Changing color of Chinese loess: Geochemical constraint and paleoclimatic significance

Two loess-paleosol sequences on the central Chinese Loess Plateau were investigated to understand spatial and temporal variations in the soil color (e.g., lightness and redness) and factors that control those variations. Color difference between the original samples and pretreated samples suggests that loess lightness is influenced by the pedogenic matter (e.g., iron oxides, organic matter, and calcium carbonate) to varying degrees, depending on the concentrations. Iron oxides and organic matter darken the loess lightness, whereas carbonate lightens the loess lightness. By contrast, the redness is dominantly controlled by the types and concentrations of iron oxides. Variations in magnetic susceptibility and redness are associated with different magnetic minerals that formed mainly during post-depositional processes, and therefore both proxies can be employed to reconstruct the history and variability of the East Asian summer monsoon. Since the sensitivity of these two proxies to the summer monsoon variation is different, we generate a stacked summer monsoon index by combining these two records and interpret the stacked index as most indicative of precipitation variability. The fidelity of this new index is supported by an independent ratio, estimated from the first-derivative values of the color reflectance spectra. Our loess-based proxies provide a new understanding of the East Asian summer monsoon variability as a two-phase strengthening of summer monsoon intensity during the penultimate deglaciation.</p

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

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

Astronomical and glacial forcing of East Asian summer monsoonvariability

The dynamics of glacialeinterglacial monsoon variability can be attributed to orbitally induced changes in summer insolation and internal boundary conditions. However, the relative impacts of astronomical and internal factors on East Asian summer monsoon variability remain controversial. Here we combine proxy data and model results to evaluate the response of East Asian summer monsoon change to these forcings. d13C of loess carbonate, a sensitive summer monsoon proxy from the western Chinese Loess Plateau, demonstrates coexistence of distinct 100-, 41- and 23-ka periods, in contrast to precession-dominated speleothem d18O records in South China. Model results indicate that insolation, ice and CO2 have distinct impacts on summer precipitation changes in East Asia, whereas their relative impacts are spatially different, with a relatively stronger insolation effect in south China and a more dominant ice/CO2 influence in north China. Combined proxy data and model results indicate that East Asian summer monsoon variability was induced by integrated effects of summer insolation and changing boundary conditions (e.g., ice sheets and CO2 concentration). Our proxy-model comparison further suggests that gradual weakening of the summer monsoon related to slowly decreasing summer insolation at astronomical timescales will be likely overwhelmed by the projected ongoing anthropogenic CO2 emissions.</p