INVESTIGATING HABITAT USE OF SHARKS AND THEIR RELATIVES IN SEYMOUR ISLAND, ANTARCTICA, ACROSS THE EOCENE EPOCH

The Southern Ocean is a crucial area for studying the changes that triggered glaciation at the close of the Eocene Epoch. One hypothesis suggests that the opening of oceanic gateways around Antarctica started this climatic transition. However, ambiguous proxy data near the remote Drake Passage complicate an understanding of when it fully opened and how that affected global climate. This climatic-tectonic transition is likely to have impacted marine vertebrates in high-latitude regions. Study of the occurrence and paleoecology in the time leading up to glaciation should help shed light on the dynamics of the opening passage, yet few studies exist.

Here, we report phosphate oxygen stable isotope values from the teeth of pelagic and benthic sharks (δ¹⁸O_p; n = 243) from the Eocene deposits (TELM 2 to 7, middle to late Eocene) on Seymour Island (Antarctic Peninsula). We compare δ¹⁸O_p values of sharks with carbonate isotope data from co-occurring bivalves and isotope-enabled climate model outputs (iCESM; δ¹⁸O_p^*) for the Eocene. Bulk δ¹⁸O_p values exhibit high variability within TELMs (mean δ¹⁸O_p = 22.0 ± 1.3‰) and no significant changes across TELMs. Unlike bivalves, the variation of shark δ¹⁸O_p values is larger than the predicted seasonal range for δ¹⁸O_p^* values based on the iCESM model for Seymour Island. Similarly, the estimated δ¹⁸O_p* distribution between South America and Antarctica shows fair agreement with empirical δ¹⁸O_p values from fossil shark teeth. Pelagic (n = 149) and benthic (n = 94) taxa exhibit similar values across TELMs, although shifts relative to predicted δ¹⁸O_p* values from bivalves and model for Seymour Island suggest movement to other areas.

The stability in mean δ¹⁸O_p values across TELMs suggests that sharks generally sought habitats with their preferred environmental conditions. In pelagic taxa, lower or higher δ¹⁸O_p values than predicted δ¹⁸O_p* indicate movements to shallow or deeper waters, respectively. Benthic taxa have either lower, higher, or aligned δ¹⁸O_p with predicted δ¹⁸O_p^*, indicating warmer waters and likely movements to low latitudes, colder temperatures and likely movements to deeper waters, or more sedentary habits, respectively. Our results provide insights into shark habitat use and highlight how climate model outputs can be coupled with empirical isotopic data to discern shark paleoecology during periods of global climate change.