Search

The aeolian loess deposits in northeastern China have substantial potential for reconstructing the paleoclimate in the East Asian monsoon boundary zone. Identifying the source of the aeolian dust is crucial to interpret the paleo‐proxies recorded in the loess deposits in northeastern China; however, traditional geochemical tracers such as Nd and Sr isotopes cannot discriminate between the potential source areas, that is, the Otindag Sandy Land, Horqin Sandy Land, and Hulun Buir Sandy Land regions. In this study, the (234U/238U) activity ratio is used to constrain the provenance of the loess deposits in northeastern China. The (234U/238U) of the studied loess‐paleosol sequences (from 0.935 to 0.910) decreases with increasing depositional age (from 10 to ∼1,000 ka), which is consistent with predictions from uranium isotope comminution age model. A derived (234U/238U) of 0.935 ± 0.003 for the newly deposited loess (zero depositional age) is most similar to that of the Otindag Sandy Land positioned at a proximal upwind direction, suggesting that the Otindag Sandy Land is the most likely source of the aeolian dust that formed the loess deposits in northeastern China. A transportation time of 432 ± 65 ka was calculated for these deposits, indicating extensive mixing of the fine dust in the source regions, which could partially explain the relative homogeneity of loess deposits in northeastern China. The transportation time longer than several glacial‐interglacial cycles suggests that reconstructions of regional paleoclimate on orbital timescales should be viewed with caution because the potential influence of climatic changes could largely be buffered. Plain Language Summary: The activity ratio of the uranium isotopes (234U and 238U) in fine‐grained particles can be used to determine the "comminution age", or the amount of time since the production of fine sediment particles (≤50 μm) from the breakdown of bedrock by weathering. The (234U/238U) activity ratio provides a new approach to track the age and origin of dust particles. In this study, we measure the (234U/238U) composition of the loess deposits in northeastern China and surface sand samples in potential source areas to better understand the transport of aeolian dust in the region. We find that the Otindag Sandy Land, which is in the proximal upwind direction, is the main source region for the loess deposits in northeastern China and that the dust materials have been resided in the source region for around 432 ± 65 thousand years before aeolian transport and deposition. Our results shed light on the transport and depositional history of loess in northeastern China and provide valuable information for reconstructing paleoclimate using the loess deposits in this region. Key Points: The (234U/238U) activity ratio of a loess sequence in northeastern China is consistent with a uranium isotope comminution age modelLoess deposits found in northeastern China have a long residence time of 432 ± 65 ka in the source regionsThe Otindag Sandy Land in the proximal upwind direction is the main source region for the studied loess sequence [ABSTRACT FROM AUTHOR]

Orbital‐scale East Asian Summer Monsoon (EASM) variations inferred from loess deposits in northern China and speleothems from southern China display different dominant periods, complicating our understanding of monsoon response to insolation and ice‐volume forcings. Here we integrate a new microcodium δ18O record from a high‐resolution last interglacial loess profile with previously published data and provide a composite microcodium δ18O record on the Chinese Loess Plateau (CLP) since the last interglacial. The composite microcodium δ18O record displays distinct precessional cycles, consistent with speleothem δ18O records, but with different amplitude contrast (particularly during the peak interglacials). We propose that both loess and speleothem δ18O records exhibit covariations at precessional timescale oscillations. The discrepancy between loess and speleothem from southern China can be attributed to the influences of other processes besides summer precipitation on the proxies. A slight difference in amplitude between microcodium and speleothem δ18O records implies that the EASM is also influenced by inland surface boundary conditions, which has important impacts on the occurrence of EASM precipitation. Therefore, microcodium δ18O from the Chinese loess–paleosol sequences can be regarded as a representative proxy of EASM precipitation in northern China and then a reliable proxy reflecting the variation of EASM intensity. [ABSTRACT FROM AUTHOR]

