Plains Of Argentina Research Articles

Morphotectonic Processes in Seismic Zones of Moderate Intensity, Argentina

This study deals with the neotectonic deformation in seismic zones of moderate intensity in some areas of the Sierras Pampeanas, Santa Bárbara System and Chaqueña Plain of Argentina. In the study region, the location of earthquakes of ≥ three, ≥ four, and ≥ six Richter magnitudes agree with the traces of regional faults, evidencing their neotectonic activity. Seismic energy is generally transmitted over inherited faults, such as in the Metan basin. Minor new faults occur in Quaternary terraced deposits,and others move, taking advantage of the Neogene sedimentary strata. The folds are generally buried, but the younger sequences are gently undulating. The seismic energy dissipated through fewer cohesion materials that form the valleys' fill, developing discrete fault scarps and strongly folded conglomerate strata. The foothills deposits and basins absorbed most of the seismic energy released during the reactivation of the faults. Tectonic activity is deforming 630 BP deposits in the Cumbres Calchaquíes piedmont. Considering deformation values, neotectonic deformation in the outcrops and seismic profiles evidence, it is possible to interpret that the seismic activity in the study area from the Pleistocene to the present remained between the current records. This seismic activity, sustained over time, is sufficient to produce deformation in the foothills, reactivate pre-existing faults with small displacements and generate minor new faults in recent conglomeratic deposits. Many faults are blind. However, the deformation of the landscape and the modification of the drainage network are direct indicators of the activity of these structures.

Effects of polyphenols on plankton assemblages and bacterial abundance representative of a pampean shallow lake: an experimental study

Shallow lakes are complex environments that play an important role in ecology. Their relationships with the surrounding areas make these lentic water bodies susceptible to alterations in response to human activity. For example, the input of polyphenols could be critical in altering species interactions. The shallow lakes located in the pampean plain of Argentina are major reservoirs of diversity, with the environmental habitats of the Salado River basin in particular having become impacted by human activities. The aim of this work was to evaluate the effects of different polyphenol concentrations (i.e. tannins) on changes in the plankton assemblages and on the abundance of bacteria in San Miguel del Monte Lake. Factorial experiments under laboratory conditions (4 tannin concentration levels and 4 response times) revealed that high tannin concentrations affected neither the phyto- and zooplankton assemblages nor bacterial abundances. Major changes both in assemblages and in the bacterioplankton abundances were dependent on exposure time. We found that the initial food web of grazers shifted to detritivorous feeding, thus producing a simplification of the community toward early succession. Although no effects of polyphenols were registered here, what proved striking was the resilience of the system and the capability to support high tannin concentrations throughout the experiment. Therefore, future investigations are necessary to elucidate the key role of such humic substances in the structuring of plankton assemblages of shallow lakes from strongly impacted areas, such as those found in the pampean region.

Taphonomic field experiment in a freshwater shallow lake: alteration of gastropod shells below the sediment–water interface

Abstract We conducted a taphonomic field experiment to evaluate the effects of (1) depth below the sediment–water interface (SWI) and (2) time of exposure on the accrual of damage (particularly through dissolution) to empty mollusc shells in freshwater environments. The experiment, which lasted 30 months, was carried out in the littoral area of Lake Nahuel Rucá, a shallow freshwater lake in the Pampa plain of Argentina. Bags (0.5-cm mesh) containing empty, cleaned shells of the freshwater gastropods Heleobia parchappii, Biomphalaria peregrina and Pomacea canaliculata were buried at three different depths (5, 20 and 35 cm) below the SWI. Damage was assessed every 3 months. All experimental shells exhibited fine-scale surface alteration, but the extent of this damage was relatively low, even in shells recovered after 30 months of exposure. Most of the damage consisted of minor pitting. For H. parchappii and B. peregrina, shell surface alteration varied significantly with depth, but not with time; in both species, alteration occurred mainly at the beginning of the experiment. For P. canaliculata, shell surface alteration varied significantly only with respect to time and this was the case for all three burial depths. All shells of this species exhibited a lower level of damage than what was observed for H. parchappii and B. peregrina. These differences may be related to the fact that P. canaliculata is characterized by a larger and thicker shell than the other two species. The shell attributes of larger size and greater thickness are known to confer a greater resistance to shell dissolution. In addition, the reactive surface area available for dissolution varies with shell size. Larger-sized shells have a lower potential for dissolution than smaller shells due to their lower surface-area-to-volume ratio. In contrast, species such as H. parchappii and B. peregrina, which have smaller and thinner shells, are likely to be more rapidly destroyed because they are more vulnerable to dissolution and have less preservation potential. Our results demonstrate that dissolution is a significant taphonomic process affecting shells even during burial and that the influence of dissolution on shell alteration might be significant in cases of long persistence within the taphonomically active zone. Consequently, we suggest that when working on taphofacies in the context of aquatic environments, assessments of taphonomic alteration should include changes at and below the SWI.

