Loess Mantle Research Articles

Meteoric 10Be as a tracer of soil redistribution rates and reconstruction tool of loess–mantled soils (SW, Poland)

Loess deposits are terrestrial archives that record progressive deposition and erosion events of varying intensities. Data on long-term erosion rates are crucial for tracking changes in the stability of a loess mantle and reconstructing the evolution of loess-enriched soils. We used meteoric 10Be to i) define the factors responsible for its distribution along the profile, ii) determine long-term erosion rates in loess-enriched polygenetic soils characterised by illuviation processes, and iii) evaluate initial soil thickness and stability over time. Distribution of meteoric 10Be along the soil profiles was mainly driven by its translocation with clay particles and accumulation in the illuvial horizons. However, in some cases (loess over serpentinite), the highest meteoric 10Be content was measured in the C horizons which may be related to the longer exposure of serpentinite to meteoric 10Be deposition before the occurrence of a major loess accumulation event. The estimated long-term erosion rates greatly depend on the assumed environmental settings and were in the range of about 0.1–3 t ha−1 yr−1. Based on the soil redistribution rates, we reconstructed the removed loess layer which was from a few dm to about 3 m. The results indicate four main soil evolutionary phases: a) pre-exposure of sediments to meteoric 10Be accumulation; b) formation of thick loess mantles during the Last Glacial Maximum; c) erosion events between 21 and 11.6 ka that significantly shallowed the initial loess mantles; d) pedogenesis (with subsoil clay accumulation) in the Holocene within the thinner relicts of the former Late Pleistocene loess mantle followed by a recent and strong erosional phase due to human impact. These phases are also believed to have occurred in several other areas of Central Europe.

Multiproxy approach to the reconstruction of soil denudation events and the disappearance of Luvisols in the loess landscape of south-western Poland

Loess landscapes are highly susceptible to soil redeposition processes and thus may provide detailed insights into the record of denudation processes. Using optically stimulated luminescence dating and the soil micromorphology of 12 soil profiles, we reconstructed a complete record of denudation processes in south-western Poland. The first episode of soil redeposition took place around 9.1 ka. The denudation events that followed were attributed to the Neolithic (6.4 ± 0.3 ka), early Bronze Age (3.8 ± 0.2 ka), early and late Middle Ages (1.5 ± 0.1 ka and 0.7 ± 0.03 ka, respectively) and early Modern (0.4 ± 0.02 ka). As a consequence of the denudation processes, the soil cover in the studied area had been strongly reshaped. The predominant Luvisols had experienced progressive erosion processes that led first to a significant shallowing of the eluvial and argic horizons (truncated Luvisol) and, after some time, to their complete removal. Further thinning of the loess mantles had exposed geological substrates with very weak pedogenic alternations, thus pushing their transformation towards Regosol types. Similarly, Regosols occurred in toeslopes where freshly eroded material had been deposited, and where diagnostic horizons had not yet developed. Modern soil erosion rates in the studied loess area have considerably increased, and it is estimated that the Luvisol status may be completely transformed within approximately 80–300 years, if not sooner, due to progressive climate change.

Clay mineralogy fingerprinting of loess-mantled soils on different underlying substrates in the south-western Poland

Loess may be integrated into slope deposits at different soil depths and may influence related physical, chemical and mineralogical soil properties. Therefore, tracking the border between deposited loess and underlying materials and estimating the depth of loess penetration is a challenge. Five soils from the Lower Silesia province in south-western Poland having different types of geologic substrate and being covered by loess deposits of various thicknesses were chosen to: 1) trace the origin of phyllosilicates in these heterogeneous soils (loess vs. underlying bedrock); 2) determine the influence of the geologic substrate and the loess mantle on clay mineral transformation in the soil; and 3) relate clay mineralogical traits to soil morphology. The loess consisted of long-distance and local aeolian sources as shown by the Ti/Zr ratios and heavy mineral composition. Geochemical and mineralogical traces of loess were easily detected in the mixed zone and in part also in the basal layer. The loess deposits are characterised by mica, chlorite, kaolinite, interstratified mica-vermiculite or hydroxy-interlayered vermiculite, vermiculite and a minor amount of smectite. Aeolian silt admixture was an important source of chlorite at all sites. The vertical distribution of mica indicated two types of sources — loess input and substrate. Smectite, however, has mostly been inherited from the underlying bedrock (serpentinite, glacio-fluvial deposits and basalt slope sediment) or was formed in the soils from mica or chlorite. Where present in the loess, smectite was only found in small amounts. The presence of kaolinite, HIV and mixed layered mica-vermiculite could be derived either from aeolian input or neoformation and thus actively occurring mineral (trans)formation reactions. Similar to mica, vermiculite was derived from both aeolian input and the geological substrate. The widespread loess deposits in this region rejuvenated the soil formation process, as evidenced by their mineralogical and chemical composition.

