Evaluation of high-resolution elemental analyses of Chinese loess deposits measured by X-ray fluorescence core scanner

Chinese loess is a unique continental archive of Quaternary climate change, and its elemental components or ratios can provide significant insights into variations in the East Asian palaeomonsoon at tectonic to millennial timescales. However, high-resolution elemental analyses of loess deposits have seldom been reported. To investigate geochemical variations of loess sediments at millennial and centennial timescales, and their potential for recording abrupt climate change, powder and U-channel (long plastic single-boxes used to sub-sample the center of split core sections) samples were collected from the Gulang loess section on the northwestern Chinese Loess Plateau for conventional and scanning X-ray fluorescence (XRF) analyses. Results suggest that although ten elements (Al, Si, K. Ca, Ti, Fe, Zn, Rb, Sr and Zr,) can be detected robustly using scanning XRF method, only five elements (Si, Ca, Fe, Sr and Zr) can be employed confidently to infer the chemical weathering and grain size sorting effects. Comparison of scanning XRF-derived elemental data with magnetic susceptibility, grain size and speleothem records indicates that high-resolution elemental records have great potential for evaluating rapid fluctuations of the East Asia monsoon.</p

δ(13)C Values of loess total carbonate: A sensitive proxy for Asian summer monsoon in arid northwestern margin of the Chinese loess plateau

We measured carbon isotope ratio of loess total carbonates (delta(13)Cic) from the Jingyuan loess profile in the arid northwestern portion of the Chinese Loess Plateau (CLP), in order to examine the effectiveness and sensitivity of loess total carbonate delta(13)Cic values as a potential paleoenvironmental indicator. The 813Cic record in the loess profile shows large variations ranging from -1%. to -7%., during the past 70 ka. The most negative delta(13)Cic peaks coincide with paleosol formation; S(0), L(1)SS(2), and L(1)SS(2), which occurred under relatively wetter climatic conditions during middle Holocene and marine delta(18)O stages 3. On the other hand, the least negative delta(13)Cic values responded to loess accumulation during relative drier phases of LGM and marine delta(18)O stages 4. While delta(13)Cic variations in the loess profile in arid area demonstrate a clear correlation with changes of total organic carbon isotope (delta(13)Coc) of bulk sediments which has been widely used as a proxy for the past monsoon precipitation intensity, the delta(13)Cic values in the loess profile are more sensitive to the changes of paleohabitat controlled by the Asian monsoon variation in the arid area. We propose that carbon isotope compositions of total carbonates in loess profile can serve as a sensitive and reliable proxy for the Asian summer monsoon in the arid northwestern Chinese Loess Plateau.</p

Impacts of grain size sorting and chemical weathering on the geochemistry of Jingyuan loess in the northwestern Chinese Loess Plateau

Major and trace elemental compositions of loess samples collected from the Jingyuan section in the northwestern Chinese Loess Plateau (CLP) were analyzed to investigate the potential impacts of grain size sorting and chemical weathering on the loess geochemistry and to extract appropriate geochemical indices for better evaluating the East Asian monsoon variability. Based on variations of major and trace elements in different grain size fractions, seventeen elements were classified into three types: (1) Si and Na display higher contents with the increased particle sizes; (2) Al, Fe, Mg, K, Mn, Zn, Rb, Cr, V are mainly enriched in fine size fractions; (3) Ti, Ba, Zr, P. Ca and Sr show irregular variations among different size fractions. Comparison of Al-normalized elemental ratios with Zr/Rb and Rb/Sr ratios (two commonly employed indicators for grain size sorting and pedogenic weathering) indicates that Si/Al, Zr/Al, Ti/Al variations match well with Zr/Rb and grain size results, whilst Ca/Al, Sr/Al, P/Al ratios display similar variability as that of Rb/Sr ratio. Comparison of loess based proxies (e.g., elemental ratios, magnetic susceptibility, grain size) of Jingyuan section with speleothem and ice-core records confirms that elemental ratios of high-resolution loess sequences developed in the northwestern CLP can be employed to address fluctuations of the winter monsoon-induced grain size sorting and summer monsoon-related weathering and pedogenesis at glacial-interglacial and millennial timescales.</p

Timing and lock-in effect of the Laschampgeomagnetic excursion in Chinese Loess

The Laschamp geomagnetic excursion (LGE, at 41 ka) is a critical age control for hemispherical comparison of paleoclimate records from various kinds of geological archives (e.g., ice core, marine, and lake sediments). The timing of the LGE in Chinese Loess, however, remains poorly constrained due to the lack of a reliable chronology and the complex acquisition processes of natural remanent magnetization. Here, we systematically compare the paleomagnetic results of three optically stimulated luminescence dated loess sequences on the Chinese Loess Plateau. Our results indicate that the timing of the LGE in the northwestern Loess Plateau is slightly older than the absolute radiometric age determination and the timing inferred from Greenland ice core (10Be flux) and marine (i.e., relative paleointensity and authogenic 10Be/9Be stack) records, but younger than the counterparts in the central Loess Plateau. We attribute the different timing of the LGE in the three loess sections to a progressive southeastward increase in the lock-in depth caused by the combined effects of postdepositional processes (e.g., surface mixing, chemical weathering, and lock-in delay) on the remanence acquisition process. We conclude that caution is needed to use the LGE in Chinese Loess as a reliable tie-event for highresolution chronological correlation to marine and ice core records, only if the potential lock-in effect can be precisely determined.</p

