Cecilia Laprida

Patagonian ostracods as indicators of climate-related hydrological variables: implications for paleoenvironmental reconstructions in Southern South America

Different precipitation regimes across Patagonia generate an environmental gradient that is expected to reflect upon the solute composition and concentration of lake water through the ensuing differences in water balance. In turn, this hydrochemical gradient could influence the occurrence and distribution of ostracods in the area. A cluster analysis on hydrochemical data of 36 Southern Patagonian waterbodies shows that low salinity, bicarbonate-dominated waters characterize Andean and ecotonal lakes, located in the wetter part of the precipitation gradient, while their steppe counterparts, which receive a much lower precipitation input, are more saline, enriched in all major anions and sodium-dominated. Kruskal–Wallis tests followed by Mann–Whitney pairwise comparisons show statistically significant differences in the ostracod species each water type hosts, with L. patagonica, E. cecryphalium, P. smaragdina and P. incae being found in the more dilute waters characteristic of Andean and ecotonal lakes and Limnocythere rionegroensis in the mesohaline, evaporatively evolved waters typical of steppe lakes. Other species studied do not appear to have such distinct distributions, having been found throughout the range of conditions studied. These findings promote the use of the aforementioned ostracods as semi-quantitative paleohydrological indicators, which could improve paleoclimatic reconstructions in Patagonia.

Benthic foraminifers and paleoecology of a Holocene shelly concentration, Salado Basin, Argentina

Foraminiferal fossil assemblages of the shelly concentration, which represents the type section of Cerro de la Gloria Member of Las Escobas Formation, were analyzed for the first time. Taxonomic, quantitative and taphonomical analyses were made in order to obtain paleoenvironmental (physical as well biological) information. The complexity of the internal structure of the taphocoenoses was analyzed to infer the history of the final concentration process. The shell bed of Channel 15 corresponds to a shoreface longshore bar deposited in a brackish, unstable, wave-dominate coast. Shoreface and beach (foreshore) regressive facies were recognized. Taphonomic processes have modified the paleobiocoenoses by the selective preservation of autochthonous elements and by the addition of parautochthonous fauna from relict beachs and lagoons. Thus, the taphocoenoses is a within-habitat time-averaged assemblage composed of species that inhabit shallow shelf marine environments. Discrimination of autochthonous and parautochthonous fauna was possible based on taphonomic signatures of tests. The addition of the parautochthonous elements was related with longshore currents and littoral drift, and with high-energy events, probably with storm-waves, when erosion of the Pre-Mid Holocene substrate was more intense. Thus, the contribution of parautochthonous fauna is directly related with lithology and therefore, with dynamic of the paleoenvironment: in the shoreface, autochthonous fauna dominates in clastic sands with lamination and parallel stratification, whereas parautochthonous elements dominate in shoreface shelly gravels and in the foreshore sands. In the upper part of the section (beach facies), mainly parautochthonous elements related with erosion of the Pre-Mid Holocene substrate was recognized, since the shoreface was only periodically affected by marine processes, i.e., during storms.

A review of the genus Limnocythere (Podocopida: Limnocytheridae) in the Pampean region (Argentina), with the description of a new species, Limnocythere cusminskyae sp. nov.

Occurrences of Quaternary and extant limnocytherids in the Pampean region of Argentina are reviewed, particularly in relation with their use as paleohydrological indicators in environmental reconstruction research. In spite of its spread occurrence, the systematic of the genus in the area is still little known, which results in most records being left in open nomenclature and paleoenvironmental inferences being made on the basis of general considerations of the genus. We describe a new species, Limnocythere cusminskyae sp. nov., which we consider to be the taxon featured in several of these works, and discuss its morphological and ecological differences with coexisting Limnocythere species, particularly the proposed indicator species L. rionegroensis Cusminsky & Whatley.

Chapter 6 The Argentine continental shelf: morphology, sediments, processes and evolution since the Last Glacial Maximum

Abstract The Argentine continental shelf is one of the largest and smoothest siliciclastic shelves in the world. Although it is largely emplaced in a passive continental margin, the southernmost regions are related to transcurrent and active margins respectively associated with the Malvinas Plateau and Scotia Arc. Sea-level fluctuations, sediment dynamics and climatic/oceanographic processes were the most important conditioning factors in the modelling of the shelf, with a minor influence from isostatic and tectonic factors that are more relevant in the southernmost regions. The shelf is shaped by diverse geomorphic features, among which the most significant are four sets of terraces genetically associated to sea-level stillstands during the post-glacial transgression; the final one occurred at around 11 ka and is associated with the Younger Dryas event. The Last Glacial Maximum (LGM) sedimentary sequence is composed of, on average, 5–15 m-thick terrigenous, siliciclastic, relict–palimpsest sands mainly sourced from the Andean region, with minor amounts of bioclast and gravels, resulting from the reworking of pre-transgressive coastal environments.

