The Middle Miocene is characterized by a long-term increase in the oxygen isotopes of benthic foraminifera (δ18Obenthic). 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 (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 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 (δ2HSSW) during the Middle Miocene Climate Transition. Indeed, when δ18Obenthic 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 δ2HSSW of 5.2 ±3.1‰ derived from δ2HC37 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.

The long-chain diol index (LDI) is a relatively new proxy for sea surface temperature (SST) which has been rarely applied in upwelling regions. Here, we evaluated its application by comparison with other SST records obtained by commonly used proxies, i.e. the Mg/Ca ratio of the planktonic foraminifera species 37 and Mg/CaG.ruber SST records revealed an increase of 0.6-3.4°C from the Last Glacial Maximum to the late Holocene with somewhat higher amplitude in the northern part of the Arabian Sea than compared to the western part. In contrast, the LDI SSTs did not reveal major changes during the last glacial-interglacial transition which was followed by a decreasing trend during the Holocene. The LGM vs the Holocene LDI SSTs ranged between -0.2 and -2.7°C. Particularly at one record, offshore Oman, the SST decrease from during the Holocene was high in amplitude, suggesting a potential cold bias, possibly related to changes in upwelling intensity during the Holocene. This indicates that care has to be taken when applying the LDI for annual mean SST reconstruction in upwelling regions.