Terrestrial carbon isotope chemostratigraphy

Funding: American Chemical Society Petroleum Research Fund ACS-PRF# 38141 -GB 8 “ Carbon isotope variability within terrestrial organic matter and its possible impact on chemostratigraphic correlation at the K/T boundary“. There are no official collaborators on this grant, but we have been working with a number of folks including Kirk Johnson, Mary Kraus, Greg Ludvigson, Rich Barclay, and Doug Nichols.

What began as a study of along-strike variability in carbon isotope ratios of bulk sedimentary organic matter near the K/T boundary here in Colorado has morphed into a much broader investigation of terrestrial Jurassic and Cretaceous sediments from western North America. The overall goal is to identify carbon isotope shifts that are applicable as markers for chemostratigraphic correlation, particularly correlation between vertebrate fossil localities.

As a test case we are still focusing on the well-studied K/T boundary. Data from our six sampled sections across the boundary at West Bijou Creek, Denver basin, CO are being compared to carbon isotope and age data from the New Mexico, North Dakota, and Montana in order to determine the timing and exact nature of the carbon isotope shift at this time in western North America, and its relation to ecological changes.

To expand the temporal coverage of this investigation in the Denver basin we have sampled the Kiowa Core, which was drilled as part of the Denver Basin Project and covers late Cretaceous through Eocene time. In order to determine if data from the core are truly representative of late Cretaceous/Maastrictian time, we have begun to compare these data with carbon isotope data obtained from organic material in North Dakota (see Dinosaur research project). Hopefully these chemostratigraphic sections will provide a foundation for correlating Hell Creek/Lance Fm. fossil localities all over western North America.

Lastly, we have recently begun studying Jurassic-aged rocks of the Morrison Fm. These rocks are very rich in dinosaur and other vertebrate fossils, yet accurate correlation between important localities in Colorado and Wyoming remains difficult. To address this problem, we are putting together carbon isotope curves from several Morrsion Fm. sections by analyzing bulk sedimentary organic material.

By the way, all of the work we have done so far indicates that it is indeed important to characterize how much carbon isotope ratios vary for any one stratigraphic level if a carbon isotope record is to be interpreted accurately.