Firstly we need to determine the partial pressure of O2:

We will now use the Henry's Law equation to determine the solubility of the gas:

Answer: Solubility is 2.7x10^-3 M
<span> the atmosphere holds about 21 per cent oxygen. Over the Earth’s 4.6 billion year history, oxygen did not appear in the atmosphere until perhaps about 2.5 billion years ago. Since then, oxygen levels have fluctuated in tandem with global geological and biological events, such as mass extinctions.</span>
Answer:
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By Using relative and radiometric dating methods hope this helps!!