Abstract
Submarine canyons are dynamic sedimentary environments influenced by sediment transport, erosion and deposition. Gravity flows can scour and erode the canyon floor, thus redistributing sediment to distal locations. In addition, submarine canyons can act as sedimentary traps where sediment transported laterally across the continental shelf and slope is advected into.
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Hence, elevated sedimentation rates and high organic carbon content are often found in submarine canyons in comparison with the adjacent open slope. Such dynamic processes must affect the ecosystems inhabiting these exceptional environments. To investigate and quantify this relationship, living benthic foraminifera were sampled from Nazaré and Lisbon-Setbal submarine canyons located on the Portuguese continental margin. The foraminiferal abundances and species distribution were correlated with a host of geochemical (e.g. organic carbon, phytopigment content, redox chemistry) and physical/sedimentological parameters (e.g. current speed, sedimentation rate, frequency of gravity flows). Eventually this information was used to reconstruct temporal variations in sedimentation processes and associated changes in foraminiferal community structure in two piston cores derived from the lower canyon. The results of these studies highlight the importance of habitat instability and food abundance in structuring the foraminiferal communities in canyons. Food, quantified in terms of the sedimentary phytopigment content, was readily available in the studied canyons, declining in abundance with increasing water depth. Food abundance was also reflected in the pore water chemistry (nitrate penetration depth used as a redox indicator), higher pigment content coinciding with shallower nitrate penetration depth in sediment. The standing stocks of foraminifera and community structure changed with these parameters, and a positive correlation was observed between foraminiferal numbers and the sedimentary pigment content and a negative with the nitrate penetration depth. Stations recording the highest pigment loads (> 15g/cm3) and the shallowest redox zones (nitrate penetration depth <1cm) were inhabited by infaunal taxa e.g. Chilostomella oolina, Melonis barleeanum, Bigenerina cylindrica and Globobulimina spp. At the deepest sites, where the pigment concentrations were very low (? 2 g/ cm3) and the redox zonation deep (nitrate penetration depth exceeding 5 cm depth in sediment) communities were dominated by agglutinated taxa and only few calcareous species were present, including Nuttallides umbonifera. Despite the high pigment concentrations however, foraminiferal numbers were low in the upper and middle Nazaré Canyon axis. At these sites the development of stable ecosystems appeared to be hindered by sedimentary disturbance. Distinct agglutinated foraminifera, Technitella spp., were found to inhabit the unstable environment. This species can be regarded as a highly opportunistic recoloniser of newly exposed habitats, and/or may be able to tolerate high sedimentation rates and frequent sedimentary disturbance (i.e. gravity flows). In the paleostudy, a postglacial increase in the sediment transport was clearly reflected in the foraminiferal record as an increase in the relative abundance of shallow water taxa. Further, a shift in faunal composition was observed as a response to changing sedimentary regime; higher sediment and organic carbon supply from the canyon leading to a development of an infaunal Pullenia spp. assemblage and decreasing activity to the hemipelagic Nutallides umbonifera assemblage.
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