Southeastern South America Research Articles

Effect of climate on tree growth in the Pampa biome of Southeastern South America: First tree-ring chronologies from Uruguay

Tree-ring research in the highland tropics and subtropics represents a major frontier for understanding climate-growth relationships. Nonetheless, there are many lowland regions – including the South American Pampa biome – with scarce tree ring data. We present the first two tree-ring chronologies for Scutia buxifolia in subtropical Southeastern South America (SESA), using 54 series from 29 trees in two sites in northern and southern Uruguay. We cross-dated annual rings and compared tree growth from 1950 to 2012 with regional climate variability, including rainfall, temperature and the Palmer Drought Severity Index – PDSI, the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM). Overall, ring width variability was highly responsive to climate signals linked to water availability. For example, tree growth was positively correlated with accumulated rainfall in the summer-fall prior to ring formation for both chronologies. Summer climate conditions were key for tree growth, as shown by a negative effect of hot summer temperatures and a positive correlation with PDSI in late austral summer. The El Niño phase in late spring/early summer favored an increase in rainfall and annual tree growth, while the La Niña phase was associated with less rainfall and reduced tree growth. Extratropical climate factors such as SAM had an equally relevant effect on tree growth, whereby the positive phase of SAM had a negative effect over radial growth. These findings demonstrate the potential for dendroclimatic research and climate reconstruction in a region with scarce tree-ring data. They also improve the understanding of how climate variability may affect woody growth in native forests – an extremely limited ecosystem in the Pampa biome.

Seasonal Predictability of Summer Rainfall over South America

The seasonal predictability of austral summer rainfall is evaluated in a set of retrospective forecasts (hindcasts) performed as part of the Minerva and Metis projects. Both projects use the European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecast System (IFS) coupled to the Nucleus for European Modelling of the Ocean (NEMO). The Minerva runs consist of three sets of hindcasts where the spatial resolution of the model’s atmospheric component is progressively increased while keeping the spatial resolution of its oceanic component constant. In the Metis runs, the spatial resolution of both the atmospheric and oceanic components are progressively increased. We find that raw model predictions show seasonal forecast skill for rainfall over northern and southeastern South America. However, predictability is difficult to detect on a local basis, but it can be detected on a large-scale pattern basis. In addition, increasing horizontal resolution does not lead to improvements in the forecast skill of rainfall over South America. A predictable component analysis shows that only the first predictable component of austral summer precipitation has forecast skill, and the source of forecast skill is El Niño–Southern Oscillation. Seasonal prediction of precipitation remains a challenge for state-of-the-art climate models. Positive benefits of increasing model resolution might be more evident in other atmospheric fields (i.e., temperature or geopotential height) and/or temporal scales (i.e., subseasonal temporal scales).

Tracing the diversification history of a Neogene rodent invasion into South America

We investigated spatial patterns of evolutionary relatedness and diversification rates to test hypotheses about the historical biogeographic processes underlying the radiation of Neotropical rats and mice (Sigmodontinae, ~400 species). A negative correlation between mean phylogenetic distance and diversification rates of rodent assemblages reveals a pattern of species co‐occurrence in which assemblages of closely related species are also the fastest diversifying ones. Subregions of the Neotropics occupied by distantly related species that are on average more slowly diversifying include Central America, northern South America, and the Atlantic forest. In southern South America, recent species turnover appears to have been higher. Ancestral locations for the main tribes of sigmodontines were also estimated, suggesting eastern South America and the Amazonian lowlands were colonized before some central Andean regions, even though the latter are now centers of species richness for these rodents. Moreover, a past connection between the tropical Andes and the Atlantic Forest is suggested by our results, highlighting a role for a hypothetical arc connecting the two biomes, which would have impacted many other groups of organisms. Whether rapid, recent speciation in some regions is related to Quaternary climatic fluctuations and the young age of sigmodontines (~12.7 Ma crown age) or instead to intrinsic traits of these rodents remains an open question. If the former is true, we hypothesize that contrasting trends will characterize older Neotropical clades.

