Processes of MISS-formation in a modern siliciclastic tidal flat, Patagonia (Argentina)

The study focus on the description of several MISS as erosional pockets and remnants, flipped-over edges, and large microbial deformation structures as roll-ups, folds and gas domes, in the context of sediment composition, hydraulics, and geomorphology. The aim of the paper is to recognize the mechanisms of formation of MISS by analyses on hydrodynamics under the influence of the geomorphology. The study was conducted in an elongated inactive tidal channel colonized by microbial mats (2.5 × 0.3 km) in a progradation environment. To continuously record the water-level fluctuations, a HOBO water level logger was placed 40 cm below the flat surface for two years. The sea water enters several times a year, during storms, where the flood currents were characterized by faster velocity than ebb currents, reaching a water depth up to 0.70 m over the tidal flat. That coastal process creates MISS over the tidal flat. The most conspicuous microbial structures are the huge deformation roll-ups several m-scale, associated to elongated rip-off mats, folds and flipped over mats. The process of fluidization was postulated to explain the sand-infilling of gas domes and folds we observed. This process would result from the wave action on the water column that produces a temporal fluid behavior of the underlying sand.Fil: Maisano, Lucia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Tecnológica Nacional. Facultad Regional Bahía Blanca; Argentin

Sediment transport inferred by submarine bedforms

By means of bathymetric and side scan sonar records, the net circulation of sediments caused by tidal currents and waves in the outer Principal Channel of Bahía Blanca Estuary is inferred. On the northern coast of the studied area there are two harbours, Belgrano and Rosales, which are simultaneously affected by internal and external hydrosedimentological processes. Through this study, present and potential negative interactions between human activities and natural processes within the studied area are demonstrated.A través del análisis de registros batimétricos y de sonar de barrido lateral se infiere la circulación neta de sedimentos causada por olas y mareas en el sector externo del Canal Principal del estuario de Bahía Blanca. Sobre la costa norte del área de estudio se encuentran dos puertos, Puerto Belgrano y Puerto Rosales, los cuales son simultáneamente afectados por procesos hidrosedimentológicos internos y externos al estuario. Por medio de este estudio se evidencian las interacciones negativas, presentes y futuras, entre las actividades humanas y los procesos naturales.Fil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Ginsberg, Silvia Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentin

Geomorfología y dinámica del Canal San Blas, provincia de Buenos Aires (Argentina)

