Clues from deep-sea sediments suggest that a rapid global warming episode millions of years ago triggered a drastic change in ocean circulation that lasted thousands of years and further baked the continents. Scientists say this window to the past might provide a view of future consequences of global warming.
Some 59 million years ago, carbon dioxide began to build up in the atmosphere, likely from volcanic eruptions. This slowly warmed the sea surface in the high latitudes, aided by slight variations in Earth's orbit. Scientists suspect the warming threw a wrench into ocean circulation and chemistry that caused extinctions of deep-sea species and land mammals alike. But exactly how circulation patterns were affected has been hard to pin down.
Flavia Nunes and Richard Norris of the Scripps Institution of Oceanography at the University of California, San Diego, wondered if the answer might lie in deep-sea sediments. These sediments contain the fossil shells of microscopic organisms, known as foraminifera, and the carbon isotope ratios in the shells can indicate whether water at the sea floor had been deep for long periods of time or had recently sunk from shallow depths.
Up until about 55 million years ago, carbon isotope ratios from samples taken from 14 sites around the globe show that deep sea currents had been generally forming in the Southern Ocean and South Atlantic Ocean and flowing northward to the Pacific Ocean. But less than 5000 years later, the currents seem to have reversed direction, the researchers report 5 January in Nature. This backward flow persisted for at least 40,000 years, and the original pattern didn't completely return for 100,000 years.
The research backs up global climate models that suggest the ancient change in ocean circulation may have warmed so-called gas hydrates on the sea floor, causing massive amounts of methane to bubble into the atmosphere, which would have in turn caused more warming on both sea and land. All told, ocean temperatures spiked by about 6 degrees Celsius. Because modern atmospheric CO2 levels are approaching those seen 55 million years ago, the team warns that people could have a long-lasting effect on deep-ocean circulation.
"Clearly there [was] something going on with deep-sea circulation patterns," says paleoceanographer James Zachos of the University of California, Santa Cruz. But correlating specific time intervals in sediments from different oceans can be very tricky, he says. It is also difficult to say whether the modern ocean will react the same way, he notes, because of differences such as the location of continents and the growth of the Antarctic ice sheet.
