Eduardo L. Piovano

Hydrological Variability in South America Below the Tropic of Capricorn (Pampas and Patagonia, Argentina) During the Last 13.0 Ka

New limnogeological results (Laguna Mar Chiquita, 30°S) at the middle latitudes in the subtropical Pampean plains of Southern South America are used to establish comparisons with records spanning Late Glacial times and the Holocene covering the Northern Andean Patagonia and the Extra Andean Southern Patagonia. Historical and archaeological data were also used to enlarge existing environmental reconstructions in the central plains of Southern South America. Two groups of climate records – at both sides of the Arid Diagonal- can be distinguished according to their hydrological response during dominant warm or cold climatic phases. The first group includes records from the Pampean region and allows to reconstruct the past activity of the South America Monsoonal System. The second group includes archives of the Patagonian climate, as well as the Salinas del Bebedero, and provides a record of past changes in the strength and latitudinal position of the Southern Westerlies.

The late Ordovician carbonate sedimentation as a major triggering factor of the Hirnantian glaciation

A new approach explaining the main forcing factor of Hirnantian glaciation is proposed herein. It follows the models associating occurrences of continental glaciations with periods of low atmospheric CO2 levels. The accumulation of great volumes of carbonates during pre-Hirnantian late Ordovician, in regions where these deposits were previously absent, is suggested as a major sink of atmospheric CO2. This would have caused an important lowering of the average temperature in the early Hirnantian, after CO2 values had attained a certain threshold. This process was maintained by other positive feedbacks, such as the short-term carbonate weathering CO2 sink. An increase of the direct flux of CO2 from the atmosphere to the oceans by means of dissolution would have been driven by the enhancement of carbonate deposition. The great inundation of the low latitude Laurentia craton during Cincinnatian times and the establishment of a temperate-water carbonate sedimentation on the North Gondwana margin during pre-Hirnantian Ashgill allowed the burying of more than 840 × 1015 kg (1.9 × 1019 mol) of dissolved CO2. This mass is equivalent to nearly 350 times the present values of atmospheric CO2. This is important enough to have greatly altered the equilibrium between the CO2 dissolved in the oceans and the partial pressure of CO2 in the air, eventually causing an important reduction of the latter. The new model also offers a simple explanation for the end of the glaciation after a short time-span. Glacioeustatic lowering of the sea level, concomitant with the glaciation, would have stopped the extra-sedimentation of carbonate due to the retreat of the oceans from the platforms, closing this CO2 sink. Pre-glacial CO2 levels would then recover, due to volcanic and metamorphic CO2 outgassing. After subsequent melting of the ice cap, oceanic circulation did not recover pre-Hirnantian Ashgill strength, resulting in a strong stratification of ocean waters and precluding the recovery of an extensive carbonate deposition. The well-known positive shift in the [omega]13C at the base of the Hirnantian is assumed to have been caused by weathering and dissolution of carbonates, relatively enriched in 13C, during the glacioeustatic regression and exposure of the platforms.

Rare earth element patterns in lake sediments as studied by neutron activation analysis

Using instrumental neutron activation analysis, the vertical distribution of La, Ce, Nd, Sm, Eu, Tb, Yb and Lu was determined in 3 well dated sedimentary cores collected from Laguna Mar Chiquita, Argentina. Validation of the methodology was done using the Soil 7 (IAEA) reference material. The REE contents vary according to core location and lithological units, depending on the proportion of fluvial terrigenous components in the lake sediments. Lithologies with higher proportion of terrigenous components show higher REE concentrations, suggesting that the dominant REE carrier phase might be a detrital mineral. There is a consistent value of about 1.2 for (La/Yb)N and (LREE/HREE)N ratios for all sediments at the three sites, indicating a slight LREE enrichment in relation to the average shale NASC. Ce/Ce* values are close to 1.0, meaning that Ce behaves similarly to other light REEs.

