Luochuan Loess Section Research Articles

Periodic variations of alpine glaciation and desertification of the NE Tibetan Plateau: Evidence from the loess-paleosol sequences in the Menyuan Basin

The NE Tibetan Plateau (NETP) is one of the most ecologically vulnerable areas, with large expansion and retreat of desert area since the Last Glacial Maximum. However, the long-term history of desertification in this area and its mechanisms are still unknown due to lack of long-term and well-dated records. Here we present the results of sand particle content (a proxy of local materials) and detrital-zircon ages from a high-resolved loess-paleosol section spanning the past 420 ka in the Menyuan (MY) Basin lying in the NETP. Our results show that the provenance of loess units (formed in glacial periods) was strongly influenced by local desertified materials that were mainly produced by alpine glaciers surrounding the basin, which was significantly different from the paleosol units (formed in interglacial periods). The strong signals of ~100 kyr and ~ 20 kyr cycles in the content of local materials suggest that the desertification of the MY Basin was controlled by both the late Quaternary ~100-kyr ice age and the ~20-kyr precession cycles, which is obviously different from the periodicities of the classic Luochuan loess section in the mainland of the Chinese Loess Plateau (CLP). The strong ~20-kyr cycle in the MY Basin suggest that the alpine glaciation activities in the NETP were driven by local summer insolation. These results allow us to characterize the long-term natural variability of desertification in the NETP, as well as the mechanisms for the spatial and glacial-interglacial variations of loess provenance on the CLP.

An absolutely dated record of climate change over the last three glacial–interglacial cycles from Chinese loess deposits

Abstract Chinese loess-paleosol sequences have long been regarded as the continental counterpart of deep-sea sediments in terms of their record of the Quaternary glacial–interglacial cycles. However, absolute chronologies for loess-paleosol sequences older than ca. 130 ka on the Chinese Loess Plateau are scarce. We conducted the first high-resolution luminescence dating, extending back to 350 ka, of the classical Luochuan loess section of the central Chinese Loess Plateau. Bacon age-depth modeling of the luminescence ages was used to obtain an age framework. The results indicate that on orbital timescales, loess accumulation at Luochuan was continuous over the last three glacial–interglacial cycles. The new age framework resolves discrepancies between orbitally tuned age models at several boundaries and provides new evidence supporting the reassignment of the Marine Isotope Stage 8/9 boundary age from 300 ka to ca. 280 ka. Our results also suggest that published astronomical timescales for the loess deposits of the Chinese Loess Plateau over the last ~2.6 m.y. are probably broadly reliable, as are the related regional and global climatic interpretations.

Hominin distribution in glacial-interglacial environmental changes in the Qinling Mountains range, central China

The Qinling Mountain Range (QMR) in central China encompasses innumerable Paleolithic sites. The hominin settlement in the QMR is comparable with that in the Nihewan Basin in northern China. The recorded information on the loess deposition in the QMR include both hominin remains and environmental changes. Since 2004, geological, geomorphological, archaeological, and chronological investigations were conducted by our team. By systematically using luminescence, paleomagnetic, and 26Al/10Be burial dating methods to obtain age controls, and by correlating the pedostratigraphy and magnetic susceptibility of the Luochuan loess section, we established the loess–paleosol sequence and chronology of the lithic artifact levels for 35 Paleolithic sites and spots in the QMR. This work remarkable found shifts from glacial-to interglacial-driving hominin settlement patterns. During the stage between 1.2 and 0.7 Ma, large drying events, such as L15 (MIS 38) and L9 (MIS 22, 23 and 24), may have driven hominin migrations when the Loess Plateau was depopulated; moreover, the southern QMR was a glacial refugium. During the stage after ∼0.60 Ma, the contrasts between glacial and interglacial scales are the greatest; furthermore, longer and warmer humid interglacial environments were dominant. S5 (MIS 13, 14, 15) and S1 (MIS 5) interglacial periods provided the optimal environments for hominin settlement and dispersal. On the basis of investigations, we also found that the hominin settlement is relatively continuous from ∼1.20 Ma to ∼0.05 Ma in the QMR. The human occupation of the QMR decreased considerably after ∼0.05 Ma, probably because of changes in climate and human adaptations.

