Scielo RSS <![CDATA[Latin American journal of sedimentology and basin analysis]]> http://www.scielo.org.ar/rss.php?pid=1851-497920140002&lang=en vol. 21 num. 2 lang. en <![CDATA[SciELO Logo]]> http://www.scielo.org.ar/img/en/fbpelogp.gif http://www.scielo.org.ar <![CDATA[Limnogeology in Southern South America: an overview]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200001&lng=en&nrm=iso&tlng=en One of the major goals of Limnogeology is to provide clues on past Earth system environmental unevenness and feedbacks on longer time scales (100s-1,000s of years) than instrumental records, thus including periods with null or low anthropic influences on the environment. The multiproxy approach in the analysis of lake records allows to gain a wider overview than could be acquired from a single proxy data. Unlike the Northern Hemisphere, reconstructions of Late Pleistocene and Holocene environmental variability across Southern South America have been hampered by the paucity of complete and well-dated paleoclimate archives. However, last decades have been marked by a substantial increase of paleoclimatic research providing new data to analyze past climate variability from a regional perspective in Southern South America. This special issue include five articles applying a variety of proxy data (physical, chemical and biological) to elucidate climate and environmental changes on various time scales. Contributions cover a wide geographic distribution from the Antarctic Peninsula, Patagonia, Pampean region and NW Argentina up to the Río de la Plata Estuary. Results provide critical elements for further assessments of latitudinal paleo-circulation dynamics and hydroclimatic changes. The recent proliferation of limnogeological studies in Argentina and Uruguay evidence the reinforcement of regional research networks providing comparative and integrative analysis. <![CDATA[El uso de parámetros magnéticos en estudios paleolimnológicos en Antártida]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200002&lng=en&nrm=iso&tlng=en En esta contribución se describen las distintas técnicas y mediciones magnéticas utilizadas en Magnetismo Ambiental y Paleomagnetismo. Tales mediciones ofrecen útiles indicadores para realizar estudios relacionados con cambios climáticos y ambientales, así como herramientas de datación. Si bien es ampliamente conocida la utilidad de la susceptibilidad magnética, en primer lugar se discute el potencial y necesidad del uso de parámetros adicionales obtenidos a partir de mediciones de magnetizaciones remanentes (natural, anhistérica e isotérmica), histéresis magnética y estudios termomagnéticos. A continuación se presentan resultados magnéticos obtenidos en sedimentos lacustres del Archipiélago James Ross (NE de la Península Antártica) como un caso de estudio. Se complementa con estudios sedimentológicos, hidroquímicos, geoquímicos y de estadística multivariada, pero se pone énfasis en los parámetros magnéticos y su relación con los distintos procesos que ocurren en los sistemas lacustres antárticos. Se analiza además el uso de las paleointensidades relativas como herramienta de datación en lagunas antárticas.<hr/>Environmental magnetism and paleomagnetism methods are useful tools for climate-related and environmental changes studies, as well as for age dating. In particular, they may be applied in magnetic monitoring and to investigate the behavior, response, and history of environments like rivers, lakes, soils, etc. Although the usefulness of magnetic susceptibility in such applications is well known, in this contribution the potential and the need of using additional parameters obtained from measurements of remanent magnetization (natural, anhysteretic, isothermal), magnetic hysteresis and thermomagnetic studies are discussed. Magnetic signals from sediments and soils are often dominated by ferromagnetic minerals sensu lato: ferrimagnetic (titano)-magnetite and/or antiferromagnetic hematite. These minerals are often not detected with standard analytical techniques due to their low concentrations (<1% wt). However, rock magnetic measurements are sensitive enough to detect and identify such minerals, with concentrations that are several orders of magnitude smaller. One important phenomenon to be taken into account is “magnetic enhancement” (Le Borgne 1955, Tite and Linington, 1975), which is caused by the in situ conversion of paramagnetic to ferrimagnetic materials. New formation of ferrimagnetic materials may occur by fermentation, heating, fire, biological activity and chemical processes (Mullins, 1977; Maher, 1986), and thus also be the result of pollutants and magnetized particles of anthropogenic origin (Chaparro et al., 2006, 2013a,b; Bucko et al., 2010; Marié et al., 2010; Zhang et al., 2011). Natural systems are complex and their behavior depends on the interaction of several independent forcing factors like climate variations and human activity affecting the environment. Parameters describing indirectly a system are called “proxies” and may be derived from physical, chemical or biological properties. Magnetic proxies have the advantage to be determined with high sensitivity combined with fast laboratory processing; sample preparation is easy, laboratory instruments are of relatively low cost, and most measurements are non-destructive. Magnetic properties are well suited as proxies of: a) magnetic mineral concentration; b) magnetic grain size; c) correlation of sediment cores and soil profiles; d) geomagnetic field records; e) climatic changes records; f) pollution records; and g) environmental processes records (Maher et al., 1999; Kravchinsky et al., 2003; Chaparro et al. 2006, 2007, 2013a,b, 2015; Irurzun et al., 2006, 2014a; Blundell et al., 2009; Bidegain et al., 2009; Torrent et al., 2010; Lascu and Plank 2013; Oldfield et al. 2014; Quijano et al., 2014). In this work, preliminary results from James Ross Archipelago lakes (NE Antarctic Peninsula) are presented. Sediment cores and water samples were taken from lake systems located on James Ross, Vega and Marambio islands (JRI, VI, MI, respectively, Fig. 1). Lake water’s pH and total dissolved solids exhibit a wide range of variation (<7 to 10 and <50 to 5000 mg/L, respectively), differentiating MI from JRI and VI in these aspects, due to the evaporation processes and a deficit of biological activity on MI. Organic and inorganic carbon contents in sediments are low and points to the dominance of siliciclastic materials. Thermomagnetic studies reveals in both JRI and VI the dominance of magnetite (Tc= 580°C) and subordinated titanomagnetite and/or iron sulfur (Tc= 290°C), goethite and hematite (Tc= 110°C and 680°C, Fig. 2). Lake Esmeralda (Fig. 1) shows a strong increase of magnetization after heating (sample ESM7-24, Fig. 2), maybe due to the initial presence of hematite or goethite and paramagnetic minerals, and their conversion to ferrimagnetic minerals. Magnetic measurements of both JRI and VI sediments, suggest the presence of very fine ferrimagnetic minerals (magnetite, <0.1 μm), while Esmeralda is characterized by larger particles (1-5 μm); and MI by fine ferrimagnetic and paramagnetic minerals (0.2-1 μm, Fig. 3). Magnetic hysteresis shows that magnetites tend to be of single domain size (SD) for JRI and Anónima (in Vega island) and of pseudo-single