Western Chinese Loess Plateau Research Articles

Long-term magnetic susceptibility trends in the western Chinese Loess Plateau and their implication on magnetic susceptibility enhancement

Long-term magnetic susceptibility trends in the western Chinese Loess Plateau and their implication on magnetic susceptibility enhancement

∼4 Ma coarsening of sediments from Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH glaciation

Abstract Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3–4 Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3–4 Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain size, sea level and benthic δ 18 O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~ 4 Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0 Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~ 4 Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2 Ma. Notably, the absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1 Ma and the gradual decrease in coarse-grained peaks since 2.1 Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8–4.0 Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 Ma. A persistent and steady cooling during 4–3 Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75 Ma ago.

Geomorphological and sedimentological precautions in radiocarbon dating of eolian and lacustrine sequences in arid Asia

Lacustrine sediments in deep lakes are suitable for establishing radiocarbon chronology. Lacustrine sediments in shallow lakes are not suitable for establishing radiocarbon chronology, simply because younger plant roots can readily penetrate into older lacustrine sediments. Three features are warning signs for shallow-lake sediments: (1) high concentration of salts, (2) abundant vertically-oriented plant residues, and (3) organic matter-enriched sand layer. Field and laboratory investigations of eolian and water-reworked eolian sequences suggest that fluvial addition and fluvial alteration on terraces may produce reversed chronologies. Fluvial layers interbedded within eolian sequences should be avoided in obtaining dating targets, and eolian layers isolated by fluvial or lacustrine layers should be the targeting layers from which dating samples should be obtained. Bulk sediment 14C dates of a paleosol at Yuanbao loess section in the western Chinese Loess Plateau are distinctively older than charcoal dates, suggesting that bulk sediment samples contain older organic carbon that was likely brought upward by animal burrowing. In contrast, bulk sediment dates were found to be systematically younger than snail and charcoal dates at the Romantic section in Eastern Kazakhstan. The younger dates likely resulted from the downward intrusion of plant roots and/or translocation of mobile organic compounds into deeper layers. The only suggestion is to avoid bulk sediments as dating targets in eolian–paleosol sequences. The misleading chronology for the upper loess–paleosol complex at Lujiagou section indicated that even continuous loess deposits may yield reversed chronologies where loess has inherited older organic carbon from deflated floodplains and from loess hills. All kinds of dating targets (including charcoals) may produce unreliable chronologies.

Quartz OSL dating of last glacial sand dunes near Lanzhou on the western Chinese Loess Plateau: A comparison between different granulometric fractions

Optically stimulated luminescence (OSL) dating technique is a reliable method to determine the ages of sand dune sediments. While it seems logical to assume that for these windblown materials (such as sand dune sediments) grains from different sized fractions are suitable for optical dating and would yield identical ages, this was not previously explicitly demonstrated yet. In this study, six samples were selected from the sand dunes intercalated in loess strata near Lanzhou, western Chinese Loess Plateau, and different grain-size quartz fractions (e.g. 38–63 μm, 90–150 μm, 150–200 μm, 200–250 μm and 250–300 μm) were extracted to compare the OSL ages of different grain-size quartz. The results show that: (1) quartz OSL ages derived from different grain-size fractions produce identical ages within errors, confirming that the ages resulting from both coarse silt-sized (or middle grain of 38–63 μm) and sand-sized (90–300 μm) quartz can represent the periods of sand dune accumulation; (2) the OSL ages of the selected sand dune samples fall into ca. 28–18 ka, suggesting that the sand dune accumulation occurred during the marine isotope stage 2 (MIS 2) in current study area, which might imply regional increased aridity on the western Chinese Loess Plateau.

