A modern-like Atlantic meridional overturning circulation (AMOC) emerged following the Eocene-Oligocene transition (EOT) about 34 million years ago, upon the initiation of large-scale Antarctic glaciation, an international research team including researchers from Chinese Academy of Sciences (the Stable Isotope Biogeochemistry Group, Institute of Earth Environment), the University of Hong Kong, and the University of Southampton have now found.

The AMOC exerts a strong control on global climate and marine ecosystems, redistributing oceanic heat and salt, interacting with the atmosphere and ventilating the ocean interior. However, the timing and cause of the inception of this system and its evolution since the start of the Cenozoic Era 65 million years ago (Ma) remain highly uncertain. 

For the new study, the international research team proposed a novel method for tracing oxygenation using microbial biomarkers. “Microbial source indicators, based on distributions of glycerol dialkyl glycerol tetraethers (GDGTs), could indicate the contribution of other archaea/bacteria relative to the ubiquitous marine Thaumarchaeota. As non-Thaumarhaeotal microbes prefer to live in anoxic environments, the relative abundance of non-Thaumarchaeota, particularly methanogenic and/or methanotrophic archaea, in geological samples, offers a means to evaluate the oxygenation status of past oceanic environments”, explained the first author  WANG Huanye from Chinese Academy of Sciences.

The team then employed the microbial source indicators to infer the evolution of early AMOC. The results show that, oxygenation of AMOC-feed waters decreased and culminate in especially poor ventilation towards the end of Eocene. AMOC-feed waters became better oxygenated at the EOT (~34 Ma) upon initiation of Antarctic glaciation, and were consistently well ventilated from ~30 Ma. “This marks the onset of more modern-like AMOC at the EOT”, said paleoceanographer LIU Zhonghui, “and the onset of modern-like AMOC is probably triggered by Antarctic glaciation”.

The timing and cause of the inception of Cenozoic AMOC would shed new light on the mechanisms driving this system. “Our work implies that both vertical mixing and Southern Ocean wind-driven upwelling are essential for sustaining modern AMOC”, saId paleoceanographer Paul A. Wilson.

This work was published in Nature Geoscience on October 16.

Contact: Bai Jie, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an, China. Email: baijie@ieecas.cn