Se estudió la geomorfología submarina y se efectuaron mediciones de corrientes de marea en el canal San Blas para determinar la dinámica sedimentaria actual. Se relevó un sector del fondo del canal con el sistema batimétrico GeoSwath para describir exhaustivamente la morfología de un campo de dunas. Su asimetría permite inferir un transporte residual del material del fondo hacia el interior de la Bahía Anegada sobre el veril NE del canal. Se realizó un estudio de la dinámica de las corrientes de marea, determinándose sus velocidades y direcciones durante un ciclo completo de marea en toda la columna de agua y a lo largo de un perfil transversal a la costa mediante un ADCP. A pesar que el ambiente corresponde a un régimen micromareal (rango de marea alrededor de 2 m), en el canal San Blas existen fuertes corrientes de marea que desde el sector medio transportan todo el material inconsolidado del fondo, formando un delta de reflujo en la boca externa del canal y un delta de flujo en la Bahía Anegada. Se reconocieron diferentes geoformas costeras que permiten inferir un transporte litoral hacia el exterior del canal sobre la costa sur del mismo. Basándose en sus características morfodinámicas, es posible definir el canal San Blas como una entrada de marea que conecta el mar abierto con la Bahía Anegada.The Bahía Anegada is the coastal zone of the southern part of Buenos Aires Province (Argentina), where several environments are recognized: islands, inlets, marshes, tidal plains and different types of beaches. The Bahía San Blas is located at the southern part of the Bahía Anegada, where the San Blas channel separates the mainland from an island (Fig. 1). The San Blas channel is 2.5 km wide and 12 km long. The study area has a tidal amplitude characterized by a mean of 1.62 m, being 2.20 and 0.58 m the high and low tide respectively. The present study shows the results of a bathymetric survey of the San Blas channel and an analysis of the behavior of the tidal currents that affect sediment transport and promote the generation of different bedforms. The aim of this study was to analyze the submarine geomorphology related to the dynamic conditions in order to characterize in detail the sedimentary conditions in this area. A detailed bathymetric study was conducted over a zone of 50 km2, covering the San Blas channel, through a digital echosounder Bathy-500 positioned by DGPS operating in real time. Fifteen transversal tracks and three longitudinal tracks were made at the San Blas channel in order to obtain the necessary bathymetric profiles to make a bathymetric chart of the area. Over a zone of 1.5 km2, characterized by a field of subaqueous dunes, a Phase Measuring Bathymetric System (FMBS), called swath bathymetry system "GeoSwath Plus" from GeoAcoustics Lt. (UK), was employed in order to determine the detailed dune morphology. This survey yielded details and disposition of the bedforms present on the channel bed with centimetric precision. Bottom sediment samples were collected and tidal currents were measured using an Acoustic Doppler Current Profiler (ADCP) mounted on a ship. The tidal currents were measured during a whole tidal cycle, obtaining the distribution of velocity and direction over the water column, on a track carried out transversally to the channel. These values were computed with the associated WinRiver software. The entire field work (bathymetry, sampling, FMBS and ADCP) was performed with the 6.5 m long boat IADO IV. The coastal features display geomorphological differences in response to a changing dynamic regimen along the channel (Fig. 2). Remarkable morphological and textural differences occur along the coast. Close to the mouth, a dissipative beach, with medium sand and a gentle slope is present. Mobile coastal dunes are common crowning the beach. Nearly the central part, the beach is steeper and composed of gravels developing then a reflective beach. Towards the north, in the inner part of the Bahía Anegada (north of Punta Ramirez), wave-cut platforms and marshes covered with Spartina alterniflora appear, sheltered by cliffs formed by deposits from the Río Negro Formation. The bathymetric map allows distinguishing the existence of significant changes in the morphology throughout the San Blas channel. Based on these differences, the area was divided into 4 zones (Fig. 3), each one with distinctive profiles (Fig. 4). Zones I and II are characterized by a 28 m-depth flat bottom, free of unconsolidated sediments and with steep flanks. These features allow establishing an analogy with a tidal throat. A subaqueous dune field, covering an area of more than 1.5 km2, was found in Zone III. Zone IV is a shallow area where sand bars are exposed during low tide. The dune field was characterized by large dunes, with spacing between 40-80 m and heights of 2.5 m, identified at 21 m depth (Fig. 5). Higher dunes occur in deeper zones, reaching values of 4.5-5 m height at around 24 m depths. Most of the subaqueous dunes located on the southern portion of the dune field exhibit a symmetrical cross section and are covered by smaller bedforms of 0.3-1 m high and 0.7 to 0.9 m of spacing. On the other hand, dunes located on the northern side of the field are asymmetrical, with the steeper side towards the inner part of the channel. The mean grain size in the field dune is between medium and coarse sand (1.8 a 0.28 phi) that is in equilibrium with the strong velocities of tidal currents. The results obtained from the current measurements over a tidal cycle show that maximum velocities measured in the San Blas channel reached 2 m s-1 during flood and 1.8 m s-1 during ebb (Fig. 6). The maximum velocities of the flood currents were attained mainly on the northern flank and central part of the channel almost reaching the bottom. On the other hand, the maximum velocities of the ebb currents were found on the deepest site of the profile, on the southern flank, from mid-water up to the surface. At both ends of the channel, sand accumulates due to the reduction in the carrying capacity which leads to the formation of an ebb and a flood tidal deltas. The sand bars exposed in low tide in the inward shallow part in the Anegada Bay is the tidal flood delta formed by the loss in the sediment transport capacity caused by the widening of the channel section. This unconsolidated granular sediment is available to be transported by the tidal currents. Towards the outer part of the channel, the bathymetric map shows shallower depths while nautical charts confirm the presence of submarine bars at the entrance of the San Blas channel (Fig. 7). These bars might be evidence of the occurrence of an ebb tidal delta. The present study allowed concluding that the San Blas channel is a narrow strait between the mainland and an island, connecting the Anegada Bay with the outer sea and showing different submarine topography. Towards the mouth, the channel presents a flat bottom over 80% of its width, showing a U shape. The bottom is free of unconsolidated sediments due to the strong currents that wash away any loose material. The submarine topography of the San Blas channel corresponds very well with the tidal inlet significance.Fil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; ArgentinaFil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Tecnológica Nacional; Argentin

Morfodinámica de un campo de dunas submarinas en una entrada de marea: San Blas, Argentina

