Nearly naked: large swath of Pacific lacks seafloor sediment.

Oceanographers have discovered a broad, almost-bare patch of seafloor in the remote South Pacific. An unusual combination of circumstances has left the region without the mineral and organic sediments hundreds of meters deep that are typical elsewhere in the world's oceans, the scientists say.

The sediment-poor region is about the size of the Mediterranean Sea and centered approximately 4,000 kilometers east of New Zealand. Researchers discovered the area, which they dubbed the South Pacific Bare Zone, during a cruise early last year, says David K. Rea, a marine geologist at the University of Michigan in Ann Arbor.

The scientists were surprised when their seismic equipment, which detects sediment only when it's at least 5 meters thick, indicated that there was no sediment in that region. The team then sent sampling equipment more than 4 km to the seafloor and discovered as little as 50 centimeters of sediment in some places.

A unique combination of factors seems to have dictated the area's dearth of sediment that's accumulated since the basalt crust below formed between 85 million and 34 million years ago, Rea and his colleagues report in the October Geology.

First, the area has nutrient-poor surface waters and so is home to few organisms. Therefore, there aren't large quantifies of plankton to die, fall to the bottom, and accumulate, as they do in seas with high biological content, says Rea.

Second, the deepest waters in this area contain less carbonate and silica than those in other locations do, so skeletons of organisms that reach the seafloor dissolve.

Third, the bare zone is far from any major landmass, so little windblown dust ends up in the surface waters and eventually sinks. Finally, the region has little if any hydrothermal activity to spew water containing dissolved minerals that would precipitate.

Rea says that he and his colleagues had expected to find at least a dozen meters of sediment in the region. "It's fun to be wrong sometimes," he notes.

Neil C. Mitchell, a marine geologist at Cardiff University in Wales, suggests another factor that may contribute to the sediment skimpiness of the area. It's out of the path of major ocean currents, so Antarctic icebergs carrying material scraped from that continent don't pass over the bare zone and drop sediment, says Mitchell.

The sparse sediments may permit researchers to find seafloor substances that are typically hidden, says David Scholl, a marine geologist at Stanford University. For instance, meteor dust, which falls evenly over Earth's surface, may be more easily detectable in the bare zone than elsewhere, says Scholl.