Quaternary mollusk assemblages from the northern Atlantic coast of Tierra del Fuego (southern South America): taphonomy and paleoecology

GORDILLO, S., ISLA, F.I

Buenos Aires

Congreso; IV Regional Commitee on Atlantic Neogene Stratigraphy; 2008

Pleistocene and Holocene marine deposits along the northern Atlantic coast of Tierra del Fuego represent different transgressive-regressive phases throughout the Quaternary. Previous works in this region provided data which permitted chronological reconstruction of these sequences. They were deposited during different sea level highstands corresponding to oxygen isotopic stages (OIS) from the Middle and Late Pleistocene to the Holocene. The objective of this work is to characterize the typical mollusk assemblages developed along the northern Atlantic coast of Tierra del Fuego and, on this basis, to analyze their relationships with environmental changes during the Quaternary. Fossil bulk samples were taken from 5 localities (Laguna Arcillosa, Laguna Las Vueltas, La Sara, Río Chico y San Pablo) previously dated in age, and studied from geological and geomorphological viewpoints. Different taphonomic attributes (articulation, fragmentation, surface alteration, bioerosion and shell coloration) were recorded for each specimen from each sample, and then were averaged over the entire sample. These signatures were observed in the field and/or in the laboratory. Results show that the faunal composition of Quaternary marine terraces consists mostly of bivalve and gastropod mollusk shells. All the species recovered are extant taxa, living in the Magellan Province. Although gastropods (15 species) outnumber bivalves (5 species), bivalves are more abundant. The highest richness of gastropods and the dominance of bivalves appear to be the result of natural ecological conditions of the fauna in the region. Taphonomic alteration of these shells is related to different energies and periods of exposure within each environment. When comparing the Pleistocene assemblages with the Holocene and modern ones, some differences arise: the first ones exhibit lower diversity and are mostly represented by infaunal bivalves (venerids), while the second ones are more diverse, with more presence of gastropods, and dominated by epifaunal taxa (mytilids). They represent typical shallow benthic communities which developed within the Atlantic fuegian marine realm during the Quaternary. The fact that they are similar to the benthic communities living today in the region reinforces the theory that the climatic conditions have been maintained without significant changes since at least the Middle Pleistocene. However, the local disappearance of Retrotapes in postglacial benthic communities from the studied area is interpreted on the basis of local environmental conditions affecting wave dynamics and sediment deposition, and not to any significant climatic change.