A Numerical Model to Estimate the Effects of Variable Sedimentation Rates on Methane Hydrate Formation-Application to the ODP Site 997 on Blake Ridge, Southeastern North American Continental Margin
Sedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and...
| Published in: | Journal of Geophysical Research: Solid Earth |
|---|---|
| Main Authors: | , , , |
| Format: | Report |
| Language: | English |
| Published: |
AMER GEOPHYSICAL UNION
2020
|
| Subjects: | |
| Online Access: | http://ir.gig.ac.cn/handle/344008/60979 https://doi.org/10.1029/2019JB018851 |
| Summary: | Sedimentation and burial rate can vary considerably at continental slopes, due to constantly changing tectonic processes on the geological timescale. This variation may affect the transport of methane in sediment, as manifested in the change of pore water flux, methanogenesis, methane diffusion and hydrate removal from hydrate stability zone (HSZ) and further affect the accumulation of hydrate in submarine sediments. Most of previous models assumed a constant sedimentation rate and thus may result in inaccurate estimates of total amount of hydrate within the HSZ. In this study, we developed a hydrate accumulation model that is capable of handling multiple sedimentation stages. Site 997 of ODP Leg 164, with data indicating its recent history of four sedimentation stages of different rates, is chosen to investigate the significance of sedimentation rate variation experienced at this location. We examined the effect of varying sedimentation rates on hydrate accumulation by taking in consideration sediment compaction, in situ methanogenesis, and composition and component transport processes. Simulation results suggest that the history of hydrate accumulation at Site 997 is characterized by a sequence of increase, decrease, and then increase till the present day. At present, the hydrate deposit has accumulated to 8.30 x 10(4) mol/m(2), and the average hydrate saturation near the base of the HSZ is 6.3%, which is in general agreement with published estimates in the literature. The accumulation of hydrate at Site 997 was significantly affected by variable sedimentation rates, and nearly one half of the hydrate deposit was accumulated during the last 2.5 Myr. Plain Language Summary Methane hydrate forms in submarine sediments where thermodynamic conditions are satisfied and adequate methane is available. Three main mechanisms have been proposed for adequate methane availability: in situ methanogenesis, advection of methane-bearing fluids, and diffusion of methane. However, in actual environments off continental margins, a variable sedimentation rate may significantly affect the above mechanisms and thus change the amount of methane supply. Therefore, the formation of methane hydrate is influenced eventually. Meanwhile, the dissociation of methane hydrate changes simultaneously with a variable sedimentation rate. Hence, it is unclear how will hydrate reservoirs respond to different sedimentation rate values. In this study, we selected Site 997 of the ODP Leg 164 due to the significant variations in sedimentation rate and developed a hydrate accumulation model reflected several sedimentary stages. We highlighted the importance of consideration of variation of sedimentation rate in the estimation of total amount of hydrate within the HSZ and drawn a conclusion that dissociation of hydrate is more sensitive to the changes in sedimentation rate compared to hydrate formation. |
|---|