In addition, a number of deep channels cut across the continental rise. Ocean sediments are composed of terrigenous, pelagic, and authigenic material. Terrigenous deposits consist of sand, mud, and rock particles formed by erosion, weathering, and volcanic activity on land and then washed to sea. These materials are largely found on the continental shelves and are thickest off the mouths of large rivers or off desert coasts. Pelagic deposits, which contain the remains of organisms that sink to the ocean floor, include red clays and Globigerina, pteropod, and siliceous oozes.
Authigenic deposits consist of such materials as manganese nodules.
They occur where sedimentation proceeds slowly or where currents sort the deposits. The salinity of the surface waters in the open ocean ranges from 33 to 37 parts per thousand and varies with latitude and season. Although the minimum salinity values are found just north of the equator, in general the lowest values are in the high latitudes and along coasts where large rivers flow intothe ocean. Maximum salinity values occur at about 25 deg north latitude. Surface salinity values are influenced by evaporation, precipitation, river inflow, and melting of sea ice.