domain size (PSD) for Esmeralda (Fig. 4). The relation between a magnetic concentration dependent parameter (χ) and a magnetic grain size dependent parameter (κARM/κ) is shown in figure 5. MI and Esmeralda exhibits a trend of increasing χ and decreasing κARM/κ, that is, higher magnetic particle concentration is accompanied by coarser magnetic grain size. In contrast, JRI and Anónima show a linear relationship between parameters χ and κARM/κ, indicating a correlation of magnetic mineral concentration with decreasing grain size. A multivariate statistical analysis of magnetic concentration, magnetic grain size, physic-chemical, and geochemical characteristics shows an inverse relationship between magnetic and physic-chemical variables. Moreover, statistically significant differences between lakes from each studied island are observed. These differences may be due to individual basin and lake environment characteristics and the processes (diagenetic, biological, geochemical, etc.) occurring in them. Finally, Lake Anónima was selected to determine a curve of relative paleointensity (RPI) of high quality, and to calibrate these Antarctic lake sediments through its correlation with a dated RPI curve from Lake Esmeralda (Fig. 9). Five cores were correlated using χ and κARM/κ variations and they were converted to a common sedimentary depth, using one core as the master core. For RPI normalization, sediments must satisfy requirements proposed e.g. by Tauxe (1998): NRM/k, NRM/ARM and NRM/SIRM ratios should only depend on the intensity of the geomagnetic field. Those ratios were normalized to the unity for comparison with records from other lakes. The NRM15mT/k results (NRM15mT is the remanence left after demagnetization with 15 mT) for the Anónima cores are shown in Figure 9 and compared to the corresponding Esmeralda mean curve (Irurzun et al., 2013). Both curves show a similar behavior, with at least three possible lines of correlation. The minimum ratio NRM15mT/k at 68 cm for Anónima is clearly correlated with Esmeralda minimum ratio at the age of 7,700 cal. years. This suggests that sediments recovered from Anónima may have recorded changes of the Earth magnetic field during the last 6,000-7,000 years. We conclude that the magnetic proxies, besides being useful environmental indicators, allow the discrimination between study areas and also to select target region for future studies (e.g. paleomagnetism, environmental changes, etc.). The RPI technique is a very promising tool for dating sediments in Antarctic lakes. This study contributed to a better understanding of Antarctic lake systems from a multidisciplinary approach. <![CDATA[Paleobioindicadores del fin del mundo: ostrácodos y quironómidos del extremo sur de Sudamérica en ambientes lacustres Cuaternarios]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200003&lng=en&nrm=iso&tlng=en El objetivo de este trabajo es resumir el estado de las investigaciones paleolimnológicas basadas en bioindicadores pertenecientes al Phylum Arthropoda, en particular ostrácodos (Crustacea) y quironómidos (Diptera), en el ámbito de la República Argentina. Si bien la mayor parte de las investigaciones se han efectuado en el ámbito de la Patagonia, donde han permitido la reconstrucción paleolimnológica de registros desde el último Máximo Glacial y el Holoceno, estos estudios se fueron extendiendo progresivamente hacia las regiones pampeanas, Cuyo y el Noroeste, abarcando distintas ventanas temporales del Pleistoceno Tardío y el Holoceno. Son cada vez más numerosas las investigaciones que integran datos actuales y fósiles de ostrácodos y quironómidos, lo que ha permitido desarrollar funciones de transferencia dando lugar a reconstrucciones cuantitativas robustas y mejorar las interpretaciones cualitativas basadas en la presencia de especies indicadoras. Ambos tipos de reconstrucciones son cada vez más frecuentemente combinadas con otros proxies biológicos, químicos o físicos, brindando un marco multiproxy que permite reconstrucciones integrales de los ecosistemas lacustres cuaternarios. Los estudios de ostrácodos y quironómidos de registros lacustres han permitido no sólo reconocer las tendencias ambientales y climáticas de los últimos miles de años, sino también reconocer eventos climáticos de media y alta frecuencia de interés global, tales como el Younger Dryas, el Antarctic Cold Reversal, el Evento Frío 4,2ka, y la Pequeña Edad de Hielo, así también como los cambios climáticos e impactos antrópicos del siglo 20.<hr/>The main goal of this contribution is to summarize our current knowledge on paleolimnological research in Argentina using bioproxies, particularly ostracods (Crustacea) and chironomids (Diptera). Additionally, we present our perspective on the strengths and weaknesses of the major approaches to reconstruct past climate from these biological proxies. Ostracod and chironomid assemblages are powerful tools in Paleolimnology. These arthropods live in continental waters where they can reflect climate and environmental changes because of their high sensitivity to variations in the physical and chemical parameters controlling aquatic systems. Based on their modern ecological requirements it is possible to apply numerical, multivariate, and qualitative techniques to reconstruct some key parameters of late Quaternary environments. For the past 20 years, the use of modern and fossil ostracods and chironomids has increased significantly allowing the development of robust and unbiased high-resolution environmental reconstructions including climate applying multivariate approaches (i.e., transfer functions) and qualitative interpretations. Indicator species often consolidate investigations. The combination of biological, physical, and chemical proxies provides a network of independent data that allow the comprehensive understanding of late Quaternary continental aquatic ecosystems. Ostracods and chironomids have permitted not only to recognize long- and mid-term climate and environmental trends but also high frequency climatic events of global interest, like the Younger Dryas (YD), the Antarctic Cold Reversal (ACR), the “4,2ka Dry Event”, and the Little Ice Age (LIA), as well as climate changes occurred during the 20th century, including anthropogenic impact. Most of these studies have been performed in North America and southern Patagonia of Argentina, permitting to reconstruct the climatic and environmental evolution since the Last Glacial Maximum (LGM). More recently, this kind of research has expanded towards lower latitudes in Argentina, like the Pampas Plain and the Northwest Argentine Altiplano or Puna, covering different temporal windows of the middle and late Holocene (Fig. 1). To date, Patagonia offers the best records due to the active research conducted by local and foreign scientist. Based on the suitability of high-resolution sedimentary archives (Table 1), these studies have proven useful for understanding the evolution of the Southern Hemisphere Quaternary climate. This is especially true for the Andean lakes that may include continuous sequences since the LGM. (Whatley and Cusminsky, 1995, 1999; Cusminsky and Whatley, 2008; Cusminsky et al., 2011; Massaferro et al., 2004, 2009, 2014). By contrast, the Patagonian steppe lakes and ponds may be more sensitive to environmental change but