Last glacial pollen record from Lanzhou (Northwestern China) and possible forcing mechanisms for the MIS 3 climate change in Middle to East Asia

The vegetation on the northeastern margin of the Tibetan Plateau is highly sensitive to climatic changes and thus represents a potentially interesting environmental archive. Pollen samples from the Fanjiaping Loess section in Lanzhou on the western Chinese Loess Plateau (CLP) were analyzed in conjunction with OSL dating. The results indicate that pollen zone B (60.6–46.0 ka, correlative to the early MIS 3) had the greatest abundances of Cupressaceae, Tsuga, Gramineae and Cyperaceae of the entire section, suggesting a warm phase during the last glacial period. These pollen taxa decreased significantly in abundance in the zones C (46.0–39.0 ka) and D (39.0–27.0 ka), reflecting a substantial climate cooling from the middle MIS 3 to MIS 2. These results correlate with climate records from the South China Sea, the CLP, Baikal Lake, North America, North Atlantic Ocean and other regions, and probably correspond with the decline of northern high-latitude insolation and the increase of global ice volume from 50 to 20 ka. In particular, arboreal pollen, fern spore and algae abundances declined sharply since ∼40 ka, while shrub and herb pollen reached the highest abundances. Conifer pollen Picea and Abies abundance also rose markedly and increased up the section. This implies significant climate deterioration and likely corresponded with substantial growth of the polar ice sheets since ∼40 ka. The decreasing temperature caused by an insolation decline during the last glacial period probably reinforced the cooling effect in a ‘snow/ice/albedo’ feedback, which would result in less climate sensitivity to radiative forcing. Meanwhile, vegetation decline in the Northern Hemisphere during the last glacial period and tundra development at high latitudes possibly caused additional cooling, enhancing the growth of polar ice sheets since 40 ka. The development of polar ice sheets increased the polar-to-equator temperature and pressure gradients, strengthening the westerlies and supplying plenty of moisture to Northwest China during 40–30 ka. Lake sediments developed widely on the Tibetan Plateau during 40–30 ka, probably related to an increase in the seasonality of middle-to-low latitude insolation which caused an enhancement of glacier melting on the Plateau.

Palaeodunes intercalated in loess strata from the western Chinese Loess Plateau: Timing and palaeoclimatic implications

Dune systems represent important palaeoenvironmental archives and therefore provide valuable information on past climate conditions of dryland regions. This study reports on several palaeodunes (thickness ca. 10–20 m) intercalated in thick loess accumulations, located in the Lanzhou area of the western Chinese Loess Plateau. The intercalated dune sands are well-sorted and have a typical aeolian dune structure, with distinct cross-bedded sand layers. The chronology for these palaeodunes and the related loess sediments was established by quartz optically stimulated luminescence (OSL) dating of 19 samples. The dating results indicate that sand dune accumulation occurred at ca. 35–25 ka, which implies a period of increased aridity during dune formation. This interpretation is in contrast to previously retrieved data from lake archives of the nearby deserts (e.g. Tengger and Badain Jaran desert), indicating a humid episode during marine isotope stage 3 (MIS 3). The previously proposed humid MIS 3a may not be universal over all of northwest China.

New eolian red clay sequence on the western Chinese Loess Plateau linked to onset of Asian desertification about 25 Ma ago

The expansion of inland Asia deserts has considerably influenced the environmental, social and economic activities in Asia. Aridification of inland Asia, especially timing of the initiation of Asian desertification, is a contentious topic in paleoclimatology. Late Cenozoic eolian loess-red clay sequences on the Chinese Loess Plateau, which possess abundant paleoclimatic and paleo-environmental information, can be regarded as an indicator of inland Asia desertification. Here we present a detailed magnetostratigraphic investigation of a new red clay sequence about 654 m in Zhuanglang located at the western Chinese Loess Plateau. Sedimentological, geochemical, mineralogical, and quartz morphological lines of evidence show that the red clay is of eolian origin. Magnetostratigraphic correlations indicate that this core sequence spans from 25.6 to 4.8 Ma, and typical eolian red clay appears as early as 25 Ma. This extends the lower limit of the red clay on the Chinese Loess Plateau from the previously thought early Miocene back into the late Oligocene. This new red clay record further implies that the inland Asia desertification was initiated at least by the late Oligocene. This sequence provides a unique high-resolution geological record for understanding the inland Asia desertification process since the late Oligocene.

