December Geology and GSA Today ...( ellfish formed a vital part of the food...)

ellfish formed a vital part of the food chain in the area and were likely an important food supply for migratory waterfowl. Islands composed entirely of gleaming white clam shells line the lower reaches of the Colorado Delta. Satellite images and field data indicate that at least two trillion clam shells make up the areas beaches and islands. Virtually all these shells are less than 1000 years old, according to radiocarbon and amino acid dating. An average of about 6 billion clams inhabited the delta before the rivers natural flow was interrupted. Clam densities formerly reached 50 specimens per square meter (5 per square foot). Now, recent surveys reveal far fewer, only 3 per square meter (0.3 per square foot). Although a few highprecipitation years since 1981 have resulted in occasional flow to the delta, the findings suggest that these rare events have not restored the ecosystem to its original productivity. The methods pioneered in this study can be used to estimate the prehistoric productivity of coastal ecosystems in other parts of the world. Such estimates will be especially valuable in areas where no biological surveys were made before humans modified the habitat.

GSA TODAY

Beneath Yellowstone: Evaluating plume and non-plume models using teleseismic images of the upper mantle.
Gene Humphreys et al.

This paper provides a synthesis of new geophysical data on the state of Earth's mantle at depths down to 700 km beneath Yellowstone; one of the most complete images, anywhere on Earth, of the nature of the mantle beneath a continental hotspot. This data set provides an opportunity to understand the mantle processes that give rise to the spectacular tectonic uplift, earthquakes, volcanism, and hydrothermal springs and geysers that characterize Yellowstone and other hotspots. The findings are surprising and controversial. Yellowstone has traditionally been interpreted to result from motion of the North American plate over a stationary
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Contact: Ann Cairns
acairns@geosociety.org
303-447-2020 ext 156
Geological Society of America
27-Nov-2000

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