Se estudió la morfología de un campo de dunas submarinas desarrollado en una entrada de marea que conecta la plataforma continental argentina con bahía Anegada (sur de la provincia de Buenos Aires). Se infiere la circulación sedimentaria a partir del desplazamiento diferencial de las geoformas medido en relevamientos consecutivos. Como parte de la metodología se utilizó un ecosonda y un sistema batimétrico por medición de fase (SBMF) denominado GeoSwath que permitieron obtener la morfología submarina en detalle. Se observaron dunas grandes sobre el límite sur del campo de dunas con altura entre 4,5 y 5,0 m y espaciamiento entre 100 y 120 m, a profundidades de 24 m, que se desplazan hacia el exterior del canal a una rapidez entre 18 y 75 m año-1. Hacia el límite norte del campo, a 21 m de profundidad las dunas eran de menor magnitud, entre 2,0 y 2,5 m de altura y espaciamiento entre 40 y 80 m, y se desplazaron a una rapidez entre 18 y 36 m año-1 hacia el interior de bahía Anegada. Se obtuvo la distribución del flujo de agua en toda la columna, en un perfil perpendicular al canal relevado, mediante ADCP. Los valores máximos de rapidez de corriente fueron durante la marea creciente, alcanzando 2,0 m s-1. Los resultados obtenidos permiten definir un modelo circulatorio de transporte de sedimentos arenosos, con la formación de un delta de marea de flujo en la cabecera de la entrada de marea y un delta de reflujo en el límite del dominio oceánico, unidos por una garganta de marea profunda, exenta de sedimentos inconsolidados.The morphology of a submarine dune field developed in a tidal inlet that connects the Argentinean continental shelf with Anegada Bay (southern Buenos Aires province) was studied. The sediment circulation was inferred from the differential displacements of the bedforms evaluated by comparing consecutive surveys. An echosounder and a Phase Measuring Bathymetric System (PMBS) called GeoSwath were used to obtain a detailed submarine morphology. Large dunes, with heights between 4.5 and 5.0 m and from 100 to 120 m separating them, were present near the southern limit of the dune field at 24 m depth. These dunes move towards the outer part of the channel at a speed of 18 to 75 m year-1. At the northern end of the dune field, at 21 m depth, the dunes are smaller, being 2.0 to 2.5 m in height and separated by 40 to 80 m. The smaller dunes move towards the interior of Anegada Bay at 18 to 36 m year-1. The distribution of the water flow in the entire water column was obtained through a perpendicular profile of the channel by means of an ADCP. Maximum current speeds were 2 m s-1, and were found during flood tide. The results obtained indicate a circulatory transport model of sedimentary sands, with the formation of ebb and flood deltas at both ends of the deep inlet throat, which lacks of unconsolidated sediments.Fil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Gomez, Eduardo Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Tecnologica Nacional; Argentin

Modern and fossil siliciclastic sediments colonized by microbial mats. A comparative study for analog determination

