ND ISOTOPIC EVIDENCE FOR SEA-LEVEL FLUCTUATIONS AND DECOUPLING BETWEEN THE OCEAN AND EPEIRIC SEAS DURING THE LATE DEVONIAN

The Nd isotopic composition of Late Devonian conodonts was used to investigate geochemical signatures of seawater in the Variscan realm. Several sections were systematically sampled in Morocco (Meseta and eastern Anti-Atlas), France (Montagne Noire), and southern Poland (Sudetes, Holy Cross Mountains). Across the Variscan realm, from the Moroccan shelf of Gondwana to the shelf of Euramerica in southern Poland, the seawater shows a wide range of variation in e_Nd values from -2 to -12. There is, however, a clear difference in the Nd isotopic signature between the oceanic waters and those covering shelves. The oceanic aquafacies was characterized by high radiogenic e_Nd values, similar to those of the modern Pacific Ocean. The shelf aquafacies yielded low e_Nd values, reflecting Nd supply from Precambrian continental sources, and a wide range of Sm/Nd ratios, comparable with those of the modern epeiric seas. Both aquafacies differed not only in their Nd isotopic signatures but also showed different C isotopic evolution. The geochemical decoupling between these aquafacies in the Variscan realm testifies a restricted water exchange between shelves and the ocean. The oceanic waters entered the shelf areas only episodically during transgressions. Fluctuations of the sea level modified the participation of both aquafacies on the shelves and produced temporal and spatial variations in the Nd isotopic signal.

During the prominent mid-Frasnian semichatovae-transgression, for instance, an ingression of oceanic waters on the shelves is documented by a significant positive excursion in e_Nd values. The Lower and the Upper Kellwasser units, however, fall within intervals of decreasing e_Nd values. This indicates that these black, mostly anaerobic or dysaerobic sediments were deposited during regression phases. These both eustatic events are responsible for an extensive erosion and origin of stratigraphic gaps on the carbonate platforms of the Anti-Atlas. Thus, Nd isotopic data do not confirm the hitherto commonly postulated transgressive character of the Kellwasser lithology.