Eastern Patagonia Research Articles

Scorched mussels (BIVALVIA: MYTILIDAE: BRACHIDONTINAE) from the temperate coasts of South America: Phylogenetic relationships, trans-Pacific connections and the footprints of Quaternary glaciations

This study addresses aspects of the phylogeny and phylogeography of scorched mussels (BIVALVIA: MYTILIDAE: BRACHIDONTINAE) from southern South America (Argentina and Chile), as well as their ecophylogenetic implications. Relationships were inferred from sequences of two nuclear (28S and 18S) and one mitochondrial (COI) genes, using Bayesian and maximum likelihood analyses. Our results indicate that the monophyletic BRACHIDONTINAE include three well supported clades: [i] Brachidontes Swainson (=Hormomya Mörch), [ii] Ischadium Jukes-Browne+Geukensia van de Poel, and [iii] Austromytilus Laseron+Mytilisepta Habe (usually considered a member of the SEPTIFERINAE)+Perumytilus Olsson. Species of clade [iii] are distributed along the temperate coasts of the Pacific Ocean. Available evidence supports divergence between Austromytilus (Australia) and Perumytilus (South American) following the breakup of Australian, Antarctic and South American shelves. Four brachidontins occur in southern South America: Brachidontes rodriguezii (d’Orbigny), B. granulatus (Hanley), and two genetically distinct clades of Perumytilus. The latter are confined to the Chile-Peru (North Clade) and Magellanic (South Clade) Biogeographic Provinces, respectively warm- and cold-temperate. The South Clade is the only brachidontin restricted to cold-temperate waters. Biogeographic considerations and the fossil record prompted the hypothesis that the South Clade originated from the North Clade by incipient peripatric differentiation, followed by isolation during the Quaternary glaciations, genetic differentiation in the non-glaciated coasts of eastern Patagonia, back-expansion over southern Chile following post-LGM de-glaciation, and development of a secondary contact zone between the two clades in south-central Chile. Evidence of upper Pleistocene expansion of the South Clade parallels similar results on other organisms that have colonized coastal ecosystems from eastern Patagonia since the LGM, apparently occupying free ecological space. We emphasize that the assembly of communities cannot be explained solely in terms of environmental drivers, as history also matters.

Ecological drivers of guanaco recruitment: variable carrying capacity and density dependence.

Ungulates living in predator-free reserves offer the opportunity to study the influence of food limitation on population dynamics without the potentially confounding effects of top-down regulation or livestock competition. We assessed the influence of relative forage availability and population density on guanaco recruitment in two predator-free reserves in eastern Patagonia, with contrasting scenarios of population density. We also explored the relative contribution of the observed recruitment to population growth using a deterministic linear model to test the assumption that the studied populations were closed units. The observed densities increased twice as fast as our theoretical populations, indicating that marked immigration has taken place during the recovery phase experienced by both populations, thus we rejected the closed-population assumption. Regarding the factors driving variation in recruitment, in the low- to medium-density setting, we found a positive linear relationship between recruitment and surrogates of annual primary production, whereas no density dependence was detected. In contrast, in the high-density scenario, both annual primary production and population density showed marked effects, indicating a positive relationship between recruitment and per capita food availability above a food-limitation threshold. Our results support the idea that environmental carrying capacity fluctuates in response to climatic variation, and that these fluctuations have relevant consequences for herbivore dynamics, such as amplifying density dependence in drier years. We conclude that including the coupling between environmental variability in resources and density dependence is crucial to model ungulate population dynamics; to overlook temporal changes in carrying capacity may even mask density dependence as well as other important processes.

