Episodic recurrence of deepwater methane seepage in the South China Sea over the past 133 k.y.

Abstract Compared to the growing evidence of what drives methane seepage along upper continental slopes, our understanding of the factors controlling seepage in deep-marine environments is insufficient; this limitation is partly caused by the lack of constraints on the history of seepage in deep-sea settings. Here, we use uranium-thorium and radiocarbon dating of seep carbonates and bivalve shells sampled from cores taken in the Qiongdongnan Basin of the South China Sea (~1700 m water depth). The carbonate and shell ages indicate two episodes of methane emission, one period during Marine Isotope Stage 5e (ca. 133–121 ka), and another period after the Last Glacial Maximum (ca. 18–3 ka). Modeling reveals that changes of bottom water temperature and sea level during periods of methane release had only limited influence on the extent of the hydrate stability zone, suggesting that—unlike for shallower water settings of the South China Sea—a change of bottom water temperature was not the main trigger of methane seepage. Considering that sedimentation rates during periods of seepage were significantly lower than during periods of dormancy, we put forward the new concept that a decline of sediment loading favored episodic seepage in the studied deep-sea setting. During periods of increased deposition, changes in gas pressure due to the gradual thickening of the gas hydrate stability zone resulted in the closure of fractures and, thus, reduced methane release. Vice versa, methane seepage increased when sedimentation rates declined, and hydraulic fracturing regained momentum. On Quaternary time scales, deepwater gas reservoirs may have been more sensitive to changes in sedimentation than they were to warming or sea level fall. The deep-sea reservoir of methane hydrate should, therefore, be considered separately when estimating the impact of hydrate stability on climate change.