Salt marsh sedimentation over the past c. 120 years along the west Cotentin coast of Normandy (France): relationship to sea-level rise and sediment supply

Natural high sediment supply to a number of macrotidal estuaries along the western coast of the Cotentin Peninsula (Normandy, France) is resulting in estuarine infilling. A previously published aerial survey in one estuary shows salt marsh laterally expanding at a rate of $4400m^ {2}/y$. Five sediment cores collected from salt marshes in three estuaries (Havre de Carteret, Havre de Surville, and Havre de Lessay) are used to investigate the relationship between lateral and vertical salt marsh accretion, and sea level. The cores are chemostratigraphically correlated with reference to is established by 210Pb and 137Cs dating of one of the cores, providing a chronologyextending back to $1880\pm23$. Through this period, mean vertical accretion rates for single cores from interior planar salt marsh surface sites at Surville and Lessay are 7.7 and 6.3mm/y respectively. Three cores from Carteret were taken from a seaward sloping salt marsh, one each from low, middle and high marsh settings, yielding mean vertical accretion rates of 5.5, 4.4, and 4.1 mm/y respectively. These data are compared with a regional rise in relative mean sea level (msl) of 3.9mm/y determined from a tide gauge on the nearby island of Jersey from 1952-2001. The high marsh at Carteret appears to be accreting relative position to msl, as there is no significant upcore change in the faunas. A core from Surville, however, indicates salt marsh emergence, demonstrated by transitions from sandflat to salt marsh faunas. The low marsh core from Cateret reveals a recent acceleration in vertical accretion (8mm/y since $1975\pm5$) and may be related to an increase in sediment availability due to the cessation of intertidal sand mining. This study demonstrates the long-term emergence of the Cotentin salt marshes, but suggests no general increase in vertical accretion despite increased sediment supply from recent mining cessation. Rather, newly available sediments support enhanced accretion along the marsh edge and rapid development of new marsh surface through lateral accretion. This study is in close agreement with a previous study of stilt marsh deposition from the macrotidal Bay of Fundy (Canada) that states high marsh accretion reflects sea-level or hydrological changes, whilst low marsh accretion is greatly influenced by changes in sediment supply. For future studies, knowledge of these relationships enables the selection of sites according to the aims of the investigation