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Franco-Fraguas P.,University of the Republic of Uruguay | Burone L.,University of the Republic of Uruguay | Mahiques M.,University of Sao Paulo | Ortega L.,Laboratorio Of Oceanografia | And 7 more authors.
Marine Geology | Year: 2014

The hydrology of the Southwestern Atlantic margin is dominated by the confluence of water masses with contrasting thermohaline properties, generating a frontal zone that extends from the Brazil-Malvinas Confluence zone (BMC) in the open ocean to the Subtropical Shelf Front (STSF) on the continental shelf. However, the hydrodynamics of the transition between these hydrographic features is still not fully understood. High-resolution morphological (multibeam) and hydrological (CTD) data were integrated with sedimentological data (textural and productivity proxies) in order to develop a sound framework to understand surface sedimentation of the slope extension of the STSF (subsurface, outer shelf and upper slope) and of the BMC (intermediate level, middle slope) on the Uruguayan continental margin (34° to 36°S). Since the detailed morphology of the study area is presented for the first time, related geological processes are briefly discussed. On the outer shelf and upper slope, and north of the STSF, the current direction of erosive mound scarps indicates the dominance of the southward-flowing Brazil Current (BC). This suggests a northernmost boundary for the distribution of the STSF. A strong flux of the BC favored by a steeper slope as well as by the occurrence of canyons incised on the upper slope is evidenced by the occurrence of coarser sediments and low values of productivity proxies, but also by the presence of deep sea coral reefs. Southwards, the effect of a less energetic Malvinas Current (MC) and the highly productive STSF is indicated by the deposition of fine sediments with high organic matter content, as well as by the absence of deep sea coral reefs. This depositional scenario is enhanced by a smooth slope and the occurrence of canyons incised deeper in the middle slope. Off-shelf sediment transport along the STSF is inferred by the similar texture registered between the outer shelf and shallow upper slope, the occurrence of biogenic shelly reworked sands and gravel, and by the observed decrease in grain size with depth. Glacial iceberg transport northwards and/or gravity processes are suggested by the occurrence of igneous/metamorphic gravel in lag deposits on the upper slope. On the middle slope, the northernmost influence of the erosive Antarctic Intermediate Water is evidenced by the vanishing of morphologic contouritic structures. This is also imprinted in a pronounced northward fining in grain size. This contribution increase our understanding of this highly dynamic and complex area, providing the first detailed analysis of the regional sediment patterns and oceanographic and morphological controls on surface sedimentation. © 2013 Elsevier B.V. Source


Lastras G.,University of Barcelona | Lastras G.,Spanish Institute of Oceanography | Acosta J.,Spanish Institute of Oceanography | Munoz A.,TRAGSA Secretaria General del Mar | Canals M.,University of Barcelona
Geomorphology | Year: 2011

In the framework of the Vulnerable Marine Ecosystems (VME) of the High Seas of the South West Atlantic, large areas of the Argentine Continental Margin (ACM) between 44°30'S and 48°S have been swath-mapped for the first time, obtaining full data coverage of the seafloor in this region between the outermost continental shelf and the middle slope down to 1600. m water depth. The slope is characterized by the presence of smooth terraces (Nagera, Perito Moreno and Piedra Buena) that widen towards the south, limited by morphological steps with evident signs of erosion in the form of scours. These terraces form part of the Argentine contourite depositional systems, generated by the interaction of the northwards flowing Antarctic water masses with the seafloor. Within the studied area, seven canyons and their multiple branches dissect the upper and middle continental slopes, from west to east, across the terraces and the steps. These canyons, which belong to the Patagonia submarine canyon system and are collected at a depth of ~. 3.5. km by a slope-parallel, SSW-NNE-oriented channel known as the Almirante Brown transverse canyon, display a large variety of morphologies. These include incisions from just a dozen of metres to 650. m, straight to highly meandering sections with sharp bends, well-developed levees and walls that reach 35° in slope gradient, hanging branches, conspicuous axial incisions and multiple knickpoints. Only the northernmost canyon indents in the continental shelf, whereas the others start at the limit between the upper and middle slopes, and are often fed by small, straight, leveed gullies.The action of both across-slope processes represented by submarine canyons and along-slope processes represented by terracing and scouring conform the ACM as a peculiar mixed margin, with the presence of both contour and gravity currents at the same place at the same time. We propose that at present, along-slope erosion and transport mainly occurs along the Perito Moreno terrace, whereas across-slope processes are much more dominant in the Nagera terrace. Erosive bedforms such as crescent scours, generated by contour currents, contribute to the progressive bottom-up erosion of the Nagera terrace and act as an initial collector of across-slope transported sediment, that later, due to flow focusing and recurrence, incise and interconnect creating definitive canyons that progress upslope by retrogressive erosion until their head indents the shelf break.Changes in the balance between across-slope and along-slope transport would imply a disequilibrium in the combination of processes leading to canyon formation, producing canyon abandonment, and partial or total filling. These changes could be produced by a variation in the depth of the main interfaces of Antarctic water masses leading to either an increase or a decrease in the erosion and transport capacity of contour currents, and/or by an enhancement of across-slope transport related to an increase of sediment availability. © 2011 Elsevier B.V. Source


Carranza A.,Museo Nacional de Historia Natural | Recio A.M.,TRAGSA Secretaria General del Mar | Kitahara M.,James Cook University | Scarabino F.,Museo Nacional de Historia Natural | And 6 more authors.
Marine Biodiversity | Year: 2012

We report the finding of monospecific scleractinian (i. e. Lophelia pertusa) reefs from the Uruguayan outer shelf and slope during an exploratory joint research cruise onboard the R/V 'Miguel Oliver' during January-February 2010. Acoustic mapping of the seafloor allowed the detailed analysis of 8,944 km 2, where some 17 structures identified as mounds were detected. Isolated cusps or groups of small mounds were the two main morphologies observed. Mound summit depths ranged from 167 to 326 m. The average height of the mounds was 35 m, reaching a maximum of 67 m. In all sampled mounds, the presence of live coral and/or coral rubble was detected, while absent from surrounding soft sediment bottoms. Some mounds were associated with fluid seepages. This is the first report of deep-sea coral reefs on the Uruguayan continental shelf and slope, and represents the southernmost Western Atlantic shelf and slope record of L. pertusa to date. © 2012 Senckenberg, Gesellschaft für Naturforschung and Springer. Source

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