Ecological Correlates of Group-Size Variation in a Resource-Defense Ungulate, the Sedentary Guanaco

For large herbivores, predation-risk, habitat structure and population density are often reported as major determinants of group size variation within and between species. However, whether the underlying causes of these relationships imply an ecological adaptation or are the result of a purely mechanistic process in which fusion and fragmentation events only depend on the rate of group meeting, is still under debate. The aim of this study was to model guanaco family and bachelor group sizes in contrasting ecological settings in order to test hypotheses regarding the adaptive significance of group-size variation. We surveyed guanaco group sizes within three wildlife reserves located in eastern Patagonia where guanacos occupy a mosaic of grasslands and shrublands. Two of these reserves have been free from predators for decades while in the third, pumas often prey on guanacos. All locations have experienced important changes in guanaco abundance throughout the study offering the opportunity to test for density effects. We found that bachelor group size increased with increasing density, as expected by the mechanistic approach, but was independent of habitat structure or predation risk. In contrast, the smaller and territorial family groups were larger in the predator-exposed than in the predator-free locations, and were larger in open grasslands than in shrublands. However, the influence of population density on these social units was very weak. Therefore, family group data supported the adaptive significance of group-size variation but did not support the mechanistic idea. Yet, the magnitude of the effects was small and between-population variation in family group size after controlling for habitat and predation was negligible, suggesting that plasticity of these social units is considerably low. Our results showed that different social units might respond differentially to local ecological conditions, supporting two contrasting hypotheses in a single species, and highlight the importance of taking into account the proximate interests and constraints to which group members may be exposed to when deriving predictions about group-size variation.

Nueva asociación de mamíferos del Paleógeno en el este de la Patagonia (provincia de Chubut, Argentina): implicancias biocronológicas y paleobiogeográficas

The continental fossil mammals found in Barrancas Blancas, (near Camarones, Southeast of Chubut Province, Argentina) constitute a new faunal association from the Paleogene of Patagonia. From the base to the top, the outcrop in this locality is composed of continental and marine sedimentites, assigned to Sarmiento Formation and Chenque Formation, respectively. The fossil mammals that represent the new association (mainly Xenarthra, Notoungulata and Litopterna) come from the lower section of the Sarmiento Formation. The following taxa have been recognized: Xenarthra, Cingulata, Dasypodidae, Euphractinae, Eutatini: Meteutatus percarinatus; Euphractini: Archaeutatus sp.; Parutaetus; Prozaedyus; Peltephilidae: Peltephilus undulatus; Glyptodontidae, Glyptatelinae indet.; Cingulata incertae sedis: Machlydotherium ater; Phyllophaga: Pseudoglyptodon; Notoungulata, ‘Notohippidae’, ‘Rhynchippinae’: Rhynchippus pumilus; ‘Rhynchippinae’ indet.; Interatheriidae: Eopachyrucos pliciferus, Proargyrohyrax curanderensis; ‘Archaeohyracidae’: Pseudhyrax eutrachytheroides?; Leontiniidae: Leontinia gaudryi?; Isotemnidae indet.; Litopterna, Proterotheriidae indet. Although the presence of Leontinia and Rhynchippus might support a Deseadan age for these levels, the high diversity of xenarthrans and many groups of tipotheres (Interatheriidae and Archaeohyracidae) could indicate a Tinguirirican mammal Age. Moreover, this association could represent an equivalent fauna to that registered in levels from La Cantera (Gran Barranca), with a radiometric estimated age of ca. 30 Ma, which would indicate a post-Tinguirirican to pre-Deseadan age. From a paleobiogeographic point of view Barrancas Blancas would correspond to the most eastern Patagonian locality with Paleogene Fauna of the ages mentioned above.

On two South-West Atlantic Diastylis (Cumacea: Crustacea), D. obliquisulcata n: sp. and D. geocostae, with remarks on this speciose genus.

A new species, Diastylis obliquisulcata from eastern Patagonia, is fully described and illustrated based on specimens collected in shallow waters. Diastylis obliquisulcata shares with 14 other species of this genus an opercular maxilliped 3, a short telson and rudimentary exopods on pereopods 3 and 4 in the females. Diastylis obliquisulcata can be easily distinguished from all these species by the females having an oblique groove or sulcus on each side of the carapace. The taxonomic position of this group of species in the genus Diastylis is discussed. The adult male of Diastylis geocostae is described for the first time, the adult female briefly redescribed, and its distribution, up to now known from the type locality only (Cabo Frio, Rio de Janeiro State), is extended to off Rawson, Argentina. Within this species there are reported ontogenetic changes in the carapace ornamentation, in the chaetotaxy of the telson, and in the length ratio between the telson and the uropod peduncle.

