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The behaviour of rare earth elements (REEs) and yttrium (Y) during chemical weathering processes has been investigated on a 4.05 m thick terra rossa profile over dolomite on the Yunnan-Guizhou Plateau, China. In this profile, ferromanganese concretions and gibbsite spots coexist in the terra rossa solum. Analyses of REEs, Y, Zr, F, S, and TOC, as well as mineralogical studies, were conducted on a suite of ferromanganese concretions, gibbsite spots, terra rossas, dolomites, and insoluble residues from the underlying dolomite. These analyses helped us to understand the mobilization, redistribution, and fractionation of REEs and Y during chemical weathering. The REEs and Y are mobilized and redistributed during the terra rossa formation. REEs, except for Ce, are removed from the upper and middle profiles, transferred downwards, and then precipitated in the base profile, resulting in remarkable enrichment in the terra rossa near the weathering front. The significant increase of pH near the weathering front was responsible for REE (excluding Ce) and Y enrichment in the base profile. Y is quite mobile during extreme chemical weathering, and most of it was carried away from the profile. Because of very efficient oxidative fractionation of Ce, a significant positive Ce anomaly in terra rossa was found in the middle of the profile, whereas the terra rossa near the weathering front exhibited notably negative Ce anomalies. Moreover, the presence of cerianite and its content in the ferromanganese concretions can explain the markedly positive Ce anomaly and the variation of Ce/Ce * values with depth. In profile, these Ce anomalies are characteristic of the geochemical environment, especially the redox condition. Studies of REE distribution in the ferromanganese concretions, gibbsite spots, and surrounding terra rossa revealed that significant REE fractionation occurred. REEs and Y were preferentially concentrated in the ferromanganese concretions rather than in the gibbsite spots. The water-rock interaction resulted in M-type tetrad effects in some of the ferromanganese concretions, gibbsite spots, and terra rossa samples. In addition, the tetrad effect in terrestrial weathering processes likely played an important role in the fractionation of REEs and Y, such as Y-Ho, Sm-Nd, and the Eu anomaly change. © 2010 Elsevier B.V. All rights reserved. Source

Hou J.,CAS Institute of Tibetan Plateau Research | D'Andrea W.J.,Lamont Doherty Earth Observatory | Liu Z.,University of Hong Kong
Quaternary Science Reviews | Year: 2012

There is a great deal of controversy regarding the fate of glaciers and ice fields on the Tibetan Plateau in the face of continued anthropogenic global warming. Paleoclimate reconstructions and spatial analyses aimed at mapping past climate changes are the key to understanding the climatic response of the Tibetan Plateau to changing conditions. Specifically, the numerous lakes distributed across the Tibetan Plateau can provide high-resolution (spatial and temporal) climate reconstructions to investigate past changes in the climate system. In this paper, we review the primary limitation to exploiting these valuable paleoclimate archives: errors in radiocarbon-based age models. We review the techniques that have been used to estimate 14C reservoir ages on the Tibetan Plateau and compile the published 14C reservoir ages to examine their spatial and temporal patterns and to assess the imposed chronological uncertainties. Using site-specific evaluations of Bangong Co and Lake Qinghai, we demonstrate that 14C age model uncertainties permit equally probable and contrasting interpretations of existing paleoclimate records. We also examine 14C-induced uncertainties in the spatial climatic response on the Tibetan Plateau to (1) the termination of the Last Glacial Maximum and (2) the Holocene Thermal Maximum. We conclude with recommendations for reducing uncertainties in future lake-based paleoclimate studies on the Tibetan Plateau. © 2012 Elsevier Ltd. Source

