Drilling hits birthplace of Pacific plate.

Drilling hits birthplace of Pacific plate

After many failed attempts over the last two decades, oceanographers have finally located the earliest portion of the Pacific plate, dating back 170 million years to Earth's Jurassic period. This piece of seafloor, identified during a drilling expedition in the Pigafetta basin southeast of Japan, holds information from an unstudied chapter in the history of the world's oceans.

During the Jurassic, a huge ocean stretched uninterrupted across most of the planet while the continents sat huddled to one side. Almost all of the seafloor from that majestic superocean has since disappeared into the Earth's interior through the process of subduction, and until now scientists have lacked any seafloor rocks from middle of that ocean.

"It's all gone, it's all been subducted, except for this part," says Roger Larson of the University of Rhode Island in Narragansett. He served as co-chief scientist on the Ocean Drilling Program's Leg 129, which ended Jan. 19.

In the early 1970s, Larson and others hypothesized that Jurassic portions of Pacific plate should lie buried in the deep western part of the ocean, near where the plate plunges into the Mariana trench. Yet he and his colleagues failed to find it in several previous expeditions. Each time, the drill bit struck a thick layer of hard volcanic rock from a more recent period in Earth's history.

This time, Larson and colleagues located thin patches in the volcanic layer through a technique known as seismic reflection profiling, which bounces acoustic waves off rock formations under the seafloor. The crew successfully penetrated the volcanic layer and drilled several hundred meters into the underlying Jurassic material.

The Jurassic rocks offer clues to the climate and biology of that era. The scientists say the sedimentary rocks contain abundant fossils from silica-shelled plankton but none from carbonate-shelled plankton. Since silica-shelled organisms withstand nutrient-poor conditions better than their carbonate counterparts, Larson suggests the Jurassic ocean had weak current systems that delivered only meager nutrient supplies from deeper waters. However, it is difficult to generalize about the entire ocean with sediments from only one site, he says.

The Pacific plate today covers about one-quarter of the Earth's surface, but during the late Jurassic it was hardly bigger than the United States, Larson says. The rest of the ocean floor consisted of other, unknown plates that have since disappeared as the Pacific plate grew.