This study identified several consistent pulsed turbidite layers and methane seep layers based on a comprehensive analysis of sedimentology and geochemistry proxies of core Q6 in the northwestern South China Sea (SCS). The coupling relationship between these layers since the last glacial maximum (LGM) is discussed. Eight potential pulsed turbidite layers were identified according to the petrographic characteristics, grain size parameters, and element geochemistry (Si/Al, Si/Fe and Zr/Rb) of Q6. On the other hand, seven obvious methane seep events were identified based on total sulfur (TS)/total organic carbon (TOC) ratio, CaCO 3 content, and total inorganic carbon isotope (δ 13 C TIC ) of Q6. With reference to the chronological framework established by foraminifera shell AMS 14 C, it was determined that the above events mainly occurred from the LGM to early Holocene, characterized by a low sea level and transgression period. These events were recorded in the cores of the northwestern, northeastern, central northern, and southern slopes of the SCS, indicating the universal occurrence of this in the slopes of the SCS. The comprehensive analysis showed that the coupling development of methane seep events and pulsed turbidite events over the past 25 ka can be divided into three stages: (1) 25–15.5 ka; (2) 15.5–7 ka; (3) 7 ka. In stage 1, there was a significant decrease in pressure over hydrate resulting from the sharp decline in sea level over the last glacial period. This may have induced methane leakage, possibly leading to instability of the overlying strata and the occurrence of turbidity events. The removal of the overlying strata may in turn have promoted methane leakage. In stage 2, after the end of the LGM, there were rapid increases in the sea level and bottom water temperature, and the methane seep remained prevalent. However, the records indicated significant decreases or stagnation of gas hydrate in almost all areas at the end of this stage. This indicated the significant pressurization and temperature rise of the storage environment resulted in a continuing relatively high temperature and low-pressure environment of the methane seep. The environment then experienced high pressure due to the significant rise in sea level. The highest sea level and bottom water temperature have been experienced since stage 3. The records indicated significant decreases in gas hydrate decomposition in various regions of the SCS, and strata stability prevented the formation of turbidite layers. This study established a relationship between the late Quaternary marine environmental change and turbidite deposition in the continental slope area through gas activities and can act as a useful reference for further understanding the continental margin sedimentary process and Quaternary climate and environment change.
Authigenic pyrite is an important recorder for methane seepage.During methane seepage, the sulfur and iron isotopic composition of pyrite will change, allowing it to be used as an indicator to identify methane seepage.However, the dissimilar behaviour of trace elements in authigenic pyrite during methane seepage remain unclear.To provide insights, we used pyrite samples from the 'Haima' seep locality to determine differences in trace element contents in pyrite obtained from the sulfatemethane transition zone (SMTZ) and that from normal (non-seepage) sedimentary environments.In the SMTZ, the content of cadmium (Cd) related to the organoclastic sulfate reduction in pyrite was low, while the molybdenum (Mo) content, which is highly sensitive to redox environments, was high.This discrepancy can be explained by the fact that sulfate ions (SO 4 2-) in the SMTZ were preferentially consumed by sulfate driven anaerobic oxidation of methane (SD-AOM), which inhibited organoclastic sulfate reduction and decreased the trace metals derived from organic matter in pore water.Simultaneously, intense SD-AOM produced more hydrogen sulfide (H 2 S), which was more conducive to Mo removal from pore water, and then more Mo adsorption onto pyrite.Further analysis shows that the Mo/Cd ratio of pyrite in the SMTZ (average value of 82.08) is significantly higher than that of the non-SMTZ (average value of 16.02).We believe that the Mo/Cd ratio has great potential to indicate methane seepage, and thus provides a new indicator for methane seepage research.
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