7.10 Chemical Characteristics of Sediments and Seawater

Abstract

The transition from an anoxic to oxygenated atmosphere was arguably the most dramatic change in the history of the Earth. This “Great Oxidation Event” (Holland 2006) transformed the biogeochemical cycles of the elements by imposing an oxidative step in the cycles, creating strong redox gradients in the terrestrial and marine realms that energised microbial metabolism. Although much past research was focused on establishing when the rise of atmospheric oxygen took place, recognition that substantial mass-independent fraction (MIF) of the sulphur isotopes is restricted to the time interval before 2.45 Ga and requires an anoxic atmosphere (Farquhar et al. 2000, 2007; Mojzsis et al. 2003; Ono et al. 2003; Bekker et al. 2004) argues the atmosphere became permanently oxygenated at this time (Pavlov and Kasting 2002). A false-start to the modern aerobic biosphere and a “whiff” of atmospheric oxygen (Anbar et al. 2007) may have occurred in the latest Archaean, as reflected in a transient enrichment in the redox-sensitive element molybdenum in marine shales and a reduction in the extent of MIF precisely coincident with the peak in Mo and FeS2 enrichment (Kaufman et al. 2007). Geochemical proxies are imperfect, and an earlier (c. 3 Ga) appearance of atmospheric oxygen is possible (Ohmoto et al. 2006) but disputed (Farquhar et al. 2007; Buick 2008).