Provenance and paleoenvironmental context of the Late Pleistocene thin aeolian silt mantles in southwestern Poland – A widespread parent material for soils

Thin loess deposits are widespread soil parent materials and important archives for paleoenvironmental reconstruction. The origin of loess in SW Poland is attributed to the Great Odra Valley (GOV), following the general concept that large rivers play a major role in regional silt supply. Yet, the precise provenance (glacier sources and/or local rocks) of silts, possibly deflated from dry GOV braided riverbeds, is not clear. Our study of thin and thick loess mantles in SW Poland for the first time indicates the provenance of thin loess based on mineralogical (MLA-SEM) and isotopic analyses (143Nd/144Nd, 87Sr/86Sr). Luminescence ages of five localities point to thin loess mantle formation during and shortly (23.0 to 17.7 ka yr) after the Last Glacial Maximum (LGM). Our isotopic data indicate that thin loess deposits in SW Poland are the mixtures of two main components – local Sudetic and Scandinavian, the latter delivered by the Fennoscandian ice sheet (FIS). Also, detailed analyses of heavy minerals show that a single mineral (e.g., hornblende) may come from both Sudetic and Scandinavian sources. This research highlights the role of the (Pleistocene) GOV in collecting and homogenizing materials, while supplying the region with fine particles to be deflated by paleowinds from open surfaces. Anomalies in mineralogy and isotopic composition are connected with influence of Sudetic mountain rivers and locally blowing silt material by katabatic winds. Regional grain size differentiation of thin loess mantles explains transport distance and altitude.

Weathering and paleoprecipitation indices in a Late Pleistocene–Holocene loess–paleosol sequence in central Argentina

A Late Pleistocene–Holocene loess–paleosol sequence in central Argentina was studied to obtain a weathering insight and to probe geochemical tools as paleoclimatic proxies. Three paleosols and a buried soil interlayered with loess mantles compose Corralito I sequence (32°00′7″ S, 64°11′08″ W, 469 m a.s.l). Several weathering indices (CIA, CIA-K, and LWI) and elemental ratios (∑Bases/Al2O3, Rb/Sr, CaO/TiO2, Na2O/TiO2) were calculated. An incipient weathering degree throughout the sequence was detected, denoting that the parent material (i.e., loess mantles) has not been substantially modified by pedogenesis. Although a statistically significant correlation between magnetic susceptibility and several weathering indices was identified, the weathering intensity was not strong enough to produce a decrease in the magnetic signal of the paleosols, which is, in part, inherited from the detrital fraction. Paleoprecipitation indices, calculated using geochemical data, suggest slightly wetter conditions during paleosol formation when compared to loess. Paleosols were formed during humid conditions (paleosols II and II during MIS 5e, and paleosol I during MIS 3) with sedimentation pause and soil development, whereas loess layers were deposited during more arid conditions.

Chronostratigraphy of silt-dominated Pleistocene periglacial slope deposits on Mt. Ślęża (SW, Poland): Palaeoenvironmental and pedogenic significance

