Loess Layers Research Articles

Environmental tracer transport ( 3H and SF 6) in the saturated and unsaturated zones and its use in nitrate pollution management

An important quantity in groundwater protection is the residence time of water in an aquifer. It relates to both the travel time of a pollutant to arrive at a well and the time span required for self-purification of a polluted aquifer after removal of pollutant inputs. Time scales for aquifers can be gained from artificial tracer experiments or from environmental tracer data, the latter offering the only realistic alternative if time scales of years or decades have to be taken into account. Different tracers show different time scales due to their different transport mechanisms especially in the unsaturated zone. While solute tracers are moved advectively with the seepage water, gas tracers pass the unsaturated zone diffusively through the air phase. Depending on the properties of the unsaturated zone (hydraulic properties, thickness) this difference in behavior can be used to separate the subsurface transport process into the unsaturated and the saturated parts. In a field study in Germany, SF 6 and 3H were used as environmental tracers. Both have a relatively well-known input function. Interpretation of data from observation wells by a box model approach led to spatially and temporally varying residence times. This was an indication that the influence of the unsaturated zone could not be neglected. While the gas tracer SF 6 shows only residence times in the saturated zone, the tracer 3H reflects the whole travel time of water including both the unsaturated and saturated zones. Using a one-dimensional plug-flow model for the unsaturated zone combined with a detailed two-dimensional flow and transport model for the saturated zone leads to a holistic and consistent interpretation of the measured tracer concentrations. The observed pattern of old water under thick loess cover and younger water under areas where the fractured basalt aquifer crops out is reproduced after adjusting only two parameters: the effective porosity of the saturated aquifer and the product of field capacity and thickness of the unsaturated zone. While the effective porosity of the saturated zone is adjusted by means of the SF 6 data, the field capacity of the loess layer is adjusted by means of the 3H observations. The thickness of the unsaturated zone is deduced from geological and pedological maps. All flow data are obtained from a calibrated flow model, which is based on geological data, observed heads and pumping tests only. The transport model for the saturated zone was calibrated by fitting the porosity by means of gaseous tracer concentrations (SF 6). The combined saturated–unsaturated zone model was then calibrated by fitting the field capacity of the unsaturated zone by means of 3H concentrations. With this model it was possible to verify the observed NO 3 concentrations at the drinking water wells and to develop predictions for their future development under various scenarios of fertilizer input reduction in specific areas.

10Be-susceptibility model and quantitative estimates of pedogenic ferromagnetic material flux in Chinese loess

The 10Be flux onto a loess plateau is composed of two different components: the atmospheric flux and the dust flux. A simple 10Be-susceptibility model shows that the magnetic susceptibility flux can be also divided into a dust related component and an in situ produced pedogenic component. From comparison with the dust 10Be flux, we can separate and quantify the detrital ( F D ′) and the pedogenic ( F X) magnetite contributions to the total magnetic flux ( F S). F X/ F S varies between 0% and 80% with low values in loess layers and high values in palaeosols and weathered loess. The pedogenic magnetic susceptibility flux could be considered to be linearly related to palaeoprecipitation and may be used as a quantitative measure for regional palaeorainfall in the Chinese loess plateau.

Separation of pedogenic and lithogenic components of magnetic susceptibility in the Chinese loess/palaeosol sequence as determined by the CBD procedure and a mixing analysis

