Limnogeology in Southern South America: an overview

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

Forzantes ambientales intervinientes en la dinámica trófica de Laguna del Plata (Córdoba)

El análisis de indicadores paleolimnológicos físicos, químicos y biológicos del registro sedimentario de Laguna del Plata (LdP), sistema de la Laguna Mar Chiquita (LMC) (Córdoba, Argentina), permitió reconstruir la variabilidad ambiental ocurrida en los últimos ca. 70 años y relacionarla con la variabilidad hidroclimática del sistema y con el impacto de las actividades antrópicas en la región. Se identificaron 4 escenarios con características ambientales diferentes. El incremento en las concentraciones de nutrientes determinado a partir del año 1984 en el registro sedimentario es sincrónico con el avance de la frontera agrícola en la región, la cual se expandió en la década de 1980. Además, los resultados muestran el control que ejerce la variabilidad hidroclimática sobre el ingreso de nutrientes a LdP permitiendo relacionar forzantes antrópicos y naturales. Asimismo, se observó que, durante períodos de alto impacto antrópico, la mayor disponibilidad de nutrientes es el factor que predomina en el control de la producción primaria de LdP.Este trabajo considera la acción simultánea de forzantes naturales y antrópicos, y aporta al entendimiento de la dinámica ambiental del sistema para los siglos XX y XXI. Por lo anterior, se considera que puede contribuir a la proyección y planeamiento de las actividades de la región

Una visión paleolimnológica de la variabilidad hidroclimática reciente en el centro de Argentina: desde la pequeña edad de hielo al siglo XXI.

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

Arqueomalacología en las costas de Ansenuza: análisis de una almeja nacarífera (Anodontites trapesialis) hallada en contexto funerario del sitio El Diquecito (Laguna Mar Chiquita, Córdoba)

Mollusk shells are relatively frequent items in archaeological sites from different regions of Argentina, as part of human diet, utilitarian or ornamental items, as well as rituals. In this paper we analyze the biological characteristics of a unique mollusk shell found in a burial context of a residential site located on the southern coast of Laguna Mar Chiquita or Mar de Ansenuza (Córdoba province) and its surrounding environment, inferring the hydroclimatic conditions during the occupation of the site. Photographs were taken of the recovered material, and observations were made using binocular microscope. The specimen was dated by AMS in 2077 ± 38 BP.  The mollusk shell was classified as Anodontites trapesialis (Lam.), previously unreported in archaeological sites in the region, which lives in rivers and freshwater lakes, and therefore would not have lived in Laguna Mar Chiquita. The specimen probably came from the tributary rivers or neighboring regions of North Pampas. The event would have taken place in a period characterized by hydric oscillations, dry and probably very cold climate conditions.Las valvas de moluscos son elementos de aparición relativamente frecuente en sitios arqueológicos de distintas regiones de Argentina, por ser parte de la dieta humana, o elementos utilitarios u ornamentales, e incluso de tipo ritual. En este trabajo se analizan las características biológicas de una única valva de molusco, hallada en contexto funerario de un sitio residencial de la costa sur de la laguna Mar Chiquita o Mar de Ansenuza (provincia de Córdoba) y su entorno ambiental, infiriendo además las condiciones hidroclimáticas durante la ocupación del sitio. Se tomaron fotografías del material recuperado y se realizaron observaciones a simple vista y utilizando lupa binocular y microscopio. El ejemplar fue datado mediante AMS en 2077 ± 38 AP. El análisis permitió concluir que la almeja nacarífera corresponde a la especie Anodontites trapesialis (Lam.), no reportada anteriormente en contextos arqueológicos de la región, que vive en ríos y lagos de agua dulce, y por lo tanto, no habría habitado la laguna Mar Chiquita. Procedería de los ríos afluentes o de regiones vecinas, del Norte de la región pampeana. El evento registrado habría tenido lugar en un período caracterizado por oscilaciones hídricas, de clima seco, y probablemente muy frío

A modern subtropical playa complex: Salina de Ambargasta, central Argentina

Salina de Ambargasta is a playa located at mid latitudes in central Argentina (29 °S-64 °W). Because of its hydrological behaviour, this playa complex can be subdivided into a closed system "sensu-stricto" and an "open-like system" due to the presence of a seasonal outflow. Geomorphological and sedimentological features enable the separation of the Ambargasta playa into well-defined western, eastern and northern zones, where aeolian processes, groundwater supply and surface inflow, respectively, define distinctive sedimentary environments and typical processes of deposition. The following depositional settings were recognized: (1) alluvial fan; (2) sandflat; (3) springs; (4) dunes and palaeo-dune field; (5) dry mudflat; (6) capillary mudflat; (7) ephemeral saline lake (includes saline mudflat and salt pan). The dry mudflat is present at the highest topographical zone, where the groundwater influence is less important and only intermittent ponds are present. In the capillary mudflat, evaporite sedimentation is limited to the development of efflorescence by evaporative pumping. The ephemeral saline lake, placed in the lowermost topographical region, alternates cycles of lake expansion and contraction responding to inter-annual hydrological variability as well as to seasonal variability. Lake expansion takes place during early austral summer (December-March), while continuous evaporation of brine leads to the growth of halite crystals during late summer. Sodium chloride-type brines result from both chemical fractionation as evaporation increases and salt dissolution. The isotopic compositions of surficial and underground waters (δ18O and δ2H) indicate that lake waters become isotopically enriched during summer when evaporation plays a significant role in the playa complex

