mongolia Autonomous Region in northeastern (NE) China, which is located outside the Chinese Loess Plateau, is chosen to reconstruct the vegetation history since ∼1.08 Ma. Temperature exhibits a threshold value, which determines the growth of C4 plants in this study area. The organic matter of the samples is derived from two different vegetation types, namely, the mixed C3 and C4 plants and the pure C3 plants. The δ13C of the organic matter shows negative values in loess units and higher values in paleosol units. This finding reflects the influence of temperature and summer monsoon intensity on the vegetation dynamics over glacial–interglacial cycles. On a longer time scale, the δ13C values are higher between ∼1.1 and ∼0.9 Ma and after ∼0.35 Ma, and lower between ∼0.9 and ∼0.35 Ma, which may be attributed to a long-term temperature variation. Our analysis shows that regional temperature is the most important limiting factor that forces vegetation changes at the glacial–interglacial time scale in NE China.

Magnetic stratigraphy of the Danube loess: A composite Titel-Stari Slankamen loess section over the last one million years in Vojvodina, Serbia

In Europe, the most complete loess-paleosol sequences over the past million years are preserved in the Serbian part of the Danube River drainage basin. The similarity in stratigraphy and climatic cycles between the Serbian and the Chinese loess sequences suggests that the loess deposits in these two regions could play an important role in the study of intercontinental climatic correlations and linkage. However, previous magnetostratigraphic studies of the Serbian loess, mainly conducted using the alternating field (AF) demagnetization approach, show significant differences with the Chinese loess, especially the stratigraphic positioning of the Matuyama-Brunhes boundary (MBB). Here, we conduct a detailed magnetostratigraphic investigation of a composite Titel-Stari Slankamen loess section in the Vojvodina (northern Serbia), using thermal and hybrid demagnetization methods, aiming to establish new chronological constraints for the Danube Basin loess. Based on the analyses of the mineral magnetic properties, anisotropy of magnetic susceptibility, progressive thermal and hybrid demagnetization, the MBB was determined within S8, broadly consistent with the MBB position seen in the Chinese loess. Present evidence supports the hypothesis that the overall normal polarity in the upper part of L9, with a thickness of 2.1 m, was possibly caused by a combination of the remagnetization of remanence-carrying coarse-grained magnetite during the Brunhes normal chron and bioturbation during the forming of the overlying paleosol S8. Thus, our magnetostratigraphic results provide solid chronological evidence for the coherence of pedostratigraphy and climatostratigraphy for the loess deposits in the Danube Basin and China. The synchronous occurrence of the strongest developed paleosol S5 (corresponding to MISs 15–13) and the least weathered thick loess unit L9 (corresponding to MISs 24–22) in the two regions confirms that extreme climatic events during these two intervals are possibly common features of the climate in the Northern Hemisphere at those times.

Mineralogical and geochemical evidence of weathering in a middle to late Pleistocene paleosol sequence in the Driftless Area of Wisconsin

AbstractPaleosols occur in the Driftless Area of southwestern Wisconsin because this area escaped direct Pleistocene glaciation, allowing long-term loess and colluvium accumulation in selected settings. The most complete known depositional sequence at Oil City, Wisconsin, contains eight lithologic units with five paleosols, all with normal remanent magnetism (i.e., <780 ka). Previous work characterized the stratigraphy, pedology, micromorphology, and clay mineralogy of the section. We investigate chemical weathering of the 8–63 µm silt fraction using X-ray diffraction (XRD) and elemental geochemistry by portable X-ray fluorescence (pXRF). Elemental ratios TiO2/CaO, Zr/Sr, and Rb/Sr generally align with the pedological evidence of weathering. Mineral ratios plagioclase/quartz and K-feldspar/quartz display greater scatter and less certainty in interpretation. The paleosols with Bt horizons have ratios indicating greater weathering than the modern soil. The most weathered paleosol is formed in the unnamed fourth loess unit stratigraphically below the Loveland Member. The stratigraphic position and higher degree of weathering support correlation of the fourth loess with the Yarmouth Geosol and Crowley’s Ridge Silt of the Middle Mississippi valley. Geochemical indices by pXRF are more consistent with established lithologic breaks than mineralogy by XRD. Our results support the use of pXRF for stratigraphic and weathering studies of Quaternary sediments.