Modeling of Atmospheric Aerosol Properties in the São Paulo Metropolitan Area: Impact of Biomass Burning

AbstractSmoke particles ejected into the atmosphere from biomass burning can modify the atmospheric composition around and even far from the sources. In late winter and early spring, biomass burning emissions from inland regions can be efficiently transported to urban areas in southeastern South America, thus affecting air quality in those areas. In this study, the Weather Research and Forecasting with Chemistry model was applied in order to investigate the impact of biomass burning sources on aerosol loadings and properties over the São Paulo Metropolitan Area (SPMA), in southeastern Brazil, during the period from 19 August to 3 September 2014. The model performance was evaluated using available aerosol measurements from the Narrowing the Uncertainties on Aerosol and Climate Change in São Paulo State project. The combined application of aerosol data and Weather Research and Forecasting with Chemistry simulations made it possible to represent some of the most important aerosol properties, such as particle number concentration and cloud condensation nuclei activation, in addition to evaluation of the impact of biomass burning by analyzing a 5‐day transport event, from 22 to 26 August 2014. During this transport event, differences in the average predicted PM2.5 concentration reached 15 μg/m3 (peaking at 20 μg/m3 during the nighttime hours) over the SPMA, compared with 35 μg/m3 over inland areas northwest and north of the SPMA. Biomass burning accounted for up to 20% of the baseline particle number concentration‐ and cloud condensation nuclei‐weighted relative differences over the SPMA (2,300 and 1,400 cm−3, respectively).

The 920–900 Ma Bahia-Gangila LIP of the São Francisco and Congo cratons and link with Dashigou-Chulan LIP of North China craton: New insights from U-Pb geochronology and geochemistry

A widespread mafic sill and dyke unit (the Pedro Lessa suite) adjacent to the southeastern São Francisco (SF) craton (eastern South America) is dated as 940 ± 42 Ma, 912 ± 17 Ma, and 986 ± 64 Ma (based on combined ID-TIMS and LA-ICPMS analyses on baddeleyite grains) on the Santa Maria de Itabira gabbroic intrusion. These ages suggest membership in the previously defined ca. 920–900 Ma Large Igneous Province (LIP) (herein referred to as the Bahia-Gangila LIP) of the reconstructed São Francisco and formerly adjacent Congo craton (Africa). Additional mafic and felsic units throughout the SF craton are also linked (based on a literature review), thus expanding the size of the reconstructed LIP to ca. 1,500,000 km2. Geochemistry reveals high Ti OIB signature for most units and low Ti E-MORB composition for the Gangila flood basalts, marking the initiation of attempted ocean opening. Geochemistry of the Bahia-Gangila LIP is also compared with the ca. 920–900 Ma Dashigou-Chulan LIP of the North China craton, and implications for three possible reconstructions of SF/Congo craton with the North China craton are considered.

First results of energy and mass exchange in a salt marsh on southeastern South America

Salt marshes are vulnerable ecosystems since they are found in locations often preferred for urban development. They produce a great amount of biomass, and also are of great interest because of the ecosystem services they provide from which carbon storage stands out. This work aims to study the energy and mass exchanged by a salt marsh located in southeastern Buenos Aires province (Argentina) and to characterize its net ecosystem production. A field campaign was carried out from February 2014 to March 2015. Sensible heat, water vapor (H2O) and carbon dioxide (CO2) fluxes were measured with eddy covariance technique at 6 m height over a Spartina densiflora canopy. Fifty five percent of the data were lost and 15% more were discarded due to low turbulence conditions. This gaps were filled with a combination of techniques (look-up tables and mean diurnal variations), which allowed the estimation of monthly mean net ecosystem exchange, gross primary production and ecosystem respiration. As in other marshes, the latent heat flux consumed more than 55% of the available energy of the system, reaching up to 85% after a large flood event. This flux systematically exceeds the values of sensible heat measured throughout the study period. Unlike other environments, this southern salt marsh behaved as a CO2 sink throughout the year. The net ecosystem production from March 2014 to February 2015 was approximately −10.5 t of CO2 ha−1 yr−1 which is greater than reported results for other wetlands. These preliminary results for a southern S. densiflora salt marsh are encouraging, although new field studies are under way to confirm their accuracy.

