Descripción: The use of the magnetic signal of soils and paleosoils as a climatic function. The correlation between an index of potential water storage (PWS) with magnetic signal in soils and paleosoils, developed in loessic sediments has been proposed. The analyzed data suggest the existence of climatic thresholds that affect the genesis, preservation or depletion of ferrimagnetic minerals. Soils and paleosoils characterized by a positive PWS have an appropriate environment that favors the depletion of ferromagnetic minerals due to mainly reductive loss. Such soils are characterized by a depletion of detrital ferrimagnetic minerals, as in northeastern Pampean plain soils and paleosoils of Argentina and SE of China. A negative PWS prevents highly reducing conditions in the soil, and the detrital ferrimagnetic minerals are preserved. On the other hand, the environmental conditions of these soils allow the formation of new nannoparticle minerals. These conditions produce a net magnetic enhancement of the soil, as observed in European soils (Russia, Czech Republic, Rumania), Asian (paleosoils from the loess plateau of China) and Northern African ones (Tunisia). The influence of parent material in the magnetic signal is also discussed.

Descripción: A geological and geophysical study in fluvial post-pampean sediments cropping out on the right bank of the Rio Areco was carried out. The sequence studied comprises the Guerrero and Río Salado members of the Luján Formation, in which three palaeosols are developed. The geophysical studies suggest an impoverishment of the detrital ferrimagnetic minerals (magnetite and titanomagnetite) in the parent material, due to the action of pedogenic processes, accompanied by the generation of two neominerales, one antiferromagnetic and the other ferrimagnetic. The ferrimagnetic mineral consists of particles of ultrafine magnetite (state of superparamagnetic domain, grain size approaching 0.01 μm) formed during periods of excess water. The antiferromagnetic minerals (hematite and/or goethite) were generated in periods of major water deficiency. The presence of these minerals would be indicating a warm and humid climate with a marked dryperiod. The oldest paleosol was developed on the Guerrero Member which correlates with the Puesto Callejón Viejo palaeosol that has an age between 10,000 and 8,000 years AP, in this profile. The Rio Salado Member and the two paleosols developed on it would have developed during the warm period known as, Megathermal, between 9,000 and 7,000 years BP. © 2004 Asociación Geológica Argentina.

Descripción: Spectral analyses performed on records of cosmogenic nuclides reveal a group of dominant spectral components during the Holocene period. Only a few of them are related to known solar cycles, i.e., the De Vries/Suess, Gleissberg and Hallstatt cycles. The origin of the others remains uncertain. On the other hand, time series of North Atlantic atmospheric/sea surface temperatures during the last ice age display the existence of repeated large-scale warming events, called Dansgaard-Oeschger (DO) events, spaced around multiples of 1470 years. The De Vries/Suess and Gleissberg cycles with periods close to 1470/7 (∼210) and 1470/17 (∼86.5) years have been proposed to explain these observations. In this work we found that a conceptual bistable model forced with the De Vries/Suess and Gleissberg cycles plus noise displays a group of dominant frequencies similar to those obtained in the Fourier spectra from paleo-climate during the Holocene. Moreover, we show that simply changing the noise amplitude in the model we obtain similar power spectra to those corresponding to GISP2 δ18O (Greenland Ice Sheet Project 2) during the last ice age. These results give a general dynamical framework which allows us to interpret the main characteristic of paleoclimate records from the last 100 000 years. © 2010 Author(s).

Descripción: Large landslides of different types not previously recorded are located in the left bank of the Río Curri Leuvú valley, north of the Tricao Malal village. There are situated among the Cerro Palao and the Palao pond and Riscos Negros site to the north. The largest landslides (Malal Cura and Arroyo Mogolillo landslides) correspond to the multiple rotational progressive type. They are developed on Mesozoic sedimentary rocks, covered by Cenozoic basalt flows. The debris and earth flows are developed in areas covered by the volcanic rocks of the Charilehue Formation (Middle Miocene) and continue north of study reaching the latitudes of the Grande and Medio streams and also the Colimamil stream valley southeast of Cerro Domuyo. The impact of these phenomena in the fluvial system was important and produced ephemeral and permanent blocking as seen in the La Turbia stream with coeval ponding. It is hypothesized that the landslide occurrence is related to excess or long-lasting periods of rain in postglacial times. It is also indicated the presence of rock glaciers in the Cerro Palao.