SUMMARY Magnetic susceptibility variations in the Chinese loess/palaeosol sequences have been used extensively for palaeoclimatic interpretations. The magnetic signal of these sequences must be divided into lithogenic and pedogenic components because the palaeoclimatic record is primarily reflected in the pedogenic component. In this paper we compare two methods for separating the pedogenic and lithogenic components of the magnetic susceptibility signal: the citrate-bicarbonate-dithionite (CBD) extraction procedure, and a mixing analysis. Both methods yield good estimates of the pedogenic component, especially for the palaeosols. The CBD procedure underestimates the lithogenic component and overestimates the pedogenic component. The magnitude of this eect is moderately high in loess layers but almost negligible in palaeosols. The mixing model overestimates the lithogenic component and underestimates the pedogenic component. Both methods can be adjusted to yield better estimates of both components. The lithogenic susceptibility, as determined by either method, suggests that palaeoclimatic interpretations based only on total susceptibility will be in error and that a single estimate of the average lithogenic susceptibility is not an accurate basis for adjusting the total susceptibility. A long-term decline in lithogenic susceptibility with depth in the section suggests more intense or prolonged periods of weathering associated with the formation of the older palaeosols. The CBD procedure provides the most comprehensive information on the magnitude of the components and magnetic mineralogy of loess and palaeosols. However, the mixing analysis provides a sensitive, rapid, and easily applied alternative to the CBD procedure. A combination of the two approaches provides the most powerful and perhaps the most accurate way of separating the magnetic susceptibility components.

Multiple origins and interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments

In most Chinese loess–paleosol sequences, high magnetic susceptibility values are found in the soil horizons, with low values in the loess layers. The susceptibility signal has been widely used as a proxy climatic indicator. However, both the causes and mechanisms which control susceptibility still remain controversial. Our recent studies challenge some earlier interpretations of the magnetic susceptibility signal in Chinese wind-blown sediments. First, the prevailing hypothesis of pedogenic origin cannot totally account for the susceptibility variations in many sections. Second, in some cases, the principal carrier of the magnetization seems not to be the ultrafine pedogenic minerals, rather coarse lithogenic magnetic minerals derived from local sources can also contribute significantly to the susceptibility signal. Finally, not all the Chinese soils have higher susceptibility values than that of loess or sand layers, opposite relationships do exist. It is concluded that the source and causes of susceptibility may be different from site to site or even at different times at a given site. Any single hypothesis cannot completely account for the susceptibility variations in the Chinese Loess Plateau and its surrounding regions. Besides the previous explanations of carbonate leaching, pedogenic processes and organic decomposition, the contribution of source materials to magnetic susceptibility must also be taken into account.

Paleo-environmental study on the growth of magnetotactic bacteria and precipitation of magnetosomes in Chinese loess-paleosol sequences

405 samples were collected from L5-S5-L6 in consideration of obvious variations in susceptibility of the geological sections, which are section Xifeng in Gansu Province and section Duanjiapo in Shaanxi Province for study of magnetotactic bacteria (MB) and magnetosomes (MS) in Chinese loess-paleosol sequences. MB in each sample were observed by TEM after being cultured under 8–18°C., room temperature (RT), 25°C., 26°C and 30°C conditions. In general, MB are distributed widely in loess-paleosol sequences, fewer in loess layers with predomination of vibriod in shape. However, there are more MB in paleosol layers with morphological varieties such as roddish, vibriod and occasionally approximately coccus. The magnetosomes (MS) in MB of paleosol are usually arranged in chains along the cells. It was also found that MB growth and MS formation are associated with the environment in which MB live. It can be inferred from the distributions of MB and MS that the paleoclimates fluctuated during the formation of loess-paleosol sequences in the Chinese Loess Plateau. The climate became gradually warmer but displayed more frequent fluctuations from the northwest to the southeast of the Plateau.

