Our understanding of the processes that affect or control the chemistry of the sea began with the obvious fact that rivers run into the sea, and they carry the washings of the land into the sea. Then came some appreciation of the evaporation of water and the rudiments of the major hydrological cycle. In the twentieth century it become generally appreciated that atmospheric dust is a major contributor to oceanic sediments, and in the latter decades of that century the transport of many sub- stances through the air between land and sea became generally appreciated. In the last half of the century it also became generally well known that throughout the floor of the ocean there is some likelihood of significant chemical exchanges between the sediment and the overlying water, and between seawater and the hot rocks at the spreading centers and other volcanic regions on the sea floor, and that these have important influences on the chemistry of the ocean. None of the fluxes involved is really well known for any substance, however, although the estimates for river inputs of most substances are certainly satisfactory for geochemical calculation, and most of the other fluxes are well enough constrained for first-order estimates and calculations. Much less is known about the factors that might cause changes in the rates of many of the important fluxes.

Deep as it is, the ocean is still a thin layer on the surface of Earth, and during the course of several thousand years nearly every drop of water has some opportunity to be exposed to the air–water boundary at the surface and to the sediments and rocks at the bottom. The chemical exchanges that influence the sea occur at all these boundaries, and in turn the sea profoundly affects the air above and the sediments and rocks below. In the sections that follow, these exchanges are examined further.