The mobilization, redistribution and fractionation of trace elements during chemical weathering processes have been investigated on a 4.05. m thick terra rossa profile overlying dolomite on the Yunnan-Guizhou Plateau, in Southwest China. In this in situ weathering profile, the ferromanganese concretions and the gibbsite spots coexist in the terra rossa saprolite. The mass-balance evaluation reveals that titanium, Nb and Hf in the terra rossa matrix are conservative elements during chemical weathering compared to Zr. The elements of Li, Sc, V, Cr, Fe, Ga, As, Mo, Cs, Ce, Ta, Tl, Pb and Th in the terra rossa matrix include additions from external sources. Beryllium, Mn, Co, Ni, Cu, Rb, Ba and La are depleted in the shallow parts of the terra rossa profile and enriched in the deep parts. The elements of Zn, Sr, Y, Cd, Sn and U in the terra rossa profile are lost during weathering. Compared to the terra rossa matrix, the ferromanganese concretions are significantly enriched in most trace elements, especially Mn, Co, Cd, Ce, Tl and Pb. In contrast, the gibbsite spots are depleted in all trace elements, except for U. The results regarding specific inter-element relationships indicate that most trace elements have different inter-element relationships in the ferromanganese concretions, the gibbsite spots and the terra rossa matrix. This suggests that the behavior of many trace elements during mobilization and redistribution differs from their behavior during incorporation into secondary mineral phases, especially the Mn and Fe oxides and/or oxyhydroxides in the ferromanganese concretions. It is worthy to note that the fractionation between Ce and Mn occurs under intensive chemical weathering conditions. Correspondingly, beryllium exhibits a similar geochemical behavior as that of rare earth elements (except for Ce) and Y during surface weathering. © 2010 Elsevier B.V. Source

Gao Y.,CAS Lanzhou Cold and Arid Regions Environmental and Engineering Research Institute | Cuo L.,CAS Institute of Tibetan Plateau Research | Zhang Y.,U.S. National Center for Atmospheric Research
Journal of Climate | Year: 2014

Changes in moisture as represented by P - E (precipitation 2 evapotranspiration) and the possible causes over the Tibetan Plateau (TP) during 1979-2011 are examined based on the Global Land Data Assimilation Systems (GLDAS) ensemble mean runoff and reanalyses. It is found that the TP is getting wetter as a whole but with large spatial variations. The climatologically humid southeastern TP is getting drier while the vast arid and semiarid northwestern TP is getting wetter. The Clausius-Clapeyron relation cannot be used to explain the changes in P - E over the TP. Through decomposing the changes in P - E into three major components-dynamic, thermodynamic, and transient eddy components-it is noted that the dynamic component plays a key role in the changes of P - E over the TP. The thermodynamic component contributes positively over the southern and central TP whereas the transient eddy component tends to reinforce (offset) the dynamic component over the southern and parts of the northern TP (central TP). Seasonally, the dynamic component contributes substantially to changes in P - E during the wet season, with small contributions from the thermodynamic and transient eddy components. Further analyses reveal the poleward shift of the East Asian westerly jet stream by 0.7° and poleward moisture transport as well as the intensification of the summer monsoon circulation due to global warming, which are shown to be responsible for the general wetting trend over the TP. It is further demonstrated that changes in local circulations that occur due to the differential heating of the TP and its surroundings are responsible for the spatially varying changes in moisture over the TP. © 2014 American Meteorological Society. Source

Nie J.,Lanzhou University | Nie J.,CAS Institute of Tibetan Plateau Research
Geochemistry, Geophysics, Geosystems | Year: 2011

Earth's climate over the last one million years experienced several ∼100-kyr glacial cycles, but no simple forcing mechanism has been identified. Numerous studies have tried to explain strong 100-kyr glacial cycles without recognizable forcing, which has come to be known as the 100-kyr problem. Few studies have examined 100-kyr band paleoclimatic signals before 1 Ma. A recent study has demonstrated that benthic oxygen and carbon isotope records are phase-locked and amplitude-coupled at the 100-kyr band, but that neither is phase-locked to and amplitude-coupled with the 100-kyr eccentricity signal between 3 and 1 Ma. This phasing and amplitude mismatch of the 100-kyr band between 3 and 1 Ma between marine records and the eccentricity forcing signal has been called the "late Pliocene-early Pleistocene 100-kyr problem." However, it remains unknown whether terrestrial paleoclimate records are consistent with marine records at the 100-kyr band. Here I show that loess monsoon records from China are amplitude-coupled with benthic oxygen and carbon isotope records at the 100-kyr band, but not with the 100-kyr eccentricity forcing between 3 and 1 Ma. This observation provides further evidence in support of a free 100-kyr oscillation as the cause of the 100-kyr band amplitude variability in paleoclimatic records between 3 and 1 Ma. In contrast, benthic oxygen isotope records and loess monsoon records at the 100-kyr band are not amplitude-coupled with 100-kyr benthic carbon isotope records over the last 0.4 million years, indicating that the late Pleistocene 100-kyr climatic cycles may not result exclusively from a free oscillation. Copyright 2011 by the American Geophysical Union. Source

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