USING STATISTICAL CLUSTERING TECHNIQUES TO DETERMINE THE FIRST OCCURRENCE DATUM OF THE PLANKTONIC FORAMINIFER ORBULINOIDES BECKMANNI AND THE BASE OF PLANKTONIC FORAMINIFERAL BIOZONE E12

Planktonic foraminiferal Biozone E12 (Middle Eocene) is defined by the first and last occurrence of Orbulinoides beckmanni. This species’ range divides what would otherwise be a 4-Myr interval without planktonic foraminiferal bioevents, into biozones E11, E12 and E13. However, O. beckmanni is the endmember of a continuously evolving lineage with many transitional specimens. Therefore, identifying the first occurrence of O. beckmanni, and the base of E12 is not straightforward.

Here, we determine the first splitting of O. beckmanni from its ancestor Globigerintheka euganea using statistical clustering methods. We compare 7 morphological traits on both species over a 1.2-Myr interval immediately preceding and including the full range of O. beckmanni to statistically determine when the species first originated. Subsequently, all younger individuals of O. beckmanni where analysed using the same technique to determine whether any distinct morphotypes exist within this morphologically variable species, that can be used as additional bioevents.

The results provide a robust age determination of the base of biozone E12 in the western Equatorial Pacific. The presented technique can be readily repeated at other sites and ocean basins to determine the global synchronicity of the base of biozone E12.