Terrestrial selenium distribution in China is potentially linked to monsoonal climate

The prevalence of terrestrial environments low in the essential trace element selenium (Se) results in large-scale Se deficiency worldwide. However, the underlying processes leading to Se-depleted environments have remained elusive. Here we show that over the last 6.8 million years (Ma) climatic factors have played a key role in the Se distribution in loess-paleosol sequences in the Chinese Loess Plateau (CLP), which lies in a severely Se-depleted region with a history of Se deficiency-related diseases. We use a combination of geochemical and paleoclimate data to demonstrate that during interglacial periods between 2.30 and 0.16 Ma, variations in the Se concentration in the CLP are potentially related to variability in Se input via East Asian monsoon-derived precipitation. Our results identify precipitation as an important controlling factor of Se distribution in monsoonal China. We suggest that atmospheric Se inputs via precipitation could also play an important role in other regions worldwide.</p

ESR signal intensity and crystallinity of quartzfrom Gobi and sandy deserts in East Asia andimplication for tracing Asian dust provenance

Electron spin resonance (ESR) signal intensity and crystallinity index (CI) of fine- (&lt;16 mm) and coarse-grained (&gt;16 mm) quartz were measured in surface samples from the Taklimakan desert in western China, the Badain Juran, Tengger and Mu Us deserts in northern China, and the Gobi desert in southern Mongolia to evaluate whether these geophysical parameters can serve as reliable provenance tracers of Asian dust. The results indicate that spatial variability of both ESR signal intensity and CI is evident within the Taklimakan deserts and the Mongolian Gobi, but less significant in the three deserts of northern China. Coarse-grained quartz from the Mongolian Gobi and northern China deserts can be differentiated from the Taklimakan desert using the ESR signal intensity. Fine-grained quartz originating from three major Asian dust sources, i.e., the Gobi-sandy deserts in western China, northern China and southern Mongolia, can be distinguished effectively using the combination of ESR and CI signals. Our results suggest that ESR signal intensity and CI can discriminate the sources of fine-grained quartz better than coarse-grained quartz, providing an effective approach to trace the provenance of fine-grained dust deposition on the land and in the ocean.</p

Hf-Nd isotopic variability in mineral dustfrom Chinese and Mongolian deserts:implications for sources and dispersal

Mineral dust provenances are closely related to the orogenic processes which may have distinct Hf-Nd isotopic signatures. Here we report the clay-sized (,2 mm) Hf-Nd isotope data from Asian dust sources to better constrain the source and transport dynamics of dust deposition in the North Pacific. Our results show that there is a more positive radiogenic Hf isotopic composition with clay-sized fractions than the corresponding bulk sample and a decoupling of the Hf-Nd couplets in the clay formation during the weathering process. The clay-sized Hf-Nd isotopic compositions of the desert samples from the Sino-Korean-Tarim Craton (SKTC) are different from those of the Gobi and deserts from the Central Asian Orogeny Belt (CAOB) due to varying tectonic and weathering controls. The Hf-Nd isotopic compositions of dust in the North Pacific central province (NPC) match closely with those from the Taklimakan, Badain Jaran and adjacent Tengger deserts, implying that the NPC dust was mainly transported from these potential sources by the westerly jet. Our study indicates that dusts from the CAOB Gobi deserts either didn&rsquo;t arrive in NPC or were quantitatively insignificant, but they were likely transported to the North Pacific margin province (NPM) by East Asian winter monsoon.</p

Stable isotope ratio measurements of Cu and Zn in mineral dust (bulk and size fractions) from the Taklimakan Desert and the Sahel and in aerosols from the eastern tropical North Atlantic Ocean

Accurate characterization of the stable isotope composition of Cu and Zn in major global mineral dust sources and in aerosols is central to the application of these isotope systems to the studies of global geochemical processes and cycles. We test here for the first time Cu and Zn isotope ratios within a well-defined source-receptor setting on the continent-ocean interface and determine the isotope composition of (i) bulk surface soil dust samples from the Sahel region, (ii) individual size fractions of surface dust samples from the Taklimakan Desert, and (iii) aerosols collected in the equatorial eastern North Atlantic region. This is achieved reducing the blank contribution during the ion exchange step using small resin and acid volumes and using a second ion exchange passage to purify the Cu fraction. We find no significant correlation between size fractions and isotope ratios in the two samples analyzed from the Taklimakan Desert. Mass balance calculations suggest that isotope ratios of bulk samples are within the analytical precision of the &lt;4 mu m size fraction and can be used to characterize atmospheric long range transport of Cu and Zn in mineral dust from the Taklimakan Desert. The &lt;1 mu m size fractions of two aerosol samples collected over the equatorial eastern North Atlantic region have Cu and Zn isotope ratios that are different to Sahel surface soil dust suggesting important non-crustal sources, in line with calculated enrichment factors, and possibly of anthropogenic origin. Using previously reported delta Zn-66 values for anthropogenic emission from Europe, preliminary calculations suggest that up to 55% of Zn arriving at the sampling points in the equatorial eastern North Atlantic region could be of anthropogenic origin.</p