Low‐level circulation and precipitation simulated by CMIP5 GCMS over southeastern South America

Global climate models (GCMs) have, in general, problems representing precipitation over southern southeastern South America (SSESA), namely southern Brazil, Uruguay and eastern Argentina. In this study, 18 models were selected from the Coupled Model Inter‐comparison Project Phase 5 to evaluate rainfall of the warm semester (October–March) when most of the rain takes place. Though GCMs reproduce the main features of the two main low‐level circulation variability modes over southeastern South America, east of the Andes, particularly the two flow patterns associated with the active and weak SACZ, most of them underestimate the northern flow that brings water vapour into SSESA, as well as the mean precipitation in this region. According to an index that stands for the northern flow towards SSESA, all models, except one, have lower northern flow into SSESA than the observed reanalysis field. In addition, this index has a significant correlation across GCMs with mean SSESA precipitation, indicating that lower northern flow into SSESA is generally associated with a greater underestimation in the model precipitation. Hence, it is concluded that the weaker northern flow into SSESA simulated by most GCMs is a cause of their simulated lower precipitation in this region.

Using Climate-Flood Links and CMIP5 Projections to Assess Flood Design Levels Under Climate Change Scenarios: A Case Study in Southern Brazil

The Intergovernmental Panel on Climate Change (IPCC) assessed with medium confidence that there has been an anthropogenic influence in the intensification of heavy rainfall at the global scale. Nevertheless, when taking into account gauge-based evidence, no clear climate-driven global change in the magnitude or frequency of floods has been identified in recent decades. This paper follows up on a previous nonstationary flood frequency analysis in the Itajai River, which is located in the Southeastern South America region, where evidence of significant and complex relationships between El Nino-Southern Oscillation (ENSO) and hydrometeorological extremes has been found. The identified climate-flood link is further explored using sea surface temperature (SST) output from CMIP5 models under different representative concentration pathway (RCP) scenarios. Results are inconclusive as to whether it is possible to make a statement on scenario-forced climate change impacts on the flood regime of the Itajai river basin. The overall outcome of the analysis is that, given that sample sizes are adequate, stationary models seem to be sufficiently robust for engineering design as they describe the variability of the hydrological processes over a large period, even if annual flood probabilities exhibit a strong year-to-year dependence on ENSO.

Historical connections among river basins and climatic changes explain the biogeographic history of a water rat

BackgroundThe water rat Nectomys squamipes (Cricetidae: Sigmodontinae) is a semiaquatic rodent from eastern South America that shows shallow genetic structure across space, according to some studies. We tested the influence of hydrography and climatic changes on the genetic and phylogeographic structure of this semiaquatic small mammal.MethodsDNA sequences of two mitochondrial genetic markers (Cyt b and D-loop) and six microsatellite loci from water rats were collected at 50 localities in five river basins in the Atlantic Forest along the eastern coast of South America. We evaluated the genetic structure within and among river basins, and we estimated divergence dates. Species distribution models for the present and past were built to identify possible gene flow paths.ResultsMitochondrial data and species distribution models showed coherent results. Microsatellite loci showed a more complex pattern of genetic differentiation. The diversification of N. squamipes haplotypes occurred during the Pleistocene and the river basin cannot explain most of the genetic structure. We found evidence of population expansion during the last glacial maximum, and gene flow paths indicate historical connections among rivers in the Atlantic Forest.DiscussionHistorical connections among rivers in the Atlantic Forest may have allowed N. squamipes to disperse farther across and within basins, leading to shallow genetic structure. Population expansions and gene flow through the emerged continental shelf during glacial period support the Atlantis forest hypothesis, thus challenging the forest refuge hypothesis.

