LATEST DEVONIAN DEMISE OF CARBONATE FACTORIES CAUSED BY BASIN-TO-SHELF ANOXIA IN WESTERN LAURENTIA
A combined stratigraphic and geochemical study of the Hangenberg anoxic event in western Laurentia provides new insight into the cause, severity and spatial extent of the anoxic event. Sections were examined, measured and sampled across the Devonian-Carboniferous boundary from two localities: (1) Alberta shelf, western Canada, representing proximal mid-ramp and distal outer ramp depositional environments; and (2) Madison shelf in Montana, USA, representing an outer ramp depositional environment. Geochemical proxies for anoxia (Mo, Cu, Ni, V and P), abundance of pyrite less than 7 μm, and trends of organic carbon isotopes (δ13Corg) were used to examine paleo-redox condition and the trophic state in the water column.
Positive shifts of δ13Corg (+3.3‰) and δ13Ccarb (+1‰), increase in abundance of pyrite framboids with less than 7μm in diameter, and increase of Mo concentrations in all sections suggest that anoxia extended landward to at least a distal mid-ramp setting, representing a relatively shallow water environment along western North America. Euxinic conditions lead to a pyrite rain-out event that may have stripped bio-essential nutrients from oxygen-depleted seawater. These combined impacts of basin wide anoxia and loss of essential nutrients likely had a catastrophic effect on the growth of carbonate ramps in western Laurentia.