A HIGH-RESOLUTION GLOBAL TIME-LINE OF BIOLOGICAL AND GEOCHEMICAL EVENTS THROUGH THE END-CENOMANIAN OCEANIC ANOXIC EVENT (OAE 2)

To resolve the sequence of biotic and geochemical events during intervals of rapid global change it is prudent to combine information from as many localities as possible. Individual stratigraphic sections and drill cores are incomplete and parochial accounts of events. Global scope requires records from many regions; using several localities in each region mitigates incompleteness. For OAE2, we have already compiled local records of 3,214 taxon range ends and 48 other events from 53 locations – a total of 8,996 local event observations, leading to 340,702 pairwise observations of superposition (one event proven to occur below another) that need to be honored in the final time-line. The taxa include ammonites, coccoliths, foraminifera, dinoflagellates, ostracods and radiolaria. The other events are uncertainty intervals on radioisotopic ages and segments of geochemical trends for stable isotopes (¹⁸C, ³⁴S) and paleoredox elements (Mo, V). As a buffer against deterioration near the ends of a timeline, the OAE2 information is nested within an Albian-Maastrichtian timeline based upon 15,205 local event records for 1,213 ammonite taxa known from at least two of 562 localities.

All this information must be correlated with better resolving power than traditional biozones. Neither geochemical signals nor taxon range ends may drive the correlation exclusively. Instead, we use the principles of graphic correlation to find sequences of events that best fit all the field data. To overcome the huge volume of information we use an automated constrained optimization (CONOP). Locally observed taxon ranges may be stretched to fit a common timeline – standard procedure in graphic correlation to recognize that true first and last occurrences most likely lie outside the locally observed range. For the other events, we developed a new data class: uncertainty intervals that may be shrunk to fit; this new procedure allows us to make initially conservative statements about, for example, the positions of peaks in stable isotope excursions. The timeline indicates that OAE2 is a time of high turnover; local extirpation and emigration may be more frequent than real extinction. The use of uncertainty intervals becomes crucial for resolving whether peaks in the waning portion of the carbon isotope excursion persist into the Turonian.