Last interglacial‐glacial climatic cycle in loess‐palaeosol successions of north‐western France

The loess series at St. Pierre‐les‐Elbeuf and St. Sauflieu are key successions for the western European Quaternary stratigraphy. The present study proposes a detailed record of the last interglacial‐glacial climatic cycle at St. Pierre and its integration into the synthetic pedosedimentary record of north‐western France using detailed correlations with the type sections of St. Sauflieu and Achenheim. Finally, comparisons with the marine isotope, Greenland GRIP chronologies and dust records are proposed. At St. Pierre, the pedostratigraphic and sedimentological analyses (total iron, organic matter, carbonate, grain size), in association with low field magnetic susceptibility measurements, demonstrate that this loess succession records the major climatic events of the Upper Pleistocene. The basal soil complex at St. Pierre is similar to those from the main successions of North (St. Sauflieu) or Northeast France (Achenheim). It shows a Bt horizon of brown leached soil, a deeply reworked grey forest soil and two isohumic steppe soils separated by a non‐calcareous loess layer. This loess level corresponds to the first aeolian event clearly observed in the succession and can be correlated with Marker II of the Central European stratigraphy located around the marine isotope stage (MIS) 5/4 boundary. The main aeolian sedimentation starts after the soil complex and ends with the top soil (brown leached soil). Finally, a good parallel is observed between the strongest peaks of the dust records of the ice cores and the main period of loess deposition in St Pierre‐lès‐Elbeuf occurring during MIS 2.

Loess–paleosol sequence of La Mesada in Tucuman province, northwest Argentina characterization and paleoenvironmental interpretation

A 42 m thick loess–paleosol sequence was studied in the dry pre-Andean valley of Tafi-del-Valle, at the La Mesada site (2280 masl), Tucumán Province in northwest Argentina. The sequence contains 28 paleosols interbedded with 26 loess layers. The latter are coarse loamy and have large hexagonal polyhedrons. The soil layers are restricted to fine loamy Bt horizons, with prismatic structure and organo-argillans on the structural surfaces. Surface (A) and eluviation (E) horizons are systematically absent from the whole sequence. Soil truncation is unlikely as no remains of such horizons or erosional uncomformities were identified. The dated part of the section encompasses a time span of 10,080 years, from 17,580 BP at 5.2 depth to 27,660 BP at 42.3 m depth. A total of 20 Bt/C pairs developed during this time interval, corresponding to a climatic change every 500 years on the average between dry–cool conditions, promoting loess influx, and moist–warm conditions favouring soil development. The depth function of the Bt/C clay ratios reflects the climate improvement during the soil formation interstadials, with an optimum in the upper part of the sequence. This might be related with recurrent northward shifts of the polar front, accompanied by the weakening of the mid-latitude South Pacific anticyclone. Similar short-term periodicity has been identified in the ice cap of Greenland, highlighting the global character of such frequent climatic changes during the Pleistocene.

580,000-year environmental reconstruction from aeolian deposits at the Mu Us Desert margin, China

A 76.7 m-thick section of aeolian sediment is exposed in the transition zone between the Mu Us Desert and the Loess Plateau of northern China. The aeolian sediments contain sand beds, loess layers, and palaeosols. The particle-size distribution data and the SEM examination of quartz grains suggest the sand beds are wind-blown origin. Palaeomagnetic measuring combined with thermoluminescence (TL) dating and pedostratigraphic correlation suggests the studied aeolian sequences were deposited within about 580 kyr. The sand layers appear to reflect the episodic extensions of Mu Us Desert during phases of active dune formation and intensified winter monsoon. Both the stratigraphy and the proxy climatic curves such as bulk magnetic susceptibility and grain size indicate that large amplitude second-order climatic oscillations occur during both glacial and interglacial periods. It is worth noting that nearly all the interglacial soils can be sub-divided by the intercalated sand or loess beds, this second-order climatic oscillations are on the order of 10,000 of years which may reflect the precession-scale variations of southeast summer monsoon driven by the orbital pacing of insolation in low latitudes.

