Marine minerals map Cretaceous climate

Sulfur isotopes on the seabed provide a prehistoric weather report.

Data just in on the chemical make-up of deep ocean sediments reveal that dinosaurs inhabited an extreme and turbulent world. The data could also help predict the planet’s response to current climate change, report US researchers.

This is the first high-resolution description of changes in seawater sulfate in the Cretaceous period. Deep ocean cores laid down from 130 to 65 million years ago (Ma) reveal rapid fluctuations in levels of sulfur isotopes present in the mineral barite, says Adina Paytan, assistant professor of geological and environmental sciences at Stanford University. The fraction of isotope 34 in prehistoric oceans was far lower than in the past 45 million years, perhaps due to increased volcanic and hydrothermal activity, says Paytan.

Superimposed on this fiery background are two sharp dips in the relative amount of isotope 34, suggesting two episodes of extreme and rapid climate change - one from 120 to 105 Ma; one from 95 to 80 Ma. Several factors could account for this: in addition to volcanoes and vents belching more and lower isotope sulfur into the atmosphere, the warmer and wetter climate may have caused rapid weathering of recently erupted deposits. The authors also argue that sulfate-reducing bacteria, which preferentially metabolise low-isotope sulfur, increasing the proportion of sulfur isotope 34, must have been less active. This would have affected the amount of oxygen released into the environment, resulting in fluctuations in atmospheric oxygen.

’There must have been more rapid changes in the atmosphere of the Cretaceous than we knew about,’ says Patyan. ’Understanding how the atmosphere, land and ocean system interacted . is very relevant if we want to understand the fate of our future climate.’

Henry Nicholls