2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 11
Presentation Time: 8:00 AM-6:00 PM

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

ELLWOOD, Brooks B., Department of Geology & Geophysics, Louisiana State University, E-235 Howe-Russell Geoscience Complex, Baton Rouge, LA 70803, TOMKIN, Jonathan H., School of Earth, Society, and Environment, University of Illinois, 428 Natural History Building, 1301 W. Green Street, Urbana, IL 61801, EL HASSANI, Ahmed, Departement de Geologie, Institut Scientifique, Avenue Ibn Batouta, B. P. 703, Rabat Agdal, Morocco and BULTYNCK, Pierre, Department of Paleontology, Royal Belgian Institute of Natural Sciences, rue Vautier 29, Brussels, BE-1000, Belgium, ellwood@lsu.edu

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.