The Loess in the Qinling Mountains and Its Records of Palaeoenvironmental Changes

Abstract: The Fengzhou loess section is very typical in the Qinling Mountains. The section, which is about 82 m thick and underlain by Neogene red clays, consists of 33 layers of loess and 33 layers of palaeosol. It covers the Brunhes normal polarity zone and Matuyama reversed polarity zone, and the B / M boundary is located in the middle part of loess layer 8 (L8). The loess of the Matuyama reversed polarity zone records the Jaramillo, Olduvai and Reunion normal polarity subchrons. The boundary between the Matuyama reversed polarity zone and Gauss normal polarity zone (M / Ga) appears on the lithological boundary between loess and Neogene red clays. Loess accumulation in the Fengzhou section started before 2.48 Ma B.P. Based on the stratigraphical structure, the material composition and magnetic susceptibility curve of the Fengzhou loess section, the palaeoclimatic changes during the last 2.48 Ma in the Qinling Mountains are subdivided into 66 cold‐dry and warm‐humid stages, equivalent to 33 climatic cycles. The above climatic fluctuations agree very well with those of the oxygen isotopic records from deep—sea deposits.

Magnetic characteristics and their paleoclimatic significance of Fengzhou loess in the Qinling Mountains of China

The Fengzhou loess section is very typical in the Qinling Mountains. The section, about 82 m thick and underlain by the Neogene red clay, consists of 33 layers of loess and 33 layers of paleosol. The section covers Brunnes normal polarity zone and Matuyama reversed polarity zone, and the B/M boundary is located in the middle of L8. Matuyama reversed polarity zone records Jaramillo, Olduvai and Reunion normal polarity subchrons. The boundary between Matuyama reversed polarity zone and Gauss normal polarity zone (M/Ga) appears in the lithological boundary between loess and the Neogene red clay. Loess accumulation in Fengzhou started before 2.48 Ma. The magnetic susceptibility curve indicates 33 cold-dry and humid-warm cycles of paleoclimatic variation in the Qinling Mountains during the last 2.48 Ma. The above climatic fluctuation may be well compared with that of the oxygen isotopic records from deep-sea deposits.

Deposits and Soils of the Past 130,000 Years at the Desert–Loess Transition in Northern China

The desert–loess transitional zone in north-central China has long been thought sensitive to Quaternary climatic change. However, reconstruction of Quaternary climates in this area has been hindered by incompleteness of geological sections. Here we report the analytical results of two recently found sand–loess–soil sections. Both sections have thick eolian deposits from the last interglacial–glacial cycle and can be correlated with one another. Field observations, thermoluminescence dating, and other laboratory analyses show that the last interglacial period produced three paleosols and two intercalated loess layers. Loess from the last glacial period is interbedded with three sand horizons that represent desert extension. The expansion and contraction of desert in northern China may have been forced by the east Asia monsoon.

Correlation between Loess profiles on the basis of mineralogical, malacological, and anthropogenic indicators: A case study from Moravany-Lopata, western Slovakia

Moravany-Lopata, located in the complex of sites in the middle Vah basin, dates to the periodimmediately preceding the LGM. The authors use this site to demonstrate the usefulness ofmineralogical, sedimentological, palaeomalacological, and anthropogenic criteria for the cor-relation of loess profiles. This analysis is especially concerned with sites in loess territorieswhere the loess layers covering archaeological levels are not thick. Suchsitesoccurfrequentlyon the plateaus of the northern part of Central Europe, though they appear most notably inthe areas in which the last loess cover, corresponding to the period after the last Pleniglacialand/or Late Glacial postloess sediments, is absent. The archaeological levels dated to beforeor directly after the LGM occur in the upper portion of the loess, overlain by the Holocenesoil. The investigations at the Moravany-Lopata site have confirmed the usefulness of theanthropogenic indicators in the fraction for the identification of occupationlevels0.1–1.0 mmextending even beyond the area of occurrence of macrofinds and evident type structures. Onthis basis, a relationship could be established between two occupational levels and two ice-wedge generations in the period from 21.4 to Gravettian settlement in western Slovakia20 ky.seems to have persisted up to the maximum of the LGM. Population groups representing theend of the shouldered points horizon (

Magnetic properties of modern soils and Quaternary loessic paleosols: paleoclimatic implications

