In Situ Raman Detection of Gas Hydrates Exposed on the Seafloor of the South China Sea

Abstract

Gas hydrates are usually buried in sediments. Here we report the first discovery of gas hydrates exposed on the seafloor of the South China Sea. The in situ chemical compositions and cage structures of these hydrates were measured at the depth of 1,130 m below sea level using a Raman insertion probe (RiP‐Gh) that was carried and controlled by a remotely operated vehicle (ROV) Faxian. This in situ analytical technique can avoid the physical and chemical changes associated with the transport of samples from the deep sea to the surface. Natural gas hydrate samples were analyzed at two sites. The in situ spectra suggest that the newly formed hydrate was Structure I but contains a small amount of C3H8 and H2S. Pure gas spectra of CH4, C3H8, and H2S were also observed at the SCS‐SGH02 site. These data represent the first in situ proof that free gas can be trapped within the hydrate fabric during rapid hydrate formation. We provide the first in situ confirmation of the hydrate growth model for the early stages of formation of crystalline hydrates in a methane‐rich seafloor environment. Our work demonstrates that natural hydrate deposits, particularly those in the early stages of formation, are not monolithic single structures but instead exhibit significant small‐scale heterogeneities due to inclusions of free gas and the surrounding seawater, there inclusions also serve as indicators of the likely hydrate formation mechanism. These data also reinforce the importance of correlating visual and in situ measurements when characterizing a sampling site.

In Situ Quantitative Raman Detection of Dissolved Carbon Dioxide and Sulfate in Deep‐Sea High‐Temperature Hydrothermal Vent Fluids

“…The Sea-Going System-RiP The details of the RiP system have been described in previous papers (Zhang et al, 2017a(Zhang et al, , 2017b. RiP System and Data Processing 3.1.1.…”

“…RiP System and Data Processing 3.1.1. The Sea-Going System-RiP The details of the RiP system have been described in previous papers (Zhang et al, 2017a(Zhang et al, , 2017b. A Raman insertion probe for hydrothermal vents (RiP-Hv probe) resistant to 4508C was designed for in situ detection experiments conducted in a hydrothermal field.…”

“…The development of new Raman instrumentation has created new opportunities to explore chemical processes occurring in hydrothermal vents, as it is highly adaptable to aqueous environments (Brewer et al, 2004;Ferraro, 2003;Lewis & Edwards, 2001;McCreery, 2005). Another adaptation of an in situ Raman detection system, the Raman insertion probe (RiP), can adapt to a variety of extreme environments and has been used for in situ detection in the Manus hydrothermal area, the Okinawa Trough hydrothermal area and the South China Sea cold seep area (Zhang et al, 2017a(Zhang et al, , 2017b. The deep ocean Raman in situ system (DORISS) designed by Brewer et al (2004) has been used on a variety of natural gas, hydrate, mineral, and pore water measurements (Hester et al, 2006(Hester et al, , 2007White, 2009;White et al, 2005, 2006a, 2006b, Zhang et al, 2010.…”

“…The deep ocean Raman in situ system (DORISS) designed by Brewer et al (2004) has been used on a variety of natural gas, hydrate, mineral, and pore water measurements (Hester et al, 2006(Hester et al, , 2007White, 2009;White et al, 2005, 2006a, 2006b, Zhang et al, 2010. Another adaptation of an in situ Raman detection system, the Raman insertion probe (RiP), can adapt to a variety of extreme environments and has been used for in situ detection in the Manus hydrothermal area, the Okinawa Trough hydrothermal area and the South China Sea cold seep area (Zhang et al, 2017a(Zhang et al, , 2017b.…”

In situ Raman Quantitative Detection of the Cold Seep Vents and Fluids in the Chemosynthetic Communities in the South China Sea

“…Nonetheless, this hydrate formation did not affect the components of the gases erupted from the cold seep vent. The gaseous and dissolved methane condensed into solid methane during gas hydrate formation, and the remaining gaseous CH 4 and H 2 S were occluded by the hydrate fabric (Du et al, ; Zhang et al, ). As a result, the in situ detection of the gases using RiP‐Cs system becomes easier.…”