Redox global change

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). 

In Chapter 25, we will consider further the important role of the continental shelves in regulating organic matter burial. Because this type of organic matter burial is probably altered by changes in sea level, it provides a feedback in the crustal-ocean-atmosphere fectory that acts on the biogenic gases involved in global climate (CO2 and CH4) and redox (O2) control. 

Sediment diagenesis reactions cause sources and sinks that are of global importance to the geochemical mass balance of manganese and some other metals sensitive to redox changes. 

Wang K., Attrep M., Jr., and Orth C. J. (1993) Global iridium anomaly, mass extinction, and redox change at the Devonian-Carboniferous boundary. Geology 21, 1071-1074. 

Arsenic moves between ditferent environmental compartments (rock-soil-water-air-biota) from the local to the global scale partly as a result of pH and redox changes. Being a minor component in the natural environment, arsenic responds to such changes rather than creating them. These changes are driven by the major (bio)geochemical cycles. 

The effects of climate change on the speciation and fate of mercury in Polar ecosystems is particularly important. Not only is mercury increasing in the atmosphere but atmospheric deposition will be favored in colder climates due to changes in atmospheric redox chemistry. This means that mercury released in equatorial areas will undergo a global distillation via a process similar to the grasshopper effect observed with semi-volatUe organic pollutants. 

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]. 

---