Publication details
Element geochemistry, gamma-ray spectrometry and magnatic susceptibility as correlative tools for the Devonian-Carboniferous boundary interval in Europe
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| Year of publication | 2013 |
| Type | Conference abstract |
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| Description | A definition of the Devonian-Carboniferous boundary (DCB) is subject of criticism for many years due to the problems with the index conodont Siphonodella sulcata, and should be changed. A multiproxy approach involving geochemical and/or petrophysical correlation methods in combination with biostratigraphy has been successfully used for definition of some Cenozoic stages. Therefore this approach should be used also for seeking for new DCB definition, especially when connected with the global Hangenberg Event (HBE). Presented research has been focused on the element geochemistry (XRF), field gamma-ray spectrometry (GRS) and magnetic susceptibility (MS) survey of DCB sections located in Moravian Karst (CZ), Carnic Alps (A), Ardennes (BE, FR), Montagne Noire and Pyrenees (FR). The terrigenous elements (e.g. Al, Ti, Zr) measured by XRF show similar component loadings in the principal component analysis, which suggests a common and stable siliciclastic provenance. The negative correlation of these elements with Ca indicates that the variability in the terrigenous elements is driven by the effect of ‘dilution’ by CaCO3. Regarding the GRS data, covariance between the dose rate (nGy.kg-1) and K and Th demonstrates that the GRS signal comes from these elements, which are indicators of siliciclastics content. On the other hand, there is none or very low correlation between the terrigenous elements and MS. Therefore more factors than dilution effect play a role in vertical changes of MS, in contrast to the commonly used paradigm. Very slight covariance shows MS with various paleoredox and paleoproductivity proxies (Mn, Sr, P, Fe, Zn, Ni) which differ between sections. These elements are easily affected during diagenetic processes. Moreover, preliminary rock magnetic analyses (IRM, ARM, s-ratio) show a strong influence of ferromagnetic minerals on MS. Strongly differentiated magnetic mineralogy seems to be a result of diagenetic changes. The general trends of GRS and XRF values in the DCB sections are as follows: low rates of terrigenous input (e.g. Al, Zr/Al, K/Al) and higher paleoproductivity (e.g. Sr/Al, Mn/Fe) proxies in the Famennian below HBE. Culmination of these values together with increasing Mn/Al proxy was documented in the sections where the HBE interval is dominated by carbonates. Low values of Mn/Al and increase of terrigenous elements take place where the HBE base is associated with shales. It demonstrates the supposed eutrophication that caused carbonate crisis at the base of the HBE s.s. An increase in the terrigenous input proxies at the top of the HBE could be connected with sea-level fall (HBE s.l.). MS failed to show correlatable patterns. The XRF and GRS methods are possible to encompass the detrital input or carbonate productivity, which can be correlated between distant areas in the case of global events. The MS signal, in contrary, can be overprinted by local diagenetic features. |
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