The magnetic properties of paleosols developed in Quaternary sequences of loess have been used for: stratigraphic definition; correlation with other terrestrial and deep-sea sequences; and paleoclimatic (paleorainfall) reconstruction. In some loess/paleosol sequences, including those of the Chinese Loess Plateau, Tajikistan and the Czech Republic, maxima in magnetic susceptibility values correspond with the paleosol horizons, and minima with the least-weathered loess layers. In other loess/soil sequences, including those of Siberia, Alaska and Argentina, the relationship is completely opposite, with susceptibility minima associated with the most developed paleosols. To account for these opposite relationships, the respective roles of: (1) magnetic enhancement and (2) magnetic depletion and/or dilution in determining soil magnetic properties are investigated for a range of modern soil types. Most magnetic enhancement is seen in the upper horizons of well drained cambisols. Absence or loss of magnetic iron oxides is apparent in acid, podsol profiles and waterlogged soils. For the cambisol profiles, significant correlation is found between susceptibility and organic carbon, cation exchange capacity and clay content. The mineralogy, morphology and grain size of soil magnetic carriers, extracted from three modern enhanced soils and a paleosol from the Chinese Loess Plateau, are also identified by independent petrographic means (microscopy and X-ray diffraction of magnetic extracts). Magnetite and maghemite of ultrafine grain size (from ∼0.4 to < ∼0.001 μm) are the major contributors to the magnetically enhanced soils. Weathering can concentrate detrital magnetic grains, especially in the fine silt size fractions of soils. The magnetic data from the modern soils indicate that interpretation of paleosol magnetic properties must be done on a site-specific basis, taking into account the possibilities of pedogenic enhancement, pedogenic dilution or depletion, and allochthonous inputs of magnetic minerals. Excessively arid, wet or acid soils are unable to form significant amounts of pedogenic ferrimagnets. Well drained, intermittently wet/dry soils, with reasonable buffering capacity and a substrate source of Fe, show most magnetic enhancement. For those soils which favour magnetic enhancement processes, correlation has been found between the maximum value of the pedogenic susceptibility and the annual rainfall. The almost unique pedogenic system in the Chinese loess plateau, where variation in soil-forming factors other than climate is reduced to a global minimum, allows use of the paleo-susceptibility values as proxy paleorainfall values. To identify the mechanism of any link between magnetic properties, for example, susceptibility, and climate change, the mineralogy and grain size of soil magnetic carriers, requires detailed investigation. Given the prospect of: (1) quantitative paleorainfall records from continental loess records; (2) predictions of present and future climate change; and (3) the value of paleoclimate data for testing of numerical climate models; such magnetic investigations can contribute significantly to our understanding of past and future environmental change.

The role of time versus climate in the formation of deep soils of the Apennine fringe of the Po Valley, Italy

