Loess Sequences Research Articles

Chronological Link Between the Abrupt Change of the Loess Grain Size Sequence and the Formation of River Terraces on the Eastern Margin of the Qinghai‐Tibetan Plateau Since the Late Early‐Pleistocene

Abstract:Based on the study of magnetostratigraphy, magnetic susceptibility and grain size of Garzê A section on the southeastern margin of the Qinghai‐Tibetan Plateau since the late early‐Pleistocene, the basal age of Garzê loess is located at ∼1.16 MaBP and a series of abrupt paleoclimatic changes is detected. The times of abrupt changes are of distinct series features, and the interval between each two adjacent abrupt changes is ∼50 kyr or ∼100 kyr. The most significant abrupt changes occur at around 1.06, 0.85, 0.6, 0.46, 0.39 and 0.14 MaBP. There is a chronological link between the abrupt changes of paleoclimate and the formation of river terraces and it is almost simultaneous with a strengthening trend of neotectonic activities. Therefore, maybe the climatic transition controll the timing of terrace formation, and the tectonic uplift originate potential energy and has a direct effect on channel incision, both the climatic transition and the tectonic uplift are important. Terraces are the products of the interaction of instable climatic variations and tectonic uplift. Like the loess‐paleosol sequences, river terrace sequences are also controlled by the climate‐tectonic coupling system and are ruled by climate‐tectonic gyration with a ∼100 kyr paracycle, which may be the short eccentricity period of the earth.

The last million years recorded at the Stari Slankamen (Northern Serbia) loess-palaeosol sequence: revised chronostratigraphy and long-term environmental trends

The Stari Slankamen loess-palaeosol section is located on the northeastern part of the Srem Loess Plateau (Vojvodina region, North Serbia). The c. 40-m thick cliff comprises loess intercalated with 9 major palaeo pedocomplexes and can be considered to be one of the most important Quaternary sections in the Carpathian (Panonnian) basin. Here we present new magnetostratigraphic and aminostratigraphic evidence that demonstrates the importance of the site in terms of its age and the long-term palaeoclimatic record it preserves. Directional palaeomagnetic data, obtained through alternating field demagnetization demonstrates the presence of reversed polarity below a profile depth of 36 m indicating a Matuyama chron age of this interval. This interpretation is confirmed by new high resolution palaeomagnetic investigations (434 oriented samples) from the lower part of the profile. The new magnetic susceptibility record and aminostratigraphy indicate a missing pedocomplex (V-S2), with an erosional unconformity represented by a distinct gravel layer. The combined new magnetostratigraphic and aminostratigraphic based age model requires a significant revision of hitherto published chronostratigraphic subdivisions at the site. The relative completeness and long time frame covered by the section is unusual in European loess sequences. Hence, the sequence could form the basis of a continental scale stratigraphic scheme that would alleviate much current chronostratigraphic uncertainty and enable more broad-scale climatic reconstructions. The section also provides a rare opportunity to investigate detailed and long-term climatic change over the Middle Pleistocene in a region influenced by air masses originating from high and middle latitudes, as well as the North Atlantic and Mediterranean. The changing relative importance of these air masses through time provides insight into local and regional atmospheric systems and their evolution through the last c.1 Ma. The section can thus be considered as one of the key climatic archives in the Europe.

North Atlantic abrupt climatic events of the last glacial period recorded in Ukrainian loess deposits

Abstract. Loess deposits are widely distributed in the Northern Hemisphere, where they have recorded not only the glacial-interglacial cycles, but also millennial-timescale changes resembling those in marine and ice cores. Such abrupt variations are clearly marked in western European series, but have not yet been evidenced in the East of the continent. Here we present results of the high-resolution investigation of a Weichselian Upper Pleniglacial loess sequence (~38–15 ka) from Stayky, Ukraine. The stratigraphy shows an alternation of loess horizons and embryonic soils, similar to sequences from western Europe. Similarities are also found between variations of a grain-size index (ratio between coarse and fine material fractions) in Stayky and in western European profiles. Based on these similarities and in agreement with the luminescence dates, the embryonic soils are associated with the Greenland interstadials (GIS) 7 to 2, and the Vytachiv paleosol at the base of the sequence, with GIS 8. Pollen analysis indicates a wetter climate for these interstadials, allowing the development of arboreal vegetation, than for the stadials, which are marked by loess formation. The grain-size index reaches the highest values for intervals correlated with the Heinrich events 3 and 2. Thus, it appears that the North Atlantic abrupt climate changes have extended their influence and modulated the loess sedimentation at least as far as eastern Europe. This result is supported by recent climate modeling experiments and recommends the Stayky sequence as a reference for further comparisons between profiles along the Eurasian loess belt centered at 50° N.

