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Paleointensity of the early geodynamo (2.45 Ga) as recorded in Karelia: A single-crystal approach.
A.V. Smirnov, Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA, et al. Pages 415-418.
The paper describes measurements of the intensity of Earth's magnetic field for rocks formed 2.45 billion years ago using a new technique that utilizes single crystals. The results are important because there is currently a debate concerning when the inner core of Earth, which tends to stabilize the field, started to grow. The study is also of general interest, as we seek to learn more about the present-day field, which is dropping rapidly in strength at an alarming rate.
Neoproterozoic biotic diversification: Snowball Earth or aftermath of the Acraman impact?
Kathleen Grey, Department of Industry and Resources, Geological Survey of Western Australia, 100 Plain Street, East Perth, Western Australia 6004, Australia, et al. Pages 459-462.
Australian scientists studying rocks near an ancient South Australian asteroid impact structure have uncovered evidence that could replace current theories explaining the diversification of complex life on Earth about 500 million years ago. Dr. Kath Grey and her colleagues challenge the idea that an intense period of glaciation about 600 million years ago triggered the evolution of simple life forms into more complex and familiar species. Instead, they have put forward the idea that an asteroid, which smashed a hole in South Australia about four times the size of Sydney 580 million years ago, played a pivotal role in this evolutionary jump. For nearly three billion years, bacteria and simple algae dominated life on Earth. The ancestors of most animals and plants appeared quite suddenly in the fossil record about half a billion years ago. The big question is what caused the rapid proliferation of life at that time? Some scientists suggest the burst of life resulted from an intense period of glaciation about 600 million years ago. However, if the findings of the study prove correct, the cause could lie beyond our planet. Dr Grey, who has studied fossil plankton (single-celled green algae) from drill holes across Australia, found that only bacterial mats and simple forms of plankton managed to survive this ice age. She believes it wasn't until 20 million years after the ice age that more than fifty new and more complex species suddenly replaced the simple ones in the fossil record. Grey and her co-workers have reasoned that since plankton is the base of the food chain, any changes observed about the time of the Acraman impact must have played a significant part in the evolution of the more complex species that followed.
Extinction of Cloudina and Namacalathus at the Precambrian-Cambrian boundary in Oman.
Joachim E. Amthor, Petroleum Development Oman, Box 81, Muscat, PC 113, Sultanate of Oman, et al. Pages 431-434.
The cause of the Cambrian radiation has long been debated. With the publication of the paper by Amthor and colleagues, it seems likely that this evolutionary radiation was immediately preceded by a wave of extinction of terminal Proterozoic calcified metazoans. This supports early suggestions for extinction based on evidence for the demise of soft-bodied Ediacaran animals. This new evidence for extinction helps fuel the old model that flattening of ecosystems during times of stress creates opportunities for new adaptive strategies, expressed in this case as the Cambrian radiation.
Humidity estimate for the middle Eocene Arctic rain forest.
A. Hope Jahren, Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, Maryland 21218, USA, and Leonel Silveria Lobo Sterberg, Department of Biology, University of Miami, Coral Gables, Florida 33124, USA. Pages 463-466.
The exquisite preservation of fossilized Metasequoia trees that grew near 80 degrees N latitude during the middle Eocene (ca. 45 million years ago) allowed researchers to perform stable isotope analyses on cellulose. These hydrogen and oxygen isotopic techniques had previously been restricted to wood <30,000 years old due to a lack of suitably preserved samples. From the results, authors calculated that the middle-Eocene Arctic atmosphere contained ~2X the water found in the region's atmosphere today. This water vapor contributed to a middle-Eocene greenhouse effect that insulated the polar region during dark polar winters.
Seismicity and time-lagged lava output at Mount Etna: A new method of long-term forecasting at a destructive volcano.
John B. Murray, Volcano Dynamics Group, Department of Earth Sciences, Open University, Walton Hall, Milton Keynes MK7 6AA, UK. Pages 443-446.
This paper describes a new type of volcano forecasting that seems to work well at Mt. Etna, where the method can most easily be tested because of the excellent historic records of earthquakes, and accurate topographic maps of the volcano, which go back to 1868. The author has found that high earthquake activity is followed by a significant drop in lava output from the volcano 25 years later, and that low earthquake activity precedes high lava output by the same time interval. This means that earthquake records can be used to predict the amount of lava that will be erupted from the volcano more than a decade in advance, very much longer than present volcano prediction methods, most of which can give warnings only hours or days in advance. The disadvantage is that the method cannot predict times of specific eruptions, only that a certain amount of lava will be erupted over a given period of years, but this kind of information would be extremely useful to civil protection authorities for forward planning at a destructive volcano. Using the method, the author predicts that 170 million cubic meters of lava will erupt from Mt Etna between the years 2007 and 2015.
Crustal structure and exhumation of the Daabie Shan ultrahigh-pressure orogen, eastern China, from seismic reflection profiling.
Yuan Xue-Cheng, Center of Research and Development, China Geological Survey, 31 Xueyuan Road, Haidian District, Beijing 100083, People's Republic of China, et al. Pages 435-438.
One of the most remarkable geological discoveries in recent years was the presence of diamonds in metasedimentary rocks in old mountain belts. Because the rocks are metasedimentary we know they formed at the surface of Earth; because they contain diamonds we know that they have been taken to depths in excess of 100 km, to produce the enormous pressures required to create diamonds; and now we find these rocks back at the surface of Earth. This geological yo-yo defies the predictions of the conventional plate tectonic cycle, which provides an easy mechanism for deep burial of the rocks ("subduction") to produce the diamonds, but lacks a simple explanation of the subsequent uplift. These ultrahigh-pressure rocks are best known from a region of eastern China. Our paper describes new geophysical data from this region that provide a convincing image of the subsurface, and show a narrow channel beneath the center of the old mountain belt through which the ultrahigh-pressure rocks must have returned to the surface.
Plate-kinematic explanation for mid-oceanic-ridge depth discontinuities.
Christopher Small, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, and Leonid V. Danyushevsky, School of Earth Sciences and Centre for Ore Deposit Research, University of Tasmania, Hobart, Tasmania 7001, Australia. Pages 399-402.
Seafloor spreading centers migrate relative to hotspots and the underlying mantle. The balance between the rate and direction of this migration and the spreading rate at which seafloor is formed at spreading centers determines where and how much of the underlying mantle is consumed to make lithospheric plates. Current plate motions (kinematics) result in geographic discontinuities in this consumption rate. The two most prominent kinematic consumption discontinuities on the global mid-ocean ridge system coincide with previously unexplained discontinuities in seafloor depth and lava chemistry. Restricted asthenospheric return flow required to balance this kinematic consumption provides a possible explanation for the depth and geochemical discontinuities.
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