Collaborative Research: Integrating Radioisotopic and Astronomical Time Scales for the Cretaceous

Northwestern University, Evanston IL

Investigators

Abstract

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.