Marcelo Zárate

Loess of southern South America

Abstract Loess and loessoid (loess-like, reworked loess) sediments extend across the Chaco–Pampean plains and the NW mountain environments of Argentina, and in neighbouring countries (Paraguay, Brazil, Uruguay, Bolivia). Loessoid sediments are much more abundant than primary loess. The beginning of the loessoid sedimentation cycle was related to a phase of Late Miocene orogeny in the Andes. The Plio-Pleistocene record is mostly composed of loessoid sediments modified by pedogenesis, which produced welded palaeosols. The Late Pleistocene/Holocene loess record reveals a heterogeneous composition across the region. Coarse textures and an Andean-derived volcanic composition prevail in the southern Pampas. Finer textures and material coming from the Andes and the other two sources (Sierras Pampeanas and the Parana basin) characterize the loess deposits of the northern Pampas and the eastern Chaco. A southern Patagonian source is also suggested for the mountain valley loess of Tucuman, although a western Andean provenance has been proposed. It is believed that the material in the western Chaco was derived from the Bolivian Andes. Loess deposition was related to a multistage transport mechanism, involving fluvial and aeolian processes. Inferred westerly and southwesterly wind directions, as dominant carriers of the aeolian deposits, are in agreement with westerly palaeowind simulations using climate models. However, the role played by westerly tropospheric winds and northerly winds remains to be established.

Late Pleistocene-Holocene eolian deposits of the southern Buenos Aires province, Argentina: A preliminary model

Abstract The aim is to reconstruct the sedimentological history of the Late Pleistocene-Holocene eolian deposits of southern Buenos Aires province. Hence, a preliminary model is proposed in order to analyze the different stages of the sedimentological cycle. The deposits include sandy loess, fine sand sheets and dunefields. The southern Buenos Aires province represents the depositional area, whereas the source and transport environments are located in the northern Patagonian Andes and northern extra-andean Patagonia. According to the mineralogical characteristics of the material, explosive volcanism is considered the predominant process of particle formation, particularly as generator of pyroclastic deposits which were reworked by different erosive processes (glacial, fluvial, eolian). During the Late Glacial Maximum and the Holocene, alluvial deposits which were mainly generated by fluvial erosion of volcaniclastic sedimentary units, were deposited along the floodplains of the Colorado and Negro rivers, including those distal segments located in the emerged offshore platform. The fluvial regime depended upon the regime of the glaciers, hence it reflected the major glacial fluctuations which took place in the Andes. Floodplains were deflated by southwest winds, transporting the material towards the southern Buenos Aires province (sandy loess and loessial sands) and to La Pampa province (dunefields).

Isotopic constraints on the source of Argentinian loess – with implications for atmospheric circulation and the provenance of Antarctic dust during recent glacial maxima

We present rare-earth element (REE) and Sr–Nd isotopic data for Argentinian loess (28–38°S) with two aims: (1) to examine the source regions of Argentinian loess and the constraints that these put on palaeo-wind directions; (2) to further investigate the source of Antarctic ice-core dust and to test the hypothesis that some of it could be derived from a region to the north of Patagonia – into which the dry, dusty, westerly dominated Patagonian climate expanded during Quaternary glacial maxima. Sr–Nd isotopic data for Argentinian loess from north of 37°S are distinct from Patagonian loess compositions in that they have more radiogenic Sr (87Sr/86Sr=0.7059–0.7123) and less radiogenic Nd (ϵNd=−0.8 to −6.4). REE patterns and Sr–Nd isotopic values are relatively homogeneous for multiple samples taken from single loess sections but show significant differences between sections. In general, there is a northward change from Patagonia-like REE patterns and isotopic values away from volcanogenic signatures and towards those that are more like the continental crust. The latitudinal Nd isotopic pattern is remarkably similar to that for Andean volcanic rocks and suggests derivation of loess from the Andes by more-or-less direct westerly transport. For loess sections in the north, the data imply a contribution from Palaeozoic gneisses to the northwest in the Chilean Altiplano. Sr–Nd data for extra-Patagonian Argentinian loess north of 37°S do not support a significant role for this source region in supplying dust to Antarctica at the last glacial maximum. This conclusion contrasts with previous studies that suggest a significant northward shift in the climatic belts – and in particular the westerlies and the Antarctic Polar Front – during Quaternary glacial maxima. Very systematic relations between the Sr–Nd isotopic composition of loess and their Andean source highlights shifts in the Sr isotopic composition of loess to more radiogenic values and strongly suggests that the slight offset between Patagonian loess and ice-core dust identified by previous workers is due to grain-size differentiation effects.

