Earth sciences: A salty solution

A new modelling study offers an elegant explanation for a curious anomaly seen in the carbon and sulphur isotope records of the Early Cretaceous period.
In modern-day sediments, the rates of pyrite (iron sulphide) and carbon-based organic matter burial are positively related. But during the Early Cretaceous period, which began approximately 140 million years ago, they appear to be negatively related — a relationship that has, until now, been difficult to explain.
Ulrich G. Wortmann and Boris M. Chernyavsky show that this puzzling relationship could be due to the deposition of evaporites — mineral sediments that precipitate from water as it evaporates — on the ocean floor as South Africa and South America moved apart and the South Atlantic Ocean basin formed. The evaporites contain sulphate, so their production removed sulphate from the surrounding waters. Enough, the model suggests, to reduce significantly the rate at which organisms in marine sediments formed pyrite and broke down organic matter, thus explaining the negative relationship between the amount of pyrite and organic matter buried in marine sediments at this time.
Evaporite deposition may have had a similar effect at other times in Earth’s history, and so may explain other changes in carbon and sulphur cycling. These are important to understand because alterations in these cycles can have a direct effect on atmospheric oxygen levels.
CONTACT
Ulrich G. Wortmann (University of Toronto, Canada)
Tel: +1 416 978 7084; E-mail: uli.wortmann@utoronto.ca