Fonction : Doctorant - thèse soutenue le 30/04/23
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CRPG UMR 7358 CNRS-UL
15 rue Notre Dame des Pauvres
54500 Vandœuvre les Nancy – France
Email : xiaobai.ruan@univ-lorraine.fr
2021 |
Ruan, X., Galy, A. On the significance of periglacial conditions in active mountain belts for chemical weathering processes : Insights from the Chayu area, SE Tibet (Article de journal) Dans: Chemical Geology, vol. 585, p. 120581, 2021. @article{Ruan+Galy2021,Chemical weathering is a key player of the long-term (Ma) timescale carbon cycle. Weathering processes under alpine periglacial environment could be important during the Quaternary glacial-interglacial cycles, but such timescale is difficult to fully apprehend for the weathering impact on C cycles. The Chayu river catchment in SE Tibet spans a geographical gradient from a periglacial environment in the north to a monsoonal environment in the south and we have investigated the various chemical weathering along this climatological gradient. The lithology of the catchment is dominated by granitoid rocks. Due to the active tectonic activity and the periglacial environment, mass wasting deposits including landslide deposits, talus and rock glaciers are found in the catchment. In order to figure out the influence of mass wasting process on water chemistry, we sampled seepage water from the mass wasting deposits and nearby stream water for comparison. Cyclic input contributes 5.7 to 8.2% to the dissolved load of the water samples, suggesting the dominance of weathering contribution within this granitoid catchment. The contributions from sulfuric acid and carbonate to chemical weathering are estimated based on the elemental data and the oxidation of sulfide as the source of dissolved sulfate. In the monsoonal South Chayu, the K*/Si, Ca*/Si ratios of the bedrock landslide seepages are higher than the streams by 265% and 117%, respectively. Together with the high contribution of carbonate (textasciitilde80%) and sulfuric acid (20 to 40%), the results indicates that landslide deposits have strong influences on local water chemistry by processes identical to bedrock landslides of greywacke and carbonate lithology in active mountain belts, suggesting that the enhanced weathering by landslide process can apply to granitoid regions as well. In the North Chayu with periglacial environment, the chemistry of the seepages and streams are nearly identical, and are similar to the landslide seepage water in the south. Such result highlights the role of the periglacial critical zone without the influence of mass wasting deposits that possibly enhances chemical weathering. The high porosity created by repeat freezethaw cycles provides more fresh mineral surfaces for water-rock interaction. But, the low temperature and the partial availability of water, restricted to thaw periods, provide strong kinetic limitations that inhibit the silicate weathering. Such features of the alpine periglacial critical zone led to the preferential weathering of the most reactive mineral phases including calcite, sulfide, and the interlayer K+ of sheet silicates (biotite, chlorite), resulting in the characteristic periglacial water chemistry. The influence of periglacial environment on local water chemistry may apply beyond the North Chayu and may play an important role in modulating the chemical weathering within the transition between Quaternary glacial and interglacial period. |
2019 |
Ruan, X., Yang, Y., Galy, A., Fang, X., Jin, Z., Zhang, F., Yang, R., Deng, L., Meng, Q., Ye, C., Zhang, W. Evidence for early (�W12.7 Ma) eolian dust impact on river chemistry in the northeastern Tibetan Plateau (Article de journal) Dans: Earth and Planetary Science Letters, vol. 515, p. 79–89, 2019. @article{Ruan_etal2019,As one of the largest dust sources on the Earthtextquoterights surface, dryland in Central Asia gives rise to thick eolian deposits over East Asia (e.g., the Chinese Loess Plateau, CLP) and significantly influences the regional hydrochemistry in the downwind drainage areas. However, the formation of thick eolian dust deposits requires not only climatic prerequisites for dust emission and transport but also climatic and topographic conditions favourable for deposition and accumulation. The scarcity of widespread eolian deposition around the CLP before 7-8 Ma hinders a full understanding of the processes and mechanisms of Central Asian aridification. The deposition of eolian dust also