Bihemispheric warming in the Miocene Climatic Optimum as seen from the Danish North Sea

The Miocene epoch witnessed major changes in climate. The marine oxygen isotope record, our best single continuous representation of the time interval, contains large shifts indicating substantial changes in the glaciation of Antarctica and/or deep ocean temperatures, with the interval of the most depleted isotopic composition occurring between ~18 and 13.9 Ma (mid-Miocene Climatic Optimum [MCO]). We present here a record of alkenone unsaturation estimates from a borehole penetrating marine sediments spanning most of the Miocene at ~56°N. Marine temperatures reconstructed for the Miocene are up to 20°C warmer than present day. A maximum occurs at the time of the MCO. The record is strikingly similar to discontinuous alkenone results published from other North Atlantic sites. As in the southern hemisphere, ocean temperature gradients were extraordinarily low in the North Atlantic in comparison to today. Our findings suggest that the MCO was a global temperature maximum and link the partial deglaciation of the Antarctic and subsequent “refrigeration” to a global forcing process. However, the small magnitude of the ocean cooling from ~14 to 10 Ma suggests a highly nonlinear (threshold) relationship between East Antarctic ice volume and global temperature.