Mantle anisotropy and asthenospheric flow around cratons in southeastern South America

Mantle anisotropy and asthenospheric flow around cratons in southeastern South America

Unraveling Deep Branches of the Sigmodontinae Tree (Rodentia: Cricetidae) in Eastern South America

Neotropical cricetid rodents of the subfamily Sigmodontinae constitute an incredibly diverse and ubiquitous clade of mammals in South America. In spite of recent advances in sigmodontine systematics, a variable collection of genera from eastern South America with no clear tribal affiliations and well-differentiated morphologies has been pervasive in most classification attempts, being generally listed as Sigmodontinae incertae sedis. Here, we assess the phylogenetic relationships of these incertae sedis genera based on supermatrix and multispecies coalescent analyses of a multilocus molecular dataset (one mitochondrial and five nuclear loci) for 76 genera, calibrated with ten fossil-based node dates. Both analytical approaches provided concordant and unprecedented resolution of suprageneric clades within the large Oryzomyalia clade (constituted by most sigmodontine genera), recovering a natural group constituted by the genera Wiedomys, Wilfredomys, and the Atlantic Forest endemics Juliomys and Phaenomys - here redefined as the Wiedomyini tribe - and a sister-relationship between this group and the tribe Abrotrichini. The enigmatic spiny rodent genus Abrawayaomys was recovered as sister to Akodontini in three of four analyses, but pending higher support. Morphological comparisons performed among the four wiedomyine genera highlighted a number of shared external, cranial, and dental similarities useful to emend the tribe diagnosis, such as the presence of chromo-genetic fields associated with the muzzle, pinnae, and rump, the long and narrow rostrum flanked by shallow and narrow zygomatic notches, and the asymmetrically divided procingulum of M1, among other characters. Molecular dating coupled with fossil evidence suggests Wiedomyini as one of the oldest sigmodontine tribes with a late Miocene autochthonous origin in eastern South America, highlighting the relevance of this region as an ancient cradle of sigmodontine lineages.

Shifts of the Brazil-Falklands/Malvinas Confluence in the western South Atlantic during the latest Pleistocene–Holocene inferred from dinoflagellate cysts

The Brazil-Falklands/Malvinas Confluence (BFMC), a highly energetic convergence of surface currents in the western South Atlantic, has shifted southward in recent years and this shift is projected to progress in the future. Palaeoecological insights documenting past changes of these currents may help in anticipating future impacts on the environment. We used dinoflagellate cyst analyses from a marine sediment core to reconstruct environmental changes in the Argentine continental margin, western South Atlantic, during the last ca. 12,600 years. The dynamics of the BFMC were reconstructed using the relative frequency of warm-water indicators for the Brazil Current (BC) versus cold-water taxa thriving in the Falklands/Malvinas Current (FMC). We found that the latitudinal position of the BFMC was relatively stable with only minor amplitude migrations between 12.6 and 8.7 cal kyr BP, followed by periods with stronger shifts to the south and the north until 0.66 cal kyr BP. After that, the BFMC shifted continuously to the south. The increase in freshwater algae abundance after ca. 5.7 cal kyr BP suggests an increase in precipitation over the adjacent Rio de la Plata drainage basin in south-eastern South America. As previously documented, such an increase in precipitation was probably related to a higher El Niño-Southern Oscillation frequency and longer, stronger El Niño events since the mid-Holocene. The dinoflagellate cyst record indicates a phase of the enhanced presence of nutrient-rich waters over the core site between ca. 6.3 and 5.7 cal kyr BP, as well as from 0.66 cal kyr BP to the recent. The highest eutrophication in the ocean surface occurred during the last ca. 100 years, most probably due to stronger human impact in the area of the Rio de la Plata drainage basin.

