IODP ³ Expedition 505S “ENIGMA: ExploratioN Into a Global early Miocene Anomaly” Scientific Prospectus

Abstract. The early Miocene (23–17 Ma) is emerging as a potentially transformative time in marine ecosystems. A lack of sampling in the early Miocene, relative stability in the genus-level richness of traditionally studied marine fossils (e.g. shallow-water hard-shelled invertebrates), and no record of perturbation(s) in traditional climate proxies such as benthic foraminifera-derived carbon and oxygen isotope ratios have led to an assumption that the early Miocene was relatively uneventful climatically. Growing evidence suggests otherwise; recently published high-resolution marine microfossil, phylogenetic, and geochemical records have revealed that the early Miocene was a time of significant ecological upheaval that led to permanent changes in marine ecosystem structure and function at a scale not observed since the Cretaceous–Paleogene Mass Extinction. Reconstructed early Miocene biotic changes spanned trophic levels from primary producers to top predators, impacting diatoms, foraminifera, reef fish, and whales, alongside other marine taxa. Specifically, open-ocean sharks were heavily impacted, suffering the largest extinction event known in their 400-million-year history, corresponding with a global disruptive event in planktonic foraminifera community composition. Though a clear environmental driver for these ecological changes is not yet known, the upheaval observed in marine biota coincides with an abrupt shift in foraminifera-bound nitrogen isotopes, signifying a change in ocean oxygenation or nutrient cycling. Together, these records suggest that the potentially rapid changes in biodiversity and marine community structure during the early Miocene were profound and may have laid the foundation for the ocean ecosystem structure observed from the later Miocene to present day. In this SPARC, we will explore the biotic and environmental changes that occurred during this critical and understudied interval in Earth's history. Spanning the breadth of the Atlantic Ocean and the Caribbean Sea, we will employ a multi- and inter-disciplinary approach to refine age models, collect new geochemical proxy data, and perform Earth system model simulations to assess the basin-wide response across preserved trophic levels to early Miocene ecosystem perturbation. ENIGMA aims for a spatiotemporally complete documentation of the ecological and environmental response of an entire ocean basin to disentangle this enigmatic Earth system change.