Coastal environmental changes associated with Holocene sea-level fluctuation: Southeastern Buenos Aires, Argentina

Abstract

Sea level was 2 m higher than present 6000 years ago at southeastern Buenos Aires Province, Argentina. This fluctuation has caused several environmental changes including: Drift reversal: Geomorphological features and radiocarbon dating suggest that the Mar Chiquita barrier grew from north to south during the regression from 4000 years BP. In contrast, mineral dispersals, coastal defenses, inlet migrations and wave statistics show that present beach drift is to the north, in the same direction as storm-induced shelf sediment transport. Coastal mollusk mass mortalities: Lagoonal deposits, that accumulated during the regression, contain abundant remains of Mactra isabelleana, Tagelus plebeius and Labiosa plicatella. These species are rare in present coastal lagoons. Many causes have been proposed for the absence of these species in present lagoons and include: sudden sea-level decline, outlet obstruction producing variation in salinity, and increased fluvial freshwater inputs. The regional character of these mortalities and growth curves indicates that taphonomic-feedback processes related to the scarce 2 m involved in the transgressive-regressive cycle are the reason for mollusk mortality. Sand consumption. During the latter stages of the Holocene transgression, sand ramps (up to 10 m height) at Mar del Plata cliffs, and sand barriers (up to 2 km width) north of Mar Chiquita village, developed as a result of high wave energy and a plentiful sand supply. The subsequent regressive phase produced lagoons, tidal flats, marshes and cheniers. The alongshore growth of barriers meaned their cannibalization (misfit foredunes) and narrowing driftwards. These processes occurred before the drift reversal mentioned above.