Sediments at marine cold seep areas provide potential archives of past fluid flow, which allow insights into the evolution of past methane seepage activities. However, signals for anaerobic oxidation of methane (AOM) might be obscured in bulk sediments in cold-seep settings due to several factors, especially flood and turbidite deposition. Comprehensive inorganic data were gathered in this study to explore the availability of related records at cold seeps and to provide insights into the evolution of past methane seepage activities. Sediments collected from the site 973-4 in the Taixinan Basin on the northern slope of the South China Sea were characterized in terms of total carbon and sulfur, δ¹³C values of total organic carbon (δ¹³C_TIC), δ³⁴S values of chromium reducible sulfur (δ³⁴S_CRS), and foraminiferal oxygen and carbon isotopes. The results confirmed a strong correlation between formation of authigenic minerals and AOM. Moreover, the ³⁴S enrichments and abundant chromium reducible sulfur (CRS) contents in the authigenic sulfides in the sulfate–methane transition zone (SMTZ) within 619–900 cm below seafloor (cmbsf) reflected past high methane fluxes supported by constant methane seepages. Lithological distribution and AMS (Accelerator Mass Spectra) ¹⁴C dating of planktonic foraminifera show that the turbidite (~35.14 ka) was related to a foraminifera-rich interval (Unit II: 440-619 cmbsf) and increased carbonate productivity during the last glacial maximum (LGM). Enrichment of Mo and U was observed accompanied by low contents of nutrient metals (Al, Ti, V, Ni, Fe, Mn, and Cu) in Unit II. The foraminifera-rich interval (Unit II) of cold seep sediments was probably linked to the phenomenon of inconsecutive sedimentary sequence due to the turbidites, which resulted in the lack of Fe, Mn, and Ba enrichment. There is no U enrichment but only Mo enrichment within Unit III, which might be related to H₂S produced by AOM during the methane seepages. Based on the above results, it can be speculated that this area has experienced multiple-episodes of methane seep events. Further exploration of AOM should focus on the risks of rapid deposition, especially the impact of turbidity current on sediments.

The Jiulong Methane Reef, located on the northern slope of the South China Sea (SCS), is characterized by a number of features, such as bottom simulating reflections (BSRs) and authigenic carbonates, which are indicative of methane seeps that are currently occurring as well as those that occurred in the past. However, to date, the effect that these methane seeps have on the sedimentary environment is still not completely clear. To provide further insights into the biogeochemical processes involved in methane seeps, a 6.73-m piston core (973-2) was retrieved from this area in 2011 to perform an in-depth analysis. The chronology of the core has been established by AMS¹⁴C dating, and sedimentary events since the last glacial period have been recorded. The results show abnormally low abundance and diversity, as well as high infaunal percentages of benthic foraminiferal assemblages in the lower part of the core from 673 to 350 cmbsf (centimeters below the sea floor), indicating impacts from methane activities. Major elemental barium (Ba), bromine (Br), and titanium (Ti), which serve as proxies of the paleoproductivity, organic contents, and terrestrial supply, respectively, are found to make little contribution in CaCO₃ content. However, regardless of the factors mentioned above, the CaCO3 content is still higher in the lower part of core, indicating the presence of authigenic carbonates. Additionally, the total organic carbon (TOC)/total sulfur (TS) ratios below 400 cmbsf are found to be beyond the normal range, which is 2.8±0.8, reflecting the effects of methane oxidation. In one of the sections, the trace metals and molybdenum profiles show enrichments, which is closely related to those of the methane seeps. The results indicate that this area experienced methane seep events in the last glacial period.

Anaerobic oxidation of methane (AOM) is a common biochemical process in the ocean and it plays an important role in global climate change, elemental circulation, and atmospheric evolution over geological time. In this paper, we analyzed of δ³⁴S, Fe, Mn, Ca/Ti, and Sr/Ti ratios, and the date of carbon and sulfur from the site SH3 of Shenhu area. Result showed that (1) 0–6 mbsf (meter blow the sea floor) was mainly affected by OSR (anaerobic oxidation of organic matters) and 7–15 mbsf was a paleo-SMTZ (sulfate–methane transition zone) position. The modern SMTZ was mainly distributed at 19–25 mbsf. The barium sulfate precipitation above the modern SMTZ indicating that the current methane leakage was stable and lasted longer during geological history. (2) By studying the change of magnetic and the different carbonate minerals, results showed that there were two AOM stages. During the early stage, Fe²⁺ were mainly produced by sulfide abiotic reductive dissolution. During the later stage, Fe²⁺ were mainly produced by the metal-AOM. (3) Study of the mineral characteristics of the paleo-SMTZ and the modern SMTZ showed that the modern SMTZ carbonate minerals were mainly low-Mg calcite and aragonite, while the paleo-SMTZ carbon minerals were mainly high Mg minerals. The reason for this difference is that the modern SMTZ layer was only experienced the first stage of anaerobic oxidation of methane. In the paleo-SMTZ layer, it has experienced two stage of anaerobic oxidation of methane. During the last stage of metal-AOM, the low Mg carbonate minerals were converted into high Mg carbonate minerals. This research confirms the presence of metal-driven methane anaerobic oxidation at the bottom of sulfate-driven methane anaerobic oxidation and during the metal-driven methane anaerobic oxidation, methane and metal oxides or hydroxides would couple to convert the in situ metal oxides or hydroxides into metal ions, meanwhile the phosphorus adsorbed on the surface of the metal oxides is released into adjacent pore water, and convert to new P-bearing minerals under suitable conditions.

Cold seep emissions of low temperature fluid from the marine sediment basins are mainly comprised of methane and other hydrocarbons. A series of biogeochemical processes related to methane lead to the formation of authigenic carbonate minerals. In this study, a self-built experimental device was used to study the formation process of carbonate minerals under cold seep conditions. The concentrations of pore water chemicals, HCO₃₋ and Ca²⁺ at different heights of the reactor under flow conditions can be observed. According to the experimental results, the formation process of carbonate minerals under cold seep conditions was estimated, that 1 m carbonate growth needs 12,000 and 7000 years, respectively, under fast (5 mL·min⁻¹) and slow emission (1 mL·min⁻¹) conditions. Furthermore, TOUGHREACT was used to simulate the diagenesis process. A 1D unsteady react-transport model was developed, and the experimental data was used to constrain the simulation. The results of simulation show that the carbonates need 17,000 and 9700 years to grow 1 m under the condition of fast and slow flow scenarios, respectively. The results of this work will contribute to the study of foundation on the formation of authigenic minerals in cold seep areas, and for the physical properties of sedimentary media as well.