The presence of thousands of (now forested) sand dunes spread across 12,000 square kilometers of eastern upper Michigan, more than 500 km east of the present forest-prairie ecotone, sounds an alarm calling for the long-accepted, pollen-based models of vegetation history in the eastern mid-continent to be reviewed. Walter L. Loope of the U.S. Geological Survey and colleagues recently used 57 optically stimulated luminescence (OSL) ages on quartz sand from stabilized dunes in eastern upper Michigan to document dune building about eight to ten thousand years ago. Noting that pollen-based models suggest that closed canopy forest covered the region throughout the last 11,500 years, Loope and colleagues show that their data require that a sharp, sustained decline in forest cover occurred between about eight to ten thousand years ago. Their signal of forest decline and dune building is roughly coincident with drought-driven hydrologic closure of the upper Great Lakes, determined by other methods. Forest openings (a minimal requirement for dune development) are rarely detected in pollen sums from eastern upper Michigan because faint signatures of non-arboreal pollen are largely obscured by abundant and highly mobile pine pollen. OSL dating of eolian sediment deposition provides a direct, spatially explicit archive of greatly diminished forest cover during the driest episode of the past 11,500 years.
Dynamic deviation of fluid pressure from hydrostatic pressure in turbidity currents
Joris T. Eggenhuisen, Dept. of Earth Sciences, Utrecht University, P.O. Box 80021, 3508 TA Utrecht, Netherlands; and William D. McCaffrey. Posted online 2 Mar. 2012; doi: 10.1130/G32627.1.
Large sediment avalanches transport gravel, sand, and mud from coastal areas i
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Geological Society of America