July 1, 2015
Geologic map of the Orchard 7.5' quadrangle, Morgan County, Colorado
By Margaret Berry (GECSC), Janet Slate, Paul Hanson, and Ted Brandt (GECSC). USGS Scientific Investigations Map 3331.
The Orchard 7.5' quadrangle is located along the South Platte River corridor on the semi-arid plains of eastern Colorado, west of Ft. Morgan. The mapped area contains surficial deposits that record alluvial, eolian, and hillslope processes that have operated through environmental changes from the Pleistocene to the present. Along with the map and georeferenced map PDFs, this publication includes downloadable GIS files.
June 10, 2015
Late Quaternary sea-level history and the antiquity of mammoths (Mammuthus exilis and Mammuthus columbi), Channel Islands National Park, California, USA
By Dan Muhs (GECSC), Kathleen Simmons (GECSC), Lindsey Groves, Jack McGeehin, Randy Schumann (GECSC), and Larry Agenbroad. Published in Quaternary Research.
Mammoth fossils are sometimes found in Channel Islands National Park, and most date to the last glacial period (Marine Isotope Stage [MIS] 2, about 25,000 to 12,000 years ago). Recently, however, a tusk was found in the lowest marine terrace of Santa Rosa Island (MIS 5.1, about 80,000 years ago), which dates it to a time of relatively high sea level. Mammoths mostly likely swam to the islands, and the favorable times to do so (other than MIS 2) would have been during glacial periods MIS 6 (~150,000 years ago) or MIS 8 (~250,000 years ago) which were times of low sea level. The date of the MIS 5.1 fossil challenges the hypothesis that climate change, vegetation change, and decreased land area from sea-level rise were the causes of mammoth extinction at the Pleistocene/Holocene boundary on the Channel Islands, because pre-MIS 2 mammoth populations would have experienced similar or even more dramatic changes at the MIS 6/5.5 transition.
June 4, 2015
Radar attenuation and temperature within the Greenland Ice Sheet
By Joseph MacGregor, Jilu Li, John Paden, Ginny Catania, Gary Clow (GECSC), Mark Fahnestock, Prasad Gogineni, Robert Grimm, Mathieu Morlighem, Soumyaroop Nandi, Hélène Seroussi, and David Stillman. Published in the Journal Geophysical Research Earth Surface.
Currently, temperature information of ice within polar ice sheets comes from direct measurements made in the few boreholes produced by deep ice-coring projects. This article presents a different method of determining depth-average temperatures using the attenuation of airborne radar-sounding data and testing results against the network of borehole temperature measurements. Applying this method to the Greenland Ice Sheet, it was found that present temperature structure depends more strongly on past temperature and accumulation changes than previously recognized, particularly in southern Greenland. This new information is expected to provide important constraints for thermomechanical models used to investigate the future stability of the Greenland Ice Sheet.