An examination of possible relationships between paleoenvironmental conditions during the Pleistocene–Holocene transition and human occupation of southern Patagonia (Argentina) east of the Andes, between 46° and 52° S

Spatial and temporal variations in evidence for human occupation of Argentinian south Patagonia closely resemble changes in past vegetation reconstructed from pollen records. The Antarctic Cold Reversal was cold and dry in the Deseado Massif and there is little evidence of humans at this time. In contrast, the Younger Dryas interval was warmer and slightly wetter and this was when humans moved into the Deseado area. Increasing temperatures during the early Holocene brought higher precipitation to much of Patagonia east of the Andes except the northern Deseado Massif, explaining the increase in evidence of human occupation in western areas and the decrease in evidence in the massif. Spatial and temporal variations in the frequency of radiocarbon dates indicating human presence, show that in southern Patagonia humans were not only influenced by the major climate shifts of the Pleistocene–Holocene transition but possibly also by distinct short-term changes lasting only a few to several centuries that are apparent in the high-resolution Antarctic Byrd ice core. Water availability may have had the greatest influence on human use of the landscape rather than temperature. However, as temperature is crucial in controlling the latitude of the core of the Southern Westerly Winds, associated with reduced precipitation over most of eastern Patagonia, it is difficult to separate water from temperature in examining impacts on the first humans in southernmost South America.

Wind variability over central eastern Patagonia during the last 1500 years

Wind variability over central eastern Patagonia during the last 1500 years

Hydrological regime of remote catchments with extreme gradients under accelerated change: the Baker basin in Patagonia

The Baker basin (27 000 km2) is located in one of the most pristine and remote areas of the planet. Its hydrological regime is poised to undergo dramatic changes in the near future due to hydropower development and climate change. The basin contains the second-largest lake in South America, and part of a major icefield. This study documents the natural baseline of the Baker River basin, discusses the main hydrological modes and analyses the potential for sustainable management. Annual precipitation varies several-fold from the eastern Patagonian steppes to the North Patagonian Icefield. The westernmost sub-basins are strongly governed by glacier melt with a peak discharge in the austral summer (January–March). The easternmost sub-basins have a much more seasonal response governed by quicker snowmelt in spring (November–December), while they exhibit low flows typical for semi-arid regions during summer and autumn. Topography, vegetation and wetlands may also influence streamflow. The strong spatio-temporal gradients and variability highlight the need for further monitoring, particularly in the headwaters, especially given the severe changes these basins are expected to undergo. The great diversity of hydrological controls and climate change pose significant challenges for hydrological prediction and management. Editor Z.W. Kundzewicz Citation Dussaillant, J.A., Buytaert, W., Meier, C., and Espinoza, F. 2012. Hydrological regime of remote catchments with extreme gradients under accelerated change: the Baker basin in Patagonia. Hydrological Sciences Journal, 57 (8), 1530–1542.

Precipitation rather than temperature influenced the phylogeography of the endemic shrub Anarthrophyllum desideratum in the Patagonian steppe

AbstractAimIn order to assess the impact of precipitation changes during Pleistocene glaciations on plant species of the Patagonian steppe, a phylogeographical study of the endemic shrub Anarthrophyllum desideratum was performed.LocationSouthern Patagonia: Argentina and Chile.MethodsChloroplast intergenic spacers trnS–trnG and rpoB–trnC were sequenced for 264 individuals from 33 localities spanning the entire distribution of A. desideratum. Phylogenetic (statistical parsimony, maximum likelihood and Bayesian inference) and population genetic analyses (spatial analyses of molecular variance, mismatch distributions, neutrality tests and Bayesian skyline plot) were performed. Divergence time estimates using a calibrated molecular clock were also conducted. Niche modelling was used to reconstruct the palaeodistribution to validate phylogeographical patterns.ResultsThirty haplotypes were identified that clustered into two main lineages, revealing a significant latitudinal phylogeographical break north and south of the Deseado River (c. 47° S). Infra‐specific diversification began in the late Miocene, with northern and southern lineages separating c. 3 Ma, after the eastern Patagonian lowlands started to become increasingly arid. Three areas of high molecular diversity were identified: one in southern and two in northern Patagonia where niche modelling indicates that the species may have survived during the Last Glacial Maximum. These putative refugia received more moisture than much of the steppe during glaciation‐associated aridization. The south‐western refugium is the more likely source for eastward range expansion during post‐glacial humidification.Main conclusionsAnarthrophyllum desideratum responded differently to historical processes north and south of the Deseado River. In the north this species survived in situ in fragmented populations, whereas in the south it survived in localized refugia that presumably avoided extreme aridization, and from which it expanded eastwards. For southern Patagonia, our results support a new historical scenario affected more by precipitation regimes than by temperature changes associated with glacial cycles. This hypothesis should be considered in future plant phylogeographical studies from the Patagonian steppe.

