The effects of biological and hydrodynamic processes on physical and acoustic properties of sediments off the Eel River, California

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The spatial trends in surficial sediment macro- and microstructure and the resultant values of sediment physical and geoacoustic properties are controlled by the water-depth-dependent interplay of biological, depositional and hydrodynamic processes along shore-normal (25-100 m water depth) and shore-parallel (70-m contour) transects north of the Eel River, northern California. Values of sediment compressional and shear wave speed, porosity, bulk density, mean grain size, and shear strength cluster within reasonably well defined surficial sediment facies: inshore sands (21-42 m water depth), offshore muds (90-100 m), flood deposits (57-80 m), and transition sediments between flood deposits and inshore sands (47-57 m). Statistical relationships among seafloor impedance (calculated from in situ and laboratory measurements or measured remotely by acoustic methods), sediment physical properties, and geoacoustic properties are sufficiently robust to allow prediction with confidence. The high local spatial variability in values of sediment physical and geoacoustic properties reflects the variability in macro- and microstructure exhibited in X-radiographs and CT imagery. Large-scale distribution of sedimentary facies is controlled by flood deposition from the Eel River and subsequent remobilization by hydrodynamic processes. High local variability in sediment properties occurs where no single process is consistently dominant (transition sediments). The high variability of seafloor properties in flood deposits reflects the high structural heterogeneity associated with the interaction among numerous flood deposits, resuspension and redeposition of flood deposits by surface gravity waves, and mixing by bioturbation. Faunal reworking of sediments rapidly (> 1 yr) compacts the sediment, destroys surficial layering, subdues vertical gradients in sediment properties created from flood deposits, and breaches flood layer horizons. High sediment permeability (created by numerous burrows and tubes) promotes sediment dewatering and exchange of pore fluids, the respective consequences of which are rapid consolidation of the sediment after deposition of flood deposits.

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Marine Geology

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