Answer to Question 1
There are a few potential feedback mechanisms for keeping O2 constant, but the most plausible one involves the burial of organic carbon and C:P ratios in surface waters. In this feedback, increasing oxygen production leads to increased oxygen concentration in deep waters, which lowers the C:P ratio in sediments, allowing less organic carbon burial (through higher rates of respiration) and less net O2 production, thus returning the system to its equilibrium point.
Answer to Question 2
Carbon isotope values in carbonates indicate the rate of organic carbon burial or, more precisely, the fraction of outgassed carbon that is buried as organic carbon (as opposed to carbonates). Since organic carbon burial leads to net O2 production on time scales of less than millions of years, the carbon isotope data can be used to determine relative rates of net O2 production. Isotopically heavy C indicates fast rates of organic carbon burial and high levels of O2 production. But, curiously, there is little or no secular change in carbon isotope values throughout geologic time. This is usually taken to mean that the rise of O2 was caused by changes in the sinks for O2, not by changes in the source. Frankly, however, there is little agreement on what exactly caused O2 to rise when it did.
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