River Chemistry of the Chayu catchment (SE Tibet) and the chemical weathering of the Trans-Himalaya batholith.
Abstract :
This thesis aims to understand better why and how the climate cooling, CO2 drawdown and the marine Sr isotopic ratio increase all happened during Eocene/Oligocene. I checked the possibility of Trans-Himalaya weathering as the candidate for explaining the early change in marine 87Sr/86Sr and atmospheric CO2 during Eocene/Oligocene, by studying the modern chemical weathering of the Trans-Himalaya batholith under monsoon climate. We choose the Chayu River Catchment locating in the SE Tibet as the study area for three reasons: 1) this catchment is mostly covered by the granitoid rocks of the Trans-Himalaya batholith, 2) it is now under the monsoon climate and 3) it is experiencing intense tectonic activity. Besides, the alpine part of the catchment is under periglacial environment, and there is a lack of knowledge about alpine periglacial weathering especially in active mountain ranges. Combining the above, we set two objects of the thesis: first, to study the periglacial weathering process in active alpine mountain of the Chayu Catchment, and second to study the weathering budget and the 87Sr/86Sr ratios of the catchment, in order to figure out the atmospheric CO2 sinks and sources, the climatic dependency of chemical weathering rates and the Sr systematics of the Chayu Catchment. Samplings for the main river, tributaries and seepages of mass wasting deposits were conducted at the end of monsoon season in 2019.
To study the alpine periglacial weathering processes, we compared the seepages from mass wasting deposits generated by freeze-thaw processes under periglacial environment and landslide under monsoon climate, and the nearby streams of these two climatic conditions. The landslide seepages under monsoon climate are more concentrated than nearby streams, and the seepages under periglacial environment are as concentrated as the landslide seepages but exhibit negligible differences than nearby streams. Our analyses on elemental ratios indicate that the freeze-thaw process provide an exposure mechanism similar to landslide to facilitate the weathering of reactive mineral phases of calcite, biotite and sulphide. The similarity between periglacial seepages and streams may indicate the widespread periglacial mass wasting deposits has a decisive influence on the river chemistry in alpine periglacial areas.
To discuss the weathering budgets of the Chayu River Catchment, we first identified and corrected the river composition for the influences of secondary calcite precipitation (SCP). It is found that the weathering budget is dominated by carbonate weathering even though the lithology is mostly granitoid, and the degree of SCP is higher in the periglacial North Chayu than in the South Chayu, and is closely related to the carbonate weathering contribution. The δ13CDIC and sulphate proportion together tell that the chemical weathering of the Chayu catchment is mostly driven by carbonic acid, although sulphuric acid weathering is widespread. Combining the budgets of substrates and acids, the chemical weathering of the modern Chayu River Catchment is a CO2 source rather than a CO2 sink. After estimating the relative discharge of each tributary by δ18O of the river water, the carbonate and silicate weathering rates in each tributary basin are calculated. The carbonate weathering rates are basically irrelevant to climatic factors while the silicate weathering rates are positive correlated with basin MAT and runoff. The 87Sr/86Sr ratios of the main river is around 0.735, suggesting the impact from radiogenic carbonate.
Combining the results, the silicate weathering of Trans-Himalaya might be more important than today during Eocene/Oligocene with warmer and wetter climate, and it is also possible that the weathering at that time was thus reversed to a CO2 sink. The radiogenic Sr signals of the Chayu River suggest the weathering of Trans-Himalaya was capable of rising the marine 87Sr/86Sr before Himalayan exhumation.
Key words: Chemical weathering, Trans-Himalaya, Tibetan Plateau, Granitoid weathering, Chayu River Catchment
Composition of the Jury :
Directeur de thèse : Albert Galy, CRPG-CNRS, Vandoeuvre les Nancy
Rapporteurs :
Oleg Pokrovsky Directeur de recherche, Université de Toulouse, GET, Toulouse
Pascale Louvat Ingénieure de recherche, Université de Pau, IPREM, Pau
Examinateurs :
Laurie Reisberg Directrice de recherche, UL-CRPG-CNRS, Vandoeuvre les Nancy
Siliang Li Professeur, Tianjin University, Tianjin (CHN)
Membres invités :
Arnauld Legout Chargé de recherche, Université de Lorraine, Inrae, Nancy
Damien Lemarchand Maïtre de conférences, Université de Strasbourg, ITES EOST, Strasbourg