The Anthropocene is the current geological period, during which mankind has become an agent of drastic modifications of the Earth’s environment. The effect of human activities are particularly visible in the so-called “Critical Zone” of the Earth surface, which is the thin veneer between rocks and the sky, and which forms the basis for the habitability of our planet and for the development of civilisation. Within this Critical Zone, water and soils are two resources that are essential for mankind. However, these resources are under great threat because of the global climate and land use changes.
With this project, we will lead an ambitious study of the sediments of the largest Chinese rivers. China is drained by many very large river basins flowing eastward and covering a large range of climatic conditions from North to South. These rivers collect the material exported by the Critical Zone (water, organic carbon, and particles formed in soils) over large areas. We aim to test the ability of the sediment transported by rivers to provide integrated information on the environmental status of the regions drained by these rivers. Although large river products somehow “average out” over the area of the drainage basin the effects of different settings such as soils, floodplains, or dams, they provide fundamental, first-order information and reflect what is transported from continental surfaces to estuaries and coastal oceans.
The SEDI-MAN project will use a series of geochemical tools to characterize river sediments: chemical concentration of many elements; isotope composition to trace sources of sediments and processes affecting them on their journey to and in the river; isotope composition of elements known to be influenced by human activities (“anthropophile elements” such as transition metals); or composition of particulate organic matter. Importantly, we will characterize the temporal and spatial variability of the sediment composition of the largest rivers in China by sampling along depth-profiles and at several points along the hydrological cycle. Therefore, we will be able to link the chemical composition of sediments with their transport properties. The sampling locations that we will investigate in detail are the mouhts of the Zhujiang, Changjiang, Huang He and HeilongJiang rivers. For the Changjiang River, an additional sampling point located upstream the Three Gorges Dam will be examined. In addition to these locations at which sampling will be performed along time series, we will carry out a few campaigns to sample the largest tributaries of these river systems.
Using our results, we will:
- estimate accurately the chemical composition of sediments exported to the ocean by the largest Chinese rivers, and the link between this composition and grain size, hence hydrodynamics;
- evaluate the role of erosion processes in China at the scale of large watersheds, as we will provide an estimate of the fraction of river sediment that is derived from soils relative to the fraction derived from bedrock;
- assess the extent of desequilibrium between soil formation by weathering and soil destrcution by erosion in the largest Chinese river basins, using simple models for Earth surface weathering and erosion;
- characterize the degree to which anthropogenic contamination in transition and heavy metals has modified the chemical composition of Chinese River sediments;
- provide constraints on the way the biogeochemical processes of “river metabolism” were modified by human activities.
This project is part of an international effort to better understand the Critical Zone and its perturbation, from the scale of soils to the scale of large watersheds. It is inparticular supported by an international project of the IAEA aiming to characterize the largest world’s rivers in terms of isotope composition, in order to estimate the extent of human perturbation on soils and water at the global scale.
Monsieur Jerome Gaillardet (Institut de Physique du Globe de Paris)
The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.
ISESS Institute of Surface-Earth System Science, Tianjin University
IPGP Institut de Physique du Globe de Paris
IGCAS Institute de Geochemistry, Chinese Academy of Science, Guiyang
Help of the ANR 246,064 euros
Beginning and duration of the scientific project: October 2015 - 48 Months