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.