On geological time scales, the rate of deposition of organic carbon in sediments exerts a major influence on the concentration of oxygen and carbon dioxide in the atmosphere and thus significantly affects the environmental conditions of the Earth. In marine deposits, about 20 percent of the organic carbon is directly bound to reactive iron oxides (Fe_R). The fate of the reactive iron-bound organic carbon (Fe_R-OC) in subseafloor sediments and its availability to microorganisms are still undetermined.

To investigate this, the team reconstructed continuous Fe_R-OC records in two sediment cores from the northern South China Sea, spanning the suboxic to methanic biogeochemical zones and reaching a maximum age of about 100,000 years.

The study shows that in the sulfate-methane transition zone (SMTZ) with high microbial activities, Fe_R-OC is remobilized during microbially mediated iron reduction processes and subsequently remineralized by microorganisms. The energy generated can support a significant portion of the microbial life in the approximately one meter thick SMTZ.

With the exception of the SMTZ, a relatively stable fraction of the total organic carbon survives the microbial degradation processes as Fe_R-OC and is stored in marine deposits over geological time periods. “This means,” says Dr. Yunru Chen, lead author of the study and now a postdoctoral fellow at the Cluster of Excellence “The Ocean Floor – Uncharted Interface of the Earth,” “that the estimated global reservoir of Fe_R-OC in microbially active Quaternary marine deposits could be 18 to 45 times larger than the atmospheric carbon pool.”

This study is a crucial step in assessing the stability of sedimentary Fe_R-OC in response to post-depositional microbial activities and sheds light on its dynamic cycles and persistence in subseafloor sediments. The results will be incorporated into the Cluster of Excellence “Ocean Floor”, which is coordinated at MARUM.

Participating institutions:

• MARUM – Center for Marine Environmental Sciences and Department of Geosciences, University of Bremen, Bremen, Germany
• Department of Geosciences, University of Bremen, Bremen, Germany
• State Key Laboratory of Microbial Metabolism, School of Life Sciences and Biotechnology, Shanghai Jiao Tong University, Shanghai, China
• Central Laboratory for Polar Ecosystems and Climate Change, Ministry of Education and School of Oceanography, Shanghai Jiao Tong University, Shanghai, China
• Guangdong (Zhuhai) Southern Laboratory of Marine Science and Technology, Zhuhai, China

MARUM MARUM provides fundamental scientific knowledge about the role of the ocean and the seabed in the Earth’s overall system. The dynamics of the ocean and the seabed significantly shape the entire Earth system through interactions of geological, physical, biological and chemical processes. This influences the climate and the global carbon cycle and creates unique biological systems. MARUM stands for basic and open-ended research in the interest of society, for the benefit of the marine environment and in line with the United Nations’ sustainable development goals. It publishes its quality-controlled scientific data and makes it freely accessible. MARUM informs the public about new findings on the marine environment and provides actionable knowledge in dialogue with society. MARUM’s cooperation with companies and industrial partners takes place in line with its goal of protecting the marine environment.