Late Quaternary loess and soils on uplands in the Canyonlands and Mesa Verde areas, Utah and Colorado

AbstractThin loess deposits on the uplands of the southeastern Colorado Plateau have previously not been well studied. We sampled deposits and soils from trenches on Hatch Point (HP) mesa near Canyonlands National Park, Utah, and from two outcrops in Mesa Verde National Park, Colorado. At HP, the oldest buried unit yielded 2 optically stimulated luminescence (OSL) ages of 10,370 and 7555 yr; the middle unit yielded 10 OSL ages from 6220 to 1385 yr; and the youngest unit had a single age of 1740 yr. At Mesa Verde (MV), three loess units are preserved in the two outcrops we examined; 6 OSL ages range from 51 to 17 ka. At least one buried soil is present between two units with ages of about 50 and 40 ka. The ages of the loess units in both study areas correspond well with OSL-dated dune sands in Canyonlands National Park and with dune sands on Black Mesa, Arizona. Particle-size distribution combined with chemical and magnetic data indicate that HP loess was derived mostly from nearby sandstone sources with a small component of far-traveled atmospheric dust, whereas MV loess was sourced both from the nearby sandstone and the San Juan River and its tributaries.

Loess deposits since early Pleistocene in northeast China and implications for desert evolution in east China

Loess deposits and deserts are regarded as coupled geological systems and loess deposits on the periphery of deserts can often be used to reconstruct desert evolution. Previous studies of desert evolution in Asia are mainly concentrated in northwest China and the China Loess Plateau, and little is known about long-term desert evolution in east China. In this study, we selected the Sishijiazi loess section in the Chifeng area in northeast China to study the long-term evolution of the desert in east China. A high-resolution magnetostratigraphy combined with optically stimulated luminescence dating indicated that the age of the section base is approximately 1.02Ma. The Brunhes-Matuyama boundary is at the depth of 39.8m in loess unit L8, and the upper boundary of the Jaramillo Subchron is at the depth of 60.8m in paleosol S10. The results of grain-size analysis indicate a coarsening grain-size trend in the past 1.0Ma. In addition, based on grain-size variations, the desert evolution in east China since ∼1.0Ma can be divided into three stages: stability from 1.0 to 0.8Ma, desert recession from 0.8 to 0.5Ma, and gradual expansion since 0.5Ma. Our results further indicate that the evolution of desert in east China was mainly controlled by changes in global ice volume, and that the uplift of the Tibetan Plateau may have had an additional effect.

Palaeoclimatic records of the loess/palaeosol sequences of the Chinese Loess Plateau

Whether during past climate stages or into a progressively warming world, changes in precipitation constitute a key component of climatic change. Quantitative proxies for palaeo-precipitation are relatively rare. The magnetic properties of the windblown loess units and interbedded palaeosols of the famous Chinese Loess Plateau provide key palaeo-precipitation data for this populous, monsoon-dominated region. The loess/palaeosol sediments record rainfall totals, directly complementing the oxygen isotope records of Chinese speleothems. These isotopic records predominantly reflect moisture source, and hence large-scale atmospheric circulation changes. The two major Asian monsoon systems appear to display antiphase behaviour. Dominance of the Indian summer monsoon system seems associated with minimum precession/maximum northern hemisphere summer heating; dominance of the East Asian summer monsoons with maximum precession. At ∼2.8 Ma, more intense development of the East Asian winter monsoon initiated major increases in dust deposition rates, and formation of relatively unweathered loess layers. Glacial-stage loess units then interleaved with interglacial/interstadial-stage palaeosols throughout the Quaternary period. Decoupling of the loess/palaeosol rainfall records from the Chinese cave records of moisture source shows that the Indian and East Asian monsoon winds were continuously driven by precessional forcing while summer monsoonal rainfall was greatly suppressed during cool, glacial stages. The timing of these East Asian climatic transitions, the subsequent intensification of northern hemisphere glaciations, and the association between monsoon circulation changes and North Atlantic temperatures, indicates a possibly leading global role for these monsoonal changes via alterations in the poleward distribution of heat and moisture.