Natural and laboratory TT-OSL dose response curves: Testing the lifetime of the TT-OSL signal in nature

This study compares natural and laboratory generated thermally transferred optically stimulated luminescence (TT-OSL) dose response curves (DRCs) for fine-grain quartz extracts from the Luochuan loess section in central China. Both DRCs saturate at high doses relative to the quartz OSL signal; the natural TT-OSL DRC saturates at about 2200 Gy and laboratory DRCs saturate at about 2700 Gy. However, the natural and laboratory TT-OSL DRCs deviate from one another at circa 150 Gy resulting in TT-OSL equivalent dose underestimation relative to palaeodoses expected from dose rates and independent age control. The lifetime of the TT-OSL signal at 10 °C, calculated from values of trap parameters E and s, is compared against the value for lifetime of the TT-OSL signal in nature at average burial temperature as determined from the age underestimation caused by deviation of the natural and laboratory generated DRCs. These two independent assessments of TT-OSL signal lifetime at Luochuan give similar values, suggesting that laboratory measurements of thermal stability reflect natural burial lifetimes and can potentially be used to correct TT-OSL ages for the difference between natural and laboratory dose response curves.

Natural pedogenic pathway of iron oxides

The exact formation pathway of iron oxides via pedogenesis has long been a contentious topic in geological and environmental studies [1]. Pedogenesis is crucial to accurately interpret the environmental significances of iron oxides in paleoclimatic archives, for example, the Chinese Loess/Paleosol sequences [2]. Several models have been developed but all lack direct constraints from natural records [2]. Hu et al. adopted a novel approach by integrating rock magnetism, dynamic dissolution and diffuse reflectance spectroscopy techniques [3]. They successfully separated and quantified major iron oxides of different origins (detrital and pedogenic) from a loess– paleosol–loesstransition sequence of the classic Luochuan Loess section in Shaanxi, China. Their major scientific contribution is 2-fold. First, they characterized, in detail, the pedogenic and detrital (or aeolian) hematite in loess/paleosols. Pedogenic hematite is nano-sized, preferentially dissolved in a weak reductive environment and, with a greater content of Al, can be easily demagnetized by the alternating field (AF) (remanence coercivity=∼130 mT). In contrast, the detrital counterpart is micro-sized, more resistant to dissolution, is almost stoichiometric and is resistant more to the AF demagnetization (remanence coercivity = ∼1 T). Second, based on the separation of iron oxides of different origins, the researchers quantified and described the relationships between different iron oxides. The results indicated that superparamagnetic (SP) pedogenic maghemite was Figure 1. Identification of pedogenic (Ped) and detrital (Det) hematite (Hm) in the Chinese loess/paleosol sequence. a) Schematic representation of Luochuan section; b) The depth variation of low field susceptibility, χ ; c) Hematite dissolution behavior in a typical loess sample; d) Isothermal remanent magnetization (IRM) decomposition of a typical loess sample, indicating the presence of detrital hematite with the central remanence coercivity around 1. 5 T. The stars defines the end of the first step where Ped–Hm is significantly dissolved, but the Det–Hm remains almost untouched; e) Hematite dissolution behavior in a typical paleosol sample; f) IRM decomposition of a typical paleosol sample, indicating the presence of both detrital and pedogenic hematitewith central remanence coercivity around 1 T and 130 mT, respectively. The black and color curves indicate the original and the dissolved samples, respectively; g) Relationship between the concentration indicator and the remanence indicator of pedogenic hematite, indicating the initial production of pedogenic hematite is in the SD region. The dashed line represents the case when SP hematites are largely produced initially. The solid line indicates the linear trend corresponding to the case that the initial production is in an SD state. (Revised from Hu et al. [3]).

