The understanding of how organic carbon is preserved in marine sediments has been a longstanding question in the study of Earth’s long-term carbon cycles. Recently, researchers have made progress in understanding the complex cycling of iron-bound organic carbon in sediments beneath the ocean floor.
Over geological time, the rate at which sedimentary organic carbon is buried plays a significant role in determining levels of atmospheric oxygen and carbon dioxide, ultimately affecting Earth’s environmental conditions. In marine sediments, approximately 20 percent of organic carbon is directly associated with reactive iron oxides (FeR). However, the ultimate fate of reactive iron-bound organic carbon (FeR-OC) in these deeper sediments, as well as its accessibility to microorganisms, remains unclear.
To investigate this, the research team analyzed continuous records of FeR-OC from two sediment cores taken from the northern South China Sea. These cores span several biogeochemical zones, from suboxic to methanic, and date back nearly 100,000 years.
The findings indicate that in areas of the sulfate-methane transition zone (SMTZ), where microbial activity is high, FeR-OC is released during iron reduction processes and is subsequently broken down by microorganisms. The energy generated from this process supports a significant amount of microbial life in this approximately one-meter-thick zone.
Apart from the SMTZ, a consistent portion of the total organic carbon survives microbial breakdown as FeR-OC, remaining stored in marine sediments over long geological periods. “This suggests,” explains Dr. Yunru Chen, the lead author of the study and a postdoctoral researcher at the Cluster of Excellence ‘The Ocean Floor – Uncharted Interface of the Earth’, “that the global reservoir of FeR-OC in microbially active marine sediments from the Quaternary period could be 18 to 45 times larger than the current atmospheric carbon pool.”
This research marks an important advancement in our understanding of the stability of sedimentary FeR-OC in relation to microbial activity after deposition, highlighting its cycling and persistence in the sediments below the ocean floor. The findings will be integrated into the “Ocean Floor” Cluster of Excellence, managed by MARUM.