Taxonomic Revision of the Neotropical Genus Campuloclinium (Eupatorieae, Compositae)

The genus Campuloclinium belongs to the tribe Eupatorieae, one of the largest and most complex tribes of the Compositae family in the Neotropical Region. This genus encompasses suffruticose perennial plants and one annual herbaceous species. They are mainly distributed in southeastern South America, the Brazilian cerrado and campos rupestres being the center of diversity. It is characterized by displaying highly hemispherical or conical, glabrous, and scrobiculate receptacles, and elongate achenes with a narrowly stipitate base, five pale prominent ribs, and carpopodium expanded in a narrow basal ring. The first worldwide revision of this genus recognizing 12 species is presented here. In this new taxonomic treatment C. eiteniorum has been synonymized to C. chlorolepis, C. hatschbachii to C. irwinii, and C. hickenii to C. macrocephalum. Twenty names are lectotypified, seven new synonyms are proposed, and the required combination C. scabrum is made. Descriptions and distribution maps are provided for all the accepted species, as well as an identification key and pictures for some of them.

Mid-Pliocene global land monsoon from PlioMIP1 simulations

The mid-Piacenzian (Pliocene) warm period (approximately 3.3–3.0 Ma) monsoon not only provides us with a useful insight into the future monsoon under a warming scenario with high atmospheric CO2 concentration but is also hypothesized to play a key role in climate feedback through impact on the carbon cycle on a geological timescale. The monsoon evolution over time is one key to test this hypothesis. In this sense, we examine the mid-Piacenzian global land monsoon area (GLMA), precipitation (GLMP), and GLMP intensity (GLMPI) using all available model data from the Pliocene Model Intercomparison Project phase 1 (PlioMIP1). Relative to the pre-industrial period, both the GLMA and GLMP increase in the mid-Piacenzian and by an average of 7.3% and 5.8%, respectively. On a large scale, the increased GLMP comes mainly from northern Africa, Asia, and northern Australia. Quantitatively, it is the variation of GLMA, not of GLMPI, that makes a major contribution to the GLMP change. Further analysis reveals that anomalous inland water vapor transportation due to the reduced large-scale meridional thermal contrast in the mid-to-upper troposphere, together with the variation of vertical moisture advection and evaporation, explains most of the global land monsoon changes. A model–data comparison shows that the simulated monsoon precipitation variations are roughly consistent with the reconstructions in northern Africa, Asia, northern Australia, Central America, and eastern South America. Moreover, the intensified mid-Piacenzian Asian monsoon, which is supported by both simulations and reconstructions, or an overall intensified global land monsoon is hypothesized to contribute to the lowering of atmospheric CO2 concentration and the subsequent onset of the sustained major Northern Hemisphere glaciation.

Mapping the transmission risk of Zika virus using machine learning models

Zika virus, which has been linked to severe congenital abnormalities, is exacerbating global public health problems with its rapid transnational expansion fueled by increased global travel and trade. Suitability mapping of the transmission risk of Zika virus is essential for drafting public health plans and disease control strategies, which are especially important in areas where medical resources are relatively scarce. Predicting the risk of Zika virus outbreak has been studied in recent years, but the published literature rarely includes multiple model comparisons or predictive uncertainty analysis. Here, three relatively popular machine learning models including backward propagation neural network (BPNN), gradient boosting machine (GBM) and random forest (RF) were adopted to map the probability of Zika epidemic outbreak at the global level, pairing high-dimensional multidisciplinary covariate layers with comprehensive location data on recorded Zika virus infection in humans. The results show that the predicted high-risk areas for Zika transmission are concentrated in four regions: Southeastern North America, Eastern South America, Central Africa and Eastern Asia. To evaluate the performance of machine learning models, the 50 modeling processes were conducted based on a training dataset. The BPNN model obtained the highest predictive accuracy with a 10-fold cross-validation area under the curve (AUC) of 0.966 [95% confidence interval (CI) 0.965-0.967], followed by the GBM model (10-fold cross-validation AUC = 0.964[0.963-0.965]) and the RF model (10-fold cross-validation AUC = 0.963[0.962-0.964]). Based on training samples, compared with the BPNN-based model, we find that significant differences (p = 0.0258* and p = 0.0001***, respectively) are observed for prediction accuracies achieved by the GBM and RF models. Importantly, the prediction uncertainty introduced by the selection of absence data was quantified and could provide more accurate fundamental and scientific information for further study on disease transmission prediction and risk assessment.

