76
Sources and Composition of Marine Sediments
Typically, diagenesis is most active in the uppennost meter of freshly deposited
sediments. "Redox" reaction usually dominate, and these reactions are the more intense the more organic carbon is present. The oxidation of organic carbon leads to
removal of dissolved oxygen from pore waters. Additional oxygen demand is satisfied by stripping oxygen from dissolved nitrate and from solid iron oxides and hydroxides (e. g., coatings on grains). If the demands is strong enough, sulfate is
stripped of its oxygen also . These reactions are all mediated by bacteria. In the
process, gases are produced (carbon dioxide, ammonia, hydrogen sulfide), as well as
iron sulfide. Thus, the escaping pore water is enriched in these gases and impoverished in nitrate and sulfate, and the remaining sediments are enriched in sulfide (e. g.,
pyrite). Fermentation of remaining organic carbon compounds can subsequently lead
to methane fonnation , which can react with water (given low temperature and high
pressure) to make clathrates. Such clathrates are thought to be widespread in sediments below coastal upwelling regions, where they produce prominent seismic reflectors. The massive escape of gases from organic-rich muds can produce "mud
volcanoes" (see Fig. 3.5) or smaller "pockmarks".
Besides redox reactions, the dissolution of carbonate and opal is of prime importance in diagenesis. During "early diagenesis", much of the dissolved matter can
leave with the escaping pore water, or depart simply by diffusion out of the sediment
into the overlying waters. Once the sediment is deeply buried, concentrations within
the interstitial waters can increase to the point where reprecipitation occurs. This
leads to cementation of remaining grains by carbonate and silica cements. These
Fig. 3.5. Mud volcano. Side scan record from (he Black Sea bottom. (Side scan principl e see FIg.
4.15). [Courtesy Dr. Glunow. Moscow. from UNESCO-IMS -Newsletter 61 . Paris]
Sources and Composition of Marine Sediments
Typically, diagenesis is most active in the uppennost meter of freshly deposited
sediments. "Redox" reaction usually dominate, and these reactions are the more intense the more organic carbon is present. The oxidation of organic carbon leads to
removal of dissolved oxygen from pore waters. Additional oxygen demand is satisfied by stripping oxygen from dissolved nitrate and from solid iron oxides and hydroxides (e. g., coatings on grains). If the demands is strong enough, sulfate is
stripped of its oxygen also . These reactions are all mediated by bacteria. In the
process, gases are produced (carbon dioxide, ammonia, hydrogen sulfide), as well as
iron sulfide. Thus, the escaping pore water is enriched in these gases and impoverished in nitrate and sulfate, and the remaining sediments are enriched in sulfide (e. g.,
pyrite). Fermentation of remaining organic carbon compounds can subsequently lead
to methane fonnation , which can react with water (given low temperature and high
pressure) to make clathrates. Such clathrates are thought to be widespread in sediments below coastal upwelling regions, where they produce prominent seismic reflectors. The massive escape of gases from organic-rich muds can produce "mud
volcanoes" (see Fig. 3.5) or smaller "pockmarks".
Besides redox reactions, the dissolution of carbonate and opal is of prime importance in diagenesis. During "early diagenesis", much of the dissolved matter can
leave with the escaping pore water, or depart simply by diffusion out of the sediment
into the overlying waters. Once the sediment is deeply buried, concentrations within
the interstitial waters can increase to the point where reprecipitation occurs. This
leads to cementation of remaining grains by carbonate and silica cements. These
Fig. 3.5. Mud volcano. Side scan record from (he Black Sea bottom. (Side scan principl e see FIg.
4.15). [Courtesy Dr. Glunow. Moscow. from UNESCO-IMS -Newsletter 61 . Paris]