Dust Accumulation Rates Research Articles

Twisting or stable quaternary boundary? A perspective on the glacial late Pliocene concept

There has been a long-running debate over the position of the Pliocene-Pleistocene boundary, i.e. 1.8 Ma versus 2.6 Ma. This study presents litho- and pedo-stratigraphic characteristics, grain size, magnetic susceptibility, and dust sedimentation rates for ‘red clay’-loess sequences over northern China, in order to assess the drastic climatic event(s) recorded in the quasi-continuous eolian deposits, and to readdress the position of the Pliocene-Pleistocene boundary. In complete Chinese loess sequences, 33 soil-loess couplets have been identified and labeled with Si-Li system. Traditionally, the base of Chinese loess has been designated at the bottom of loess unit L33. However, a typical loess layer within the uppermost ‘red clay’ is identified and formally designated here as L34 and the soil unit between L33 and L34 as S33, which lowers the basal age of Chinese loess from 2.6 Ma to ∼2.8 Ma. Although the Neogene ‘red clay’ and the overlying Quaternary loess are both of windblown origin, they have quite different pedogenic characteristics. The loess consists of alternating reddish soil and yellowish loess layers, while the ‘red clay’ is composed of soils with much redder colour and carbonate nodule layers. In general, the “red clay” has been subjected to stronger pedogenic processes than the overlying loess, and can be regarded as an extremely thick soil complex. The alternation of loess and soils reflects large-scale oscillations between glacial and interglacial conditions, whereas the well-developed ‘red clay’ deposition indicates a relatively stable climate condition. At ∼2.8 Ma, shortly before the Gauss/Matuyama boundary, the eolian sequences show a sedimentary transition from the well-weathered ‘red clay’ to the alternating loess and paleosol deposits, accompanied by coarser particle size, increased dust accumulation rates, and change of magnetic susceptibility signature. These indicate a large climatic shift from long-lasting warm-humid conditions to large-amplitude cold-dry and warm-humid fluctuations around 2.8 Ma, and a dramatic increase in aridity over the dust source region. In contrast, such a drastic event is not identified around the top of the Olduvai event or at any other time of the past 7.7 Ma. As the Gauss/Matuyama boundary offers a sound foundation for global time correlation and chronostratigraphic classification, these results still support the proposal of ICS and INQUA, that the conventional Pliocene-Pleistocene boundary should be lowered from 1.8 Ma to 2.6 Ma.

Simulating Magnetic Susceptibility Profiles in Loess as an Aid in Quantifying Rates of Dust Deposition and Pedogenic Development

Magnetic susceptibility profiles χ(z) in loess sequences reflect a combination of two climatically modulated processes: dust deposition and pedogenic development. Prominent soils with high χ values, for example, likely reflect periods of slow dust deposition, and warm wet conditions favorable for rapid chemical weathering. To refine our understanding of the climate records contained in loess, we develop a numerical model as a tool for exploring quantitatively the integrated record of the temporal variation in rates of loess aggradation and soil development contained in χ profiles. In our model, the aggrading loess is pedogenically altered in a reactive zone near the ground surface. The strength of the χ signal is dictated by both the depth-dependent intensity of pedogenic processes and the rate of dust accumulation, which dictates the total time a loess parcel spends in the near-surface reactive zone. The model can accommodate both pedogenic production of magnetically susceptible minerals and arrival of magnetically susceptible grains as eolian dust. To explore the model performance and develop a sense of time and length scales implicit in χ profiles, we first examine simple synthetic cases with idealized steady and cyclic climatic forcing. Reported χ profiles in three Chinese loess sequences at varying distances from the western China dust source are then modeled in two illustrative ways: (i) by imposing a specific dust deposition rate history that is proportional to the dust accumulation rate history reported from western Pacific deep-sea cores, allowing the time variation of pedogenic rates to be calculated directly from χ profiles; and (ii) by imposing both dust accumulation and pedogenic rate histories that are independently scaled by the deep-sea δ18O history, which reflects global climate cycles to which regional climate forcing is linked. We find that the zone of pedogenic activity is roughly 0.5–1.0 m thick, both deposition rate and pedogenic intensity have varied dramatically over the last 140,000 yr, the age structure of the Luochuan loess sequence is best fit by driving the model with the δ18O record, and environmental conditions must have been anomalously favorable for pedogenesis during isotope stage 3 at all three sites. Finally, we advocate the assembly of a variety of data types at a suite of sites within any loess field that taken together will better constrain the temporal and spatial patterns of climatically modulated deposition and pedogenic processes.

Thermoluminescence ages of loess and associated sediments in central Nebraska, USA

Abstract Thick deposits of Quaternary loess (maximum 60 m, more typically 20–30 m) occur in the Loess Hills and south of the Platte River in central Nebraska. A programme of field sampling and thermoluminescence dating has provided new age estimates for the younger aeolian stratigraphic units in the upper part of the sequence. Dates from aeolian sands below the Gilman Canyon Formation at two sites indicated ages ranging from 64 ± 7 to 48 ±5 ka, providing a maximum age for this unit. A date of 35 ± 3 ka was obtained from near the top of a truncated Gilman Canyon sequence in one section, but radiocarbon evidence from other sites suggests that slow accumulation of dust, accompanied by pedogenesis and syn-depositional reworking, continued in some areas until at least 23 ka ago. The Gilman Canyon Formation is overlain by up to 25 m of grey Peoria Loess which accumulated between c. 22 ka and 10.5 ka ago. Between c. 10.5 ka and 9 ka ago the rate of dust accumulation decreased, apparently due to a reduction in sediment supply from the Great Plains river valleys, thereby allowing the widespread development of a soil (the Brady Soil). After 9 ka ago the rate of dust deposition again accelerated, resulting in the accumulation of up to 5 m of Bignell Loess. The main source of Bignell Loess was probably also the valleys of ephemeral Great Plains rivers during periods when the region experienced a drier climate than present. Dating evidence suggests that some time after 3 ± 2 ka ago the rate of dust accumulation again slowed, allowing development of the modern soil. It is suggested that this change reflected a reduction in dust availability, related to a change towards slightly wetter climate and associated changes in river regime.