En este estudio se presenta la comparación de depósitos de ambientes silicoclásticos actuales con los correspondientes al Pleistoceno-Holoceno del SE de la costa bonaerense donde se ha reconocido actividad microbiana en los sedimentos. Se comparan las diversas características sedimentológicas de las acumulaciones, entre ellas la fábrica, granulometría, composición mineralógica, presencia de microorganismos o rasgos derivados y estructuras sedimentarias. El objetivo del trabajo es comprender el paleoambiente, la ocurrencia de los mecanismos de formación que actuaron en el pasado y su relación con el marco ambiental donde se han producido mediante evidencias biosedimentarias en comparación con análogos actuales. En el ambiente actual del estuario de Bahía Blanca y el costero marginal de Paso Seco se han identificado diversas estructuras sedimentarias inducidas por actividad microbiana, generadas por los procesos hidrodinámicos propios de la planicie supramareal donde se encuentran. También se han estudiado depósitos asociados a la presencia de actividad microbiana del Pleistoceno tardío en el sector costero de la Reserva Paleoicnológica de Pehuen Có, y del Holoceno temprano-medio en los sitios arqueológicos La Olla 3 y 4 y Monte Hermoso 1. En los primeros se ha verificado la preservación de huellas de mega-mamíferos extintos y aves con notable calidad y en los segundos se ha comprobado una excelente conservación de restos óseos, vegetales y de pisadas humanas. Además de la observación directa de estructuras sedimentarias, fábrica sedimentaria y otros rasgos texturales se han llevado a cabo diferentes análisis que permitieron corroborar la existencia de actividad microbiana en los sedimentos. Entre ellos se han realizado análisis microscópicos, petrográficos y de rayos X. Las observaciones y micrografías realizadas en microscopio electrónico de barrido (MEB) han permitido la identificación de la relación de los sedimentos con microorganismos, reconociendo la presencia de SEP (sustancias poliméricas extracelulares) y de minerales autigénicos como la pirita. Los análisis de secciones delgadas bajo microscopio petrográfico, han permitido la comparación de la fábrica entre depósitos actuales y de los paleoambientes asociado a la presencia de cianobacterias. Los análisis de rayos X permitieron determinar minerales autigénicos como calcita, calcita magnesiana y dolomita. Los resultados del trabajo permiten reconocer las características distintivas de losdepósitos asociados a la presencia de actividad microbiana. Entre ellas se destaca la identificación de laminación de espesor milimétrico a centimétrico, que es la característica más sobresaliente de la presencia de microorganismos, reconocida como biolaminación, tanto en visión macroscópica como en secciones delgadas. Se reconocieron estructuras sedimentarias relacionadas con el metabolismo bacteriano (que involucran procesos de crecimiento, de decaimiento, etc.), el efecto impermeabilizante del biofilm, de plasticidad y cohesividad. Se documentaron estructuras microbianas como respuesta a los procesos físicos reinantes y se reconoció la importancia de las matas microbianas en la preservación de icnitas por medio de la litificación temprana.The present study is based on the comparison between siliciclastic sedimentary deposits of both, paleo- and modern environments in which microbial activity has been recognized in sediments, leading to the establishment of analogues. This fact leads up to the characterization of microbial sedimentary structures and the comprehension of the physicalchemical and biological processes of their formation and modification. Different characteristics are compared in both environments: sedimentary fabrics, grain size, mineralogical composition presence of microorganisms or microbial activity remnants, and sedimentary structures. The objective is to understand the occurrence of such characteristics, the mechanisms of formation that acted in the past and their relationship with the environmental conditions. In addition, the presence of microbial mats in the fossil environment is discussed as an agent for the preservation of the ichnites and archaeological remains after an early lithification. The archaeological localities of “La Olla” and “Monte Hermoso I” are studied (Fig. 1). They are characterized by an excellent conservation of organic remains associated with human activity, and the preservation of human footprints, respectively. Both deposits are Holocene sediments, currently located in the lower- and middle- intertidal area of a modern beach. The remarkable preservation of traces of extinct mammals (the most remarkable is the huge ground sloth Megatherium), birds, and vertebrates at the Pehuen Có paleoichnological site, has also been documented. These sediments correspond to late Pleistocene, and are currently located in the upperintertidal zone where is sometimes covered by sand after storm events. All these fossil sedimentary deposits are located on the coast of SE Buenos Aires. Modern environments, Bahía Blanca Estuary and the paleo-channel of Paso Seco, were studied (Fig. 1). Several microbial induced sedimentary structures have been found and were related to the hydrodynamic processes. Direct observation of microbial sedimentary structures present in the supra-tidal zone of Bahia Blanca Estuary and coastal area in Paso Seco were compared with sedimentary structures expose in the fossil deposits. Also, various analyses to determine the sedimentary arrangement and textural features have been applied. Micrographs obtained from