The remaining open question

On geological timescales, the burial rate of sedimentary organic carbon exerts major control on the concentrations of atmospheric oxygen and carbon dioxide and thus substantially influences Earth’s environmental conditions. In marine sediments, about 20 percent of the organic carbon is directly bound to reactive iron oxides (Fe_R). It has been shown that reactive iron bound organic carbon (Fe_R-OC) can persist even in late Paleocene to early Eocene sediments (Longman et al.2021, 2024). However, in anoxic marine sediments, Fe_R is one electron acceptor actively involved in biogeochemical processes (Liang et al., 2019, 2022). The question to what degree the Fe_R-OC reservoir interacts with sedimentary biogeochemical processes, in particular those involving redox reactions of iron and sulfur, remains unresolved.

Cycling and persistence of iron-bound organic carbon in subseafloor sediments

The new study from Chen et al. (2024) reveals that in sulfate-methane transition zone (SMTZ) with high microbial activities, the content of Fe_R-OC decreased dramatically, accompanied by notable ¹³C depletion of Fe_R-OC. These results indicate that Fe_R-OC is remobilized and quickly remineralized upon remobilization. The remobilization of Fe_R-OC could be resulted from iron reductive dissolution mediated by microbial processes in the SMTZ, e.g., dissimilatory iron reduction, abiotic iron reduction caused by sulfide produced during sulfate reduction, and iron reduction coupled with anaerobic methane oxidation.  The energy produced from the remineralized Fe_R-OC in the SMTZ is estimated to be enough to support a substantial fraction of subseafloor microbial life in this zone.

Except for in the SMTZ, a relative stable fraction of total organic carbon survives microbial processes during early diagenesis as Fe_R-OC and is sequestered in long buried marine sediments. The estimated global reservoir of Fe_R-OC in microbially active Quaternary marine sediments could be 19 to 46 times the size of atmospheric carbon pool.

Implications for Future Research

This study takes a critical step in assessing the stability of sedimentary Fe_R-OC in response to post-depositional microbial activities and sheds lights on its dynamic cycling and persistence in subseafloor sediments. This has profound implication in understanding the role of reactive iron oxides in organic carbon sequestration in marine environments not only in the past but also in a warming world with increasing iron inputs into the ocean.

References

Longman, J., Gernon, T. M., Palmer, M. R. & Manners, H. R. Tephra deposition and bonding with reactive oxides enhances burial of organic carbon in the Bering Sea. Global Biogeochemical Cycles 35, e2021GB007140 (2021).

Longman, J. et al. Production and preservation of organic carbon in sub-seafloor tephra layers. Mar. Chem. 258, 104334 (2024).

Liang, L., Wang, Y., Sivan, O. & Wang, F. Metal-dependent anaerobic methane oxidation in marine sediment: Insights from marine settings and other systems. Science China Life Sciences 62, 1287-1295 (2019).

Liang, L. et al. Iron (oxyhydr)oxides shift the methanogenic community in deep sea methanic sediment - insights from long-term high-pressure incubations. Sci. Total Environ. 848, 157590 (2022).

Chen, Y., Dong, L., Sui, W. et al. Cycling and persistence of iron-bound organic carbon in subseafloor sediments. Nat Commun 15, 6370 (2024). https://doi.org/10.1038/s41467-024-50578-5