A METHOD FOR DETERMINING WHICH CLUSTERS OF LAST OCCURRENCES IN THE STRATIGRAPHIC RECORD LIKELY REPRESENT PULSES OF EXTINCTION

The Late Ordovician mass extinction (LOME) is commonly expressed as two clusters of last occurrences in the fossil record. Both clusters appear to coincide with sequence-stratigraphic surfaces generated by the glaciation and deglaciation of Gondwana. These clusters are commonly interpreted as two pulses of extinction. However, stratigraphic architecture can produce clusters of last occurrences that can be misinterpreted as extinction pulses. These clusters typically occur at abrupt changes in facies or sequence-stratigraphic surfaces. Such clusters could lead to a misunderstanding of the pattern, timing, and tempo of extinction resulting in a misunderstanding of the causes of an extinction event. Recent studies have proposed single-pulse, multi-pulse, and prolonged extinction scenarios for the LOME. Thus, there is a growing need to explore how stratigraphic architecture may affect the expression of plausible LOME scenarios.

We use a modelling approach to test how glacioeustatic changes would affect the expression of the LOME. Using the sedimentary basin model Sedflux 2.1, we simulate stratigraphic columns based on an inferred Late Ordovician sea-level curve. We combine these with a branching model of evolution and extinction, as well as water-depth preferences for each taxon, to simulate plausible LOME scenarios in an Upper Ordovician stratigraphic record.

First, we show that stratigraphically-generated clusters of last occurrences are expressed in individual stratigraphic columns in accordance with sequence-stratigraphic principles. However, compiling the data from individual columns into a basin-wide record of the extinction event does not remove or reduce these stratigraphically-generated clusters of last occurrences as proposed by previous work. To correct for this clustering of last occurrences at the basin-wide scale, we coarsen the stratigraphic resolution to the systems-tract level and remove taxa whose last occurrences coincide with a basin-wide loss of their preferred facies. This enables us to consistently and correctly identify the pattern of extinction for a variety of extinction scenarios. We are undertaking the field work to determine whether this approach will enable the identification of the extinction pattern in the Upper Ordovician stratigraphic record.