Slope deposits with aeolian silt admixture are a widespread parent material of soils in the temperate zone but may be neglected when rates of soil production are quantified. The concept of periglacial cover beds differentiates slope deposits with or without aeolian silt admixture; yet there is a remaining debate on processes and the timing of their formation. A previous study done by us at Mt. Ślęża, SW Poland, concluded that slope deposits with variable aeolian silt admixture, or its lack, have a significant influence on the pathway of soil formation. The present work builds upon this finding, by adding further granulometric and micromorphological data from three representative profiles along a toposequence, in order to refine our understanding of local slope deposits and soil formation. Additionally, seven numerical ages using luminescence dating provide a chronological framework for our reconstructions and allow linking the forming processes of these pedosedimentary records to regional palaeoenvironmental conditions. The oldest aeolian deposits are of Middle Pleistocene age (>280 ± 19 ka) with interlayered palaeosol (marine isotope stage [MIS] 9 or older). Late Pleistocene slope deposits encompass the maximum loess thickness and are dated to MIS 2. Luminescence ages from the upper layers indicate shallow reworking, which we tentatively correlate to the Younger Dryas (YD). Two profiles with thick loess mantles have strong clay illuviation features, presumably formed during the Holocene. However, weak clay illuviation in the third profile with a thin loess mantle (having an age of YD) over granite regolith seems to have occurred before the Holocene, as only fragmented clay coatings (probably MIS 2 pedogenesis) could be found.

The paleoclimatic message from the polymodal grain-size distribution of late Pleistocene-early Holocene Pampean loess (Argentina)

Wind-blown dust deposits are considered one of the most important terrestrial archives for past climate change studies. In the Southern Hemisphere, the Pampean loess is the most extensive paleo-dust record, whose origin is still a matter of debate. In this paper, grain-size was carried out at three high-resolution loess profiles deposited during the late Pleistocene-early Holocene and for present-day dust collected on the Pampean plain. Based on comparing loess records with in situ present-day dust, this work aims to provide constraints on the climatic conditions that allowed deposition of the Argentinean loess mantle. Unmixing methods allow differentiating three grain-size subpopulations/end members in the Pampean loess which are comparable to end members found in present-day dust. The provenance and transport conditions observed for present-day dust were used to infer that the fine silt components of Pampean loess were transported by high-level air stream associated with the deflation of the Puna-Altiplano Plateau. On the other side, the coarse-silt sediments were carried by dust storms associated with high transport energy events taking place in proximal dust sources. In the central Pampas, the increased abundance of coarse-silt sediments during the Antarctic Cold Reversal (ACR) may indicate the existence of an increased frequency of dust storm episodes. During the beginning of the Holocene, the rise in fine-silt loess could be linked to the activation of distant sources associated with lake desiccation in the Puna-Altiplano Plateau.

Geochemistry of a Late Quaternary loess-paleosol sequence in central Argentina: Implications for weathering, sedimentary recycling and provenance

A loess-paleosol sequence in the Pampean Plain, central Argentina was analyzed geochemically, in order to evaluate the weathering signature and sedimentary recycling as well as to analyze the origin of these loessic sediments. Corralito I sequence is composed of three paleosols and a buried soil separated by loess layers. The loess-paleosol sequence shows a similar geochemical composition throughout, suggesting that pedogenesis has not been intense enough to mask the chemical signature of the parent material. Weathering indices (CIA, CIW, PIA and LWI), elemental ratios (Rb/Sr, CaO/TiO2 and Na2O/TiO2), and the A-CN-K ternary diagram indicate incipient chemical alteration for this sequence, compatible with a weathering-limited regime. As expected, paleosols (CIAs ~60) exhibit a slightly higher chemical alteration than loess layers (CIAs ~58). Mass balance calculations reveal the losses of some major oxides (i.e., CaO, Na2O) and Sr in the buried soil and paleosols compared to its loess mantles, reflecting the weathering of plagioclase. Besides, the losses of K2O and Ba in the buried soil and paleosols I and II, when compared to its loess layers, suggest the chemical alteration of K-feldspar. The chemical alteration of volcanic glass is evidenced by the relative losses of As in almost all levels. Conversely, the relative gains of Fe2O3 and MgO reveal that pyroxenes and amphiboles or Fe oxy-hydroxides are concentrated in paleosols and the buried soil compared to its loess mantles. The geochemical approach used to constrain the origin of these Pampean Plain loessic sediments indicates they derive mainly from the Andean region. Conversely, the Pampean ranges, the Paraná River basin and Uruguayan Precambrian outcrops, which were pointed out as minor local sources, do not contribute significantly to the loess deposits. This work also shows that Corralito I loess-paleosol sequence and other loess samples from Argentina do not evidence sedimentary recycling, suggesting that these Pampean Plain loess deposits are mainly composed by young materials derived from undifferentiated volcanic rocks.

