Loess, an accumulation of wind-blown silt, forms cliffs that are more than 50 meters high in Fairbanks, Alaska. These fine-grained sediments, which were generated by glacial grinding and polishing of rocks, have been dated from ca. 3 Ma to 140 ka. What remains unknown is where the dust came from.
This project is aimed at tracing the origins of the Fairbanks loess. We will use U-Pb dating of detrital zircons that are separated from intervals of the loess. For these same samples, we will also use Ar/Ar dating on muscovite grains. We have 6 river samples in the area—from the Chena, Yukon, Delta, Nenana, and upper and lower Tanana rivers—and will compare their grain-age populations with those of the loess, since glacial sediments are thought to be carried by rivers. All of the resulting sediment grain-age populations will be compared with grain-age populations from the neighboring Alaska Range and Yukon-Tanana Uplands bedrock.
The source of the Fairbanks loess will tell us how far the sediments were carried before they were deposited here. Were the sediments first carried nearby by rivers, and then only blown around locally? Or were they carried by wind all the way from the mountains, more than 100 miles away?
Many fossils are preserved in the Fairbanks loess, recording transitions in life over the past few million years. One collaborator, Dorothy Peteet of NASA, will look for pollen that may record transitions in plant life during this climatically variable length of time.