Spatial soil variability in the Cajon Pass chronosequence: implications for the use of soils as a geochronological tool

Spatial variation of soil properties on a single geomorphic surface has a significant influence on surface correlations and age determinations made on the basis of soil properties. Few chronosequence studies have been based on a sufficient number of soils to evaluate the influence of soil variability. We described and evaluate 35 soils on four terrace surfaces which constitute part of a previously described, well-dated soil chronosequence in Cajon Pass in southern California. Soils were described on both bar and swale sites, removed from the influence of deposition adjacent slopes. The influence of eolian silt on the rate of pedogenesis in Cajon Pass indicates that this sequence of soils cannot be considered a chronosequence in the strict sense of the term. Furthermore, the evidence for a pedogenic threshold in these soils indicates that the soils do not develop monotonically with time and that soil development cannot be defined by a continuous univariant chronofunction. The discrete nature of the data from a chronosequence and the complex nature of soil development are better analyzed by multivariate statistical procedures. Systematic variation in soil properties between soils from bar sites and soils from swale sites is Attributed to geomorphic processes which affect either the rate or duration of eolian dust accumulation. Soils from bar sites were less variable and generally less strongly developed that soils from swale sites from the same surface. The degree of variation of soil properties determined for a given landscape position on a surface is sufficient to raise questions concerning the usefulness of chronofunctions for determining the nature of time-dependent pedogenic changes and for providing numerical age determinations. Multivariate statistical methods were used to differentiate among soils developed on three surfaces which differ in age by only 1500 years.

Loess is not just the accumulation of dust

Most regrettably there is much misuse of terminology in connection with the concept of loess and the interpretation of its origin. People often speak and write incorrectly about loess accumulation — even loess specialists who are aware of the fact that loess as a concept has to satisfy at least 10 criteria. Loess is not simply dust carried and deposited by wind. Dust only becomes loess after the passage of a certain amount of time in a given geographical zone. i.e. only through diagenesis in certain ecological environments. To state that loess is of aeolian origin is an oversimplification and an incorrect definition because an aeolian origin applies only to the dust from which the loess has been formed. We are aware of the fact that airborne dust cannot be transformed into loess in every geographical environment, but only under those conditions typical of semi-arid grassland or steppe or forested steppe. The process primarily occurs where the rate of dust accumulation exceeds sheet wash or weathering (soil formation) rates. If the rate of dust accumulation is less than that of surface erosion or of soil (biogenetic) processes, the dust then develops into soil or, through intensive weathering and increased precipitation, into loam or clay. Only part of the aeolian dust transported and deposited in a zone suitable for loess formation remains there permanently and is transformed in situ into loess. Dust not affected by diagenesis is usually further transported by snowmelt or rainwash and is only transformed into loess after it has been redeposited. The loess itself, however, is easily erodible and its minerals are readily reworked and reaccumulated and, given the appropriate conditions, it readily undergoes diagenesis again. We do not always have sufficient evidence to determine whether a given loess body is of primary or secondary origin. Traditionally, the minerals making up ‘primary loess’ have been regarded as originating in dust accumulated by aeolian processes. ‘Secondary loess’ by contrast is different from typical loess in many ways and it is not unusual to find various loess series in which superimposed dust fractions have been transformed into loess by different processes. Recently we have observed that the origin of various types of loess is governed by differences in (litho) ecological conditions rather than by the way in which the mineral material from which the loess is derived (dust) has been primarily accumulated. Loessification is therefore determined by environmental conditions.

Effects of dust on the performance of concentrator photovoltaic cells

The effect of dust on the performance of photovoltaic concentrators has been investigated. The dust concentration in the air was measured continuously during the test period, and the rate of dust accumulation on the concentrator's surface was determined. For the purpose of comparison, an identical concentrator, the surface of which was kept clean, was evaluated simultaneously with the dusty concentrator. The change of the I/V characteristics as a result of dust accumulation was related to the amount of dust accumulated per unit area of the collector surface (g/m2). It has been shown that the major reductions in the short-circuit current, cell temperature and the efficiency occur as the dust starts to deposit onto the collector surface, but the rate of decrease is slower for dust accumulations beyond 2 g/m2. It is argued that the dust accumulation per unit area is a more representative parameter than the exposure time for such studies.

On the dust-equivalent series resistance of a photovoltaic concentrator

A new concept for treating the effects of dust on the electrical performance of photovoltaic concentrators is presented in the paper. The dust concentration in the atmospheric air around the concentrator is measured continuously during the test period. The rate of dust accumulation on the concentrator surface is determined. The concentrator performance degradation, as a result of dust accumulation, is related to the amount of dust accumulated per unit area of the collector surface (in g/m2) rather than the exposure time. It has been shown that major reductions in the short-circuit current and the efficiency are observed for dust accumulations up to 5.4 g/m2. The accumulation of dust on the photovoltaic concentrator causes a successively larger `rounding? of the I/V characteristic at constant incident direct normal radiation intensity and constant cell temperature. This effect is equivalent to an increase in the internal series resistance of the concentrator. This dust-equivalent series resistance increases with increasing dust accumulation.