Records of Asia Summer Monsoon (ASM) from the Chinese loess and the speleothem display distinct features. The very different proxies that were applied to the two archives may be responsible for this discrepancy. A direct comparison between the speleothem and the loess records under the same proxy system of rainwater δ 18 O may help to resolve this puzzle. Here we show that the calcified microcodium in the loess deposits may record the oxygen isotopic composition of the summer rainwater. A microcodium based δ 18 O record covering the past 140 kyrs was generated, which shows similar magnitude of the overall variation to that of the speleothem records. However, much weaker precession variability was registered in the microcodium record during the last interglacial period. Instead, the microcodium δ 18 O record is more consistent with the widely used summer monsoon proxy of magnetic susceptibility in the loess deposits with clear glacial-interglacial pattern. This similarity may originate from the low sedimentation rate of the interglacial paleosol layer that preferentially record the peak ASM signals on the precession band. It is also possible that the orbital variability of ASM between the North China and South China is inherently different with more ice-volume related influence in the north. A longer microcodium δ 18 O record in sequences of higher sedimentation rate and a reliable record of summer rainfall may help to resolve these possibilities. [ABSTRACT FROM AUTHOR]

The loess-paleosol sequence on the Chinese Loess Plateau provides a unique archive that records climate change in East Asia in the Quaternary, yet absolute dating of the loess deposits is challenging due to the lack of directly datable materials. Fossil land snail shells, which are made from aragonite, are widely preserved in the loess deposits and have long been used to reconstruct past environmental changes. U-series dating of fossil land snail shells has the potential to provide a route for absolute dating of the loess deposits but remains largely unexplored. In this study, we present the first systematic investigation on the U-series isotope geochemistry as well as the early diagenetic imprints of fossil land snail shells (Cathaica sp.) from the Mangshan loess-paleosol sequence in Henan province, central China. Several geochemical techniques, including Raman microscopy, SEM, LA-ICPMS, LA-MC-ICPMS, solution-MC-ICPMS, and AMS 14C dating, were employed to investigate the mineralogy, chemical and isotopic compositions of both modern and fossil snail shells to micrometer level. Our results show that the fossil Cathaica sp. shells are overall characterized by a higher degree of porosity and elevated contents of organic matter compared to live-collected shells of the same species. The layers with higher porosity and organic matter content in the fossil Cathaica sp. shell are also found to be enriched in Na, Mg, Mn, Ba, and U, indicating diffusion and adsorption of these elements by specific surface binding sites of either aragonitic lattice or organic compounds of the fossil shell. Combining in-situ measurements using LA-MC-ICPMS with solution U-series determination, we further demonstrate that fossil Cathaica sp. shell is relatively homogeneous regrading both [234U/238U] and [230Th/238U] values although the distribution of U in the fossil shell is sample-specific. The comparison of different dating results suggests that the calculated apparent closed system U–Th ages are all systematically younger (∼6000 to 13,000 years) than the corresponding shell 14C ages and quartz SAR (single-aliquot regenerative-dose) ages from the Mangshan section. We suggest that the underestimation of U–Th ages of fossil Cathaica sp. shells is very likely caused by diagenetic uptake of U that started immediately after the burial of the shell and effectively ceased when the fossil shell was isolated from the pore waters due to persistent deposition of eolian dust at the Mangshan section. Our work on both modern and fossil Cathaica sp. shells thus provides detailed morphological and geochemical characterization for the diagenetic alteration of fossil snail shells and suggests that U-series dating of fossil land snail shells may provide age constraints for dust deposits in the semi-arid region although the timing of early diagenetic U-uptake by the fossil shells need to be better quantified for reliable age determination. [ABSTRACT FROM AUTHOR]

