Southwestern Buenos Aires Province Research Articles

Neogene communal rodent burrow systems from central Argentina

ABSTRACT Fossil tetrapod burrows from the Cerro Azul Formation (Late Miocene-Early Pliocene) of southwestern Buenos Aires Province, Argentina, are described and interpreted. The local sedimentary sequence is composed of massive siltstone, with two interbedded calcareous palaeosols, that were deposited in a loessic plain under a semiarid climate. The burrow systems are recognised as Maneraichnus pampeanum igen. and isp. nov., an ichnotaxon that is typified by long ramps that lead to a main chamber located at bifurcation, with chambers forming at least one-third of the total tunnel/ramp volume. A rodent producer is indicated by the presence of interconnected burrow systems with a subcircular cross-section (average RDI = 0.98) and by paired, blunt, incisor traces. The large ratio of chamber volume to tunnel/ramp volume (0.45) is indicative of a communal species and a body mass ranging from 0.9 to 4.6 kg is inferred for the producer. Considering the mentioned constrains and the fossorial mammals of the Cerro Azul Formation, it is proposed that the burrow systems belonged to Lagostomus (Late Miocene-Recent). The fossil burrows also match extant L. maximus (‘vizcacha’) burrow systems in overall architecture, passive fill, and similar size of claw traces. Miocene species of Paedotherium (Notoungulate) is discussed as an alternative potential digger.http://zoobank.org:act:C3588BE1-BAAF-46D7-92FA-0ADEF295AED3 http://zoobank.org:act:2A9F3301-9EC8-4FD1-B875-078CD9324049

The Phreatic Aquifer In The Coastal Dunes Of Southwestern Buenos Aires Province, Argentina. Factors Conditioning Groundwater Quality

In the southwest of the province of Buenos Aires, a chain of coastal dunes contains a phreatic aquifer, which is the only source of drinking water in most of the seaside cities. The objective of this study is to analyze the geological, hydrogeological and anthropic activities that affect the quality of groundwater for human consumption, between the cities of Monte Hermoso and Punta Alta. Methodologically, the research is developed based on the study of antecedent information, field recognition, well census, cutting analysis, interpretation of electrical tomography and hydrochemical data. The results relate the quality of the groundwater with the lithological and stratigraphic characteristics of the area, recognizing the natural presence of trace elements such as F, As, Fe and Mn as well as undesirable organoleptic characteristics (smell, color and taste). In addition, the occupation of the land with potentially contaminating anthropic actions (cesspool or absorbent wells, farmyards, open-air dumps, etc.) is observed. This research aims to contribute to hydrogeological knowledge, providing the conceptual basis for the exploitation and management of the aquifer, in an area where water supply is entirely based on the groundwater resource.

Geochemical, palaeontological, and sedimentological approaches of a syn-orogenic clastic wedge: Implications for the provenance of the Permian (Cisuralian) Tunas Formation, Ventania System (Argentina)

The Tunas Formation crops out in the eastern sector of the Ventania System (southwestern Buenos Aires Province, Argentina) and represents the last unit of the Pillahuincó Group (Pennsylvanian–Cisuralian). This work integrates sedimentological, palaeontological, geochemical, and geochronological studies of the uppermost levels of the unit with the aim to enhance its palaeoenvironmental and stratigraphic framework within the Southwest Gondwana context. The analysed succession, of 250 m thick, is dominated by lithic arkoses and feldspathic litharenites composed of crystalline quartz and minor feldspars and volcanic fragments, interbedded with muddy-siltstones and heterolithic deposits with plant fossils. The succession is organized in three thickening- and coarsening-upward packages that correspond to fluvio-deltaic deposits. The plant fossil assemblage includes fronds of Gangamopteris obovata, Glossopteris angustifolia, Glossopteris wilsonii, and Asterotheca sp. cf. A. andersonii, in association with articulate ribbed stems of Paracalamites australis, which are here assigned to the Gangamopteris Zone. The low plant-diversity of the Tunas Formation could be related to the change from postglacial to semiarid-arid conditions evidenced during the Cisuralian in the Southwest Gondwana region where the study area is located. The obtained geochemical results indicate low recycling and sorting of continentally derived sediments with dominant input from the upper continental crust and acidic to intermediate magmatic precursors related to an active continental margin. Previous studies pointed out that the North Patagonian Massif, located SW of the Ventania System, was the source of such materials. Additionally, new LA-ICP-MS U–Pb zircon ages from two tuff levels of the Tunas Formation were considered together with previous U–Pb zircon ages. These suggest that the source of the ash fall deposits could be as far as the pre-Choiyoi volcanism (Late Mississippian–middle Cisuralian) of the Chilean Precordillera and Frontal Cordillera and as close as the Choiyoi volcanism (middle Cisuralian–Lower Triassic) of the Las Matras and Chadileuvú blocks, all located west of the Ventania System. The integrated analyses of the available U–Pb geochronological data also allowed us to constrain the sedimentation age of the uppermost levels of the Tunas Formation to the Artinskian. However, the age interval of the stratigraphic unit should be wider.

