Late Holocene record from a Loire River incised paleovalley (French inner continental shelf): Insights into regional and global forcing factors

Sediment core KV14bis was retrieved from the Fosse du Croisic, a Loire River incised paleovalley located in the inner shelf of the Bay of Biscay. This core (5.4 m length) covers the past 2.6 cal kyr BP with high sedimentation rate. A multiproxy approach (X-ray imagery, XRF data, grain size analyses, and benthic foraminiferal assemblages) was applied in order to reconstruct the hydrological regime of the Loire River during the late Holocene. This hydrological regime was further linked to climate forcing (e.g. solar activity, North Atlantic Oscillation - NAO) and human activity. Our data describe a succession of several dry/humid periods. The humid periods are centered on ~2.2, 1.3, and 0.5 cal kyr BP and are characterized by high terrigenous flux, fine silty sedimentation, relatively high organic carbon content and high proportions of the epiphytic foraminiferal species Planorbulina mediterranensis. This species was probably brought to the study site by the Loire River plume, attached on floating algae. The dry periods are centered on ~2.5, 1.5, 1.2 cal kyr BP and 1900 CE and are characterized by low terrigenous input, and high carbonate content due to abundant shell debris. During periods of low river runoff, deposition of a sand-particle population (grain size ~200 μm) could indicate increased storminess. This latter may have remobilized sediments from shallower depths during these generally dry periods on the continent. The overall coherence of our results with several European continental records supports a pluri-centennial scale regional climate control of humidity and storminess resembling that of the NAO. Spectral analyses emphasized a possible additional control of solar activity. Through deforestation and soil erosion, early human impact was also probably superimposed on this climatic forcing.