Sandra Gordillo

Quaternary of Tierra del Fuego, Southernmost South America: an updated review

Abstract The Quaternary of Tierra del Fuego is represented by glacial, glaciofluvial, glaciolacustrine, marine and aeolian deposits. Six drift units have been described, the oldest dating from the Late Pliocene: these are, starting from the oldest, the Rio Grande, Sierra de los Frailes, Cabo Virgenes, Punta Delgada, Primera Angostura and Segunda Angostura Drifts. Neoglacial and “Little Ice Age” events are represented in cirques and higher mountain valleys. Marine deposits and raised beaches were formed during Middle Pleistocene, Late Pleistocene and Holocene interglacial stages. After the definitive ice-retreat (10xa0ka ago) vegetation changed from tundra and cold steppe to subantarctic forest environments. Marine deposits reflect also colder conditions than the present ones. Aeolian processes prevailed in northern Tierra del Fuego, where semiarid conditions and the frequent westerlies favoured the retransportation of finer materials coming from pre-existing deposits. Before the opening of the Magellan Straits, earliest human colonization occurred in northern Tierra del Fuego ca. 11xa0ka BP, in tundra-like environmental conditions. Pedestrian hunters of camelids and foxes co-existed with Pleistocene fauna that became extinct during Late Glacial–Earliest Holocene times. The steppe area (inland and Atlantic coast) was successively occupied since then until recent times. On the other hand, the Beagle Channel coasts were occupied since 6xa0ka BP by hunter–gatherer groups adapted to maritime littoral conditions. Finally, the easternmost area of the island was inhabited at least since 1.5xa0ka BP. The human settlements in these latter areas occurred under environmental conditions similar to the present ones, when the Fuegian forest was definitively established.

Holocene raised beaches along the northern coast of the Beagle Channel, Tierra del Fuego, Argentina

Abstract Raised beaches developed during Holocene times along the northern Beagle Channel coast. These deposits contain a rich marine fauna, especially shelled organisms such as mollusks. Four Holocene marine sites along the northern coast of the Beagle Channel have been analysed; from west to east they are: Bahia Ensenada, Playa Larga, Bahia Brown and Cutalataca airfield. They were studied in relation to relative altitude, sediment type, faunal composition and radiocarbon dates. The area is characterized by several discontinuous terraces with elevations varying from about 1.5 to 10 m above sea level. Their possible origin (1) due to tectonic uplifting, in comparison with raised beaches of the northern part of Tierra del Fuego, and (2) resulting from glacio-isostatic recovery, taking into account the ice retreat, is discussed. Seventeen species of mollusks (eight bivalves, nine gastropods) were collected from the study sites. All of them are living species from the Beagle Channel and have been interpreted as representative of Holocene paleocommunities in this channel. These species show no evidence of substantial climatic changes during the Holocene, although minor temperature fluctuations cannot be ruled out yet.

Late quaternary evolution of a subantarctic paleofjord, Tierra del Fuego

Lago Roca-Lapataia valley (54°50′S, 68°34′W) is a paleofjord that was occupied by a valley-glacier system during the glacial maximum of the late Pleistocene (estimated ca. 18–20 ka BP). Deglaciation began before 10,080 ± 270 BP. The marine fauna in several marine terraces found in the area shows that early-middle Holocene climatic conditions were basically the same as at present. Species found are characteristic of cold and shallow waters, although minor temperature fluctuations cannot be ruled out for this period. A recent radiocarbon date of 7518 ± 58 BP on Chlamys patagonica (NZ # 7730) confirms that Lago Roca was transformed into a fjord ca. 7500–8000 BP. The sea reached its maximum level of 8–10 m a.s.l. around 6000 BP and at 4000–4500 BP was at least above 6 ± 1 m a.s.l. Later, when sea level fell, Lago Roca was occupied by fresh water and was no longer tidal. The relative land-sea positions during this period are a consequence of combined eustatic and neotectonic processes.

