Research
Gas hydrate research from pore water chemistry
Dissolved ions in interstitial water (water among sediment grains) are influenced by gas hydrate growth. For gas hydrate growth, methane concentration is the most important factor. Interstitial water needs to be saturated with methane. For methane saturation near the seafloor adjusting to oxic seawater, development of anoxic condition is necessary. One of the geochemical influences is change in interstitial water chemical compositions owing to the oxic/anoxic condition shift. Anther change is caused by gas hydrate growth in anoxic deep sediment depth or after development of anoxic condition.
Interstitial water chemistry change related with development of anoxic condition
Chemical composition of interstitial water gradually change with sediment depth (distance from the seafloor adjusting to the oxic seawater). This is due to microbial activity which changes oxidizer for decomposition of organic matters step by step. After oxygen consumption, oxides of manganese (Mn) and iron (Fe), and nitrate (NO3-) follow. Because anaerobic oxidation with sulfate ion (SO42-) is the last reaction (owing to the lowest energy generation) and microbes can also oxidize methane with SO42-, interstitial water needs to be depleted in SO42- for increase of methane concentration.
Sediment depth is geochemically divided into two zone, sulfate zone with SO42- in interstitial water and methane zone with dissolved methane. The zones overlap each other and the overlap is called sulfate-methane transition (SMT) (in some researches, sulfate-methane interface, SMI is used owing to ignorable thickness of SMT).
Depth of SMT represents methane flux (e.g., Borowski et al., 1996). This is because (1) SO42- concentration in seawater is almost constant and SO42- is not limiting material, (2) Anaerobic oxidation of methane (AOM) with SO42- occurs only at the SMT (outside of SMT, one material is depleted). Development of methane zone (increase of methane flux) shoals the SMT depth and finally, overlaps with Fe and Mn oxides. In such case, AOM with these oxides supplies Fe and Mn ions to interstitial water. Because Fe oxides are carrier of rare earth elements (REEs) (Haley et al., 2004), change in methane flux influences various chemical compositions in interstitial water.
Interstitial water chemistry change induced by gas hydrate growth
Methane hydrate excludes dissolved ions which is charged. This phenomena is called ion exclusion (Ussler and Paull, 2001). Active gas hydrate formation increases chloride ion (Cl-) concentration in interstitial water. Although concentrations of other ions are also increased due to the ion exclusion, Cl- is focused because this is the most inert ion in seawater. In natural environment, Cl- diffuses in interstitial water and such anomaly disappears (Cl- concentration finally return to initial value).
Methane hydrate formation causes another geochemical anomaly. For crystal stabilization, isotopically higher water molecules are preferred for the lattice (isotopic fractionations about oxygen and hydrate of water). As that results, oxygen and hydrogen isotopic compositions or residual interstitial water become lower.
In case of gas hydrate dissociation, opposite phenomenon (decrease in ion concentrations and change in isotopic compositions owing to release of isotopically high water) is observed. The magnitude of geochemical anomaly related with hydrate dissociation depends on amount of geochemically anomalous residue caused by gas hydrate formation.
Because both gas hydrate growth and dissociation influence interstitial water chemistry, interpretation of geochemical data from gas hydrate bearing cores needs great consideration. We need to consider various information for that.