Sr and Nd Isotopes as Tracers of Chemical and Physical Erosion

Abstract

The applications of radiogenic isotopes to investigate chemical and physical erosion processes, particularly in river basins of the Himalaya, have led to interesting inferences on the relationship between tectonics, weathering and climate. The chemical weathering studies rely more on Sr isotopes because of their widely different ratios in various end members, their uniform distribution in the oceans and the availability of continuous and robust record of marine 87Sr/86Sr through much of the geological past. The record for the Cenozoic shows steady increase in 87Sr/86Sr; one of the hypotheses suggested to explain this is enhanced continental silicate weathering due to the uplift of the Himalaya. This hypothesis linking tectonics-weathering-climate, based on 87Sr/86Sr as an index of silicate weathering, however, is being challenged by the recent observations that there are a variety of carbonates in the river basins of the Himalaya with 87Sr/86Sr similar to that of silicates which have the potential to contribute significantly to the high 87Sr/86Sr of rivers such as the Ganga-Brahmaputra. Further, the non-stochiometric release of Sr isotopes during chemical weathering of minerals and rocks, the imbalance of Sr isotope budget in the oceans and temporal variations in riverine fluxes due to impact of glaciations all have compounded the problem. Studies on the provenance of sediments and physical erosion pattern employ both Sr and Nd isotopes under the assumption that their source signatures are preserved in sediments. Though there are concerns on how well this assumption is satisfied especially by the Sr isotope system, both Sr and Nd systems are being used to learn about physical erosion in the Himalaya, its variability and causative factors. The results show that at present the major source of sediments to the Ganga plain and the Bay of Bengal is the Higher Himalayan Crystallines and that physical erosion among the various sub-basins is very heterogeneous with maximum rates in regions of intense precipitation and high relief. There are three such “hot-spots”, one each in the basins of the Ganga, Brahmaputra and the Indus, which unload huge amount of sediments promoting rapid uplift of regions surrounding them and enhance chemical weathering by exposing fresh rock surfaces. The pattern of physical erosion and its temporal variations shows that it is influenced by climate change both on ky and My time scales though during the latter periods the erosion regime has been by and large stable. This article reviews investigations on the present and past chemical and physical erosion in river basins of the Himalaya using Sr and Nd isotope systematics in water and sediments.