Lead atmospheric deposition rates and isotopic trends in Asian dust during the last 9.5 kyr recorded in an ombrotrophic peat bog on the eastern Qinghai-Tibetan Plateau

A full understanding of the atmospheric Pb cycle in Asia during the Holocene is key to palaeoclimate studies of past atmospheric circulation patterns, as well as to assess the impact of increasing industrial activities in this region. However, long-term records of atmospheric Pb isotopic trends in Asia are still sparse. Consequently, we study changes in the concentrations, fluxes and isotopic signature of deposited Pb contained in a (14)C-dated peat core from the eastern Qinghai-Tibetan Plateau, dating back to 9.5 kyr BP, and present the first peat record of the changing isotopic composition of atmospheric Pb in dust in Asia during this time. Lead concentrations and fluxes vary between 2.96-21.58 mu g g(-1) and 0.06-3.52 mg m(-2) y(-1), respectively, with an average Pb baseline of 6.80 +/- 4.18 mu g g(-1). These values agree with other Pb studies of lake and peat archives in China but are one order of magnitude larger than early and mid-Holocene values measured in Europe. Lead isotopic variability throughout the core is small, varying between (206)Pb/(207)Pb = 1.190-1.197, (206)Pb/(204)Pb = 18.648-18.786, (207)Pb/(204)Pb = 15.666-15.694 and (208)Pb/(204)Pb = 38.890-39.090. The application of Bayesian trans-dimensional changepoint modelling to the Pb dataset enabled the identification of eight significant changes in the isotopic composition of the deposited Pb. Such changes mark different phases of atmospheric Pb deposition, and hence variations in atmospheric circulation patterns and environmental conditions. Temporal variations in the potential natural and/or anthropogenic Pb sources are assessed based on the (206)Pb, (207)Pb, (208)Pb and (204)Pb isotopic composition and the 1/[Pb] ratio of the peat samples. Our results suggest that Pb deposition throughout the Holocene was governed by local deposition and long-range input from natural dust sources in northwestern (Taklamakan desert and Qaidam basin) and northern China (Badain Jaran and Tengger deserts). Input from the northern sources seems to have been particularly important between 3.1-2.7 kyr BP and 1.7-0.9 kyr BP, suggesting a possible strengthening of the East Asian winter monsoon, in agreement with previous reconstructions in Asia. Based on the Pb/Sc and isotopic composition profiles we do not note any evidence for anthropogenic Pb derived from the thriving mining or smelting activities in northern and eastern China in the last few millennia, suggesting that atmospheric deposition to this region of the eastern Tibetan Plateau was dominated by natural Pb fluxes. These can serve as a true Asian &quot;background&quot; value against which anthropogenic impacts can be quantified. Our results confirm that the combination of radiogenic isotopes (Pb) and trace elements in peat bogs enables observational reconstructions of changes in past regional atmospheric circulation. Such records will enable more refined interpretations of marine and terrestrial palaeorecords in Asia and the Pacific and consequently provide further constraints for changes in ocean and atmospheric circulation and for the testing of palaeoclimate models of circulation patterns.</p

Provenance fluctuations of aeolian deposits on the Chinese Loess Plateausince the Miocene

The evolution of the provenance of aeolian deposits on the Chinese Loess Plateau (CLP) is closely linked to changes in source aridity and dust transport dynamics. Although previous studies have revealed that the provenance of Chinese aeolian deposits may have fluctuated on tectonic timescales, the exact timing and cause of the provenance shifts remain poorly constrained due to limitations of the isotopic and mineralogical tracers used. Here we report the results of electron spin resonance (ESR) signal intensity and crystallinity index (CI) of fine-grained (<16 lm) quartz isolated from two aeolian sequences on the CLP, in order to address tectonic-scale shifts in dust provenance over the last 23.5 Ma. The ESR–CI results spanning the interval 7–5 Ma for two aeolian sequences are comparable, implying a broadly similar provenance of dust deposits over the entire CLP. The ESR–CI values are lower after 7 Ma than before 9.5 Ma, indicating that a significant provenance shift occurred during 9.5–7 Ma. Comparison of the ESR–CI results for fine-grained quartz in desert surface samples and for the loess and Red Clay sequences indicates that the provenance shift may have been caused by increased dust input from the Mongolian Gobi and western China (i.e., the Taklimakan desert) since the late Miocene. The combination of our results with regional tectonic evidence and global climate record suggests that tectonically-driven climate changes in the dust sources may have played a dominant role in driving the late Miocene provenance shift