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The Wetteldorf section is the Global Stratotype Section and Point (GSSP) locality of the Lower–Middle Devonian boundary. The section is characterized by an alternation of marls, limestones, and siltstones, with the base of the Eifelian stage defined by the first occurrence of the conodont Polygnathus costatus partitus. The well-established conodont biostratigraphy for this section permits correlations with other time-equivalent sections; however, a cyclostratigraphic framework does not yet exist for this section. Here, we construct such a framework that provides an additional tool for long-distance correlation, and at the same time constrains the amount of geologic time represented by this stratigraphic interval. Microfacies analysis indicates that the entire section has been deposited in an open-marine shelf paleoenvironment, below the storm wave base and under oxygen-rich conditions. We collected high-resolution (5-cm spaced) geochemical data, using a handheld X-ray fluorescence (XRF) spectrometer, and quantified net intensity results for 23 chemical elements. By using factor analysis, we explain 52% of the observed variance in those elements by a model that consists of four factors. The most important factor, factor 1, is driven by geochemical elements related to detrital input (e. g. K, Rb, Si, Ti). Factor 2 is interpreted as a pyrite-indicator, while Factor 3 is driven by Ca and Sr and is thus tied to lithological variations in carbonate content. Factor 1 and 3 are used for cyclo-stratigraphy, as their respective power spectra show similar characteristics. Both factors exhibit a strong 58-cm period, which can be interpreted, according to the available biostratigraphic constraints, either as the imprint of the 18-kyr precession cycle or as the imprint of the 33-kyr obliquity cycle. However, the ratio between different spectral peaks in the different proxies’ power spectra strongly advocates the first option. Our interpretation in terms of astronomical climate forcing implies that the 8.65 meter thick Wetteldorf section (within the Happel Hut) is characterized by an average sedimentation rate of ∼ 3.2 cm/kyr and represents ∼ 250 kyr. From a paleoclimatological point of view, our results indicate that detrital input into the Rhenohercynian Basin was spurred under high-eccentricity configurations, which allowed for strongly enhanced seasonal rainfall. Under astronomical configurations that avoided extremes in seasonality for a prolonged period of time (simultaneously low eccentricity and obliquity), Factor 2 indicates the occurrence of pyrite, which we associate with temporary deteriorations of generally well-oxygenated conditions.
- Astronomical forcing
- Elemental geochemistry
- Magnetic susceptibility
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Philippe Claeys (Manager)Analytical, Environmental & Geo-Chemistry