Anoxic deep water

Formation of the shallow-water concretions is associated with anoxic conditions that range in duration from seasonal to nearly continuous. For example, in the Baltic Sea, nodules and crusts are mostly found around the margins of the deep anoxic basins. They form from Mn and Fe that accumulates from the reduction of Mn and Fe oxides in the anoxic deep waters. When the basin is periodically flushed with oxic water from the North Sea, about once a decade, the concretions undergo growth as the fresh supply of metals is oxidized. 

Fenchel T, Bernard C, Esteban G, Finlay BJ, Hansen PJ, Iversen N (1995) Microbial diversity and activity in a Danish fjord with anoxic deep water. Ophelia 43 45-100 Hackstein JHP, Akhmanova A, Voncken F, van Hoek A, van Alen T, Boxma B, Moon-van der Staay S, van der Staay G, Leunissen J, Huynen M, Rosenberg J, Veenhuis M (2001) Hydrogenosomes convergent adaptations of mitochondria to anaerobic environments. Zoology 104 290-302... 

Stein R. (1986) Organic carbon and sedimentation rate, further evidence for anoxic deep-water conditions in the Cenoma-nian/Turonian Atlantic Ocean. Mar. Geol. 72, 199-209. 

The decreasing redox potential favors the reduction of iron-3-phosphate to iron-2-phosphate, which is dissolved from the sediments causing a strong increase of phosphate in the anoxic deepwater. Nitrate and nitrite are unstable under these conditions. Ammonium resulting from the microbial decomposition of organic compounds is the only stable nitrogen compound in the presence of hydrogen sulfide. Microbial processes also seem to be responsible for the accumulation of silicate in anoxic deep waters. 

Stein, R., 1986a. Surface-water paleo-productivity as inferred from sediment deposited in oxic and anoxic deep-water environments of the Mesozoic Atlantic Ocean. Mitt. Geol.-Palaont. Inst. Univ. Hamburg, 60 55-70. 

As a first approach a model was used to test whether anoxic deep water can be the sole explanation to the high concentrations of organic matter in the sapropel layers. Data from the literature has been combined to make an algorithm using Model Maker ver. 3 software. The model uses the present primary productivity of 40 mg 10 mg C day This is an average of the 12 g C given by Bethoux (1989) and 16.7 g C m a by Krom et al (1992). 

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