Biomarker-based paleoclimate reconstruction during the mid-miocene climatic optimum

Abstract

Cenozoic climate witnessed a step-wise cooling process interrupted by several warming periods. As the most warming interval in the Neogene, the Mid-Miocene Climate Optimum (MMCO) from 17 to ~14.7 million years ago (Ma) is commonly regarded as an analogue for future climate change. However, paleoclimate records centered on this interval are very limited. In this study, 4 sites from global ocean were selected to reconstruct temperature and oceanic changes using biomarkers, attempting to provide further constraints on global ocean conditions during the MMCO. Reconstructed temperature records in the Arctic Ocean displayed an apparent cooling trend from ~10°C to ~3°C during the Mid-Miocene, despite some ambiguity in chronology. Around 12 Ma, sea surface temperature (SST) at the Arctic Coring Expedition (ACEX) site had already decreased to a relative low value (~3°C), but simultaneous SST at Ocean Drilling Program (ODP) Site 907 stayed at a warm level (~20°C), suggesting that central Arctic Ocean SST already declined significantly at ~12 Ma, accompanied by a large SST gradient existence between central Arctic Ocean and eastern Greenland. Meridional SST gradient during the MMCO was obtained by compiling the results reported in this research and other published records. The results showed a smaller SST gradient than that during the Pliocene and modern period, with tropical warmth level extending to the mid-latitudes, suggesting substantial warmth during the MMCO. The compiled SST results from the present study provide a new benchmark for model simulations during the MMCO. Glycerol dialkyl glycerol tetraether (GDGT)-based records from the ACEX site, Sites 1208A, U1352C and U1405A were also presented. The results suggest enhanced stratification at Sites 1208A and U1405A, and reduced stratification at Site U1352C during the MMCO, revealed by the relative changes of BIT values and other GDGT-based proxies. This exploration provides another perspective for deciphering oceanic conditions during the MMCO. Finally, a composite SST record covering the past 40 million years at a moderate resolution was obtained from the mid-latitudes of the Northern Hemisphere. SST reached ~29°C in the Mid-Eocene, subsequently decreased to ~24°C at ~32 Ma, followed by high-amplitude oscillations from 32 Ma to 18 Ma. A moderate increase by ~2°C could be identified when entering the MMCO. A dynamic warming phase during the Middle Miocene was revealed by SST records from Sites 1208A and U1405A, followed by substantial cooling during the Late Miocene. At Site U1405A, Late-Miocene SST declined by ~6°C, consistent with existing results, implying the large-scale impact of transient glaciation in the Northern Hemisphere. Based on the results generated from this study and existing ones, the MMCO is characterized by exceptional warmth with tropical waters extending to the mid-latitudes and perhaps even high latitudes, and thus a much reduced meridional SST gradient, together with diverse ocean water column stratification status in the mid-latitudes. The generated results above provide fundamental insight into the basic oceanic conditions during the MMCO.