Luminescence dating of Chinese loess beyond 130 ka using the non-fading signal from K-feldspar

A multi-elevated-temperature post-IR IRSL (MET-pIRIR) protocol, which utilizes the IRSL signals measured by progressively increasing the stimulation temperature from 50 to 300 °C in a step of 50 °C, was applied to date the potassium-rich feldspar (K-feldspar) extracts from loess samples at the Luochuan section of the Chinese Loess Plateau. It was observed that the MET-pIRIR ages obtained at elevated-temperatures (250 and 300 °C) are consistent with independent chronological control for the samples from the first loess layer (L1) to the third paleosol layer (S3), which correspond to the marine isotope stages (MIS) 2–9. Our results indicate that the MET-pIRIR protocol can provide reliable ages for the Chinese loess up to ∼300 ka. The results suggest that the MET-pIRIR signal measured at 250 and 300 °C gives the most reliable ages for older samples (>130 ka). For samples below L3, the natural MET-pIRIR signals measured at high temperatures reach the saturation level in dose response curves, suggesting a dating limit of ∼300 ka for the Luochuan loess section.

Climatic sub-cycles recorded by the fourth paleosol layer at Luochuan on the Loess Plateau

The Chinese loess–paleosol succession, deep-sea sedimentary sequences and ice cores are the three types of paleoclimatic records. Measurements were made of the ancient climatic proxies of magnetic susceptibility, median diameter of grain size and content >40 μm, and the CaCO3 content of samples collected at 1 cm depth intervals in the fourth paleosol layer of the Luochuan loess-paleosol section on the Chinese Loess Plateau. Wavelet analysis of the data identified cycles of climate change, which were compared to the marine δ18O record of SPECMAP. The results showed that: (1) when the fourth paleosol layer was forming, the paleoclimate was relatively stable but five extreme climatic events were identifiable as occurring around 408, 381, 376, 368 and 361 kaBP; (2) two of these regional events differed from the global trend; (3) during marine isotope stage (MIS)11, the fourth paleosol layer of the Luochuan loess section recorded a regional quasi-19 ka climatic sub-cycle and (4) this sub-cycle was also reflected globally in the SPECMAP data indicating that, during MIS11, this sub-cycle was regionally and globally synchronized.

Chronology and the upper dating limit for loess samples from Luochuan section in the Chinese Loess Plateau using quartz OSL SAR protocol

Luminescence dating of loess older than 100 ka has long been a challenge. It has been recently reported that, using optically stimulated luminescence (OSL) of fine-grained quartz (4–11 μm) extracted from loess, the range of luminescence dating could be pushed to ∼0.6 Ma with OSL ages being in agreement with independent ages [Watanuki, T., Murray, A.S., Tsukamoto, S., 2005. Quartz and polymineral luminescence dating of Japanese loess over the last 0.6 Ma: comparison with an independent chronology. Earth and Planetary Science Letters 240, 774–789]. The aim of this study is to provide a luminescence chronology (20 samples) for the standard Luochuan loess section, and to further examine the upper limit of quartz OSL dating for Chinese loess. The growth curve does not saturate at 700 Gy, and should allow reliable equivalent dose ( D e) determination up to at least 400 Gy. However, when compared with independent chronological control, the D e that could be treated as reliable is less than ∼230 Gy (corresponding to ∼70 ka in age for Chinese loess), and the D e larger than ∼230 Gy should be underestimated. Ages for samples from the lower part of palaeosol S1 are severely underestimated, with the maximum age of 95 ka for a sample from the bottom of this palaeosol, much younger than the expected age of ∼128 ka. The maximum D e obtained for sample L9/M, collected from loess layer L9 which is below the Matuyama–Brunhes (B/M) boundary whose age is ∼780 ka, is only 403 Gy which corresponds to an age of 107 ka. The cause of underestimation is not yet clear. The previous results by Watunuki et al. (2005) on the extension of OSL dating of loess to 0.6 Ma is not confirmed. When evaluating the validity of OSL ages in S1, another possibility is to question the already established chronological frame for Luochuan section, which is based on the hypothesis of continuous dust deposition. The assumption of an erosion hiatus between L2 and S1 could make the OSL ages look reasonable. However, if this is the case, then it is difficult to explain why the age of sample L9/M is only 107 ka which could be treated as a saturation age, while the OSL can provide a correct age for loess as old as 94.9 ka for sample LC22 collected from the bottom of S1. Much work is required to clarify these confusions.

