A Magnetostratigraphic Susceptibility Climate-Driven Model and Development of a Floating Point Time Scale for the Middle Devonian Eifelian Stage

Here we propose an approach using high-resolution data sets to establish an age chronology that can be applied to any geologic Stage - using cyclostratigraphic methods and a geochemical or geophysical data set serving as a climate proxy. To demonstrate and test the method, we use the magnetic susceptibility (MS) technique as the basis for fitting a climate model to the Eifelian Stage of the Middle Devonian. We establish a Milankovitch ~400 ky eccentricity climate model for the Eifelian - tested using MS data pinned to time-series analysis from outcrop samples (1) from its lower (Emsian/Eifelian, Germany) and (2) upper (Eifelian/Givetian, Morocco) GSSPs, and (3) data for Eifelian samples from Jebel Bou Tchrafine near Erfoud, Anti-Atlas Morocco. For comparison with other data sets, methods and instruments, we present the MS data as dMS, where

dMS = (MS - MSstandard)/MSstandard

and MSstandard = 5.5 x 10-8 m3/kg, the median value for over 10 k measurements of marine sedimentary samples measured by us, including limestone, mudstone and siltstone samples. Using time-series analysis of these data we construct a uniform cyclicity model designed to conform to a ~405 ky cyclicity with a duration corresponding to the published duration for the Eifelian of ~ 5.7 m.y. (Gradstein et al., 2004). To this model we fit a well-established conodont zonation developed independently using graphic correlation (Gouwy and Bultynck, 2003), thus allowing time estimates for conodont ranges for the Eifelian. This comparison yields a range from ~2.8 m.y. (costatus zone) to ~50 ky (kockelianus zone) for individual conodont zones. 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. In addition, these data allow direct evaluation of sedimentation rates throughout the Eifelian Stage.