~ 4 Ma coarsening of sediments from Baikal, Chinese Loess Plateau and South China Sea and implications for the onset of NH glaciation

Intense tectonic movement occurred along the northeastern margin of the Tibetan Plateau 3–4 Ma. During the same time period, global climate changed from a period of stability to a period of frequent and abrupt changes, which prevented fluvial and glacial systems from establishing a state of equilibrium. Thus, it is difficult to directly attribute the 3–4 Ma coarse-grained sediment accumulation in East Asia to climate change or tectonic activity. This study compares Late Cenozoic multi-proxy records from the Chinese Loess Plateau (CLP) to the low-latitude grain size, sea level and benthic δ 18O records from the South China Sea (SCS) and the high latitude grain-size record from Lake Baikal. All records suggest a steady and persistent cooling since ~ 4 Ma, which correlates well with those cooling records from other regions around the world. This coincided in timing with the modeling results that the closure of the Central American Seaway (CAS) initiated strengthening of Atlantic meridional overturning circulation between 4.8 and 4.0 Ma which led to both warming of the Northern Hemisphere (NH) and cooling of the Southern Hemisphere (SH). Cooling of the SH would induce a marked development of the Antarctic ice sheets at ~ 4 Ma, pushing the Intertropical Convergence Zone (ITCZ) northward. This was superimposed on warming of the NH and brought more precipitation to the middle latitudes of the NH, resulting in increases in coarse-grained sediments in the Sikouzi section from the western CLP since 4.2 Ma. Notably, the absence of coarse-grained sedimentation in the Sikouzi section during 3.0-2.1 Ma and the gradual decrease in coarse-grained peaks since 2.1 Ma reflected by the Sikouzi grain-size record probably resulted from stepwise increase in the NH ice volume pushing the ITCZ southward. On the other hand, development of the Antarctic ice sheets would induce global cooling and enhancement of physical weathering, initiating increases in sedimentation rates as well as increases in grain size from Lake Baikal to the CLP to the SCS. Therefore the closure of the CAS during 4.8–4.0 Ma and its influence on ocean heat transport was possibly the major forcing factor for global cooling since 4 Ma. A persistent and steady cooling during 4–3 Ma probably made a significant contribution to the establishment of the NH ice sheets at 2.75 Ma ago.

Pliocene land snail record from western Chinese Loess Plateau and implications for impacts of the summer insolation gradient between middle and low latitudes on the East Asian summer monsoon

The East Asian monsoon probably existed as early as at the Palaeogene/Neogene boundary. However, its evolutionary process is still less well known owing mainly to the lack of long, continuous palaeoenvironmental records. The recently reported Miocene (22–6.2 Ma) and late Miocene–Pliocene (7.1–3.5 Ma) loess–palaeosol sequences from the western Chinese Loess Plateau (CLP) provide new insights into the evolution of the monsoon system. However, reports on the bioclimatic indicators from these deposits and the subsequent reconstruction of the palaeomonsoon are rare. Here we present a Pliocene terrestrial mollusk record from the western CLP and discuss the possible impact of isolation gradients on the East Asian summer monsoon. Our results show that most peak values of the dominant thermo-humidiphilous mollusk taxa, Metodontia and Punctum, a proxy of the East Asian summer monsoon, approximately correspond to maxima of mean summer insolation gradient between middle and low latitudes as well as some maximum values of the 35°N insolation, suggesting a possible causal link between the summer monsoon and the insolation parameters. The major frequencies from spectrum analysis of the sum of the two warmth- and moisture-loving taxa through the loess–palaeosol succession match those obtained from the mean summer insolation-gradient variations between middle and low latitudes over this geological period, providing further evidence for such a causal relationship. Mean summer insolation gradient between middle and low latitudes could influence atmospheric circulation (in the present condition the East Asian summer monsoon). Any elevated mean summer insolation gradients between middle and low latitudes would have intensified the East Asian summer monsoon and the flux of moisture and heat over the oceans to the interior region including the CLP, creating favorable conditions for the expansion of the mollusk fauna. As such, the mollusk record from the loess–palaeosol deposits in the western CLP provides evidence for insolation-gradient impacts on the development of the East Asian monsoon system in the Pliocene.