the records are shorter, younger, and fragmentary (Massaferro and Laroque, 2013; Ramón Mercau et al., 2010, 2012b). In spite of the good quality of records in Patagonia, the vastness of the region, and the unattainability of some areas, it makes sometimes difficult to assess the systematic position and autoecology of the taxa involved at the expense of the reconstruction’s quality. On the other hand, the Pampean records (Fig. 1, Table 2) include mostly Holocene sediments allowing the understanding of the trophic history (Laprida et al., 2014), rainfall patterns (Laprida and Valero Garcés, 2009) and climate gradients (Laprida et al., 2009) in one of the most social-economically important regions of the country. The results obtained in this study corroborate the potential value of these arthropods as proxies of late Quaternary environmental change, and suggest directions of future research to improve our understanding of the biological interactions with the environment and climate. New investigations are required on the taxonomy of modern ostracods and chironomids, their ecological requirements and distribution to be able to extrapolate this knowledge to the past generating calibration methods that would cover broad gradients. The development of quantitative techniques for inferring past environments from paleoecological data and of multiproxy studies using several variables, can allow the study of biotic responses to and the testing of hypotheses about climate change. <![CDATA[Historia ambiental de los lagos someros de la llanura Pampeana (Argentina) desde el Holoceno medio]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200004&lng=en&nrm=iso&tlng=en En este trabajo se presenta un estudio paleolimnológico basado en el análisis de múltiples indicadores en múltiples sitios con el objetivo de reconstruir la historia evolutiva de cinco lagos someros de la llanura Pampeana (Argentina) desde el Holoceno medio, y así realizar inferencias paleoclimáticas para esta región. Los cinco lagos estudiados respondieron sincrónicamente y mostraron el mismo patrón de evolución paleoambiental durante el Holoceno medio y tardío. Entre los ca. 7.000 y 700-500 años cal AP, se evidencia una primera fase clara, dominada por carofitas del género Chara, que cambia a los ca. 700-500 años cal AP hacia una fase turbia dominada por fitoplancton y con una importante presencia de carofitas sumergidas del grupo de las angiospermas, que indican fases claras que alternan con el estado turbio general. En la vegetación circundante, se evidencia un cambio desde una comunidad halófita que indica condiciones salobres e inestables hacia la comunidad de macrófitas que caracterizan en la actualidad a estos lagos, por lo cual sugiere ambientes con mayor estabilidad. Durante el Holoceno medio y por largos períodos (alrededor de 6.500 años) la estabilidad del paisaje se mantuvo debido a las condiciones climáticas más secas que las actuales, en las que predominaron fluctuaciones de sequías e inundaciones y/o condiciones de alta evaporación. Estas fluctuaciones son las que mantuvieron a las comunidades tanto acuáticas como terrestres. A partir de ca. 2.000 años cal AP el incremento de la diversidad de macrófitas sumergidas (carofitas y angiospermas) indica mayor aporte de nutrientes y materia orgánica, causado por un aumento en la intensidad y/o duración de las precipitaciones. A partir de este momento comienza un cambio gradual que se efectiviza a los ca. 700-500 años cal AP, con el establecimiento de las comunidades de macrófitas emergentes, flotantes y sumergidas, características de los sistemas actuales.<hr/>High-resolution paleolimnological studies based on multi-proxy analysis constitute an important tool to reconstruct the evolution of aquatic systems as well as evaluating their responses to natural and/or anthropogenic forcing factors (Lotter, 2003; Birks and Birks, 2006). While individual palaeoecological studies reveal local developments, general patterns often only emerge when information from several sites is combined together. Numerous shallow lakes occur throughout the Pampa plain of Argentina, between 33° to 39°S and 57° to 66°W. These lakes present two alternative states of equilibrium (Scheffer and Jeppesen, 2007; Scheffer and van Ness, 2007). Some of them are turbid lakes due to the high amount of algae, while others are clear macrophytedominated lakes. A third type of lake that can be recognized within the region is inorganic-turbid lakes, in which turbidity is caused by high amount of suspended inorganic material (Quirós et al., 2002; Allende et al., 2009). These lakes are characterized by low productivity, with scarce phytoplankton and macrophytes. Most of the lakes developed on deflation basins that originated during the late Pleistocene by the prevailing westerly winds. Often, the lakes are associated to lunettes, fixed palaeodunes that developed at the shore, on the windward side of the basins (Tricart, 1973; Zárate and Tripaldi, 2012). During the Holocene, the basins became areas of groundwater discharge and surface water accumulation gradually filling with sediments. Today, the lakes are nutrient-rich, eutrophic to hypereutrophic, and polymictic, too shallow to develop thermal stratification. Water depth and salinity are highly variable. During summer and/or during episodic droughts the lakes suffer significant reduction in water volume, and thus strong fluctuations in their water levels (Quirós and Drago, 1999; Sosnovsky and Quirós, 2006). These aquatic systems are able to support an abundance of macrophytes and phytoplankton, which have proven to leave an exceptional fossil record of environmental changes (Stutz et al. 2002, 2006, 2010, 2012; Fontana, 2005). For more than a decade the authors have been investigating these aquatic systems in order to reconstruct their evolutionary history and the regional environments with the main goal of inferring the past climatic conditions. In order to achieve this, multi-proxy analysis of diverse biological indicators were carried out in several lakes of the south-east region of the Pampa plain. Pollen, non-pollen palynomorphs and plant macrofossil remains and associated fauna were analysed in sediment sequences from lake Hinojales- San Leoncio (37º23’S; 57º23’W) and lake Tobares (37°30’S; 57°28’W). These new results were then combined with former investigations from lakes Lonkoy (37º12’S; 57º25’W), Nahuel Rucá (37º37’S; 57º26’W) and Hinojales (37º34’S; 57º27’W) (Fig. 1). The studied lakes range in suface from 200 to 300 ha and a water depth of about 1 m. The regional vegetation is characterized by temperate subhumid grasslands, knowns as pampas. The modern landscape is strongly influenced by human activities since the establishment of European settlements in the XVI century. The land has been intensively used, in particular for grassing, as well as for agriculture. Native trees are absent in the vegetation, except for Celtis ehrenbergiana, a deciduous tree known as tala, which occurs on Pleistocene lunette dunes and Holocene ridges of shell debris. The aquatic plant communities of the studied sites are characterized by several species of emergent, free-floating leaved and submerged macrophytes. The lakes are surrounded by a ring of Schoenoplectus californicus, among which other emergent macrophytes occur: e.g. Zizaniopsis bonariensis, Typha latifolia, Hydrocotyle bonariensis, H. ranunculoides, Alternanthera philloxeroides, Solanum glaucophyllum, Bacopa monnieri, Polygonum punctatum, Ranunculus apiifolius, Triglochin striata, and Phyla canescens. Near the shore, in sheltered areas, free floating plants like Ricciocarpus natans, Azolla filiculoides, Limnobium laevigatum, Lemna valdiviana, Wolffia brasiliensis, Wolffiella lingulata and W. oblonga form a dense carpet. Diverse submerged macrophytes occur when the water tends to be clear, so the light available is enough for their development: e.g. Myriophyllum elatinoides, Ceratophyllum demersum, Potamogeton pectinatus, and Zannichellia palustris together with diverse Charophytes. The studied sediment records were recovered with different samplers: Dacknovsky (Hinojales), vibracorer (Nahuel Rucá, Hinojales-SL and Tobares) and Livingston-type corer (Lonkoy). The chronology of the records is based on AMS radiocarbon age determinations on terrestrial plant remains, where possible (Table 1). Gastropod shells of Heleobia parchappii have also been used for dating, since previous studies have yielded reliable results (Fontana, 2005, 2007). Age-depth models (Fig. 2) are constructed using CLAM 2.2 (Blaauw, 2010) with the Southern Hemisphere calibration curve, SHCal13 (Hogg et al., 2013). The studied records, spanning the time since the middle Holocene, document similar evolutionary pattern. Changes in the different proxies are consistent, showing similar trends in the evolution of the aquatic ecosystems (Figs. 3-7). At the beginning of the records a clear phase characterized the water bodies, dominated by the green algae Chara, the dinoflagellate Peridinium and aquatic plants such as Myriophyllum, Ruppia and/or Potamogeton. At ca. 2,000 cal yr BP the diversity of submerged macrophyte communities increased, indicating clear water lakes with increasing nutrient content. Towards ca. 700-500 cal yr BP the lakes switched to a turbid phase dominated by phytoplankton together with water fleas and flatworms. Among the phytoplankton the main constitutes are: Pediastrum, Scenedesmus and Tetraedron species of green algae Desmidiaceae and the cyanobacterium Gloeotrichia. On land, halophyte plant communities, dominated by Chenopodiaceae, surrounded the water bodies during the first clear-water phase, suggesting instable periods with brackish conditions. Towards the present, the surroundings of the lakes are characterized by dominance of Cyperaceae together with Bacopa, Ranunculus, Polygonum, Typha and Apiaceae. This vegetation is characteristic of more stable environmental conditions. During the middle Holocene and part of the late Holocene, brackish-shallow lakes with clear water phases characterised the landscape of the south-east pampean plain. After ca. 2,000 cal. yr BP, water run-off and/or wind action increased, incorporating organic matter from the surroundings into the basins and initiating a gradual change towards the next phase. This dynamics could be associated to periods of pronounced drought followed by floods. A marked seasonality in the annual precipitation regimen would explain this dynamics indicating a regionally instable environment with dryer climatic conditions than present (Zárate et al., 1998; Zárate, 2005; Vilanova et al., 2010; Laprida et al., 2014). The change to turbid conditions at around 700-500 cal yr BP was probably caused by a significant input of nutrients into the basin, impeding the development of submerged aquatic plants, characteristic of the clear phase, which in turn favoured the expansion of phytoplankton. However, the presence of submerged macrophytes that tolerate some degree of turbidity like Miryophyllum, Ceratophyllum and Potamogeton would suggest that during some periods the level of turbidity did not reach critical values. The presence of submerged flowering plants may also suggest alternate periods of clear phases within a general turbid state. The synchronous change to a turbid face in all studied sites suggests a climatic regional trigger like an increase of precipitation, with a more stable seasonal regime. Similar values of precipitation compared to today did not occur until historical times (Irurzun et al., 2014; Laprida et al., 2014). This study constitutes the first of its type basing the reconstruction of paleoenvironmental and paleoclimatic conditions on the dynamics and functioning of the shallow lakes, studied in the context of multiproxy and multi-site analysis. <![CDATA[Una visión paleolimnológica de la variabilidad hidroclimática reciente en el centro de Argentina: desde la pequeña edad de hielo al siglo XXI]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200005&lng=en&nrm=iso&tlng=en En este trabajo se presenta una revisión de diferentes estudios paleolimnológicos previamente publicados y desarrollados en la región subtropical Argentina, con la finalidad de efectuar comparaciones latitudinales de la respuesta hidrológica de estos sistemas lacustres frente a las variaciones de la circulación atmosférica asociada a la dinámica del Sistema Monzónico Sudamericano desde la Pequeña Edad de Hielo (PEH) hasta la actualidad. Con este objetivo se analizan y comparan los registros paleolimnológicos recientes (~ 250 años) de las lagunas Mar Chiquita (Córdoba), Melincué (Santa Fe) y Encadenadas del Oeste (Buenos Aires). La necesidad de analizar integradamente la variabilidad hidroclimática de alta (1 a 101 años) y baja frecuencia (102 años en adelante) en el centro de Argentina toma relevancia a partir del gran “salto hidroclimático” ocurrido durante la década de los años 70 en el sudeste del continente sudamericano. Este cambio caracterizado por un aumento notable en las precipitaciones ha sido registrado como uno de los mayores saltos hidrológicos ocurridos en ambientes continentales a nivel global. Debido a que algunos patrones de variabilidad climática se caracterizan por períodos largos, es difícil discernir si la variabilidad ambiental observada es natural o bien corresponde a una señal de cambio con múltiples forzantes (antrópicos + naturales). En este sentido las reconstrucciones hidroclimáticas basadas en indicadores múltiples (sedimentología, geoquímica, bioindicadores, isótopos estables) permiten conocer la variabilidad ambiental durante un período superior al percibido por los habitantes de una región afectada, aportando a la sociedad el conocimiento básico para abandonar la idea del clima estacionario, suministrando además herramientas para efectuar eficientemente la Gestión Integrada de los Recursos Hídricos. Las reconstrucciones paleohidrológicas y paleoambientales de las secuencias estudiadas indican que en general durante la finalización de la PEH predominaron condiciones áridas a lo largo de la región Pampeana, reflejadas por los niveles bajos a extremadamente bajos de las lagunas, con episodios de fases lacustres de niveles intermedios de menor duración. La PEH se habría extendido hasta la década de 1870/80 AD, momento que es indicado por el pasaje de sistemas lacustres efímeros a perennes. A partir de ca. 1870/80 AD y hasta 1976/77 AD se produce un mejoramiento climático progresivo a partir de un incremento sostenido de la humedad efectiva, que se refleja en los niveles intermedios alcanzados por las lagunas desde la segunda mitad del siglo XIX. Durante los últimos ~ 40 años se establecieron los niveles lacustres más altos registrados desde la PEH, dando lugar al establecimiento de las condiciones actuales de las lagunas. Los resultados obtenidos permiten perfeccionar los modelos planteados sobre la variabilidad hidroclimática pasada en las regiones ubicadas hacia el E-SE de la Diagonal Árida Sudamericana y aportan información crucial para descifrar la actividad del Sistema Monzónico Sudamericano en su zona de influencia más austral.