Two very deep, red soils on a loess-mantled fluvial terrace of the Trebbia River in northern Italy were sampled and analyzed to determine their history and pedologic significance. Magnetostratigraphy confirms that the upper part of the terrace gravel and all of the 4-m-thick loess cover is normally magnetized and therefore less than about 780,000 yr. Artifacts collected from the oldest loess suggest an age of about 400,000 yr for the inception of loess deposition in this area. The soil horizons in loess dominantly have argillic or albic character, with redoximorphic features that include mottles, glossic tongues, and zones of ferromanganese concretions. Soil colors become progressively redder with depth in the loess cover and are most red, a 2.5 YR color hue, in the top of the terrace gravel. The upper horizons in gravel have abundant ghosts of completely weathered marl and sandstone. Rubification and weathering of stones diminishes with depth to fresh, slightly carbonate enriched parent material at 12 m. The heavy mineral composition of the sand fraction suggests minor discontinuities in loess-derived horizons. Abundance of opaque heavy minerals and picotite, which are resistant to weathering, increases from the soil surface to the base of the loess, is highest in the upper part of the terrace gravel, and decreases from there to the C horizon in gravel. Depth functions of major elements to Si in the medium silt fraction (10–30 μm) indicate a downward increase in degree of decalcification and of dissolution of primary minerals. Micromorphology adds detail to the interpretation of these complex soils: yellow complex ferriargillans are present in almost all horizons; they are rare at the top of the sequence in loess and become progressively more abundant with depth, reaching a maximum in the upper 3 m of the gravel and then decreasing with further depth to 12 m. We interpret this peak in abundance of yellow cutans to result from long continued clay illuviation to this depth from a succession of eluvial zones in the aggrading loessial sequence above. Distinctive red, hematite-rich argillans are abundant only in the upper horizons in the gravel. The red argillans relate to initial development of the soil in gravel, because they are included as pedorelicts in the deepest loess layer, where they are embedded in the yellow argillans. Stratigraphic and pedologic evidence indicates that these soils began forming in terrace gravel for perhaps 300,000 yr during the early middle Pleistocene. From this relict core these soils continued developing, both downward by continuing deep decalcification and argilluviation and upward as a sequence of wet Alfisols developed in a succession of loess layers that began aggrading about 400,000 yr ago. A paleoclimatic record can be perceived weakly in these soils, mainly by thick horizons of FeMn nodules that mark boundaries between episodes of loess deposition that took place during glacial cold phases. Paleofeatures from one unit to another are similar in kind and intensity, indicating a degree of continuity of a basic set of pedogenic processes, principally declcification, argilluviation, and redoximorphism. These soils can be regarded as Vetusols because a single dominant set of pedogenic features are represented in all parts of these soils and deeper parts of these aggrading super profiles have never been isolated from continuing pedogenic change to the present.

Aeolian particle input to the soil surface at the northern limit of the Negev desert

In arid and semiarid regions, aeolian particle input is one of the major sources of soil nutrients. The particle input also improves the soil depth and strongly influences the water‐holding capacity of the soil. The authors studied the airborne particle accumulation dynamics on the soil surface in the Northern Negev desert (Sayeret Shaked Park) in comparison to the Central Negev highland (Sede Boqer and Shivta). The measurements of the components of the aeolian deposition and accumulation at the study site of the Sayeret Shaked Park were carried out for 3 years from August 1992 to July 1995. The average soil accumulation for this period was 0.22 g m−2 d−1 (80.64 g m−2 year−1). The minimal quantity during the research period was during July 1993 (1.35 g m−2 month−1) and the maximal was in March 1993 (24.42 g m−2 month−1). Significant differences were obtained during the whole study period regarding the slope directions of the watershed. The aeolian accumulation and the deposition on the north‐facing slope was significantly lower (P < 0.05) than on the south‐facing slope (average 5.09 g m−2 month−1 to 6.97 g m−2 month−1, respectively). The highest accumulation was obtained in the wadi (average 827 g m−2 month−1). Almost all the granulometric analyses conducted on monthly collected samples for the study period showed significant amounts of clay‐size (<16 μm) and silt‐size (between 16 to 50 μm) particles. Chemical composition of the airborne particles showed that two major constituents, Si and Ca, dominate all the samples. Particle‐size analysis of the aeolian particles accumulated during the study period at the Sayeret Shaked area showed a median diameter varying between 25 and 35 μm. This leads to a layer of fine airborne particles or loess, as scattered silt or clay particles, of 14 μm m−2 year−1.

