Diatom-based paleoenvironmental reconstruction from the coast of Northern Patagonia, Argentina

Abstract

Estuaries and deltas are common along modern coasts and are well represented in the geologic record because of the high preservation of their deposits. Worldwide, estuaries are more abundant than deltas because of the relatively short time available for Holocene coastal sedimentation due to sea level variations. The evolution of estuaries and deltas during the recent geological past can be reliably reconstructed. Among others, paleostudies in estuaries aim to reconstruct sea level and freshwater input variations, and the timing and frequency of extreme events (e.g., tsunamis). A significant number of studies have been published in the past 20 years, which used preserved remains of microorganisms accumulated during long intervals in sediments. The preserved record of fossil remains evidences the response of microorganisms to environmental changes at the time of their deposition. Amongst the proxies preserved in sediments, diatoms are one of the most reliable bioindicators to trace environmental changes. Due to rapid reproduction rate under favourable conditions and their siliceous frustule, they are abundant in recent and older sediments from estuaries and deltas. In this manuscript, results from previous studies were reviewed, integrated and statistically analyzed with the aim to reconstruct Holocene coastal changes in relation to sea level curves. These studies were conducted along a latitudinal transect on four estuarine mesotidal systems from Northern Patagonia, Argentina: Colorado River delta (39°40´S; 62°08´W), San Blas Bay (40°33´S; 62°13´W), Negro River (40°57´S; 62°49?W) and Chubut River estuaries (43°22´S; 65°03´W). The focus is on the preserved diatom assemblages and the paleoecological information they represent. Seven coastal sedimentary cores were correlated. Based on multivariate analysis (NMDS), fossil and modern diatom assemblages were integrated and compared. These estuary systems are dominated by littoral drift, where the plains became flooded by the sea about 7,000 years BP ago. When the sea transgression occupied the area, Colorado delta and San Blas Bay evolved from tidal channels flooded to tidal flats, sand flats and marshes during the late Holocene regression. At Colorado River, the gradually increase of the fluvial influence is associated with the eastward progradation of the deltaic lobe during the last ca. 7,000 years BP. At the Negro River estuary, a shallow vegetated brackish/freshwater environment evolved into marine conditions, tidal channels and marshes during the last 2,000 years BP. A marine coastal environment (estuary) at ca. 4,400 years BP that evolved into a salt marsh was inferred at the inlet of the Chubut River. Diatom salinity-preferences showed how connections between the wetland and the sea varied over time with sea level changes and gradual infilling. Diatom-inferred paleoenvironments and radiocarbon dates allowed reconstruction of the coastline position to the east in the late Holocene and to the west during transgressive-regressive events.