BALTIMORE CANYON AS A MODERN CONDUIT OF SEDIMENT TO THE DEEP-SEA
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An integrated study using moored current meters, transmissometers and sediment traps revealed that sediment in Baltimore Canyon was resuspended regularly when energy from internal tides was focused along the canyon axis. Such focusing occurred primarily during late winter and early spring. During resuspension events particle concentrations at 275 m in the canyon axis typically increased from a background level of 100 g l-1 to over 1000 g l-1 within a 5-min period and often exceeded 5000 g l-1. Net transport of both water and sediment 5 m above bottom (mab) at 275 m in the canyon axis was consistently downcanyon, while 5 mab at 600 m in the canyon axis net transport was upcanyon, implying convergence between the two sites. The distribution of suspended sediment was determined using a CTD/transmissometer during hydrographic transects along the canyon axis and across the slope adjacent to either side of the canyon. Resuspension on the shelf, canyon walls, and along the slope was minimal, but in the canyon interleaving tongues of turbid water were always present between 200 and 800 m. Although concentrations of resuspended sediment were high, they were not sufficient to generate downcanyon flows. Turbid plumes moved seaward along density surfaces at the convergence between 275 and 600 m and other regions of the canyon. The passage of atmospheric storms and Gulf Stream rings may have changed the pattern of currents of resuspension in portions of the canyon sometimes, but not always. Fluxes of particles collected in sediment traps along the canyon axis increased exponentially toward the buttom, but the size of particles collected in traps decreased with distance from the canyon head, suggesting that resuspended sediment was advected, but depleted in larger, denser particles away from the canyon. An exponential increase with depth in trap fluxes at 1000 m, despite fairly uniform concentrations of suspended particles measured by transmissometers, suggests that many particles are advected as large aggregates (marine snow) that are not detected quantitatively by transmissometers. Currents outside the canyon walls quickly dilute and dissipate sediment moving out of the canyon. Subamrine canyons can act as conduits of sediment to the deep ocean not only by channeling turbidity currents, but also by focusing internal tides that resuspended sediment, after which it is advected to deeper water. 1989.
