Carpet of algae over ancient ocean.

A ship cruising through the eastern Pacific Ocean 10 million years ago might have found itself plowing through mats of green algae spread across vast areas of the ocean, according to two researchers who have found the fossilized remains of these ancient algal carpets. Alan E.S. Kemp of the University of Southampton in England and Jack G. Baldauf of Texas A&M University in College Station discovered numerous algal layers in cores of seafloor sediments pulled up from the eastern tropical Pacific. Evidence of such layers stretches across an area of the equatorial ocean several thousand kilometers in length, they report in the March 11 NATURE.

Formed by a single species of silica-shelled algae called diatoms, the mats developed when the long, thin cells tangled together in a mesh that blanketed the ocean surface in calm waters. The mats eventually sank and were preserved in the seafloor ooze that slowly accumulates over thousands of years. Kemp and Baldauf do not know how large these algal sheets grew. In one case, they detected evidence of the same distinctive layer in holes 2,000 kilometers apart, but they cannot tell whether the mats existed in these locations during the same year or even the same millennium. Kemp and Baldauf have found the algal layers in sediments going back 15 million years, which is as far as they have checked. The layers disappear in sediments younger than 4.4 million years old.

Oceanographers on cruises have seen diatom mats in the same area of the Pacific, a fertile region known for its high concentrations of nutrients. But researchers do not know how large the modern mats grow, or whether they rival the size of those that formed millions of years ago, says oceanographer Constance Sancetta of the National Science Foundation in Washington, D.C. Because evidence of the mats does not appear in sediments less than 4.4 million years ago, something in the ocean must have changed then, she says. Either vast algal sheets formed less often during the last few million years, or seafloor sediments failed to preserve them once they sank to the ocean floor.