Subduction carbonate mass

Much of the chemical, mineralogical, and isotopic data on which our ideas of the evolution of Earth s surface environment are based come from analysis of sedimentary carbonates. Unfortunately because of the increased susceptibility of the sedimentary carbonate mass to alteration and destruction by the processes of weathering, subduction, and metamorphism, much original information is lost from the rock record with increasing age of the carbonate mass. We are left with a biased record of the preserved sedimentary carbonate rock mass. The Precambrian Eon, which includes more than 85% of Earth history, contains only about 25% of the mass of sedimentary carbonates in existence ... 

The data presented above can be used to set some limits on C02 levels over geological time. The 2.75 Ga paleosol evidence sets an upper limit of 0.04 bars on COa levels whereas the weathering rind study of Hassler et al. (2004) may set a lower limit of 0.0025 bars for 3.2 Ga. Calculations by Mel nik (1982) show that C02 levels were not greater than 0.1-0.15 bars, for otherwise BIFs would be present as iron carbonates rather than oxides. The nahco-lite study of Lowe and Tice (2004) lies within this range, although this result must be regarded as a maximum, as does the estimate of Zahnle and Sleep (2002) based purely on a mass balance between volcanic COa and subducted, carbonated ocean crust (Fig. 5.15). 

The input fluxes of various volatile species (Table 10) can now be compared to various output fluxes through arc volcanism (Table 9) to assess the extent of volatile mass balance. At this stage, we ignore possible volatile losses at the back-arc given the large uncertainty in actual values. Also, we note that in the case of subducted carbon, it is important to distinguish between reduced sedimentary carbon, sedimentary carbonate and carbonate of the altered oceanic crust. 

The above view is clearly supported by the mass/age distribution of lithologies within the same tectonic domain. For example, carbonates, chert, red clay, and terrigeneous sediments on the ocean floor (Hay et al., 1988) all have the same type of age distribution pattern that is controlled by a single variable, the rate of spreading and subduction of the ocean floor. This sedimentary mass also differs lithologically from its continental counterpart, because it is comprised of... 

Hydrothermal reactions between seawater and young oceanic crust have been proposed as an influence on atmospheric O2 (Walker, 1986 Carpenter and Lohmann, 1999 Hansen and Wallmann, 2002). While specific periods of oceanic anoxia may be associated with accelerated hydrothermal release of mantle sulfide (i.e., the Mid-Cretaceous, see Sinninghe-Damste and Koster, 1998), long-term sulfur and carbon isotope mass balance precludes substantial inputs of mantle sulfur to the Earth s surface of a different net oxidation state and mass flux than what is subducted at convergent margins (Petsch, 1999 Holland, 2002). 

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