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External Envelopes Geochemistry

External Envelopes Geochemistry

The G2E team works on the geodynamics of the Earth’s surface envelopes. It has a resolutely geological approach, focusing on the interaction between the water cycle (driven by solar energy) and the Earth’s internal cycle (driven by mantle convection). The geological mechanisms studied are erosion and weathering, transport of material from land to ocean, sedimentation and diagenesis through the chemical element cycle. These cycles are used as tracers and their isotopic abundances as markers.

Our tools are therefore elemental and isotopic geochemistry but also magnetic properties. As man has become a geological force of the “Anthropocene”, the G2E team incorporates him as one of the factors forcing the geodynamics of the outer envelope.

The G2E team shares with other IPGP teams an analytical platform to measure the isotopic ratios of a large number of chemical elements with high precision. These isotope ratios are used as passive tracers of mechanisms and fluxes at the Earth’s surface.

Research themes

The strong erosion of volcanic islands has not only an environmental impact (sediment flow in the lagoons, pollution etc.) but also a societal one. Growing populations are migrating, due to lack of space, to the highlands where erosion is most intense. It is therefore imperative to quantify and understand the erosion processes in volcanic islands to help land management. The use of cosmogenic isotopes makes it possible to quantify the residence time of materials in the critical zone. In a watershed, this exposure time to cosmic radiation is translated into erosion rates. The coupling of this technique with the geochemical balance technique for rivers makes it possible to understand the share of chemical alteration and physical erosion in the overall erosion of volcanic islands (Reunion Island, Tahiti, etc.).

The Earth is the planet of the water cycle. Chemical weathering is the chemical reaction that occurs when water and atmospheric gases react with rocks that outcrop on the surface of continents. This geological mechanism, which consumes carbon dioxide and releases soluble elements, is important because it can affect the evolution of the chemical composition of the atmosphere, rivers, the ocean, sedimentary rocks and, through recycling, the evolution of the continental crust and mantle. Chemical potamology is the study of the chemical and isotopic composition of rivers. Our team is developing various geochemical approaches to understand how rivers record the weathering processes affecting the most superficial part of the planet we live on, which is therefore a critical area for planetary geodynamics and its future.

The joint study of chemical alteration, physical erosion and fluvial transport is one of the strong points of our group, made possible by the thematic and geographical proximity with the Geological Fluid Dynamics Team of the IPGP, and in particular its geomorphological component.

We are interested in the cycling of radiogenic tracers in the ocean. Radiogenic tracers are derived from the radioactive decay of a parent element. The various differentiation processes of the Earth have created chemical heterogeneities between the parent and daughter elements which, over the course of time, are transformed into isotopic heterogeneities of the daughter element. Thus ancient continental crusts or recent volcanic rocks have different isotopic compositions. Erosion and weathering of the continents transfers these heterogeneities to the ocean. Once in the ocean, these elements undergo different processes and will sediment more or less quickly depending on their residence time. Thus, elements with long residence times such as Strontium have homogeneous isotopic compositions at the scale of the global ocean, reflecting continental and hydrothermal inputs. Elements such as Neodymium or Lead have shorter residence times compared to the mixing time of the ocean, which is why the isotopic compositions of these elements are different in different ocean basins, reflecting both local continental inputs and regional ocean circulation. Osmium has an intermediate residence time, thus spatially and temporally integrating the different processes we seek to understand in an intermediate way.