Rare rocks drilled from Pangaean time.

Rare rocks drilled from Pangaean time

This summer, while drilling into an underwater plateau off the northwest coast of Australia, an international scientific team pulled up a rich haul of sediments chronicling the breakup of the supercontinent Pangaea, the waxing and waning of global seas and the evolution of one of the most important marine plants. Investigators on Leg 122 of the Ocean Drilling Program also collected the oldest section of sedimentary rock ever obtained during scientific ocean drilling.

"There has never been a leg where they have recovered such a diverse set of sediments," says paleontologist Timothy J. Bralower from Florida International University in Miami, who returned last week from the drilling.

For Leg 122, the drilling program focused on the Exmouth plateau. This submerged feature sits on a sunken piece of the Australian continent that was once attached to India when all the continents were assembled into a single land mass called Pangaea.

For safety reasons, previous scientific drilling projects have avoided this kind of location. Continental margins often contain pockets of oil or gas, and the program's drillship JOIDES Resolution is not equipped to deal with hitting a hydrocarbon deposit. For Leg 122, however, the investigators drilled near oil-company holes that had already shown an absence of deposits. The co-chief scientist of the cruise, Bilal U. Haq of the National Science Foundation, says the success of Leg 122 provides incentive for planning more drilling into the scientifically important continental margins.

The oldest sediments collected during the leg date back approximately 220 million years to the later part of the Triassic period. Made of clay and silt, these rocks formed from deposits at the bottom of shallow seas or river deltas along the coast of Pangaea.

Within the Triassic sections of rock, the shipboard crew found some of the oldest fossilized shells of calcareous nan-noplankton-single-celled marine plants with a hard covering of calcium carbonate. Through the new specimens, scientists hope to gain clues about the early evolution of these tiny plants, which have been the dominant plankton in the ocean during certain periods in Earth's history, says Bralower.

Researchers also hope to use some sediments from Leg 122 to help decipher how global sea levels have wavered over time. Findings from the cores will bear on a controversy concerning whether scientists can compare sea level changes recorded on different continental margins around the world (SN: 3/7/87, p.154).