A lizard of the genus Stenocercus (Iguania: Pleurodonta) from the Pliocene of Argentina

ABSTRACT The Pliocene record of iguanian pleurodont lizards of South America is scarce, although it is plentiful during the Miocene, including extant genera. Here we describe a partial dentary from the Pliocene Saldungaray Formation of the southwestern Buenos Aires province, Argentina. It is assigned to the extant tropidurid Stenocercus. This is the first fossil attributed to this genus and together with Uquiasaurus from northern Argentina are the only pleurodont iguanians from this age.

A teratorn (Aves: Teratornithidae) from the early late Miocene of Buenos Aires, Argentina

ABSTRACT We present new specimens of a teratorn (Aves, Teratornithidae) from the Chasicoan (early late Miocene) of southwestern Buenos Aires Province, Argentina. The specimens consist of an incomplete left ulna without its proximal end and two fragments of a radius. The ulna is very similar to, but larger than, Vultur, and its size and the relative slenderness of the distal shaft excludes it from Cathartidae and is diagnostic of Teratornithidae. Its length is very similar to Teratornis merriami, so its wingspan would have been around 4 m. Coming from sediments of the Barrancas Member of the Arroyo Chasicó Formation is older than Argentavis magnificens and so far the oldest record for Teratornithidae in Argentina.

Do disturbed environments affect density of the tunnel-web spider Acanthogonatus centralis (Mygalomorphae: Nemesiidae) from native grasslands in Argentina?

Disturbance is an important factor affecting community composition and biodiversity in natural ecosystems. The Ventania hill system in central Argentina presents several ecosystem disturbances. Spiders are good candidates for studies related to human impact and conservation, as they can be good biological indicators of certain ecosystems. Therefore, we tried to elucidate if the abundance and spatial distribution of the mygalomorph spider Acanthogonatus centralis (Nemesiidae) are affected by different degrees of disturbance in these hilly grasslands. We studied 3 sites with different levels of disturbance located in the Ventania system in southwestern Buenos Aires Province, Argentina. We did not find differences in the mean density of A. centralis among sites. The highest density of this spider species was found in the site with the highest level of disturbance. In addition, the spatial arrangement indicated an aggregate distribution. In conclusion, we found that A. centralis populations are not affected by different levels of disturbances.

Modern plains vizcacha (Lagostomus maximus, Chinchillidae, Rodentia) as a bone accumulating agent in the Argentine Pampas: Application to the study of fossiliferous sites

Lagostomus maximus (Rodentia, Chinchillidae), or Plains vizcacha, is a caviomorph rodent that inhabits semi-arid grasslands and desert scrub of central and northern Argentina, southern Paraguay and southeastern Bolivia. A particular behavior of the Plains vizcacha is the collection and transport of bones, dried dung and branches, among other objects, which are accumulated near the vizcacheras entrances. A bone concentration, originated in active vizcacheras of the southwestern Buenos Aires Province (Argentina), was studied from a taphonomic point of view for one year. The obtained results reflect that Plains vizcacha represents an important bone accumulating agent in the Argentine Pampas, and it is capable of producing large concentrations. Moreover, their vizcacheras act as natural traps that may favor the accumulation and preservation of the bone remains. The information obtained through this actualistic study provides new information on the ecology of this rodent and, in turn, introduces a framework of reference for the identification of similar concentrations in the fossil record.

