The Emsian – Eifelian (Lower – Middle Devonian) boundary occurs in a 100-kyr eccentricity maximum: A potentially useful secondary marker for the GSSP section (Wetteldorf Richtschnitt, Germany).

Abstract:

A Global Boundary Stratotype Section and Point (GSSP) of a stage is chosen such that it can be accurately correlated with widely separated sections. To make such correlation, one can use the primary marker (usually the first appearance of a species). However, more often than not, secondary markers of chemo-, magneto-, cyclo- or biostratigraphic nature prove useful in establishing accurate and precise correlations. The GSSP for the Emsian - Eifelian stage boundary is exposed in the Wetteldorf Richtschnitt, in the Eifel Hill in Germany. The Emsian-Eifelian boundary is defined by the first occurrence of the conodont Polygnathus costatus partitus (Ziegler and Klapper, 1985). However, no secondary markers are currently available for this GSSP section. Our aim is to construct a cyclostratigraphic framework for this GSSP section, and thus to facilitate future correlations. Therefore, we collected 4-cm spaced elemental data of an 8.65 m thick interval, using a Bruker Tracer handheld XRF. We complemented our data with previously-published magnetic susceptibility (MS) data (Ellwood et al., 2006).

A strong anti-correlation exists between the calcium-iron ratio (Ca/Fe) and the MS data. These variations mainly reflect the lithological variability between carbonate-poor mud- and siltstones and argillaceous limestone beds or calcareous sandstones. These proxies, as well as Ti/Al and K/Al, reflect strong 0.5 - 1 m periodicity. However, the orbital calibration of this dominant periodicity is challenged by the few time constraints available for the studied section. To circumvent this problem, we apply the evolutive average spectral misfit method (eASM; Meyers and Sageman, 2007) to the 4 different proxy records. This method tests the null hypothesis of no orbital signal and provides an estimate of sedimentation rate throughout the section. For all proxies, the eASM method provides similar results and we use the obtained sedimentation rates to convert stratigraphic height into time. We observe a mixed forcing of obliquity and eccentricity-modulated precession. Moreover, the Emsian – Eifelian boundary clearly occurs shortly after a 100-kyr eccentricity maximum. This cyclostratigraphic observation can be used as an additional mean in future long-distance correlations between Emsian – Eifelian boundary sections.