Holocene (~4.5–1.7 cal. kyr BP) paleoenvironmental conditions in central Argentina inferred from entire-shell and intra-shell stable isotope composition of terrestrial gastropods

The isotopic fingerprint of terrestrial gastropods has been increasingly used as a credible natural paleoenvironmental archive. Most published work has used this proxy at tropical and temperate latitudes of the Northern Hemisphere, and focused on entire-shell analysis. The present study provides entire-shell and intra-shell isotopic profiles to infer average and seasonal late Holocene environmental conditions in central Argentina (30°S). Shells of Plagiodontes daedaleus (Gastropoda: Odontostomidae) were retrieved from the Alero Deodoro Roca–Sector B site, one of the few archaeological sites in central Argentina rich in shells collected by pre-Hispanic hunter-gatherer groups. Ancient entire shells exhibited values that were ~2.5‰ higher in δ13C and ~1.8‰ higher in δ18O than modern individuals, pointing to higher abundance of C4 plants and overall drier conditions (lower relative humidity and/or higher rain δ18O) during 4.5–1.7u2009cal.u2009kyr BP than today, in agreement with published regional proxies. Intra-shell isotopic profiles suggest that modern and fossil specimens deposited their shells throughout two-to-three summer/winter cycles. Intra-shell δ18O values varied ~5‰, matching with the seasonal variation of rain δ18O values. The extent of seasonality was similar during 4.5–1.7u2009cal.u2009kyr BP and today. Intra-shell δ13C values varied ~2–3‰ and did not portray distinct seasonal cycles, depicting minimal seasonal variations in the snail diet. This work illustrates that South American terrestrial gastropods have great potential for paleoenvironmental studies.

MURICID DRILLING PREDATION AT HIGH LATITUDES: INSIGHTS FROM THE SOUTHERNMOST ATLANTIC

ABSTRACT Drilling predation is frequently studied in the fossil record. Less information is available from recent environments, however. Previous studies have indicated that drilling predation is usually higher in the tropics but little research has been undertaken in high latitudes. To address this hypothesis, we examine muricid-drilling predation along a 1,000 km transect in southern South America. Drilling frequencies ranged between 3% and 36%, and they were not correlated with the abundance of the predator (Trophon geversianus) or the abundance of its preferred prey. The only locality with exceptionally high predation (36%) was a heavily anthropogenically impacted site. Trophon exhibited different drilling strategies on different prey, and edge drilling represented 27%–56% of the drill holes in mytilids. Drilling frequencies were not correlated with latitude or water temperature. Our results, however, show that drilling frequencies are indeed lower at high latitudes compared to the tropics, and these data provide a recent baseline to compare and interpret spatial variability in muricid drilling predation from past environments. The fact that dead-shell assemblages seem to be recording human-related impacts in this system strengthens their relevance as potentially valuable conservation tools.

REGIONAL-SCALE COMPOSITIONAL AND SIZE FIDELITY OF ROCKY INTERTIDAL COMMUNITIES FROM THE PATAGONIAN ATLANTIC COAST

Abstract The use of rocky intertidal assemblages in paleoecology and conservation paleobiology studies is limited because these environments have low preservation potential. Here, we evaluate the fidelity between living intertidal mussel bed communities (life assemblages or LAs) and mollusk shell accumulations (death assemblages or DAs) from the environmentally harsh Patagonian Atlantic Coast. LAs were sampled from rocky mid-intertidal and mussel-dominated habitats while DAs were collected from the high water mark at beaches in close proximity to the living intertidal community to assess live-dead mismatch at regional scales. DAs were restricted to the subset of species in the DAs that inhabit rocky intertidal habitats. A total of 37,193 mollusk specimens from 15 intertidal species were included in the analysis. Ten species were present in LAs, 14 in DAs, and nine were shared by LAs and DAs. DAs showed higher diversity, less dominance, and more rare species than LAs. Despite finding good agreement in species composition between DAs and LAs within the same region, smaller species are underrepresented, as shown by differences in size-frequency distributions. Our findings indicate that the composition of DAs is a result of the combined effects of spatial and temporal averaging, size-related biases, and biases related to low detectability of boring and vagile species in LAs. Thus, DAs do not accurately detect within-provincial latitudinal gradients in composition. However, DAs clearly capture differences between the Argentine–Magellanic Transition Zone and the Magellanic Province, indicating that DAs are informative tools at regional scales despite the environmental harshness to which they are subjected.