Tracking textural, mineralogical and geochemical signatures in soils developed from basalt-derived materials covered with loess sediments (SW Poland)

This study investigates the scale of inheritance of signatures typical of loess- and basalt-derived substrates in soils having both components present as parent material due to past periglacial processes. Based on field description, particle size distribution, mineralogy (heavy minerals and clay minerals) as well as geochemistry we track the signal of both materials (local basalt-derived and loess-derived), estimate the maximum thickness of loess mantles and mixed zones (having both loess and basaltic components), and note the presence or absence of basal layers without allochtonous additions. The results show that geochemistry and heavy minerals are the most reliable proxies for a verification of aeolian silt contributions and to define individual layers in complex soil parent materials.Soil formation in the studied landscape depends on the type of slope sediments. Typical fine-grained weathering of basalt without input from aeolian silt would promote rather vertic development. However, in thick loess deposits as well as in thin silt-textured materials superimposed on clay loam beds, clay illuviation dominates. Development of argic horizons, however, results in decreasing permeability, promoting water stagnation and enhanced degradation processes in clay-rich subsoil. Instead of Luvisols, Stagnosols and Retisols developed in the study regions. Basaltic block covers mixed with loess host cambic horizons. Based on our findings it seems that at the edge of thin loess deposits no soils exist that developed exclusively from basaltic parent materials.

Paleoenvironments reconstructed from the analyses of loess sequences on Susak Island, Adriatic Sea

Susak Island is the outermost member of the archipelago of the Kvarner Bay, North Adriatic Sea, Croatia. Its long-term landscape evolution is defined by tectonic movements. Most characteristic are partly exhumed landforms of an ancient terrane (scarps, uplifted limestone cliffs) in a still active neotectonic environment, but the most appropriate tools for the reconstruction of Quaternary paleoenvironments are loess and loess-like deposits. The Quaternary sequence is up to almost 100 m thickness locally and intensively studied by numerous disciplines (from structural geology to geochemistry and geomorphology) today. The special location of the island makes it a key area of research into the evolution of the broader environment, including the Po Plain as well as other Italian source areas of wind-borne and redeposited dust. The loess mantle was also of great importance for a paleoecological reconstruction of floral and faunal evolution, on which efforts of nature conservation are founded.

Primeras citas de ostrácodos (Crustacea) del Pleistoceno tardío de la provincia de Formosa, Argentina

First records of ostracods (Crustacea) from the late Pleistocene of the Formosa province, Argentina. We report the presence of Quaternary ostracods in the Province of Formosa, Argentina, found on the banks of the Bermejo Rriver, in the town of Villa Escolar. Tthe fossil site is placed 100 km from the city of Formosa, in the region of Laishi. Tthese outcrops were deposited at different moments of the Quaternary period during arid climate, causing the development of dunes and loess mantles. Is likely that the analyzed outcrops correspond to the Bermejo Formation (late Pleistocene). ostracods come from clay-silt levels and belong to the genera: Cytheridella, Cyprideis, Cypridopsis and Candona. Tthis ostracod association would indicate environments of continental influence, freshwater, shallow, silt-clay background with low energy streams.

Loess and floods: late Pleistocene fine-grained valley-fill deposits in the Flinders Ranges, South Australia

