Anoxic seawater

Brewer and Spencer [428] have described a method for the determination of manganese in anoxic seawaters based on the formulation of a chromophor with formaldoxine to produce a complex with an adsorption maximum at 450 nm. Sulfide (50 xg/l), iron, phosphate (8 ig/l), and silicate (100pg/l) do not interfere in this procedure. The detection limit is 10 pg/1 manganese. 

Solutions containing Sbm and Sbv in deionised water at 0°C and 25°C in polyethylene containers were stable for 1 year (De la Calle Guntinas and Camara, 1992). However, samples of natural water, acidified to pH 2 or less, required rapid freezing to -4°C to avoid oxidation of Sbm. In anoxic seawater, concentrations of antimony species (Sbm and Sbv) in stored samples were about 49% lower than those determined at sea soon after the samples had been obtained (Cutter et al., 1991). Samples of particulate material were placed in acid-cleaned plastic bags or vials and then preserved by freezing. 

Inorganic arsenic species, As111 and Asv, in natural water and anoxic seawater samples were not stable (Cutter et al., 1991). Rapid freezing and storage at —4°C was recommended as a means of preservation. Particulate samples were collected in acid-cleaned plastic bags, and then frozen. 

Sun, M.Y., Zou, L., Dai, J., Ding, H, Culp, R.A., and Scanton, M.I. (2004) Molecular carbon isotopic fractionation of algal lipids during decomposition in natural oxic and anoxic seawaters. Org. Geochem. 35, 895-908. 

Krysell M., Fogelqvist E., and Tanhua T. (1994) Apparent removal of the transient tracer carbon-tetrachloride from anoxic seawater. Geophys. Res. Lett. 21, 2511-2514. 

Tanhua T., Fogelqvist E., and Basturk O. (1996) Reduction of volatile halocarbons in anoxic seawater, results from a study in the Black Sea. Mar. Chem. 54, 159-170. 

Another potential mechanism responsible for the formation of laterally continuous, strata-bound calcite-cemented sandstones is the episodic up>-welling of anoxic seawater (see Kempe, 1990 Grot-zinger Knoll, 1995). The upwelling of such high alkalinity waters to shelf and coastal areas may occur subsequent to periods of sea water stratification accompanying sea-level rise. 

Fossing H. and Jorgensen B. B. (1990) Isotope exehange reactions with radiolabeled sulfur compounds in anoxic seawater. Biogeochemistry 9, 223 -245. 

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. " ... 

This determination will test for the presence of naturally occurring reducing agents in seawater which by their action on iodonium ions could lead to an underestimate in iodate concentration. (The use of the method on anoxic waters containing sulfide is a prime example of when this precaution should be taken.)... 

In the analysis of seawater, the only significant interference arises from turbidity caused by particles in the sample. Prior filtration of the sample is therefore necessary. For anoxic waters, however, sulfide concentrations over 2 pm were found to decrease the absorbance. This was overcome by adding an excess of either Cd2+ or Hg2+ to the sample [171,172],... 

Fig. 22.5. Concentrations of components (sulfate, sulfide, carbonate, methane, and acetate) and species (O2 and H2) that make up redox couples, plotted against temperature, during a model of the mixing of fluid from a hot subsea hydrothermal vent with cold seawater. Model assumes redox couples remain in chemical disequilibrium, except between 02(aq) and H2(aq). As the mixture cools past about 38 °C, the last of the dihydrogen from the vent fluid is consumed by reaction with dioxygen in the seawater. At this point the anoxic mixture becomes oxic as dioxygen begins to accumulate.

Because of its high organic content, the marine snow acts as a microhabitat that supports enhanced rates of heterotrophic microbial activity. The associated nutrient remineralization causes the seawater within and aroimd the marine snow to be characterized by elevated nitrogen and phosphorus concentrations and low levels of O2. The importance of these suboxic and anoxic microzones to the marine cycling of the biolimiting elements is unknown but potentially significant. 

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. 

The concentrations of the major inorganic ions in seawater are well known in estuarine and coastal areas as well as in interstitial waters anomalies in their constant ratios may occur. The major cations are Na+, Mg2+, Ca2+, K+ and Sr2+, the major anions Cl-, SCty2-, HCO3", B(OH) ", F" and Br". Ion pairs involving these elements and H+, OH", CO32-, POif3- and SiOz -. Under anoxic conditions the S2- - ion and bi- and polysulphides become important. A summary of the major ion speciation in seawater is given by Kester et al. (1975). 

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). 

Further evidence for the addition of H2S to carbon-carbon double bonds very early in sediments, and further insights into reaction mechanisms, have been reported by Vairavamurthy and Mopper in 1987 and 1989 (109.110). They identified 3-mercaptopropionic acid (3-MPA) as a major thiol in anoxic intertidal marine sediment and demonstrated that the thiol formation could occur by the reaction of HS with acrylic acid in sediment water and seawater at ambient temperature The formation of 3-MPA was hypothesized to occur by a Michael addition mechanism whereby the nucleophile HS adds to the activated double bond in the a,/3-unsaturated carbonyl system ... 

Care must be taken, however, in pore waters and special areas such as coastal waters, carbonate banks, and lagoons where significant deviations from normal seawater concentration ratios can occur. As an aside, it should also be noted that the apparent constants are not accurate where seawater composition is altered. This composition change can be especially important in anoxic environments where extensive sulfate concentration changes occur. 

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