The movement of water masses in the ocean, its circulation, is an essential component of the global climate system. In a study recently published in the journal Proceedings of the National Academy of Science (PNAS), researchers were able to show that circulation in the deep ocean was significantly slowed down during the last glacial period. Analyses of sediment samples show that the decomposition of organic carbon in the water masses of the deep sea consumed the oxygen available there. Scientists from Oklahoma State University (U.S.), GEOMAR Helmholtz Centre for Ocean Research Kiel and MARUM—Center for Marine Environmental Sciences at the University of Bremen are involved in the publication.