The Rare Gas Geochronology Lab
 

Evolution and Chronostratigraphy of Lacustrine and Marine Basins

We are seeking graduate students to participate in geochronology research on Cretaceous strata. If interested, email: bsinger@geology.wisc.edu

Cyclostratigraphy in the Wilkins Peak Member,

Cenomanian-Turonian boundary section, which includes four bentontite (volcanic ash) beds, Pueblo Colorado (courtesy Brad Sageman, August, 2007.
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Dating of volcanic ash beds deposited in lakes or oceans using the 40Ar/39Ar geochronometer is revolutionizing our ability to determine rates of sediment accumulation, biological evolution, and environmental change. A major focus of this research has been undertaken in collaboration with Professor Alan Carroll on the Eocene Green River Formation in Wyoming, Utah, and Colorado. An array of more than two dozen precisely dated ash beds has allowed for the first time to track the evolution of lakes that occupied the Green River, Piceance Creek, and Uinta basins over eight million years of time.

For example, we found that dry, evaporative conditions which led to extensive brine deposition migrated southward over time in response to changing climate and topgraphy. Our laboratory participates in the NSF-supported EARTHTIME initiative (http://www.earth-time.org/)—a community-based effort to improve temporal resolution throughout the past 800 Ma of Earth history. We have recently begun to focus on developing a new radio-isotopic time scale for the entire Cretaceous Period, beginning with dating ash beds in late Cretaceous strata preserved in the Western Interior Basin of Colorado and the Rocky Mountains.


Age model for the Eocene Green River Formation, Wyoming-Colorado-Utah, based on 22 40Ar/39Ar ages determined from ash beds. From Smith et al. (2008) GSA Bulletin.
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The following targets and goals are exemplary of the larger effort:

Biogeochemical and paleobiological rates across the Cenomanian-Turonian boundary
Sedimentary rocks spanning the Cenomanian-Turonian boundary that crop out near Pueblo, CO, preserve the type locality for oceanic anoxic event II (OAE II), a major perturbation of the global carbon cycle that rapidly drew down atmospheric carbon dioxide, caused mass extinction of marine fauna, and left behind distinctive organic, oil-rich, black shale deposits. The timing and duration of OAE II —critical to understanding burial fluxes of carbon to the sea floor, evolving redox states of the water column, and rates of biotic change in times of environmental stress—are known best from a “floating” orbital chronology. The orbital chronology suggests that anoxic conditions began abruptly and lasted for at least 600 thousand years. We plan to determine new 40Ar/39Ar ages for several sanidine-bearing ash beds within this sequence to constrain rates of sediment accumulation and hence environmental change associated with OAE II.

Cyclostratigraphy in the Wilkins Peak Member , Green River Formation, near Rock Springs Wyoming. Cycles defined by coarsening upwards beds with arkosic tops. From Pietras et al. (2003) Geology.
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Sea level change and climate cycles during the peak of greenhouse conditions
Cyclostratigraphy in sediments deposited for several million years before and after the Cenomanian-Turonian boundary is expressed by alternations of limestone and shale widely interpreted to reflect periodic rise and fall of sea level. A fundamental question is to what extent these sea level changes, which took place during peaks in atmospheric temperature and carbon dioxide, were forced by the same quasi-periodic orbital behavior—Milankovitch cycles—thought to dominate our current icehouse world. The only way to test the orbital chronology being created for these 100 - 89 Ma strata is via radio-isotopic dating of ash beds. Thus, we plan to determine a new set of precise 40Ar/39Ar ages from a dozen additional ash beds found in late Cretacous sediments from Saskatchewan, Montana and Wyoming to Colorado. This work is being undertaken in collaboration with Professor Brad Sageman at Northwestern University.


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