Sedimentation and erosion processes in Middle to Late Pleistocene sequences exposed in the brickyard of Langenlois/Lower Austria

The correlation of sedimentary and pedogenetic processes in Lower Austria is difficult due to significant discontinuities and local variability in soil formation. This hampers landscape reconstruction at a regional scale. However, at a local scale distinct landscape formation processes represented by a shift from fluvial to aeolian deposition can be observed in the brickyard of Langenlois, Lower Austria. Sedimentological and mineralogical analyses in combination with palaeontological finds suggest that the fluvial deposition took place during the Middle Pleistocene. This attribution is confirmed by infra-red stimulated luminescence (IRSL) dating, which gives a minimum age of 300 ka for the palaeosurface on which the fluvial sediments were deposited. This is consistent with a small faunal assemblage including Stephanorhinus sp., Dama sp. and an alcine cervid. Such a fauna is previously unknown in Austria; it indicates a Middle Pleistocene interglacial period. The low degree of weathering as well as Cryosols found in the loess sequence point to loess accumulation during the Last Glacial; the dating results (35–55 ka) indicate prolonged loess deposition. No signs of pedogenesis could be found; this is surprising because in other areas this period is known for weak soil development. The lack of soil formation seems to be specific to the western part of Lower Austria, as is the complete erosion of the last glacial maximum (LGM) loess, which can not be found at Langenlois.

First Warm, Then Wet

Climate Science Data derived from Asian loess sequences and stalagmites from caves in China have provided a valuable record of changes in the East Asian monsoon since the Last Glacial Maximum but have not been equally informative about the relationship between the timing of monsoon precipitation and warming in that region. Knowing the relative timing of those changes is an important part of understanding how climate changed during the last deglaciation. Peterse et al. present a record of temperature in central China for the past 34,000 years, in an effort to better define its relation to monsoon activity there. They find that the intensification of east Asian summer monsoon precipitation lagged deglacial warming and the increase of Northern Hemisphere summer insolation by about 3000 years. Intense soil formation, which depends on both higher temperatures and available moisture, lagged deglacial warming by even longer, around 7000 years. Earth Planet. Sci. Lett. 301 , 256 (2011).

Loess-soil sequences as a source of climatic proxies: an example from SW Poland

Loess in SW Poland occurs in several isolated patches which diff er in sediment thickness, stratigraphy and physical properties. Th e most representative loess sequences provide a record of climate changes for the Last Glacial cycle (Eemian–Weichselian). A representative loess section in SW Poland can be subdivided into four units: two polygenetic fossil soil sets and two, usually calcareous, loess units. On the top of the younger loess unit, recent soil has formed. Diff erences in the physical properties of these units result from the climatic conditions during sedimentation, which infl uenced the intensity of pedogenic processes and the rate of loess accumulation. Combined climatic proxies obtained from palaeosols, lithological composition and periglacial structures enable reconstruction of Late Pleistocene climate conditions as well as of proper correlations between continental, marine and ice-core events.