Quantifying paleoprecipitation of the Luochuan and Sanmenxia Loess on the Chinese Loess Plateau

Paleoprecipitation is one of the most important paleoclimatic factors for monsoon studies. Magnetic properties of the Chinese loess/paleosol sequences have been demonstrated to be controlled strongly by the mean annual precipitation (MAP). In this study, temporal variations of MAPs were systematically investigated for the Luochuan (LC) and Sanmenxia (SMX) sequences from the loess unit L1 (~10ka) to the paleosol unit S5 (~620kyr) by applying the new climofunction MAP=8.06×χFD/HIRM+254 (where χFD and HIRM are frequency-dependent magnetic susceptibility and “hard” isothermal remanent magnetization, respectively). For both the LC and SMX sequences, our climofunction indicates paleosols have higher MAPs than loess units and the maximum MAP value occurs in S5, indicating that the East Asian summer monsoon was the strongest during the formation of S5. In agreement with previous studies, overall, MAPs of SMX are higher than those of LC, as SMX is in the southeast of LC and receives more precipitation. The positive correlation between the reconstructed MAPs and χFD and ARM suggests that precipitation is a key factor controlling degree of pedogenesis. MAPs of paleosols from SMX display a negative correlation with synchronous insolation maxima and a positive correlation with corresponding insolation minima. However at LC, MAPs have a less significant relationship with insolation. This may be because the summer monsoon signals are relatively weak in LC and covered by local climate and environmental alterations. In interglacials, the summer monsoon, associated with precipitation, is controlled by the corresponding insolation maxima and minima. In addition, the MAP gradient between LC and SMX is also modulated by the insolation amplitude.

Stratigraphy, morphology, and geochemistry of late Quaternary buried soils on the High Plains of southwestern Kansas, USA

This study investigated two stratigraphic sequences that record the complex interplay of sedimentation and pedogenesis over the past ca. 84ka on the High Plains of southwestern Kansas. Up to eight eolian sand and loess units with associated soils were identified in two cores collected from an upland setting. Soil morphological and geochemical data were used to quantitatively assess and compare soil development between multiple buried soils. Chronostratigraphic relationships indicate that 1) loess and eolian sands were episodically deposited during late Marine Isotope Stage (MIS) 5 (ca. 84–70ka), 2) the Sangamon soil at this site formed between MIS 3/4 (ca. 70–52ka), which is consistent with age estimates for renewed Sangamon pedogenesis in the Mississippi River valley, and 3) soil development in the Gilman Canyon Formation began at ca. 44ka and continued until at least 29.2ka. Multiple lines of evidence, including grain-size distributions, structure, clay content, and chemical weathering index data, indicate that the morphology and chemistry of the Ak horizon developed in the Gilman Canyon Formation is a product of cumulization by slow loess additions. Similar evidence suggests that cumulization processes are also responsible for the morphology and chemistry of other buried soils in the stratigraphic sequence. Overall, loess inputs during pedogenesis complicate the quantification of weathering processes in these soils.

Plagioclase sub-species in Chinese loess deposits: Implications for dust source migration and past climate change

Plagioclase mineral sub-species in the Lingtai Section in central Chinese Loess Plateau are examined using Mineral Liberation Analyzer techniques, showing that loess and paleosol samples exhibit similar patterns in terms of plagioclase feldspar sub-species content. This suggests that both loess and paleosol units have preserved their primary Ca-bearing plagioclase compositions of loess source regions. Weighted average CaO (%) in Ca-bearing plagioclase lies within a narrow range and is equivalent to the average plagioclase composition for upper continental crust. This fact supports the hypothesis that Chinese loess deposits are the result of a thorough mixing of dust sources. The sum of Ca-bearing plagioclase content exhibits a general increasing trend superimposed by glacial–interglacial oscillations. In combination with observed plagioclase data in the deserts, the variations of Ca-bearing plagioclase minerals might be used as a proxy for dust source migration and climate changes in the loess source regions. Furthermore, linear relationship between lithogenic magnetic susceptibility (MS) component input and contents of Ca-bearing plagioclase in loess units revises a MS proxy for reconstructing paleo-monsoon precipitation history. The revised MS and plagioclase sub-species records help in understanding the mechanism of glaciation across northern Tibetan Plateau.