Terrace remnants of late Pleistocene fine-grained valley-fills, at present eroded by ephemeral traction-load streams, are reported from many semi-arid and arid parts of the world. While they present promising palaeo-environmental archives for recent geological times such as the Last Glacial Maximum (LGM) for which few other terrestrial depositional records exist, their poorly understood nature has limited their significance. This study examines the fine-grained valley-fill deposits from the Flinders Ranges in South Australia, here called Flinders Silts. It establishes the timing, mode and environmental controls of deposition as opposed to their advancing erosion under the current climate. A regional chronostratigraphy based on 124 numerical dates is discussed, of which 43 radiocarbon and 22 luminescence ages were obtained from 12 sections across three major catchments within the scope of this thesis. Regionally significant intervals of rapid aggradation, relative surface stability and erosion are established. Regional climatic controls are differentiated from intrinsic catchment- and site-specific effects on the system. Further, individual age proxies and age models are critically assessed in how far they reflect depositional events. The final aggradational interval bracketing the extended LGM is discussed in detail on a continuous layered to laminated stratigraphic sequence. The provenance question of the fine-grained sediments and the depositional environment of the Flinders Silts are further addressed by high-resolution particle-size analysis. In order to study the subtle variations within the fine-grained partially-aggregated material, an original parametric sediment-sizing approach is employed. Finally, a range of traditional and emerging analytical techniques are applied to improve our understanding of palaeo-environments promoting aggradation. In conclusion, arid intervals throughout the last glacial cycle resulted in significant quantities of proximal dust being deposited as loess mantles within the catchments of the Flinders Ranges, acting as a near-longitudinal dust trap in the centre of the late Pleistocene “dust bowl”. The fine-grained aeolian accessions were repeatedly eroded by low-frequency high-magnitude precipitation events and redistributed as loess-derived alluvium, congesting narrow gorges and raising the base level for tributaries. Locally, backflooding resulted in the aggradation of layered to laminated slackwater deposits, the most continuous recording at least 12 large and numerous smaller flood events between 24 ka and 18 ka. The synchronous termination of the Flinders Silts coincides with early Deglacial climatic amelioration. The re-establishment of a perennial plant cover stabilising both dune fields and slope mantes is discussed as a potential scenario that would have discontinued dust supply to the fluvial system, in turn promoting incision and erosion. The studied aeolian-fluvial interplay of loess and floods has large implications for our understanding of landscape evolution in…

Loess mantle spatial variability and soil horizonation, southern Wisconsin, USA

Loess is an important Quaternary sediment that records evidence of landscape instability in sediment source areas and landscape stability in accumulation areas. In Wisconsin, USA loess mantles glaciogenic deposits dating to the last glaciation, but the landscape distribution and effects of the loess mantle on soil formation have received very little attention. This study investigates the influence of glacial landforms on the spatial pattern of loess thickness and how the presence or absence of a loess mantle overlying glaciogenic sediments has influenced soil profile morphology, namely thickness and nature of horizonation. Loess thickness follows systematic trends along hillslopes, with slope percentage explaining most loess thickness variation between summit and backslope positions. In the presence of a loess mantle, solum and Bt horizon thickness is proportional to the thickness of the loess mantle in well-drained profiles. In the absence of a loess mantle, landscape and sediment characteristics that control permeability and water throughflow appear to be the most important determinants of profile characteristics. The loess mantle modulates soil formation above the lithologic discontinuity with sandy calcareous glacial sediments by controlling soil water behavior and having physical and chemical characteristics conducive to transformation to soil genetic horizons.

Characteristics and paleoenvironmental significance of a thin, dual-sourced loess sheet, north-central Wisconsin

Abstract We provide the first extensive data on the characteristics, distribution, and origin(s) of a thin but extensive loess sheet in north-central Wisconsin, USA. Unlike the relatively thick loess deposits downwind of broad meltwater valleys that reflect ice sheet dynamics, thin loess sheets far from such rivers may provide unique information about local paleoenvironmental change due to their smaller, and often environmentally sensitive, source areas. Silt loam textured and thin ( 70 cm) and coarsens towards the west and northwest, such that, on its western margins, the loess mantle is dominated by very fine and fine sands. Collectively, data on loess particle size distributions, thickness, and silt mineralogy suggest that this loess sheet had sources in two distinct and disjunct landscapes: (1) the late Wisconsin terminal moraine to the northwest, and (2) the sandstone-dominated landscapes to the west and southwest. Post-glacial thawing of these permafrost-affected landscapes probably led to draining of ice-walled lake plains on and behind the moraine, as well as destabilization of slopes on the sandstone landscape; in both cases, exposing large quantities of sediment for deflation. Our study makes two significant contributions to aeolian research: (1) we determine that the north-central Wisconsin loess sheet, unlike almost all others, has two distinctly different and disjunct loess sources, and (2) neither of these two loess sources fall within the traditional mode of “glacial meltwater valley.”

