Oxygen fluctuations in oceans linked to mass extinctions throughout history.
Sedimentary pyrite records show that ocean oxygen levels have cycled over the past 700 million years. Four major cycles of oxygenation occurred, with peaks at 540, 390, 310, and 0 million years ago, and lows at 700, 455, 365, and 200 million years ago. Low oxygen periods led to severe deficiencies in essential elements, potentially contributing to mass extinctions in the oceans.
