Search

Copyright of Cuaternario y Geomorfología is the property of Cuaternario y Geomorfologia and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Pleistocene permafrost has been recognized in the lowlands of extra-Andean Argentina from Tierra del Fuego to the Rio Negro valley at 40°S, and to the Sierras Australes at 38°S. Features that could have formed only by cryogenic activity at elevations between 230-400 m above sea level in surficial deposits and in the bedrock beneath are described here as far north as 36°S. These features are not as pronounced as they are farther south because most of central Argentina was a cold desert during the glacial episodes, and therefore little ice formed. Calcareous dust, formerly considered as pedogenic and now known to be glaciogenic, is closely associated with these features. Secondary precipitates, such as lamellar crystals of calcite and gypsum, and other microscopic features like those observed in perpetually frozen ground also confirm that this region experienced permafrost at some time. These new findings mean that the area affected by periglaciation is much larger than previously thought and expanded more than 200 km farther to the north. Stratigraphic evidence and geomorphological features place both deposits and cryogenic features within them as early-middle Pleistocene age. [ABSTRACT FROM AUTHOR]

Fossil‐rich sediments of the Santa Cruz Formation, Patagonia, Argentina, span the initiation of the Miocene Climatic Optimum (MCO), the most recent period of warm and wet conditions in the Cenozoic. These conditions drove the expansion of tropical and subtropical ecosystems to much higher latitudes, with the fossiliferous Santa Cruz Formation recording one of the southernmost examples. We collected new carbon and oxygen isotope compositions of herbivore tooth enamel from fossils ~17.4 to 16.4 Ma in age to investigate ecological and climatic changes across the initiation of the MCO. Enamel δ13C values are consistent with a C3‐dominated ecosystem with moderate precipitation and a mix of wooded and more open areas. Serially sampled teeth reveal little zoning in δ13C and δ18O values, suggesting little seasonal variation in water and plant isotope compositions or seasonal changes in diet. Carbon isotope‐based estimates of mean annual precipitation (MAP) are consistent with aridification, with MAP decreasing from ~1,000 ± 235 mm/yr at 17.4 Ma to ~525 ± 105 mm/yr at the start of the climatic optimum (~16.9 Ma). This decrease corresponds to increasing global temperatures, as indicated by marine proxy records, and was followed by a rebound to ~840 ± 270 mm/yr by ~16.4 Ma. In comparison to a modern mean annual temperature (MAT) in the region of ~8°C, oxygen isotopes indicate high MAT (at least 20°C) at the onset of the MCO at 16.9 Ma and a significant increase in MAT to ~25°C by 16.4 Ma. Key Points: The Miocene Climatic Optimum interrupted a trend toward arid conditions in PatagoniaStable isotope‐based estimates of precipitation and temperature significantly changed at the onset of the MCOGeneral circulation models underestimate temperature for Miocene Patagonia [ABSTRACT FROM AUTHOR]

This study presents the first detailed glacial geomorphological map of the sediment-landform assemblages formed by three eastern outlet glaciers of the former Patagonian Ice Sheet. These glaciers occupied the Río Corcovado, Río Huemul and Lago Palena/General Vintter valleys, Chubut province, Argentina (43°S, 71°W). By combining remote sensing and field-mapping, we build on previous ice-sheet scale mapping and geological surveys to provide high-resolution spatial information on local ice-contact glaciogenic, glaciofluvial, glaciolacustrine, and subglacial landforms. Twenty-five landform types, many of which are newly mapped in the region, were digitized as georeferenced shapefiles over a 5300 km2 area. This map enables the identification of former ice-flow directions, relative ice-margin positions and glaciofluvial drainage pathways for each preserved Quaternary glaciation. It also elucidates the former areal extent, geolocation and spillways of glaciolacustrine bodies formed during the last deglaciation. The map delivers an essential framework on which to build robust glacier-scale geomorphological and geochronological reconstructions. [ABSTRACT FROM AUTHOR]