SPATIAL VARIATION IN BAT SPECIES RICHNESS IN A FOREST-GRASSLAND TRANSITIONAL REGION OF SOUTHEASTERN SOUTH AMERICA

The latitudinal diversity gradient is the most well-known ecological pattern. Several hypotheses have been proposed to explain this biodiversity gradient. However, the predictors of species richness at continental scales may have different effects at regional scales, and even lose importance. Here we tested the effects of climate, energy, and habitat heterogeneity on the spatial variation of bat species richness in a forest-grassland transitional region, in southeastern South America. Our main goals were to assess which variables better explain bat species richness, and to analyze redundancy and complementarity among hypotheses. We generated three regression models, being each model related to a hypothesis, and compared R-squared among models. Then, we estimated redundancy and complementarity among hypotheses by partitioning the variation in species richness into unique and shared effects among hypotheses. Climate explained a larger proportion of the spatial variation of bat species (R² = 0.97, p < 0.0001), followed by heterogeneity (R² = 0.94, p < 0.0001), and energy (R² = 0.93, p < 0.0001), respectively. Variation partitioning analysis showed that climate explained the largest proportion of richness variation (83%). Energy and heterogeneity explained 55% and 51% of bat richness, respectively. The amount of variation explained uniquely by climate and heterogeneity were identical (R² = 0.09) whereas energy explained a small fraction of the variation (R² < 0.01). We conclude that climatic conditions coupled with habitat heterogeneity were the main predictors of bat richness in a forest-grassland transitional region and that the variables explaining regional richness gradient were the same at continental scale.

Plant diversity and community structure of Brazilian Páramos

In Eastern South America, high altitude grasslands represent a mountain system that has a high number of endemic species. However, studies on the ecology of plant communities in these environments remain scarce. We aimed to evaluate the patterns of biodiversity and structure of plant communities from rocky outcrops in high altitude grasslands of three areas at the Caparao National Park, southeastern Brazil, by sampling 300 randomly distributed plots. Then, we compared the floristic composition, relative abundance, and biological and vegetation spectra among areas. We classified species as endemic and non-endemic and verified the occurrence of endangered species. Species richness was evaluated by rarefaction analysis on the sampling units. The importance value and species abundance distribution (SAD) models were assessed. We also performed an indicator species analysis. We sampled 58 species belonging to 49 genera and 32 families. The number of species decreased with increasing altitude, with significant differences being observed among areas regarding richness, abundance, and cover. Of the total number of species, 10 are endemic to the Caparao National Park and 17 are listed on the Brazilian Red List of endangered species. The dominant families on all peaks were Asteraceae and Poaceae. The SAD models showed lognormal and geometric distributions, corroborating the fact that 10 species that were common to all three areas were also the most dominant ones in the communities and showed the highest importance values, which ranged between 35% and 60%. Indicator species analysis revealed that 28 species (48.27%) were indicators. Of these, 42.85% had maximum specificity, meaning that they occurred only in one area. Thus, the number of species per life form ratio was similar among areas, yet vegetation spectra differed, especially for hemicryptophytes. The altimetric difference among the areas showed to be a very important driver in the community assembly, influencing the evaluated variables, however, other drivers as soil depth, slope and water could also influence the community structure on a smaller and local spatial scale.

