The Nature of Marine Sediments

Abstract

The discussion in this chapter will be restricted to surficial sediments (up-per few meters) found on the continental shelf (beach to the continental shelf-slope break, approximately to 100-130 m water depth) and in other nearshore environments (lagoons, estuaries, and bays). This coastal area is approximately 29 ×10 6 km 2 , or 8% of the world's oceans and is characterized by high spatial and temporal variability in both morphology and sediment distribution [Reineck and Singh 1973]. Short-term temporal changes in the seafloor, which occur at time scales of minutes to decades, are the result of the interaction of hydrodynamic (waves, currents and tides, and deposition from rivers), biological (bioturbation), and biogeochemical (mineral dissolution , precipitation, and in situ gas bubble formation) processes acting on the seafloor. Longer-term changes in seafloor characteristics result from global changes in sea level which alter regional sediment erosion, transport, and depositional patterns and occur over geological time scales. These coastal areas are the likely locations for high-frequency sonar operations and high-frequency acoustic experiments. The general trends in sediment morphology and characteristics described herein should reflect corresponding trends in high-frequency acoustic behavior. This chapter will include a general discussion of coastal morphology and will describe global patterns of sediment distribution. Next, the structure of sediments is discussed with the hope of providing developers of acoustic propagation theory with a realistic picture of the morphology, fabric, and particle interactions of sediments. This structure provides the building blocks for coastal marine sediment and has a great influence on the sediment physical and geoacoustic properties discussed in Chs. 4 and 5. Bubbles in sediments are also considered, including their morphology and the conditions that promote their existence. The chapter concludes with a discussion of the hydrodynamic and biological processes that alter sediment morphology , structure, and relief. Other diagenetic (post-depositional) changes that occur in near-surface sediments related to consolidation and biogeochemical processes are discussed within appropriate sections on sediment structure. Together these environmental processes are responsible for vertical gradients in sediment physical properties, create much of the random spatial variability