<hr/>This paper provides a review of the hydroclimatic variability reconstructions along the subtropical Argentinean region based on paleolimnological records from Laguna Mar Chiquita (Córdoba; Piovano et al., 2002, 2004, 2009), Laguna Melincué (Santa Fe; Guerra, 2015; Guerra et al., 2015) to Lagunas Encadenadas del Oeste (LEO; Buenos Aires; Córdoba, 2012; Fig. 1). Lake records span two climatologically interesting periods, the so-called Little Ice Age (LIA; Grove, 2001; Wanner et al., 2008) and the 20th century. Regional climate in the studied area is mainly defined by the South American Monsoon System that rules the precipitation regime and is one of the major atmospheric features driving seasonal climatic variability in southeastern South America (Vera et al., 2006; Garreaud et al., 2009; Carvalho et al., 2011; Fig. 1). The need for an integral analysis of the high (1-101 years) and low (>102 years) frequency hydroclimatic variability associated with the South American Monsoon System activity becomes relevant when considering the significant “hydroclimatic jump” occurred during mid-1970s in the southeast of South America (Castañeda and Barros, 1994; Boulanger et al., 2005; Piovano et al., 2009; Carvalho et al., 2011; Jacques-Coper and Garreaud, 2014). This “jump” toward humid conditions (Figs. 2, 3) has been recorded as one of the largest instrumentally recorded hydrological changes occurred globally in continental environments (Giorgi, 2002). Because some climate variability patterns are characterized by long periods, it is difficult to distinguish whether the observed environmental variability is natural or corresponds to a climatic change with multiple forcing factors (natural plus anthropogenic). In this sense hydroclimatic reconstructions based on multiple proxies (sedimentology, geochemistry, biomarkers, stable isotopes) provide insight into how was environmental variability during a longer period than that perceived by the people of an affected region. Results of instrumental data blended with multiproxy studies on sedimentary cores from Laguna Mar Chiquita (Fig. 5), Laguna Melincué (Fig. 6) and Lagunas Encadenadas del Oeste (Fig. 7) indicate that Pampean lake systems have clearly recorded hydrological variations around the end of the LIA (since AD 1770) to the present. Sedimentological, geochemical and isotopic data (Figs. 5, 6 and 7) combined with robust chronologies based on 210Pb profiles (Fig. 4) and historical data (Piovano et al., 2002, 2004; Guerra, 2015; Guerra et al., 2015; Córdoba, 2012; Córdoba et al., en revisión) provide the framework for building a sedimentary model for Pampean shallow lakes with highly variable water depth and salinity (Fig. 8). Intervals with either negative or positive hydrological balances control lake water levels, salinity and primary productivity, and also the isotopic composition of both authigenic carbonate (d18Ocarb and d13Ccarb) and sedimentary organic matter (d13Com). Extensive evaporation during lowstand stages results in an enrichment of 18O and 13C in the lake waters, and is recorded in the sediments as the most positive d18Ocarb and d13Ccarb compositions. Conversely, more negative d18Ocarb and d13Ccarb values are the result of increasing freshwater input into the lake system. Relatively low d13Com values correspond with high lake levels, low salinity, low alkalinity and high lake productivity. High water salinity during lowstands diminishes the amount of primary production and the d13Com value is correspondingly high. Lake water level drops and concurrent increases in salinity promoted the development of evaporitic layers and a marked decrease in primary productivity. The deposits of these dry stages are evaporite-bearing sediments with a low organic matter content. Conversely, highstands are recorded as organic matter-rich muds. These results show that Pampean lakes are good sensors of high- and low-frequency changes in the recent hydrological budget and, therefore, document climatic changes at middle latitudes in south-eastern South America. The paleohydrological reconstructions based on these Pampean lacustrine sedimentary sequences (Figs. 5, 6 and 7) allowed identifying three major environmental periods (Fig. 9). The paleolimnological records indicate that during the end of the LIA arid conditions prevailed along the Pampean region, reflected by low to extremely shallow-water depths, with shorter intermediate lake-level phases (Period III; Fig. 9). The LIA would have extended until AD 1870/1880, as indicated by the passage from ephemeral to perennial lake systems. From ca. AD 1870/1880 to 1976/1977 a progressive climate improvement after a sustained increase in effective moisture occurred along the region, reflected by the intermediate lake levels achieved since the second half of the nineteenth century (Period II; Fig. 9). During the last ~ 40 years the highest lake levels of the analyzed period were established, leading to the development of the present-day hydrological conditions (Period I). These results allow improving the models based on past hydroclimatic variability in areas located east-southeast of American Arid Diagonal (Bruniard, 1982; Piovano et al., 2009), and provide critical information to decipher South American Monsoon System activity in its southernmost influence area. <![CDATA[Cambios históricos en el aporte terrígeno de la cuenca del Río de la Plata sobre la plataforma interna Uruguaya]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200006&lng=en&nrm=iso&tlng=en El Río de la Plata (RdlP) presenta significativas variaciones naturales (hidrodinámicas y oceanográficas) asociadas a diferentes condiciones climáticas. El propósito de este trabajo es inferir los cambios de aportes continentales de sedimentos y su relación con las variaciones hidrológicas del Río de la Plata, a través del análisis de proxies sedimentológicos y geoquímicos en testigos de sedimentos de la plataforma interna uruguaya que registran los últimos 100 años, aproximadamente. A partir de la datación por 210Pb de dos testigos de sedimentos (GeoB 13813-4 y BAR1) se reconstruyó la geocronología del ambiente, y se relacionó con datos de las forzantes climáticas Pacific Decadal Oscillation, El Niño/La Niña Southern Oscillation, Atlantic Multidecadal Oscillation, y las anomalías hidrológicas de los ríos Paraná y Uruguay. Los valores más positivos y estables del Southern Oscillation Index, los cuales corresponden a fases La Niña, se observan en el periodo correspondiente entre 1910-1970, respecto al resto de la serie, donde se aprecia una mayor variabilidad y una tendencia hacia valores más negativos (eventos El Niño). Se hicieron dendrogramas (clustering) jerárquicos para ambos testigos. Para el testigo GeoB 13813-4, se utilizó la relación Ca/Ti y la granulometría, mientras que para BAR1 se