The effects of diagenetic processes and fluid migration on rare earth element and organic matter distribution in seep-related sediments: A case study from the South China Sea

Abstract

Methane seepage is an important factor governing microbial activity in marine sediments, although its impact on the distribution of rare-earth elements (REEs) and organic matter in sediments is unclear. Here, we report isotope signatures of organic (δ13CTOC) and inorganic (δ13CTIC) carbon as well as total REEs contents of core sediments collected from a well-characterized seep area on the northern South China Sea. Previous studies have identified sediments corresponding to three methane-release events (MREs) in the studied core: MRE I at 4.0–5.5 m, MRE II at 7.4–8.2 m, and MRE III at 10.0–11.5 m depth intervals. Increased methane flux and reducing bottom waters during MRE I resulted in a higher accumulation of marine-sourced organic matter, as indicated by lower C/N ratios and higher δ13CTOC values compared with those of MREs II and III. Sediments deposited during MRE I have relatively low total REE contents, which may be related to reduced REE adsorption on Mn oxyhydroxides. However, the absence of Ce anomaly at MRE I is more likely due to the detrital input that obscures the original Ce anomaly. In the MRE III sediments, a weak negative Eu anomaly relative to that in the MRE I and II may be attributed to the reduction of Eu3+ to Eu2+ under strongly reducing conditions that caused by intense methane seepage. These observations demonstrate that fluid flow may have an important effect on the distribution of both organic matter and REEs in methane-rich marine sediments.