Beach ridge evolution during the Holocene Climatic Optimum at Río de la Plata estuary, Argentina: Former answers for future questions?

Abstract

The Holocene shows rapid climatic changes associated with alternating intervals of glacier advances and retreats. The coastal regions, where beach ridges constitute common preserved landforms, are highly sensitive to register such changes and bring light into past littoral environments. Excellent marine Argentinean Holocene deposits associated with the last climate optimum are preserved at the Río de la Plata estuary (Argentina). In this beach ridge, fourteen detailed sedimentary logs were studied in order to generate an palaeoenvironmental model. The analysis of major hierarchy surfaces of the ridge was performed combined field work and Virtual-Outcrop analysis. The ridge presents internal clinoforms that subdivide it into eleven clinothems. The first two clinothems correspond to the development of a sand ridge as a response to the erosion and reworking of previous aeolian Late Pleistocene sediments. Subsequently, a bioclastic ridge was amalgamated with the sand ridge (clinothems III to XI). Simultaneously, in a landward position, a lagoon and washover deposits were developed. Finally, after the stabilization of the beach ridge the continuous sea level fall generated the actual coastal plain deposits. Six 14C ages were obtained at the base of selected clinothems, dated between 5,240 ± 110 cal BP and 3,900 ± 90 cal BP; meaning that in 1,340 years the beach prograded 290 m. Stable isotope analyses (δ13C and δ18O) allowed to infer that the temperature during the evolution of the ridge has two maximum values of 22.5 °C and a minimum of 18.5 °C. The salinity range between 32.53? and 33.16?, showing short variability. The sudden change in the composition of the ridge, from sandy to carbonate sediments, may 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 around 5ka B.P., in coincidence with the Holocene Climatic Optimum (as recorded in Argentina) when propitious climatic conditions may have led to the proliferation of large communities of warm and warm temperate benthic organisms. We stress that, the strong activity of the Brazilian current during the Mid-Holocene thermal maximum enabled excellent conditions for the development of a carbonate warm beach similar to that occurring at tropical and subtropical areas nowadays. 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 Late Quaternary in eastern South America.