The Plio-Pleistocene record of the central eastern Pampas, Buenos Aires province, Argentina: The Chapadmalal case study

Abstract The Plio-Pleistocene record of central eastern Pampas, Argentina, is preserved in loess-like sediments exposed in the sea cliffs south of Mar del Plata. The uplift of the Andes during Miocene triggered the deposition of wind-blown material. The source areas are located about 1000 km away, in western Argentina. Volcanic ashfalls play a significant role in the formation of eolian sediments, which were reworked and redeposited by aqueous transport in a flat, low relief landscape under varying climatic conditions. Vertebrate fossil assemblages document warm and wetter Pliocene and Early Pleistocene climates during Montehermosan and Uquian land-mammal ages. More arid and cooler conditions alternating with humid and warmer intervals prevailed during the Middle to Late Pleistocene in Ensenadan and Lujanian land-mammal ages. The Chapadmalal locality is the best exposed and most complete section. Almost all the units have been modified by pedogenesis. Truncated paleosols as well as pedogenic and non-pedogenic calcretes are very frequent throughout the stratigraphic succession. Several paleosurfaces with prominent paleosols and calcretes suggest that sedimentation was an episodic process with numerous depositional gaps, recording relatively long lasting intervals of landscape stability. The depositional events represent alternations of fluvial and eolian sedimentation. Low frequency catastrophic episodes played an important role in the sedimentation process.

The Quaternary impact record from the Pampas, Argentina

Abstract Loess-like deposits cover much of central Argentina and preserve a rich record of impacts since the late Miocene. The present contribution focuses on two localities containing Quaternary impact glasses: along the coastal sequences near Centinela del Mar (CdM) and from near Rio Cuarto (RC). These highly vesicular glasses contain clear evidence for an impact origin including temperatures sufficient to melt most mineral constituents (1700°C) and to leave unique quench products such as β-cristobolite. The CdM glasses occur within a relatively narrow horizon just below a marine transgression expressed by a series of coastal paleo-dunes and systematic changes in the underlying sediments. High-resolution 40Ar/39Ar dating methods yielded an age of 445±21 ka (2σ). Glasses were also recovered from scattered occurrences lower in the section but were dated to 230±40 ka. This inconsistency between stratigraphic and radiometric age is most likely related to a nearby outcrop of glass that had been exposed and locally re-deposited in coastal lagoons during the last marine transgression at 125 ka. Sediments containing the original impact glass layer are now missing due to an unconformity, perhaps related to subsequent marine transgressions after the impact (410 ka and 340 ka) and hiatuses in deposition. Two different types of impact glasses from RC yield two distinct dates. High-resolution 40Ar/39Ar dating of fresher-appearing glasses (well-preserved tachylitic sheen) indicates an age of 6±2 ka (2σ). Independent fission track analyses yielded a similar age of 2.3±1.6 ka (2σ). More weathered glasses, however, gave significantly older ages of 114±26 ka (2σ). Consequently, materials from two separate Quaternary impacts have been recovered at Rio Cuarto. The younger glasses are consistent with previously reported carbon dates for materials on the floor of one of the large elongate structures. The depths of excavation for the RC and CdM impacts are very different. While the RC glasses are largely derived from near-surface materials, the CdM glasses from the upper level contain added components consistent with Miocene marine evaporites at a depth of about 400–500 m (e.g., high CaO and P2O5). The CdM glasses also incorporated older loess-like sediments from depth based on the geochemistry. Several ratios of key trace and rare earth elements of sediments of different ages from the Miocene to the Holocene indicate a systematic compositional change through time. Such changes calibrate the observed differences in glass composition from their host sediments and further indicate incorporation of materials from depth. Consequently, the Argentine loess-like sediments preserve evidence for at least four separate Quaternary impacts. Based on foreign components in the glasses, the CdM impact very likely produced a crater (now buried or eroded) once as large as 6 km in diameter. The younger RC glasses, however, are consistent with shallower excavation consistent with an oblique impact.

Functional morphology and palaeobiology of the pliocene rodent Actenomys (Caviomorpha: Octodontidae): the evolution to a subterranean mode of life

Abstract The Pliocene caviomorph rodent Actenomys has long been recognized as an early fossorial representative of the subfamily Ctenomyinae (Octodontidae), which includes one living genus, Ctenomys (tuco-tucos), and several species widely distributed in South America. To assess the degree of specialization for digging in Actenomys , we performed morphological comparisons with other octodontid genera ( Octodon , Spalacopus , and Ctenomys ) of known mode of life and behaviour. As a whole, our results indicate that, in terms of morphological specializations for digging, Actenomys occupies an intermediate position between Octodon , a generalized semi-fossorial rodent which forages above ground, and Ctenomys-Spalacopus , two highly specialized subterranean forms. The position of the deltoid process (humerus) and the length of the olecranon process (ulna)—two traits which affect the out-forces exerted by several forelimb muscles—were in Actenomys intermediate between non-subterranean and subterranean taxa. The skull, particularly the rostrum, appears to be strong, and sagittal and nuchal crests are well marked. The zygomatic arches are as flared as those of the generalized Octodon . Notably, the upper and lower incisors of Actenomys were extremely procumbent, as in many highly specialized chisel-tooth digging species. Based upon the stratigraphic and sedimentological analysis of the palaeosoils containing its fossil remains, we suggest that Actenomys lived in an environment of moderate to hig primary productivity. The texture of the palaeosoils indicate that theywere hard and highly cohesive. This situation, which contrasts with that observed in living Ctenomyinae, has relevant implications for burrowing cost. The integration of morphological and palaeoenvironmental data allowed testing of hypotheses about the palaeobiological attributes of this ancestral ctenomyine.

