small areas at these oceans' northern boundaries, and subsequent vertical transport and dissolution of their siliceous tests at depth.
The flux of particles to the seafloor is apparently the most important mechanism by which elements are delivered to the sedimentary reservoir. However, many complex processes, primarily biological, can occur after initial deposition. Particles falling from above are the only source of nutrition for many bottom-dwelling organisms. Thus this material is ingested and excreted repeatedly by a variety of species and is also subject to continuous bacterial degradation, reducing its carbon content and destroying the functional groups responsible for scavenging reactive elements from the water column.
The two most important properties of the ocean system that control the uptake of chemical species by the sediments and oceanic crust are its oxidation state and its temperature.
Chemical reactions in sediments involving oxidation and reduction depend on the amount of oxygen present, which in turn depends on metabolic activity and diffusion from the overlying water column through the pore fluids. Where metabolic processes exceed the oxygen supply through diffusion, the sediments become anaerobic. This condition creates diffusion gradients in the oxic-anoxic transition zone from above and below, affecting a range of chemical reactions. The transition zone rises through the sediment column as sediment accumulation progresses. Understanding and quantifying these processes are crucial for studies of global change because of their implications for interpreting the sedimentary record.
The character of the crustal sink for oceanic dissolved material changes with the temperature of the water-rock interactions. At high temperatures, the reaction environment is anoxic. Sulfide-forming elements are precipitated; elements that form insoluble oxides in the reduced state, such as uranium and chromium, are also precipitated. Soluble materials, such as boron and alkali compounds, are completely removed from the rocks. Magnesium is removed but calcium is released. At the very high temperatures (>400°C) associated with recent eruptions, phase separation can occur, producing a dilute aqueous phase and a residual brine. Subsequent mixing of these components appears to be responsible for the large variations in salinity observed in hydrothermal flu-