Vegetation and environmental changes in western Chinese Loess Plateau since 13.0 ka BP

Pollen records from the Chinese Loess Plateau revealed a detailed history of vegetation variation and associated climate changes during the last 13.0 ka BP. Before 12.1 ka BP, steppe or desert-steppe vegetation dominated landscape then was replaced by a coniferous forest under a generally wet climate (12.1–11.0 ka BP). The vegetation was deteriorated into steppe landscape and further into a desert-steppe landscape between 11.0 and 9.8 ka BP. After a brief episode of a cool and wet climate (9.8–9.6 ka BP), a relatively mild and dry condition prevailed during the early Holocene (9.6–7.6 ka BP). The most favourable climate of warm and humid period occurred during mid-Holocene (7.6-∼4.0 ka BP) marked by forest-steppe landscape and vegetation alternatively changed between steppe and desert-steppe from ∼4.0 to ∼1.0 ka BP.

A mineral tracer toward high-resolution dust provenance on the Chinese Loess Plateau: SEM, TEM, and sulfur isotopes of sulfate inclusions in biotite

The most important terrestrial archive of the Quaternary climate is the Chinese Loess Plateau (CLP) distributed over a vast region peripheral to the deserts of Inner Asia. Numerous paleoclimatic studies have examined magnetic, chemical, and isotopic proxies from deep loess-paleosol sections. However, the highly uniform mineralogy of the CLP has made it difficult to track the provenance, transportation, and deposition of dust particles. Here, we report that the micrometer-scale mineralogical heterogeneity of the loess particles has a significant potential in determining high-resolution loess provenance. Scanning and transmission electron microscopy of the Chinese loess reveals that biotite particles commonly have microinclusions of Ba-Sr sulfates displacing the micaceous layers along cleavages. Chemical compositions varied widely from barite to celestine within the same biotite grain. The Ba-rich sulfates precipitated first, followed by an overgrowth of Sr-rich sulfates. The frequency of sulfate-bearing biotite varied greatly over several loess sections, implying a specific source and sedimentation process. Their abundance on the western CLP and sulfur isotopic composition support their derivation from porous sedimentary sequences reacted with hypersaline solutions, e.g., arkosic sandstone, probably located west of the CLP covering the Qilian Mountains, Qaidam Basin, and surrounding mountains of the Tibetan-Qinghai Plateau. The sulfate-bearing biotite is the first microscopic mineral tracer in the CLP, provoking further search for other microscopic heterogeneities of the Chinese loess and their equivalents in the source regions.

An 11-Ma-old red clay sequence on the Eastern Chinese Loess Plateau

Older red clay deposits on the Chinese Loess Plateau contain information of early paleomonsoon evolution and tectonic history of the Ordos platform and the Tibetan Plateau. A well-conserved eolian red clay sequence was recently discovered at Shilou, on the Eastern Chinese Loess Plateau. Paleomagnetic measurements demonstrate that polarity zones recorded in the Shilou red clay profile correspond well with polarity zones between C2An.1n and C5n.2n of the GPTS, and the basal age of the study profile is approximately 11.0 Ma. Field observation, grain-size frequency distribution and quartz grain morphology suggest a wind-blown origin for this sequence. This indicates that the Eastern Loess Plateau began to accumulate dust deposits at 11 Ma ago, implying earlier establishment of the East Asian paleomonsoon probably owing to uplifting of the Tibetan Plateau and ongoing global cooling. It also demonstrates that the Eastern Loess Plateau belonging to the Ordos platform was slowly uplifted and suffered from erosion in the early Neogene, and has no conserved eolian deposits older than 11 Ma as in the Western Chinese Loess Plateau.

