As a graduate student in Germany, Ulrich Wortmann collected rock samples in the Bavarian Alps. Little did he know that these rocks would one day help him solve a geochemical mystery.

Now based at the University of Toronto in Canada, Wortmann has studied the marine system that converts organic matter and sulphur, burying them as sediments in the sea floor.

In today's oceans, the burial rates of organic matter and sulphur are interrelated. But 120 million years ago, in the Early Cretaceous period, data suggest that this was not the case — a phenomenon that had never been satisfactorily explained.

Analysing his alpine rock samples, which in the Early Cretaceous would have been part of the sea floor, Wortmann saw that there was a time when the amount of sulphur buried in the sea bed fell to almost zero.

This, he argues on page 654, coincided with the emergence of the South Atlantic ocean basin. The massive gypsum deposits that formed on the new shores of Africa and South America removed sulphur from the oceans — leaving little to be buried and so apparently disrupting the carbon–sulphur relationship. “I was only able to do this work because I have a background in seemingly unrelated fields such as plate tectonics, Cretaceous carbon cycling and biosphere research,” says Wortmann.