LATE HOLOCENE RELATIVE SEA-LEVEL RISE AND THE GEOLOGICAL DEVELOPMENT OF TIDAL MARSHES AT WELLS, MAINE, USA

Tidal marshes along the Webhannet and Little Rivers of the Welts Natio nal Estuarine Research Reserve, Maine, U.S.A., are protected by barrie r beaches and underlain by a relatively thick sequence of Late Holocen e estuarine and back-barrier sediment. More than 50 cores from the sal t marshes at Wells were collected to evaluate geological and botanical development of the estuaries, and to determine the local, late Holoce ne rate of relative sea-level rise. New radiocarbon dates were employe d to construct a local relative sea-level curve. Sea level rose at an estimated rate 0.8 mm/yr at 4,000 BP, slowed to 0.4 mm/yr at 2,000 BP, and 0.2 mm/yr at 1,000 BP. Modern tide gauges in Maine began to measu re a contemporary rate of relative sea-level rise of around 1.0-2.0 mm /yr between 1940 and 1990, suggesting a much accelerated rate of rise analogous to mid-Holocene rates. During the mid-Holocene period of rap id sea-level rise, sediment collected in the deeply cut fluvial channe ls remaining from the lowstand of sea level. Interfluves of the estuar ies, floored by glacigenic sediment, were covered by tidal flat deposi ts and the low marsh plant, Spartina alterniflora. Tidal channels migr ated very slowly and remained confined to channels cut by rivers durin g the sea-level regression. By about 2,500 BP, the rate of sea-level r ise slowed, and the marsh increasingly became dominated by the high ma rsh plant, Spartina patens. Rapid contemporary sea-level rise is assoc iated with narrowing of the barrier beach and widening of tidal creeks , as the back-barrier and estuarine system adjusts to the new sea-leve l regime.