High temperatures in the Late Cretaceous Arctic Ocean

To understand the climate dynamics of the warm, equable greenhouse world of the Late Cretaceous period, it is important to determine polar palaeotemperatures. The early palaeoceanographic history of the Arctic Ocean has, however, remained largely unknown, because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice. A shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated Upper Cretaceous organic-rich sediment from Alpha ridge1,2,3,4, a submarine elevated feature of probable oceanic origin5. A lack of carbonate in the sediments from this core has prevented the use of traditional oxygen-isotope palaeothermometry. Here we determine Arctic palaeotemperatures from these Upper Cretaceous deposits using TEX86, a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton, Crenarchaeota6. From these analyses we infer an average sea surface temperature of ∼15 °C for the Arctic Ocean about 70 million years ago7. This calibration point implies an Equator-to-pole gradient in sea surface temperatures of ∼15 °C during this interval and, by extrapolation, we suggest that polar waters were generally warmer than 20 °C during the middle Cretaceous (∼ 90 million years ago).