Coastal deposits of Punta Hermengo, Argentina: A record of holocene sea level changes in southern South America.

ARISTEGUI, D., ESPINOSA, M.A., ISLA, F.I. & DE FRANCESCO, C.G.

Davos

Simposio; 21st. IAS Meeting of Sedimentology; 2001

International Association of Sedimentology

Holocene sea level variations on the Atlantic coast of southern South America are relatively well described. Their extension and forcing mechanism, however, remain unsolved and several hypotheses have been proposed. First step towards achieving a better understanding of these changes in sea level during the Holocene is to recognise their regional extension. In an attempt to accomplish this task, the emerging coastal pro_le of Punta Hermengo has been studied using a multiproxy approach. Located ca. 40 km south of Mar del Plata, Argentina, the sequence has a channel shape. The ca. 2 m pro_le comprises organic-rich muds at the base coarsening to laminated silty and _ne sandy sediments towards the top of the sequence. Based on the diatom content it has been interpreted as a tidal channel (dominated by brackish and planktonic taxa) in_lled into a brackish swamp (dominated mesohalobous, benthic and epiphytic diatoms). Two radiocarbon dates of 6680 _ 136 and 3395 _ 107 years BP have been obtained from bulk organic matter and Heleobia parchappii shells, respectively. The latter is the dominant gastropod present in the sequence and can tolerate a wide range of salinities. The oxygen isotopic composition of these shells, therefore, provides a valuable record of variations in salinity through time (i.e. relative sea level uctuations). _18O compositions varied from maximum values of 3.5 to 1.0 0=00 V-PDB from the top of the organic rich section and the middle of the sequence, respectively. Average values of 0.5 0=00 V-PDB characterise the rest of the pro_le and a general covariance can be observed between both _18O and _13C compositions. Furthermore, the observed variations in salinity are consistent with documented changes in diatom assemblages throughout the sequence. The sediments overlying the organic-rich deposits at the base of the sequence contain gastropod with comparatively higher oxygen isotopic compositions and diatom species indicative of a more saline environment. The interpreted conditions are followed by a sharp drop in salinity as indicated by the lowest _18O compositions that suggest an increasing contribution of fresh water to the coastal system. The regressive interval presents a higher marine inuence that continues towards the top of the pro_le. This sequence of events seems to be consistent with previous views for the region. The results of the Punta Hermengo study point towards the use of coastal sedimentary sequences to obtain records of marine-continent interactions throughout time in a high-resolution fashion.