Ancient Arctic Was Warm, Wet and Ice-Free

10 August 2006

Swedish drilling vessel, Vidar Viking, led the Arctic Coring Expedition in Aug. and Sept. 2004, when scientists set out to retrieve subseafloor sediment records to support their investigations into climate change. Photo: IODP/ESO

A new analysis of ocean-floor sediment cores collected near the North Pole finds that the Arctic was extremely warm, unusually wet and ice-free the last time massive amounts of greenhouse gases were released into the Earth’s atmosphere during a prehistoric period 55 million years ago. The findings appear in the 10 August issue of Nature.

Current climatic evidence and computer models suggest the modern Arctic is rapidly warming, gaining precipitation and becoming ice-free because of carbon emissions. Scientists have been keen to unlock the mysteries of the Arctic when this last happened—an interval known as the Paleocene/Eocene thermal maximum, or PETM.

Researchers have long known that a massive release of greenhouse gases, probably carbon dioxide or methane, occurred during the PETM. Surface temperatures also rose in many places by as much as 15 degrees Fahrenheit in the relatively short geological time of about 100,000 years.

Past analyses of seafloor sediments and sedimentary rocks worldwide have given scientists many clues about the PETM, but sediments from the Arctic remained elusive until 2004, when the $12.5 million Arctic Coring Expedition (ACEX) recovered the first deep sediments from 430 meters beneath the Arctic Ocean near the North Pole.

To recover the sediments that yielded the prehistoric climate records, the research team needed to manage three ice-breakers, one of which was equipped with a drill rig. The sediment records were recovered from the Lomonsov Ridge, in water about 1,000 meters deep.

Building a picture of ancient climatic events is a lot like putting together a jigsaw puzzle, and what ACEX allowed us to do was fill in a blank section of the PETM picture. The ACEX cores clearly show that the Arctic got very warm and wet during the PETM. Even tropical marine plants thrived in the balmy conditions.

—Gerald Dickens, Rice University, co-author

Certain species of microscopic plants in today’s oceans are known to rapidly multiply and create algal blooms, including “red tides,” under certain conditions. Dickens said that fossils of these plants—known only from the tropics before the PETM—suddenly become common in the ACEX cores.

Furthermore, the chemistry of the organic carbon in the ACEX cores may rule out some earlier theories about what caused the PETM. The diminution of these alternate explanations strongly suggests that an enormous amount of carbon entered the atmosphere at the beginning of the PETM, either from volcanic eruptions or the melting of oceanic gas hydrates.

In previous research, Dickens and colleagues have estimated that the amount of methane carbon trapped in ocean gas hydrates worldwide likely exceeds all the carbon in all the world’s oil, coal and natural gas reserves combined. Given the magnitude of carbon trapped in oceanic gas hydrates, and the fact that hydrates are susceptible to melting when adjacent seawater warms by as little as 3-4 degrees Fahrenheit, Dickens said it is probable that at least some of the PETM greenhouse gases came from methane that bubbled up from the seafloor.

The magnitude of the carbon input at the PETM outset is truly enormous. If it were all volcanic, you'd need something like a Vesuvius-sized eruption each day for centuries, which seems very unlikely.

—Gerald Dickens

Co-authors on the paper include Mark Pagani and Nikolai Pedentchouk of Yale University, Matthew Huber of Purdue University, Appy Sluijs and Henk Brinkhuis of Utrecht University, and Stefan Schouten and Jaap Sinninghe Damsté of the Royal Netherlands Institute for Sea Research.

This is the second paper in Nature in as many months resulting from the ACEX work. In the 1 June issue, the research team reported that:

• Evidence of ice in the Arctic Ocean was found much earlier than formerly believed, about 45 million years ago;

• At one time about 55 million years ago, Arctic temperatures rose to subtropical levels (about 23 degrees Centigrade); and that

• At one time about 49 million years ago, the Arctic was green, with fresh surface water and large amounts of fern covering the water, at least in summer months.

The Integrated Ocean Drilling Program’s (IODP) ACEX expedition was conducted by the European Consortium for Ocean Research Drilling (ECORD). IODP, an international program that supports scientists from 21 countries, receives its primary funding from the U.S. and Japan, through the National Science Foundation and the Ministry of Science, Education, Culture, Sports, Science and Technology, respectively.

Resources:

• Proceedings of the Integrated Ocean Drilling Program, Volume 302 (summary of the Arctic Coring Expedition)

• ACEX website

• “The Cenozoic palaeoenvironment of the Arctic Ocean”; Kathryn Moran et. al.; Nature 441, 601-605 (1 June 2006) | doi:10.1038/nature04800