El registro sedimentario Pleistoceno tardío-Holoceno de la Salina de Ambargasta (Argentina central): una aproximación paleolimnológica

Salina de Ambargasta is a playa system located at the middle latitude of Argentina (29°S; 64°W). Two sedimentary cores retrieved at the eastern border (AB-1) and the central area of the salina (AB-2) were studied using a multi-proxy approach, an actualistic sedimentary model and radiocarbon dates allowed reconstructing the paleohydrological history of the Salina de Ambargasta since the late Pleistocene. The paleoenvironmental reconstruction for the last ca. 45,000 years to the present suggests six main environmental stages: 1) between 44,700 and 39,600 cal. year BP, a dry period is represented by the development of a capillary mudflat; 2) between ca. 39,600 and 26,700 cal. year BP a more humid period is recorded by lacustrine facies with carbonate and sulfate precipitation; 3) between ca. 26,700 and 23,600 cal. year BP occurs the most humid phase in Ambargasta, characterized by the sedimentation in an ephemeral sulfate lake, relatively deep, enriched with microbial mats and fringed by mudflats; 4) between 23,600 and 18,500 cal. year BP, a change to drier conditions with humid pulses is evidenced by the development of saline mudflats alternating with ephemeral lakes and associated with mudflats in the supralittoral areas; 5) between 18,500 and 8,600 cal. year BP a drier period is identified by the dominance of capillary and saline mudflats, and 6) since 8,600 cal. year BP to the present the most negative hydrological balance is recorded in the saline system, represented by an expansion of capillary mudflats The control of the hydrological balance in the Salina de Ambargasta is mostly associated with the variation of the South America Monsoon–like System. Thus, the record presented here allows us to provide new clues in order to decipher former fluctuations of this important driver of the climatic system of South America since the late Pleistocene.La Salina de Ambargasta es un sistema playa localizado a latitudes medias de Argentina (29°S; 64°W). El estudio de dos testigos sedimentarios, AB-1 (borde este de la salina) y AB-2 (área central), efectuado mediante un enfoque de multi-indicadores ambientales, la aplicación de un modelo sedimentario análogo y edades radiocarbónicas, permitió reconstruir la historia paleohidrológica de la salina de Ambargasta a partir del Pleistoceno tardío. La reconstrucción paleoambiental de los últimos ca. 45,000 años hasta el presente, indica seis estadios ambientales principales: 1) entre 44,700 y 39,600 años cal. AP, un intervalo seco está representado por el desarrollo de una planicie fangosa capilar; 2) entre 39,600 y 26,700 años cal. AP, un período más húmedo es registrado por facies lacustres con precipitación carbonática y sulfatada; 3) entre 26,700 y 23,600 años cal. AP ocurre la fase más húmeda en Ambargasta, caracterizada por la sedimentación en una laguna efímera sulfatada, relativamente profunda, rica en tapices microbianos y marginada por planicies fangosas; 4) entre 23,600 y 18,500 años cal. AP se presenta un cambio a condiciones más secas con pulsos húmedos intercalados, evidenciado por el desarrollo de planicies fangosas salinas que alternan con lagunas efímeras, y planicies fangosas capilares en las zonas supralitorales; 5) entre 18,500 y 8,600 años cal. AP se identifica un intervalo más seco caracterizado por el dominio de planicies fangosas salinas y capilares, y 6) desde los 8,600 años cal. AP al presente se registra el balance hídrico más negativo en el sistema salino representado por una expansión de las planicies fangosas capilares. El control del balance hidrológico en la Salina de Ambargasta está vinculado con la variación del Sistema de tipo Monzónico Sudamericano, por lo cual la reconstrucción efectuada desde el Pleistoceno tardío permite aportar nuevas claves para comprender las fluctuaciones pasadas de este importante forzante del sistema climático de Sudamérica.Fil: Zanor, Gabriela Ana. Consejo Nacional de Invest.cientif.y Tecnicas. Centro Cientifico Tecnol.conicet - Cordoba. Centro de Invest.en Cs.de la Tierra; Argentina;Fil: Piovano, Eduardo Luis. Consejo Nacional de Invest.cientif.y Tecnicas. Centro Cientifico Tecnol.conicet - Cordoba. Centro de Invest.en Cs.de la Tierra; Argentina;Fil: Ariztegui, Daniel. Universidad de Ginebra. Section des Sciences de la Terre et de l’environnement; Suiza;Fil: Pasquini, Andrea Ines. Consejo Nacional de Invest.cientif.y Tecnicas. Centro Cientifico Tecnol.conicet - Cordoba. Centro de Invest.en Cs.de la Tierra; Argentina;Fil: Chiesa, Jorge Orlando. Universidad Nacional de San Luis; Argentina

