UC Riverside

Rapid changes in Earth’s marine environment are sufficiently rare that the scope and consequences of these events cannot be inventoried without recourse to the stratigraphic record of deep pre-historic time. Traditional stratigraphy divides geologic time into a succession of coarse biozones – uneven time bins, each recognized by distinctive index fossils and spanning several hundred thousand to even a few million years. This thesis uses the CONOP software to resolve sequences of global appearance and disappearance events at the species level across the late Cenomanian oceanic anoxic event (OAE 2) at a much finer resolution than traditional biostratigraphy. The resulting time-lines include information for the Aptian to Maastrichtian from 501 locations, 4,962 taxa, and 25,112 local last occurrences. This dataset is large enough to represent the global environment and to compensate for the inevitably patchy and incomplete record preserved at any one location. Six geographically widespread clades – ammonites, calcareous nannofossils, dinocysts, benthic forams, planktonic forams, and radiolaria – were analyzed; they span a range of trophic levels, composition and preferred habitat.

Extinction pulses and intervals of falling diversity are not unique to OAE 2 or even uniquely severe. Several occur throughout the Albian-Santonian interval. The key is to monitor trends in diversification rate. OAE 2 is distinguished as a boundary between more positive (or equal in the ammonite case) and more negative diversification regimes. It may have been part of a general habitat deterioration or disturbance, but it would be too simplistic to describe it as a coordinated pelagic extinction event.