The aeolian deposits in the North Pacific Ocean (NPO) serve as important archives for the surface processes in the arid Asian interior. Aeolian flux, which is usually based on the ‘operationally defined aeolian dust’ (ODED) extracted from the pelagic sediments, is a widely used paleo-proxy that reflects aridity of the source regions. However, such reconstruction of aeolian flux is subject to large uncertainty associated with the age model due to the low sedimentation rate and lack of calcareous nannofossil of the pelagic sediments. Precipitation of authigenic minerals and contribution of volcanic ash also complicate interpretation of the reconstructed ODED flux. This work extracts ODED from the sediments recovered at Ocean Drilling Program (ODP) site 1208 on the Shatsky Rise in NPO. The high sedimentation rate at ODP site 1208 enables a high-resolution age model. The resulting ODED flux, which shows a progressive increasing trend over the past 25 Ma, is very different from the previous reconstructions. The study indicates that authigenic phillipsite contribute a significant portion to the sediment of 25–18 Ma, but the relative contribution of Asian dust to the ODED is roughly constant (60–80%) over the past 18 Ma. Thus, the progressive increasing trend of ODED flux at the ODP site 1208 is not contributed by authigenic phillipsite and volcanic ash but reflect the increasing flux of Asian dust. We propose that the increasing flux of Asian dust in NPO reflects the progressive aridification of Asian interior in response to global cooling and/or regional mountain building. [ABSTRACT FROM AUTHOR]

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: 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]

Abstract: Oxygen isotopic compositions of rhizoconcretions and land snail shells were measured to reconstruct the δ 18O profiles of paleorainwater (δ 18Op) for well-dated loess-paleosols in Luochuan, China. To assess the paleoclimatic significance of δ 18Op, factors that control the spatial and temporal δ 18O variations of rainwater in East Asian monsoon regions were investigated. Modern records indicate that the spatial δ 18O variations of modern rainwater (δ 18Om) are closely related to near surface temperature (T) and the amount of precipitation (P). In order to separate the effects of T and P on the δ 18Om, a linear relationship between δ 18Om, T and the natural logarithm of P is established by multiple regressing analyses: The δ 18O of paleorainwater over the past 130 Ka in Luochuan area is calculated by the modern-spatial δ 18O–T–P relationship using estimated T and P from other paleoclimate records. The calculated δ 18O match well to those reconstructed from the authigenic carbonates, which implies that the spatial and temporal δ 18O variations of rainwater in East Asian monsoon regions have similar δ 18O–T–P relationships and the authigenic carbonates in Chinese loess-paleosol sequences can serve as valid records of δ 18Op. As an application of the δ 18O–T–P relationship, P variation in the past 130 ka in Luochuan was evaluated. The reconstructed P is in good agreement with other studies using different approaches. Our results suggest higher precipitation in Marine Isotope Stages 1, 3 and 5, reflecting intense summer monsoon influences during these periods of time. [Copyright &y& Elsevier]

The rare earth elements (REEs) in loess sediments may preserve key information of paleoenvironmental changes in East Asia but remain poorly explored. Here we present the REE compositions of sequentially leached phases of loess sediments from a 16 m long loess-paleosol sequence collected from Weinan at the southern margin of the Chinese Loess Plateau (CLP). The REEs associated with carbonates, easily reducible oxides, reducible oxides, clay minerals, and the residual silicate minerals were sequentially leached with Na-acetate, hydroxylamine-HCl, Na-dithionite/Na-citrate, 12 mol L−1 HCl and concentrated HF-HNO 3 solutions. The results show that the REE concentrations of different phases vary in the loess-paleosol layers and are most enriched in the easily reducible phase (except for the residual phase), which is likely explained by the association of REEs with Mn-oxides. Significant Eu anomalies can only be found in the carbonate and easily reducible phases, with the carbonate phase being characterized by appreciable positive Eu anomalies and the easily reducible phase being characterized by negative Eu anomalies, indicating the incorporation of Eu2+ into secondary carbonates that leaves the authigenic Mn-oxides with depleted Eu relative to other REEs. The carbonate phase shows significant negative Ce anomalies, which is likely due to the preferential removal of Ce4+ by Mn oxides before the formation of secondary carbonates. Nevertheless, appreciable positive Ce anomalies for the easily reducible phase can only be found at the bottom of the paleosol S1 while significant negative Ce anomalies were found within the paleosol S1, indicating the downward leaching of Ce4+ that shows stronger complexation with the organic matters than REE3+ in the soil solution. This may partly account for the negative Ce anomalies previously found in the paleosols. Overall, our results suggest a two-stage evolution of Eu and Ce anomalies of the carbonate phase, with the first stage controlled by the formation of both Mn-oxides and secondary carbonates and the second stage dominated by the precipitation of secondary carbonates. The comparison of carbonate Eu and Ce anomalies with other paleoclimatic proxies indicates that the combination of Eu and Ce anomalies of the carbonate fraction of loess may be applied as novel proxy records of climate change in East Asia. [ABSTRACT FROM AUTHOR]

