On Forced Instabilities of Sediment-Resuspending Marine Gravity Currents
Published:January 01, 2009
Federico Falcini, Pierluigi Maponi, Ettore Salusti, 2009. "On Forced Instabilities of Sediment-Resuspending Marine Gravity Currents", External Controls on Deep-Water Depositional Systems, Ben Kneller, Ole J. Martinsen, Bill McCaffrey
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We present the time and space evolution, i.e., stability properties, of gravity currents flowing downslope over an erodible sea bottom. This bed consists of noncohesive fine-grained sediments, entrainment of which can increase the current density. To schematize such a complex interaction, we assume that initially the current is slow, such that no sediment entrainment occurs. At a fixed time an external forcing is applied, giving rise to hydrodynamic perturbations. These can increase the water velocity, generate turbulence, and in turn entrain bottom sediments, further increasing the water velocity. In our model turbulent energies are not considered explicitly since we assume that density variations, ultimately due to turbulent bottom erosion or deposition, are given by an empirical formula discussed by Itakura and Kishi (1980), Parker et al. (1986), and Garcia and Parker (1993). A complex, but solvable, equation is thus obtained, in which both time and space variability for a realistic two-layer model of gravity currents flowing over an erodible slope below less dense ambient water are considered. On mathematical grounds, this consists in a diffusion equation, but with a peculiar type of nonlinear “time-delayed” evolution. Comparison with the experimental data suggests that the most interesting quantity is probably the “ignition” time, i.e., the time necessary for an external forcing to generate nonlinear explosive effects and sediment entrainment. Finally a numerical study of the evolution of current thickness supports the evidence of these nonlinear effects.
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External Controls on Deep-Water Depositional Systems
The principal objective of the meeting from which this set of papers arose was to gain an overview of the current state of knowledge of the roles and interplays of external controls on deposition in deep marine environments. By external controls we mean allocyclic or allogenic factors, i.e., those that are unrelated to the self-organization of the depositional system (autocyclic or autogenic); principal among these are climate, sea level, sediment supply, and tectonics. One of the big questions that the meeting sought to address concerned the comparability of the recent high-frequency, high-resolution record with the older, generally lower-frequency stratigraphic record of “deep time”; to what extent are the apparent differences a function of resolution, or of comparisons between a glacial and a nonglacial Earth? In fact, as the papers in this volume illustrate, the variability between individual systems, even in Late Glacial time, and the paucity of constraints on older systems makes these questions difficult to answer, but some useful conclusions can be drawn. The papers presented at the meeting were organized into themes that included: overviews of glacial sea-level change, and of climate modeling; external controls on large river-fed submarine fans, including the effects of climate and sea level on the fluvial system itself; influences of climate, sea level, and tectonics on a range of smaller modern systems; deep marine processes; the outcrop record of the pre-Pleistocene Earth; the subsurface record of the pre-Pleistocene Earth; and syntheses. The organization of the volume largely reflects this structure.