Claudio G. De Francesco

Distribution and abundance of hydrobiid snails in a mixed estuary and a coastal lagoon, Argentina

This study examined the environmental factors influencing the distribution and abundance of hydrobiid snails in two estuaries on the northeastern coast of Argentina in a coastal lagoon (Mar Chiquita, 37°40′S, 57°20′W) and a partially mixed estuary (Quequén Grande, 38°30′S, 58°45′W). Five intertidal study sites in each estuary represented a gradient in environmental conditions. Variations in the main environmental factors and in the abundance of hydrobiids were assessed both spatially and seasonally. The three species wereHeleobia australis, Heleobia conexa, andHeleobia parchappii, and they were primarily distributed across a salinity gradient. This pattern was clearly recognizable in the partially mixed estuary, where the abundance ofH. australis decreased as salinity decreased, and the abundance ofH. conexa gradually increased towards the inner reaches of the estuary.H. parchappii was restricted to areas far away from the influence of the tide. Slight differences in the distribution patterns of these species between Quequén Grande and Mar Chiquita were refated to the different dynamics of environmental factors in each estuary.

Paleoenvironmental reconstruction of Holocene coastal deposits from the Southeastern Buenos Aires Province, Argentina

An estuarine sequence outcropping at La Ballenera Creek (BuenosAires Province), dated between 6,800 and 4,100 14C years BP, wasinvestigated for diatoms and molluscs. The sea level history along the BuenosAires coastline has been reconstructed from discrete beach ridge sequencesalong low-lying plains. The La Ballenera profile is located on a cliffcoast where a sequence recorded environmental changes. Fifty-eightdiatom species were grouped based on their salinity and life formcharacteristics. Cluster analysis allowed the division of the sequence intothree diatom zones. Mollusc tolerances were also used to discern theenvironmental changes induced by sea-level fluctuations. The base of thesequence recorded the initiation of the marine influence about6,790–6,200 years ago. Diatom assemblages consist ofbrackish-freshwater tychoplankton accompanied by brackish benthicdiatoms. Among the molluscs, the freshwater-brackish species Heleobiaparchappii dominates. Between 6,200 and ca. 4,800 14C years BP, anestuarine lagoon environment is indicated by benthic and epiphyticmarine-brackish diatoms, as well as by the estuarine snail Heleobiaaustralis. After 4,800 14C years BP, the diatom assemblages and therelative abundance of freshwater molluscs indicate a marshy environment withlower salinity content. The La Ballenera records the salinity changes thataffect an estuary that infilled during the 2,700 years after the maximum sealevel reached in mid-latitudes of South America.

Environmental significance of freshwater mollusks in the Southern Pampas, Argentina: to what detail can local environments be inferred from mollusk composition?

The aim of the present contribution is to assess whether the abundance of mollusk species in the Southern Pampas (Argentina), an environmentally homogeneous region, reflect the local conditions of water bodies. In order to test this hypothesis, a comprehensive study was conducted in 30 sites regularly distributed across the region. At each site, the abundance of mollusk species was determined, and a series of physico-chemical measurements taken. Principal Component Analysis (PCA) and Cluster Analysis (CA) were used for the ordination of sampling sites based on the measured environmental variables. In addition, Canonical Correspondence Analysis (CCA) was conducted to explore the relationships between environmental variables and mollusk abundances. Mollusk species were represented by the gastropods Biomphalaria peregrina, Chilina parchappii, Heleobia parchappii, Physa acuta, Pomacea canaliculata, Stenophysa marmorata, Uncancylus concentricus and the bivalve Musculium argentinum. Although aquatic vegetation cover, conductivity, and substrate were among the main parameters influencing mollusk distribution, their effect was insufficient to explain the spatial distribution pattern of the species in a regional scale. It is because the Southern Pampas is a very homogeneous area, and the ranges of these environmental conditions are within the range of ecological tolerance of most of the species represented. Yet, some species resulted good indicators of environmental conditions at local (microhabitat) scale, i.e., particular microhabitats that occur in different water bodies as well. In fact, even distributed in many different water bodies along the Southern Pampas C. parchappii is always linked to lotic environments, and U. concentricus is exclusively restricted to hard substrata. On the other hand, H. parchappii is the only species represented in mesohaline waters and P. acuta appeared to be a good indicator of pollution in the area.

