Research Interests

Discovery at Snowmass Village, Colorado - A Window into the Ice Age Rockies

Snowmass 1. Snowmass 2. Snowmass 3.

In October 2010, construction crews working to expand the capacity of a reservoir near Snowmass Village, Colorado unearthed several bones of a juvenile Columbian mammoth. In the months that followed, scientists and volunteers from the Denver Museum of Nature & Science (DMNS) uncovered more than 5000 bones of 41 different kinds of animals, including mastodons, mammoths, ground sloths, camel, horse, and two different species of bison. The bones are exceptionally well preserved, as are plant, insect and aquatic invertebrate fossils recovered from the site - beetle parts are iridescent, plants are still green, and conifer cones are intact. USGS and other scientists are currently working to reconstruct climate and environmental conditions at the site during the times the animals were alive. The Snowmastodon site provides a unique opportunity to understand what life was like for the animals high in the Colorado Rockies during the Ice Age.

Related links

USGS Snowmastodon Project website

Press release - final details of field work (7 July 2011)

Denver Museum of Nature and Science "Snowmastodon" project

PBS Newshour with Jim Lehrer (29 Nov 2010)

USGS podcast interview (9 Dec 2010)

Evidence for the 12.9 ka "Impact Event": Catastrophe or Natural Processes?

Cosmo 1. Cosmo 2. Cosmo 3.

The Younger Dryas impact hypothesis contends that an extraterrestrial (ET) object exploded over North America at 12.9 ka (thousands of calendar years before present), initiating the Younger Dryas cold event, the extinction of many North American megafauna, and the demise of the Clovis archeological culture. Although the exact nature and location of the proposed impact or explosion remain unclear, alleged evidence for the fallout comes from multiple sites across North America and a site in Belgium. At six of the ten original sites, elevated concentrations of various “impact markers” were found in association with black mats that date to the onset of the Younger Dryas. Black mats are common features in paleowetland deposits and typically represent shallow marsh environments. In this study, we investigated black mats ranging in age from ~6 to >40 ka in the southwestern U.S. and the Atacama Desert of northern Chile. At ten of thirteen sites, we found elevated concentrations of iridium in bulk and magnetic sediments, magnetic spherules, and/or titanomagnetite grains within or at the base of black mats, regardless of their age or location. This suggests that elevated concentrations of these markers arise from processes common to wetland systems, and not a catastrophic ET impact event.

ScienceNOW article (23 Apr 2012)

LiveScience article (23 Apr 2012)

NGS proposal summary

Quaternary Climate Change - Paleowetlands

Wetlands 1. Wetlands 2. Wetlands 3.

Spring discharge (or "paleowetland") deposits form in arid environments as water tables rise and breach the ground surface during periods of enhanced effective precipitation. In addition to providing an important water source for local fauna, emergent water tables support hydrophilic vegetation, which in turn acts as a natural catchment system for eolian sediments. The interplay between emergent water tables, ecological and biological systems, and wind-derived sediments results in a unique and complex depositional environment that contains information on the timing (age of deposits) and magnitude (faunal and ostracode assemblages, isotopic data) of past climate change. Spring deposits also clearly demarcate the position of past water tables on the landscape, which provides direct evidence of past hydrologic conditions. To date, I have worked on the chronology, stratigraphy, and paleoclimatic interpretations of spring discharge deposits in the American Southwest and the Atacama Desert of northern Chile. I am currently working paleowetland deposits in the Mojave Desert (USGS), southern Great Basin (San Bernardino County Museum, BLM), and Jordan (Miami University) to further investigate climate and hydrologic changes during the Pleistocene and Holocene in these arid lands.

Mendenhall Project Profile

U.S. Geological Survey Mendenhall Post-Doctoral Fellowship Proposal #2006-26

Photograph by Peter Essick, National Geographic Magazine, September 2004

Cover Photograph, Geological Society of America Bulletin, October 2002

¹⁴C Dating - Gastropod Shells

Gastropods 1. Gastropods 2. Gastropods 3.

Terrestrial gastropods are one of the most successful animal groups on Earth. They occupy and thrive in incredibly diverse habitats, ranging from marshes and grasslands to upland forests and arctic tundra. Their distribution in the fossil record is equally diverse. Yet despite their widespread occurrence, fossil gastropod shells are usually avoided for radiocarbon dating because many taxa incorporate ¹⁴C-deficient (or “dead”) carbon from limestone when building their shells. This is referred to as the “Limestone Problem” and can cause ¹⁴C ages to be up to ~3,000 years too old. Recently, I analyzed the carbon-14 content of nearly 250 aliquots of modern shell material (3750 individual shells) from 46 different species. In sum, most of the small shells, including some of the most common gastropods in North America did not contain dead carbon from limestone or other carbonate rocks even though it was readily available at all sites. Fossil shells of these and other small terrestrial gastropods are common in a wide range of Quaternary deposits in North America and, therefore, our results may have broad chronologic applications to Quaternary geology and New World archeology. Along with Jason Rech, Dan Muhs, and others, I am currently testing whether these shells can be used to date late Pleistocene and Holocene loess and glacial deposits in midcontinental North America and Alaska.

NSF proposal #EAR 0614840 (PDF 0.57 Mb)

Tumamoc Hill research highlight