This study was carried out on sediment cores collected with a gravity corer from Laguna Cháltel, an almost circular crater lake located in Patagonia, Argentina (49.9°S, 71°W). The main magnetic carrier was Ti-magnetite in the pseudo–single domain range. A model using magnetic grain size and concentration, previously applied to Laguna Potrok Aike to infer lake-level changes, was used for Laguna Cháltel. The main requirement to apply the model is uniform magnetic mineralogy, which is the case for Laguna Cháltel. After magnetic data were compared with previously studied lake levels, it was found that the magnetic proxies that best follow hydrologic changes are ARM/SIRM (anhysteretic remanent magnetisation/saturation of isothermal remanent magnetisation) and ARM. The concentration proxy (ARM measured with a 100 mT alternating field and 0.05 mT direct field) was also used as wind indicator. High wind strength was found at around 3650 cal yr BP, and low wind strength for the last century. ARM/SIRM and ARM were used to infer the strength of fluvial runoff into the lake for a core collected close to the shore and near a tributary. The results show that the magnetic model is promising for inferring lake-level variations, particularly in Patagonian lakes. [ABSTRACT FROM AUTHOR]

Calcareous dust occurs in Argentina as layers and pockets closely associated with Pleistocene deposits and periglacial features from southernmost Patagonia to at least the Mendoza Precordillera and has been traditionally interpreted as a soil horizon resulting from postdepositional pedogenesis during interglacials. Detailed field and microscopic observations and sedimentological and geochemical analyses of more than 100 samples collected from lower to upper Pleistocene deposits between 51°S and 33°S and from near sea level to 2800 m asl allow us to interpret the dust as synchronous with the host sediment. All observations and analyses lead us to conclude that: (1) the cryogenic morphology and the chemical signatures of the calcite component show that the dust is glaciogenic, (2) the dust was carried by southeasterly Antarctic winds, and (3) it was deposited over most of southern and central Argentina. Field observations, geomorphic evidence, and radiocarbon dates suggest that the dust was deposited during several Pleistocene glacial episodes. [ABSTRACT FROM AUTHOR]

Aquatic sediments record past climatic conditions while providing a wide range of ecological niches for microorganisms. In theory, benthic microbial community composition should depend on environmental features and geochemical conditions of surrounding sediments, as well as ontogeny of the subsurface environment as sediment degraded. In principle, DNA in sediments should be composed of ancient and extant microbial elements persisting at different degrees of preservation, although to date few studies have quantified the relative influence of each factor in regulating final composition of total sedimentary DNA assemblage. Here geomicrobiological and phylogenetic analyses of a Patagonian maar lake were used to indicate that the different sedimentary microbial assemblages derive from specific lacustrine regimes during defined climatic periods. Two climatic intervals (Mid-Holocene, 5 ka BP; Last Glacial Maximum, 25 ka BP) whose sediments harbored active microbial populations were sampled for a comparative environmental study based on fossil pigments and 16S rRNA gene sequences. The genetic assemblage recovered from the Holocene record revealed a microbial community displaying metabolic complementarities that allowed prolonged degradation of organic matter to methane. The series of Archaea identified throughout the Holocene record indicated an age-related stratification of these populations brought on by environmental selection during early diagenesis. These characteristics were associated with sediments resulting from endorheic lake conditions and stable pelagic regime, high evaporative stress and concomitant high algal productivity. In contrast, sulphatereducing bacteria and lithotrophic Archaea were predominant in sediments dated from the Last Glacial Maximum, in which pelagic clays alternated with fine volcanic material characteristic of a lake level highstand and freshwater conditions, but reduced water column productivity. Comparison of sedimentary DNA composition with that of fossil pigments suggested that post-depositional diagenesis resulted in a rapid change in the initial nucleic acid composition and overprint of phototrophic communities by heterotrophic assemblages with preserved pigment compositions. Long DNA sequences (1400-900 bp) appeared to derive from intact bacterial cells, whereas short fragments (290-150 bp) reflected extracellular DNA accumulation in ancient sediments. We conclude that sedimentary DNA obtained from lacustrine deposits provides essential genetic information to complement paleoenvironmental indicators and trace post-depositional diagenetic processes over tens of millennia. However, it remains difficult to estimate the time lag between original deposition of lacustrine sediments and establishment of the final composition of the sedimentary DNA assemblage. [ABSTRACT FROM AUTHOR]