impacts the hydrogeochemistry of fluvial systems and the precipitation of authigenic phases in continental sedimentary systems could be an archive for studying eolian dust dynamics when pure eolian deposits are scarce. Here, we present the Ca-Mg-Sr concentrations and 87Sr/86Sr isotope compositions of bulk carbonates in a new fluvial sequence (12.7-4.8 Ma) of the Xining Basin. The Mg/Ca and Sr/Ca ratios of the carbonate describe a power law relationship with a power coefficient of �`u0.8, lower than the coefficient characteristic of prior calcite precipitation (PCP). An input of eolian dust with the dissolution of Mg-rich carbonate is likely responsible for the deviation from a pure PCP process. The bulk carbonates also show a general decrease of 87Sr/86Sr ratios from 12.7 to 4.8 Ma, with a transition around 8.6 Ma revealed by lower Sr/Mg ratios. The comparison of these proxies to a previously reported fluvial section (12.2-5.1 Ma) in the Linxia Basin, �`u200 km to the southeast, shows that the 87Sr/86Sr ratios of the bulk carbonates and water-soluble salts in the Linxia Basin are around 0.7098, which is 0.0018 lower than those in the Xining Basin before 8.6 Ma, but shows a significant rise between 8.6 and 7.0 Ma. The two basins share the same range of carbonate 87Sr/86Sr ratios when sediments are younger than 7 Ma. For the last 7 Myrs, the evolution of the 87Sr/86Sr ratios in bulk carbonates of fluvial sediments and Pliocene-Quaternary eolian deposits found in the Xining Basin are similar to those in typical eolian red clays/loess-palaeosol sequences on the CLP. These results suggest a transition of the hydrochemical regime at 8.6 Ma in the Linxia Basin from a catchment only influenced by the weathering of its bedrock to one significantly impacted by eolian dust input. In the Xining Basin, the carbonate elemental and 87Sr/86Sr ratios are consistent with a hydrochemistry more impacted by the presence of the eolian dust. There, the dust input occurred earlier, at �W12.7 Ma, though it has strengthened since 8.6 Ma. The eolian dust impact on fluvial systems in the Xining Basin was much earlier than in the Linxia Basin and also preceded the initial accumulation of widespread eolian red clays on the CLP (7-8 Ma), suggesting a temporally propagating and spatially stepwise expansion of eolian dust delivery across the Asian inland during the late Cenozoic. |
Yang, R., Yang, Y., Fang, X., Ruan, X., Galy, A., Ye, C., Meng, Q., Han, W. Late miocene intensified tectonic uplift and climatic aridification on the northeastern Tibetan plateau: Evidence from clay mineralogical and geochemical records in the Xining Basin (Article de journal) Dans: Geochemistry Geophysics Geosystems G3, vol. 20, no. 2, p. 829–851, 2019. @article{Yang_etal2019,The uplift of the Tibetan Plateau (TP) during the late Cenozoic is thought to be one of crucial factors controlling Asian climate. However, the complex interaction between tectonics and climate change is still unclear. Here we present the first record of clay mineralogy and elemental geochemistry covering textasciitilde12.7--4.8 Ma in a fluvial‐lacustrine sequence in the Xining Basin. Geochemical provenance proxies (Th/Sc, Zr/Sc, and Cr/Zr) in the <2‐$mu$m fraction show a significant provenance change at textasciitilde8.8 Ma. Silicate‐based weathering indexes (CIA, CIW, and PIA) displayed coeval changes with provenance but discrepant changes with regional climate. Since the clay mineralogy exhibits significant change at textasciitilde7.8 Ma uncorrelated with modifications in provenance, it can be employed to reveal regional climate change. The rise in illite and associated decrease in the sum of smectite and illite/smectite mixed layers reflect gradual and slow aridification since textasciitilde12.7 Ma with intensified drying since textasciitilde7.8 Ma until approaching the modern climate status. Our results, together with other regional climatic and tectonic records, clearly illustrate that accelerated uplift of the northeastern TP since textasciitilde8--9 Ma has mainly modulated the regional erosion, weathering, transportation, and sedimentation and amplified the global cooling and drying trend toward the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a comprehensive mineralogical and geochemical investigation of the fine‐grained fraction of the basin sediments could retrieve the interactions between tectonics and climate behind the complex change in exhumed lithology and sedimentary routing systems. |