The Cretaceous period, between about 146 and 66 million years ago, was a time of sustained global warming during which atmospheric CO2 levels likely surpassed 800 part per million, similar to the amount predicted for 2100 by ?business-as-usual? emission scenarios. Cretaceous strata record evidence of high sea level, remarkably cyclic sedimentation thought to reflect orbitally-forced climate oscillation, high biological productivity, large scale igneous activity, and major perturbations of the carbon cycle during ocean anoxic events (OAEs). Understanding these phenomena, and their influence on fluxes into and out of the marine carbon reservoir, can provide deep-time analogues and a rich context for possible greenhouse scenarios of the future. However, a major limitation on our understanding of these deep-time analogues is the lack of accurate chronologies for assessing timing and rates of climatic and oceanic change. This project comprises a complete re-calibration of the Cretaceous time scale via new radioisotopic dating of volcanic ash beds integrated with chemical profiling and time-series analysis of sediment sequences aimed at detecting the influence of earth?s orbit and solar energy budget on sediment deposited in a shallow ocean setting. This will provide a solid foundation for the Cretaceous in the next iteration of the geologic time scale and should help resolve a number of outstanding problems. Moreover, the intercalibration of the 40Ar/39Ar and U-Pb radioisotopic chronometers - based on a common set of sanidine and zircon-bearing volcanic ash samples - will provide geologic tests of these decay systems, thereby addressing a primary goal of the NSF-sponsored EARTHTIME initiative and the entire geochronology community. This project will support two graduate student theses at UW-Madison and forge collaborations with active research groups in Europe, Canada and the US. Because Cretaceous strata also supply most of the global petroleum and natural gas reserves, our efforts to improve the 4-D understanding of Cretaceous rocks has high societal relevance with respect to these energy resources. To raise public awareness we have developed an outreach plan with staff of Lake Pueblo State Park, CO, which had 1.8 million visitors in 2008 and is home to one of the world's most classic exposures of Cretaceous strata, including the Cenomanian-Turonian Global Boundary Stratotype Section and Point (GSSP). Working in collaboration with the Park and Pueblo County school kids, as well as scientists at the USGS in Denver, we will design and install signs that highlight the science of geologic age determination and significance of the GSSP.
This project has been a collaborative effort between Prof. Brad Singer and Prof. Steve Meyers at the University of Wisconsin-Madison and Prof. Brad Sageman at Northwestern University. The Northwestern portion of project funding was expended this year and the grant closed, initiating the outcomes report request, but the UW-Madison team requested and recieved a one year no cost extension in order to complete some of the final objectives of the project. The major goal of this project has been improvement of the geologic time scale for the Cretaceous Period. Accurate time scales provide the critical foundation for a very broad spectrum of other studies of the geologic past, including paleoclimate reconstructions, tectonic analyses, studies of key resources such as gas shale deposits, and many others. This project focused on Cretaceous deposits of the Western Interior basin of North America because they contain both abundant volcanic ash beds with datable minerals that are intercalated within biostratigraphically well-constrained marine, as well as non-marine strata spanning at least Aptian through Maastrichtian time (about 75% of the Cretaceous Period), and significant thicknesses of rhythmically bedded chalk-marl marine strata that preserve astronomical (Milankovitch) cycles. This confluence of physical attributes allows the integration of radioisotopic and astrochronologic methods. Advances in radioisotope techniques, including improvements in instrumentation and the application of multiple chronometers (Ar-Ar and U-Pb methods) has resulted in revision of many ages and significant decrease in the uncertainty assigned to age data within the study interval. The integration of astrochronologies with new radioisotope dates, where possible, has resulted in far more accurate interpolation of ages between new radioisotope dates. In addition, over the course of the project some new methods to improve estimation of time scale uncertainties have been introduced. Importantly, the agreement between independent chronometers demonstrated in this study significantly increases confidence in the new time scales and provides support for current radioisotope and astrochronologic methodologies. Thus far, three papers have been published presenting results from the project: (a) a revised time scale for the Cenomanian-Turonian (C-T) boundary interval (Meyers, et al., 2012); (b) extension of the C-T time scale down into the Middle Cenomanian (Ma, et al., 2014); and (c) a revised time scale for the Late Turonian through Early Campanian (Sageman et al., 2014). Ongoing efforts are focused on filling the gap between the Early and Late Turonian and extending the time scale into both older and younger strata. In addition to these intellectual merit contributions, the project had a very specific objective for broader impacts beyond the training of next generation geochronologists (Seiwert and Ma, student at UW-Madison), and the dissemination of scientific results (many presentations at conferences and the aforementioned publications, plus more to come): the PI's collaborated with a media firm (ECOS Inc.), the State of Colorado (Parks and Wildlife division), the International Commission on Stratigraphy, and numerous other stakeholders to create an educational display marking the Global Stratotype, Section and Point (GSSP) for the Cenomanian-Turonian boundary in the Lake Pueblo State Park. The display and GSSP marker were commemorated in association with the 2013 annual meeting of the Geological Society of America (see write up in GSA Today, 2014, v. 24, no. 2). The Lake Pueblo State Park receives almost 2 million visitors per year, and the new GSSP display offers an accessible introduction to the Geololgic Time Scale and the methods used to develop it, including key results from this project.
