'Record grooves' on ocean floor document Earth’s ice ages
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
“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.”