Under the influence of meteoric waters and many others factors like glaciers and wind, continents are continuously damaged. Geologists call this erosion. Erosion is both chemical, also called chemical weathering, and physical. During chemical weathering, protons contained in rainwater interact with the base cations of minerals from the Earth’s crust and new minerals appear during this acid-base reaction. These minerals are clays and iron or aluminium-oxides that accumulate in soils or are removed by physical erosion and transported by rivers to sedimentary basins. At geological time-scales, chemical weathering participates in the shaping of landscapes, the evolution of the composition of continental crust and acts as a long term feedback on the Earth’s climate through the consumption of atmospheric CO².

As isotopes of lithium are fractionated during water/rock interactions (ie the light isotope is preferentially incorporated in solid weathering products like clays), isotopes of lithium revealed to be a powerful tracer of chemical weathering. Recently, they have been proposed as a potential proxy for reconstructing past chemical weathering rates. However, interpretation of the variation of Li isotopes in the geological record and its link to change in chemical weathering as a function of climate or tectonic activity, remain a challenge and are still debated.

Because rivers drain large surface area of the continents, the study of products of erosion transported by rivers both in dissolved and solid phases provides integrated information about the processes occurring at the Earth surface. A research team from the Institute of Geochemistry of the Chinese Academy of Sciences (IGCAS) in Guiyang conducted a systematic research on Li isotopes in sediments and waters from the rivers of the Changjiang basin. As the basin is characterized by a climatic gradient and a contrasted geomorphology between the lower reaches and the upper reaches, it offers the possibility to test these different hypothesis.

The results have shown that the evolution of the isotopic composition of river water from the Upper Reaches to the Lower Reaches is mainly controlled by the fraction of Li incorporated into secondary minerals, and in fine by probably the difference in weathering regimes related to the difference of geomorphology. In the upper reaches, characterized by steep slopes, high physical erosion rates prevent soil formation resulting in short residence time of material before being removed. Thus products of erosion display similar Li isotopic compositions with the bedrock.

By contrast, in the lowlands, lower physical erosion rates allow the development of thicker soils, resulting in longer residence time of material and this is evidenced by products of erosion having isotopic compositions highly fractionated relative to the bedrock. Mass balance calculations also revealed that the bedrock is mainly derived from the recycled sedimentary rocks which have undergone previous cycles of erosion-deposition and have lost most of their soluble cations limiting their capacity to consume atmospheric CO².

This study provides not only new insights into the functioning of the erosion engine at the Earth’s surface and the coupling between chemical weathering and physical erosion but also into the interpretation of lithium isotopes variation in the geological record and its link with chemical weathering. The importance of sedimentary recycling highlighted in this study has strong implication for the evolution of composition the continental crust through ages and also for the long term atmospheric CO² consumption by chemical weathering.

This study has been published in Geochimica Cosmochimica Acta.

Reference:
Q.-L. Wang, B. Chetelat, Z.-Q. Zhao, H. Ding, S.-L. Li, B.-L. Wang, J. Li and X.-L. Liu (2015), Behavior of lithium isotopes in the Changjiang River system: Sources effects and responses to weathering and erosion, Geochim. Cosmochim. Acta .151, 117-132.

Contact:
Benjamin Chetelat^*
ZHAO Zhiqi
State Key Laboratory of Environmental Geochemistry
Institute of Geochemistry, CAS