Oxygen in Pore Waters of Deep-Sea Sediments

Profiles of dissolved oxygen are presented from contrasting ocean areas, and compared with data on the distribution of trace elements in sediment and pore water. The release of manganese starts when oxygen is depleted to about 1 μ M. In the Sulu Sea (Philippines), the manganese flux to this interface balanced 48 ± 16% of the oxygen flux. Large excess O₂ fluxes in the east Atlantic are attributed to non-steady-state diagenesis. At the transition from O₂ to manganese reduction, cobalt and nickel are mobilized, and many trace elements, including rare earth elements, undergo a phase transition as a result of the dissolution of the manganese oxyhydroxide carrier phase. In the deep Angola Basin, Weddell Sea and west equatorial Pacific, oxygen penetrates the sediment to well over 1 m. In the semi-enclosed Sulu Sea with a low oxygen concentration in the bottom water (O_bw = 55 μ M), z_o increases from 1 cm at 1000 m water depth to 10-15 cm at 4000 m, and decreases again to 1 cm in the deepest spots covered by turbidites. Generally, the depth where oxygen is depleted (z_o) increases with increasing water depth; z_o decreases at the approach of the continent and under the influence of turbidites; it increases at topographic highs exposed to winnowing. A model is presented that relates z_o to O_bw, primary production and water depth. z_o is especially sensitive to changes in organic matter input and O_bw when oxygen breaks through the bioturbated zone. The large differences in z_o at the same water depth in the Sulu Sea and adjacent South China Sea and west equatorial Pacific (6, 14 and 40 cm at 3500 m depth) can be explained merely by the differences in O_bw (55, 105 and 150 μ M respectively). It is concluded that glacial to interglacial changes in O_bw must have had major effects on diagenesis and infauna throughout the world oceans.