Tectonic versus climatic control on the evolution of a loess–paleosol sequence at Beremend, Hungary: an integrated approach based on paleoecological, clay mineralogical, and geochemical data

The role of two major factors, climate and tectonism, in controlling loess–paleosol evolution has been evaluated in the present study based on a multi-proxy dataset comprising mollusc, bulk and clay mineralogical, and geochemical data. A recent trend has been to use chemical indices such as the chemical index of alteration (CIA) for paleoclimate reconstructions in loess–paleosol sequences spanning several hundreds of thousands of years, but without any tectonic interpretation. Possible effects of geodynamic factors on physical erosion, chemical weathering and consequently weathering proxies are discussed. Clearly, the relative rates of physical erosion and chemical weathering in a sedimentary environment could be significantly influenced by geodynamic factors and governed not just by climate. Intensifying tectonic uplift gives rise to enhanced physical erosion, leading to increased supply of fresh mineral surfaces having less time to react with weathering agents. In theory, this process may point towards decreasing chemical weathering in a sequence without any real changes of climate. In the studied loess–paleosol sequence at Beremend (Hungary) a trend of decreasing chemical weathering has been observed, which can be explained by a global climate deterioration and accelerating uplift in and around the sedimentary basin. Increasing dust sedimentation could theoretically also contribute to this process as a result of increasing aridification and wind strength reflecting climate change in weathering records following a non-linear amplification. The findings refer to the fact that the impact of tectonism should also be taken into consideration in tracing past chemical weathering trends on timescales of hundreds of thousands (or millions) of years. This is because tectonic effects may impose on the CIA signal and distort it suggesting a potential bias in reconstructing paleoclimate change based only on this proxy from certain loess sequences associated with young orogenic belts.

Paleomonsoon route reconstruction along a W–E transect in the Chinese Loess Plateau using the anisotropy of magnetic susceptibility: Summer monsoon model

The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along a W–E transect (400 km). The loess–paleosol sequences in our study represent the most recent 130 kyr interval. The measured AMS was compared to expected theoretically derived magnetic fabrics occurring in strong and weak current airflow conditions. The major and minor AMS ellipsoid axis orientations were used to evaluate the paleowind direction along the transect. Previously published models assumed that cold and dry winter monsoons bringing dust from northwestern desert areas were responsible for the magnetic fabric formation of loess sequences. In our new interpretation, the stronger summer monsoons from the southeast played the major role in magnetic fabric orientation in the studied west and central parts of the Chinese Loess Plateau. Although the material was brought to the area by the winter monsoon, the AMS was generated during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged, settled, and fixed. We reconstruct the summer paleomonsoon routes for the last 130 kyr. These winds prevail from SE to NW but appear to be affected by regional topographic factors. In the western section, the corridor between the north and south Liupan Mountains disturbed the summer monsoon route, shifting it from SE to SEE.

Malacofauna continental holocena y paleoambientes en Villa Valle María (Diamante, Entre Ríos, Argentina)

The molluscan fauna recovered from Holocene continental sediments at Villa Valle María fossiliferous locality (Diamante, Entre Ríos, Argentina), eolian in origin and dated at 2,490 ± 45 14C yrs. B.P. on Bulimulus sp, is reported. It represents the first illustrated record of Quaternary molluscs from loess sequences in Argentina. It is represented exclusively by land snails which belong to five gastropod families (Pupillidae, Succineidae, Ferussaciidae, Orthalicidae and Scolodontidae), 7 genera and 9 species: Gastrocopta nodosaria (d’Orbigny), Succinea meridionalis d’Orbigny, Cecilioides consobrina (d’Orbigny), Bulimulus apodemetes (d’Orbigny), B. bonariensis bonariensis (Rafinesque), Bulimulus sp, Naesiotus pollonerae (Ancey), N. deletangi (Parodiz), Miradiscops brasiliensis (Thiele) and Scolodonta semperi (Doering). The first record of Miradiscops is reported for Entre Ríos province. The gastropod assemblage, typical of habitats with plant cover and leaves and trunks as substrate, is similar to the modern molluscan fauna found today along southern Entre Ríos province (Mesopotamian District in the Subtropical Dominium), an area characterized by a warm-temperate steppe continental scenario. However, the associated record of S. meridionalis, typical of environments with higher humidity contents like ponds, suggests local more humid temporary environmental conditions such as palustrine habitats during the time span considered. Our results provide additional evidence regarding the occurrence of a dominant eolian event which produced the reworking and subsequent accumulation of superficial deflated sediments on the Pampean region. They add to previous models considering a dry period between 3,500 and 1,400 yrs. B.P. over the central Argentine plains.