Dune stabilization in central and southern Yukon in relation to early Holocene environmental change, northwestern North America

Eolian deposits of central and southern Yukon, northwestern Canada, consist of loess mantles, small areas of active dunes, and larger stabilized dune fields. Dune fields in valley settings within the region are situated both within and beyond the limit of the last glaciation. Infrared stimulation luminescence (IRSL) dating in central and southern Yukon reveals that these dune fields stabilized as late as 9–8.5 ka, well after the retreat of Cordilleran glaciers. These findings are comparable to other valley-setting dune fields and loess from central Alaska, which record activity during the period from the Lateglacial to the Holocene Thermal Maximum (HTM), and reduced or altered activity after 9–8 ka. Post-glacial dune activity was most likely related to warm, dry conditions during the HTM, under predominantly shrub-tundra vegetation. Early Holocene stabilization of these dunes probably occurred in response to cooler, moister conditions, and replacement of predominantly tundra by boreal forest cover, dominated by spruce. Stabilization of dune fields in southern Yukon and Alaska most likely represented an extension of the time-transgressive stabilization of dune fields that occurred across northwestern North America with the post-glacial expansion of the boreal forest.

‘Of droughts and flooding rains’: an alluvial loess record from central South Australia spanning the last glacial cycle

Abstract Deposits of proximal dust-derived alluvium (alluvial loess) within the catchments of the now semi-arid Flinders Ranges in South Australia record regionally synchronous intervals of fluvial entrainment, aggradation and down-cutting spanning the last glacial cycle. Today, these floodplain remnants are deeply entrenched and laterally eroded by ephemeral traction load streams. The north–south aligned ranges are strategically situated within the present-day transitional zone, receiving both topographically enhanced winter rainfall from the SW and convectional downpours from summer monsoonal incursions from the north. We develop a regional chronostratigraphy of depositional and erosional events emphasizing the Last Glacial Maximum (LGM). Based on 124 ages (94 accelerator mass spectrometry radiocarbon and 30 optically stimulated luminescence) from the most significant terrace remnants on both sides of the Ranges, we conclude that the last glacial cycle including the LGM was characterized by major environmental changes. Two pronounced periods of pedogenesis between c . 36 and 30 ka were followed by widespread erosion and reworking. A short-lived interval of climatic stability before c . 24 ka was followed by conditions in which large amounts of proximal dust (loess) were deposited across the catchments. These loess mantles were rapidly redistributed and episodically transported downstream by floods. The termination of this regime c . 18–16 ka was marked by rapid incision.

Late Quaternary loess landscape evolution on an active tectonic margin, Charwell Basin, South Island, New Zealand

Loess deposits constitute an important archive of aeolian deposition reflecting wider patterns of glacial atmospheric circulation, and more localised interactions between riverine source areas, loess trapping efficiency and geomorphic controls on erosion rate. Conceptual models have been formulated to explain the coeval evolution of loess mantles and associated landscapes (loess landscape models) but none apply to areas of tectonically induced base-level lowering. This study uses an age sequence of alluvial fill terraces in the Charwell Basin, north-eastern South Island New Zealand, which straddles the transpressive Hope Fault, to investigate geomorphic controls on loess landscape evolution in an active tectonic region. We hypothesize that the more evolved drainage networks on older terraces will more effectively propagate base-level lowering by way of a greater areal proportion of steep and convex hillslopes and a smaller proportion of non-eroding interfluves. Eventually, as the proportion of interfluves diminishes and hillslope convexity increases, terraces shift from being net loess accumulators to areas of net loess erosion. We investigate the nature of erosion and the geomorphic thresholds associated with this transition. Morphometric analysis of alluvial terraces and terrace remnants of increasing age demonstrated geomorphic evolution through time, with a decrease in extent of original planar terrace tread morphology and an increase in frequency of steeper slopes and convexo-concave land elements. The number of loess sheets and the thickness of loess increased across the three youngest terraces. The next oldest (ca. 150 ka) terrace remnant had the greatest maximum number of loess sheets (3) and loess thickness (8 m) but the loess mantle was highly variable. A detailed loess stratigraphic analysis and the morphometric analysis place this terrace in a transition between dominantly planar, uniformly loess-mantled landforms and loess-free ridge and valley terrain exemplified by the oldest terrace remnant. Variations in thickness and preservation of loess sheets demonstrated spatially and temporally variable erosion during loess accumulation. To test our hypothesis of loess persistence we calculated critical steady-state hillslope curvatures from a soil transport model, calibrated for the study area, above which the uppermost loess sheet (L1, max. thickness 1.8 m) should be completely eroded. We compared loess distribution mapped in the field to values of slope curvature calculated from topographic surveys and found two of three critical curvature values had acceptable predictive ability. Where predictions failed this is probably due to transient responses related to active steam incision. Because all critical curvature values predicted presence of loess on the oldest terrace remnant where there is none we conclude that important factors other than morphometric ones are important in determining loess distribution in loess landscapes in active tectonic regions. These may include internal changes to regolith affecting erodibility, or vegetation or topographic interaction with wind patterns affecting loess trapping.

