GLOBAL BIOGEOGRAPHIC RESPONSE OF MARINE PLANKTON TO THE EOCENE-OLIGOCENE TRANSITION

Anthropogenic climate change is associated with a large suite of detrimental changes to climate, ocean circulation, and water column structure. The compound effect of these changes has the potential to significantly alter the physical structure of the ocean and its ecosystems, upon which billions of people around the globe rely on for their livelihood.

The planktonic foraminifera have the most-complete Cenozoic species-level fossil record of any organism. Therefore, their record is incredibly important to our understanding of the macroevolutionary responses of marine ecosystems to global climate perturbations of similar scale to those expected on anthropogenic timescales. Moreover, their calcareous skeletons represent functionally significant morphological and ecological data which can be used to assess changes in marine ecosystems on geological timescales.

Permanent, global-scale changes in ocean structure through geological time have shaped the biodiversity of important marine groups, including the planktonic foraminifera. Within this clade, extinctions associated with the global reorganization of ocean circulation associated during the Eocene-Oligocene Transition (EOT, ~34 Ma) represent the largest loss of biodiversity since the Cretaceous-Paleogene mass extinction event.

Here, we use the Triton dataset to reconstruct macroperforate planktonic foraminiferal occurrences from 45-25 Ma. Occurrence data were placed in temporal bins of 1 million years, and paleolatitudinal bins of 5°. All species were grouped by genera, and the loss/gain of occurrences between temporal bins was recorded to determine how the EOT effected genera spatiotemporally.

As the Earth cooled following the Middle Eocene Climatic Optimum (~40 Ma), genera with ecological affinity to warmer waters show contraction of biogeographic ranges towards the equator, whilst cool water species show biogeographic expansion, specifically within the higher mid-high latitudes. These patterns precede the significant loss of biodiversity at the EOT. With the continuation of anthropogenic climate warming and the likely associated changes in ocean structure triggered by ice sheet loss, we may expect to detect patterns antithetical to those at the EOT, with biogeographic shifts preceding a more rapid loss of global biodiversity.