GEOCHEMISTRY OF LATE DEVONIAN OCEANS

The Middle and Late Devonian are characterized by development of vast carbonate platforms with extensive reef constructions. The decline of the Devonian reef ecosystem in the latest Frasnian represents one of the most severe reef crisis in Earth history. The ultimate cause of this mass extinction event is unclear. Anoxic conditions, climatic changes, and comet showers have been proposed as forcing mechanisms. Isotope and trace element geochemistry, as well as organic biomarkers, indicate that anoxic conditions prevailed episodically in the deeper water column of Frasnian oceans. Enhanced deposition of organic material is evidenced by positive excursions in inorganic and organic carbon isotopes (Kellwasser horizons). A prominent excursion in the sulfur isotope composition of sulphide is observed across the Frasnian-Famennian boundary and points to increased sulfate reduction as consequence of a higher input of organic carbon. Short-term cooling events were deduced from high resolution oxygen isotope records of conodont apatite. The d¹⁸O excursions parallel the positive excursions in d¹³C of carbonate and suggest that enhanced organic carbon burial had a significant effect on atmospheric CO₂ concentration and climate. The long-term d¹⁸O record indicates moderately warm tropical sea- surface temperatures for the Givetian. Climatic warming is observed during the early Frasnian with surface water temperatures reaching >30° C during the middle Frasnian to middle Famennian. These warm tropical surface water temperatures are interpreted to result in a sluggish oceanic circulation that may have contributed to low dissolved oxygen contents in the water column. The ⁸⁷Sr/⁸⁶Sr record reveals a prominent increase starting in the late Givetian and reaching a maximum in the late Famennian. The gradual increase to more radiogenic values is interpreted to reflect a higher input of continent derived material. Enhanced nutrient delivery from the continents is another factor that may have stimulated primary productivity, export of organic carbon from the photic zone and initiated anoxic conditions in the lower water column. Available data suggest that late Devonian oceans experienced major changes in ocean geochemistry that may have affected marine ecosystems during the Frasnian-Famennian crisis.