Long-term biogenic soil mixing and transport in a hilly, loess-mantled landscape: Blue Mountains of southeastern Washington

In soil-mantled landscapes, downslope sediment transport occurs via disturbance-driven processes that vary with climate and vegetation change. To help constrain the long-term (≫ 10 yr) pattern and rate of soil mixing and transport in forests, we analyzed the distribution of tephra grains in soil along a hillslope transect in the Blue Mountains, SE Washington. Deposited within a loess mantle, tephra associated with Mt. Mazama (7.7 cal. kyr B.P.) serves as a marker bed for estimating erosion and transport rates. Moving downslope, the buried tephra horizon is progressively exhumed and becomes increasingly mixed in the upper soil layer, reflecting disturbance and transport via tree root growth and turnover. This pattern also implies increasing erosion rates downslope and our hillslope transect becomes increasingly convex coincident with progressive exhumation of the tephra layer. This systematic correspondence between topographic form, specifically, local convexity, and surface lowering is consistent with theoretical models for which soil transport rates depend on slope inclination. From our analysis, calibrated coefficients for a linear, slope-dependent transport model are on the order of 10 − 3 m 2 yr − 1 , consistent with previous work in forested loess-mantled landscapes. In addition, our results reveal both the high degree of soil mixing over millennial timescales and the local variability of mixing in forested landscapes. Furthermore, the results enable us to quantify the amount of energy expended by trees in mixing and transporting soil and the net sediment transport fraction of the net primary productivity NPP of the ecosystem.

Pedogenic alteration of aeolian sediments in the upper loess mantles of the Russian Plain

Study of the upper loess strata within the profile of surface soils highlighted the role of pedogenesis in the formation of characteristic features of loess. Loess-paleosol sequences within the study area are influenced by their position in paleocryogenic microrelief. Clear evidence of sequential loess sedimentation, accompanied by slope processes and pedogenesis, is present in soil profiles within former thermokarst depressions. Different stages of loess sedimentation are marked by cryomorphic features, solifluction stripes and buried humus horizons. The balance between the rate of sedimentation, intensity of slope processes and pedogenesis changed within the upper 3 m of loess strata. Corresponding loess strata in inter-depression areas were also formed by sequential accumulation of aeolian dust, gradually altered by initial pedogenesis that left weakly developed soil profiles without clear horizonation. Pedogenesis resulted in diverse complexes of secondary carbonates, loose soil fabric and microfabric with abundant pores of biogenic and cryogenic nature, as well as other features, characteristic of soils of cold arid environments. The uniformity of these features throughout the upper loess strata confirms the synlithogenic nature of pedogenesis that accompanied loess accumulation.

Interhorizon Transport of Elements in Ultisols Developed in a Loess Mantle Mixed With Rhyolite Residuum in the St. Francois Mountains of Missouri

Four soils of the Ultisol order in the St. Francois Mountains of Missouri, which were developed in loess and rhyolite residuum, were characterized to: (i) describe the dominant soil series and landscape characteristics having mass-wasting features, (ii) identify the types of parent materials, and (iii) evaluate the soil profile descriptions and associated geochemical data to estimate the influence of eluviation-illuviation processes on soil profile expression. In particular, we wished to document the interhorizon transport of an array of elements associated with eluviation-illuviation processes and its importance to understanding soil genesis. The presence of argillic horizons and nutrient accumulations in the near-surface horizons infers stable landscapes. Aqua regia digestion Fe concentrations of the four pooled pedons show a positive and linear relationship with clay content, suggesting that the parent materials and their weathering histories are similar across the sampled pedons. Iron, Al, Cu, Zn, K, Ni, Cr, Ga, Sr, Sc, As, Li, Rb, V, P, S, Sn, Cs, Th, and U are preferentially partitioned in the clay fraction. For the pooled pedons, elemental ratios of V, Sc, Ni, As, Cr, Cu, Cs, Li, Rb, Th and U with respect to Fe are presented as evidence that the parent materials are similar and that loess weathering greatly contributes to the fine earth fraction.