Sedimentary Rocks: Deep Ocean Pelagic Oozes

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Deep-ocean pelagic (from the Greek pelagios, meaning ‘of the sea’) sediments are areally and volumetrically the dominant sediment type found on the ocean floor. They comprise three main types, depending on their primary composition: deep-sea siliceous oozes, calcareous oozes, and deep-water red clays (Fig. 1). Pelagic sediments mixed with terrigenous material derived from continental weathering are termed ‘hemipelagic’. Siliceous and calcareous oozes are largely composed of test and test debris of planktonic micro-organisms such as foraminifera, coccolithophores, pteropods, diatoms and radiolaria (Fig. 2). Formation of pelagic sediments involves settling of material derived from biological surface productivity, but also includes wind-derived material, that travels through the water column (‘pelagic rain’) to the seafloor (Fig. 3). This process occurs throughout the world's oceans and true pelagic deposits tend to blanket seafloor topography. Local remobilization of pelagic sediments on topographic highs due to slope instability may result in pelagic turbidites (redeposited units) that pond in adjacent lows. The distribution of pelagic sediments is, however, strongly depth controlled, as calcium carbonate shows increasing solubility with depth. In contrast to terrigenous sediments (the other main type of deep-sea sediment, largely composed of detrital material derived from continental weathering), pelagic sediments are characterized by low sedimentation rates and frequently contain a high proportion of authigenic minerals, extraterrestrial material, and, where physico-chemical conditions allow, a substantial biogenic component.