Stable Middle Miocene Seawater Isotopes in the Eastern North Atlantic Ocean

The Middle Miocene is characterized by a long-term increase in the stable oxygen isotopic composition of benthic foraminifera (δ¹⁸O_benthic). However, it is unclear to what extent this increase reflects changes in seawater isotopic composition or deep water temperature. We present a high-resolution alkenone hydrogen isotope (δ²H_C37) record of the Middle Miocene from a core taken at the upper slope edge (about 409 m water depth) of the Porcupine Basin continental margin in the eastern North Atlantic Ocean, Site U1318 of the Integrated Ocean Drilling Program. The δ²H_C37 values vary between −174 and −200‰ with an average of −191 ± 5‰, similar to modern open-ocean values. Importantly, they do not show a long-term increase in surface seawater isotopes (δ²H_SSW) during the Middle Miocene Climate Transition. Indeed, when δ¹⁸O_benthic is corrected for subsurface temperature, the bottom seawater oxygen isotopes (δ¹⁸O_BSW) show no significant increase in this period. When the latter record is translated into the hydrogen isotopic composition of bottom seawater using the modern open-ocean waterline, it has an average value of 5.8 ± 1.5‰, similar to the δ²H_SSW of 5.2 ± 3.1‰ derived from δ²H_C37:2, suggesting a relatively small difference between bottom and surface waters. Our results suggest a stable global surface seawater isotope evolution during the Middle Miocene, coupled with a long-term decrease in bottom water temperature.