Prior to about 1870, it was thought that the depths of the ocean were devoid of oxygen and devoid of life. In 1873, C. Wyville Thompson published The Depths of the Sea, an account of three expeditions in 1868–1870. He and his colleagues dredged and collected water samples for the first time to depths of about 2400 fathoms (more than 4400 m) and found life at all depths. Measurements of gases in water samples all showed the presence of oxygen. Then, during the great Challenger expedition, the major ocean basins were surveyed to considerable depths, and all the samples contained oxygen. The statement by Dittmar, above, is still true, because there is always some movement of the water, but there are important areas where oxygen is absent.

Living things exist at all depths. Indeed (with the exception of the extremely hot water feeding the black smokers; see Chapter 13), there is no single liter of seawater anywhere in the ocean that is devoid of life, and all living things metabolize. If oxygen is present it will be used in respiration, and will eventually all be used, if it is not replaced. If all oxygen is gone, life continues, but the types of organisms present are different.

Rates of oxygen consumption

The first serious attempt to estimate the rate of oxygen consumption by evaluating the distributions of oxygen in the ocean relative to the physical motions of the water was presented by Gordon Riley in a classic paper published in 1951 (see Table 12.1). Another approach is to use the modern sediment-trap data and the equations fitted to these to calculate the oxygen consumption that must be associated with the observed decrease of particle flux as the depth in the water column increases (Figure 12.1). The agreement between the early estimate by Riley and this estimate from sediment-trap data is remarkable. It is, of course, possible or even likely (see the discussion in Chapter 11) that both estimates are too low, and in any case the fluxes are certainly quite variable spatially. Nevertheless, these estimates illustrate the point that, given enough time, there would be no oxygen in the deep sea unless it is somehow replaced.