Investigating the penultimate and last glacial cycles of the Süttő loess section (Hungary) using luminescence dating, high-resolution grain size, and magnetic susceptibility data

The loess–paleosol record at Süttő provides an excellent high-resolution archive of climate and environmental changes. Loess deposits with a thickness of 20 m cover the travertine at Süttő, in the northern part of Hungary, next to the right bank of the Danube River. The loess sequence is intercalated with two greyish stratified horizons, three brownish steppe-like soils and a pedocomplex, including a reddish-brown paleosol covered by a chernozem-like paleosol. Detailed infrared stimulated luminescence (IRSL) dating was carried out, revealing more or less continuous sedimentation from Marine Isotope Stage (MIS) 6–MIS 2. Sedimentological and paleomagnetic investigations have been performed to provide a high-resolution study of the Süttő loess–paleosol sequence, in order to reconstruct the paleoclimate and environmental changes during the penultimate and last glacial cycles. Grain size and magnetic susceptibility (MS) records of the Süttő profile basically show similar patterns to those of other European loess sections. However, small differences are traced at Süttő, including more intensive deposition of coarser grained materials during MIS 6–4. The loess record at Süttő has a transitional position within the European loess belt, as its sediment was deposited under an intermediate climate between the loess successions from the southern Carpathian Basin, which accumulated under drier climate, and the loess sequences from Western Europe, which developed under wetter climate during the Middle and Late Pleniglacial.

Geo-archaeological investigations of Palaeolithic sites along the Ural Mountains – On the northern presence of humans during the last Ice Age

We review geo-archaeological results from six Palaeolithic sites along the western flank of the northern Ural Mountains. The oldest traces of human activities, dated to around 36–35 14C ka BP (43–40 cal ka), were found in alluvial strata at Mamontovaya Kurya at the Polar Circle - their connection to cultures further south remains uncertain. Slightly younger artefacts were found at the site Zaozer’e, nearly a thousand km further to the south, where a rich archaeological assemblage, dated to 34–33 14C ka BP (39–37 cal ka), was uncovered from underneath several meters of loess. The assemblage contains some small “Middle Palaeolithic like” bifaces alongside distinct Upper Palaeolithic traits, such as well-defined blades. This site also contains some perforated “pendants” made of freshwater molluscs. At the Byzovaya site, located at 65°N and radiocarbon dated to about 30–29 14C ka BP (34–32 cal ka), more than 300 artefacts and several thousand animal remains, mostly of mammoth, were incorporated in coarse-grained debris-flow deposits, sealed by aeolian sand. Pending the results from a new technological analysis of the whole artefact assemblage we can yet not decide whether Byzovaya should be categorized as a Middle- or Upper Palaeolithic site. The finds from Garchi, located in a loess sequence near Zaozer’e, have a similar or slightly younger age than the material from Byzovaya. Also at this site bifacial tools are present; alongside some characteristic triangle projectile points as well as some other elements which have nearly identical counterparts in the Upper Palaeolithic Kostenki/Streletskaya and Sungirian complexes, unambiguously associated with Modern humans. We conclude that the initial human colonisations along the Ural Mountains took place during a relatively favourable period of Marine Isotope Stage 3, when only small mountain glaciers existed in this region. The finds from the Medvezhia Peshera rock shelter have a completely different age (19–16 14C ka BP, 23–19 cal ka) and character, indicating that humans also were present along the Urals close to the Last Glacial Maximum (26–19 ka BP), the coldest and driest period of the last ice age. The few artefacts that were uncovered at Pymva Shor, the northernmost site investigated by us, are probably a little younger than those at Medveshia Peshera, but the timing of human presence is not precisely constrained at this site. In view of the obtained results it appears that humans were at least temporarily present in these northern landscapes from more than 40,000 years ago and onwards.

