Detrital oxygen

From the perspective of the global rock cycle (Figure 1.2), volcanic activity is the ultimate source of minerals comprising the crust. The crust is 27.7% by mass silicon and 46.6% oxygen, so it is not surprising that silicates are the dominant mineral type. Weathering of these minerals generates siliclastic particles. These are also referred to as detrital silicates. 

Carbonates deposited in freshwater lakes exhibit a wide range in isotopic composition, depending upon the isotopic composition of the rainfall in the catchment area, its amount and seasonality, the temperature, the rate of evaporation, the relative humidity, and the biological productivity. Lake carbonates typically consist of a matrix of discrete components, such as detrital components, authigenic precipitates, neritic, and benthic organisms. The separate analysis of such components has the potential to permit investigation of the entire water column. For example, the oxygen isotopic... 

The evidence for the oldest zircons, and their interaction with water, is described in S. A. Wilde, J. W. Valley, W. H. Peck, and C. M. Graham, Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago , Nature, 409 (2001), 175, and S. J. Mojzsls, T. M. Harrison, and R. T. Pidgeon, Oxygen-Isotope Evidence from Ancient Zircons for... 

Geochemical evidence suggests that there were delays of several hundred million years between the rise of oxygenic photosynthesis, the oxygenation of the atmosphere, and the oxygenation of the deep ocean. Photosynthesis (evidenced by cyanobacterial microfossils and biomarkers) rose as early as 3.5 Ga (billion years before present (Schopf, 1993)) and had been sofidly established by 2.7—2.5 Ga (Brocks et al, 1999 KnoU, 1996 Schopf, 1993 Summons et al, 1999). Data from red beds, detrital mineral deposits, and sulfur isotopes indicate the rise of atmospheric oxygen around 2.4 Ga (Bekker et al, 2004 Chandler, 1980 Des Marais et al, 1992 ... 

Mojzsis S. J., HarrisonT. M., and Pidgeon R. T. (2001) Oxygen isotope evidence from ancient zircons for liquid water at the Earth s surface 4300 Myr ago Jack Hills, evidence for more very old detrital zircons in Western Australia. Nature 409, 178-181. 

Figure 1 Conceptual model for the origin of mixed detrital-biogenic facies relating the three major inputs to the processes that control them. The major inputs are shown in boxes with bold-type labels. ControlUng factors are shown in italics. Large and medium scale arrows represent fluxes of key components involved in sedimentation and the biogeochemical cycles of carbon, sulfur, and oxygen. Thin arrows illustrate relationships between major controlling factors and depositional processes and/or feedback. Dashed thin arrows apply to major nutrient fluxes only. Dotted thin arrows apply to major authigenic fluxes only. See text for further explanation.

Several lines of geochemical evidence support low to negligible concentrations of atmospheric O2 during the Archean and earliest Proterozoic, when oxygenic photosynthesis may have evolved. The presence of pyrite and uraninite in detrital Archean sediments reveals that the atmosphere in the earliest Archean contained no free O2 (Cloud, 1972). Although Archean-age detrital pyrites from South Africa may be hydrothermal in origin, Australian sediments of the Pilbara craton (3.25-2.75 Ga) contain rounded grains of... 

The band of buff-colored, translucent pallisade calcite from 11 mm to 32 mm. Zone C, is relatively event-free, with only a few minor dissolution horizons or detrital drapes (not visible with the naked eye) representing short-term local flood events but not longterm glacial advance flooding. The isotopes show parallel oscillations and are somewhat more enriched than the upper zones of the sample. The oxygen isotopic values lie close to those of modern calcite in the cave, suggesting a moderately cool, wet climate similar to that of today. The oscillations in the carbon isotopic values follow the simple pattern of... 

Minor amounts of purpurin and chlorin acids, typical of oxidative pigment destruction in marine (5,20,28) and terrestrial (41) ecosystems, were also found. These most likely reside in broken detrital cells exposed to oxygen. Further study is required to clarify destructive pathways. 

The next stages in the evolution of atmospheric oxygen and consequently in the evolution of chemical composition of the Ocean, lasted from approximately 2.4 billion years to 1.7 billion years. During this time, the atmosphere appears to have been oxidizing (based, again, on red beds and detrital mineral deposition, see 3.2), but the deep ocean remained reduced. The evidence for an anoxic deep ocean comes from the occurrence of massive banded iron formations (BIFs), which are considered to require a deep ocean rich in dissolved ferrous iron (Figure 14). 

Fig. 7. Oxygen and carbon isotopic data for the stratigraphical tmits examined in this study. All data, excepting Antognola and Loiano, are for whole rocks (uncorrected for detrital versus authigenic content).

---