Combined U-Pb SHRIMP and Hf isotope study of the Late Paleozoic Yaminué Complex, Rio Negro Province, Argentina: Implications for the origin and evolution of the Patagonia composite terrane

We have carried out zircon U-Pb SHRIMP dating and Hf isotope determinations on a biotite paraschist and on a tonalitic orthogneiss of the Yaminué Complex, and re-evaluate this complex in the broader context of the tectonic evolution of the Patagonia composite terrane. In the metasedimentary unit (msuYC), the youngest detrital zircon dated at 318 ± 5 Ma (Mississippian/Pennsylvanian boundary) indicates a Pennsylvanian (or younger) depositional age. The three main age populations peak at 474, 454 and 374 Ma. Preliminary Hf isotope data for two detrital zircons (447 and 655 Ma) yielded ε(Hf) values of −0.32 and 0.48, indicating that their primary sources contained small amounts of recycled crustal components (of Calymmian age; TDM 1.56 Ga). Zircons from the orthogneiss (miuYC; intrusive into msuYC) show a crystallization age of 261.3 ± 2.7 Ma (Capitanian; late middle Permian) which is broadly coeval with deformation, and Neoarchean–Paleoproterozoic inheritance. Meaningful core-rim relationship between Neoarchean zircon cores and late Permian rims is well defined, indicating the occurrence of Archean crust in this sector of Patagonia. Hf TDM of Permian zircons is mainly Meso-Paleoarchean (2.97–3.35 Ga), with highly negative ε(Hf) values (ca. −33). Hf TDM of inherited Neoarchean zircon cores is also Meso-Paleoarchean (3.14–3.45 Ga) but more juvenile (ε(Hf) = −0.3). Hf isotopes reinforce the presence of unexposed ancient crust in this area.Combining geological and isotope data, as well as geophysical models, we identify the Yaminué Complex within the La Esperanza-Yaminué crustal block flanked by two other, distinct crustal blocks: the Eastern block which forms part of the Patagonia terrane sensu stricto, located in the eastern Patagonian region, and the Western block forming part of the Southern Patagonia terrane. Their origins and timing of amalgamation to form the Patagonia composite terrane are also discussed.

Dining hall at sea: feeding migrations of nektonic predators to the eastern Patagonian Shelf

Seasonal changes in relative abundance and biomass of nektonic predators were analysed on the eastern Patagonian Shelf and continental slope; one of the most productive large marine ecosystems of the southern hemisphere. Several migratory types were revealed for species belonging to either temperate or sub-Antarctic faunas. Despite high productivity, only a few large nektonic predators spend their entire life cycle on the eastern Patagonian Shelf and use only a small proportion of the meso-nektonic resource. Most of the resource is exploited by non-resident nektonic migrants, which move to the area from distant spawning grounds. Pelagic and demersal sharks and skates, the squid Illex argentinus, tunas and gadoids migrate to the eastern part of the Patagonian Shelf to feed at different times of the year; arriving in seasonal waves according to their life cycle and spawning seasonality. Some deepwater fishes and squid migrate onto the shelf as juveniles to harvest the resource, and then return to deepwater habitat as adults. It is hypothesized that the large biomass of meso-planktonic and meso-nektonic consumers prevents most higher-trophic level predators from establishing spawning populations in this area, as their larvae and fry would be overwhelmed by predation. Instead, the higher-trophic level predators establish spawning and nursery grounds elsewhere and arrive to feed on the meso-planktonic and meso-nektonic resources after they have outgrown their own stages of predation vulnerability.

