Oxygen levels Archaean

The oxygen burden of the Archaean air (the level in the bathtub Fig. 8) remains an unsettled controversy. Much opinion (e.g. Kasting 1993 Holland 1999) holds that oxygen levels rose sharply in a sudden event that took place in a short time, somewhen around 1.8—2.4 Ga ago. Much evidence, such as the need for reduced... 

The opposite view has been championed by Ohmoto (1997) on the basis of isotopic evidence, interpreted as suggesting relatively high oxygen levels. There is a reasonable case that oxygen levels in the Archaean were higher than thought in orthodox opinion. 

Bjerrum and Canfield (2004) have used the relationship in Fig. 5.13 to calculate the ratio of organic carbon burial to the total amount of carbon buried in the sedimentary record over time. They show (Fig. 5.13b) that organic carbon burial was less in the Archaean than at the present day. These authors also calculated the effects of a third carbon isotope reservoir - that of carbonate in the oceanic crust - and showed that in this case the degree of organic carbon burial was much less in the Archaean than at present. This result would imply that oxygen levels were low in the Archaean, not because... 

Figure 2 Geological timeline expanding the mid-Precambrian period (Archaean and early Proterozoic). Note the burst of evolutionary activity in the period 2.3 to 2 billion years ago, as oxygen levels rose to about 5-18% of present atmospheric levels.

After the first appearance of cyanobacteria, presumably in microbial mats, the arrival of unicellular cyanobacterial plankton must surely have been rapid. In the modern warm tropical and subtropical oceans, cyanobacterial picoplankton are ubiquitous (Capone et al. 1997), supporting complex microbial consortia, and in the Archaean they could have formed the upper 100 m layer of an open ocean biological community, which may have had great diversity (Karl 2002). Archaeal plankton would have been out-competed for occupancy of the topmost levels, but could have occupied a now more productive underlying lower layer, 100-300 m thick, dependent on the redox debris (including dissolved chemical species) from the overlying oxygenic photosynthesizers. The immediate results. 

There are two reasons why it is likely that the Archaean sulfur cycle might have been very different from that which is observed in the modern. First, as will be discussed later in this chapter, the Archaean atmosphere had very low levels of oxygen, so that there was no oxidative weathering of sulfide in crustal rocks at this time. This means that no sulfate was delivered to the Archaean oceans through weathering and consequently the Archaean ocean was probably very low in sulfate. Second, the removal of sulfate from the ocean is bacterially mediated. The operation of this... 

The regular precipitation of iron oxides from the Archaean oceans is also thought to have had a strong control on phosphate levels. Phosphate is important because it is a major nutrient which controls oxygen productivity. Bjerrum and Canfield (2002) calculated the dissolved phosphate content of the Archaean... 

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