Pedosedimentary and palaeoenvironmental significance of a Holocene alluvial sequence in the southern Pampas, Argentina

A representative section of the Holocene alluvial record of the southern Pampas (Argentina) com prises a sequence of clastic sediments, a diatomite and two palaeosols. Detailed macro- and micromorphological descriptions provide the basis for the reconstruction of its pedosedimentary history. Combining the pedosedi mentary reconstruction with palaeoecological data allows a detailed overview of alluvial landscape development and associated palaeoenvironmental change in the region during the Holocene. The early-Holocene shift from a subhumid dry to humid climate was marked initially by a diminution in clastic sedimentation, the development of the Puesto Callejón Viejo Soil and then re-establishment of fluvial aggradation with a dominance of bioclastic sedimentation. Loess inputs increased during the mid-Holocene as the climate reverted to subhumid dry, and temporary subaerial exposure of the sediments led to the formation of the Puesto Berrondo Soil. More variable environmental conditions followed with natural fluvial aggradation ceasing in the late Holocene, though aeolian sedimentation has continued until the present day. The resultant surface accretionary soil has been modified by recent alluvial inputs from flood events linked to agricultural disturbance in adjacent catchments.

THE PLEISTOCENE–HOLOCENE TRANSITION AND HUMAN OCCUPATIONS IN THE SOUTHERN CONE OF SOUTH AMERICA

Abstract The archaeology of the Pleistocene–Holocene Transition in the Pampas and Fuego-Patagonia is analyzed in light of the results obtained at recently excavated archaeological sites. These sites offer detailed artifactual, faunistic, chronological, and paleoecological information. Results from paleoecological research performed off site are also presented. Changes in climate, vegetation, and faunal composition are recorded, but the archaeological assemblages do not exhibit any clear-cut correspondent changes.

The Pleistocene/Holocene transition and human occupation in the Central Andes of Argentina: Agua de la Cueva locality

In the Central Andes of Argentina, evidences of early human occupations come from the Precordillera, a low mountain system which was under periglacial conditions prior to 14,000 14C yr BP, when the Cordillera Principal and Cordillera Frontal were glaciated. The corridors for human migration across the Andes may have opened before 13,000 14C yr BP. The Pleistocene/Holocene transition gave rise to a substantial variability of environments and consequently of natural resources for early people, who found in the Precordillera new ecosystems with suitable conditions for living. The first human groups arrived ca. 11,000 14C yr BP at Agua de la Cueva rockshelter, located at 2900 m above sea level in the western flank of the Precordillera. The occurrence of raw material outcrops close to the site and the archaeofaunistic record indicate a local and likely seasonal exploitation of mountain resources. These early people lived in a shrub steppe environment under cooler and probably wetter conditions in which camelids were the most important food resource. Since 9000 14C yr BP, warmer and drier conditions were dominant. Except for a lower intensity of human occupations, the site function of Agua de la Cueva seems to have remained the same.

Identification and differentiation of Pleistocene paleosols in the northern Pampas of Buenos Aires, Argentina

Abstract A combined macromorphological and micromorphological approach was used to identify and differentiate paleosols at a representative exposure of the Pleistocene sedimentary succession in the northern Pampas of Buenos Aires (La Plata area). Five pedological units (Pu), which apparently represent four discrete paleosols plus the surface soil, were initially differentiated on the basis of field-scale morphological properties. The succession was divided into B and C horizon with an A horizon only clearly identified in the surface soil and a weak A horizon at depth. Micromorphology suggests a complex pedosedimentary history of welding, with some degree of water reworking indicated by fragments of sorted layers and the significant grain-size heterogeneity of the parent material. The micromorphological data do not support the field differentiation of Pu4, Pu3, and Pu2 into discrete paleosols. Pedological features (i.e. excrements, secondary carbonate coatings, illuvial clay coatings) occur throughout without any obvious breaks or patterns. Pu4, Pu3, and Pu2 are therefore interpreted as an accretionary and/or welded pedocomplex. The Gorina section does not conform to the simple classical model proposed by other authors in the region of alternating loess and paleosol units associated with arid (loess deposition) and wet (soil formation) intervals, respectively.