Ocean redox state

Rouxel, O.J., Belcker, A., and Edwards, K.J., 2005. Iron isotope constraints on the Archaean and Palaeoproterozoic oceanic redox state. Science, 307, 1088-91. 

Wallmann, K., 2003. Feedbacks between oceanic redox states and marine productivity a model perspective focused on benthic phosphorus cycling. Glob. Biogeochem. Cycles, 17, 1084, doi 10.1029GB001968. 

The chemical behavior of U and its daughter nuclides in the ocean environment was extensively studied in the 1960s and 1970s and has been well summarized (Cochran 1992). The most important mechanism by which nuclides are separated from one another to create disequilibrium is their differing solubility. For U, this solubility is in turn influenced by the redox state. The process of alpha-recoil can also play an important role in producing disequilibrium. 

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. 

Figure 8.35 shows the redox state and acidity of the main types of seawaters. The redox state of normal oceanic waters is almost neutral, but they are slightly alkaline in terms of pH. The redox state increases in aerated surface waters. Seawaters of euxinic basins and those rich in nutrients (eutrophic) often exhibit Eh-pH values below the sulfide-sulfate transition and below carbonate stability limits (zone of organic carbon and methane cf figure 8.21). We have already seen (section 8.10.1) that the pH of normal oceanic waters is buffered by carbonate equilibria. At the normal pH of seawater (pH = 8.2), carbonate alkalinity is 2.47 mEq per kg of solution. 

Getting the scale right links between metallogenesis, planetary degassing and the redox state of Earth s oceans ... 

Links between Earth Degassing, Gold and the Redox State of Late Archean Oceans... 

Walshe, J.L. Kendrick, M.A. 2009. Links between planetary degassing, gold and the redox state of Late Archean Oceans 19 V.M. Goldschmidt Conference - Challenges to Our Volatile Planet, 2009. 

The formation of marine sediments depends upon chemical, biological, geological and physical influences. There are four distinct processes that can be readily identified. Firstly, the source of the material obviously is important. This is usually the basis for classifying sediment components and will be considered below in more detail. Secondly, the material and its distribution on the ocean floor are influenced by its transportation history, both to and within the ocean. Thirdly, there is the deposition process that must include particle formation and alteration in the water column. Finally, the sediments may be altered after deposition, a process known as diagenesis. Of particular importance are reactions leading to changes in the redox state of the sediments. 

This requirement is fulfilled for electric discharges in a reduced atmosphere containing methane, ammonia, and water, as in the original Miller experiment. It has also been observed for atmospheres based on N2 and CO or CO2 on the condition that H2 or methane is also present in snfflcient amonnts (19). A neutral atmosphere (based on N2, CO2, and water) wonld produce much lower yields of organics (by several orders of magnitude). In the absence of other species to be oxidized, the rednction of CO2 reqnires the concomitant thermodynamically nnfavorable conversion of water into O2 (as in photosynthesis). However, even if the atmosphere was nentral when life arose, as nsnaUy believed, the Earth was not nniform with respect to redox state simply becanse the rednced state of the mantle and the high volcanic activity favored the occnrrence of locally rednced environments (for instance, in hydrothermal vents in the oceans). Then, a preservation of the hydrogen content of the early atmosphere or the diversity of environments on the early Earth is likely to have made amino acid formation possible, at least at specific places. 

The overall consequence of the co-evolution of oxygenic photosynthesis and the redox state of the ocean is a relatively well-defined trace-element composition of the bulk phytoplankton. Analogous to Redfield s relationship between the macronutrients, trace-element analyses of phytoplankton reveals a relation for trace elements normahzed to cell phosphorus of (C125N16P1S13... 

Species of the more soluble and kineticaUy labile Fe(II) redox state are intermittently present in seawater as a result of Fe(III) reduction by a variety of processes in different ocean environments. Chemical and/or microbial reduction of Fe(III) occurs on a large scale in anoxic basins and sediments (Sections 3.1.4 and 3.3.4) and on a microscopic scale within the fecal pellets of zooplankton. In the surface ocean reduction occurs via absorption of high visible-low UV light (photo-reduction) [51,59-65], and via biologically-mediated reactions at cell surfaces [12,66-68]. 

In an oversimplified way, it may be stated that acids of the volcanoes have reacted with the bases of the rocks the compositions of the ocean (which is at the fkst end pokit (pH = 8) of the titration of a strong acid with a carbonate) and the atmosphere (which with its 2 = 10 atm atm is nearly ki equdibrium with the ocean) reflect the proton balance of reaction 1. Oxidation and reduction are accompanied by proton release and proton consumption, respectively. In order to maintain charge balance, the production of electrons, e, must eventually be balanced by the production of. The redox potential of the steady-state system is given by the partial pressure of oxygen (0.2 atm). Furthermore, the dissolution of rocks and the precipitation of minerals are accompanied by consumption and release, respectively. 

Anderson RF (1987) Redox behavior of uranium in an anoxic marine basin. Uranium 3 145-164 Anderson RF, Fleisher MQ, LeHuray AP (1989) Concentration, oxidation state, and particulate flux of uranium in the Black Sea. Geochim Cosmochim Acta 53 2215-2224 Back W, Hanshaw BB, Pyler TE, Plummer LN, Weiede AE (1979) Geochemical significance of groundwater discharge in Caleta Xel Ha, Quintana Roo, Mexico. Water Res 15 1521-1535 Barnes CE, Cochran JK (1990) Uranium removal in oceanic sediments and the oceanic U balance. Earth. Planet. Sci. Lett 97 94-101... 

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