The Nd isotopic composition of conodonts was used to investigate geochemical signatures and circulation of seawater in the Variscan Sea during the Late Devonian. Material selected for analysis included almost 300 samples of Frasnian and Famennian conodonts collected in Morocco (eastern Anti-Atlas, Meseta), southern France (Montagne Noire) and in southern Poland (Holy Cross Mts, Sudetes, Cracow area). Although significant variation in Nd concentrations occurs between different conodont elements within a single conodont sample, each element in the sample yields identical Nd isotopic composition. Nd concentrations in conodont crowns (from 25 to 280 ppm) depend on their morphology; the higher the surface /volume ratio of conodont elements the higher is their Nd content.
Nd isotope data from two horizons in the eastern Anti-Atlas, the base and the top of the Kellwasser facies, were selected to reconstruct the seawater circulation on the Moroccan shelf. During the onset of the Kellwasser facies (Zone 11) a general seawater flow from the southwest towards the east has been recognized. The circulation pattern was complex due to local riverine input from the West African Craton in the south and from the exposed Precambrian basement in the west. During the end of the Kellwasser facies (rhomboidea Zone) seawater circulation was dominated by a general westward flow and a much weaker countercurrent in the central Tafilalt. The episodic entrance of oceanic waters on the shelf during the Frasnian (Zone 11) documents the connection of the Anti-Atlas shelf area with the Rheic Ocean to the southwest and with the Variscan Sea to the north.
The seawater showed in the Variscan realm a wide range of variation in epsilon-Nd values, from - 2 to -12, and Sm/Nd ratios, from 0.16 to 0.85. However, there was a clear difference in the Nd isotopic signatures between the oceanic water and that covering shelves. The ocean aquafacies was characterised by strong radiogenic signatures with epsilon-Nd values from -2 to -5, similar to those of the modern Pacific Ocean, and low Sm/Nd ratios. The oceanic waters entered the shelf areas only episodically during transgressions. In contrast, the shelf aquafacies yielded epsilon-Nd values from -6 to -12 and high Sm/Nd ratios, between 0.28 and 0.8. Both aquafacies differed not only in their Nd isotopic signatures but also showed different carbon isotopic evolution and REE fractionation.
Temporal trends in Nd isotopic composition of conodonts reflect sea-level fluctuations. The epsilon-Nd values decreased during regression phases when enhanced erosion accelerated supply of low radiogenic Nd from old continental sources. A rise in sea level generated a positive shift in epsilon-Nd values due to input of more radiogenic oceanic water into the seawater reservoir of the shelf areas. The eustasy appears to be the most effective factor that governed temporal changes in the Nd isotopic composition of seawater. The method has a great potential for the sea-level research and palaeoceanography because sea-level changes can be recognized independently from facies, biota and lithology. Effects of other factors on temporal changes in epsilon-Nd values can be minimized or even excluded by analysis of trends in different sections located at various palaeoceanographic positions.
On the basis of temporal fluctuations in the Nd isotopic composition of conodonts from the western part of the Variscan realm an eustatic sea-level curve for the late Frasnian and early Famennian has been constructed. Although, it reveals a general similarity to previous schemes, there is significant difference that the lower and the upper Kellwasser horizons, as known from Variscan Europe and northern Meseta, fall within intervals of decreasing epsilon-Nd values. This strongly indicates that these black limestones were deposited during regression phases.
Verknüpfung zu Publikationen oder weiteren Datensätzen