Astronomical dating of the Xiantai, Donggutuo and Maliang Paleolithic sites in the Nihewan Basin (North China) and implications for early human evolution in East Asia

Magnetostratigraphic studies have established a first-order chronological framework for the Paleolithic sites in the Nihewan Basin (North China), which enabled tracking early human evolution in East Asia. However, to fully understand how well early humans were adapted to climate change, a truly precise dating of the Paleolithic sites is required. Here, we established a high-resolution astronomical timescale for the Xiantai and Donggutuo fluvio-lacustrine successions at the eastern margin of the Nihewan Basin employing low-field magnetic susceptibility ( χ) as a climatic indicator, aiming to further refine the ages of the Xiantai, Donggutuo and Maliang Paleolithic sites. Starting from an initial age model constrained by geomagnetic reversals, larger-scale χ cycles were firstly tuned to orbital obliquity using an automatic orbital tuning method. This first-order tuning was followed by simultaneously tuning χ to both obliquity and precession. The finally tuned χ records can be correlated almost cycle-by-cycle with the quartz grain-size record of the Chinese loess sequence and the marine δ 18O record. The astronomically estimated age of the Xiantai Paleolithic site is ca. 1.48 Ma, corresponding to paleosol layer S 20 of the Chinese loess sequences or marine oxygen isotope stage (MIS) 49, an interglacial period. The astronomical estimate for the Donggutuo Paleolithic site ranges from ~ 1.06 Ma to 1.12 Ma, corresponding to paleosol/loess layers S 11–S 12 or MIS 31–33, spanning both interglacial and glacial periods. The astronomically estimated age of the Maliang Paleolithic site is ~ 0.79 Ma, corresponding to loess layer L 8 or MIS 20, a glacial period. This astronomical finding further implies that early humans may have permanently occupied China as far north as 40 oN since at least 1.1 Ma, and before this time the occupation may be intermittent.

Climatic interpretation of a 1.9 Ma environmental magnetic record of loess deposition and soil formation in the central eastern Pampas of Buenos Aires, Argentina

Much of what we know about Quaternary climate has been learned from sedimentary records from the world’s oceans. With the exception of the extensive studies of the Chinese loess/paleosol sequence and more recent studies of long lake records, there are few long terrestrial climate records, particularly from the southern hemisphere. The loess record of Argentina provides an important opportunity to further our understanding of climate change from a terrestrial environment, but its complexity and discontinuity have led to difficulty in formulating a climatological model of depositional and pedogenic processes. In this study, we present one of the longest and most continuous loess/loessoid records from the central eastern Pampas of Argentina. Our age model is based on optically stimulated luminescent dates and a paleomagnetic reversal stratigraphy and indicates a basal age around 1.9 Ma. Within the age model uncertainties, we characterize the environmental magnetic properties associated with loess deposition and soil formation with respect to wind patterns, moisture availability, and temperature. Major changes in magnetic grain size are linked to a differential northward shift of the subtropical high-pressure cell during glacial periods. We suggest that coarser (finer) magnetic grains correspond to weaker (stronger) glacial periods when the high-pressure cell is located in a more southerly (northerly) position and the source region is more proximal (distal) to our study area. An abrupt increase in the ultrafine-grained magnetic material around 0.9 Ma is related to an increase in moisture transport from the South Atlantic driven by an increase in summer sea surface temperatures at the mid-Pleistocene transition (∼1 Ma). In addition to these grain size variations, there is a relative decrease in the amount of goethite compared to hematite beginning around 0.5 Ma, which has been related to the temperature increase observed after the mid-Brunhes Event (∼450 ka) in the EPICA ice core temperature record. A more detailed comparison to insolation indicates that, for portions of the record, ferrimagnetic minerals are depleted during periods of low insolation. This result suggests that Argentine loess deposition and soil formation follows a model more similar to the Alaskan loess sequences than the Chinese loess sequences. Although further work is needed to validate the models and mechanisms proposed in this study, our record indicates that the mineral magnetic properties of the loess and paleosol deposits record major changes in deposition and soil formation and provide insight into possible mechanisms relating to global and/or hemispheric climate change.

An extensive late Cenozoic terrestrial record of multiple glaciations preserved in the Tintina Trench of west-central Yukon: stratigraphy, paleomagnetism, paleosols, and pollenThis is a companion paper to Barendregt et al., also in this issue.Geological Survey of Canada Contribution 20100035.

