Sources of Seawater Substances

Let us now consider how water in the ocean was composed and how its composition is regulated. Broadly speaking, the initial sources of the substances in the ocean must have been the primordial atmosphere, the original rock of the earth, and gases released from the earth's inter or (Fig. 4-3). A small amount may have come to the ocean as cosmic dust from extraterrestrial sources.

In all probability, water evolved from vapor released from volcanoes, fumar- oles, and hot springs. Water combined with chemicals has been released also during the erosion of primordial rocks. The present rate of discharge of this juvenile water from the interior of the earth is about 66 x 10¹⁵ g per year.

Figure 4-3. Sources of seawater substances and their cycles of transport throughout continents, atmosphere, and ocean

This rate can easily account for all the water on earth—provided that contributions have remained the same throughout geologic time and provided that allowance is made for considerable recycling of water through the ocean, atmosphere, rivers, lakes, and groundwater. If such assumptions are true, the total amount of water in the world ocean has been increasing at a rate of about 0.4 x 10¹⁵ g per year throughout its history.

Most of the major cations (positively charged ions) and minor elements were derived from weathered igneous rocks (see Chap. 15, page 398). This fact has been deduced by comparing the average compositions of igneous rocks, seawater, and sediments. However, to account for the chloride, sulfate, and bromide anions (negatively charged ions) in seawater, another source is required. These substances are present in volcanic discharges, so it is likely that the major anions in seawater have been added from past emissions of volcanoes.

The most abundant anions in the world ocean—chloride, sulfate, and bicarbonate—are quite soluble in water but are not saturated in seawater. Consequently their proportions in the ocean should reflect the proportions in which they are released from the earth They are released from the earth in the proportions in which they exist in the earth's interior, which in turn represents the proportions in which they were formed from the solar system event that formed the earth.

Today, the total amount of dissolved material observed in the ocean is about 5 x 10²² g. Rivers carry about 2.5 x 10¹⁵ g to the sea annually. If we assume that dissolved substances began accumulating in the ocean about 3 billion years ago and that the rate of transport by rivers rose steadily to the present value, somehow a large excess of dissolved materials has been removed from the sea.

Obviously, therefore, the sea does not trap all the ions brought to it by rivers and by other sources. Instead, there must be regulatory processes that maintain the concentrations of dissolved materials at the levels observed today. Other lines of evidence (noted in a later section) indicate that the composition of seawater has not changed much during the last 1.5 billion years. We can infer that the regulatory processes have been active at least that long.