Molecular fossil and paleovegetation records of paleosol S4 and adjacent loess layers in the Luochuan loess section, NW China

Using gas chromatography-mass spectrometry (GC-MS) technique, a series of biomarkers were identified, including n-alkanes, n-alkane-2-ones, isoprenoid etc. from the loess-paleosol samples collected from the S4 and adjacent L5, L4 of the Luochuan loess section, Northwestern China. Based on these data, especially n-alkanes and high-resolution magnetic susceptibility and grain size data, the paleoenvironment and paleovegetation history during S4 was reconstructed. The CPI (Carbon Predominance Index) and correlation between n-alkanes and magnetic susceptibility and grain size data demonstrated that the molecular fossils in paleosol and loess layers can reflect the vegetation condition during the loess-paleosol formation, if the allochthonous organic inputs could be excluded reasonably. The ACL (average chain length) index is correlated well with paleomagnetic susceptibility and grain size variations, displaying their good synchrony with warm and humid climate. However, it relatively lagged behind the paleomagnetic susceptibility and the grain size variations when the climate began to deteriorate. During the formation period of paleosol, the n-alkanes was dominated by C31 homologue, indicating that the primary organic input originated from herbs. Our study also demonstrated that the herbs were more flourish than wood plants in Loess Plateau, especially in the Luochuan area during the warm and humid phase, and there was no typical forest vegetation developed in the studied period.

Phosphorus geochemistry in the Luochuan loess section, North China and its paleoclimatic implications

Total P (P t) on a carbonate-free basis in an entire loess–paleosol sequence and P t, organic P (P o) and inorganic P (P i) in the S 0–L 1–S 1 sequence were investigated in detail with different resolutions for the Luochuan loess section from northern China. P t content varies between 393 and 786 ppm throughout the loess–paleosol sequence, and is generally higher in the loess than in interstratified paleosols, showing fluctuation cycles of 100 ka in correspondence to loess–paleosol alternations. P t variations on a carbonate-free basis in the loess–paleosol sequence could indicate variations in atmosphere precipitation resulting in different leaching loss of P from paleosols. P i has an average value of 499 ppm with a range of 324–560 ppm, accounting for more than 70% of P t in the S 0–L 1–S 1 sequence, where the minimum of P i in the Malan loess is higher than the maximum of P i in S 1. P o ranges between 59 and 233 ppm with an average of 132 ppm in the S 0–L 1–S 1 sequence. Phosphorus (P) was initially delivered to the Luochuan loess section via influx of aeolian dust from the northern desert and Gobi areas by the East Asian winter monsoon, and then was modified by pedogenesis associated with the East Asian summer monsoon during the last 130 ka. “Preserved P t” in the loess L 1 is tightly correlated with grain size without leaching loss of P due to enrichment of P in fine-grained fractions, as well as “initial P t”. “Leaching P t” data show that paleosol S 1 had lost 15–40% of its “initial P t”, and that there was much more precipitation in S 1 than in L 1. P i subject to slightly weak pedogenesis was completely transformed into P o without leaching loss of P in loess L 1. By contrast, much P i disappeared from paleosol S 1 due to strong pedogenesis, partly through leaching and partly through conversion to organic forms during P cycling processes. P o variation is similar to those of MS and the <7.8 μm fraction in L 1, but contains more information on the East Asian winter monsoon due to weak pedogenesis without leaching of P. P o in S 1 lower than L 1SS 1 as a consequence of strong decomposition of the organic matter kept constantly in the middle of S 1 where P i kept at the lowest of 423 ppm, suggesting that there existed a very warm and humid climate related to the enhanced summer monsoon during that period. The mean organic P/ inorganic P ratio (P o/P i) is lower in the L 1LL 1 and L 1LL 2 than in the S 0, S 1 and L 1SS 1, indicating that low P o/P i ratios coincide with weak weathering-pedogenesis, and higher P o/P i ratios correspond to strong weathering-pedogenesis. P o/P i ratio can eliminate the effect of grain size on aeolian dust because of chemical uniformity of aeolian dust and enrichment of P o and P i in the fine-grained fractions. Thus, P o/P i ratio is solely linked to pedogenesis of the Luochuan loess section. Variation in P o/P i ratio is similar to those of MS and the Marine Oxygen Isotope composition, indicating the summer monsoon evolution during the last 130 ka and providing the biogeochemical evidence for further understanding the genetic links between the East Asian monsoon and global climate change.

