Freshwater and Ice

Although there is an arbitrariness in the definition of freshwater used here (salinity <0.05%, Table 10.2), for most purposes there is little difficulty in making the distinction because most of the water we consider is either well below or clearly above this limit. The only significant doubt is the fraction of groundwater that can be identified as fresh. The flow of groundwater into the sea is not well constrained by observations but is generally believed to be of order 10% of the river flow. However, much groundwater is saline and its contribution to ocean salt probably exceeds that of the river input. Table 11.1 summarises the distribution of the Earth’s water. An estimate of the total water content of the Earth requires some information about the water in the mantle, which probably exceeds all the rest put together. Table 11.1 reports an upper limit, obtained from reported water contents of mineral inclusions in diamonds of very deep origin, which give more than 0.2% (Shirey et al. 2013), assuming this to be representative of the deep mantle. Water in Hawaiian lavas identified by Kokubu et al. (1961) as juvenile amounted to 0.09%. If this is a better representation of the mantle average, then the water content is 3.6 × 10⁶ in units of 1000 km³. The hydrogen content of the mantle listed in Table 7.3, derived from geochemical evidence of element partitioning, gives a value of 2.4 × 10⁶ in the same units if it is assumed all to be in the form of molecular H₂O or as related H⁺ and OH⁻ ions. Mantle rheology is controlled by water, or its separated ions, but does not give an estimate of concentration.