Descripción: This paper discusses the general atmospheric circulation over Patagonia on the basis of the principal palaeoclimate forcings: continental drift, orography, variations in the greenhouse gases in the Earth's atmosphere, evolution of the atmosphere and the oceans, changes in the orbit of the Earth, albedo feedbacks, and the land surface. These processes affect climate on time scales of millions to hundreds of thousands of years. Additionally, orbital forcing has had a major influence on climate during the Quaternary. The palaeo-atmospheric circulation of Patagonia is analyzed for the Early to Late Jurassic, the Cretaceous, the Late Palaeocene-Eocene Thermal Maximum, the Tortonian-Oligocene cooling, the Pliocene, the Quaternary including the Last Glacial Maximum, the Holocene Optimum, and the last millennium changes. Alternative palaeo-atmospheric circulations from climatic modelling scenarios through the ages are reviewed and compared with proxy data. Detailed and updated reference information on the topics analyzed is also provided. © 2011 The Linnean Society of London.

Descripción: An environmental magnetic study of mainly fluvial "postpampean sediments" assigned to the Lujan Formation (Luján, Buenos Aires Province, Argentina) was carried out. The results obtained suggest a depletion of detrital ferromagnetic minerals during the pedogenetic processes. A characteristic magnetic signal can be distinguished in Holocene paleosols and soils of both eolian and fluvial "post-Pampean sediments" (Fm. Luján and Fm. La Postrera . This signal is analogous to that determined in loessic Pampean sediments assigned to the Buenos Aires Formation. It is inferred that the depletion of detrital ferromagnetic minerals is a general process in the Chacopampean area. This process may be associated with the genesis of a superparamagnetic fraction (SP), which could indicate dry periods during the climatic event. The depletion could also be associated with the genesis of a high coercitivy fraction, analogous with those determined in paleosols in the Pampean loess.

Descripción: The Little Ice Age (LIA) is a climate episode between the 16th and middle 19th centuries, characterized in Europe by colder temperatures and occasionally stormy weather. In certain areas of Europe, long instrumental observations record the Little Ice Age partially; however, in the pampean region meteorological data only started about one hundred years ago. The objective of this contribution is to provide new evidences about the Little Ice Age in the Pampean plain based on lake cores. Short cores of Chascomús and Monte lakes were analyzed and, according to AMS data, they span the last 500 years. The Chascomús core consists of three fining upward sequences. Sediment logical and biological proxies demonstrate a benign period from the end of the 15th century. Around 1700 AD lake level decreased and a dry period started and lasted for almost 150 years. After 1850 AD higher lake levels and increased moisture dominate the pampean scenario. The del Monte lake core is also composed of three fining upwards sequences, and AMS chronology assigned a basal age between 1441-1494 AD. Although the model age needs improvement, the events of Chascomús and del Monte lakes seem to be correlated and show coherent regional tendencies of humidity and droughts during the last 500 years.

Descripción: The continental deposits found in southern and western Uruguay show important climatic changes along the Cenozoic. The sequence begins with Paleocene palustrine carbonates known as the Queguay Formation, associated with calcretes of phreatic origin which developed mainly on fluvial sediments of Mercedes Formation (Late Cretaceous), and indicates a semiarid climate, seasonally contrasted. The Asencio Formation (Early Eocene) is separated from Mercedes Formation by the Yapeyu paleosurface, limiting two sedimentary cycles. Fluvial deposits lies above this surface, on which Ultisols developed under a warm and humid climate; periods of intense dryness would provoked their induration and formation of ferricretes, which under subsequent increased precipitation were dismantled. Above, Fray Bentos Formation (Oligocene-Early Miocene) lies unconformably. It is composed by loessic sediments deposited in a semiarid climate, paleosols and diverse pedogenic calcretes developed on these sediments, among which a new type named tubular calcrete, are here described; in the latter the tubular units are related to a coarse prismatic structure derived from shrink-swell processes and the surface morphology of this calcrete refers to a gilgai microrelief, typical for Vertisols. The pedogenic calcretes point to a seasonal semiarid climate. The Raigón Formation (Late Pliocene- Middle Pleistocene) of fluvial origin was formed in a humid period, and shows a paleosol at the top developed in a seasonally contrasted climate. Lying unconformably, the Libertad Formation (Early to Middle Pleistocene) is composed by loesses deposited during glacial periods that were subsequently modified by pedogenesis during interglacial periods.