Consistent grain size distribution of pedogenic maghemite of surface soils and Miocene loessic soils on the Chinese Loess Plateau

AbstractThere is agreement that ultrafine maghemite grains (<100 nm) are responsible for the magnetic enhancement of Chinese loess. Recent studies show that grain size distribution of ultrafine pedogenic maghemite grains within the aeolian sequences on the central Chinese Loess Plateau deposited in the last 8 Ma is consistent. However, whether the observed grain size distribution of ultrafine pedogenic maghemite grains can be observed in the western Chinese Loess Plateau and modern soils is not entirely clear. Here we find that young surface soil samples across the Chinese Loess Plateau and early Neogene loessic soils from the western Chinese Loess Plateau also show consistent grain size distribution with that of ultrafine pedogenic maghemite grains on the central Chinese Loess Plateau. The fact that young surface soil samples show apparent magnetic enhancement suggests that the ultrafine pedogenic maghemite grains derive from oxidation of ultrafine magnetite grains. Copyright © 2009 John Wiley & Sons, Ltd.

Optically stimulated luminescence dating of sediments from the Yellow River terraces in Lanzhou: Tectonic and climatic implications

The stratigraphic chronology of Yellow River terraces was investigated and studied in Lanzhou Basin, western Chinese Loess Plateau. The optically stimulated luminescence (OSL) dating results show that terraces T 1, T 2 and T 3 formed at 8 ka, 20 ka and 70 ka, respectively. Lateral accretion of the riverbed facies gravel sediments occurred during interglacial periods while vertical aggradations of the terrace sediments deposited predominantly under cold and dry glacial period. A thick layer of aeolian loess with a basal age about 35 ka indicates a remarkable drop of air temperature and a dry, cold climate. The temporal correlation between terrace formation and tectonic movement has not yet been established in this research, but the stratigraphic chronology of the terrace sections provides the timing of the terrace formation, the incision rate of the Yellow River, and the slip rate of the fault horizon.

Last glacial climate instability documented by coarse-grained sediments within the loess sequence, at Fanjiaping, Lanzhou, China

Optically Stimulated Luminescence dating, grain-size analysis and magnetic susceptibility measurements were conducted on the Fanjiaping loess section, from the western Chinese Loess Plateau. The results confirm that last glacial high-frequency climatic shifts were documented in mid-latitude continental archives. The grain-size record indicated that coarse-grained sediments with horizontal bedding and channel-fill structures were only deposited in several short intervals, equivalent to the beginning of marine oxygen isotope stage (MIS) 4 and the early to middle MIS 3. This probably implies brief rainfall intensification of the Asian summer monsoon, and its disappearance since the late MIS 3 to MIS 2 may have been a response to significant glacial cooling in the Northern Hemisphere. Previous investigations revealed high sea-surface temperatures at high latitudes at the start of MIS 4, and the early to middle MIS 3 intensification of summer insolation in the Northern Hemisphere, implying evident climate amelioration. Climate improvement favors boreal forest recovery, enhancing both winter and summer air temperatures. The resultant smaller equator-polar temperature gradient probably helped the moisture-laden summer monsoon to penetrate northward. This study thus provides new significant information about the response of terrestrial loessic palaeoenvironments to millennial-timescale climatic fluctuations during the last glacial period.

Holocene natural rhythms of vegetation and present potential ecology in the Western Chinese Loess Plateau

