A shipboard method for measuring the oxygen uptake of undisturbed cores of sediment with their original overlying water and intact mud-water interface gave the same results as in situ measurements at 22-m depth. Hydrostatic pressures up to 18 atm have no effect on total oxygen uptake of cores from 180-m depth. Partitioning experiments with intact cores show that oxygen uptake by inorganic chemical oxidation is often greater than respiratory consumption; in these experiments the rate of chemical oxidation decreases with decreasing oxygen tension, while the rate of respiration is relatively constant down to a critical oxygen concentration of about 1 ml/liter. Sweeping away the surface layer and exposing subsurface sediment, either in situ or aboard ship, leads to increased rates of oxygen uptake. Where chemical oxidation predominates, the rate of uptake is a function not only of oxygen concentration but also of stirring of the water overlying the sediment. Undisturbed cores where respiration predominates show a constant rate of uptake for many hours, even after cessation of stirring for 1 hr. Presumptive evidence is presented that active denitrification could be taking place as deep as 20-21 cm in the sediment of the continental shelf off Peru. The unknown relationship between chemical oxidation and anaerobic metabolism clouds understanding of the biological meaning of total oxygen uptake by the sediment, but this is still the best measure of the interated biological activity in the seabed.
