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Natural geochemical cycles : fluxes and compositions of the ocean and atmosphere

The carbon cycle is fundamental in the interactions between the different reservoirs of the earth’s surface. Regarding the long-term carbon cycle, the mechanisms involved are complex because they involve chemical and biological processes that control the geochemical fluxes between continents, oceans, atmosphere, and the inner Earth. The carbon cycle is intimately linked to that of other major elements (such as Mg, Ca, and K). Continental erosion, hydrothermal exchanges on ridges, and the precipitation of bio-minerals are the main components that control the composition of the ocean and its evolution over geological time. Our understanding of global cycles therefore requires a better quantification of these fluxes, their geochemical and isotopic compositions, and their controls. We propose to document this quantification through different projects using the latest analytical developments that enable the fine isotopic analysis of some key elements in the carbon cycle.

The geochemical fluxes of continents toward the ocean are closely related to the chemical weathering of rocks and soils. Quantifying the consumption flow of CO2 by the continental weathering of the silicates of Ca and Mg is a central concern for the modelling, for any time scale, of the forces and interactions between the inner Earth and the climate. We continue the process and tracing of the alteration with the use or development of new isotopic tracers called "non traditionals" in the biogeochemical cycle (Mg, Li, K, Ca, and Si), and their coupling with more conventional tracers (Sr and Os isotopes). In particular, the study of water samples and sediments in rivers should allow us to refine the quantification of the weathering rates of silicates (versus carbonates) in areas with complex lithologies that are related to regional sites studied in the EARTH study. We benefit from an experience in the Himalayas and from a collection of samples that were taken in a variety of seasonal, lithological, and climatic contexts. We also want to focus a portion of our activities on the weathering of volcanic areas (e.g., Hawaii, the Traps of Ethiopia), which are probably the most effective in terms of CO2 consumption via the chemical weathering of basaltic or andesitic lithologies.

We also want to quantify the role of additional sources of acid, i.e., deep CO2 (metamorphic or volcanic) and sulphuric acid from the oxidation of sulphide, as an alternative source of acid for weathering. These components are still rarely taken into account in calculations of atmospheric CO2 consumption. Specific tools, such as δ13C and 14C, the Ge/Si ratio, and the isotopes of lithium, which are very specific tracers of the weathering of silicate minerals or the origin of C, or which are sensitive to reaction temperature, should help better understand and quantify these effects. To better understand the fluxes involved in the long-term carbon cycle, we want to emphasise that three new tracers are being developed at CRPG : the 40K-40Ca system, the 186Pt-186Os system, and isotopic measurements of helium in the air. The recent arrival of a next-generation TIMS at CRPG (Triton, Thermo) has enabled the development of ultra-precise measurements of Ca isotopes. This progress allows us to precisely quantify the amount of 40Ca generated from the decay of 40K (half-life of 1.25 Gy). This approach should provide new constraints on the biogeochemical cycling of calcium and, in particular, should help distinguish the flux of dissolved calcium coming from the weathering of silicates (rich in K) and coming from that of carbonate phases, which is a major problem in the carbon cycle context. The recent development of 186Os measurements, resulting from the decay of 186Pt, is expected to overcome the significant erosion contribution of a minor organic phase, black shales (which controls the 187Os-187Re system), and, therefore, to better trace the weathering of the silicate crust. We will also use the helium isotopes in the air to locate and quantify the flux of CO2 from the inner Earth to the atmosphere. Thus, a particular aspect of the helium programme is to measure the isotopic variations of this noble gas in the vicinity of volcanic provinces to quantify the cryptic volcanic degassing in key regions (Afar, Hawaii, Mount Etna). This research project is a gateway to the theme Magmas and deep fluids. It is also an integral part of the ERC NOGAT project and is supported by the Deep Carbon Observatory of the Sloan Foundation.

We also plan to look for new archives of PCO2 in geological time. This information is fundamental for a better understanding of the forcing between the climate and inner Earth during the Cenozoic period. To this end, we will test new geological archives to assess their ability to trap and preserve palaeo-atmospheres over geological time. We are also interested in tektites, in evaporites, and possibly in amber. The archives of PCO2 will be tested in modern times by normalising the CO2 content in fluid inclusions to a gaseous isotope whose past composition is fairly well known (e.g., 20Ne, 36Ar, or N2).

Another motivation for better understanding the external geochemical cycles lies in the fact that the palaeoclimatic and palaeoenvironmental reconstructions made from the isotopic signatures of sediments or marine organisms require a thorough knowledge of the ocean cycle and registration processes (biomineralisation and authigenic sediments) of the elements studied (B, Mg, Ca, Li, K, and Os). We propose to refine the quantification of inflows and outflows to the ocean (and their associated isotopic signatures) by a study (1) of detrital sediments that are exported by rivers and deposited in the estuarine areas to better quantify the importance of reverse alteration, (2) of authigenic marine sediments and/or those high in organic matter that record the isotopic variations of the ocean, (3) of hydrothermal fluids and samples of oceanic crust altered at various temperatures, and (4) to better understand how marine organisms recorded palaeo-environmental conditions. This last point will be achieved through measurements of the elementary and isotopic compositions of foraminifera and corals grown in a laboratory under controlled conditions. In particular, we will measure the isotopic composition of boron (pH marker) in coral grown at different pH values to understand their strategies of precipitation, particularly in "acid" pH values, which may be relevant to the current acidification of the oceans. It is also important to know whether these corals have, similarly to the foraminifera, a pH threshold, which could explain the different vital effects for different species of corals and thus establish a possible link between the proportions of boron incorporated as B3 and B4 in the skeleton and the pH of the solution in which the corals grew. In all of these studies, it is essential to work in collaboration with biologists who are specialised in corals and foraminifera to better understand the geochemical data in relation to the living constraints.

The Great Barrier Reef and the cliffs of Etretat, examples of biomineralisation, having a major role in the ocean-atmosphere dynamics. Foraminifera observed in a scanning electron microscope.