Ecological restoration (reconstruction) must have a scientific basis on the biological zones of vegetation mainly controlled by the climatic factors (humidity and thermal conditions) and the sere of it. Holocene natural rhythms of vegetation in the Western Chinese Loess Plateau were reconstructed using pollen assemblages, a powerful proxy for paleo-environmental reconstruction and the potential natural vegetation (PNV) modeled using environmental variable-enveloping approaches with the Holdridge model. The succession of vegetation and geographical pattern of the potential vegetation are discussed. Both have very important implications for ecological restoration and management. During the Holocene (i.e., past 10,000 years), the virgin vegetation in the Western Chinese Loess Plateau has experienced a series of changes. A desert-steppe of late glacial was succeeded by a forest-steppe from 8850 to 7540 14C BP, and a Pinus-dominated forest occupied the landscape from 7540 to 6560 14C BP. A deciduous forest of high density and diversity existed from 6560 to 5790 14C BP. Afterwards the vegetation changed to a Pinus-dominated forest-steppe (5790–4950 14C BP) and then to an Ulmus-dominated forest-steppe (4950 to ∼4000 14C BP). The vegetation subsequently changed to a steppe from ∼4000 to 3120 14C BP and further to a desert-steppe from 3120 to 2900 14C BP. After a period of vegetation improvement (steppe) from 2900 to 2460 14C BP, a desert-steppe resumed (2460–2020 14C BP). Steppe vegetation was re-established around 2020 14C BP and seems to have then deteriorated again around 1000 14C BP. The potential vegetation pattern modeled using environmental variable-enveloping approaches with the Holdridge model indicates that, in the Western Chinese Loess Plateau except for the hill and mountain areas, there are three south–north potential ecological conditions zones corresponding to the S–N gradient of modern climate of this area. They are forests and steppe forest, steppe, and desert, respectively. In the hills and mountains, zonal vegetation is readily observable. Specifically, the lower elevations are dominated by steppes, the middle elevations by steppe forests, and higher elevations by forests. The lower limits of steppe-forest and forest zones are lower in east- and south-facing slopes due to the influence of the East Asian summer monsoon that brings precipitation from the southeast. Results indicate that the southeastern corner and the western fringe should be deciduous forests, the southeastern half steppe forests, and the northwestern half steppes, which will become the main regions of ecological restoration and agricultural development. Only the northwestern corner should be desert-steppe, and in such places human activities should be reduced. In hills and mountains, efforts will be made to prohibit hunting and grazing and plant trees on mountains, return the grain plots to forestry, and carry out water and soil conservation at present.

Late Neogene evolution of the East Asian monsoon revealed by terrestrial mollusk record in Western Chinese Loess Plateau: From winter to summer dominated sub-regime

More and more evidence indicates that the onset of the East Asian (EA) monsoon can be traced back to the Oligocene–Miocene boundary (at about 23 Ma). However, the process of its evolution is still less well-known until now. Here we investigate its late Neogene evolution by analyzing a terrestrial mollusk sequence, from the Chinese Loess Plateau (CLP), covering the period between 7.1 and 3.5 Ma. Considering the modern ecological requirements of these organisms, we were able to define two groups of cold-aridiphilous (CA) and thermo-humidiphilous (TH) species, representing the EA winter and summer monsoon variations, respectively, as previously defined in the Quaternary glacial–interglacial cycles. Variations in these two groups indicate two different monsoon dominated periods during 7.1–3.5 Ma. First, between 7.1 and 5.5 Ma, the EA winter monsoon, with a 100-kyr periodicity, was dominant. Second, between 5.1 and 4 Ma, the EA summer monsoon dominated, with a 41-kyr periodicity. Furthermore, our mollusk record yields valuable evidence for a late Miocene–Pliocene transition of about 400 kyr from winter monsoon dominated towards summer monsoon dominated, associated with a periodicity transition from weak 100 kyr to 41 kyr. The strengthened winter monsoon interval, with a 100-kyr periodicity, is coeval with orbital-scale global ice-volume changes, in conjunction with the uplift of the Tibetan Plateau which probably reinforced the winter monsoon sub-regime. Conversely, closures of the Panama and Indonesian seaways, associated with changes in obliquity between 5.1 and 4 Ma, are probably major forcing factors for the observed dominant summer monsoon with 41-kyr frequency, favoring heat and moisture transports between low and high latitudes to allow TH mollusks to grow and develop in the CLP.

