Huayu Lu

Bimodal grain-size distribution of Chinese loess, and its palaeoclimatic implications

Grain-size analysis indicates that Chinese loess generally shows a bimodal distribution with a coarse and a fine component. The coarse component, comprising the main part of the loess, has pronounced kurtosis and is well sorted, which is interpreted to be the product of dust storms generated by low-altitude northwesterly winds. Its grain-size reflects the strength of the low-altitude circulation in the dust seasons of the year, and its percentage provides an indicator of the source area aridity and the frequency of dust storms. Conversely, the fine component has a wide grain-size range and is poorly sorted. Sedimentary illustrations based on the grain-size distribution characteristics of bulk samples and of detrital quartz suggest that the fine component probably represents the background dust load of the atmosphere and is mainly transported by high-altitude westerly airstreams. Its grain-size provides an estimate of the westerly air stream intensity. The coarse and fine components of a loess sample can be mathematically separated by fitting a designated mathematical distribution function to the measured grain-size data, and this procedure constitutes an approach for reconstructing the palaeowind system of Northern China

Astronomical calibration of loess-paleosol deposits at Luochuan, central Chinese Loess Plateau

Abstract The 140 m loess–paleosol profile at Luochuan in the central Chinese Loess Plateau was sampled at 5-cm intervals in loess units and at 3 cm in paleosol units, in order to obtain a high resolution climatic record covering the past 2.5 million years. All samples were measured for magnetic susceptibility, which is regarded as a good proxy index of the East Asian summer monsoon strength. On the basis of the astronomical theory of Pleistocene climatic change, an age model of the Luochuan loess–paleosol sequence was developed by tuning the magnetic susceptibility record to time-series of insolation changes. The results show that the ages of the boundaries between the Malan and Lishi, and Lishi and Wucheng loess formations are 71 and 1320 kyr BP, respectively. The onset of loess accumulation is at 2470 kyr BP. Our age model was tested by comparing the orbitally derived ages with absolute age determinations of magnetic reversals, and cross-spectrum analyzing with solar radiation variations for summer at 65°N. These indicate that the calibration provides a reliable time scale for the Luochuan loess–paleosol deposit.

Lower temperature as the main cause of C4 plant declines during the glacial periods on the Chinese Loess Plateau

The distribution of C3 and C4 plants changed in regionally contrasting ways during the last glacial period. C4 plant expansion in low-latitude Africa and America coincided with C4 plant decreases in Mesoamerica and the US Great Plains. This C4 plant expansion has been attributed to lower pCO2 and increased aridity and the decline in C4 plants is believed to have been caused by increased winter precipitation and lower temperatures. However, it is still difficult to generalize whether pCO2, temperature, or aridity was mainly responsible for C3 vs. C4 vegetation changes during glacial periods. Paleoclimate conditions on the Chinese Loess Plateau (CLP) offer opportunities to further evaluate the role of temperature in the C3 vs. C4 competition. Detailed climate records have shown that the CLP region was drier and colder during glacial periods, with aridity favoring C4 expansion and lower temperatures favoring C3 expansion. Here, we present two high-resolution compound-specific carbon isotope records of n-alkanes from the CLP as vegetation biomarkers to estimate the relative abundance of C4 plants during the last two glacial/interglacial transitions. More negative δ13C values during the glacial periods from two CLP sites clearly show that, in contrast with low-latitude sites, C4 plants declined. This decrease in C4 abundance was caused by lower temperature, despite the lower pCO2 and increased aridity on the CLP during glacial time.

Variation of East Asian monsoon precipitation during the past 21 k.y. and potential CO2 forcing

Paleoclimatic research can provide critical insight on causes of change in the East Asian monsoon, which influences the lives of 1.6 billion people today. In this study, we use paleoclimatic indexes from Chinese loess deposits, which have clear climatic implications and are independently dated, to reconstruct the monsoon precipitation since 21 ka. Our results show that monsoon precipitation persistently decreased from 21 ka to ca. 8 ka, and increased after ca. 8 ka, with a precipitation peak at 8–3 ka. These changes in East Asian summer monsoon precipitation are synchronous with changes in high-northern-latitude ice volume/ice cover and atmospheric CO2. These new data suggest that variation of the monsoon precipitation was probably driven by CO2-forced high-northern-latitude temperature changes, shifting the location of the intertropical convergence zone that dominates monsoon precipitation. Our TraCE-21000 modeling experiment supports this interpretation.

Loess plateau storage of northeastern Tibetan plateau-derived Yellow River sediment

Marine accumulations of terrigenous sediment are widely assumed to accurately record climatic- and tectonic-controlled mountain denudation and play an important role in understanding late Cenozoic mountain uplift and global cooling. Underpinning this is the assumption that the majority of sediment eroded from hinterland orogenic belts is transported to and ultimately stored in marine basins with little lag between erosion and deposition. Here we use a detailed and multi-technique sedimentary provenance dataset from the Yellow River to show that substantial amounts of sediment eroded from Northeast Tibet and carried by the rivers upper reach are stored in the Chinese Loess Plateau and the western Mu Us desert. This finding revises our understanding of the origin of the Chinese Loess Plateau and provides a potential solution for mismatches between late Cenozoic terrestrial sedimentation and marine geochemistry records, as well as between global CO2 and erosion records.