Magnetic stratigraphy constraints on the Matuyama–Brunhes boundary recorded in a loess section at the southern margin of Chinese Loess Plateau

Although the Matuyama-Brunhes boundary (MBB) in the Chinese Loess Plateau (CLP) is very important in reliably correlating Quaternary loess with other sediments in the world, particularly with marine and polar ice cores, its exact stratigraphic position remains controversial. Previous investigations usually placed the MBB between paleosol unit S8 and loess unit L8 in various locations. To better understand the spatial differences in the MBB position, a high-resolution paleomagnetic study was conducted in a loess section of the Lantian Basin at the southern margin of the CLP. The results show that the MBB is situated in the middle of the relatively weak paleosol unit S7, consistent with a recent report on the MBB based on a Be-10 study from the Xifeng and Luochuan loess sections of the central CLP. However, the regional anomalously low magnetic susceptibility in paleosols S7 and S8 indicates that it is more reliable to determine the paleoclimate boundaries between loess and paleosol horizons of this segment with median grain size. Then, the MBB in the Yushan section can be correlated with the bottom of paleosol S7, corresponding to the older part of interglacial marine isotope stage 19. This result temporally reconciles the striking discrepancy of the position of the MBB recorded in between loess and other typical sedimentary sequences, and further confirms that the stratigraphic position of the MBB could spatially vary to a certain extent due to regional sedimentary or paleoclimatic conditions in the marginal areas of the CLP. In the Yushan section, the high-frequency variations of paleomagnetic directions during a long period of similar to 31 ka before the MBB, however, could not be attributed to a genuine response to the true geomagnetic behaviour. Moreover, the climate offset defined by the magnetic susceptibility and median grain size of the section can be preliminarily attributed to the regional geology and paleoenvironment background. A multiproxy-based stratigraphic division is considered very necessary when paleomagnetic and climatic boundaries are defined exactly in a specific area of the southern CLP.

Color characteristics of Chinese loess and its paleoclimatic significance during the last glacial–interglacial cycle

The soil color is widely used in paleoclimate and paleoenvironment reconstructions in the Chinese Loess Plateau. To better understand the color spatial changes during the glacial–interglacial cycle, the soil color lightness (L*), characteristic spectra, magnetic susceptibilities and mean grain sizes of three loess–paleosol sequences were compared. Results showed that high L* and low hematite to goethite ratios (Hm/Gt) appeared in loess units, and low L* and high Hm/Gt ratios accompanied paleosol layers, indicating glacial–interglacial hydrothermal oscillation. L* in the Yulin section was higher than in the Chaona and Lihuacun sections, indicating that different precipitations have great effect on L*. Furthermore, Hm/Gt, magnetic susceptibility (χlf), and mean grain size are correlated closely with L*. L* and Hm/Gt not only document climatic variations in the glacial–interglacial cycle vis-à-vis loess–paleosol sequences, but also can identify Heinrich cold events and millennial scale Dansgaard–Oeschger (D–O) warm events. It suggests that soil color responds sensitively to global climate change driven by ice volumes. L* and Hm/Gt curves exhibit higher frequencies and larger amplitudes than magnetic susceptibility (χlf) curves, indicating that L* and Hm/Gt can be regarded as sensitive and reliable proxies for characterizing high-resolution climate change during the last glacial–interglacial cycle.

Physical Model Test Study of the Relationship Between Loess Landslide and Rainfall

Launching research has important theoretical and practical value on model test study of the relationship between loess landslide and rainfall. According to the monitoring data of model test of loess slope, formula expression of safety factor has been fitted on landslide. Firstly, the loess slope damage process was monitored at different rainfall intensity, rainfall duration through rainfall landslide model tests. Secondly, some parameters were determined by laboratory tests, such as loess unit weight, moisture content, shear strength. Thirdly, the relational expression was fitted between moisture and rainfall intensity, rainfall duration, cohesion, internal friction angle, unit weight. Finally, moisture content being an intermediate variable, the expression was got between landslide safety factor and rainfall intensity, rainfall duration.