Descripción: Between 1870 and 1884, Florentino Ameghino consecrated all his efforts to document and to prove the coexistence of man and Pampean megafauna in the Luján valley. He described several sites in channel and alluvial plain settings in valleys incised on Pampean loess. The geoarchaeological context of these sites, characterized by the abundance of megafauna modified bones, were never studied again after the publication of La Antigüedad del Hombre en el Plata in 1880-81. Here we present a stratigraphical reevaluation and reinterpretation of his "Piso Lujanense" based on a sequence stratigraphy analysis and AMS, OSL and ESR dating. We conclude that the Pleistocene valley fill spans from OIS 4 to OIS 2 with sequence boundaries at 75, 55, 30, 17 and 13 ky BP, reflecting the OIS 4 to OIS 2 glacioeustatic falls in the pampas valleys. These deposits were sealed by a rich organic matter layer or black mats, indicating an abrupt climatic change with a dramatic increase of humidity starting sometime between 12.5 and 13 ky BP. The associated ecological changes probably triggered the extinction process.

Descripción: The paleoenvironmental and paleoclimatic significance of Cretaceous flora in the Neuquén Basin is summarized. Berriasian-Barremian terrestrial palynological assemblages are dominated by Araucariaceae and Cheirolepidiaceae, with subordinated podopcarps, ferns, lycopsids and bryophytes that progressed in a mild and humid climate. Tithonian-Barremian dinoflagellates mostly possess Mediterranean affinities. Upper Barremian evaporitic strata represent the final cycle of marine deposition related to the Pacific ocean in the basin. In the Aptian appear the first angiosperm pollen grains. Aptian-Cenomanian terrestrial palynological assemblages are characterized by the presence of northern Gondwana forms such as Afropollis and elaterate pollen grains. Tempskyaceae and Ginkgoales are also present in the late Early Cretaceous. Turonian-lower Campanian continental strata mostly include petrified wood and charophytes. Upper Campanian-Maastrichtian strata yield a rich terrestrial palynological assemblage composed by Nothofagaceae, Podocarpaceae, Cheirolepidiaceae, Araucariaceae, Salviniales and angiosperms, but also dinoflagellates that reflect the beginning of a marine transgression related to the Atlantic ocean. The presence of locally abundant cycads and palms suggests a warm and relatively humid climate towards the end of the Cretaceous.

Descripción: In the right bank of the Arroyo Tapalqué (Salto de Piedra, Olavarría, Buenos Aires province) is a section through fluvial sediments of the Luján Formation (upper and lower members) and aeolian deposits of La Postrera Formation. A geological and geophysical study was carried out to determination the magnetic parameters of the sediments of both formations, in order to evaluate the environmental and climatic conditions at the time of deposition. Levels with high concentrations of ferromagnetic minerals are thought to be related to an increase in capacity of river transport probably due to higher rainfall. By contrast, a particular magnetic behaviour was detected in levels closely associated with pedogenetic processes (Suelo Puesto Callejón Viejo, SPCV, Suelo Puesto Berrondo, SPB and present soils). This behaviour is attributed to the partial dissolution of the ferromagnetic fraction (magnetic/titanomagnetite) and to possible formation of a high c oercitivity magnetic fraction (hematite?) as an associated process. These results are similar to those obtained for paleosols from the Buenos Aires Formation, probably because of parental material common to both. On the basis of present field and laboratory observations, together with previously available information, a detailed correlation is proposed between features observed in this section and palaeoclimatic fluctuations identified for Pleistocene/Holocene.

Descripción: Samples obtained from four wells located at Bancalari, Buenos Aires Province, have been analysed in order to define textural parameters, petrofacies, palaeoenvironments and palaeoclimate evolution. The samples were obtained from the upper part of the Puelches Formation and from the Pampean and Postpampean sediments; in particular this paper highlights the informally named Preensenadense interval. Six textural groups (I to VI) and two petrofacies (A and B) have been recognised. Groups I to III represent fluvial sands (I), gradually overlain by backswamp and lake deposits (II -III), generated on very low slope and well-drained floodplains, with oxidizing conditions. Groups II and III are assigned to Preensenadense interval. The three groups are included in petrofacies A (Qt:F:L = 43:49:8), of sedimentary and basement provenance, which has an igneous metamorphic heavy suite, scarce hornblende and dominance of smectite over illite. By contrast the upper groups (IV to VI) show more varied depositional settings, including aeolian, fluvial and marine environments. Group IV is loessic and corresponds to the Ensenada Formation. Group V, the Lujan Formation (Río Salado Member), is composed of fluvial plain deposits deposited during a wet and temperate climate. Marine silts and clays (group VI) represent a transgression over terrestrial environments and record of the so-called Querandino event. The three upper groups are included in petrofacies B (12:56:32), and have a magmatic arc provenance, with plagioclase dominance and contributions of volcaniclastic and vitroclastic material. It contains a heavy-mineral suite mainly composed of hornblende and it has more illite than smectite. © 2005 Asociación Geológica Argentina.