Capture of high-altitude precipitation by a low-altitude Eocene lake, western U.S.
Alan R. Carroll et al., Department of Geology and Geophysics, University of Wisconsin, 1215 W. Dayton St., Madison, Wisconsin 53706, USA. Pages 791-794. NSF funding received.
The oxygen isotopic composition of minerals precipitated by surface waters has been used in many studies to estimate the magnitude of past uplift (paleoaltimetry), based on the decreased relative abundance of 18O versus 16O in precipitation falling at higher elevations. Such studies generally assume that these minerals were formed at, or at least near, the site of the original precipitation. However, mountain rivers can carry water hundreds or even thousands of kilometers away from its origination point (for example, the Columbia River in the U.S. Pacific Northwest). Eocene Lake Gosiute in the western United States appears to have captured such a river ~49 million years ago, based on a relatively sudden (circa 100 thousand year) change to sedimentation indicative of more open hydrology. This change is associated with a -5 ppm shift in delta-18O, indicating that the captured river originated at higher elevations than the lake. However, rather than reflecting a change in regional paleoaltitude, this large and rapid change appears to have resulted from stream piracy. Carroll et al.'s study therefore indicates that geomorphic processes alone may generate isotopic records similar to those sometimes interpreted as evidence for tectonic uplift.
Carbonate-hosted Avalon-type fossils in arctic Siberia
Dmitriy V. Grazhdankin et al., Institute of Petroleum Geology and Geophysics, Koptyug Avenue 3, Novosibirsk 630090, Russia. Pages 803-806.
Our present understanding of the o
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