Past Hydrological Conditions in a Fluvial Valley: Records from C-O Isotope Signatures of Holocene Sediments in the Loire River (France)

Multi-proxy indices (grainsize distribution, mineralogy, δ18O, δ13C) in sediments from a meander infill in the Middle Loire alluvial plain of central France are used to highlight some aspects of the basin evolution over the period from 0 to 10,000 years BP. During the Late-Glacial and Holocene period, the lacustrine carbonate substratum of the alluvial plain was incised by the Loire River, creating numerous oxbows and channels related to meander migration. The channel fills consist mainly of fine clayey sediments deposited during flooding of the river, with an almost total absence of coarse-clastic and sandy material, except in the basal part. The record of isotope ratio variations together with the distribution of particle sizes allows the evolution of the river dynamics to be constrained. The strong decrease of carbonate δ13C in the upper part of the record is ascribed to a progressive closure of the meander and, thus, an increasing control of the C-isotope signature by biological activity in a local environment. Variations in carbonate δ18O, rather, reflect paleohydrological/paleoclimatic changes at the basin scale. The isotope record of the river dynamics also agrees with the variations in clay mineralogy.