Decision support model for assessing aquifer pollution hazard and prioritizing groundwater resources management in the wet Pampa plain, Argentina

This paper gives an account of the implementation of a decision support system for assessing aquifer pollution hazard and prioritizing subwatersheds for groundwater resources management in the southeastern Pampa plain of Argentina. The use of this system is demonstrated with an example from Dulce Stream Basin (1,000 km(2) encompassing 27 subwatersheds), which has high level of agricultural activities and extensive available data regarding aquifer geology. In the logic model, aquifer pollution hazard is assessed as a function of two primary topics: groundwater and soil conditions. This logic model shows the state of each evaluated landscape with respect to aquifer pollution hazard based mainly on the parameters of the DRASTIC and GOD models. The decision model allows prioritizing subwatersheds for groundwater resources management according to three main criteria including farming activities, agrochemical application, and irrigation use. Stakeholder participation, through interviews, in combination with expert judgment was used to select and weight each criterion. The resulting subwatershed priority map, by combining the logic and decision models, allowed identifying five subwatersheds in the upper and middle basin as the main aquifer protection areas. The results reasonably fit the natural conditions of the basin, identifying those subwatersheds with shallow water depth, loam-loam silt texture soil media and pasture land cover in the middle basin, and others with intensive agricultural activity, coinciding with the natural recharge area to the aquifer system. Major difficulties and some recommendations of applying this methodology in real-world situations are discussed.

Heterozygosity and gene flow in populations of Calornys laucha (Rodentia, Cricetidae)

Enzyme polymorphism was studied by means of starch gel electrophoresis on five population samples of Calomys laucha collected in different sites and seasons. C. laucha, a cricetid rodent inhabiting preferentially cultivated fields in the central-eastern plains of Argentina, is a reservoir-host of Junin virus, agent of Argentine hemorrhagic fever. Results show high levels of genic variability in all the samples (P from 62.50 to 77.27; H from 0.118 to 0.163) and low genetic distance among populations (D from 0.0024 to 0.0167). Estimations of gene flow (mean Nm = 10.78) indicate a high migration capability of the species and the possibility of further dispersal of Junin virus.

Regional patterns and controls of ecosystem salinization with grassland afforestation along a rainfall gradient

Vegetation change affects water fluxes and influences the direction and intensity of salt exchange between ecosystems and groundwater. In some conditions it can also lead to an intense accumulation of salts in soils and aquifers, as has been documented for the conversion of native grassland to tree plantations in the plains of Argentina, Hungary and Russia. In this paper we present a hierarchical framework to predict salt accumulation following vegetation change that is based on climatic, hydrogeological and biological factors. We evaluated this spatially explicit framework in temperate South America using a network of 32 pairs of adjacent plantation and grassland stands studied with detailed field measurements and remotely sensed imagery from MODIS. Our sites cover a broad precipitation gradient (770 to 1500 mm a−1) and are underlain by shallow water tables (<2.5 m of depth). At the regional scale, geoelectric surveying revealed that the salinization of plantation soils depended strongly on climate, occurring only where the annual water balance (mean precipitation‐Penman‐Monteith potential evapotranspiration) was <100 mm a−1 (p < 0.0001, n = 24). At the local scale, we observed that groundwater salinities observed under ∼50‐year old plantations of different species were associated with their tolerance to salinity (p < 0.001, n = 10). Salinization occurred rapidly where rainfall was insufficient to meet the water requirements of tree plantations and where groundwater use compensated for this deficit, driving salt accumulating in the ecosystem. A general understanding of the vegetation‐groundwater relationship will help predict and manage the negative and positive consequences of groundwater use from stand to regional levels of analysis.

Effects of the Glyphosate Active Ingredient and a Formulation on Lemna gibba L. at Different Exposure Levels and Assessment End-Points

The use of formulations of the herbicide glyphosate in transgenic crops of the Pampa's plains of Argentina has extensively increased, though there is scarce information of its impact on non-target vascular plants from agro-ecosystem related surface waters. The sensitivity of a local clone of the macrophyte Lemna gibba L. to glyphosate active principle and Roundup Max formulation was studied in standardized laboratory conditions. Phytotoxic effects, considering the aquatic route, at a concentration range of glyphosate between 0.5 and 80 mg L(-1) as active ingredient during 10 days of exposure were assessed on plant population growth, frond growth, shape and number, total chlorophyll content and colony architecture. Exposure to 1 mg L(-1) of glyphosate (an expected environmental concentration) affects all the studied assessment endpoints, except for population growth and chlorophyll content. Equivalent concentrations of this herbicide as the active ingredient or RoundupMax indicate higher phytotocity of the formulation. Exposed plants at concentrations of herbicide between 1 and 7.5 mg L(-1) exhibit after two days a recovery of the multiplication rate. Frond aggregation and longer stipe was detected between 1 and 15 mg L(-1) of glyphosate, determining more open colony architecture. At higher concentrations of the herbicide fronds break-up. Comparisons with literature data indicate a higher sensitivity of the L. gibba local clone with respect to L. minor and algal species, and also a similar response to the herbicide in field experiments with the same species.

