Anoxic basin

Owing to the stability of the uranyl carbonate complex, uranium is universally present in seawater at an average concentration of ca. 3.2/rgL with a daughter/parent activity ratio U) of 1.14. " In particulate matter and bottom sediments that are roughly 1 x 10 " years old, the ratio should approach unity (secular equilibrium). The principal source of dissolved uranium to the ocean is from physicochemical weathering on the continents and subsequent transport by rivers. Potentially significant oceanic U sinks include anoxic basins, organic rich sediments, phosphorites and oceanic basalts, metalliferous sediments, carbonate sediments, and saltwater marshes. " ... 

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. 

Force, E.R. Maynard, J.B. 1991. Manganese Syngenetic Deposits on the Margins of Anoxic Basins. In Force, E.R., Eidel, J.J., Maynard, J. B. (eds.), Sedimentary and Diagenetic Mineral... 

The stability of PAH is also apparent when one examines sediment samples taken in such a way as to preserve the historical record (11). This can be done by carefully coring sediments, particularly at anoxic locations where there is little bioturba-tion, segmenting the core into 2-4 cm sections, and analyzing each section for PAH quantitatively. An example of such data is shown in Figure 5 this represents a core from the Pettaquamscutt River in Rhode Island, a highly anoxic basin (12). The total PAH concentration ranges from 14,000 ppb near the sediment surface to... 

Sulfur is present in several oxidation states, usually as sulfide and sulfate. Locally native sulfur may occur in significant quantities. Sulfate is present in large concentrations in seawater and is probably the most important oxidizer in the water of anoxic basins such as the Black Sea, Kaoe Bay, and many fjords. 

Bacterially produced elemental sulfur can also react with hydrogen sulfide form polysulnde ions. Thus, polysulfide ions should constitute a significant fraction of sulfur nudeophiles in reducing sediments especially where sulfide oxidation is incomplete, such as in intertidal and salt marsh sediments (31321. The polysulfide ions should also be important at redox boundaries (anoxic/ suboxic) in the water column of marine anoxic basins, such as the Black Sea. 

The hydrogen sulfides (H2S, SH-, S2 and their metal complexes) are well known in restricted reducing regions of the world ocean such as anoxic basins (1), but they have traditionally been dismissed as unimportant for, or even nonexistent in, most oxic seawaters 12-41. Several lines of reasoning are now beginning to suggest that sulfides actually do exist in the surface ocean, and enter into a rich metal chemistiy there. Extensive measurements of carbonyl sulfide (OCS) in seawater (5.61 permit the quantification of a mixed layer source, the hydrolysis reactions f7-111... 

Several types of anoxic environment harbor H2S along the periphery of the bulk ocean, but in order to exist in oxic waters more than a few hours of mixing removed from these isolated localities, the sulfides must be kinetically supported. They have generally been ignored in the open sea, because no internal sources nave been known for them. In an early paper on trace metals, for example, Krauskopf (21 restricted metal sulfide interactions to anoxic basins. Ostlund and Alexander (2) later determined an oxidation lifetime of about half an hour in Or saturated seawater, and noted that sulfide generated in sediments was expected to reach the atmosphere only from very shallow waters (< a few meters). With no sources between the sea bottom and surface, the sulfides were presumed absent from most of the sea. This sentiment is echoed almost to the present (1). 

Studies such as those of Berger and Soutar (1970) and Sholkovitz (1973) also point to the importance of chemical parameters in controlling calcium carbonate preservation. These authors noted that carbonate preservation is substantially greater in the sulfidic Santa Barbara basin sediments than in adjacent slope sediments that are overlain by oxic waters. This observation probably results from the fact that oxic degradation of organic matter and oxidation of sulfides are not likely to occur in this anoxic basin. 

Nonconservative elements elements that do not remain in constant proportion due to biological (e.g., uptake via photosynthesis) or chemical (e.g., hydrothermal vent inputs) processes. In estuaries, as well as other oceanic environments (e.g., anoxic basins, hydrothermal vents, and evaporated basins), the major components of seawater can be altered quite dramatically due to numerous processes (e.g., precipitation, evaporation, freezing, dissolution, and oxidation). 

Cohen, Y. (1978). Consumption of dissolved nitrous oxide in an anoxic basin, Saanich Inlet, British... 

Ward, B. B., and Kilpatrick, K. A. (1991). Nitrogen transformations in the oxic layer of permanent anoxic basins The Black Sea and the Cariaco Trench. In Black Sea oceanography (Izdar and Murray, eds.). Kluwer, Dordrecht, The Netherlands, p. 111-124. 

Much attention has recently been focused on interactions between various manganese and nitrogen species because solute profiles through the suboxic zones of anoxic basins such as of the Black Sea and marine sediments suggest that reactions involving these two species may be occurring (Murray et ai, 1995 Luther et ai, 1997, 1998). Luther et al. (1997, 1998) have proposed two reactions with manganese that result in denitrification ... 

There is also water column denitrification in Black Sea, Cariaco Trench and smaller anoxic basins and fjords, as well as within the more transient hypoxic zones over the continental shelves. The recent work by Kuypers et al. (2005) indicated that the denitrification rate off south west Africa is about 1.5 Tg a . Codispoti et al., 2002 have estimated the total denitrification in these smaller/more transient sites to be 10 Tg a. Finally, denitrification has been reported to take place in microenvironments within the oxygenated water column such as organic aggregates Aldredge and Cohen, 1987 Wolgast et al, 1998). However no quantitative estimates of the global importance of microsite denitrification are available. 

Anderson, J. J., and Devol, A. H. (1973). Deep-Water renewal in Saanich Inlet, an intermittendy anoxic basin. Estuar. coast, mar. sci. 1, 1—10. 

Although the major components of seawater are relatively constant, a number of factors can cause the ocean and estuarine waters to be nonconservative. These processes occur in estuaries, anoxic basins and sediments, hydrothermal vents, and evaporation basins and include precipitation, dissolution, evaporation, freezing, and oxidation processes. Some examples will be discussed briefly. 

Reduced forms of iron, Fe(II), can be produced in natural waters in anoxic basins and hydrothermal waters and by the photoreduction of Fe(III) (Waite and Morel, 1984 Wells and Mayer, 1991 Kingetal., 1995 Johnson etoL, 1994 Miller et ah, 1995 VoeUcer et al., 1997). Once this reduced iron is formed, its lifetime will be related to its rate of oxidation. The kinetics of oxidation of Fe(II) with O2 have been studied by a number of workers (Stumm and Lee, 1961 Kester eta/., 1975 Murray and GUI, 1978 Sung and Morgan, 1980 Davison and Seed, 1983 Roekens and Van Grieken, 1983, 1984 Waite and Morel, 1984 MUlero et al., 1987, 1990 MUlero and Izaguirre, 1989 ... 

Lee B. S., Bulhster J. L., and Whitney F. A. (1999) Chlorofluorocarbon CFC-11 and carbon tetrachloride removal in Saanich Inlet, an intermittently anoxic basin. Mar. Chem. 66, 171-185. 

Figure 17 Generalized relationship between weight percent OC (% OC) and specific mineral surface area (SA) for marine sediments. The shaded area represents the boundaries (OC SA ratio of 0.5-1.1 mgCrn ), within which most continental shelf and upper slope sediments (outside the direct influence of rivers) fall. Sediments underlying anoxic basins and OMZs associated with high productivity (upwelUng) margins tend to exhibit OC SA ratios greater than l.lmgCm whereas deltaic and abyssal sediments exhibit OC SA ratios of less than 0.5 mg C m (after Mayer, 1994 ...

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