DIATOM AND SEDIMENTARY RECORD DURING THE HOLOCENE EVOLUTION OF THE DESEADO RIVER ESTUARY, SOUTHERN PATAGONIA, ARGENTINA

FAYÓ ROCÍO; ESPINOSA MARCELA A.; VELEZ AGUDELO CAMILO; ISLA FEDERICO I.

La Plata

Congreso; XVIII Reunión Argentina de Sedimentología IX Congreso Latinoamericano de Sedimentología; 2023

Asociacion Argentina de Sedimentologia

Diatoms have been successfully used as proxies for Holocene paleoenvironmental reconstructions in numerous coastal evolution studies worldwide. A recent review and analysis of modern and fossil diatom assemblages in Northern Patagonia (Argentina), showed that tidal effects are very important factors in the distribution of species in estuarine environments. This analysis identified characteristic tidal and non-tidal diatom assemblages. In order to reconstruct environmental conditions in response to Holocene sea level changes, diatom assemblages from a coastal sedimentary succession were studied in the macrotidal estuary of Deseado River in Southern Patagonia, Argentina. Twenty-one samples were selected from a 200˗cm core taken at Cañadón La Mina (47°46’49.4” S, 66°4’59.5” W). These gullies were excavated in volcanic rocks and infilled during the initial stages of the Holocene transgressive-regressive cycle, with a significant aeolian supply. The chronology was established based on two radiocarbon dates of bulk samples, taken at depths of 55 cm and 164 cm, in the DIRECT-AMS Laboratory (USA). Radiocarbon ages obtained were 5,791±30 14C years BP (6,450-6,655 cal. years BP) and 8,145±35 14C years BP (8,982-9,141 cal. years BP) respectively. Gray muds were the dominant sediment in the core. The clay and fine sand were more abundant grain sizes composing laminations and lenticular beddings. Towards the top the presence of gravel was identified. Based on the sediments thickness and the temporal range covered in the upper section of the core, the presence of an erosive discordance was recognized. Preliminary diatom analysis showed the dominance of coastal marine assemblages typical of tidal environments, including marine tychoplankton such as Paralia sulcata, Cymatosira belgica and Raphoneis amphiceros. Additionally, accompanying flora included epiphytes such as Epithemia sorex, Shionodiscus oestrupii, Dimeregramma minor, and brackish-freshwater taxa such as Cocconeis placentula and Amphora spp. The presence of small ´fragilarioids´ were observed near the top. The site was significantly affected by Holocene sea-level changes during the last ca. 8,000 14C years BP. During the Late Holocene regression, the tidal channels were infilled with the emplacement of tidal flats and marshes with a low-vegetation cover.