Recently, the Geomicrobiology Team from the School of Oceanography at Shanghai Jiao Tong University published a research paper titled "Dynamics of Iron-Bound Organic Carbon Across Different Development Stages of Marine Cold Seeps" in the AGU journal Global Biogeochemical Cycles. PhD student Wenqi Ye from the School of Oceanography and Dr. Yunru Chen, a postdoctoral researcher from the Bremen University, are the co-first authors. Tenure-Track Associate Professor Longhui Deng from the School of Oceanography and Professor Qianyong Liang from the Guangzhou Marine Geological Survey are the co-corresponding authors. Other important collaborators include Professor Fengping Wang and her team members. 

This study presents the first investigation into the dynamic changes in content, source, and binding mode of iron-bound organic carbon (Fe-OC) across the non-seep, early-stage seep, and mature seep areas in the South China Sea. The "rusty carbon sink", through which reactive iron oxides (FeR) sequester organic carbon, was found to remain effective even in these highly dynamic seep ecosystems throughout their developmental stages. Key findings include:

(1) Enhanced production of Fe-OC acts as a crucial mechanism for preserving methane-derived OC in the early-stage seep ecosystem. Fe-OC contents in early-stage seep sediments are over 40% higher than those in non-seep and mature seep sediments. This enrichment is closely associated with strong invertebrate bioturbation, which intermittently introduces oxygen into the sediments, promoting the re-oxidation of Fe(II) to iron oxides and their subsequent co-precipitation with the newly generated OC (Fig. 1). Isotope-based end-member model further indicates that methane-derived OC contributes up to ~40% of the Fe-OC pool in the early-stage seep, suggesting that iron binding provides an important temporary reservoir for methane-derived OC (Fig. 2). This process helps prevent its microbial degradation or loss via fluid migration, thereby likely facilitating the maintenance and development of cold seep ecosystems.

Fig. 1. Conceptual diagram illustrating the interplays between organic carbon from different sources and iron oxides across different development stages of marine cold seep.

(2) Marine-origin Fe-OC are major contributor to long-term OC sequestration even in the geochemically highly dynamic cold seep ecosystems. As cold seeps succeed to mature stage, sediments gradually transit to a persistently reducing state accompanied by sulfide accumulation. During this procedure, both FeR and Fe-OC contents decreased markedly. The proportion of methane-derived OC in Fe-OC pool dropped to 17 ± 7% (Fig. 2). The residual Fe-OC pool is predominantly composed of marine-origin OC with significantly higher OC/Fe molar ratios, suggesting it persists mainly in the form of co-precipitation. This shift underscores the stage-dependent role of Fe-OC: it acts as an important temporary reservoir for methane-derived OC in the early-stage seep sediments, and transforms into a key mechanism for the long-term preservation of marine-origin OC in the mature seeps.

Fig. 2. Potential sources of organic carbon to form the Fe and OC associations. (a) Differences between δ13CFe‐OC and δ13CΤOC for evaluating the preferential binding of Fe with methane‐derived carbon. (b) Three end‐member mixing model illustrating the relative contributions of marine OC, terrestrial OC, and methane‐derived carbon to Fe‐OC.

(3) Marine cold seep sediments sustain high Fe-OC contents compared with other deep-sea sedimentary habitats. Both the content of Fe-OC (0.15% ± 0.05%) and its proportion to total organic carbon (14% ± 5%) in cold seep sediments do not differ significantly from values reported in other deep-sea sedimentary environments (Fig. 3). It demonstrates that the "rusty carbon sink" mechanism remains effective even across the dynamic geochemical and redox regimes that characterize different seep developmental stages. Extrapolation of our data suggests that the long-term burial of Fe-OC in global cold seep sediments (top 0–30 cm) amounts to 142–218 Tg C, which is on the same order of magnitude as the annual OC burial flux in marine sediments (200–400 Tg C), underscoring the quantitative significance of iron-associated carbon sequestration in cold seep systems.

Fig. 3. Box plots showing the values of fFe‐OC, Fe‐OC, TOC, FeR, and OC:Fe in the upper 30 cm sediments of different marine environments. Data points from this study are shown as red dots.

This research was supported by the National Natural Science Foundation of China (Grants 42276087, 42306057, 42576121, 42230401), the Shanghai Science and Technology Commission "Science and Technology Innovation Action Plan" projects (23ZR1428500, 24HC2810200), the China Geological Survey project (DD20230065), and the UN "Ocean Decade" Programme Global Seafloor Ecosystem and Sustainability (GSES) and Ocean Negative Carbon Emissions (ONCE).

Citation:

Ye, W., Chen, Y., Yang, C. et al. (2026). Dynamics of iron-bound organic carbon across different development stages of marine cold seeps. Global Biogeochemical Cycles, 40, e2025GB008889. 

Link:

https://doi.org/10.1029/2025GB008889

Contributed by: Geomicrobiology Team