PRELIMINARY HIGH-RESOLUTION BIOLOGICAL AND GEOCHEMICAL TIMELINES THROUGH THE CENOMANIAN-TURONIAN (LATE CRETACEOUS) OCEANIC ANOXIC EVENT (OAE2)

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 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 a few million years. Using heuristic optimization we more fully resolve sequences of global appearance and disappearance events at the species level across the late Cenomanian oceanic anoxic event (OAE2). The resulting timelines include information from several hundred locations to represent the global environment and to compensate for the inevitably patchy and incomplete record preserved at any one location. Six geographically widespread clades were analyzed using 48,226 local range ends for 4,962 Albian-Campanian taxa; they span a range of trophic levels, composition and preferred habitat.

Extinction pulses and intervals of falling diversity are not unique to OAE2 or even uniquely severe. Several occur throughout the Albian-Santonian interval. A unifying OAE 2 signature is seen in net diversification rate. OAE2 is distinguished as an interval of reduced diversification rate in all six clades whether diversification had been increasing or decreasing in the time prior to OAE2 and whether or not the OAE2 interval is characterized by positive or negative diversification rates. Ammonites, benthic forams, and radiolaria approached OAE2 with increasing diversification. Diversification rates for calcareous nannofossils, dinoflagellates, and planktic forams were already falling prior to OAE2. For ammonites, radiolaria and planktic forams, diversification rates never fell to negative values during OAE2. For calcareous nannofossils and dinoflagellates OAE2 was a time of negative diversification. For benthic forams, diversification rates fluctuated on either side of zero. OAE2 was not conducive to increasing diversification rates in any of the pelagic clades examined, although some maintained a net diversification. It may have been part of a general habitat deterioration or disturbance, but it would be too simplistic to describe it as one coordinated pelagic extinction event.