The oceans are by far the largest reservoir of the hydrosphere (see Fig. 1.4) and have existed for at least 3.8 billion years. Life on Earth probably began in sea-water and the oceans are important in moderating global temperature changes. Rivers draining continental land areas carry both dissolved and particulate matter to the oceans and the average input of dissolved major ions can be estimated by considering the input from some of the rivers with the largest discharges (see Section 5.2). The transport of particulate matter to the oceans depends on both the discharge and the supply of suspended sediment. Some large rivers such as those in central Africa carry rather small amounts of sediment because of the relatively low relief and dry climates in the catchments (Fig. 6.1). By contrast, rapidly eroding areas of South East Asia carry a disproportionate volume of sediment compared to the volume of water, due to high relief and heavy rainfall in the catchments (Fig. 6.1). Much of this sediment falls to the seafloor, usually in estuaries and on continental shelves, although in some parts of the ocean where the shelf area is small, this material may reach deep-sea environments. Most river-water enters the oceans through estuaries and here freshwater mixes with sea-water. The chemical composition of seawater is quite different from that of freshwater, a difference that affects the transport of some dissolved and particu-late components. In addition, humans often perturb the natural chemistry of coastal areas, either through contamination of the freshwater runoff, or due to activities located close to estuaries and shallow seas.

We begin this chapter by examining the chemistry of seawater close to continental areas, in the transition zone between terrestrial and open-ocean environments, before moving on to discuss open-ocean environments.

Fig. 6.1 Annual river sediment flux from large drainage basins to the oceans. Numbers in 106 tonnesyr-1 and arrow size proportional to the numbers. After Milliman and Meade (1983), with permission from the University of Chicago Press.
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