| Summary: | The Middle Miocene is characterized by a long-term increase in the oxygen isotopes of benthic foraminifera (δO). However, it is unclear to what extent this increase reflects changes in seawater isotopes or deep water temperature. We present a high-resolution alkenone hydrogen isotope (δH) 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 δH values vary between -174 to -200‰ with an average of -191 ±5‰, similar to modern open-ocean values. Importantly, it does not show a long-term increase in surface seawater isotopes (δH) during the Middle Miocene Climate Transition. Indeed, when δO is corrected for subsurface temperature, the average bottom seawater oxygen isotopes of 0.9 ±0.2‰ also show no significant increase. When the latter record is translated into hydrogen isotopes of bottom seawater using the modern open-ocean waterline, it shows an average value of 5.8 ±1.5‰ similar to the δH of 5.2 ±3.1‰ derived from δH suggesting 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.
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