The Tintina Trench in west-central Yukon is a late Miocene graben formed along the antecedent early Tertiary Tintina fault. Since its formation the trench has served as a sediment trap for alluvial and glacial deposits. An extensive record of preglacial, glacial, and interglacial sediments spanning the late Pliocene to late Pleistocene has been preserved and is exposed today in modern landslide scars. This sedimentary record comprises multiple sequences of tills, outwash, mudflows, loess, and paleosols. The glacial sediments are the product of both local (montane) and regional (Cordilleran) ice advances that channeled into the trench, while loess and well-developed paleosols (brunisols and luvisols) reflect nonglacial and interglacial conditions, respectively. The Tintina Trench exposures provide the most complete record of glaciations for the region. Paleomagnetism, paleosols, and palynology provide age constraints for the geological events. A formal stratigraphic nomenclature is proposed for this region. The name West Tintina Trench Allogroup is assigned to the glacial–interglacial and nonglacial strata that occurs above a major regional Miocene–Pliocene unconformity. The allogroup spans the late Pliocene (3.6 Ma) to middle Pleistocene (0.126 Ma), based on magnetostratigraphy and pollen data. The sequence includes an alluvial deposit at the base, overlain by an extensive sequence of tills and outwash, and capped by loess. Paleosols and weathering horizons occur throughout the sequence. Tintina Trench; Beringia; glacial chronology; magnetostratigraphy; early and middle Pleistocene; Yukon paleoenvironments; Yukon paleosols; Yukon pollen; North American glaciations; West Tintina Trench Allogroup.

The thickest and the most complete loess sequence in the Carpathian basin: the borehole Udvari-2A

Abstract The key geological borehole Udvari-2A is located in Hungary at the Tolna Hegyhat Hills, in the central part of the Carpathian basin. Its detailed interpretation yielded valuable information on the stratigraphy and the post-Pannonian continental sedimentation regime of this territory. Stratigraphic data provided by the borehole facilitated the drawing up of a more precise picture on the post-Pannonian paleoenvironment and history of the evolution of the area. The borehole cut a thick series of the post-Pannonian Tengelic formation and the overlying, presumably complete sequence of the continental loess sediments of this region. The magnetostratigraphic correlation indicates that the 97 m thick loess sequence in the borehole Udvari-2A is the thickest and most complete so far recorded in Hungary and the deposition of loess started approximately at 1.1 Ma. In this sequence each of the four separable sedimentary periods of the loess are represented by distinct sediments. A theoretical stratigraphic column is also outlined to describe the Paks Loess Formation using data from the SE Transdanubia, defining and characterizing its lithological units. Since the formation is widely known, it was possible to correlate the most complete loess sequence in the Carpathian basin with the Marine Isotope Stages, and this opens up new opportunities for global correlation.

Characteristics of Pleistocene climate cycles identified in Cérna Valley loess–paleosol section (Vértesacsa, Hungary)

A climate cycle model was calculated using different methods (e.g., paleomagnetic, sedimentological and heavy mineral studies) on the loess sequence containing four paleosol and five loess layers in Cérna Valley. The model is based on the development of the sediment sequence created by the influence of the climatic, geological and geomorphological phenomena, and contains the following cycles: • The homogeneous, non-laminated loess was developed during the semiarid (350–400 mm/y) and cold periods in the Pleistocene. The magnetic fabric was built up by multidomain, ferromagnetic minerals and reflects the direction of dust deposition or/and the paleoslope. • The accumulation period of dust was followed by a more humid (650 mm/y) pedogenic period, indicated by the enrichment of superparamagnetic minerals and by the disturbed or inverse magnetic fabric developed during pedogenesis by different processes (e.g., leaching, bioturbation). • The third period following the pedogenic period is a humid erosional phase indicated by the fine layered reworked loess. The magnetic fabric built up by multidomain ferro- and superparamagnetic minerals was characterized by better aligned directions of principal susceptibilities than in the wind-blown material. Sheet wash and other water logged surface processes appeared in the fabric of these layers. This process is possibly connected to non-periodic, sudden events with high precipitation and absence of the vegetation. • The next cycle started with the following dust accumulation period.