A new luminescence chronology for the Mangshan loess-palaeosol sequence on the southern bank of the Yellow River in Henan, central China

An unusually thick loess-palaeosol sequence was found at Mangshan on the southern bank of the Yellow River, central China. Previous attempts to establish its chronology using TL and IRSL techniques have left major uncertainty over the stratigraphic division of the sequence. Here, we report results of a quartz and polymineral luminescence dating study on medium-grain (45–63 μm) fractions from the upper five loess units of the sequence. The base of the first loess unit is dated to 68.6 ± 4.9 ka with quartz extracts. While the quartz SAR age for the top of the second loess unit is 87.2 ± 6.1 ka, the application of an elevated temperature post-IR IRSL (pIR200IR290) SAR procedure to polymineral fraction from the same sample yields an age of 137 ± 10 ka. Using this procedure, the upper five loess units are dated. Our new luminescence dates show that the uppermost ∼41 m loess accumulated during the last glaciation. This disagrees with the previously published luminescence ages but is consistent with the proposed stratigraphy based on the correlation with sections in the Loess Plateau. The accumulation rates of the Mangshan loess during the last two climate cycles are found 4–7 times higher than those in the central Loess Plateau as well as those in the lower part of the sequence. Measurements of quartz OSL/TL sensitivity reveal significant differences among the samples in different loess units, hence providing evidence for the changes in the supply of source materials as a cause for the dramatic acceleration in loess accumulation.

Evidence for bioturbation of luminescence signals in eolian sand on upland ridgetops, southeastern Minnesota, USA

This study sought to identify the age of eolian sand on narrow ridges in the Root River valley of southeastern Minnesota. The ridges in the region are cored by Ordovician dolomite and Cambrian sandstone, but the ridge tops are typically covered with Late Wisconsin Peoria and older loess units. In some locations Peoria loess is absent and the ridgetops are covered with up to 3 m of eolian sand that was likely sourced from local river valleys and transported to the uplands by sand ramps. We studied nine ridgetop soil profiles and collected seventeen OSL samples from eolian sand at depths ranging from 0.3 to 2.6 m below the present ground surface. All OSL samples were analysed using small aliquots where 90–150 μm quartz grains were applied to the inner 2 mm of 10 mm aluminum disks. The OSL ages ranged from 12.3 to 1.5 ka indicating a significant amount of age variability, and potentially suggesting nearly continual eolian deposition throughout the Holocene. However, several key differences were identified between those samples taken from within 1 m of the ground surface compared with samples that were more deeply buried. Those samples collected from depths of greater than 1 m yielded ages that were tightly clustered between 12.3 and 10.3 ka, while samples taken from depths of less than 1 m showed ages with much higher spread that ranged from 1.5 to 10.1 ka. The samples collected from within 1 m of the present ground surface also commonly showed higher spread in their equivalent dose distributions and higher overdispersion values relative to the samples that were more deeply buried. This suggests the luminescence signals from the upper portions of these deposits were reset after burial, most likely by bioturbation, and that the OSL ages are not depositional ages. This interpretation is supported by evidence from a core sample collected from one of our sites that shows primary eolian lamina are preserved below 1.4 m depth but not above this depth. Presumably, the bioturbation agents were effective at both resetting the luminescence signal and disturbing the primary bedding to depths of at least 1 m. If bioturbation occurred below this threshold it apparently did not impact either of these indicators. Our findings suggest the upper 1 m of these profiles were impacted by bioturbation and that all of the eolian sand on these ridgetops was likely deposited prior to 10.1 ka.

Middle Miocene eolian sediments on the southern Chinese Loess Plateau dated by magnetostratigraphy

We present the results of a magnetostratigraphic grain size and geochemical investigation, of a well-preserved sequence of eolian sediments of Miocene age at Duanjiapo, on the southern Chinese Loess Plateau. The paleomagnetic results demonstrate that the basal age of the studied interval is about 11Ma. The results of the grain size and geochemical analyses show strong similarities between the eolian Miocene silts and the overlying Red Clay and loess units. This leads us to conclude that all formations have a similar eolian origin, although the Miocene silts include evidence of fluvial sediments derived from river originating in the Qinling Mountains, to the south of the study site. Our results indicate that the accumulation of eolian dust in the southern Chinese Loess Plateau area began at least 11Ma ago.