Sediment transport in the Mississippi Canyon: the role of currents and storm events on optical variability

Abstract

Two modes of sediment transport were found to exist in the Mississippi Canyon: the offshelf transport of material in intermediate nepheloid layers originating at depths of 50-175 m and the resuspension and transport of material within the canyon. Large- and small-particle intermediate nepheloid layers were consistently present in the canyon axis and were not observed on the slope to either side of the canyon. The temporal variability in currents, temperature, and particulate matter was measured at a station located at 300 m depth in the canyon axis during consecutive deployments in May-July and August-November 1998. Two moored current meters, one at 3.5 mab and one at 50 mab, recorded flow, while thermographs, a light-scattering sensor, and sediment traps gathered information about the characteristics of the flow and movement of particulate matter. Currents in the upper Mississippi Canyon were oscillatory, with alternating periods of up-canyon and down-canyon flow. Harmonic analysis revealed that the diurnal tidal signal was the dominant component of the flow. Currents were most intense at 3.5 mab. Mean current speed at this depth was approximately 8 cm s⁻¹ during both deployments, reaching maximum speeds of over 50 cm s⁻¹. Current velocities generated sufficient shear stress to resuspend canyon floor sediments about 30% of the time during both deployments. During the second mooring deployment, Hurricane Georges passed 150 km NE of the study site. Near-bottom current velocities and temperature fluctuations were intensified. As the hurricane passed, maximum current speed reached 68 cm s⁻¹ and a temperature decrease of approximately 7 degrees C occurred in less than 2 hours. Conditions were favorable for sediment resuspension approximately 50% of the time during the five days of hurricane influence. Further evidence for sediment resuspension was provided by similarities between canyon floor core samples and sediment trap collections.