COLLABORATIVE RESEARCH: Paleometeorological Records from Sand Dunes and Eolian Sandstones

Nevada System Of Higher Education, Desert Research Institute, Reno NV

Investigators

Abstract

Abstract Collaborative Research:Paleometeorological Records from Sand Dunes and Eolian Sandstones This three-year proposal requests support for collaborative research on modern and ancient sand dunes by earth scientists at University of Nebraska-Lincoln and University of Nevada's Desert Research Institute. Field and laboratory work will concentrate on the 190-million-year-old Navajo Sandstone of the southwestern United States and on modern dunes in California and Nevada. The Navajo Sandstone accumulated in the subtropics, near the western edge of Pangea, the largest continental mass known from Earth history. The ancient rocks contain evidence for annual cycles in the direction and strength of the dominant wind, as well as evidence for heavy rainfall events, brief wind reversals, and a surprisingly high level of animal activity. Because we can recognize annual cycles of deposition, we can ascertain the migration rates of the sand dunes, and the seasonality and frequency of rainfall events. The rains may have occurred during long rainy episodes similar to conditions that prevailed in the Sahara desert between 14,000 and 5,000 years ago. A high water table between dunes may explain why abundant animal and plant life was present even while the dunes were actively migrating. The brief wind reversals may represent the passage of fronts. We will monitor modern sand dunes during frontal passages to see if they record these events with deposits similar to those we have seen in the rocks. Modern dune studies may also allow us to better estimate the height of ancient dunes. Although many features in the Navajo Sandstone can be interpreted to be similar to modern analogs, others indicate very different conditions. It appears that the Jurassic dunes were migrating more rapidly than most large modern dunes, indicating strong, persistent winds. Plants apparently could not stabilize these dunes, even during the rainy episodes. The dominant winds were out of the northwest, but nowhere do westerlies prevail in areas so far to the south today. High winds are commonly attributed to strong temperature gradients, and the thermal contrast between the poles and equator today causes strong atmospheric circulation. In the Jurassic, however, few, if any, glaciers existed and there is evidence that temperatures at high latitude positions were considerably milder than today. We hypothesize that in the southern summer, the supercontinent Pangea developed an effect similar to that over Asia today, and drew in air from the northwest across the equator. Our field studies of modern and ancient dunes will help us to develop a global climate model for the Jurassic. Experiments with the model will test several hypotheses relating to atmospheric circulation over Pangea. Ancient dune deposits record meteorological, hydrological, and biological events in amazing detail. Our study will attempt to mine this information with an eye toward better understanding atmospheric circulation under conditions different from those of today's Earth.