the scanning electron microscope (SEM) have allowed the identification of the interaction of the sediments with microorganisms, recognizing the substances generated by them (EPS, extracellular polymeric substances) and the presence of authigenic minerals, such as pyrite as geochemical product of the bacterial metabolism. Analysis of thin sections show the sedimentary fabric associated with the presence of cyanobacteria. X-ray analysis allowed determining authigenic minerals such as biogenic calcite, magnesian calcite, and dolomite. The relevant characteristics of the deposits associated with the presence of microbial activity are documented in detail. The identification of millimeter to centimeter thickness lamination, fine and coarse sedimentation stands out (Fig. 2) which is the most remarkable characteristic of the presence of microbial activity recognized as biolamination (Fig. 4). Thisalternation can be clearly documented in modern environment with specific details as light crinkled lamination and the presence of EPS that amalgamate the siliciclastic grains. Thin and coarse alternation contributes to another common characteristic that is the bimodal distribution in granulometric sediment analyses (Fig. 4). Petrographic thin sections were observed under a petrographic microscope (Fig. 5). Both environments, ancient and modern sediments, present similar characteristics. The biolamination with diffuse border in the fossil sample, sometimes waving, in light and dark brown laminae can be appreciated. The biolaminites can be discriminated in modern samples. The alternation with coarse sediment (sands) reflects the hydrodynamic conditions of the area, a change towards a greater energy level. Micrographs obtained from SEM under fossil samples reveal the presence of bacteria, EPS, diatoms, and authigenic minerals as pyrite (Figs. 6, 7). The micrographs from the modern environment show the sedimentological-biological relationship in the microbial mat and authigenic precipitates. Several microbial sedimentary structures were recognized in La Olla site (Fig. 8). They are: a) biolaminites; b) presence of bubbles generated by microorganism metabolisms and entrapped by biofilm; c) elephant skin texture; d) plant and arqueological rests entrapped by the microbial mat colonization; e) microbial folds; f) mat deformation. Also, in El Pisadero site in the archaeological locality Monte Hermoso I, an special configuration were observed that was defined as deformed structure by load pressure and injection of underlying mat, related to the weight of individuals that had walked on the microbial mat surface (Fig. 8h, i). In the Pehuen Có paleo-ichnological site, the most conspicuous structures are the huge ground sloths footprints with a marginal erosioned rim exposing the lamination (Fig. 9). There is also a high ichnodiversity preservation of mammal and bird tracks showing the high vertebrate biodiversity during the late Pleistocene in southern South America. To confirm the presence of microbial mats, several others microbial sedimentary structures and textures were recognized, biolaminites, wrinkle marks, bubbles, colonized plant rests, mat chips, desiccation cracks, and preserved ripples. The laminated siltstone erosion is common. In the colonized modern supra-tidal plain in Bahía Blanca Estuary, several microbial sedimentary structures related to physical processes as storm events were documented: flipped-over mats, erosional pockets, microbial mat chips, and desiccation cracks with recolonization due to successive inundation. In Paso Seco, the characteristic cohesive and flexible behavior of mats was recognized by the presence of microbial domes with diameter > 12 cm (Fig. 10), and deformed microbial structures as folds, and rollups. These structures are formed as a consequence of water friction associated to high tide-related currents flooding in an ancient channel. The mat growing in the supratidal area during calm conditions may involve several days or months when the tide reaches the zone during spring-tides. It can also grow over shells or plants in the modern or fossil sites (Figs. 8c, 11). The mat activity can be inferred by the formation of bubbles or domes with different sizes depending on the microbial thickness and different stages of evolution (Fig. 12). Similar bubbles were found in the fossil environment and analogous process can be suggested to their formation (Fig. 13). Also, the ancient or modern desiccation cracks represent constant periods of exposition in a supratidal zone. Conversely, other sedimentary structures are formed as a response to physical process as water currents or waves acting on microbial mats (Fig. 14). In that case, erosional pockets and remnants, microbial chips, and folds were compared in the fossil and modern environments. The characteristically plasticity and cohesiveness of the microbial mat under wet conditions favors mat deformation. Finally, the relevance of microbial mats in modifying the erosional threshold, and the conditions needed for the formation of footprints over a microbial mat are discussed. Also, the role of microbial mats in the preservation of ichnites by means of stability and early lithification is recognized.Fil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur. Departamento de Geología; ArgentinaFil: Blasi, Adriana. Universidad Nacional de La Plata. Facultad de Ciencias Naturales y Museo. Instituto de Recursos Minerales. Provincia de Buenos Aires. Gobernación. Comisión de Investigaciones Científicas. Instituto de Recursos Minerales; Argentin

