Seawater is an extremely complex solution, its composition being determined by an equilibrium between rates of addition and loss of solutes, evaporation and the addition of fresh water. The original source of seawater is uncertain, but was probably by condensation of water vapour and solutes released into the atmosphere from hot rocks and volcanic action at an early stage of the Earth's history. At the present time many constituents of seawater are continually added from various sources; for instance, in 'juvenile water' released from basalts which flow into the sea floor along the separating boundaries of the Earth's crustal plates (see page 7), in volcanic gases escaping into both oceans and atmosphere and in processes of weathering and erosion of the Earth's surface. Loss of solutes from the water occurs by precipitation on the bottom (Glasby, 1973; MacIntyre, 1970). Short-term, minor fluctuations of composition occur through biological processes involving absorption and release of solutes by organisms and detritus. There are also interchanges of gases between sea and atmosphere.
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