Geoscientists have long been puzzled by the middle Miocene epoch — a time that stretched from 16 million to 12 million years ago, and that saw some of the most dramatic changes to life on Earth. “We went from green-house conditions to ice-house conditions,” says Ann Holbourn, a geoscientist at Christian Albrechts University in Kiel, Germany. “It seemed impossible to know how long it took for this change to occur and what mechanisms triggered these changes.”

What has made it so difficult to find out, says Holbourn, is the lack of clear geological evidence. Core samples obtained by drilling into the sea floor weren't long enough to cover a large enough chunk of the Miocene, and the sediment within them was often too disturbed to present a clear picture.

But for their latest work (see page 483), Holbourn and her colleagues were able to reap the benefits of advances in drilling technology developed by the Ocean Drilling Program. The group obtained passage on the JOIDES Resolution, a ship designed to take samples from the deep-ocean floor. The ship was equipped with a new kind of corer, which Holbourn thought wouldn't disturb the ancient sediment. And by drilling several holes at different depths but in close proximity, the team hoped to recreate a solid ‘splice’ of the Miocene.

Once out in the Pacific Ocean, the success of the drilling approach exceeded their expectations. “It was very exciting to be on the ship and see those cores coming up,” Holbourn says. “We recovered amazing sedimentary archives.”

Earlier sampling techniques typically recovered cores 10 metres long and resulted in gaps in the record. But Holbourn and her colleagues' approach allowed them to drill a few hundred metres into the sea floor, which itself lay up to 3,000 metres below sea level. The JOIDES team spliced together samples to construct a core several hundred metres long — big enough to capture a significant chunk of the Miocene. The excitement on board the ship was infectious, Holbourn says. “You're working with a whole party of specialists in geology. There's so much effort, so much energy.”

Once back on shore, besides examining the nearly continuous core, her group selected tiny pieces of marine organisms scattered throughout it. They used mass spectrometry to monitor stable oxygen and carbon isotopes to track the course of climate change over the middle Miocene.

Holbourn and her team found drastic changes in both oxygen and carbon isotopes indicating shifts in deep-water temperatures, ice volume and carbon dioxide. “It was an unusual time of low seasonal contrast on Earth,” Holbourn says. Her group's interdisciplinary approach helped draw this picture, with Wolfgang Kuhnt, at Christian Albrechts University, helping with the palaeontology, Michael Schulz, of the University of Bremen in Germany, conducting modelling and statistical analysis, and Helmut Erlenkeuser running the mass spectrometry lab at Christian-Albrechts. This approach allowed Holbourn and her group to ask bigger questions of the data. Next her group plans to peer more closely into the ‘greenhouse world’ of the Miocene and look for analogous present-day warming conditions.