Charcoal and wood remains for radiocarbon dating Upper Pleistocene loess sequences in Eastern Europe and Central Siberia

The present paper concerns long Upper Pleistocene loess sequences from Eastern Europe with multiple Upper Palaeolithic occupations, rich in charcoal, as well as loess sequences from Central Siberia with abundant wood remains. These complementary records have allowed establishing a high-resolution climatic sequence integrating 24 interstadial episodes between ca 42.5 and 10 kyr BP. Here, we discuss the methodology of dating used to fix the chronological framework of this climatic sequence, based on a set of 240 available radiocarbon dates, mainly produced on charcoal and wood remains. Special attention is paid to the strategy of sampling charcoal and wood material in strict accordance with stratigraphy, as well as to the preparation process in the laboratory for extraction, cleaning, identification and selection of the best fragments to date. Careful stratigraphic drawing and detailed positioning of the samples for each geological layer are also considered in order to clarify the relationship of the obtained dates with respect to any sedimentary, pedological or archaeological event to be dated. The reliability and accuracy of the dates obtained from loess sequences are further controlled by the internal consistency with regard to stratigraphy. Palaeoenvironmental implications are also discussed.

Alluvial sequence in the north piedmont of the Chinese Tian Shan over the past 550 kyr and its relationship to climate change

A new division of Middle and Late Pleistocene alluvial sequence in the north piedmont of the Chinese Tian Shan based on geomorphologic, stratigraphic, and chronologic criteria provides a framework for examining their relationship to climate change during glacial–interglacial transitions. Over the past 550 kyr at least four major alluviation episodes occurred within the piedmont. Along the major river valleys in this region, each episode of alluvial fan deposition morphologically correlates with a major river terrace. These correlations create a regionally applicable framework for subdivision of the Quaternary alluvial sequence in the study area, where seven stepped river terraces are defined. Our new chronology of this fluvial sequence suggests that, following intervals of aggradation, three highest river terraces and equivalent alluvial fans were abandoned at ∼ 530 ka, ∼ 300 ka and ∼ 10 ka, respectively. Paleosols at the base of the loess sequences that directly overlie the older terraces and fans suggest that episodes of aggradation occurred late in the glacial cycles. The subsequent incision that caused abandonment/stabilization of these terraces and fans occurred near to glacial–interglacial transitions. A relatively high degree of synchrony in major river incision events across the piedmont, despite disruption by several discrete structure zones with asychronous tectonic activities, supports the dominant control exerted by climatic conditions on alluvial deposition and terrace creation during the Quaternary across the north piedmont of the Chinese Tian Shan.

Impacts of post-depositional processes on rapid monsoon signals recorded by the last glacial loess deposits of northern China

Rapid East Asian Monsoon oscillations recorded by Chinese loess are thought to be dynamically linked to north Atlantic climate. However, few efforts have been made to assess the effects of post-depositional processes (e.g., surface mixing and pedogenesis) on loess paleoclimatic records. Here a detailed optically stimulated luminescence dating of a thick loess sequence from the western Loess Plateau is presented, offering a reliable chronology for last glacial deposits. Magnetic susceptibility and mean grain size records from three loess–paleosol sequences along a northwest–southeast transect are investigated to evaluate impacts of post-depositional processes on these loess-based proxy records. Our results indicate that: (1) loess sequences developed within the flat tableland of the central and western Loess Plateau are nearly continuous during the last glaciation; and (2) post-depositional processes have distinct impacts on rapid monsoon signals recorded in loess sequences from different regions. In the central Loess Plateau, rapid monsoon signals have been attenuated to various degrees depending on the sedimentation rate and pedogenic intensity. In the northwestern Loess Plateau, however, due to high sedimentation rate and relatively weak pedogenesis, high-resolution grain size oscillations reliably record rapid monsoon changes and can be well correlated to rapid climate changes recorded in the Greenland ice core and Hulu cave stalagmite.

Drastic climatic shift at ∼2.8 Ma as recorded in eolian deposits of China and its implications for redefining the Pliocene-Pleistocene boundary

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 S i-L i 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.