With a little training, it's easy to see how ice age glaciers sculpted the land, scouring valleys and heaping up debris. This week, researchers revealed that the ancient cycles of ice also left their mark on the sea floor, thousands of meters below the ocean surface.
The evidence comes from seafloor spreading centers: sites throughout the ocean where plates of ocean crust move apart and magma erupts in between, building new crust onto the plates' trailing edges. Parallel to these spreading centers are “abyssal hills”: long, 100-meter-high ridges on the diverging plates, separated by valleys. On bathymetric maps of seafloor topography, they look like grooves on a record. These grooves, it now turns out, play the tune of Earth's ice ages.
During ice ages, which are mainly driven by rhythmic variations in Earth's orbit and spin that alter sunlight in the Northern Hemisphere, growing ice caps and glaciers trap so much frozen water on land that sea levels can drop a hundred meters or more. As the pressure on the ocean floor eases, magma erupts more readily at the spreading centers, thickening the plates and creating the abyssal hills, say the authors of two new studies, one published online this week in Science (http://scim.ag/JCrowley) and another posted online in Geophysical Research Letters.
“Step back and think about this: Small variations in the orbital parameters of the Earth—tilt and eccentricity and wobble—are recorded on the sea floor,” says Richard Katz, a geodynamicist at the University of Oxford in the United Kingdom and a co-author of the Science paper. “It kind of blows my mind.” Outside scientists are also impressed. “Their data provides evidence that the link is real,” says David Lund, a paleoceanographer at the University of Connecticut, Avery Point. “I'm very excited about it.”
The studies suggest that the bursts of seafloor volcanism could in turn affect climate—and even play a role in bringing each ice age to an abrupt end. They may also change some minds about the origin of the abyssal hills. Ever since scientists discovered the hills more than half a century ago, many thought they resulted from cracks in Earth's crust, or faults. As new oceanic crust is made in the spreading centers (the story went), it cools, fractures, and slips along faults, creating downward-dropped blocks that could be responsible for the ridge-and-valley topography.
Scientists still agree that faults play a role in shaping the seafloor topography, but the new work emphasizes the importance of volcanism in creating the hills in the first place, Lund says. “Faulting could be a secondary process as opposed to the primary one,” he says.
The study in Science was based on fresh ocean-floor data gathered by a Korean icebreaker ship during 2011 and 2013 surveys across the Australian-Antarctic ridge, a spreading center south of Tasmania. In a transect representing more than a million years of seafloor spreading, the researchers found topographic highs and lows that seemed to have formed in synchrony with all three of Earth's astronomical cycles, which have periods of 23,000, 41,000, and 100,000 years.
Wondering whether the climate cycles might somehow be boosting seafloor volcanism at regular intervals, the researchers created a computer model to test the idea. When sea level drops during ice ages, they found, the decreased pressure on the mantle through the thin ocean floor would increase the rate of mantle melting. That would boost the delivery of magma to the seafloor surface by just enough to explain the bands of thicker crust that form the abyssal hills. Paul Asimow, an igneous petrologist at the California Institute of Technology in Pasadena, says the model is physically plausible. “It's additional confirmation that the basic [sea level] signal is felt by the mantle,” he says.
In the Geophysical Research Letters paper, Maya Tolstoy, a marine geophysicist at Columbia University, reports that she found the same correlation at the East Pacific Rise, a fast seafloor spreading region off the coast of Mexico. When she estimated the ages of abyssal hills flanking the ridge, she found that they matched the strong 100,000-year ice age cycle. “It shows up very clearly,” she says.
One of the Science co-authors, Peter Huybers, a climate scientist at Harvard University, says he was pleased by the confirmation—especially because it comes from a fast-spreading center, where the ice age signal is more difficult to observe. Because fast-spreading centers discharge more magma, lava sometimes fills valleys, overprinting the grooves left by previous lulls in volcanism during interglacial periods. Slow-spreading centers can also be hard to interpret, Huybers says, because the colder crust there is more prone to the faulting that can confuse the record. Ridges like the Australian-Antarctic are ideal, Huybers says, because they fall “in this Goldilocks range of not too fast, not too slow.” Huybers and his colleagues are now working to tease out the ice age signal from another medium-rate spreading center, the Juan de Fuca Ridge, off the coast of the U.S. Pacific Northwest.
The seafloor eruptions—big sources of carbon dioxide and other gases—might also help clear up an enduring mystery about ice ages: why they start gradually and end suddenly. Perhaps extra carbon dioxide from a period of heightened seafloor eruptions eventually percolates through the ocean and into the atmosphere, allowing warming that would deliver a coup de grâce to the massive ice sheets.
Scientists had ignored that possibility because until now they assumed seafloor volcanism is constant over time, Lund says. “It's a very seductive idea, and an interesting one: The ice sheet gets so big that it seeds its own destruction.”