Eastern Patagonia Seasonal Precipitation: Influence of Southern Hemisphere Circulation and Links with Subtropical South American Precipitation

AbstractSome aspects of the seasonal precipitation over eastern Patagonia, the southernmost area of South America east of the Andes Cordillera, are examined in this paper. Results indicate that the central-north areas, the southern continental region, and the southernmost islands are three independent regions of seasonal precipitation, and that each of them is associated with specific patterns of atmospheric circulation. Precipitation over the central-north region is significantly related to the precipitation over a wide area of southern South America east of the Andes during the four seasons. Enhanced (reduced) precipitation over this area is associated with weakened (intensified) westerly flow in the region. Precipitation over the southern continental area has a close connection with the dipolar pattern of precipitation over subtropical South America during spring, summer, and autumn. The anomalies of atmospheric circulation at low and upper levels associated with the subtropical dipole are also able to modulate the intensity of the westerlies over the south of eastern Patagonia, affecting the regional precipitation. Precipitation over the islands of the southernmost part of eastern Patagonia is connected with subtropical precipitation in summer and winter. The activity of frontal systems associated with migratory perturbations moving to the east along the Southern Hemisphere storm tracks modulates the variability of seasonal precipitation over this region.

Skate assemblage on the eastern Patagonian Shelf and Slope: structure, diversity and abundance

The eastern Patagonian Shelf and continental slope of the south-west Atlantic Ocean support a high biodiversity and abundance of skates. In this study, meso-scale differences in the assemblages, spatial and seasonal distributions of skates are revealed among six habitat zones of the eastern Patagonian Shelf characterized by distinctive oceanographic conditions. Most skates belonged to temperate fauna, and their abundance was much greater in habitats occupied by temperate waters (north-western outer shelf) or mixed waters (northern slope) than in habitats occupied by sub-Antarctic waters (SASW) (south-eastern outer shelf and southern slope). Sub-Antarctic skates were not abundant on the shelf even in habitats occupied by SASW, occurring mainly in deep areas of the lower continental slope. The majority of temperate skates migrated seasonally, shifting northward in winter and spreading southward with warming waters in summer. Most temperate species had two peaks in female maturity (mainly spring and autumn) and spawned in the same habitats where they fed. It is hypothesized that the high biodiversity and abundance of skates on the Patagonian Shelf and Slope are due to the practical absence of their natural competitors, flatfishes, which occupy similar eco-niches elsewhere.

Paleomagnetism and geochemistry from the Upper Cretaceous Tres Picos Prieto locality (43ºS), Patagonian Plateau Basalts

A paleomagnetic study on nine from lavas from the Upper Cretaceous Tres Picos Prieto locality (43o50'S and 70o3'W), Patagonian Plateau Basaltic Province, revealed that a ca 300 m thick pile of basalts accumulated during three discrete (i.e., temporally separated) volcanic episodes. The chemistry of these lavas shows characteristics compatible with both intraplate and subduction-related magmatism, with the latter having a more important contribution in the younger lavas. Overall, we interpret these rocks to be transitional basalts generated in a suprasubduction environment. The Late Cretaceous to Cenozoic Patagonian Plateau Basaltic Province crops out from eastern Patagonia to the cordillera, and their genesis have been associated to different tectonic processes, even for spatially separated rocks of the same age. Considering that the development of the magmatic province is contemporary with the stage of westward drift of South America, we proposed that magma generation and upwelling due to rapidly shearing asthenosphere inducing circulatory flow associated to asperities in the lithosphere-asthenosphere interface (shear-driven upwelling) can be evaluated as a further potential mechanism to account for many outcrops of these backarc to intraplate lavas.