Form changes in Amiantis purpurata (Bivalvia, Veneridae) shells over the past 100,000 years in North Patagonia (Argentina)

Fil: Bayer, Maria Sol. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Cordoba. Centro de Investigaciones en Ciencias de la Tierra. Universidad Nacional de Cordoba. Facultad de Ciencias Exactas Fisicas y Naturales. Centro de Investigaciones en Ciencias de la Tierra; Argentina

Moluscos en contextos arqueológicos de la costa atlántica de Tierra del Fuego: consumo prehistórico e implicancias de su distribución actual

Este trabajo analiza informacion arqueomalacologica proveniente de 3 sitios –dos concheros (tambien denominados conchales) y un sitio a cielo abierto– con fechados radiocarbonicos del Holoceno medio y tardio, ubicados en la estepa fueguina al norte de la ciudad de Rio Grande. Se compara esta informacion con la fauna malacologica procedente de otros sitios arqueologicos del norte de la Isla Grande de Tierra del Fuego y con los moluscos que habitan actualmente en la region. Para verificar la distribucion actual se realizaron seis muestreos en las inmediaciones de las localidades arqueologicas mejor conocidas para el norte de la isla. Se realizan interpretaciones sobre la distribucion y paleoecologia de diversas especies de moluscos y su potencialidad como recurso economico en el pasado para los cazadores recolectores pedestres de la isla.

Benthic mollusc assemblages in West Antarctica: taxa composition and ecological insights

Although different studies in Antarctica have dealt with benthic communities, few studies have focused on molluscan assemblages and their ecology. During the austral summer of 2011, 17 stations between depths of 68.5 and 754m were sampled in West Antarctica using a demersal bottom trawl pilot net on board RV ARA Puerto Deseado. In all, 1848 specimens of shelled molluscs were recorded. Gastropods were the most diverse group (species richness=74) and bivalves were the most abundant (n=1344). Shannon–Wiener diversity index values ranged between 0.58 and 2.99, with great variation at different stations. Cluster analysis using the Bray–Curtis coefficient showed three distinct assemblages types: one dominated by suspension feeders; a second with representatives from different trophic groups, including suspension feeders, grazers, scavengers, predators and deposit feeders; and a third, more differentiated, with few taxa and dominated by deposit feeders. Finally, multivariate analysis suggests that bivalves were more sensitive to temperature, whereas gastropods were more sensitive to depth.

Late Quaternary Faunal Changes in Northeastern Patagonia (Argentina) according to a Dynamic Mosaic of Benthic Habitats: Taphonomic and Paleoecological Analyses of Mollusk Assemblages

Abstract. The aim of this study is to describe and interpret the paleoenvironmental history of the San Matías Gulf (SMG), in northern Patagonia, Argentina, which is associated with possible biotic and abiotic changes that occurred during the late Quaternary. In this regard, a taphonomic (disarticulation, right/ left valve ratio, fragmentation, abrasion, teeth preservation) and paleoecologic (alpha diversity, species abundance, life habit, substrate preference, feeding mode) analysis of Amiantis purputata (the target species) and its accompanying faunal remains (the non-target species) in modern, Holocene and Pleistocene shell assemblages, was performed. The general trend of the SMG throughout the late Quaternary was that of a low energy environment with varying water energy intensity depending on the study area. Additionally, each study site contains different substrates marked by patches of sand and rock which would have also determined the presence of certain species and, in turn, the proportion of sandy and rocky patches may have changed over time thus leading to the development of different local paleocommunities. Therefore, this gulf presents a dynamic mosaic of environments over time. Its waters would have been sufficiently rich in nutrients to allow the development of the filter feeding fauna which dominated the late Quaternary, especially A. purpurata. Crepidula species, which feed in two distinct ways (herbivore and filter feeding), increased in proportion throughout the Holocene. These changes, among others, coincided with the Last Glacial Maximum, the final configuration of the SMG, the Climatic Optimum of the Holocene and the impact of humans in the area.