Micromorphological variation of the S 1 paleosol across northwest China

A micrornorphological approach, based upon sets of closely-spaced thin sections taken from three sites located along a climatic and depositional gradient, has been utilised to reconstruct the sequence of pedogenic and sedimentary events leading to the development of the S1 paleosol in northwest China. Two main pedosedimentary stages related to regional changes in monsoonal controls are evident at each site, although the resolution, processes and resultant features vary in a manner reflecting differences in climate and rates of loess deposition. Around Xining in the west, where loess accumulation rates were high, and pedogenic processes were restricted to shallow leaching and limited bioturbation by the low rainfall, S 1 comprises two discrete weakly-developed paleosol units separated by a less-pedogenically altered layer of loess. In contrast, at sites farther away from the loess source area in the more humid southeast, S 1 is a thinner, welded and more pedogenically-developed unit dominated by the effects of extensive leaching, bioturbation and sometimes even clay translocation processes.

High-resolution multi-proxy climate records from Chinese loess: evidence for rapid climatic changes over the last 75 kyr

The loess/paleosol sequence in loess layer L1 (Malan Loess) is investigated in three regions of the Western Chinese Loess Plateau. Nine pedogenic layers are found in L1 and three proxy climate indices, magnetic susceptibility (MS), grain size (GS) and CaCO3 content, are measured at intervals of 0.2 kyr in order to recover records of monsoon climate variations. Time series of MS, GS and CaCO3 content document the high resolution history of summer and winter monsoon climate variations over the last 75 kyr. The records show a high degree of similarity to the warm interstadials recorded in ice cores from Greenland and the Antarctic, and with Heinrich events in the North Atlantic, during the last glacial cycle. Bond cycles are also recorded by the Chinese loess records. Overall, our results indicate that numerous rapid changes in climate occurred in China during the last glacial cycle, but that the range of climate variations was smaller than recorded in Greenland.

Geochemical records of palaeoclimatic changes in Weinan section since the last interglacial

Weinan section consists of Holocene soil S0, last glacial loess L1, and last interglacial soil S1, furtherly L1 can be subdivided into three layers of loess (L1–1, L1–3, and L1–5), and two layers of weakly developed soil (L1–2 and L1–4). Based on studying the content variations of carbonate, free Fe2O3, and the total organic content in Weinan section, the environmental evolution of this region has been discussed over the last glacial-interglacial cycle. Our results indicate that the chemical parameters can be used as climatic proxy data, and the variations of these indexes reflect the cyclic nature of the Quaternary climatic change. In addition, the climatic records of Weinan section can also be correlated with that of the marine oxygen isotope records, so the environmental evolution of the Loess Plateau is also consistent with the global climatic changes. The unexpected finding is that the climatic curve of L1–5, which can be correlated with stage 4 of marine oxygen isotope records, consists of three troughs and two sandwiched peaks, and may suggest the existence of the second-order warm-cold oscillations of this period.

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.

Reconstruction of Microenvironmental Changes in the Kopasz Hill Loess Area at Tokaj (Hungary) Between 15 and 70 ka BP

We collected 11 Kopasz Hill loess profiles for paleoecological and geochronological analysis. The loess accumulation and development formed during the last (Weichselian) glacial period between 70 and 15 ka bp. We found that the majority of the loess profiles were composed of three typical loess strata and two well-developed paleosol horizons. Based on vertebrate remains, the lowest loess layer formed between 70–50 ka bp, during the first cool and dry climatic phase of the last glacial period, when forest steppe vegetation dominated in the Kopasz Hill area. On the surface of the lowermost layer, a paleosol developed between 50 and 40 ka bp as an indication of a more humid and warmer climatic phase. This paleosol layer was buried by a new loess layer that developed between 40 and 32 ka bp. The upper paleosol horizon developed between 32 and 26 ka bp. Molluscs preferring a mild climate were found in this layer, suggesting that this phase was wet and relatively temperate. A number of fired macrocharcoal remains can be found on the top of this paleosol layer. Charcoal samples from nine sites were dated by radiocarbon analyses. These results reflect the presence of a charcoal-rich horizon that developed 28–26 ka bp. Ca. 26 ka bp, loess formation resumed. We analyzed 14 samples from 6 sites by the 14C method. Based on 14C data, the uppermost part of loess profiles developed between 26 and 15 ka bp.