El registro arqueológico de la gruta de El Manzano y sus implicancias para la arqueología de Nordpatagonia

In this paper we present the results of the analysis carried out with the materials from Gruta de El Manzano archaeological site, located beside Grande River, in Malargue, southern Mendoza province. Using the new information, we discuss the site function, changes trough time, and remark his importance for the northern Patagonia discussion. The last radiocarbon data place the beginning of the occupation in more than 8.000 years BP and confirm the existence of the mid holocene regional hiatus. The anali -

DOMINANCE OF REWORKED FOSSIL SHELLS IN MODERN ESTUARINE ENVIRONMENTS: IMPLICATIONS FOR PALEOENVIRONMENTAL RECONSTRUCTIONS BASED ON BIOLOGICAL REMAINS

Abstract Death assemblages from contemporary marginal marine settings carved into ancient shell deposits are composed of fossil shells exhumed by currents or tides and shells derived from living populations. A better understanding of the bias produced by such a mixing process is of interest for studies that use modern death assemblages as analogues of similar past habitats. In order to evaluate the magnitude of reworking and redeposition of fossil shells in modern environments, a taxonomic (composition, abundance, and richness) and taphonomic (taphofacies) study was carried out in the Mar Chiquita coastal lagoon, Argentina (37°40′S, 57°20′W). The nature and extent of reworking was explored along a gradient in tidal energy from the outer to the inner reaches of the coastal lagoon. Results indicate that modern death assemblages in the lagoon are composed mostly of fossil (late Holocene) reworked shells and that reworking varies along a gradient in tidal energy, being higher in the outer reaches of the coastal lagoon, where tidal action is more significant. Temporal mixing in the coastal lagoon appears to be associated with condensation (remanié) rather than with a subtle mixing of shells, as occurs in time-averaged deposits. This reworking process leads to an abundance of old shells in modern death assemblages, which has negative consequences for their utilization as modern analogues of past lagoons. Multidisciplinary studies involving various biological indicators need to take this type of bias into consideration in order to avoid erroneous inferences on the Quaternary evolution of coastal lagoons.

Compositional Fidelity and Taphonomy of freshwater Mollusks from Three Pampean shallow Lakes of Argentina

Abstract. Three shallow lakes from the southeastern pampas, which are different in their physicochemical characteristics, were studied with the aim of comparing live/dead fidelity and preservation of mollusk shells. The studied lakes were Las Mostazas, Los Carpinchos and Nahuel Rucá, which proved different in terms of their specific richness, abundance, compositional fidelity and taphonomic alteration. The lake that presented the higher values of conductivity (3.34±0.72), hardness (286±83 mgl-1) and carbonates saturation index (SIcalcite;= 1.95±0.21) was the one for which lower specific richness in living assemblages was recorded and differences in richness and diversity between the living and the dead assemblages became apparent. It was also the lake where the number of species was higher in death assemblages (R= 5) than in living communities (R= 2). Such characteristics suggest a higher residence time in the taphonomically active zone. On the other hand, the lakes featuring lower values of the same characteristics (conductivity= 2.2±1.2; 0.8±0.5; hardness= 157±55; 250±127 mgl-1; saturation index of calcite= 0.35±0.15; 0.43+0.36, for LC and NR respectively) presented a higher species richness, lower numbers of species in death assemblages (R= 8) than in living communities (R= 10) and taphonomic attributes that suggested recently dead and deposited individuals. All these results indicate that water conditions in these lakes result in the decrease of the preservation of carbonate remains in comparison with the previously mentioned lake. Even though the lakes are located close to each other and are of similar shape and depth, such differences in water characteristics may be affecting shell preservation.