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Deciphering the eolian sources is critical to understand the paleo-significance of the Quaternary eolian deposits (the Xiashu loess) in the lower reaches of the Yangtze River. Both a local source from the fluvial sediments of the Yangtze River and a distal source from the northern deserts similar to that of the loess on the Chinese Loess Plateau (CLP) have been proposed. Recent researches show great advantages of detrital zircon ages as a source tracer for Asian dust based on the laser ablation U-Pb isotopic dating technique. This work presents the U-Pb ages of zircon grains extracted from the Xiashu loess. The results indicate that the Xiashu loess has a very different age distribution of zircon grains from that of the loess on the CLP as well as the materials in the arid lands of the Asian Interior. Instead, the zircon ages of the Xiashu loess are indistinguishable from the fluvial sediments of the Yangtze River, indicating the dominance of proximal dust source. Proximal source of the Xiashu loess implies that extensive eolian processes might have existed in the currently wet South China, possibly in response to the full glacial conditions after the middle Pleistocene transition of global climate. [ABSTRACT FROM AUTHOR]

Abstract: The age distribution of detrital zircon has been used to trace sediment sources. Existing datasets show great similarity of zircon ages between the loess on the Chinese Loess Plateau (CLP) and the sediments from the North Tibetan Plateau (NTP), implying that eolian dust is delivered from the NTP to the CLP by westerly winds or via the Yellow River. However, NTP dust can also be transported by northwesterly winds from the Alxa arid lands (AALs), where materials are received from both the NTP and the Gobi Altay Mountains (GAMs). Here we report U–Pb zircon ages for AALs sands and NTP and CLP loess. The results show that the zircons in the AALs are mixed from NTP and GAMs zircons. NTP loess is mainly derived from local sources. Mixing of materials from the NTP and GAMs defines the zircon ages of the loess on the CLP better than the pure NTP source. No temporal and spatial heterogeneities of zircon ages have been observed for the loess on the CLP, which suggests that the well-mixed materials in the AALs likely have an eolian source. [Copyright &y& Elsevier]

Abstract: The thick deposits of eolian dust on the Chinese Loess Plateau provide a rare opportunity to explore the past changes of surface processes in response to climatic and tectonic oscillations. Here we report a 22Ma-long Nd and Sr isotopic records of the eolian sources. Decreasing ε Nd values and increasing 86Sr/87Sr ratios of the eolian deposits has been observed from early Miocene to about 7Ma, which indicates growing material contribution from Qilian Mountains relative to the Gobi Altay Mountains. The source shift during 22–7Ma is interpreted to reflect the erosional response of the Qilian Mountains to its surface uplift. Between 7Ma and 1.2Ma, the results show relative constant Nd and Sr isotopic compositions of the eolian deposits, and thus suggest a constant source contribution between Qilian Mountains and Gobi Altay Mountains. Synchronized uplift of the two mountain ranges during this period may explain the constant ratio of source contribution. Since 1.2Ma, Nd and Sr isotopic records indicate that the relative debris input from Qilian Mountains drops rapidly. As large-scale topographic changes would not be expected in such a short time period, the source shift since 1.2Ma might be by the differing erosional responses in Qilian Mountains and Gobi Altay Mountains to the development of full glacial climate after the middle Pleistocene transition. This work shows that combined Nd and Sr isotopic signatures of the dust records are valuable proxies to monitoring past surface processes on the million years time scale. [Copyright &y& Elsevier]

Abstract: Carbonate in the eolian deposits on the Chinese Loess Plateau carries a significant amount of information on the evolution of paleo-environment. However, the paleo-proxies based on total carbonate are largely complicated by the coexistence of primary and secondary carbonate. This work provides a new method to quantify the relative proportion of primary and secondary carbonate in these eolian deposits according to the Mn/Ca and Mg/Ca ratios of bulk carbonate. The carbonate associated Mg and Mn, which can be extracted by selective dissolution using diluted weak acid, are mostly derived from primary carbonate because the secondary carbonate contains very little Mg and Mn. The amounts of Mg and Mn associated with carbonate could thus serve as proxies for the pedogenic dissolution of primary carbonate, which reflect clear glacial–interglacial changes of summer monsoon. A positive correlation between the Mn/Ca and Mg/Ca ratios of bulk carbonate has been observed, indicating binary mixing between secondary carbonate with the low Mn/Ca and Mg/Ca ratios and primary carbonate with high Mn/Ca and Mg/Ca ratios. The high Mn/Ca and Mg/Ca ratios of primary carbonate endmember also indicate Alxa arid lands as the main source of eolian deposits on Chinese Loess Plateau. The Mn/Ca and Mg/Ca ratios of carbonate could be employed as a quick method to check the purity of the secondary carbonate used for paleo-environmental studies and help to interpret the paleo-proxies based on the bulk carbonate. The results reveal significant amount of primary carbonate included in the traditionally believed pure authigenic carbonate phases such as carbonate concretion, rhizolith, and the fine grain sized carbonate. The new method also helps to appraise the control of the relative portion of primary and secondary carbonates on the stable carbon and oxygen isotopes of bulk carbonate. High Mg/Ca ratio and low Mn/Ca ratio of the carbonate in red clay formation is observed, which confirms the existence of authigenic dolomite. [Copyright &y& Elsevier]

