Ancient ocean oxygen tracked using thallium
Cretaceous oceans gradually lost oxygen before the dramatic anoxic event that led to mass extinction
Around 94 million years ago, oxygen levels in many oceans fell dramatically. The 600,000-year spell of suffocation caused many marine organisms to go extinct, and scientists are still divided over the exact cause. Now, however, researchers in the US have studied the thallium isotope ratio in sedimentary rocks laid down around 50,000 years before the main event, and concluded that the sediment-water column was becoming gradually deoxygenated. They warn that fertiliser pollution and climate change are causing similar deoxygenation on Earth today.