Direct correlation of the marine oxygen isotope record with the Chinese Loess Plateau iron oxide and magnetic susceptibility records

Monsoons are a major feature of global atmospheric circulation and derive heat and moisture from the oceans and transports much of that heat and moisture to nearby continents. The east Asian Monsoon derives its heat and moisture from the eastern equatorial Pacific and transfers it to Southeast Asia and the Loess Plateau of central China. In order to examine the phasing of marine and terrestrial climatic events associated with this monsoon we have analyzed a core from the western Pacific, V21-146, downwind from the Loess Plateau. Iron oxides and magnetic susceptibility were analyzed because they have been used as Loess Plateau paleoclimate proxies. Comparison of temporal changes in both iron oxide and susceptibility signals in V21-146, to the well-known Luochuan loess section, reveals an excellent correlation, both in the timing and direction of the variations. Comparison of our Loess Plateau paleoclimatic proxies in V21-146 to previously published oxygen isotope data from this core suggests that through Marine Isotope Stage (MIS) 9, coincident with Loess Plateau interglacial soil level 3 (S3), ocean temperature increases and the onset of monsoonal conditions on the Loess Plateau were synchronous. However, for MIS 11/12 (interglacial/glacial S4/L5) changes in the ocean appear to significantly lead those on land and there is a lack of correlation between MIS 13 and S5. These out of phase events may be a response to a reorganization of the ocean/atmosphere system as indicated by changes in the δ 13C record. It has previously been suggested that changes in eolian flux record eolian transport from the Loess Plateau, but we find little correlation between eolian flux and our Loess Plateau proxies, suggesting that some of the eolian sediments were derived from outside the Loess Plateau.

A Quaternary climate record based on grain size analysis from the Luochuan loess section on the Central Loess Plateau, China

Important climatic information is stored in the Luochuan sediment record on the Central Loess Plateau in China. Large quantities of dust have registered the activity of the summer and winter monsoon. In this paper, the process that caused the loess deposition is explained and the influence of (palaeo)climate on this sedimentation process is discussed. The concept of establishing a time scale is based on the fact that grain size and sedimentation rate (SR) appear to correlate very well with the strength of the winter monsoon in northern China. Grain-size analysis does not only offer the opportunity to construct an independent numerical Quaternary time scale, but also to reconstruct temporal palaeoclimatic changes. In a reference curve, the correlation between grain size and SR is determined, afterwards the SR of all layers could be calculated based on their grain size. In this work, the climatic results of the Luochuan loess section are determined by making two important adjustment steps. Firstly, a general grain-size trend has been removed since the layers tend to coarsen upwards in the section. Secondly, the real quantity of sediment deposited is calculated by multiplying the SR with the bulk density of the layer, which results in dust flux values. The bulk density of the loess layers appears to fluctuate both in depth as well as with palaeoclimate during dust deposition. Finally, climatic results derived from this improved time scale are discussed.

Geochemical study of boron isotopes in the process of loess weathering

In this paper the boron contents and boron isotopic composition of acid-soluble phases in loess and paleosol samples are determined for the first time. The boron contents of acid-soluble phases in the Luochuan loess section (S0–S2) vary within the range of (0.8–2.7)×10−6 and their δ11 B values vary from −1.8‱ to +18.6‱, mostly within the range of 0−+10‱. The boron contents and δ11 B values of paleosol layers are higher than those of loess layers, especially in the loess layer S1. Varying chemical weathering intensity and loess adsorption capability are the main factors leading to the variations of boron contents and δ11B values of acid-soluble phases in the loess section. The variation of chemical weathering intensity in response to the variation of climatic conditions seems to be the main factor leading to the variations of boron contents and δ11B values of acid-soluble phases in the loess section.

