1 Scientific RepoRts | 7: 14475 | DOI:10.1038/s41598-017-13821-2 www.nature.com/scientificreports Enhanced Arctic Amplifcation Began at the Mid-Brunhes event ~400,000 years ago t. M. Cronin 1 , G. S. Dwyer 2 , E. K. Caverly 1 , J. Farmer 3,4 , L. H. DeNinno 1 , J. Rodriguez-Lazaro 5 & L. Gemery 1 Arctic Ocean temperatures infuence ecosystems, sea ice, species diversity, biogeochemical cycling, seafoor methane stability, deep-sea circulation, and CO 2 cycling. Today’s Arctic Ocean and surrounding regions are undergoing climatic changes often attributed to “Arctic amplifcation” – that is, amplifed warming in Arctic regions due to sea-ice loss and other processes, relative to global mean temperature. However, the long-term evolution of Arctic amplifcation is poorly constrained due to lack of continuous sediment proxy records of Arctic Ocean temperature, sea ice cover and circulation. Here we present reconstructions of Arctic Ocean intermediate depth water (AIW) temperatures and sea-ice cover spanning the last ~ 1.5 million years (Ma) of orbitally-paced glacial/interglacial cycles (GIC). Using Mg/Ca paleothermometry of the ostracode Krithe and sea-ice planktic and benthic indicator species, we suggest that the Mid-Brunhes Event (MBE), a major climate transition ~ 400–350 ka, involved fundamental changes in AIW temperature and sea-ice variability. Enhanced Arctic amplifcation at the MBE suggests a major climate threshold was reached at ~ 400 ka involving Atlantic Meridional Overturning Circulation (AMOC), infowing warm Atlantic Layer water, ice sheet, sea-ice and ice-shelf feedbacks, and sensitivity to higher post-MBE interglacial CO 2 concentrations. Arctic amplifcation involves changes in albedo, ocean-atmosphere heat exchange, sea ice and other factors 1,2 expected from climate model simulations due to rising greenhouse gas concentrations. An amplifed climate response in the Arctic to past climate changes is consistent with paleoclimate evidence for cold temperatures during glacials 3 and high temperatures during warm periods such as the early Holocene, Last Interglacial 4 and the Late Pliocene 5 . To examine the development of Arctic amplifcation, this study examines the 1.5 million year history of intermediate depth ocean temperature, sea ice and marine productivity in the central Arctic Ocean using sediment core paleoceanographic records. Today’s central Arctic Ocean is characterized by a shallow halocline that separates the Polar Surface Layer and its perennial sea-ice from the warm Atlantic Water (200–500 m), which fows into central basins from the North Atlantic via the Fram Strait and Barents Sea. Variability in its strength has been linked to changes in the underlying AIW 6 and the growth and decay of ice shelves 7 . Moreover, radionuclide proxy data indicate contin- uous deep-water exchange between the Arctic Ocean, the Nordic Seas and the North Atlantic Ocean over the last 35 ka 8 . To understand the changes in ocean circulation during the MBE, we reconstructed Arctic Ocean temperatures from benthic ostracode magnesium/calcium ratios (Mg/Ca), a tool for Arctic marine paleother- mometry (Extended Data), and sea ice history from abundance patterns of ostracode species from 7 sediment cores from the western and central Arctic Ocean (Fig. 1). Both reconstructions mainly record interglacial and interstadial intervals, which are refected in the abundances of calcareous microfossils 9 . Our new paleoceano- graphic records extend those of the last 50 ka 6,10 back to ~1.5 Ma and reveal a fundamental shif in Arctic Ocean temperature, surface productivity and sea-ice cover variability at the MBE (Appendix 1). Te age models for stud- ied cores, which were based on radiocarbon ages in the upper parts of cores and orbitally tuned tiepoints based 1 Eastern Geology and Paleoclimate Science Center, MS 926 A US Geological Survey Reston, Virginia, 20192, USA. 2 Division of Earth and Ocean Sciences, Nicholas School of the Environment, Duke University, Durham, North Carolina, USA. 3 Lamont-Doherty Earth Observatory, Palisades, New York, USA. 4 Present address: Princeton University Department of Geosciences, Princeton, New Jersey, USA. 5 Dept. Estratigrafía y Paleontología, Facultad de Ciencia y Tecnología, Univ. País Vasco, UPV/EHU, Apartado 644, 48080, Bilbao, Spain. T.M. Cronin and G.S. Dwyer contributed equally to this work. Correspondence and requests for materials should be addressed to T.M.C. (email: tcronin@usgs.gov) or G.S.D. (email: gary.dwyer@duke.edu) Received: 16 March 2017 Accepted: 2 October 2017 Published: xx xx xxxx opeN