Estudio preliminar de los efectos del dragado sobre aspectos bio-sedimentológicos de las planicies de marea

Se llevó a cabo un estudio preliminar interdisciplinario donde se relacionan resultados biológicos y sedimentológicos para analizar el efecto del dragado a inyección de agua, un método recientemente implementado a nivel mundial. Los resultados de este estudio son pioneros en evaluar las consecuencias del dragado sobre las planicies de marea. Las conclusiones obtenidas permiten señalar que las tareas de dragado influyen en las comunidades microfitobentónicas de las planicies de marea, afectando por consiguiente la estabilidad de los sedimentos que son factibles de ser transportados y depositados en el medio circundante. Los resultados y su posterior análisis a través de componentes principales indican que la alteración ambiental por el dragado hidráulico afecta sólo la zona vecina al sector dragado y parece no alcanzar una distancia perpendicular al canal dragado de 1000 m sobre las planicies de marea adyacentes.A preliminary interdisciplinary study was carried out relating biologic to sedimentologic results with the objective of analyze the water injection dredging effect, a method recently applied in the world. These results are pioneer in the evaluation of consequences of dredging over the tidal plains. The conclusions let to state that the dredge works affect the microphytobenthos communities lying at the tidal plains, and consequently, the stability of the sediments that might be transported and deposited in the surroundings. The results of a statistical method (Principal Component) pointed that the environmental effect produced by dredging only influence the adjacent zone, being negligible in the tidal flat at a distance of 1000 m perpendicular to the dredged channel.Fil: Pizani, Natalia Veronica. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; ArgentinaFil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; ArgentinaFil: Parodi, Elisa Rosalia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentina. Universidad Nacional del Sur; Argentin

Application of the Lagrangian tracers model in Anegada Bay, Argentina

En el presente trabajo se ha realizado un experimento numérico utilizando un sistema de modelación hidrodinámico con trazadores lagrangianos para estudiar el transporte y circulación de partículas a partir de dos puntos de emisión. El objetivo fue establecer si las partículas pueden ser expulsadas de la Bahía Anegada (Argentina) dentro del plazo estimado para el período disponible antes de su sedimentación. Se obtuvo la distancia recorrida por las partículas, transportadas por las corrientes inducidas por la marea y el viento, y se determinó las posibles zonas de depósito. Se concluyó que las partículas emitidas en los dos puntos elegidos para este estudio permanecieron dentro de los límites de la Bahía Anegada.In this study we have applied a hydrodynamic numerical model of Lagrangian tracers to study the dispersion and subsequent settlement of particles from two different points of emission. The main objective was to define whether the particles, within the sedimentation period, could be exported out of the Anegada Bay (Argentina). We obtained the distance traveled by the particles, transported by currents induced by the tide and wind, and determined possible settlement areas. It was concluded that particles released from two different points do not leave Anegada Bay boundaries.Fil: Alvarez, Laura Inés. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; ArgentinaFil: Perillo, Gerardo Miguel E.. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Bahía Blanca. Instituto Argentino de Oceanografía. Universidad Nacional del Sur. Instituto Argentino de Oceanografía; Argentin

