Alkenone-derived Mediterranean SST during the Serravallian to Messinian Stages of the late Miocene (6.3-12.9Ma) reveals cooling prior to northern hemisphere glaciation

The Miocene epoch is a time period of our planet’s history characterized by climates warmer than those of today and unipolar glaciation. We present the results of an alkenone paleotemperature proxy reconstruction of Mediterranean sea surface temperature (SST) between the Serravallian and Messinian stages of the late Miocene (~6.3-12.9Ma). The geographical position of the basin in addition to its relative isolation from the global ocean make the climate record obtained from it extremely valuable - having regional, global and orbital forcing components. We extend a successful outcrop-based alkenone approach to the well-dated Monte dei Corvi beach section in Northern Italy. The paleotemperature record we have generated shows that during the late Miocene the Mediterranean SSTs were significantly higher than those of today; not falling below 20.6°C and at times above 28.2°C, compared to the modern SST of 19.5°C. Very warm Mediterranean temperatures were validated by applying the Tex86 paleothermometer to the same samples. Our record shows that late Miocene Mediterranean climate was characterized by a prolonged period of elevated SST before a sustained cooling began at at ~8.1 Ma. At that time we observe a dramatic temperature drop of ~5 °C occurring over a period of approximately 1 million years. Following the cooling the remainder of the record oscillates around 25.5°C by ~1°C. We believe that the large step cooling shown by the alkenones is indicative of a significant northern hemisphere cooling preceding the development of major northern hemisphere glacial cycles at ~2.7 Ma, as indicated by the global marine δ18O record - possibly because this cooling did not result in significant enough ice accumulation to register in global δ18O values . Notably, the big cooling step we observe in the SSTs also coincides with the terrestrial C3/C4 vegetation transition placed between 8-6 Ma. We consider the contemporaneous SST drop and the C3/C4 transition as an indication that the data shows not only northern hemisphere cooling but a broader climate reorganization as well. Given that the two pathways for photosynthesis are not equally effective under all circumstances, C4 being more favorable at lower CO2 levels and more arid conditions, we hypothesize that between 6 and 8 Ma climate conditions in the Miocene world changed to colder and possibly dryer, perhaps connected to a drop in CO2. Our SST reconstruction provides constructive information regarding steps leading to Northern Hemisphere glaciation - the beginning of the climate conditions of the world we inhabit today. The data agree with previously documented North Atlantic ice rafted debris from the same time period showing evidence of short-lived glaciation prior to 2.7 Ma. The Mediterranean, and by inference, the Eurasian and northern African climates, underwent a significant change between 8 and 6 Ma, solidifying this time period as pivotal in the progression of our planet’s climate history.