The occurrence of organic-rich mudrocks overlying carbonate deposits in the rock record has typically been ascribed to changes in relative sea level. We propose, alternatively, that progressive eutrophication of the euphotic zone caused the ecological demise of a Late Devonian marine carbonate ramp system in Alberta and its subsequent burial in organic-rich mudrocks. Similar lithologic transitions elsewhere in the stratigraphic record may also reflect eutrophication rather than major shifts in sea level. The transition from Upper Devonian carbonates to Devonian-Carboniferous black mudrocks in Alberta is characterized by a gradual decrease in net carbonate accumulation as indicated by (1) an upward increase in mud, phosphate, and glauconite, (2) the existence of firmground surfaces, and (3) a change from a dominantly phototrophic to heterotrophic benthic assemblage. An upward trend to lower bulk δ13Corganic and δ15N values reflects the eutrophication of surface waters by upwelling of nutrient-laden deeper waters. Resultant primary production propelled higher organic-matter flux to the sea floor, caused the demise of carbonate-producing benthic organisms, and promoted burial of carbonate deposits by organic-rich mudrocks.

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