Abstract: Morphological characters and multi-element isotopic compositions of carbonates from the loess–paleosol sequences in Northwestern China are examined to explore the origin of the minerals. Samples are collected from various sections ranging from Holocene to 0.9Ma within the sequences and fractions with grain sizes >45 and <2μm are separated from the bulk soil and examined by SEM and TEM. The results show that the grains >45μm exhibit an almost perfect spherical shape while those <2μm are dominated by nano-rods having diameters of 30–50nm and lengths of 0.3–2μm, presumably indicating the detrital origin of the coarse fractions and the authigenic characters of the fine ones. Such implications are corroborated by the multi-elemental isotopic compositions of the carbonate minerals. A comparison of the δ13C and δ18O values between minerals and biologically originated samples indicates that the <2μm fractions have a similar composition to those of coexisting land snail shells. Additional differences between the two size fractions also manifest in the ratios of 206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb, and 87Sr/86Sr. These results suggest the utility of fine carbonate particles in the soil profile in reconstructing a potentially higher resolution δ13C and δ18O time series to elucidate the paleoclimatic fluctuation in the Chinese Loess Plateau during Pleistocene. The discovery of the nano-rod calcite in Chinese loess, together with previous findings of the similar mineral form in Asian dust, strongly suggests the possibility that these highly reactive CaCO3 form may alter the aerosol properties during transport. [Copyright &y& Elsevier]

• Homogenous reactive Fe isotope signatures revealed by acid leaching of Asian dust. • A first synthetic model on Fe release kinetics during acid leaching of particles. • Low δ56Fe found in reactive components of North Pacific pelagic sediments. Iron released from wind-blown dust has been widely suggested to be an important nutrient source for oceanic primary productivity. Once introduced into the seawater, the reactive Fe might get dissolved and impart its isotope signature into the water column. Characterization of the reactive iron isotope signatures of eolian dust will thus help to understand the budget of Fe in the global ocean. Yet, there are few systematic studies on the Fe isotope systematics of the dust sources across large spatial scale. Here, we report Fe isotope data from HCl leaching experiment on surficial samples from deserts and loess deposits of different locations spanning >3000 km spatially in east Asia, one of the most important dust source regions. Our data suggest most of the samples have relatively similar leaching signatures (δ56Fe = –0.09±0.07‰, n=9, 2 SD). To understand the kinetics of Fe release during proton promoted leaching of natural particles, a synthetic model taking into account of different dissolution dynamics of easily dissolvable Fe and silicate Fe was developed. The model was tested by continuous leaching of two representative pelagic sediment samples recovered from core LL44-GPC3 in the North Pacific. When introducing only one tunable parameter (i.e., the initial dissolution rate of Fe from hydro(oxides), with other parameters constrained by the dissolution kinetics), the model successfully reproduces leachate δ56Fe evolution and agrees reasonably well with Fe speciation in the studied marine sediment samples. We thus suggest that the leaching process can be well characterized by our kinetic model, while Fe isotopes in the initial leachates of the samples are dominated by endmember composition of the easily dissolvable Fe species. The small but distinct fractionation of Fe isotopes in the desert/loess samples from the parental upper continental silicate materials are suggested to record signatures of the secondary Fe species produced during weathering in the oxic environment. The significantly lighter Fe isotope compositions in the initial leachates of the North Pacific pelagic sediment samples compared to the east Asian desert/loess deposits indicate apparent isotope exchange of the dust reactive components with seawater dissolved species. The sedimentary light reactive Fe isotope signatures in turn point toward the contribution of sources characterized by low δ56Fe to the deep North Pacific and/or preferential uptake of 56Fe by seawater ligands from the dust reactive components particularly at the seawater-sediment interface. [ABSTRACT FROM AUTHOR]