Aquatic sediments record past climatic conditions while providing a wide range of ecological niches for microorganisms. Although marine sedimentary microbial assemblages are often defined by their surrounding geochemical conditions, the influence of environmental features upon microbial development and post-depositional survival remains largely unknown in the lacustrine realm. Due to long-term microbial activity, the composition of environmental DNA can be expected to evolve with sediment depth and over time and therefore should reflect both ancient and extant microbial populations, but this hypothesis has rarely been tested using a multiproxy approach. Here geomicrobiological and phylogenetic analyses of a Patagonian maar lake were used to indicate that the different sedimentary microbial assemblages derive from specific lacustrine regimes during defined climatic periods. Two well defined climatic intervals whose sediments harboured active microbial populations and measurable ATP were sampled for a comparative environmental study based on fossil pigments and 16S rRNA gene sequences. Bacterial and archaeal 16S rRNA gene sequences recovered from the Holocene record revealed a microbial community adapted to subsaline conditions actively producing methane during organic matter degradation. These characteristics were associated with sediments resulting from endorheic lake conditions with high evaporative stress and concomitant high algal productivity. Moreover, archaeal clone libraries established throughout the Holocene record indicate an age-related stratification of these populations, consistent with a gradual use of organic substrates after deposition. In contrast, sulphate-reducing bacteria and lithotrophic Archaea were predominant in sediments dated from the Last Glacial Maximum, in which pelagic clays alternated with fine volcanic material characteristic of a lake level highstand and fresh water conditions, but reduced water column productivity. These patterns reveal that microbial assemblages identified from environmental DNA stemmed from a variety of sedimentary niches associated with climate-dependent factors (catchment inflows, water column conditions, productivity), but that initial assemblages underwent structural changes and selective preservation during early diagenesis to result in the final composition entombed in the sediments. We conclude that environmental DNA obtained from lacustrine sediments provides essential genetic information to complement paleoenvironmental indicators and trace climate change and post-depositional diagenetic processes over tens of millennia. [ABSTRACT FROM AUTHOR]

The Southern Hemisphere Westerlies (SHW) play a crucial role in large-scale ocean circulation and global carbon cycling. Accordingly, the reconstruction of how the latitudinal position and intensity of the SHW belt changed during the last glacial termination is essential for understanding global climatic fluctuations. The southernmost part of the South American continent is the only continental mass intersecting a large part of the SHW belt. However, due to the scarcity of suitable palaeoclimate archives continuous proxy records back to the last glacial are rare in southern Patagonia. Here, we show an oxygen isotope record from cellulose and purified bulk organic matter of submerged aquatic moss shoots from Laguna Potrok Aike (52°S, 70° W), a deep maar lake located in semi-arid, extra-Andean Patagonia, covering the last glacial-interglacial transition (26000 to 8500 cal BP). Based on the highly significant correlation between oxygen isotope values of modern aquatic mosses and their host waters and abundant well-preserved moss remains in the sediment record a high-resolution reconstruction of the lake water oxygen isotope (δ18Olw-corr) composition is presented. The reconstructed δ18Olw-corr values for the last glacial are ca. 3%o lower than modern values, which can best be explained by generally cooler air temperatures and changes in the moisture source area, together with the occurrence of permafrost leading to a prolonged lake water residence time. Thus, the overall glacial δ18Olw-corr level untill 21000 cal BP is consistent with a scenario of weakened or absent SHW at 52° S compared to the present. During the last deglaciation, reconstructed δ18Olw-corr values reveal a significant two-step rise describing the detailed response of the lake's hydrological balance to this fundamental climatic shift. Rapid warming is seen as the cause of the first rise of ca. 2% in δ18Olw-corr during the first two millennia of deglaciation (17600 to 15600 cal BP) owing to more 18O enriched precipitation and increasing temperature-induced evaporation. Following this interpretation, an early strengthening of the SHW would not be necessary. The subsequent decrease in δ18Olw-corr by up to 0.7% marks a millennial-scale transition period between 15600 and 14600 cal BP interpreted as the transition from a system driven by temperature-induced evaporation to a system more dominated by wind-induced evaporation. The δ18Olw-corr record resumes its pronounced increase around 14600 cal BP. This further cumulative enrichment in 18O of lake water could be interpreted as response to strengthened wind-driven evaporation as induced by the intensification and establishment of the SHW at the latitude of Laguna Potrok Aike (52° S) since 14 600 cal BP. δ18Olw-corr approaching modern values around 8500 cal BP reflect that the SHW exerted their full influence on the lake water balance at that time provoking a prevailing more arid steppe climate in the Laguna Potrok Aike region. [ABSTRACT FROM AUTHOR]

