Yunru Chen, Liang Dong, Weikang Sui, Mingyang Niu, Xingqian Cui, Kai-Uwe Hinrichs, Fengping Wang

Reactive iron (FeR) has been suggested to serve as a semi-persistent sink of organic carbon (OC) in surface marine sediments, where approximately 10-20% of total OC (TOC) is associated with FeR (FeR-OC). However, the persistence of FeR-OC on geological timescales remains poorly constrained. Here, we retrieved FeR-OC records in two long sediment cores of the northern South China Sea spanning almost the past 100 kyrs. Most prominently, the downcore marine-sourced FeR-OC contributes a relatively stable proportion of 13.3±3.2% to TOC. However, distinctly lower values of less than 5% of TOC, accompanied by notable 13C depletion of FeR-OC are observed in the sulfate-methane transition zone (SMTZ), where active sulfate reduction and potential iron reduction are indicated by microbial composition and geochemical modelling. FeR-OC is suggested to be remobilized by microbially mediated reductive dissolution of FeR (and might be substituted with freshly formed 13C-depleted OC). The global budget of FeR-OC in microbially active Quaternary marine sediments could be as large as 219±133×10^17 g, which is 18-48 times the size of the atmospheric carbon pool. Thus, sedimentary FeR-OC as an exchangeable reservoir of labile OC may support deep life and play a role in regulating Earth’s carbon cycle over geological timescales.