Geochemical tracers of source rocks in a Cretaceous to Quaternary sedimentary sequence (Eastern Sierras Pampeanas, Argentina)

Abstract Metamorphic rocks, granitic rocks, and sediments from the Eastern Sierras Pampanas, Argentina, were analyzed for major and trace element concentrations, including rare earth elements (REE). Parental rocks exhibit distinctive REE normalized diagram patterns and elemental ratios, and some elemental ratios reveal significant differences between rock sources. For example, ratios such as Th/Sc, Cr/Th, and La/Cr have a mean value of 0.7, 8.4 and 0.4 in metamorphic rocks, whereas granitic rocks exhibit means of 1.4, 0.7 and 4.9, respectively. These ratios are also useful in linking detrital materials with the corresponding parental rocks. Metamorphic sources yield sediments with lower Th/Sc and La/Cr, and higher Cr/Th ratios than sediments derived from granitic sources. REE and other elements are enriched in the silt-size fraction, whereas they are diluted by quartz in the sand-size fraction. The size of the Eu/Eu* anomaly can be used as a stratigraphical correlation tool in the sedimentary record: Cretaceous rocks show a mean value of 0.9±0.1, whereas Tertiary rocks have a mean value of 2.9±0.3. The Eu anomaly in Quaternary and modern sediments ranges from 0.5 to 0.8.

A complete hydro-climate model chain to investigate the influence of sea surface temperature on recent hydroclimatic variability in subtropical South America (Laguna Mar Chiquita, Argentina)

Abstract During the 1970s, Laguna Mar Chiquita (Argentina) experienced a dramatic hydroclimatic anomaly, with a substantial rise in its level. Precipitations are the dominant driving factor in lake level fluctuations. The present study investigates the potential role of remote forcing through global sea surface temperature (SST) fields in modulating recent hydroclimatic variability in Southeastern South America and especially over the Laguna Mar Chiquita region. Daily precipitation and temperature are extracted from a multi-member LMDz atmospheric general circulation model (AGCM) ensemble of simulations forced by HadISST1 observed time-varying global SST and sea-ice boundary conditions from 1950 to 2005. The various members of the ensemble are only different in their atmospheric initial conditions. Statistical downscaling (SD) is used to adjust precipitation and temperature from LMDz ensemble mean at the station scale over the basin. A coupled basin-lake hydrological model (cpHM) is then using the LMDz-downscaled (LMDz-SD) climate variables as input to simulate the lake behavior. The results indicate that the long-term lake level trend is fairly well depicted by the LMDz-SD-cpHM simulations. The 1970s level rise and high-level conditions are generally well captured in timing and in magnitude when SST-forced AGCM-SD variables are used to drive the cpHM. As the LMDz simulations are forced solely with the observed sea surface conditions, the global SST seems to have an influence on the lake level variations of Laguna Mar Chiquita. As well, this study shows that the AGCM-SD-cpHM model chain is a useful approach for evaluating long-term lake level fluctuations in response to the projected climate changes.

Coupling statistically downscaled GCM outputs with a basin-lake hydrological model in subtropical South America: evaluation of the influence of large-scale precipitation changes on regional hydroclimate variability

We explore the reliability of large-scale climate variables, namely precipitation and temperature, as inputs for a basin-lake hydrological model in central Argentina. We used data from two regions in NCEP/NCAR reanalyses and three regions from LMDZ model simulations forced with observed sea surface temperature (HadISST) for the last 50 years. Reanalyses data cover part of the geographical area of the Sali-Dulce Basin (region A) and a zone at lower latitudes (region B). The LMDZ selected regions represent the geographical area of the Sali-Dulce Basin (box A), and two areas outside of the basin at lower latitudes (boxes B and C). A statistical downscaling method is used to connect the large-scale climate variables inferred from LMDZ and the reanalyses, with the hydrological Soil Water Assessment Tool (SWAT) model in order to simulate the Rio Sali-Dulce discharge during 1950-2005. The SWAT simulations are then used to force the water balance of Laguna Mar Chiquita, which experienced an abrupt level rise in the 1970s attributed to the increase in Rio Sali-Dulce discharge. Despite that the lowstand in the 1970s is not well reproduced in either simulation, the key hydrological cycles in the lake level are accurately captured. Even though satisfying results are obtained with the SWAT simulations using downscaled reanalyses, the lake level are more realistically simulated with the SWAT simulations using downscaled LMDZ data in boxes B and C, showing a strong climate influence from the tropics on lake level fluctuations. Our results highlight the ability of downscaled climatic data to reproduce regional climate features. Laguna Mar Chiquita can therefore be considered as an integrator of large-scale climate changes since the forcing scenarios giving best results are those relying on global climate simulations forced with observed sea surface temperature. This climate-basin-lake model is a promising approach for understanding and simulating long-term lake level variations.