Modern and fossil siliciclastic sediments colonized by microbial mats. A comparative study for analog determination

En este estudio se presenta la comparación de depósitos de ambientes silicoclásticos actuales con los correspondientes al Pleistoceno-Holoceno del SE de la costa bonaerense donde se ha reconocido actividad microbiana en los sedimentos. Se comparan las diversas características sedimentológicas de las acumulaciones, entre ellas la fábrica, granulometría, composición mineralógica, presencia de microorganismos o rasgos derivados y estructuras sedimentarias. El objetivo del trabajo es comprender el paleoambiente, la ocurrencia de los mecanismos de formación que actuaron en el pasado y su relación con el marco ambiental donde se han producido mediante evidencias biosedimentarias en comparación con análogos actuales. En el ambiente actual del estuario de Bahía Blanca y el costero marginal de Paso Seco se han identificado diversas estructuras sedimentarias inducidas por actividad microbiana, generadas por los procesos hidrodinámicos propios de la planicie supramareal donde se encuentran. También se han estudiado depósitos asociados a la presencia de actividad microbiana del Pleistoceno tardío en el sector costero de la Reserva Paleoicnológica de Pehuen Có, y del Holoceno temprano-medio en los sitios arqueológicos La Olla 3 y 4 y Monte Hermoso 1. En los primeros se ha verificado la preservación de huellas de mega-mamíferos extintos y aves con notable calidad y en los segundos se ha comprobado una excelente conservación de restos óseos, vegetales y de pisadas humanas. Además de la observación directa de estructuras sedimentarias, fábrica sedimentaria y otros rasgos texturales se han llevado a cabo diferentes análisis que permitieron corroborar la existencia de actividad microbiana en los sedimentos. Entre ellos se han realizado análisis microscópicos, petrográficos y de rayos X. Las observaciones y micrografías realizadas en microscopio electrónico de barrido (MEB) han permitido la identificación de la relación de los sedimentos con microorganismos, reconociendo la presencia de SEP (sustancias poliméricas extracelulares) y de minerales autigénicos como la pirita. Los análisis de secciones delgadas bajo microscopio petrográfico, han permitido la comparación de la fábrica entre depósitos actuales y de los paleoambientes asociado a la presencia de cianobacterias. Los análisis de rayos X permitieron determinar minerales autigénicos como calcita, calcita magnesiana y dolomita. Los resultados del trabajo permiten reconocer las características distintivas de losdepósitos asociados a la presencia de actividad microbiana. Entre ellas se destaca la identificación de laminación de espesor milimétrico a centimétrico, que es la característica más sobresaliente de la presencia de microorganismos, reconocida como biolaminación, tanto en visión macroscópica como en secciones delgadas. Se reconocieron estructuras sedimentarias relacionadas con el metabolismo bacteriano (que involucran procesos de crecimiento, de decaimiento, etc.), el efecto impermeabilizante del biofilm, de plasticidad y cohesividad. Se documentaron estructuras microbianas como respuesta a los procesos físicos reinantes y se reconoció la importancia de las matas microbianas en la preservación de icnitas por medio de la litificación temprana.The present study is based on the comparison between siliciclastic sedimentary deposits of both, paleo- and modern environments in which microbial activity has been recognized in sediments, leading to the establishment of analogues. This fact leads up to the characterization of microbial sedimentary structures and the comprehension of the physicalchemical and biological processes of their formation and modification. Different characteristics are compared in both environments: sedimentary fabrics, grain size, mineralogical composition presence of microorganisms or microbial activity remnants, and sedimentary structures. The objective is to understand the occurrence of such characteristics, the mechanisms of formation that acted in the past and their relationship with the environmental conditions. In addition, the presence of microbial mats in the fossil environment is discussed as an agent for the preservation of the ichnites and archaeological remains after an early lithification. The archaeological localities of “La Olla” and “Monte Hermoso I” are studied (Fig. 1). They are characterized by an excellent conservation of organic remains associated with human activity, and the preservation of human footprints, respectively. Both deposits are Holocene sediments, currently located in the lower- and middle- intertidal area of a modern beach. The remarkable preservation of traces of extinct mammals (the most remarkable is the huge ground sloth Megatherium), birds, and vertebrates at the Pehuen Có paleoichnological site, has also been documented. These sediments correspond to late Pleistocene, and are currently located in the upperintertidal zone where is sometimes covered by sand after storm events. All these fossil sedimentary deposits are located on the coast of SE Buenos Aires. Modern environments, Bahía Blanca Estuary and the paleo-channel of Paso Seco, were studied (Fig. 1). Several microbial induced sedimentary structures have been found and were related to the hydrodynamic processes. Direct observation of microbial sedimentary structures present in the supra-tidal zone of Bahia Blanca Estuary and coastal area in Paso Seco were compared with sedimentary structures expose in the fossil deposits. Also, various analyses to determine the sedimentary arrangement and textural features have been applied. Micrographs obtained from the scanning electron microscope (SEM) have allowed the identification of the interaction of the sediments with microorganisms, recognizing the substances generated by them (EPS, extracellular polymeric substances) and the presence of authigenic minerals, such as pyrite as geochemical product of the bacterial metabolism. Analysis of thin sections show the sedimentary fabric associated with the presence of cyanobacteria. X-ray analysis allowed determining authigenic minerals such as biogenic calcite, magnesian calcite, and dolomite. The relevant characteristics of the deposits associated with the presence of microbial activity are documented in detail. The identification of millimeter to centimeter thickness lamination, fine and coarse sedimentation stands out (Fig. 2) which is the most remarkable characteristic of the presence of microbial activity recognized as biolamination (Fig. 4). Thisalternation can be clearly documented in modern environment with specific details as light crinkled lamination and the presence of EPS that amalgamate the siliciclastic grains. Thin and coarse alternation contributes to another common characteristic that is the bimodal distribution in granulometric sediment analyses (Fig. 4). Petrographic thin sections were observed under a petrographic microscope (Fig. 5). Both environments, ancient and modern sediments, present similar characteristics. The biolamination with diffuse border in the fossil sample, sometimes waving, in light and dark brown laminae can be appreciated. The biolaminites can be discriminated in modern samples. The alternation with coarse sediment (sands) reflects the hydrodynamic conditions of the area, a change towards a greater energy level. Micrographs obtained from SEM under fossil samples reveal the presence of bacteria, EPS, diatoms, and authigenic minerals as pyrite (Figs. 6, 7). The micrographs from the modern environment show the sedimentological-biological relationship in the microbial mat and authigenic precipitates. Several microbial sedimentary structures were recognized in La Olla site (Fig. 8). They are: a) biolaminites; b) presence of bubbles generated by microorganism metabolisms and entrapped by biofilm; c) elephant skin texture; d) plant and arqueological rests entrapped by the microbial mat colonization; e) microbial folds; f) mat deformation. Also, in El Pisadero site in the archaeological locality Monte Hermoso I, an special configuration were observed that was defined as deformed structure by load pressure and injection of underlying mat, related to the weight of individuals that had walked on the microbial mat surface (Fig. 8h, i). In the Pehuen Có paleo-ichnological site, the most conspicuous structures are the huge ground sloths footprints with a marginal erosioned rim exposing the lamination (Fig. 9). There is also a high ichnodiversity preservation of mammal and bird tracks showing the high vertebrate biodiversity during the late Pleistocene in southern South America. To confirm the presence of microbial mats, several others microbial sedimentary structures and textures were recognized, biolaminites, wrinkle marks, bubbles, colonized plant rests, mat chips, desiccation cracks, and preserved ripples. The laminated siltstone erosion is common. In the colonized modern supra-tidal plain in Bahía Blanca Estuary, several microbial sedimentary structures related to physical processes as storm events were documented: flipped-over mats, erosional pockets, microbial mat chips, and desiccation cracks with recolonization due to successive inundation. In Paso Seco, the characteristic cohesive and flexible behavior of mats was recognized by the presence of microbial domes with diameter > 12 cm (Fig. 10), and deformed microbial structures as folds, and rollups. These structures are formed as a consequence of water friction associated to high tide-related currents flooding in an ancient channel. The mat growing in the supratidal area during calm conditions may involve several days or months when the tide reaches the zone during spring-tides. It can also grow over shells or plants in the modern or fossil sites (Figs. 8c, 11). The mat activity can be inferred by the formation of bubbles or domes with different sizes depending on the microbial thickness and different stages of evolution (Fig. 12). Similar bubbles were found in the fossil environment and analogous process can be suggested to their formation (Fig. 13). Also, the ancient or modern desiccation cracks represent constant periods of exposition in a supratidal zone. Conversely, other sedimentary structures are formed as a response to physical process as water currents or waves acting on microbial mats (Fig. 14). In that case, erosional pockets and remnants, microbial chips, and folds were compared in the fossil and modern environments. The characteristically plasticity and cohesiveness of the microbial mat under wet conditions favors mat deformation. Finally, the relevance of microbial mats in modifying the erosional threshold, and the conditions needed for the formation of footprints over a microbial mat are discussed. Also, the role of microbial mats in the preservation of ichnites by means of stability and early lithification is recognized.Facultad de Ciencias Naturales y MuseoComisión de Investigaciones Científicas de la provincia de Buenos Aire

