A climate-driven model and development of a floating point time scale for the entire Middle Devonian Givetian Stage: A test using magnetostratigraphy susceptibility as a climate proxy

Here we propose an approach to establish a preliminary age chronology for complete stages within the Phanerozoic by applying cyclostratigraphic methods that employ high-resolution data sets. This requires use of geochemical or geophysical data known to serve as a cyclic climate proxy. To demonstrate the method, we use the magnetostratigraphic susceptibility technique as the basis for fitting a climate model to the Givetian Stage of the Middle Devonian System. We show a Milankovitch eccentricity climate zonation for the Givetian that is pinned to time-series analyses from outcrop samples from its upper (Givetian/Frasnian, France) and lower (Eifelian/Givetian, Morocco) stage boundaries. Using these data sets we construct a uniform cyclicity model designed to conform to a ~. 405. kyr cyclicity, with a duration corresponding to the published duration for the Givetian of ~. 4.4. myr (Kaufmann, 2006). To this model we fit two well-established conodont zonation schemes, thus allowing time estimates for conodont ranges for the Givetian, indicating a range from ~. 1.8. myr to ~. 100. kyr before extinction of an individual conodont species. We then test and adjust the model using independent data sets from the eastern United States. Adjustments to the initial model yield a duration of ~. 28 climate half-cycles (zones) for the Givetian Stage and an increased age to ~. 5.6. myr. These zones allow high correlation among sections to better resolve timing of major bio-events, and provide a Floating Point Time Scale to which absolute ages can be applied.