Determination of optimal doses of glyphosate for controlling weeds at several stages in southwestern Buenos Aires province (Argentina)

AbstractEfficient weed management is essential for avoiding competition for water, light, and nutrient resources in semiarid zones. Chemical weed control with glyphosate was evaluated on perennial wall-rocket (Diplotaxis tenuifolia), artichoke thistle (Cynara cardunculus), slender wild oat (Avena barbata), and perennial ryegrass (Lolium perenne). Plants at early, middle and advanced vegetative stages were used in this study. Glyphosate potassium salt was applied at rates of 0.0675 (1/16x), 0.135 (1/8x), 0.27 (1/4x), 0.54 (1/2x), 1.08 (x) and 2.16 (2x) kg acid equivalent (ae) · ha−1. Glyphosate combined with 2,4-D amine salt was evaluated at rates of 1.08 kg ae · ha−1and 0.53 kg active ingredient (ai) · ha−1, respectively. The volume of the spray was 100 l · ha−1with 86 droplets · cm−2and a Volume Median Diameter (VMD) of 421.19 μm. In general, all the tested weeds were controlled with a quarter of the label rate. Three sizes of tested plants were controlled in a similar way at the same glyphosate dose rate. Moreover, the addition of 2,4-D to glyphosate did not produce an increase in the control of broadleaf weeds. The results showed that glyphosate was effective in controlling the tested weed species, including low application rates for all the growth stages in the southwestern Buenos Aires province.

The presence of the Patagonian seahorse Hippocampus patagonicus Piacentino & Luzzatto, 2004 (Teleostei: Syngnatidae) in Monte Hermoso, southwestern Buenos Aires province, Argentina

Based on data gathered during interviews with local artisanal fishermen and seahorses collected by them, Hippocampus patagonicus Piacentino & Luzzatto 2004 was found to occur in coastal waters of Monte Hermoso, southwestern Buenos Aires province, Argentina. High catches spanning several years and the identification of specimens confirm the presence of H. patagonicus in Monte Hermoso.

Estimación de la biomasa fúngica en un suelo del sudoeste de la provincia de Buenos Aires (Argentina) con una tinción directa con blanco de calcoflúor

Soil microorganisms are vital for ecosystem functioning because of the role they play in soil nutrient cycling. Agricultural practices and the intensification of land use have a negative effect on microbial activities and fungal biomass has been widely used as an indicator of soil health. The aim of this study was to analyze fungal biomass in soils from southwestern Buenos Aires province using direct fluorescent staining and to contribute to its use as an indicator of environmental changes in the ecosystem as well as to define its sensitivity to weather conditions. Soil samples were collected during two consecutive years. Soil smears were prepared and stained with two different concentrations of calcofluor, and the fungal biomass was estimated under an epifluorescence microscope. Soil fungal biomass varied between 2.23 and 26.89μg fungal C/g soil, being these values in the range expected for the studied soil type. The fungal biomass was positively related to temperature and precipitations. The methodology used was reliable, standardized and sensitive to weather conditions. The results of this study contribute information to evaluate fungal biomass in different soil types and support its use as an indicator of soil health for analyzing the impact of different agricultural practices.

Seasonal and Inter-Annual Analysis of Chlorophyll-a and Inherent Optical Properties from Satellite Observations in the Inner and Mid-Shelves of the South of Buenos Aires Province (Argentina)