Lake sediments are excellent sources of palaeoenvironmental and palaeoclimatic information because they provide continuous and high-resolution records. South America is of particular interest because it is the only landmass that stretches southward into the Pacific and Atlantic Oceans towards Antarctica. The aim of this study is to explore the relationship of magnetic parameters with elemental and palaeobiological data of Laguna Potrok Aike to develop a model of lake-level changes and related hydrological and climatic fluctuations. Magnetic measurements were performed on subsamples from 15 500 cal. BP to the present, and associated rock magnetic parameters were calculated to infer magnetic mineralogy, concentration and grain size. According to the model, parameters dependent on magnetic concentration and grain size are directly related to lake-level changes. During dry periods, the remanent coercivity displays high values, whereas the proportion of magnetite is relatively low. Low percentages of greigite are observed, indicating that the water of the lake was stratified at least four times during the studied period, at approximately 10 300, 8900, 8500 and 8300 cal. BP. The preservation of greigite by inhibiting its complete transformation into pyrite is associated with a rapid burial that occurs with high sedimentation rates. Thermal stratification could be caused by a slight cooling in the area triggered by a weakening of the Westerlies and/or low activity of the sun, sum to the effect of a flood of melt water in the North Atlantic. [ABSTRACT FROM AUTHOR]

Methanogenic populations were investigated in subsaline Laguna Potrok Aike sediments, southern Argentina. Microbial density and activity were assessed via cell count andin situATP detection for the last ∼11K years. Methanogen phylogenetics highlighted species stratification throughout depth, whereas CO2reduction was the major pathway leading to methane production. Organic substrates, characterized using pore water analysis, bulk organic fractions and saturated fatty acids, showed a clear link between sediment colonization and initial organic sources. Concentrations and δ13C compositions of methane and fatty acids provided final evidence of a microbial imprint on Holocene organic proxies in the most colonized intervals. [ABSTRACT FROM AUTHOR]

Rainfall and geology of the catchment exert a dominant control on the trophic state of endorheic basins. River inflows and runoff provide nutrients, influencing primary productivity in the water column. Through time, paleoenvironmental conditions are recorded as variations within the sedimentary organic fraction. Thereafter, microbial populations settle and develop within sediments and lead to degradation processes as long as they remain active. However, their presence is generally not considered in Quaternary studies. The present study is based on the sedimentary record of the maar lake of Laguna Potrok Aike, southern Patagonia. We investigate the relationship between paleoenvironmental conditions and colonization of the corresponding sediments by microbes. Microbiological and geochemical analyses were combined to determine factors allowing microbes to sustain their activity over time. The study of Holocene sediments, containing dense and active microbial communities, provided means to evaluate the potential of microbial communities as agents of early diagenesis. We show that phosphorus released during organic matter degradation is essential for microbial growth. In highly colonized sediments, microbial communities appear capable of recycling the excreted ammonium, thus accounting for nitrogen fractionation toward high values in bulk sediment. Microbial activity in Laguna Potrok Aike still persists in 30 ka old sediments. Thus, we proposed that future lacustrine studies should include some microbial indicators to assess their impact in diagenetic processes. [ABSTRACT FROM AUTHOR]

Abstract: Relict sand wedges are ubiquitous in southern Patagonia. At six sites we conducted detailed investigations of stratigraphy, soils, and wedge frequency and characteristics. Some sections contain four or more buried horizons with casts. The cryogenic features are dominantly relict sand wedges with an average depth, maximum apparent width, minimum apparent width, and H/W of 78, 39, 3.8, and 2.9cm, respectively. The host materials are fine-textured (silt loam, silty clay loam, clay loam) till and the infillings are aeolian sand. The soils are primarily Calciargidic Argixerolls that bear a legacy of climate change. Whereas the sand wedges formed during very cold (−4 to −8°C or colder) and dry (ca. ≤100mm precipitation/yr) glacial periods, petrocalcic horizons from calcium carbonate contributed by dustfall formed during warmer (7°C or warmer) and moister (≥250mm/yr) interglacial periods. The paleo-argillic (Bt) horizons reflect unusually moist interglacial events where the mean annual precipitation may have been 400mm/yr. Permafrost was nearly continuous in southern Patagonia during the Illinoian glacial stage (ca. 200ka), the early to mid-Pleistocene (ca. 800–500ka), and on two occasions during the early Pleistocene (ca. 1.0–1.1Ma). [Copyright &y& Elsevier]

