Influence of water flow on gas hydrate accumulation at cold vents

Abstract

A cold vent is an area where methane-rich fluid seepage occurs. This seepage may alter the local temperature, salinity, and subsequent accumulation of the gas hydrate. Using a kinetic gas hydrate formation model and in situ measurement of temperature, salinity and fluid flux at the southern summit of Hydrate Ridge, we simulate the gas hydrate accumulation at three distinct fluid sites: clam, bacterial mat, and gas discharge sites. At the clam sites (pore water flux < 20 kg m 2 yr 1 ), pore water advection has little influence on temperature and salinity. However, the salinity and temperature are increased (peak salinity > 0.8 mol kg 1 ) by the formation of gas hydrate causing the base of the hydrate stability zone to move gradually from ~115 to ~70 meters below seafloor (mbsf). The gas hydrate saturation at the clam sites is relatively high. The water flux at the bacterial mat sites ranges from 100 to 2500 kg m 2 yr 1 . The water flow suppresses the increase in salinity resulting in a salinity close to or slightly higher than that of seawater (< 0.65 mol kg 1 ). Heat advection by water flow increases temperature significantly, shifting the base of the hydrate stability zone to above 50 or even 3 mbsf. The gas hydrate saturation is relatively low at the bacterial mat site. At the gas discharge sites, the pore water flux could reach 10 10 kg m 2 yr 1 , and the temperature could reach that of the source area in 9 min. There is no gas hydrate formation at the gas discharge sites. Our simulative analysis therefore reveals that a lower pore water flux would result in lower salinity, higher temperature, and a shallower base of the hydrate stability zone. This in turn induces a lower gas hydrate formation rate, lower hydrate saturation, and eventually less gas hydrate resources.cold vent, fluid seepage, temperature, salinity, gas hydrate, numerical simulation Citation:

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

“…However, the in situ biogenic methane by itself falls short in supplying the amount and concentration of methane that are necessary for forming and maintaining the vast quantity of observed and inferred methane hydrate there (Claypool & Kvenvolden, 1983; Kvenvolden & Lorenson, 2001; Chen et al, 2006; Sloan & Koh, 2008; Wallmann et al, 2012). Therefore, additional mechanisms of sustained methane supply into HSZ from below are required (Xu & Ruppel, 1999; Kvenvolden & Lorenson, 2001; Chen et al, 2006; Sloan & Koh, 2008; Su, Cao, & Wu, 2012, Su, Cao, & Yang, 2012; Bhatnagar et al, 2011; Cao et al, 2013). Diffusion of dissolved methane and advection of methane‐carrying pore fluids through marine sediment are two such transport (Davie & Buffett, 2003a, 2003b; Wallmann et al, 2012; Xu & Ruppel, 1999).…”

Impact of anaerobic oxidation of methane on the geochemical cycle of redox-sensitive elements at cold-seep sites of the northern South China Sea

“…Masuzawa et al, 1992;Chen et al, 2010;Luo et al, 2013). Methane that ascends upwards along seabed sedimentfractures is intermediately stored in hydrate deposits (Tryon et al, 2002;Cathles, 2003, 2005;Cao et al, 2013aCao et al, , 2013b, which host about 500-2000 Gt of carbon worldwide (Wallmann et al, 2012). The total reservoir of methane along continental slopes is even larger, as much of the methane can be found dissolved or in gas bubbles below and above the hydrate stability zone (Buffett and Archer, 2004).…”