Palaeo-oceanography and biogeography in the Tremadoc (Ordovician) Iapetus Ocean and the origin of the chemostratigraphy of Dictyonema flabelliforme black shales

High concentrations of vanadium, molybdenum, uranium, arsenic, antimony with low concentrations of manganese, iron and cobalt heretofore restricted to Dictyonema flabelliforme bearing Tremadoc black shales in Balto-Scandia, have been found in coeval black shales in the Saint John, New Brunswick area. Prior palaeogeographic reconstructions place these areas about 400 km. apart in high southern latitudes in the Iapetus Ocean, with New Brunswick in proximity to Avalonia (southeastern Newfoundland). These geochemical similarities are not found in coeval Tremadoc black shales of Bolivia, New York, Quebec, Wales, and Belgium. Palaeo-oceanographic reconstructions of Iapetus support the proximity of Balto-Scandia and the Saint John area during the early Tremadoc and Gee's (1981) suggestion that the signature is a feature of eastern Iapetus. Furthermore, first-order modelling of the major surface currents and related primary productivity in the Tremadoc Iapetus Ocean explain the apparent wide latitudinal range of D. flabelliforme (Fortey, 1984) and the anomalous trace metal content of certain black shales of that time. Variations in the elemental content of these black shales is produced by oceanographic and geologic conditions unique to the geographic site. The distinctive Balto-Scandic geochemical signature resulted from the coincidence of anoxic waters transgressing the shelf at latitudes of high organic productivity at the polar Ekman planetary divergence. This produces the conditions for concentrations of V, U, and Mo in the shales. Metal enriched anoxic bottom waters produced by leaching of volcanics or through hydrothermal activity may be the source of the other enhanced signature elements such as As and Sb. The absence of this geochemical signature in younger non-D. flabelliforme Tremadoc and later black shales in Balto-Scandia and other areas suggests that the closing of Iapetus moved the depositional sites into less productive oceanic areas.