Methane seeps are widespread on the seafloor in continental margins. However, the responses of methane flux and diagenetic processes at cold seeps to drastic environmental changes in the geological history are still poorly understood. Here, we report the geochemical composition of porewaters and sediments in a sediment core (CL3A) collected from methane-rich sediments in the southern South China Sea, in order to assess the variations in upward methane flux since the last glacial period. A sediment horizon at depths of ca. 510–660 cm below seafloor just above the current sulfate-methane transition zone (SMTZ) is identified, which shows geochemical anomalies including high total inorganic carbon (TIC) and total sulfur (TS) contents, extremely negative δ¹³C_TIC values and positive sulfur isotopic ratios of acid insoluble sulfur (δ³⁴S_{acid-insoluble}). We propose that the diagenetic records of sulfate-driven anaerobic oxidation of methane within a paleo-SMTZ were recorded by the accumulation of authigenic carbonate and iron sulfide within this zone. Such geochemical pattern implies a fixation of the SMTZ at a certain depth for a prolonged time, which is estimated as ~13,000 years. The stagnation of the SMTZ is probably attributed to a large decline in sedimentation rate during the transition of last glacial/interglacial period, which is supported by the ¹⁴C ages throughout the core. Moreover, a non-steady-state modelling for the concave-up depth profiles of porewater species suggests that a recent enhancement in upward methane flux has lasted for at least 550 years. In summary, this study suggests that the fixation of SMTZ during the glacial/interglacial transitions in dynamic sedimentation regimes can lead to the accumulation of large amount of authigenic carbonate and iron sulfide in continental margins and should be considered in global carbon and sulfur biogeochemical cycles.

甲烷渗漏普遍存在于大陆边缘的海底。然而，冷泉处的甲烷通量和成岩过程对地质历史中剧烈环境变化的响应仍然知之甚少。在这里，我们报告了从南海南部富含甲烷的沉积物中收集的沉积物核心 (CL3A) 中孔隙水和沉积物的地球化学组成，以评估自上次冰期以来向上的甲烷通量的变化。约深度处的沉积层。在当前硫酸盐-甲烷过渡带 (SMTZ) 正上方的海底下方 510–660 cm 处被识别，显示地球化学异常，包括高总无机碳 (TIC) 和总硫 (TS) 含量，δ ¹³ C _TIC极负值酸不溶性硫（δ ³⁴ S_酸不溶性）的值和正硫同位素比。我们建议古 SMTZ 内硫酸盐驱动的甲烷厌氧氧化的成岩记录是通过该区域内自生碳酸盐和硫化铁的积累来记录的。这种地球化学模式意味着 SMTZ 长时间固定在某个深度，估计约 13,000 年。SMTZ 的停滞可能归因于末次冰期/间冰期过渡期间沉积速率的大幅下降，这得到了¹⁴C 在整个核心中老化。此外，孔隙水物种的上凹深度剖面的非稳态模型表明，最近甲烷向上通量的增强已经持续了至少 550 年。综上所述，本研究表明，在动态沉积体系中冰期/间冰期转变期间 SMTZ 的固定可导致大陆边缘大量自生碳酸盐和硫化铁的积累，应在全球碳硫生物地球化学循环中予以考虑。