Past environmental changes during the Late Holocene sea-level fall (last 2.7 Ka) at Bahía Samborombón, NE coastal plain (Argentina)

Here we present a reconstruction of vegetation history and environmental changes at the central zone of Bahía Samborombón during the Late Holocene sea-level regressive phase (last 2.7 Ka) corresponding to MIS 1. This study is an integrative approach based on the combination of geomorphological and micropaleontological (pollen, non-pollen palynomorphs and foraminifera) proxy data. We found agreement among the environmental inferences from each proxy, which clearly show the evolution of the estuarine saltmarsh from an intertidal environment under significant marine influence towards a supratidal brackish-freshwater environment as sea-level was falling. The evidence of this evolution is (1) coastal progradation encompassing at least 5.7 km as revealed by relict shorelines. Saltmarsh elevation is another evidenced as a result of accretion due to fine sediments accumulation of at least 1.2 m thick. We posit based on these values, significant progradation and accumulation processes. (2) Plant succession, characterized by the replacement of low saltmarsh vegetation by that typical of high saltmarsh; which then is associated to brackish-freshwaters plant communities in shallow water-bodies environments. (3) A gradual decrease of dinocysts and foraminifera assemblages, the latter absent during the last millennium indicating a decrease in marine influence and the consequent lower salinity values. Bahía Samborombón coastal plains became a regional scale character during the last 2.7 Ka due to its surface increase, turning into one of the most extensive southeastern-South America saltmarshes, currently constituting a biosphere reserve.

Future hydroclimatological changes in South America based on an ensemble of regional climate models

Changes between two time slices (1961–1990 and 2071–2100) in hydroclimatological conditions for South America have been examined using an ensemble of regional climate models. Annual mean precipitation (P), evapotranspiration (E) and potential evapotranspiration (EP) are jointly considered through the balances of land water and energy. Drying or wetting conditions, associated with changes in land water availability and atmospheric demand, are analysed in the Budyko space. The water supply limit (E limited by P) is exceeded at about 2% of the grid points, while the energy limit to evapotranspiration (E = EP) is overall valid. Most of the continent, except for the southeast and some coastal areas, presents a shift toward drier conditions related to a decrease in water availability (the evaporation rate E/P increases) and, mostly over much of Brazil, to an increase in the aridity index (Ф = EP/P). These changes suggest less humid conditions with decreasing surface runoff over Amazonia and the Brazilian Highlands. In contrast, Argentina and the coasts of Ecuador and Peru are characterized by a tendency toward wetter conditions associated with an increase of water availability and a decrease of aridity index, primarily due to P increasing faster than both E and EP. This trend towards wetter soil conditions suggest that the chances of having larger periods of flooding and enhanced river discharges would increase over parts of southeastern South America. Interannual variability increases with Ф (for a given time slice) and with climate change (for a given aridity regimen). There are opposite interannual variability responses to the cliamte change in Argentina and Brazil by which the variability increases over the Brazilian Highlands and decreases in central-eastern Argentina.

Occurrence of Dicolpomys (Echimyidae) in the late Holocene of Argentina: The most recently extinct South American caviomorph genus

Echimyidae is an ancient, widely diversified hystricomorph clade whose evolutionary history is linked to the Neotropical forested areas. Its fossil record, mostly from southern South America, shows temporal stages concurrent with Cenozoic climatic trends. Fossils from the late Oligocene to middle Miocene are related to living arboreal species, whereas those known since the late Miocene are restricted to lineages currently inhabiting the Chacoan diagonal of open vegetation in eastern South America. Since the late Quaternary, echimyid faunas were already composed of currently living genera, with the only exception of Dicolpomys. Here we report the discovery of a mandibular fragment of Dicolpomys in the late Holocene of Argentina. In accordance with phylogenetic affinities, geographic distribution, and fossorial adaptations, the new record, like those of the late Miocene-Pleistocene echimyids of Argentina, can be interpreted as a southward expansion of open-adapted Brazilian lineages through the Chacoan corridor. This supports a previously proposed pattern of changes of the southern boundary of these Chacoan faunas linked to Cenozoic paleoclimatic fluctuations. The occurrence of Dicolpomys in the late Holocene represents the most recent record of an extinct South American caviomorph genus. Since the phylogenetic pattern of echimyids among Octodontoidea supports the hypothesis of the survival of lineages at lower latitudes after their extinction at higher latitudes, the absence of Dicolpomys in Brazilian sites contemporaneous with, or younger than the new Argentinian site, is here interpreted as a case of taphonomic or sampling bias.