SEDIMENTARY PROCESSES/Deep Water Processes and Deposits 643 Figure Distribution of the principal types of pelagic sediment sediment is supplied by sediment derived from subglacial melt water discharge and directly from ice, including meltout from icebergs thousands of kilometres from their source In mid-latitudes, wind-blown dust (exceptionally including sand) is a significant component of deep-ocean sediment Locally, volcanic material is a major component of seafloor sediment Pelagic sediments (see Sedimentary Rocks: Deep Ocean Pelagic Oozes) consist principally of the skeletal material of pelagic organisms, notably calcium carbonate from Foraminifera, Coccolithophoridae (coccoliths or nannofossils), and pteropods, and opaline silica from Radiolaria, diatoms, and silicoflagellates In oceanic areas with low sedimentation rates, various authigenic minerals form in situ in the sediments or on the ocean floor and may become significant components of the sediment These include: phosphorites formed in slow-sedimentation environments in areas of active upwelling; metal-rich sediments and iron oxides associated with hydrothermal discharge in areas of active volcanism, particularly mid-ocean ridges (see Tectonics: Hydrothermal Vents At Mid-Ocean Ridges); manganese nodules and crusts of iron–manganese oxides, which precipitate in areas of slow sedimentation on the deep ocean floor (see Sedimentary Rocks: Oceanic Manganese Deposits); zeolites, which form principally from the alteration of volcanic ash and glass; and barite, which may be related to either hydrothermal activity or high organic productivity The dispersal of terrigenous sediment from various sources can be tracked by studying sediment petrology The gross distribution of clay minerals within the ocean has long been used to infer sediment source and dispersion In recent years, the signature of radiogenic isotopes (particularly lead and neodymium) and mineral geochronology in detrital terrigenous sediment have been used to understand further source and dispersion, including the tracking of icerafted detritus and aeolian dust Tephra beds have been characterized geochemically and mineralogically, both to identify source and as stratigraphic markers Sediment Transport Processes in Deep Water The transport of sediment to deep water involves both the normal processes of oceanic circulation and episodic sediment gravity flows The resulting terrigenous deposits depend on the supply of sediment from the continent and shallow continental margin and on the complex interaction between flows and ocean morphology Surface currents in the ocean are primarily winddriven, whereas overall circulation in the oceans is a consequence of density differences caused by temperature and salinity variations in the ocean water Deep circulation is driven by the sinking of cold saline water in the North Atlantic Ocean and around Antarctica, with net upwelling in the North Pacific Ocean The effect of the Coriolis force is to intensify ocean circulation on the western sides of oceans, where the thermohaline circulation generally