Organic geochemical investigation of sediments in the Japan Sea: Tracking paleoceanographic and paleoclimatic changes since the mid-Miocene

The mid- to late Miocene marks one of the last warm periods of the Neogene, before the descent in to the ice house climate of the late Pliocene and Pleistocene. The mid- to late Miocene climate was long overlooked, but receives increasingly more interest. This study used organic geochemical biomarker distributions and their isotopic signatures in sediment samples from the Japan Sea, recovered during IODP Expedition 346 and ODP Leg 127/128. It was the aim to reconstruct paleoceanographic and paleoclimatic changes over the last 18 Ma. Reconstructed sea surface temperatures revealed cooling temperatures by ca 10C since the Mid-Miocene Climatic Optimum. A pronounced decline is found during the late Miocene cooling. Reconstructed pCO2 using alkenone paleobarometry, revealed a late Miocene decline from circa 520 µatm to 380 µatm. Stable isotope analysis of n-alkanes revealed an intensified summer monsoon from 5-3 Ma, and a 15% increase in C4 plants over the late Miocene. Over the last 2.5 Ma, the Japan Sea appears to receive n-alkanes from the High Arctic as well as the Eurasian continental margin. Overall, the Japan Sea is a suitable study area for the late Miocene climate change.