Abstract: Eolian deposits of loess on the Chinese Loess Plateau are investigated to decipher the influence of sequentially changed monsoon climate on silicate weathering process. Detailed hydrochloric acid dissolution experiments are applied to establish a sensitive proxy for silicate weathering. Combined elemental and mineralogical studies show that the minerals which are susceptible to incipient chemical weathering can be totally dissolved in hot hydrochloric acid (80°C, 3mol/L) after 4h, while other stable minerals are nearly unaffected. Thus, the hydrochloric acid dissolvable fractions (ADF) of loess and paleosol are more sensitive than bulk samples in the weathering reaction. Since Mg is tend to be leached out from the ADF while Al is conserved during the incongruent weathering, Mg/Al ratio of the ADF (Mg/AlADF) could indicate the weathering intensities of the Mg-bearing minerals, mainly chlorite. The Mg/AlADF of the modern soils on the Chinese Loess Plateau is closely correlated to the local precipitation amounts, suggests that the intensity of summer monsoon is a key factor in the weathering of Mg-silicate. The silicate weathering intensity of the loess-paleosol deposits of past 130Kyrs shows strong procession cycle of ∼ 23Kyrs coupled by glacial-interglacial variation. The ∼ 23Krys cycles are coherent to the variations of the intensity of summer monsoon, while the ∼ 100Kyrs glacial-interglacial variation may relates to the changes in winter monsoon. Phased intensification of East Asian summer monsoon, and thus enhanced silicate weathering and atmosphere CO2 drawdown, in response to the uplift of Tibetan Plateau, may be another mechanism relating the late Cenozoic tectonics to global cooling. [Copyright &y& Elsevier]

Deciphering the sources of eolian dust on the Chinese Loess Plateau (CLP) is fundamental to reconstruct paleo-wind patterns and paleo-environmental changes. Existing datasets show contradictory source evolutions of eolian dust on the CLP, both on orbital and tectonic timescales. Here, the silicate Sr and Nd isotopic compositions of a restricted grain size fraction (28-45 μm) were measured to trace the source evolution of the CLP since ~2.7 Ma. Our results revealed an unchanged source on orbital timescales but a gradual source shift from the Qilian Mountains to the Gobi Altay Mountains during the past 2.7 Ma. Both tectonic uplift and climate change may have played important roles for this shift. The later uplift of the Gobi Altay Mountains relative to the Qilian Mountains since 5 ± 3 Ma might be responsible for the increasing contribution of Gobi materials to the source deserts in Alxa arid lands. Enhanced winter monsoon may also facilitate transportation of Gobi materials from the Alxa arid lands to the CLP. The shifting source of Asian dust was also reflected in north Pacific sediments. The finding of this shifting source calls for caution when interpreting the long-term climate changes based on the source-sensitive proxies of the eolian deposits. [ABSTRACT FROM AUTHOR]

The U 234 / 238 U ratio of fine particles can record the time since their separation from bed rock because of the disruption of uranium series equilibrium introduced by the recoil of daughter 234 Th nuclei (precursor of 234 U) out of particle surfaces during the decay of 238 U. Application of the uranium comminution age method, which has great potential in tracing production and transportation of sediments is however complicated by the weathering dissolution of U 234 depleted particle surfaces, the difficulty in determining the fraction of recoiled nuclei, and the precipitation of exogenetic U 234 . Here we minimize these complications by using a newly developed precise size separation using electroformed sieve, and a chemical protocol that involves reductive and oxidative leaching. Eolian deposits collected from the Chinese Loess Plateau (CLP) were used to test the validity of our method. Possible effects of weathering dissolution were also evaluated by comparing samples with different weathering intensities. The results show decreasing 234 U/ 238 U ratios in fine eolian particles with increasing sedimentation age, agreeing well with the theoretical prediction of the comminution age model. This successful application of the uranium comminution age approach to the eolian deposits on the CLP is also aided by a stable dust source, the low weathering intensity, the lack of consolidation, and the well-defined age model of the deposits. A transportation time of 242 ± 18 ka was calculated for the eolian deposits, which indicates a long residence time, and thus extensive mixing, of the dust particles in source regions, partly explaining the stable and homogeneous composition of the eolian dust over glacial–interglacial cycles. [ABSTRACT FROM AUTHOR]