Gene flow among Calomys musculinus (Rodentia, Muridae) populations in Argentina.

Calomys musculinus is a Sigmodontinae rodent inhabiting periodically disturbed habitats in the central eastern plains of Argentina. It is the natural reservoir host of Junin virus, the etiological agent of Argentine Hemorrhagic Fever (AHF). In order to analyze the levels of gene flow among populations of this species, allozymic variability at 26 loci was studied in 291 individuals from the endemic zone of AHF and localities outside it. All populations showed high levels of polymorphism (He between 0.107 and 0.144; P95% between 38 and 54%). Individual loci f values were in most cases negative, although not significantly different from zero. Mean genetic differentiation among populations was low, but statistically significant (theta = 0.020; P < 0.01). There was no correlation between genetic and geographic distances between pairs of populations, and scatter of the pairwise points suggests that, at the regional scale, genetic drift is more influential than gene flow. This result can be interpreted as indicative of a relatively recent expansion of C. musculinus habitat and restricted on-going gene flow, which would be compatible with a relatively slow expansion rate of AHF.

Quinua and Relatives (Chenopodium sect.Chenopodium subsect.Celluloid)

Traditionally viewed as an Andean grain crop,Chenopodium quinoa Willd. includes domesticated populations that are not Andean, and Andean populations that are not domesticated. Comparative analysis of leaf morphology and allozyme frequencies have demonstrated that Andean populations, both domesticated(quinua) and free-living(ajara), represent an exceptionally homogeneous unit that is well differentiated from allied domesticates of coastal Chile(quingua) and freeliving populations of the Argentine lowlands(C. hircinum). This pattern of relationships indicates that Andean populations represent a monophyletic crop/weed system that has possibly developed through cyclic differentiation (natural vs. human selection) and introgressive hybridization. Relative levels of variation suggest that this complex originated in the southern Andes, possibly from wild types allied withC. hircinum, with subsequent dispersal north to Colombia and south to the Chilean coast. Coastal populations were apparently isolated from post-dispersal differentiation and homogenization that occurred in the Andes. Other data point toward a center of origin in the northern Andes with secondary centers of genetic diversity subsequently developing in the southern Andes and the plains of Argentina. Comparative linkage of South American taxa, all tetraploid, with North American tetraploids of the subsection will eventually clarify this problem. While the possibility of a direct phyletic connection betweenC. quinoa and the Mexican domesticate(C. berlandieri subsp. nuttalliae,) cannot be excluded, available evidence indicates that the latter represents an autonomous lineage that is associated with the basal tetraploid, C. b. subsp.berlandieri, through var.sinuatum, whereas South American taxa show possible affinities to either var. zschackei or var.berlandieri. An extinct domesticate of eastern North America,C. b. subsp.jonesianum, represents either another instance of independent domestication, possibly from subsp. b. var.zschackei, or a northeastern outlier of subsp.nuttalliae.

Allozyme Variation and Morphological Relationships of Chenopodium hircinum (s.l.)

The domesticated Andean grain chenopod, Chenopodium quinoa, is sympatric with closely related wild types that are classified as two species. Large-fruited (1.5-1.8 mm diam.), broad- leaved plants are placed with C. quinoa, as either subsp. milleanum or var. melanospermum. Plants with smaller fruits (1.0-1.5 mm) and rhombic to tri-lobed leaves are treated as Andean varieties of C. hircinum, a species originally described from the eastern plains of South America. This alignment was evaluated by comparative analysis of electrophoretic and morphometric data taken from pop- ulations occurring throughout the range of C. hircinum (s.l.). Samples from the eastern Andean slopes and plains of Argentina (C. hircinum s. str.) form a coherent group that is positioned at one extreme in the pattern of leaf and fruit variation. These samples, characterized by small, obtuse-margined fruits and acute, lobed leaves, are linked to material from the high Andes through a series of intermediate Andean samples. Andean material shows a wide range of variation in fruit size and shape. Leaf shape is more conservative, although samples from Bolivia trend toward the acute, tri- lobed condition. In contrast to morphological relationships, allozyme variation is simple and well defined. Andean material is homogeneous (I > 0.98), with unique alleles widely dispersed, mainly in southern populations. Populations of C. hircinum (s. str.) are more variable (I = 0.951) and well differentiated from the highland group (I = 0.825) by a relatively common set of unique alleles. Minimal electrophoretic differentiation among Andean samples indicates that the companion weed of C. quinoa is a single biological entity that may represent an Andean element of C. hircinum,