Chronology of the stratum containing the skull of the Dali Man

The field investigations of geomorphology and Quaternary sediment sequences show that the sediment stratigraphic profile containing the skull of the Homo sapiens Daliensis (Dali Man) at Tianshuigou trench, Jiefang Village (109° 44′E, 34°52′N), Dali County, Shaanxi Province, is composed of the loess-paleosol sequence, 17.3 m thick, in the upper part, and the alluvial silts, fine sands and sandy gravels interbedded, 15.4 m thick, in the lower part. There are the well developed layers L1, S1, L2 and S2 in the loess-paleosol sequence of the profile, which are comparable; to those of the Luochuan loess section on the basis of magnetic succeptibility measurements and IRSL dating of the loess samples. An alluvial gravel layer, where the skull of the Dali Man was discovered, is in 13 m beneath the bottom of paleosol S2, implying that, the age of the Dali Man must be older than about 247 kaBP when the paleosol S2 began to be developed. The samples of mammal animal tooth and shell fossils from the gravel layer containing the skull of the Dali Man have been dated by uranium series and ESR methods respectively. Based on the multiple dating results and the stratigraphic chronology, the age of the Dali Man, therefore, would be estimated as older than 250 kaBP and younger than 350 kaBP, and reasonably estimated as 260–300 kaBP.

Sr isotope ratios of acid-leached loess residues from Luochuan, China: A tracer of continental weathering intensity over the past 2.5 Ma.

We present 87Sr/86Sr ratios of acid-washed residues, Rb/Sr, magnetic susceptibilities and deposition rates of loess and paleosols from the Luochuan loess section, China, covering the last 2.5 Ma. Paleosol residues have consistently higher 87Sr/86Sr than loess residues. This is attributed to the paleoclimate-related differences in chemical weathering intensity of the source area. This study indicates that the 87Sr/86Sr ratio of residues of loess and paleosols may be a proxy for chemical weathering intensity. This investigation reveals a long-term decrease in residue 87Sr/86Sr ratios and a general increase in deposition rates from 2.5 Ma to the present. This may imply a general rise in mechanical erosion intensity coupled with a general decline in chemical weathering intensity over the past 2.5 Ma. The regular oscillations of residue 87Sr/86Sr, Rb/Sr, and magnetic susceptibilities of loess and paleosols began at ∼1.0 Ma and display a ∼100 Ka cyclicity, which is consistant with the climate change being associated with the eccentricity-driven insulation cycle of ∼100 Ka.

Response of mollusk assemblages from the Luochuan loess section to orbital forcing since the last 250 ka

This note presents a new result of terrestrial mollusk study from the Luochuan loess section since the last 250 ka. A total of 213 samples, taken at intervals of 10 cm in the S0-L3 portion, were analyzed for fossil mollusks. Generally, 150–600 individuals were counted in each sample. According to the distribution of mollusk fossil assemblages in the loess section, 11 mollusk fossil zones have been recognized, representing different climatic and ecological conditions. Three main ecological groups were identified according to the temperature and moisture requirements of each taxon. The cold-aridiphilous group shows maxima at about 240–220, 190–182, 150–140 and 74–66 ka BP. The thermo-humidiphilous set has high abundances for at least 6 times in the section at about 246–240, 220–216, 170–158, 92–86, 60–44 and 10 kaBP. Our results show that variations in mollusk ecological groups are related with changes in the Earth orbital parameters at the 41 and 20 ka frequencies. Maxima in thermo-humidiphilous taxa are in phase with accretion intervals of obliquity (41 ka period), reflecting the East Asian summer monsoon with 41 ka period in controlling variations in terrestrial mollusk ecological groups. In addition, maxima in thermohumidiphilous taxa appearing at about 246–240, 220–216, 60 and 10 kaBP, are consistent with procession (20 ka period) maxima, indicating that the procession period also plays an important role in adjusting the ecological pattern of mollusk groups.