A high-resolution palaeoclimate record for the last 4800 years from lake la Brava, SE pampas plains, Argentina

Resumen Los cambios climaticos son reflejados en las variaciones de diferentes parametros. Las secuencias sedimentarias de lagos son buena fuente de esta informacion debido a que proveen grabaciones continuas y detalladas de cambios paleoclimaticos.Para determinar los cambios en el clima al SE de la llanura Pampeana, se presentan estudios de magnetismo de rocas realizados en un testigo colectado del fondo de la Laguna La Brava (Argentina). Tambien se midieron contenidos totales de sulfuro, carbon organico e inorganico (TS, TOC y TIC), elementos alcalinos, metales pesados y livianos, y cambios en las comunidades de vegetacion. Se realizaron cinco determinaciones de edades radiocarbonicas y se calcularon las edades calibradas. La tasa promedio de acumulacion de sedimento es 1.3 mm/ano y la secuencia representa los ultimos 4800 anos en edades calibradas (cal. BP).El objetivo principal fue reconstruir el balance M. A. Irurzun * C. S. G. GogorzaA. M. SinitoM. A. E. ChaparroInstituto de Fisica Arroyo SecoUniversidad Nacional del Centro de la Provincia de Buenos AiresPinto 399, 7000 Tandil, Argentina

Principles of Paleoceanographic Reconstruction

This chapter introduces the principles of paleoceanographic reconstructions and proxy data, focusing on the concepts of climatic, biological, geological, geochemical as well as other large-scale proxies, tracers and records useful for such reconstructions in the western South Atlantic. Different proxies particularly useful for the Argentine margin, including physical and chemical properties of sediments, microfossils and geochemical and isotopic properties, are described.

Climate and Oceanographic Background

The regional climatic and oceanographic characteristics that regulate the climate-ocean system in the region are explained. The climatic system has different features depending on regional and local forcings, such as the Intertropical Convergence Zone, the South American Monsoon System, the Southern Hemisphere Westerlies, and the South Atlantic Convergence Zone. The hydrographic structure is dominated by different water-masses of Antarctic and Tropical origin, related to the major circulation system of the Western South Atlantic linked to the Atlantic Meridional Overturning Circulation. The main water-masses constituting the hydrographic structure are the Tropical Water-South Atlantic Central Water, the Antarctic Intermediate Water, the North Atlantic Deep Water and the Antarctic Bottom Water, being most of the Antarctic-sourced water-masses genetically related to the circulation of the Antarctic Circumpolar Current.

Morphosedimentary Configuration of the Argentina Continental Margin

The major regional features of the margin are coastal plains, shelf, slope and rise. These features show varied characteristics depending on their location on each type of margin. They contain diverse subordinated morphosedimentary features related to the prevailing genetic processes acting on each of them and shaping their relief. The shelf is shaped in terraces at increasing stepping depths toward offshore, they having been genetically related to fluctuations in the sea-level rise during the post-glacial transgression. The slope is also shaped in terraces, but in this case, these features are genetically associated to the development of alongslope complex systems of mixed depositional and erosive contouritic features formed at different depths, possibly related to highly energetic interfaces between adjacent water masses that constitute the thermohaline oceanic system. Gravitational, downslope processes generating turbidites, mass transport and debris flows deposits, most of them acting inside or close to submarine canyons, actively interact with the contouritic processes. The rise is partially formed by gravity-driven deposits at the base of the slope, although in the southern part of the margin the alongslope, contouritic processes are strong enough to shape the rise and imprinting it with particular current-driven features.

The Argentina Continental Margin

The Argentina Continental Margin, located in the Southwestern Atlantic Ocean, is inserted in a key region of the World Ocean due to its significance in the global oceanographic–climatic interaction. It is the only place where Antarcticand Equator-sourced water-masses interact in mid-latitudes with a net transport of meridional heat between the Southern Pole and the Equator. On the other hand, the geotectonic history of the region imprints it with significant geological characteristics. As a result, the climatically, oceanographically and physically driven sedimentary processes occurred in the region have originated particular and almost unique morphosedimentary features, which constitute complete records of the processes involved in its evolution. Those features contain different kinds of proxies, tracers and records (biological, geochemical, sedimentological, morphological and structural) which provide valuable quantitative and qualitative evidences for detailed paleoceanographic, paleoclimatic and paleoenvironmental reconstructions. Therefore, the Argentine margin potentially behaves as a complete archive for understanding most of the unique oceanographic and climatic characteristics that occur in the region and impact int he rest of the Southern Hemisphere.

The Argentina Continental Margin: Location and Significance

The Argentina Continental Margin (ACM) is one of the largest margins on the Earth. It is located in a key region of the World Ocean that is highly significant in the planet’s oceanographic-climatic system. The evolution of the Argentine margin is explained in terms of the combination and interaction among geotectonic, oceanographic and climatic factors. Because of that evolution, four types of margins develop in the region: a passive-volcanic rifted, a transcurrent-sheared, a mixed and an active margin. These aspects determine particular tectonic and stratigraphic characteristics for each of them.