Chapter 9

8. The delta 13 of carbon isotopes (based on concentrations of ¹²C:¹³C in organic matter and carbonates) can be used to infer the CO₂ levels in the past. The carbon isotopes also indicate the burial rate of organic and carbonate carbon – which can also be used to indicate the production rate of oxygen. Because Figure 9-15 shows that rate of carbon burial has been constant, we know that the source of oxygen has been relatively stable and that one or more of the removal pathways must have fluctuated. From the geological records of uraninite and pyrite, we know that oxidative weathering processes were significantly less prior to 2.2 billion years ago than after that time. Volcanic activity was likely greater in the early history of the earth.

10.    We do not know the definite answer to this question.

FYI—Whatever the control mechanism is—it’s very efficient, because O₂ levels appear to be relatively constant since the Devonian, 360 million years ago. If O₂ levels were above 35% of the atmosphere by volume, then most of the biomass would be destroyed by fires. However, forests have existed since the Devonian and no evidence of significant global forest fires has been found (other than at the KT boundary, which was a special exception). Therefore, O₂ levels must have been below this percentage. There is evidence inform of charcoal of minor forest fires throughout time since the Devonian. Therefore, oxygen levels must have above 13% by volume, which is the oxygen limit at which fires will ignite.

To answer the question—

The loss process surface weathering and oxidation of reduced volcanic gases are thought to be independent of oxygen concentrations within the range of 13%–35%, the “fire window.” Therefore, the production rate of oxygen (photosynthesis and the burial rate of carbon) must be associated with the control mechanism of oxygen in the atmosphere.

Photosynthesis + burial processes = respiration + decay of biomass + weathering

Most carbon burial occurs in the oceans, so the oceans have a role in carbon burial rates. Because there is no direct correlation between atmospheric oxygen levels and the organic carbon content of marine sediments, we need to examine ways in which 02 levels may indirectly affect photosynthetic and burial rates. Recall the Redfield ratio (from chapter 7) and that a lack of nutrients can limit photosynthesis. The concentration of phosphorous (P) in seawater is controlled by the rate of weathering of rocks on the continents and by the loss of P due to incorporation in sediments. In addition the C:P ratio also depends on the dissolved oxygen concentration, which is influenced by the atm. O₂ concentration and temperature. See Figure 9-19. So the deep ocean dissolved oxygen concentration has a negative feedback on atmospheric O₂ via C:P ratios in sediments and burial of organic carbon.