Environmental change in subtropical South America for the last two millennia as shown by lacustrine pigments

Organic matter accumulation and preservation in aquatic systems can be linked to variations in organic matter sources as well as primary productivity. These changes can be used to determine contemporaneous environmental variations in the catchment area. The source, quality and distribution of lacustrine organic matter (LOM) have been determined in a sedimentary core covering the last ~1,500 years from Laguna Mar Chiquita, a saline lake in central Argentina. Petrophysical, sedimentological and geochemical data along with results of high-resolution pigment analyses provide a unique dataset that allow characterization of both source and type of LOM. Climatically triggered changes in the Laguna Mar Chiquita catchment led to changes in the water salinity and lake trophic state that in turn influenced primary productivity and thus organic matter accumulation. Distinctive high lake water levels and associated low salinity characterize the Medieval Climatic Anomaly (MCA) as well as the last quarter of the twentieth century and beginning of the twentyfirst century. Conversely, extremely low lake levels with resulting high salinity correspond with the Little Ice Age (LIA). High-resolution sedimentary pigment analyses in these two end–members (i.e., LIA and MCA) do not indicate major changes in the structure of phototrophic communities signifying that the biota survives a large range of salinity and temperature. High lake stands (low salinity) correspond with increased primary productivity as shown by high pigment concentrations along with a decrease in authigenic carbonates and evaporites. These high lake level intervals have been previously linked to a warmer and more humid climate. Conversely, low lake levels (hyper saline waters) correlate with decreasing lake productivity and precipitation of both authigenic carbonates and evaporites. These sediments correspond to cooler intervals with a negative moisture balance. The relative similarity between LOM indicated by the pigment stratigraphy for both intervals show no clear evidence of a linkage between human impact and lake productivity. Hence, Laguna Mar Chiquita has been naturally productive even prior to agriculture expansion by the end of the twentieth century

Independent time markers validate Pb-210 chronologies for two shallow Argentine lakes in Southern Pampas

International audienceWe report here the results of detailed Pb-210 analysis from two lake sediment cores collected from Lagunas Encadenadas del Oeste (LEO), located in the southern Argentinean Pampas (37 degrees S, 62 degrees W). Non-exponential and non-monotonic Pb-210, depth-profiles in the sedimentary record of these shallow lacustrine systems indicate the interaction of complex processes behind radionuclide fluxes and lake sedimentation. The chronologies and sediment accumulation rates (SAR) for the lake sediment sequences were modeled using different Pb-210-based mathematical models. Since modeled Pb-210 ages must be validated against independent stratigraphic markers of known age, two chrono-stratigraphic markers were selected to control the upper and lowermost parts of the Pb-210 dated sediments. The younger stratigraphic marker of AD 1980 +/- 2 corresponds to an uppermost organic carbon-rich mud accumulated by 1977-78 matching the most noticeable lake highstand occurred during the 20th century, registered on instrumental and paleolimnological records across the Pampean Plains. The oldest stratigraphic marker corresponds to AD 1878 +/- 10 matching with a high lake productivity and wet phase related to anomalously intense rainfall and flooding events in southeastern South America for AD 1877-1878. When comparing the resulting ages to the stratigraphic markers, our results show that CRS-model (constant rate of supply) performs better than the other models. Thus, these independent chronostratigraphic markers used along the sedimentary records were critical to validate Pb-210-derived ages and therefore to select the most appropriate model to establish the chronological framework of the recent sediments of the LEO system. The CRS-model appears to be an adequate technique for deriving ages and SAR in the LEO sedimentary record and other Pampean lacustrine systems under strong hydrological variability. (C) 2016 Published by Elsevier Ltd

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

International audienceDuring the 1970s, Laguna Mar Chiquita (Argentina) experienced a dramatic hydroclimatic anom- aly, with a substantial rise in its level. Precipitations are the dominant driving factor in lake level uctuations. The present study investigates the potential role of remote forc- ing through global sea surface temperature (SST) elds in modulating recent hydroclimatic variability in Southeast- ern South America and especially over the Laguna Mar Chiquita region. Daily precipitation and temperature are extracted from a multi-member LMDz atmospheric gen- eral 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 in uence 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 uctuations in response to the projected climate changes