Origin of illites in the Luochuan loess section

Morphology and composition of illite from the Luochuan loess section were investigated by TEM. The results show that most of the illite grains are characterized by rounded shapes and chemical compositions are closely similar to those of anchizonal illites. The enrichment of illites in paleosol over loess was mainly caused by the weak winter monsoon during the interglacial periods, and was not related to thein situ pedogenesis. Illite can serve as an indicator for tracing eolian dust from Loess Plateau in pelagic and lacustrine sediments.

Stable isotope composition of the carbonate concretion in loess and climate change

Carbonate concretions are commonly found in the loess in China. Oxygen and carbon isotopic compositions of carbonate concretions collected from 14 different paleosols (S1–S14) in the Lishi Formation of Luochuan loess section, China were analysed. The isotopic compositions of concretion in loess are useful tools in paleoclimatic study. Usually the more developed the paleosol is, the less negative the δ18O values and the more negative the δ13C values are. The oxygen isotopic composition of these paleosols varies within a range of about 1.3‰. The carbon isotopic composition, however, shows larger variations from one paleosol to the other, within a range of 5.3‰. An attempt was made to semi-quantitatively estimate the paleosol formation temperature and precipitation based on the carbon and oxygen isotopic data obtained.

Land mollusk records from the Luochuan loess sequence and their paleoenvironmental significance

Terrestrial mollusk fossil assemblages of the Luochuan loess section have been investigated so as to improve the reconstruction of environmental changes in the last 110ka. A total of 118 samples, taken at an interval of 10cm in the S0-S1 portion, have been analyzed. Results show that mollusk fossil assemblages have recorded detailed paleoclimatic and paleoecological changes. The Luochuan area bas experienced cold-dry and warm-humid alternations in the last 110 ka. However, even under the prevailing cold-dry condition, short-term or seasonal warm-humid climates frequently occurred in this area, reflecting the impact of summer monsoons temporarily. Comparison of mollusk records from the Luochuan loess sequence with those of grain-size and aridity index from RC 27-61 core in the Indian Ocean reveals obvious similarities in several time intervals, suggesting that climate variations in the Loess Plateau may have been also subjected to the influences of the Indian monsoon to a certain extent in the geological past. The presence of some specific cold-humid and cold-dry species represents deteriorated climate conditions, which might be the cooling events similar to the Heinrich events recorded in the Northern Atlantic. Six principal factors obtained by correspondence analysis and their related environmental significance have been discussed.

High frequency pulses of East Asian monsoon climate in the last two glaciations: link with the North Atlantic

Three loess sections in the Loess Plateau of China have been studied to characterize the variations of the East Asian monsoon climate in the Late Quaternary period. Paleo-weathering profiles based on two weathering indices reveal a series of spatially correlative intervals of lower weathering intensity (LW), in the last glacial and late penultimate glacial loess, indicating significantly cooler or/and drier conditions. Most of them are identified at or near the transitional boundaries between loess and soil units, suggesting that climatic conditions favorable for LW events tended to occur at or near major climatic boundaries. Twenty-eight radiocarbon and thermoluminescence measurements, combined with Kukla's magnetic susceptibility age model date these events at ∼14, ∼21, ∼29, ∼38, ∼52, ∼71, ∼135, and ∼145 ka BP, with ∼5–10 ka frequency inlaid within the orbitally induced ∼20-ka periodicity. The ages of the first six events in the last glacial period are therefore close to those of the coarse-grained Heinrich layers in the North Atlantic Ocean, which resulted from massive discharges of icebergs. The results indicate that the climate in the area affected by the East Asian monsoon has experienced high-frequency changes more or less synchronous with the Heinrich events. These high-frequency changes have been reported by Porter and An, based on the grain-size time series from the Luochuan loess section. Our data also reveal that similar events also occurred during the penultimate glaciation. The mechanisms linking the Heinrich events and the East Asian monsoon climate may be similar to that driving the Younger Dryas event which has been identified in a large number of records in China and in the surrounding oceans.