The Late Middle Devonian (Givetian) Global Taghanic Biocrisis in its Type Region (Northern Appalachian Basin): Geologically Rapid Faunal Transitions Driven by Global and Local Environmental Changes.

摘要

The late Middle Devonian Global Taghanic Biocrisis marks the onset of extinction and a loss of faunal endemism that culminated in the subsequent Frasnian-Famennian extinction. Global environmental changes recognized at this time include increased warming and aridity, as well as rapid eustatic sea level fluctuations. In the type region, the northern Appalachian Basin, the biocrisis is recorded within the deposits of the uppermost Hamilton, Tully, and lowermost Genesee Groups over an interval of ∼0.5 million years. A high-resolution stratigraphic framework reconstructed along a complete onshore through offshore gradient has resulted in the recognition of three main pulses (bioevents) of faunal transition in the type region: (1) the incursion of the tropical Tully Fauna into eastern Laurentia and temporary loss of the endemic Hamilton Fauna; (2) Tully Fauna extermination and replacement by a recurrent Hamilton Fauna; and, ultimately, (3) extinction of large portions of the Hamilton Fauna and the beginning of cosmopolitan Genesee Fauna. Similar faunal patterns of incursion, recurrence, and cosmopolitanism have also been noted for other regions, albeit with somewhat different regional characteristics.;Global environmental changes during the biocrisis are recognized in the type region through reconstruction of δ18O(conodont apatite) and δ13C(carbonate) records. However, regional faunal incursion, replacement, and recurrence patterns during the first and second bioevents, as well as corresponding sedimentological observations, are best explained by basinal-scale water mass changes in response to the global environmental changes. During the third bioevent, eustatic sea-level rise was accentuated regionally by renewed Acadian tectonic activity. Quantitative paleoecological analysis demonstrates that Hamilton Fauna survivors of this bioevent were those taxa adapted to nearshore, siliciclastic-dominated settings. Persistence of these taxa was a direct result of the persistence of their preferred habitat through the biocrisis and subsequent tectonically-driven expansion of this facies. Similar, multi-disciplined studies of the Taghanic Biocrisis in other regions will increase our understanding of regional response to global change on geologic timescales.