Community Replacement in Estuarine Pleistocene Deposits of Eastern North Carolina

Abstract

Nearly all examples of succession described from studies of fossiliferous sequences are, in fact, instances of a wholly different synecologic process. This process is community replacement, the gradual to abrupt substitution of one benthic community for another during changes in environmental settings. Requiring time spans intermediate between those needed for succession and community evolution, replacement yields a fossil record that can be termed a community replacement sequence. Such subevolutionary sequences arise by 1) gradual replacement, mainly involving adjustments in species-abundance distributions with some minor species turnover, caused by slight alterations in habitats; and 2) attritional replacement, dominated by species turnover in habitats subjected to more rapid and drastic environmental change. A sequence of fossil beds in the middle Pleistocene Flanner Beach Formation, Neuse River valley, North Carolina, represents a case of gradual community replacement. The community replacement sequence is reflected in the following vertical order of estuarine fossil associations within the Smith Gut and Beard Creek members. (1) open bay; (2) slightly restricted bay; (3) restricted lagoonal; (4) open lagoonal; and (5) lagoonal firm-ground. The proximal causes of replacement of the Flanner Beach communities were long-term changes in average salinity levels and intensity of seasonal environmental rigor, and, to a lesser extent, changes in substrate properties. These were ultimately related to a changing geomorphic configuration as barrier islands formed and isolated this segment of the western Atlantic coastline during an interglacial high-stand of sea level.