The ‘Rodados Patagónicos’ (Patagonian shingle formation) of eastern Patagonia: environmental conditions of gravel sedimentation

The ‘Rodados Patagónicos’ (Patagonian shingle formation) of eastern Patagonia: environmental conditions of gravel sedimentation

Uplift of quaternary shorelines in eastern Patagonia: Darwin revisited

During his journey on the Beagle, Darwin observed the uniformity in the elevation of coastal Eastern Patagonia along more than 2000 km. More than one century later, the sequences of Quaternary shorelines of eastern Patagonia have been described and their deposits dated but not yet interpreted in terms of geodynamics. Consequently, we i) mapped the repartition of the Quaternary coastal sequences in Argentinean Patagonia, ii) secured accurate altitudes of shoreline angles associated with erosional morphologies (i.e. marine terraces and notches), iii) took into account previous chrono-stratigraphical interpretations in order to calculate mean uplift rates since ~ 440 ka (MIS 11) and proposed age ranges for the higher and older features (up to ~ 180 m), and iv) focused on the Last Interglacial Maximum terrace (MIS 5e) as the best constrained marine terrace (in terms of age and altitude) in order to use it as a tectonic benchmark to quantify uplift rates along the entire passive margin of Eastern South America. Our results show that the eastern Patagonia uplift is constant through time and twice the uplift of the rest of the South American margin. We suggest that the enhanced uplift along the eastern Patagonian coast that interested Darwin during his journey around South America on the Beagle could originate from the subduction of the Chile ridge and the associated dynamic uplift.

Multiple periglacial refugia in the Patagonian steppe and post‐glacial colonization of the Andes: the phylogeography of Calceolaria polyrhiza

AbstractAim We perform a phylogeographical study of an endemic Patagonian herbaceous plant to assess whether geographical patterns of genetic variation correspond to in situ Pleistocene survival or to glacial retreat and post‐glacial expansion. We also seek to determine the locations of potential glacial refugia and post‐glacial colonization routes.Location Southern Andes and Patagonian steppe.Methods We used Calceolaria polyrhiza, a widely distributed Patagonian herbaceous plant that occurs mainly in the understorey of Nothofagus rain forests and in the arid Patagonian steppe, as our model system. The chloroplast intergenic spacer trnH–psbA was sequenced for 590 individuals from 68 populations. Sequence data were analysed using phylogenetic (maximum parsimony, maximum likelihood and Bayesian inference) and population genetic (spatial analyses of molecular variance, mismatch distributions and neutrality tests) methods. Nested clade phylogeographic analyses, and divergence time estimates using a calibrated molecular clock, were also conducted.Results A total of 27 haplotypes identified in the present study clustered into four primary genealogical lineages, revealing three significant latitudinal phylogeographical breaks. The two high Andean lineages probably split first, during the late Miocene, and the Patagonian lineage split around 4 Ma, coincident with the establishment of the Patagonian steppe. Within each haplogroup, major diversification occurred in the Pleistocene. The Patagonian groups show a pattern consistent with a rapid post‐glacial expansion and colonization of the Andean flanks, achieved independently by four lineages. The highest haplotype diversity was found along a longitudinal transect that is remarkably congruent with the limit of the ice‐sheet extension during the Greatest Patagonian Glaciation. A north‐east expansion is evident, which is probably associated with the ‘Arid Diagonal’ fluctuations.Main conclusions Glacial climate fluctuations had a substantial impact on the diversification, distribution and demography of the study species. A scenario of multiple periglacial Pleistocene refugia and subsequent multiple recolonization routes, from eastern Patagonia to the Andean flanks, may explain the phylogeographical patterns observed. However, current genetic structure also preserves the imprints of older events that probably occurred in the Miocene and Pliocene, providing evidence that multiple processes, operating at different spatial and temporal scales, have moulded biodiversity in Patagonia.

