Isotopic study (Sr-Nd) of the origin of Holocene fine-grained deposits on the Atlantic littoral, SW France.

The fine-grained sediments of the West Gironde mud patch derive mainly from materials supplied via the Gironde estuary. Between 1500 and 1200/1300 yea rs BP and 300/400 years BP and the present day, the Sr isotopic and chemica l compositions of the estuarine silty clay inputs are the same. In contrast , these compositions were different during the period 1200/1300 to 300/400 years BP These variations probably reflect anthropogenic activities includi ng cultivation and agriculture, in the Garonne and Dordogne drainage basins during the medieval period. Modern silty clay sedimentation in the mud fla ts of the Marennes-Oleron basin and Perthuis Breton is characterised by the occurrence of important quantities (between 40 and 90 % of the total) of d etritus that issues from the Gironde estuary. The autochthonous silty clays derived from outcrops of Jurassic green marls and Flandrian deposits are m ixed with estuarine allochthonous clays delivered by coastal surface curren ts. In the Anse de l'Aiguillon, the muds are mainly composed of these autoc hthonous silty clays. The silty clay infilling of these mud flats has been highly influenced by the development of the oyster and shellfish aquacultur e. (C) Elsevier, Paris.