Reconstructing seawater circulation on the Moroccan shelf of Gondwana during the Late Devonian: Evidence from Nd isotope composition of conodonts

The Nd isotope composition of conodonts is used for the first time as a paleoceanographic proxy to reconstruct seawater circulation. Conodonts from two stratigraphic levels in the eastern Anti-Atlas, the lower part of the Frasnian Zone 11 (base of the Kellwasser facies) and the boundary interval of the Uppermost crepida/rhomboidea zones (top of the Kellwasser facies), were selected for reconstruction of seawater circulation on the Moroccan shelf of Gondwana during Late Devonian times. The recognized lateral variations in e Nd values are in very good agreement with current-related sedimentary features. They provide evidence for communication of the Moroccan shelf with the Rheic Ocean to the southwest and with the Variscan Sea to the north. In the Frasnian, the general seawater flow was directed from the southwest to the east and was influenced by a local riverine input of low radiogenic Nd from the south (the West African Craton and/or the Pan-African Ougarta Belt) and from the northwest (the Precambrian basement of the Anti-Atlas). The combined set of Nd isotopic records and directional data from the middle Famennian reveals a circulation pattern different from that of the Frasnian. It was dominated by a general westward flow with a countercurrent in the central Tafilalt area. The establishment of a new circulation system during the Famennian suggests essential changes in paleogeography of the western part of the Variscan realm.