The Late Frasnian Kellwasser horizons of the Harz Mountains (Germany): two oxygen-deficient periods resulting from different mechanisms.
Résumé
In the Harz Mountains (Germany), three Late Devonian sections, Aeketal, Hühnertal and Kellwassertal, were analyzed for major- and trace-element concentration. The inorganic geochemical data were used to determine the environmental conditions, associated with the deposition of the two Late Frasnian black Kellwasser horizons in a submarine-rise environment. Low Ti/Al and Zr/Al values and slight enrichment in nutrient- and organic matter-related trace elements (e.g., Ba, Cu, and Ni) suggest that the Kellwasser horizons were deposited during two periods of minimum detrital input but relatively elevated primary production. Enrichments in U, V, and Mo in both black Kellwasser horizons and values of redox indices, including U/Th, V/Cr, and Ni/Co, indicate oxygen-restricted conditions prevailed during the Late Frasnian times. Under oxygen-depleted conditions, reductive diagenesis caused precipitation of iron sulphides, such as pyrite, that are observed with SEM. However, variations in major and trace element concentrations are not similar in the two Kellwasser horizons, particularly regarding redox markers. The Lower Kellwasser horizon seems to be more O2-depleted than the Upper Kellwasser horizon in some German sections. On the basis of these results, we propose that the two Kellwasser horizons are not caused by identical conditions. The Lower Kellwasser horizon results from increased primary productivity, enhanced by land-derived nutrient-loading and which triggered the anoxic conditions recorded in drowned platform setting. In contrast, the Upper Kellwasser horizon results from the onset of oxygen-impoverished bottom water in the deepest part of the ocean, due to episodic water stratification during maximum Frasnian flooding. Oxygen-depleted water may have impinged shallower environment during the linguiformis transgressive pulse. Nutrients would have been released from organic matter decay under reducing conditions and brought up to the photic zone during episodic water mixing.