Marine paleoredox proxy

The concept for a marine paleoredox proxy based on variations in the U isotope ratio in sediments is very similar to the Mo isotope paleoredox proxy described earlier in this chapter. Since incorporation of U in oxidizing sediments such as ferromanganese crusts results in preferential removal of isotopically light U, and incorporation of U in reducing sediments results in preferential removal of isotopically heavy U, changes in the relative proportions of these two sinks over time should drive systematic shifts in the isotopic composition of U in seawater. In turn, these temporal shifts in the isotopic composition of seawater should be manifested as vertical fluctuations in in the sedimentary record. Excursions in the sedimentary record towards lighter values of (in any of these... 

Frei et al. [37] hypothesized that a paleoredox proxy based on Cr isotope ratios would likely reflect atmospheric oxygenation, rather than marine oxygenation. When oxygen is abundant in the atmosphere, Cr(lll) in exposed rocks on land is converted during oxidative weathering to Cr(Vl), which is highly soluble in water... 

If the interpretation of Cr isotope ratio variation by Frei et al. [37] is valid, Cr isotope ratios provide an extremely valuable complement to other geochemical proxies for paleoredox conditions, since they reflect atmospheric, rather than marine, oxygenation and can be measured in a rock type that is not suitable for applying other paleoredox proxies, such as Mo or S isotope ratios. Some critical assumptions in Frei et al. s study [37] must be verified, namely (i) that oxidative weathering of Cr(III) from rocks indeed results in an isotopically heavy pool of dissolved Cr and (ii) that reduction and coprecipitation of Cr with Fe oxyhydroxides either does not fractionate Cr isotopes or results in complete removal of Cr from the water column in continental shelf environments where BIF are deposited. Although Cr isotope ratio behavior is not yet fully understood at a fundamental level, several studies have been published that provide important insights into this system. 

---