Insight of past hydrological conditions in Holocene fluvial sediments in the Loire River (France) through signatures of stables isotopes
Résumé
Multi-proxy indices (δ18O, δ13C, granulometry, mineralogy) in the sediments from a channel infill in the Middle Loire alluvial plain are used to highlight some aspects of the basin evolution over the period from 0 to 10000 yr. BP. The Loire River in central France is 1010 km long and drains an area of 117,800 km2. The catchment is composed of plutonic rocks (500 - 300 Ma), a large volcanic area in the upstream section and carbonate deposits (200 - 6 Ma old) in the central part. During the Late-glacial and Holocene, the lacustrine carbonate substratum in the Middle Loire alluvial plain was incised by the Loire River and numerous incisions, oxbows and channels related to meander migration have been identified. We investigated the channel fill that spans part of the Boreal, the Atlantic, the Subboreal and probably a large part of the Subatlantic through coring of sediment. The channel fill seems to consist mainly of fine clayey sediments deposited during flooding of the river; there is a total absence of coarse clastics and sandy materials in the 5m M25B core excepted in the lower part. The distribution of particle sizes shows a decrease up-core with a coarser size occurring in the first 2/3th of the record. Semi-quantitative XRD analysis of the sediment showed that quartz, K-feldspar, plagioclase and calcite are the main mineral components and that the <2µm fraction is dominated by illite-smectite and kaolinite. The distribution of the clays highlights an opposite behavior with an increase in the illite-smectite up-core. The historical record of isotope ratio variations together with the distribution of particle sizes allow to constraint the evolution of the river dynamics. The large decrease of the δ13C in the upper part of the record has been ascribed to the progressive closure of the meander and thus the control of the C isotope signature by a local environment. Furthermore, we can postulate that the variations in the δ18O are related to the origin of the water giving the signature to the carbonate and being less dependent of the meander evolution. This functioning also agrees with the variation in the clay-mineralogy.
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