EVIDENCE FOR A SAND TIGER SHARK NURSERY IN THE EOCENE ARCTIC OCEAN

Temperatures are rising at an unprecedented rate in the Arctic, but the effects on marine vertebrates are not well understood. A deep time analogue often used to understand and predict the potential impacts of future global warming is the early – middle Eocene Greenhouse (ca. 53-38 Ma). Although the Eocene Arctic terrestrial biota is well known, marine records are rare and largely based on one central Arctic Ocean site at Lomonosov Ridge. We reconstructed paleoenvironmental conditions of a marine vertebrate – sand tiger sharks (Striatolamia macrota and Carcharias sp.) - based on oxygen isotope analysis of 30 teeth from the early-middle Eocene Cyclic Member of the Eureka Sound Formation on northern Banks Island, Canada’s westernmost Arctic island (~76˚N paleolat.). We estimated the mean paleosalinity for the coastal waters inhabited by these sharks by incorporating the δ¹⁸O values of the shark teeth with estimates of Eocene Arctic paleotemperature and freshwater δ¹⁸O values into a previously published salinity model. The mean paleosalinity for western Arctic coastal waters of 12.8 PSU (range = 5–20 PSU) is substantially lower than previous estimates by others for the Eocene central Arctic Ocean (21–25 PSU) and today’s Arctic Ocean (32–35 PSU). This unusually low paleosalinity, when considered alongside the coastal deltaic depositional environment (inferred from sedimentology), an overwhelming dominance of sand tiger sharks in the fossil assemblage (>99% of teeth), and their small body size (inferred from tooth height), suggests that coastal waters of the western Arctic Ocean could have served as a sand tiger shark nursery. A plausible modern analogue is Carcharias taurus, which seasonally migrates to a nursery in the Delaware Bay, which has a reduced salinity. Surprisingly, the largest sand tiger shark teeth had some of the lowest δ¹⁸O values. Though rare, these large teeth are likely from subadult and adult sharks (22mm crown height ~ 250–280cm total length), which, based on extant C. taurus, should migrate the farthest distances. The very low paleosalinity estimated from even the largest shark teeth in the Banks Island assemblage corroborates earlier studies that suggest the Eocene Arctic Ocean had a much lower salinity than today.