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Chemistry

Reevaluating of Hydrate-Controlled Methane Seepage from Study off Svalbard

Article: C. Berndt et al. Temporal Constraints on Hydrate-Controlled Methane Seepage off Svalbard. Science 343, 284 (2014);  DOI: 10.1126/science.1246298

Introduction

Methane hydrate is the methane gas trapped in cages formed of water molecules and its formation usually occurs in the sediment, where the pressure is high and the temperature is low. While at the sediment surface the bubbles can easily diffuse into the water, methane is more stable several meters downward in the sediment. This process requires a significant amount of organic matter and commonly happens over continental and island slopes from depth greater than 350 m to considerable depth. As this is the main source of methane, some methane can also be formed through deep water gas reservoir leakage[1].

Methane, which is an even more powerful greenhouse gas than carbon dioxide, has been a great concern as climate change may lead to large quantities of emissions of methane. Due to the temperature-dependent stability of methane, it is claimed that a fraction of methane would be released from the warming up ocean water. This will have the largest impact on shallowest sediments.

There exists an interesting phenomenon that gas bubbles coming up from the water column at a depth precisely where the dissociation of methane hydrate should happen. Meanwhile a 1°C rise of bottom-water temperature for the past 30 years is suggested. Even though the gas release was believed to be the consequence of global warming, it is worth reevaluating this issue from a long time scale.

Fig. 1. The Svalbard gas hydrate province is located on the western margin of the Svalbard archipelago (inset). At water depths shallower than 398 m, numerous gas flares have been observed in the water column (color-coded dots for different surveys) by using EK60 echo sounders and high-resolution side scan sonar. The gas flares are located between the contour lines at which gas hydrate is stable in the subsurface at 3° (brown) and 2° (blue) C average bottom water temperature.

Studies and Findings

The margin of Svalbard is a great place to study temperature related gas release since water temperature in this area is affected greater than the other places (shown in Fig 1). Using echo sounders and high resolution side scan sonar, gas flares have been observed aligning between 380 m and 400 m water depths. This finding matches well with the calculated termination of gas hydrate stability zone (GHSZ). GHSZ describes the places where methane can remain stable instead of diffusing into the water column and further into the atmosphere.

Gas flares happen at seasonal GHSZ where gas hydrate formation and dissociation alternate periodically with seasonal water temperature fluctuations. The bottom-water temperature difference between spring and fall/winter was around 1.5°C over two years. Because of the efficient heat transfer from the bottom water to the sediment, the sediment temperature changes with bottom-water, resulting in lateral shifts of GHSZ. GHSZ reaches its maximum in summer when sediment becomes cold and minimum in winter (shown in Fig 2). Supply of methane came from below the GHSZ during winter through summer, then hydrate dissociated again in the second half of the year. The decreasing level of GHSZ augments methane emissions by opening pathways of gas ascending from underneath and by releasing gas from hydrate phase.

Fig 2. Temperature and the GHSZ. (Top) Daily means of bottom-water temperature recorded by the MASOX observatory. The times when the extent of the GHSZ was at its maximum and minimum are marked by solid red and dashed blue lines, respectively. (Bottom)  The seasonal dynamics of the GHSZ.  Driven by changes in bottom-water temperature, the GHSZ advances and retreats in the course of the year. The solid red lines and the dashed blue lines indicate maximum and minimum extents of the GHSZ, respectively. The area in which gas hydrates are stable in the long-term is shaded in yellow. The difference between maximum and minimum extents of the hydrate stability zone is shaded in orange and corresponds to the seasonal GHSZ, in which gas hydrate dissociation and formation alternate periodically. The triangles filled in magenta represent the projected locations of all flares detected within 1000 m of the transect line. The green diamond shows the position of the MASOX observatory.

Fig 2. Temperature and the GHSZ. (Top) Daily means of bottom-water temperature recorded by the MASOX observatory. The times when the extent of the GHSZ was at its maximum and minimum are marked by solid red and dashed blue lines, respectively. (Bottom) The seasonal dynamics of the GHSZ. Driven by changes in bottom-water temperature, the GHSZ advances and retreats in the course of the year. The solid red lines and the dashed blue lines indicate maximum and minimum extents of the GHSZ, respectively. The area in which gas hydrates are stable in the long-term is shaded in yellow.

Although it is discovered that seasonal bottom-water temperature variations are capable of modulating the gas emissions, it is not evident that the slope sediment is undergoing a decadal-scale warming. Scientists also found that seepage is a process that happened long before anthropogenic carbon release. No sufficient data has proved annual, decadal or centennial changes in this process.

Through carbonate crust analyze, 13C was found of low contribution to the carbonate. It is characterized by microbial activities using methane for oxidization and results in a methane-related carbon precipitation. This was found happening at least 500 years before the present in samples from all three sites, meaning methane gases came to the surface sediment long time ago. A pronounced warming was indicated since the end of 19th century. This 100 years spam is far less than the history of methane gas seepage.

Significance and Future Work

Observations of large scale gas emissions cannot be a proof of whether methane hydrate destabilization is accelerating or not, though many people believe that methane emission is occurring at a faster speed resulted from climate change. For future work, this study points out the sensitiveness of shallow hydrate to water temperature. An estimate of methane hydrate trapped in shallowest part of gas hydrate stability zone would be of great significance in understanding the feedback mechanism of global warming.

 

Reference

[1]  Michael Pilson.  An Introduction to the Chemistry of the Sea.  2nd Edition.  New York: Cambridge University Press, 2013.  Print.

Caoxin Sun
Caoxin is a graduate student in the Graduate School of Oceanography at the University of Rhode Island. Her research interest lies in persistent organic pollutants in the environment. When she is not doing research she likes to create new cuisines.

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