Geochemical decoupling of water masses in the Variscan oceanic system during Late Devonian times

Neodymium isotope data obtained from conodonts collected in Morocco and Poland attest a geochemical decoupling of seawater in the Variscan realm during the Late Devonian. The water masses on the shelves were characterized by unradiogenic εNd values, from about − 7 to − 12. The surface water in the bordering oceans (the Rheic Ocean and the Variscan Sea) yielded more radiogenic εNd values, ranging from about − 1 to − 6. A compilation of Nd and C isotope signatures from the Moroccan Meseta suggests that the shelf and the surface oceanic water differed also in their carbon isotope evolution. The geochemical decoupling constrains a restricted water exchange between shelves and the ocean. Oceanic waters entered far into the shelf areas only during the semichatovae transgression in the Frasnian Zone 11. Sea-level fluctuations constituted the major force that modified circulation, and thus controlled the mixing of shelf and surface oceanic seawater. The Nd conodont data reveal that the surface water of the Variscan oceanic water resembled the seawater of the modern Pacific Ocean rather than those of the Atlantic and Indian oceans. The geochemical character of the Variscan oceanic water is consistent with the geotectonic regime of the Variscan realm in Late Devonian and Carboniferous times.