Tidal effects on short-term mesozooplankton distribution in small channels of a temperate-turbid estuary, Southwestern Atlantic

The short-term variability of mesozooplanktondistribution and physicochemical variables wasexamined in two different channels of the BahíaBlanca Estuary, Argentina, during two tidalcycles. All the physicochemical measurementsand mesozooplankton sampling were performedat a fixed site during approximately 22-23 h at3-h intervals. Pumps were used to obtain surfaceand bottom mesozooplankton samples and thewater speed of each stratum was measured withan Acoustic Doppler Current Profiler (ADCP). Inall, 23 mesozooplanktonic taxa belonging to fourphyla (Arthropoda, Annelida, Echinodermata andChordata) were identified. The most abundant taxaduring the two tidal cycles were Balanus glandulalarvae, Eurytemora americana and Acartia tonsa.A discernible variability in the water conditionsand vertical mesozooplankton distribution (VMD)different from that known for the estuary´s mainchannel, was found in the other two selectedchannels. VMD varied during the tidal cycle inboth channels in accordance with the channel?sgeomorphology and water dynamic characteristicsof each of them. The variation of the abundance ofthe different taxa during ebb and flood currents mightindicate the existence of a tidal vertical migrationof the mesozooplankton as a response to particulardynamic water conditions.A variação de curto prazo da distribuição do mesozooplâncton e das variáveis físico-químicas foi examinada em dois diferentes canais do Estuário de Bahia Blanca, Argentina, durante dois ciclos de maré. As medidas físico-químicas e a amostragem do zooplâncton foram realizadas em dois locais fixos durante aproximadamente 22-23 horas, com intervalos de 3 horas. Bombas foram utilizadas para obter amostras de zooplâncton na superfície e no fundo e a velocidade da água em cada estrato foi medida com o Perfilador Acústico de Corrente (ADCP). Ao todo, foram identificados 23 taxa mesozooplanctônicos pertencentes a 4 filos (Arthropoda, Annelida, Echinodermata e Chordata). Os grupos mais abundantes durante os dois ciclos de maré foram larvas de Balanus glandula, além de Eurytemora americana e Acartia tonsa. Nos dois canais selecionados foi identificada uma variação nas condições da água e na distribuição vertical do mesozooplâncton (DVZ) diferente daquela conhecida para o canal principal do estuário. DVZ variou durante o ciclo de maré em ambos os canais de acordo com a geomorfologia e características da dinâmica das águas existente em cada um deles. A variação na abundância dos diferentes taxa durante as marés vazante e enchente poderia estar indicando a existência de migração vertical do mesozooplâncton em resposta a uma determinada condição dinâmica da água.Fil: Chazarretta, Javier. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); ArgentinaFil: Hoffmeyer, Monica Susana. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); ArgentinaFil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); ArgentinaFil: Berasategui, Anabela Anhi. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); Argentin

Interaction between estuarine microphytobenthos and physical forcings: the role of atmospheric and sedimentary factors

The goal of this study was to analyze microbial mats and biofilms from the lower supratidal area of the Bahía Blanca estuary (Argentina), and explore their relationship with sediments and other physical forcings. Thirteen monthly sediment samples (uppermost 10 mm) were taken and their composition and abundance in microorganisms was determined by microscopy. Physical parameters (solar radiation and sediment temperature at -5 cm) were recorded with a frequency of 5 minutes by a coastal environmental monitoring station. Additionally, sediment grain size and moisture content were determined for distinct layers in the uppermost20 mm, and the rate of inundation of the supratidal area was estimated from tidal gauge measurements. There were significant seasonal differences in the biomass of the microphytobenthic groups considered (filamentous cyanobacteria and epipelic diatoms), with the former consistently making up >70% of the total biomass. The relationships between microphytobenthos and sediment temperature and solar radiation fitted to linear regressions, and consistently showed an inverse relationship between microphytobenthic abundance and either one of the physical parameters. The granulometric analysis revealed a unimodal composition of muddy sediments, which were vertically and spatially homogeneous; additionally, there were significant seasonal differences in water content loss with drying conditions prevailing in the summer. Several Microbially-Induced Sedimentary Structures (MISS) were identified in the supratidal zone such as shrinkage cracks, erosional pockets, gas domes, photosynthetic domes, mat chips and sieve-like surfaces. In contrast to studies from analogous environments in the Northern Hemisphere, we found reduced microphytobenthic biomass in summer, which were explained by increased evaporation/desiccation rates as a consequence of increased radiation, despite frequent tidal inundation. In conclusion, the observed density shifts in the benthic microbial communities are attributable to physical forcings dependent upon seasonal variations in interplaying factors such as sediment temperature, solar radiation and tidal inundation.Fil: Pan, Jeronimo. Universidad Nacional de Mar del Plata. Facultad de Ciencias Exactas y Naturales. Departamento de Ciencias Marinas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; ArgentinaFil: Bournod, Constanza Naimé. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); ArgentinaFil: Cuadrado, Diana Graciela. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); Argentina. Universidad Nacional del Sur; ArgentinaFil: Vitale, Alejandro José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); Argentina. Universidad Nacional del Sur; ArgentinaFil: Piccolo, Maria Cintia. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Argentino de Oceanografía (i); Argentina. Universidad Nacional del Sur; Argentin