A statistical study of <i>Weinmannia</i> pollen trajectories across the Andes

Abstract. Recent airborne pollen records data from Northern Patagonia (San Carlos de Bariloche, Argentina, Lat. 41.1435° S, Long. 71.375° W, 800 m elevation) suggest that pollen transport takes place from the west to the east slope of the Andes. However, the atmospheric characteristics responsible of this transport have not yet been studied. The aim of this paper is to assess potential source areas and to describe the involved atmospheric mechanisms of the trans-Andean pollen transport. Methodology relies on the analysis of backward trajectories of air masses calculated with the HYSPLIT 4.9 regional model for particular days where airborne pollen of Weinmannia trichosperma Cav. was detected east of the Andes. This pollen type was selected because it is found regularly at localities in eastern Patagonia beyond its present-day distribution. Weinmannia's substantial presence during early Holocene times would also benefit from better knowledge of its transport mechanisms. Correspondence between atmospheric trajectories and the position of sources was checked using GIS maps. Mode T, Principal Component Analysis (PCA) with Varimax rotation was used to identify the main spatial structure of geopotential height anomalies producing the calculated trajectories. Eighty-eight cases showed that the calculated directions of trajectories trended from the Northwest to Southwest passing over the Chilean region of W. trichosperma distribution. PCs results showed two patterns of negative anomalies over southern Patagonia. The prevailing circulation pattern which drives airborne transport is the presence of a trough located south of 37 to 40° S with its axis over western Patagonia. The synoptic situations for two cases highly correlated with principal component scores were described.

Neogene uplift of central eastern Patagonia: Dynamic response to active spreading ridge subduction?

The Chile Triple Junction (CTJ) episodically migrated northward during the past 14 Ma from 54°S to its present‐day position at 46°30′S, as different almost trench‐parallel spreading segments entered in subduction. This migration resulted in the opening of an asthenospheric window below Patagonia, inducing a disturbance in the regional mantle convection. On the overriding plate, the middle Miocene time corresponds to a major change in the central Patagonian basin dynamics, with a transition from subsidence to generalized uplift. The detailed mapping and the morphological study of post‐middle Miocene terraces evidence regional‐scale trench‐parallel tilt histories that differ depending on latitude. South of 46°30′S, the slopes of the fluvial terraces indicate a change from northward to southward tilt during late Miocene. Terraces younger than the early Pliocene do not show any trench‐parallel tilt. North of 46°30′S, in contrast, only northward tilt, active until the Pleistocene, is recorded. We use a semianalytical model of dynamic topography which suggests that the northward migration of the Chile Triple Junction should be accompanied by a dynamic uplift of the central Patagonian basin. Uplift would cause both north directed and south directed tilt, north and south of the triple junction, respectively, with values of ∼0.1–0.2% 500 km east of the trench. Tilt measured in the southeastern part of the central Patagonian basin (∼0.1%) is comparable to values predicted using the semianalytical model. The dynamic topography associated to the Chile ridge subduction may have exerted a major control on the Neogene dynamics of Patagonia.

Fossil pollen grains of Asteraceae from the Miocene of Patagonia: Barnadesioideae affinity

New fossil pollen grains were recovered from marine Miocene deposits from eastern Patagonia (southern South America). Sculpture and structure exine features indicate a close relationship with modern Barnadesioideae, a basal lineage within Asteraceae. Barnadesioideae is confined to South America and is represented mainly by shrubs, herbs and some trees occurring in different habitats under a wide range of climatic conditions. It has recently attracted a great deal of attention as it was considered the sister-group to the remaining members of the family based on molecular data. Barnadesioideae has not previously been described in the fossil record. One new genus and three species are erected in Quillembaypollis gamerroi, Q. tayuoides and Q. stuessyi to assemble distinct pollen types clearly similar to those produced today by extant Chuquiraga, Dasyphyllum and Schlechtendalia, respectively. These are the first fossil records of these genera, taking them back 23–20 Ma ( Dasyphyllum and Chuquiraga types) and 11–9 Ma ( Schlechtendalia type). The new morphotaxon is clearly distinguishable by being microechinate, and by having a thick sexine formed by one ( Q. tayuoides), two ( Q. gamerroi) or three ( Q. stuessyi) layers, as the most prominent features. Their closest living relatives today grow far from the studied site (eastern Patagonia), with the exception of Chuquiraga type which is the sole surviving group in the region. Pollen and spore assemblages of Early Miocene age (23–20 Ma) from southern South America indicate that the climate was sub-humid and temperate to warm–temperate. This climatic trend may have allowed Dasyphyllum species to radiate in eastern Patagonian forests, while Chuquiraga probably occupied more open areas along the coast. Late Miocene (11–9 Ma) palynological assemblages suggest warm but seasonally dry conditions, in which Schlechtendalia developed probably in the hinterland vegetation joined with low trees, and halophytic/xerophytic shrubs and herbs.