Paleomagnetic and mineral magnetic investigation of the Baicaoyuan loess‐paleosol sequence of the western Chinese Loess Plateau over the last glacial‐interglacial cycle and its geological implications

Closely spaced paleomagnetic and mineral magnetic studies were conducted on the Baicaoyuan (BCY) loess‐paleosol sequence since the last interglacial in the western Chinese Loess Plateau. The multiproxy records of mineral magnetic parameters clearly indicate loess‐paleosol alternations with the fine structure of pedostratigraphy over the last glacial‐interglacial cycle. However, the paleomagnetic results reveal the absence of any short‐lived geomagnetic excursions in the BCY section. This absence could be interpreted by two alternative mechanisms, such as magnetic overprinting and discontinuous sedimentation. The absence of the Mono Lake and Laschamp excursions in the loess unit L1 is probably due to the viscous overprinting of the remanence carried by coarse‐grained magnetite of eolian origin and/or to the episodic nature of loess accumulation during the last glacial period. However, discontinuous sediment accumulation may more probably lead to the absence of the Blake excursion in the last interglacial soil S1 as well as of the Mono Lake and Laschamp excursions in the last glacial loess L1 at the BCY section. Together with previously published paleomagnetic results from other sections over the Loess Plateau, the findings probably further indicate the discontinuous nature of loess accumulation in the western Loess Plateau, where sedimentation of the last interglacial soil may be episodic at the timescale equivalent to the duration of the Blake geomagnetic excursion (4–6 ka), much longer than the timescale for episodic loess sedimentation in the last glacial loess of both the western and eastern Loess Plateau due to the reduced sediment accumulation rate compared to that during the last glacial period. Thus, sediments with relatively high sedimentation rate in some parts of the western Loess Plateau do not consistently guarantee the faithful recording of high‐frequency geomagnetic variations.

Contribution of inherent organic carbon to the bulk δ 13C signal in loess deposits from the arid western Chinese Loess Plateau

The isotopic composition of total organic carbon (TOC) in loess deposits has been widely used to reconstruct palaeovegetation and palaeoclimate for terrestrial ecosystems. The possible significance of the distribution and contribution of organic carbon from dust source regions to loess TOC remains unclear, but may potentially affect interpretation of stable carbon isotope distributions in loess records with respect to local vegetation. Two loess profiles from arid areas in China with sparse vegetation coverage have been investigated. After removing the surface loess affected by recent farming, we detected a narrow range of carbon isotope (δ 13C) values from −21.4‰ to −20‰ in the Lanzhou area and from −21.5‰ to −19.7‰ in the Xining area. The values are inconsistent with that of the established vegetation in these areas, where C 3 plants are dominant. In contrast, the δ 13C values of the loess TOC closely resemble those from dust sources. Together with data from loess TOC and magnetic susceptibility, we demonstrate that the isotopic composition of loess TOC in arid areas was largely controlled by the carbon isotope composition of the dust source, with only a minor contribution from local native vegetation. The results suggest that background carbon isotopic signals should be taken into account when interpreting carbon isotope compositions of loess deposits in arid areas.

Holocene vegetation cover in Qin’an area of western Chinese Loess Plateau revealed by n-alkane

Previous studies have found that wetlands prevailed in western Chinese Loess Plateau and pine pollen could reach up to 80% in Qin’an area of the plateau during middle Holocene. It was then deduced that forest vegetation covered Qin’an area in a warm and wet climate during middle Holocene. The proxies of molecule biomarkers from two Holocene sections, a swamp-alluvial loess section and a typical loess-paleosol section are used to reconstruct regional vegetation history. It is found that the heavy-molecular-weight (HMW) homologues of the n-alkanes in all samples exhibit a pronounced odd-over-even predominance, maximizing at C31 and the abundance of the nC27-alkanes is the lowest in nC27, nC29, nC31, i.e. C27<C29<C31, a typical grass n-alkanes model. The results are the same as those of model surface soil samples under grass cover but different from both forest cover and modern pine leaves, which yield preferentially nC29-alkanes peak. It could be inferred that the area was dominated by grass cover but not forest cover during whole Holocene epoch. The study shows that combining both molecule biomarkers and pollen analysis can avoid the disadvantage of pollen analysis in reconstruction of regional vegetation cover.