Scientists probing volcanic rocks from deep under the frozen surface of the Arctic Ocean have discovered a special geochemical signature until now found only in the southern hemisphere. The rocks were dredged from the remote Gakkel Ridge, which lies under 3,000 to 5,000 meters of water; it is Earth's most northerly undersea spreading ridge. The study appears in the May 1 issue of the leading science journal Nature.
The Gakkel extends some 1,800 kilometers beneath the Arctic ice between Greenland and Siberia. Heavy ice cover prevented scientists from getting at it until the 2001 Arctic Mid-Ocean Ridge Expedition, in which U.S and German ice breakers cooperated. This produced data showing that the ridge is divided into robust eastern and western volcanic zones, separated by an anomalously deep segment. That abrupt boundary contains exposed unmelted rock from earth's mantle, the layer that underlies the planet's hardened outer shell, or lithosphere.
By studying chemical trace elements and isotope ratios of the elements lead, neodymium, and strontium, the paper's authors showed that the eastern lavas, closer to Siberia, display a typical northern hemisphere makeup. However, the western lavas, closer to Greenland, show an isotopic signature called the Dupal anomaly. The Dupal anomaly, whose origin is intensely debated, is found in the southern Indian and Atlantic oceans, but until now was not known from spreading ridges of the northern hemisphere. Lead author Steven Goldstein, a geochemist at Columbia University's Lamont-Doherty Earth Observatory (LDEO), said that this did not suggest the rocks came from the south. Rather, he said, they might have formed in similar ways. "It implies that the processes at work in the Indian Ocean might have an analog here," said Goldstein. Possible origins debated in the south include upwelling of material from the deep earth near the core, or shallow contamination of southern hemispheric mantle with certain elements during subduction along the edges of the ancient supercontinent of Pangea.
At least in the Arctic, the scientists say they know what happened. Some 53 million years ago, what are now Eurasia and Greenland began separating, with the Gakkel as the spreading axis. Part of Eurasia's "keel"--a relatively stable layer of mantle pasted under the rigid continent and enriched in certain elements that are also enriched in the continental crust--got peeled away. As the spreading continued, the keel material got mixed with "normal" mantle that was depleted in these same elements. This formed a mixture resembling the Dupal anomaly. The proof, said Goldstein, is that the chemistry of the western Gakkel lavas appear to be mixtures of "normal" mantle and lavas coming from volcanoes on the Norwegian/Russian island of Spitsbergen. Although Spitsbergen is an island, it is attached to the Eurasian continent, and its volcanoes are fueled by melted keel material.
"This is unlikely to put an end to the debate about the origin of the southern hemisphere Dupal signature, as there may be other viable explanations for it," said Goldstein. "On the other hand, this study nails it in the Arctic. Moreover, it delineates an important process within Earth's system, where material associated with the continental lithospheric keel is transported to the deeper convectiing mantle."
The other authors are based at LDEO; Harvard University; Oregon State University; and the University of Tulsa. The study was supported by the National Science Foundation. Copies of the study, "Origin of a 'Southern Hemisphere' Geochemical Signature in the Arctic Upper Mantle," may be obtained from Nature, Dr. Goldstein, or the press office at Columbia University's Earth Institute, of which LDEO is a member.
Nature: Katie McGoldrick, email@example.com 202-737-2355
Earth Institute: Kevin Krajick firstname.lastname@example.org 212-854-9729
Primary author, Lamont-Doherty Earth Observatory: Steven Goldstein email@example.com 845-729-4846
The Earth Institute's mission is to mobilize the sciences, education and public policy to achieve a sustainable earth. Through interdisciplinary research among more than 500 scientists in diverse fields, the Institute is adding to the knowledge necessary for addressing the challenges of the 21st century and beyond. With over two dozen associated degree curricula and a vibrant fellowship program, the Earth Institute is educating new leaders to become professionals and scholars in the growing field of sustainable development. We work alongside governments, businesses, nonprofit organizations and individuals to devise innovative strategies to protect the future of our planet. For more information, visit www.earth.columbia.edu.
The Lamont-Doherty Earth Observatory, a member of The Earth Institute at Columbia University, is one of the world's leading research centers seeking fundamental knowledge about the origin, evolution and future of the natural world. More than 300 research scientists study the planet from its deepest interior to the outer reaches of its atmosphere, on every continent and in every ocean. From global climate change to earthquakes, volcanoes, nonrenewable resources, environmental hazards and beyond, Observatory scientists provide a rational basis for the difficult choices facing humankind in the planet's stewardship. For more information, visit www.ldeo.columbia.edu.
AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert! system.