recurrió a variables granulométricas y la tasa de sedimentación. El mayor aporte continental hacia la región de la plataforma adyacente al Río de la Plata registrado a partir del año 1970, podría ser el factor principal de los agrupamientos observados en los clusters para ambos testigos. Las agrupaciones mostraron una diferenciación en la década de 1970, lo que estaría asociado al aumento de los caudales de los ríos Paraná y Uruguay, durante las últimas tres décadas del siglo XX. Por otra parte se observa que la granulometría del testigo BAR1 presentó un mayor tamaño de grano y más variabilidad que en el caso del testigo GeoB 13813-4. También se determinó una mayor acumulación de sedimentos a través del tiempo en el cinturón de barro del Río de la Plata (plataforma continental adyacente), comparado con aquel registrado en la Barra del Indio (límite entre zona intermedia y externa del estuario). Estas diferencias podrían estar relacionadas con la influencia del Río de la Plata, el cual genera un ambiente altamente dinámico sobre la Barra del Indio y un ambiente más estable sobre el cinturón de barro en la plataforma continental.<hr/>The Río de la Plata Estuary (RdlP) is a fluviomarine system that drains into the Southwestern Atlantic Ocean with the Paraná and Uruguay rivers as main tributaries. The estuary is fed by a 3,100,000 km2 catchment area which extends over the territories of Argentina, Paraguay, Brazil, Bolivia and Uruguay (Acha et al., 2008). The RdlP exhibits significant natural decadal- and annual-scale, hydrodynamic and oceanographic variability associated with the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO) and the El Niño/ La Niña Southern Oscillation (ENSO) (Depetris and Pasquini, 2007b; Chiessi et al., 2009; Garreaud et al., 2009). Such variability affects the moisture budget over the surrounding continental areas and leads, thus, to changes in the river discharge. PDO is associated with ENSO as both appear to display similar hydrological responses, though their inherent mechanisms are not yet fully understood (Garreaud et al., 2009). In this sense, warm and cold PDO phases strongly resemble El Niño and La Niña events, respectively (Garreaud et al., 2009). During El Niño episodes, an increase in precipitation over the RdlP drainage basin is commonly observed (Boulanger et al., 2005; Camilloni, 2005; Garreaud et al., 2009; García-Rodríguez et al., 2014), and consequently increased Paraná and Uruguay river discharges are displayed (Depetris and Pasquini, 2007a). Campos et al. (1999) have recorded a freshwater plume of low salinity and temperature (32, 18 ºC respectively) associated with an increase in RdlP discharge during the El Niño event of 1997, expanding northwards up to 23ºS. Furthermore, during negative AMO phases it was recorded an increasing trend on the precipitations over the SE South America (SESA) and, as a consequence, a concomitant increase in the Rivers Paraná and Uruguay discharge was recorded, while the opposite pattern was observed during positive phases (Chiessi et al., 2009). The aim of this paper is to infer the link between changes in the delivery of terrigenous sediment to the adjacent Atlantic Ocean with recorded hydrological variability of the RdlP. To achieve this, we used sedimentological and geochemical proxies from two sediment cores, which were retrieved from the inner continental shelf off Uruguay and encompass the past 100 AD. Sediment Core GeoB 13813-4 was taken from the inner-shelf “RdlP paleo-valley mudbelt” (Fig. 1;34°44’13’’S, 53°33’16’’W) during research cruise M76/3a (Krastel et al., 2012; Lantzsch et al., 2014). Sediment Core BAR1 was retrieved in the innershelf “Barra del Indio” zone (Fig. 1; 35°03’00’’S, 56°09’00’’W), performed by the Faculty of Sciences, Universidad de la República (Uruguay). For Core GeoB 13813-4, we analyzed the relative abundance of major elements (Ca and Ti) obtained by an X-ray fluorescent sediment core scanner AVAATECH and the Ca/Ti ratio was used to infer continental versus marine influence. This chemical elemental ratio was chosen according to previous successful applications within the Atlantic Ocean (Chiessi et al., 2009; Mahiques et al., 2009; Govin et al., 2012; Bender et al., 2013; Burone et al., 2013). Furthermore, the arithmetic mean grain size distribution was studied from both sediment cores, using the GRADISTAT program (Grain Size Distribution and Statistics Package for the Analysis of Unconsolidated Sediments) version 8 (Blott and Pye, 2001) (Fig. 4). For Core GeoB 13813-4 grain size were obtained by Laser Particle Sizer LS200 and for Core BAR1 were obtained by Malvern Mastersizer 2000 Laser analyser. The chronology from both cores was assessed by 210Pbxs dating (Table 1 and Fig. 2; Appleby, 2001; 2008). For core BAR1 we selected the CRS (Constant Rate Supply) model (which is highly used for estuarine systems), while for GeoB 13813-4 the CFCS (Constant Fluxe: Constant Sedimentation Rate) model was applied (Appleby, 2008; Bernal et al., 2010). In the last case, the decision of using the CF-CS model was due to the lack of a complete 210Pb dataset, which would bring very high sedimentation rates uncertainties (Sanchez-Cabeza and Ruiz-Fernández, 2012). To assess the climatic variability over the past century as inferred from the sediment proxies, we evaluated the climatic indices PDO and Southern Oscillation Index (SOI), of the Joint Institute for the Study of the Atmosphere and Ocean, University of Washington (http://jisao.washington.edu), as well as the AMO from the NOAA (http://www.aoml. noaa.gov/phod/amo_faq.php). We further compared these data with temporal series (encompassing the last century) of the Paraná and Uruguay fluvial discharges (http://www.hidricosargentina. gov.ar/acceso_bd.php), river-flow anomalies were calculated following the approach of Piovano et al. (2004). The generated proxy data were analyzed by running cluster analyses using the stratigraphically constrained Moristia similarity index, in PAST program version 3 (http://folk.uio.no/ohammer/ past/). The generated groups are represented with red lines in figure 3. The sedimentation rate of core GeoB 13813-4 was assumed to be constant with a mean value of 1.3 cm yr-1 (Table 1; Perez Becoña, 2014), while for the sedimentation rate of core BAR1 three groups were observed: 1911-1973; 1976-1984 and 1986-2010. The mean sedimentation rate for the above groups showed an increasing trend from 0.24 ± 0.13 cm yr-1 to 0.31 ± 0.14 cm yr-1 and 0.37 ± 0.10 cm yr-1, respectively. The most positive and stable values of SOI (La Niña events) were recorded during 1910-1970. After 1970, a higher variability and a trend towards more negative values was observed (El Niño events). After the year 2005, very negative SOI values occurred (Fig. 4). PDO showed either negative or close to zero values during the early period 1910-1970 (cold phase). During the subsequent interval, i.e., 1970- 2005, positive values (warm phase) were observed. Regarding with AMO, a positive phase was observed from 1925-1960, followed by a negative phase (1960- 2000), but then a shift to a positive phase until the present was observed. The Paraná river discharge anomalies for the years 1910-1970 were mostly associated with negative values (Fig. 4), while between 1970 and 2010 positive anomalies were documented. Between