Stable Middle Miocene Seawater Isotopes in the Eastern North Atlantic Ocean

Abstract

The Middle Miocene is characterized by a long‐term increase in the stable oxygen isotopic composition of benthic foraminifera (δ18Obenthic). 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 (δ2HC37) 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 δ2HC37 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 (δ2HSSW) during the Middle Miocene Climate Transition. Indeed, when δ18Obenthic is corrected for subsurface temperature, the bottom seawater oxygen isotopes (δ18OBSW) 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 δ2HSSW of 5.2 ± 3.1‰ derived from δ2HC37:2, 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.