Laguna Potrok Aike, located in southernmost Patagonia (Argentina, 52°S) is a 100 m deep hydrologically closed lake that probably provides the only continental southern Patagonian archive covering a long and continuous interval of several glacial to interglacial cycles. In the context of the planned ‘International Continental Scientific Drilling Program’ initiative ‘Potrok Aike Maar Lake Sediment Archive Drilling Project’, several seismic site surveys that characterize in detail the sedimentary subsurface of the lake have been undertaken. Long sediment cores recovered the material to date and calibrate these seismic data. Laguna Potrok Aike is rimmed steeply, circular in shape with a diameter of ∼3·5 km and is surrounded by a series of subaerial palaeoshorelines, reflecting varying lake-level highstands and lowstands. Seismic data indicate a basinwide erosional unconformity that occurs consistently on the shoulder of the lake down to a depth of −33 m (below 2003ad lake level), marking the lowest lake level during Late Glacial to Holocene times. Cores that penetrate this unconformity comprise Marine Isotope Stage 3-dated sediments (45 kyrbp) ∼3·5 m below, and post-6800 cal yrbp transgressional sediments above the unconformity. This Middle Holocene transgression following an unprecedented lake-level lowstand marks the onset of a stepwise change in moisture, as shown by a series of up to 11 buried palaeoshorelines that were formed during lake-level stillstands at depths between −30 and −12 m. Two series of regressive shorelines between ∼5800 to 5400 and ∼4700 to 4000 cal yrbpinterrupt the overall transgressional trend. In the basin, mound-like drift sediments occur after ∼6000 cal yrbp, documenting the onset of lake currents triggered by a latitudinal shift or an increase in wind intensity of the Southern Hemispheric Westerlies over Laguna Potrok Aike at that time. Furthermore, several well-defined lateral slides can be recognized. The majority of these slides occurred during the mid-Holocene lake-level lowering when the slopes became rapidly sediment-charged because of erosion from the exposed shoulder sediments. Around 7800 and 4900 cal yrbp, several slides went down simultaneously, probably triggered by seismic shaking. [ABSTRACT FROM AUTHOR]

A high-resolution multiproxy geochemical approach was applied to the sediments of Laguna Potrok Aike in an attempt to reconstruct moist and dry periods during the past 16 000 years in southeastern Patagonia. The age-depth model is inferred from AMS 14C dates and tephrochronology, and suggests moist conditions during the Lateglacial and early Holocene (16 000–8700 cal. BP) interrupted by drier conditions before the beginning of the Holocene (13 200–11 400 cal. BP). Data also imply that this period was a major warm phase in southeastern Patagonia and was approximately contemporaneous with the Younger Dryas chronozone in the Northern Hemisphere (12 700–11 500 cal. BP). After 8650 cal. BP a major drought may have caused the lowest lake level of the record. Since 7300 cal. BP, the lake level rose and was variable until the ‘Little Ice Age’, which was the dominant humid period after 8650 cal. BP. [ABSTRACT FROM AUTHOR]

The volcanogenic lake Laguna Potrok Aike, Santa Cruz, Argentina, reveals an unprecedented continuous high resolution climatic record for the steppe regions of southern Patagonia. With the applied multi-proxy approach rapid climatic changes before the turn of the first millennium were detected followed by medieval droughts which are intersected by moist and/or cold periods of varying durations and intensities. The ‘total inorganic carbon’ content was identified as a sensitive lake level indicator. This proxy suggests that during the late Middle Ages (ca. AD 1230–1410) the lake level was rather low representing a signal of the ‘Medieval Climate Anomaly’ in southeastern Patagonia. At the beginning of the ‘Little Ice Age’ the lake level rose considerably staying on a high level during the whole period. Subsequently, the lake level lowered again in the course of the 20th century. [ABSTRACT FROM AUTHOR]