The early Eocene was a time during which the Cenozoic peak warming was reached, and coincident with the warming were several episodes of major faunal turnover within mammalian communities on different continents such as North America, Asia and Europe. We are focused on establishing a reliable, high-precision timescale using he Eocene record preserved in the Green River Basin in Wyoming. We hope to place such records of faunal turnover in a reliable chronostratigraphic framework to document patterns of mammalian evolution during the super greenhouse world of the Eocene. There are two main aspects to this project. geochronology and vertebrate paleontology. we use a combination of paleomagnetism (magnetostratigraphy), ID-TIMS U-Pb zircon geochronology (in collaboration with MIT) and Ar-Ar chronology on volcanic ash beds from the Greater Green River Basin in Wyoming to establish a high resolution chronostratigraphic framework for the basin and also to improve the resolution of calibration of the Eocene Geomagnetic Polarity Time Scale.
High precision 40Ar/39Ar dating of volcanic ash beds
We apply high precision 40Ar/39Ar dating method to volcanic ash beds in conjunction with U-Pb zircon geochronology and paleomagnetic polarity determination in order to 1) fine-tune the calibration of the Eocene segment of the Geomagnetic Polarity Time Scale, 2) develop a high precision chronology for the fossiliferous Green River, Bridger, and Washakie Formations in the Greater Green River Basin, WY in order to construct a high precision stratigraphy and chronology of the Eocene geological records.
Calibration of the Geomagnetic Polarity Time Scale
PhD student Kaori Tsukui is working on high precision dating of the Eocene volcanic ash beds from the Greater Green River Basin in southwestern Wyoming using a combination of 40Ar/39Ar and U-Pb zircon geochronology and paleomagnetic polarity determination. The Geomagnetic Polarity Time Scale (GPTS) plays an integral role in interpreting geological records ranging from biological evolution, climate change to sea floor spreading, making the precision of its calibration a matter of fundamental importance to our understanding of Earth history. However, for the pre-Neogene GPTS where theoretical uncertainties in the orbital calculations do not permit reliable astronomical calibration of magnetic reversals, absolute calibration has been achieved mainly via interpolation between a limited number of discrete tiepoints using an assumption of smoothly varying seafloor spreading (Cande and Kent, 1992). While this method has proven very practical over the last few decades, according to the interpolation method, calibration of intervening magnetic chrons is highly sensitive to the number and accuracy of the tie points being used, making the interpolated segments poorly constrained. However, a more precise and stable timescale is needed in order to resolve geological, paleontological and paleoclimatological records that are being recovered at an increasingly fine temporal resolution. Since the GPTS serves as a global reference to which radioisotopic, magnetostratigraphic and marine and continental biostratigraphic data are correlated, uncertainties in the GPTS calibration can quickly propagate and potentially affect studies that rely on it for chronostratigraphic purposes.
The Greater Green River Basin in southwestern Wyoming provides one of the most complete stratigraphic records from the early to middle Eocene. The record also includes rich vertebrate fossils, especially land mammals that underwent a few turnover events as represented by the Wasatchian/Bridgerian and Bridgerian/Uintan North American Land Mammal Age (NALMA) boundaries. By providing a high resolution chronology to the fossil bearing beds, Tsukui hope to elucidate the patterns and mechanisms of the mammalian faunal turnover events and also consider them in the larger context of paleoenvironmental evolution and intercontinental immigration/migration.
Machlus, M., Hemming, S.R., Olsen, P.E., and Christie-Blick, N., 2004, Eocene calibration of geomagnetic polarity time scale reevaluated: Evidence from the Green River Formation of Wyoming: Geology, v. 32, no. 2, p. 137-140.
Tsukui, K., Clyde, W.C., and Smith, M.E., 2007. Testing different calibrations of the early to middle Eocene Geomagnetic Polarity Time Scale: new paleomagnetic results from tuff deposits in the Green River Basin. Geological Society of America Abstracts with Programs, v. 39, no. 6, p. 336.
Tsukui, K., Clyde, W.C., and Smith, M.E., 2007. Testing different calibrations of the Eocene Geomagnetic Polarity Time Scale using new paleomagnetic results from Green River Basin tuffs. Geological Society of America Abstracts with Programs, v. 39, no. 1, p. 38.
Tsukui, K. and Clyde, W.C., 2008. Ecological stability among early Eocene mammalian faunas from the Bighorn Basin, WY, Journal of Vertebrate Paleontology 28 (Supplement to no. 3): 154A.
Machlus, M.L., Olsen, P.E., Christie-Blick, N., and Hemming, S.R., 2008, Spectral analysis of the lower Eocene Wilkins Peak Member, Green River Formation, Wyoming: Support for Milankovitch cyclicity: Earth and Planetary Science Letters, v. 268, no. 1-2, p. 64-75.
Tsukui, K. and Clyde, W.C., 2009, Paleomagnetism of Eocene tuffs from Laramide foreland basins: Implications for the Geomagnetic Polarity Time Scale, Eos Trans. AGU, 90(52), Fall Meet. Suppl., Abstract GP22A-05.
Tsukui, K., and Clyde, W.C., Fine-tuning the calibration of the Eocene Geomagnetic Polarity Time Scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins. in review.