the years 2000 and 2010, we mostly registered values close to zero. Furthermore, the trends in AMO and SOI indices were negatively associated with the anomalies of both Paraná and Uruguay rivers flows, while PDO index were positive associated with such anomalies. A change in mode polarity observed for PDO and AMO took place by the middle 1970s, in addition to more frequent and intense El Niño events that led to the increased rainfall over SESA (Garreaud et al., 2009). Thus, the increase in rainfall over SESA was concomitant with positive anomalies in the Paraná and Uruguay river discharge rates after 1970 (Camilloni, 2005). In this sense, the Paraná river discharge was 20% higher during the past 30 years than the historical average of the 20th century (Mauas et al., 2008). The results of the cluster analyses groups (Fig. 3) showed a differentiation in both sediment cores that correspons to the beginning of the 1970s, which could be associated with the increasing discharge trend recorded for the Parana and Uruguay rivers over the last three decades. The increase in RdlP discharge led to a higher accumulation rate of terrigenous sediments, as inferred from the high sedimentation rate and mean grain size (BAR1), and the lowest Ca/ Ti ratio (GeoB 13813-4), and explains both the spatial and temporal sedimentological and geochemical variability. Ca/Ti ratio in the RdlP was successfully used to infer marine vs. continental influence, as Ti is associated with a continental RdlP discharge, while Ca is associated with autochthonous marine productivity (e.g. foraminifera, Burone et al., 2013). Thus, the highest continental sediment supply to the inner continental shelf is observed in GeoB 13813-4 after 1970, associated with a decrease in the Ca/Ti ratio (Fig. 4). Regarding with Core BAR1 the grainsize distribution and the sedimentation rate were both associated with the estuarine hydrodynamic changes. After 1970, the highest and most variable sedimentation rate and mean grain size was found, probably associated to an increase in both the Paraná and Uruguay river discharges during the past three decades, while the lowest and more stable sedimentation rates and mean sediment grain size recorded before 1970, is indicating a reduced RdlP freshwater supply to the study area. This study shows that both sediment cores contain a distinct continental runoff record as the result of climatic changes (PDO, AMO and ENOS), which have influenced the precipitation patterns over SESA. Both sites reflect similar responses to these environmental changes for the last 100 yr in continental terrigenous sediment supply from the RdlP watershed towards the inner continental shelf. We conclude that it is possible to assess the temporal? RdlP discharge patterns variability within the estuarine and adjacent oceanic area through the study of terrigenous proxies from sediment cores retrieved within the continental shelf. <![CDATA[Dr. Gualter Chebli: (1940-2014)]]> http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S1851-49792014000200007&lng=en&nrm=iso&tlng=en El Río de la Plata (RdlP) presenta significativas variaciones naturales (hidrodinámicas y oceanográficas) asociadas a diferentes condiciones climáticas. El propósito de este trabajo es inferir los cambios de aportes continentales de sedimentos y su relación con las variaciones hidrológicas del Río de la Plata, a través del análisis de proxies sedimentológicos y geoquímicos en testigos de sedimentos de la plataforma interna uruguaya que registran los últimos 100 años, aproximadamente. A partir de la datación por 210Pb de dos testigos de sedimentos (GeoB 13813-4 y BAR1) se reconstruyó la geocronología del ambiente, y se relacionó con datos de las forzantes climáticas Pacific Decadal Oscillation, El Niño/La Niña Southern Oscillation, Atlantic Multidecadal Oscillation, y las anomalías hidrológicas de los ríos Paraná y Uruguay. Los valores más positivos y estables del Southern Oscillation Index, los cuales corresponden a fases La Niña, se observan en el periodo correspondiente entre 1910-1970, respecto al resto de la serie, donde se aprecia una mayor variabilidad y una tendencia hacia valores más negativos (eventos El Niño). Se hicieron dendrogramas (clustering) jerárquicos para ambos testigos. Para el testigo GeoB 13813-4, se utilizó la relación Ca/Ti y la granulometría, mientras que para BAR1 se recurrió a variables granulométricas y la tasa de sedimentación. El mayor aporte continental hacia la región de la plataforma adyacente al Río de la Plata registrado a partir del año 1970, podría ser el factor principal de los agrupamientos observados en los clusters para ambos testigos. Las agrupaciones mostraron una diferenciación en la década de 1970, lo que estaría asociado al aumento de los caudales de los ríos Paraná y Uruguay, durante las últimas tres décadas del siglo XX. Por otra parte se observa que la granulometría del testigo BAR1 presentó un mayor tamaño de grano y más variabilidad que en el caso del testigo GeoB 13813-4. También se determinó una mayor acumulación de sedimentos a través del tiempo en el cinturón de barro del Río de la Plata (plataforma continental adyacente), comparado con aquel registrado en la Barra del Indio (límite entre zona intermedia y externa del estuario). Estas diferencias podrían estar relacionadas con la influencia del Río de la Plata, el cual genera un ambiente altamente dinámico sobre la Barra del Indio y un ambiente más estable sobre el cinturón de barro en la plataforma continental.<hr/>The Río de la Plata Estuary (RdlP) is a fluviomarine system that drains into the Southwestern Atlantic Ocean with the Paraná and Uruguay rivers as main tributaries. The estuary is fed by a 3,100,000 km2 catchment area which extends over the territories of Argentina, Paraguay, Brazil, Bolivia and Uruguay (Acha et al., 2008). The RdlP exhibits significant natural decadal- and annual-scale, hydrodynamic and oceanographic variability associated with the Pacific Decadal Oscillation (PDO), the Atlantic Multidecadal Oscillation (AMO) and the El Niño/ La Niña Southern Oscillation (ENSO) (Depetris and Pasquini, 2007b; Chiessi et al., 2009; Garreaud et al., 2009). Such variability affects the moisture budget over the surrounding continental areas and leads, thus, to changes in the river discharge. PDO is associated with ENSO as both appear to display similar hydrological responses, though their inherent mechanisms are not yet fully understood (Garreaud et al., 2009). In this sense, warm and cold PDO phases strongly resemble El Niño and La Niña events, respectively (Garreaud et al., 2009). During El Niño episodes, an increase in precipitation over the RdlP drainage basin is commonly observed (Boulanger et al., 2005; Camilloni, 2005; Garreaud et al., 2009; García-Rodríguez et al., 2014), and consequently increased Paraná and Uruguay river discharges are displayed (Depetris and Pasquini, 2007a). Campos et al. (1999) have recorded a freshwater plume of low salinity and temperature (32, 18 ºC respectively) associated with an increase in RdlP discharge during the El Niño event of 1997, expanding northwards up to 23ºS. Furthermore, during negative AMO phases it was recorded an increasing trend on the precipitations over the SE South America (SESA) and, as a consequence, a concomitant increase in the