Light reaches deep in southeast Pacific.

An oceanographic survey of the southeastern Pacific has discovered a region where ultraviolet radiation penetrates deeper than has been measured in any other ocean locale.

Sunlight streaming onto the ocean's surface is either absorbed by water molecules or dissolved substances, or else scattered sideways when it reflects off objects such as microorganisms. In ocean regions teeming with life, 90 percent of the light at certain ultraviolet wavelengths is blocked before it reaches a depth of 3 meters, says Richard Sempere, a marine biogeochemist at the University of the Mediterranean in Marseilles, France.

Sailing across a 3,000-kilometer-wide stretch of the southeastern Pacific, however, Sempere and his colleagues encountered waters so clear that those wavelengths penetrated to 28 m. That's a record for seawater and rivals the clarity of ultrapure lakes such as Antarctica's Lake Venda. The dearth of life in the southeastern Pacific is what renders the waters there so dear, Sempere and his colleagues note in the June 28 Geophysical Research Letters.

Researchers are particularly interested in how far ultraviolet light penetrates into the ocean because radiation at those wave-lengths stimulates reactions that break down carbon-bearing compounds dissolved in the water. Such processes contribute to the return of planet-warming carbon dioxide to the atmosphere.--S.P.