The aim of this study is to describe and understand the seasonal and inter-annual physical and biological dynamics of the inner and mid shelves of the Southwestern Buenos Aires Province (Argentina). We used chlorophyll-a (chl-a) concentrations and inherent optical properties (IOPs), derived from ocean color products between 2002 and 2010, as a proxy for the physical and biological parameters of interest. This study focuses on the absorption by phytoplankton, aph(443), particulate backscattering, bbp(443), and absorption due to dissolved and particulate detrital matter, adg(443), and chl-a derived from a multiband quasi-analytical algorithm (QAA). A regionalization based on the coefficient of variation and the Census X-11 method were applied to define regions and to analyze the inter-annual and seasonal variability of the ocean color parameters, with regards to climate variability. The coastal zone presents the highest values of chl-a with two maxima in winter and autumn, while the mid-shelf shows a strong spring chl-a maximum. After 2009, all parameters under study shifted their seasonality and their magnitude changed over the entire area. In the coastal zone, mean values of aph(443) and bbp(443) increased, while in the mid-shelf, chl-a and aph(443) decreased. The observed inter-annual and seasonal behavior of the parameters is tightly related to climate variability of the study area.

Taxonomy and distribution of the sigmodontine rodents of genus Necromys in central Argentina and Uruguay

The taxonomic status of the species of Necromys Ameghino, 1889 (= Bolomys Thomas, part), inhabiting the pampean region of Argentina and southern Uruguay is not clear. The two most recent systematic hypotheses both distinguish two species. Massoia and Fornes (1967) recognized Necromys obscurus (Waterhouse, 1837) with allopatric populations in southern Uruguay and southeastern Buenos Aires Province (Argentina), and N. benefactus (Thomas, 1919) distributed in a strip across northwestern -southwestern Buenos Aires Province. Reig (1987) argued that the populations of Uruguay and northwestern Buenos Aires Province belong to N. obscurus (including N. benef actus as a subspecies), while those of the southeast and southwest are referred to an unnamed taxon. To solve this problem, 152 specimens from Argentina and Uruguay were studied using morphometric data and qualitative characters. Craniodental variables were measured in 97 adult specimens of both sexes. Discriminant-function analysis among preestablished geographic groups and cluster analysis using Mahalanobis distances revealed the presence of two groups with contrasting morphological characteristics. The first group, which corresponds to N. benefactus, includes the populations from northern La Pampa, southern Santa Fe, and northwestern, centralwest, and southwestern Buenos Aires Province (Argentina). The second group, N. obscurus, includes the populations from Uruguay (N. obscurus obscurus) and from southeastern Buenos Aires Province (a new subspecies).

Paleovegetational and paleoclimatic changes during the Late Quaternary in southwestern Buenos Aires Province and Southern Tierra del Fuego (Argentina)

Pollen analysis from a sedimentary section in the valley of the Sauce Grande River (southwestern Buenos Aires Province) and a peat core from the Andorra Valley (southern Tierra del Fuego) provides evidence for the apparent large‐scale synchroneity of major past vegetational changes and inferred climatic trends during the late Pleistocene and Holocene in this region. Climate before 10,000 years B.P. was relatively cold and dry, both in southwestern Buenos Aires Province and in southern Tierra del Fuego. A climatic amelioration to wanner and wetter conditions during the late Pleistocene and early Holocene is recorded in both regions. Late Holocene climates show greater variability and they suggest the onset of modern environmental conditions as represented by cool and wet conditions in southern Tierra del Fuego, and by arid to semi‐arid conditions in southwestern Buenos Aires Province.

Sigmodontinos (Mammalia, Rodentia) pleistocenicos del sudoeste de la provincia de Buenos Aires (Argentina): aspectos sistematicos, paleozoogeograficos y paleoambientales

Severa1 sigmodontine remains from Ensenadan sediments (Early-Middle Pleistocene) of Bajo San Jose (southwestern Buenos Aires province) are described. The oldest records of Lundomys (Tribe Oryzomyini), Oxymycterus (Tribe Akodontini) and Phyllotis (Tribe Phyllotini) were recovered, together with Akodon cf. A. iniscatus, Akodon cf. A. azarae, Akodon sp., Reithrodon auritus, and Calomys cf. C. laucha-C. musculinus. Paleoenvironmental and paleozoogeographic data provided by these sigmodontines, together with those of &he remaining vertebrate fauna, prove a mixture of Brazilian and Patagonian species. The generic-specific composition characteristic of the modern cricetid associations of the Pampean Region was settled during the Ensenadan Age.