Paleoredox proxy

There are other enviromnental samples in which the uranium content, and especially its isotopic composition in enviromnental samples, is a useful indicator of different phenomena. For example, the uranium content and U/ U isotope ratio in shallow carbonate sediments can serve as paleoredox proxy (Romaniello et al. 2013). 

RomanieUo, S.J., Herrmann, A.D., and Anbar, A.D. (2013). Uranium concentrations and U/ U isotope ratios in modem carbonates from the Bahamas Assessing a novel paleoredox proxy, Chem. Geol. 362, 305-316. http //dx.doi.Org/10.1016/j.chemgeo.2013.10.002. 

Mass-dependent isotope fractionation effects provide insight into the physical processes and chemical reactions during which they occurred and the prevailing conditions (such as pH, temperature, salinity, and oxidation potential). As a result, determination of isotope ratios affected by mass-dependent fractionation in samples that are chronological archives, such as speleothems, corals, or forams, can be used as paleoproxies for the conditions mentioned above, as will be discussed for paleoredox proxies in Chapter 11. 

Establishing the Basis for Using Stable Isotope Ratios of Metals as Paleoredox Proxies... 

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... 

Those studies in the second and third categories have clearly demonstrated that robust application of the Mo isotope paleoredox proxy may not be perfectly straightforward and requires careful consideration of at least three complications. First, the isotopic composition of Mo in riverine waters flowing into the ocean is... 

Most of the experimental and theoretical work published thus far concerning the Mo isotope paleoredox proxy relates to isotope fractionation during removal of Mo from seawater to the oxic sink, represented by ferromanganese sediments... 

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. 

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... 

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