CH4–C3H8 mixed gas hydrate behavior in natural marine sediments: influence of sediment type and dissociation pathways

The dissociation behavior of CH 4 – C 3 H 8 mixed gas hydrates in natural marine sediments is critical to global carbon storage and marine biogeochemical cycles, but the effects of sediment composition and dissociation pathways remain unclear. In this study, dissociation experiments were conducted using marine sediments from the South China Sea, including foraminifera-rich sands, mud-foraminifera sand mixtures, and muds, under controlled heating and depressurization. Hydrate dissociation dynamics and gas release were monitored using in situ and ex situ Raman spectroscopy, supplemented by microscopic observations. Our results show that dissociation of sII hydrate crystals in all sediments begins within the hydrate stability range. The morphological changes in hydrate crystal surfaces correlate with compositional shifts in sediments characterized by high heterogeneity and a broad particle-size distribution. In muddy sediments, dissociation behavior remained uniform regardless of the triggering mechanism, while hydrate crystals in foraminifera-rich sands exhibited distinct behaviors under heating compared to depressurization. Sediment composition influences gas release, although the L (CH 4 )/ S (CH 4 ) ratio remains nearly constant across all sediments. Specifically, coarse quartz particles enhance CH 4 and C 3 H 8 release, while clay minerals have negligible effects. Foraminifera-rich sands preferentially facilitate CH 4 release under heating, whereas they promote CH 4 and C 3 H 8 release under depressurization. These findings provide mechanistic and quantitative insights into sediment-carbon interactions in marine systems, with implications for sustainable carbon management and predicting ocean responses to anthropogenic and climate-driven perturbations.