Rivers Paraná and Uruguay discharge was recorded, while the opposite pattern was observed during positive phases (Chiessi et al., 2009). The aim of this paper is to infer the link between changes in the delivery of terrigenous sediment to the adjacent Atlantic Ocean with recorded hydrological variability of the RdlP. To achieve this, we used sedimentological and geochemical proxies from two sediment cores, which were retrieved from the inner continental shelf off Uruguay and encompass the past 100 AD. Sediment Core GeoB 13813-4 was taken from the inner-shelf “RdlP paleo-valley mudbelt” (Fig. 1;34°44’13’’S, 53°33’16’’W) during research cruise M76/3a (Krastel et al., 2012; Lantzsch et al., 2014). Sediment Core BAR1 was retrieved in the innershelf “Barra del Indio” zone (Fig. 1; 35°03’00’’S, 56°09’00’’W), performed by the Faculty of Sciences, Universidad de la República (Uruguay). For Core GeoB 13813-4, we analyzed the relative abundance of major elements (Ca and Ti) obtained by an X-ray fluorescent sediment core scanner AVAATECH and the Ca/Ti ratio was used to infer continental versus marine influence. This chemical elemental ratio was chosen according to previous successful applications within the Atlantic Ocean (Chiessi et al., 2009; Mahiques et al., 2009; Govin et al., 2012; Bender et al., 2013; Burone et al., 2013). Furthermore, the arithmetic mean grain size distribution was studied from both sediment cores, using the GRADISTAT program (Grain Size Distribution and Statistics Package for the Analysis of Unconsolidated Sediments) version 8 (Blott and Pye, 2001) (Fig. 4). For Core GeoB 13813-4 grain size were obtained by Laser Particle Sizer LS200 and for Core BAR1 were obtained by Malvern Mastersizer 2000 Laser analyser. The chronology from both cores was assessed by 210Pbxs dating (Table 1 and Fig. 2; Appleby, 2001; 2008). For core BAR1 we selected the CRS (Constant Rate Supply) model (which is highly used for estuarine systems), while for GeoB 13813-4 the CFCS (Constant Fluxe: Constant Sedimentation Rate) model was applied (Appleby, 2008; Bernal et al., 2010). In the last case, the decision of using the CF-CS model was due to the lack of a complete 210Pb dataset, which would bring very high sedimentation rates uncertainties (Sanchez-Cabeza and Ruiz-Fernández, 2012). To assess the climatic variability over the past century as inferred from the sediment proxies, we evaluated the climatic indices PDO and Southern Oscillation Index (SOI), of the Joint Institute for the Study of the Atmosphere and Ocean, University of Washington (http://jisao.washington.edu), as well as the AMO from the NOAA (http://www.aoml. noaa.gov/phod/amo_faq.php). We further compared these data with temporal series (encompassing the last century) of the Paraná and Uruguay fluvial discharges (http://www.hidricosargentina. gov.ar/acceso_bd.php), river-flow anomalies were calculated following the approach of Piovano et al. (2004). The generated proxy data were analyzed by running cluster analyses using the stratigraphically constrained Moristia similarity index, in PAST program version 3 (http://folk.uio.no/ohammer/ past/). The generated groups are represented with red lines in figure 3. The sedimentation rate of core GeoB 13813-4 was assumed to be constant with a mean value of 1.3 cm yr-1 (Table 1; Perez Becoña, 2014), while for the sedimentation rate of core BAR1 three groups were observed: 1911-1973; 1976-1984 and 1986-2010. The mean sedimentation rate for the above groups showed an increasing trend from 0.24 ± 0.13 cm yr-1 to 0.31 ± 0.14 cm yr-1 and 0.37 ± 0.10 cm yr-1, respectively. The most positive and stable values of SOI (La Niña events) were recorded during 1910-1970. After 1970, a higher variability and a trend towards more negative values was observed (El Niño events). After the year 2005, very negative SOI values occurred (Fig. 4). PDO showed either negative or close to zero values during the early period 1910-1970 (cold phase). During the subsequent interval, i.e., 1970- 2005, positive values (warm phase) were observed. Regarding with AMO, a positive phase was observed from 1925-1960, followed by a negative phase (1960- 2000), but then a shift to a positive phase until the present was observed. The Paraná river discharge anomalies for the years 1910-1970 were mostly associated with negative values (Fig. 4), while between 1970 and 2010 positive anomalies were documented. Between the years 2000 and 2010, we mostly registered values close to zero. Furthermore, the trends in AMO and SOI indices were negatively associated with the anomalies of both Paraná and Uruguay rivers flows, while PDO index were positive associated with such anomalies. A change in mode polarity observed for PDO and AMO took place by the middle 1970s, in addition to more frequent and intense El Niño events that led to the increased rainfall over SESA (Garreaud et al., 2009). Thus, the increase in rainfall over SESA was concomitant with positive anomalies in the Paraná and Uruguay river discharge rates after 1970 (Camilloni, 2005). In this sense, the Paraná river discharge was 20% higher during the past 30 years than the historical average of the 20th century (Mauas et al., 2008). The results of the cluster analyses groups (Fig. 3) showed a differentiation in both sediment cores that correspons to the beginning of the 1970s, which could be associated with the increasing discharge trend recorded for the Parana and Uruguay rivers over the last three decades. The increase in RdlP discharge led to a higher accumulation rate of terrigenous sediments, as inferred from the high sedimentation rate and mean grain size (BAR1), and the lowest Ca/ Ti ratio (GeoB 13813-4), and explains both the spatial and temporal sedimentological and geochemical variability. Ca/Ti ratio in the RdlP was successfully used to infer marine vs. continental influence, as Ti is associated with a continental RdlP discharge, while Ca is associated with autochthonous marine productivity (e.g. foraminifera, Burone et al., 2013). Thus, the highest continental sediment supply to the inner continental shelf is observed in GeoB 13813-4 after 1970, associated with a decrease in the Ca/Ti ratio (Fig. 4). Regarding with Core BAR1 the grainsize distribution and the sedimentation rate were both associated with the estuarine hydrodynamic changes. After 1970, the highest and most variable sedimentation rate and mean grain size was found, probably associated to an increase in both the Paraná and Uruguay river discharges during the past three decades, while the lowest and more stable sedimentation rates and mean sediment grain size recorded before 1970, is indicating a reduced RdlP freshwater supply to the study area. This study shows that both sediment cores contain a distinct continental runoff record as the result of climatic changes (PDO, AMO and ENOS), which have influenced the precipitation patterns over SESA. Both sites reflect similar responses to these environmental changes for the last 100 yr in continental terrigenous sediment supply from the RdlP watershed towards the inner continental shelf. We conclude that it is possible to assess the temporal? RdlP discharge patterns variability within the estuarine and adjacent oceanic area through the study of terrigenous proxies from sediment cores retrieved within the continental shelf.