Oceans redox conditions

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

Finally, because the Mo ocean budget should be strongly sensitive to bottom water redox conditions, particularly perturbations associated with expanded deep sea anoxia, determination of Mo concentrations or their variations in ancient seawater, via sedimentary proxies, has been proposed as an indicator of global ocean paleoredox change (e.g., Emerson and Huested 1991). 

At 20 °C, K = 10 - and so water of pH=8.1 in equilibrium with atmospheric O2 (p02 — 0.21 atm) has pe = 12.5. This conforms to surface conditions, but the pe decreases as the O2 content diminishes with depth. The oxygen minimum is particularly well developed beneath the highly productive surface waters of the eastern tropical Pacific Ocean, where there is a large flux of organic material to depth and subsequently considerable oxidation. The O2 becomes sufficiently depleted i.e., hypoxia) that the resulting low redox conditions causes NOs to be reduced to N02 - Aeolian transport of nitrate to Chesapeake Bay can lead to low O2 conditions. Similarly, intermittent hypoxia develops in parts of the Gulf of Mexico due to the riverine transport of nutrients derived from agricultural uses in the Mississippi catchment. 

It is likely that the progressive oxidation of the atmosphere also influenced the oceans. Certainly the chemistry of the Archaean oceans was different from that of modern oceans and was richer in Ca, Mg, K, Fe, Ba, Ce, and Si and lower in S, P, Mo, Re, U, and Os. These differences, in part, reflect the different redox conditions of the Archaean Earth... 

These compositional differences reflect the different processes operating in the early Earth. In part they reflect the different redox conditions that existed but also indicate a different biology of the oceans and a different balance between hydrothermal and weathering inputs from that of the modern. 

A particularly appealing aspect of paleoredox proxies based on isotope ratios of metals is the possibility of interpreting records of global redox conditions in the oceans or atmosphere, as opposed to the local snapshots in time and space... 

Also motivated by the desire to constrain further the history of oxidation of the atmosphere and oceans, Frei et al. [37] determined the isotopic compositions of Cr in banded iron formation (BIF) samples, representing the Archean (>2.5 Ga ago) and Proterozoic (2.5 Ga to 542 Ma ago) Eons. They anticipated that isotopic variations in those rocks would correlate with changes in global redox conditions over time because of previous work reporting isotope ratio variations correlated with the Cr oxidation state in natural samples [38, 39], experiments [40, 41], and theoretical calculations [42]. 

Redox potential (oxidation-reduction) is considered a master variable with respect to controls on the concentration and speciation of many trace elements in natural waters (Stumm and Morgan 1981). Shifts between oxic, suboxic and anoxic conditions represent one of nature s most dramatic chemical variations. The response of lanthanides to variations in redox conditions has been studied in many of the world s classic anoxic and suboxic basins. These include (1) the Black Sea (German et al. 1991, Schijf et al. 1991, 1994, Schijf and De Baar 1995), (2) Saanich Inlet (Canada) (German and Elderfield 1989), (3) Chesapeake Bay (Sholkovitz and Elderfield 1988, Sholkovitz et al. 1992), (4) the Cariaco Trench (De Baar et al. 1988), (5) the Mediterranean Sea (Schijf et al. 1995) and (6) the northwest Indian Ocean (German and Elderfield 1990). The latter two regions are located on ocean shelves while the first three basins are estuarine and coastal. Data from the papers cited above are compiled in table A12. 

E. L. Shock (1990) provides a different interpretation of these results he criticizes that the redox state of the reaction mixture was not checked in the Miller/Bada experiments. Shock also states that simple thermodynamic calculations show that the Miller/Bada theory does not stand up. To use terms like instability and decomposition is not correct when chemical compounds (here amino acids) are present in aqueous solution under extreme conditions and are aiming at a metastable equilibrium. Shock considers that oxidized and metastable carbon and nitrogen compounds are of greater importance in hydrothermal systems than are reduced compounds. In the interior of the Earth, CO2 and N2 are in stable redox equilibrium with substances such as amino acids and carboxylic acids, while reduced compounds such as CH4 and NH3 are not. The explanation lies in the oxidation state of the lithosphere. Shock considers the two mineral systems FMQ and PPM discussed above as particularly important for the system seawater/basalt rock. The FMQ system acts as a buffer in the oceanic crust. At depths of around 1.3 km, the PPM system probably becomes active, i.e., N2 and CO2 are the dominant species in stable equilibrium conditions at temperatures above 548 K. When the temperature of hydrothermal solutions falls (below about 548 K), they probably pass through a stability field in which CH4 and NII3 predominate. If kinetic factors block the achievement of equilibrium, metastable compounds such as alkanes, carboxylic acids, alkyl benzenes and amino acids are formed between 423 and 293 K. 

The nodules are formed by the oxidation and precipitation of iron and manganese. The oxidation of Mn24 is catalyzed by a reaction surface io a tetravalent state that absorbs additional Fe2+ or Mn2+ which, in turn, becomes oxidized. A surface is required and the initial deposition may be of iron oxide, possibly from volcanic or geothermal sources. Proper conditions of pH, redox potential, and metal ion concentration are found in deep ocean waters. The rate of accumulation appears to be very slow. The growth also may be discontinuous, and is estimated at a faster rater rate near the continental margins. 

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