Sedimentation and diagenesis of Chinese loess : Implications for the preservation of continuous, high-resolution climate records

Chinese loess has been extensively utilized to produce continuous and high-resolution climate records of the late Cenozoic. Such work assumes uninterrupted loess deposition and limited diagenesis. Here, closely spaced optically stimulated luminescence (OSL) dates are used to characterize the Holocene and Late Pleistocene sedimentation histories of three sites across a NW-SE transect of the Chinese Loess Plateau. The results suggest that sedimentation is episodic at subglacial-interglacial time scales, with rates rapidly varying within units and between sites. Unconformities, noneolian deposition, and mixing of sediments also appear to be common. Existing understanding of loess deposition therefore requires reexamination, while previous reconstructions of rapid climate change, not dated using absolute methods, should be regarded with caution. Loess deposits may still yield detailed climate records from specific high-sedimentation-rate strata, and evidence for rapid climate change may yet be obtainable by targeting these units through absolute dating. The rapid changes in sedimentation presented here indicate the East Asian Monsoon has the capacity to vary on millennial scales.

Dune mobility and aridity at the desert margin of northern China at a time of peak monsoon strength

Wind-blown sands were mobile at many sites along the desert margin in northern China during the early Holocene (11.5–8 ka ago), based on extensive new numerical dating. This mobility implies low effective moisture at the desert margin, in contrast to growing evidence for greater than modern monsoon precipitation at the same time in central and southern China. Dry conditions in the early Holocene at the desert margin can be explained through a dynamic link between enhanced diabatic heating in the core region of the strengthened monsoon and increased subsidence in drylands to the north, combined with high evapotranspiration rates due to high summer temperatures. After 8 ka ago, as the monsoon weakened and lower temperatures reduced evapotranspiration, eolian sands were stabilized by vegetation. Aridity and dune mobility at the desert margin and a strengthened monsoon can both be explained as responses to high summer insolation in the early Holocene.

Ice core evidence for a very tight link between North Atlantic and east Asian glacial climate

[1] Corresponding millennial-scale climate changes have been reported from the North Atlantic region and from east Asia for the last glacial period on independent timescales only. To assess their degree of synchrony we suggest interpreting Greenland ice core dust parameters as proxies for the east Asian monsoon systems. This allows comparing North Atlantic and east Asian climate on the same timescale in high resolution ice core data without relative dating uncertainties. We find that during Dansgaard-Oeschger events North Atlantic region temperature and east Asian storminess were tightly coupled and changed synchronously within 5–10 years with no systematic lead or lag, thus providing instantaneous climatic feedback. The tight link between North Atlantic and east Asian glacial climate could have amplified changes in the northern polar cell to larger scales. We further find evidence for an early onset of a Younger Dryas-like event in continental Asia, which gives

The Late Miocene and Pliocene climate in East Asia as recorded by grain size and magnetic susceptibility of the Red Clay deposits (Chinese Loess Plateau)

Abstract Grain-size characteristics of the aeolian Red Clay sediment series in China demonstrate the existence of warm–cool alternations during the late Miocene and Pliocene. From the dissimilarity between the temporal patterns of grain size and magnetic susceptibility it is inferred that the finest grain-size fraction in the Red Clay (clay and very fine silt) is transported by the wind, as is the case of the coarser (silty) component. Relatively cool climatic conditions are indicated by increased loess supply (represented by the silty component) from the NW, caused by a stronger winter monsoon, as in the overlying Quaternary system. It is striking that at the same time the amounts of clay also increased, probably due to transportation by intensified westerly circulation. Relatively warm periods are characterized by opposite grain-size properties. These warm–cool alternations on the Asian continent, dated by palaeomagnetic analyses, correlate well with already established changes in sea level and ice sheet volumes up to ∼6 Ma. In addition, time series analysis of the grain-size signal points to an orbitally forced climatic cyclicity (precession, eccentricity and (weak) obliquity).

Aeolian origin and palaeoclimatic implications of the 'Red Clay' (north China) as evidenced by grain-size distribution.

The ‘Red Clay’ is an important deposit underlying the Quaternary loess–palaeosol sequence in the Chinese Loess Plateau, being regarded as an excellent record of palaeoclimate changes in the late Tertiary. Several properties of the ‘Red Clay’ have been measured previously in order to derive climatic information. However, the sedimentary processes involved and the origin of the materials remain controversial. Here we present results of grain-size analyses of the ‘Red Clay’ from four representative sites in the Chinese Loess Plateau. In particular their grain-size distribution is compared with that of typical Quaternary aeolian loess–palaeosol, as well as lacustrine and fluvial sediments. It appears from the sedimentological evidence that the major part of the ‘Red Clay’ is of aeolian origin. It is rather similar in some of its properties to the Quaternary loessic palaeosols. The dust forming the ‘Red Clay’ was transported by a wind system that was weaker than that involved in the accretion of the Quaternary loess. Furthermore, the ‘Red Clay’ sediment has been modified by post-depositional weathering. Copyright