High resolution model for beach ridge evolution, an example during the Holocene climatic optimum from Argentina

Abstract

Beach ridges constitute landforms that provide worthy information about the littoral environment where they formed. At Bahía Samborombón area (Argentina), one of the most spectacular marine Argentinean Holocene deposits associated with the last climate optimum are preserved. Geomorphologic analyses based on satellite images allows recognizing a series of beach ridges parallel to the current coast up to around 30 km inland. Twenty detail sedimentary logs (perpendicular to main axis) were measured and sampled in order to generate an environmental sedimentary and chronostratigraphic model. In order to constrain the time of the beach ridge evolution, paleontological and taphonomical strict sampling methods combined with 14C ages were carried out. At the study area it is possible to recognize a unique beach ridge geoform, which presents internal surfaces, recognized from a 3D photogrammetric outcrop model, which can be divided into eight bodies. The first two bodies correspond to the development of a sand ridge as a response to the erosion and reworking of previous aeolian Pleistocene sediments. Subsequently, a bioclastic ridge was amalgamated with the sand ridge (bodies 3 to 8). Simultaneously to these eight bodies, in a landward position, a lagoon and washover deposits were developed. Finally, after the deposition of the ridge the continuous sea level fall generated the modern coastal plain deposits. Six 14C ages were analyzed in the main bodies, ranging between 5,240±110 BP and 3,910±90 BP. These results imply that in ca.1,330 years the beach prograded ca. 290 meters. Results from stable isotope analyses suggest that the sea surface temperature during the evolution of the chenier has two maximum values at bodies 1 and 6 (ca. 22.5 ºC), with a minimum at body 7 (18.5ºC). The salinity values range between 32.53 and 33.16?, showing little variability. We document a novel precise age-controlled model for the Holocene beach ridge evolution in the southwestern Atlantic coast. The sudden change in the composition of the ridge, from sandy (bodies 1-2) to carbonate (bioclastic in bodies 3-8) sediments, can be interpreted as a combined result of an increase of carbonate productivity along with a decrease of siliciclastic supply at the coast. This stage would have been developed approximately 5ka B.P., in coincidence with the Holocene Climatic Optimum or Mid-Holocene Thermal Maximum when the very special climatic oceanic-atmospheric conditions may have led to the proliferation of large communities of benthic organisms. In particular, bivalve mollusks, which are absent or not so abundant today in the Argentine coast. In this way, the increased extension and intensification strong activity of the Brazil current during the Mid-Holocene Thermal Maximum generated excellent conditions for the development of a carbonate warm beach similar to that occurring at tropical areas northwards nowadays. In turn, this study provides an example of the strong changes occurred in coastal environments as a result of climate change, particularly in the context of global warming episodes which characterized interglacial periods of the Quaternary in South America.