PALEOENVIRONMENTAL AND EUSTATIC CONTEXT OF MIDDLE DEVONIAN BIOEVENTS IN EASTERN NORTH AMERICA AND EUROPE

Although, much attention has been focused on the cyclicity and bioevents of the Late Devonian, it is becoming increasingly evident that the Middle Devonian was also a critical interval in physical and life history, characterized by both intervals of strong local stability as well as episodes of abrupt change. Recent detailed comparative studies of stratigraphic and faunal patterns in eastern Laurentia, Avalonia, northern Gondwana , and elsewhere, have documented comparable patterns that point to the global nature of climatic, eustatic and biotic events within this interval. A refined, high-resolution record of sea level changes has been elucidated in recent years, providing evidence for high order cycles with probable Milankovitch periodicities, despite the fact that the Middle Devonian lies solidly within a “greenhouse”phase of global climatic history. The present review considers the paleoecological and paleoenvironmental contexts of three major Middle Devonian bioevents: the early Eifelian Chotek, the late Eifelian Kacak, and the late Givetian Taghanic events, in eastern North America and Europe.

Within the generally warm Mid Devonian interval, there is also accumulating evidence for periods of abrupt temperature change, especially in the late Eifelian and even more markedly in the late Givetian . These thermal events are associated with episodes of transgression to highstand, and expansion of dysoxic to anoxic environments as evidenced by widespread black shale deposits, again, in the late Eifielian and in the Givetian-early Frasnian. Such hypoxic intervals appear to have global expression and to coincide with severe extinctions, immigrations and accelerated speciation. The linkage of intervals of biotic overturn, hypoxia, climatic, and sea level fluctuation provides evidence for integrated global change and ecological-evolutionary response.