Experimental study on isotopic fractionation in water during gas hydrate formation

Abstract

Isotopic fractionation of oxygen and hydrogen in water caused by gas hydrate formation was investigated experimentally. Two different gas hydrates in structures, Structure I hydrate and Structure II hydrate, were formed with methane and krypton gases in a NaCl solution. Isotopic fractionation during gas hydrate formation was observed by measuring oxygen and hydrogen isotopic compositions in the solutions sampled before and after gas hydrate formation. Heavy isotopes of oxygen and hydrogen in water were depleted in the solution resulting from that those isotopes were concentrated in the gas hydrate. The isotopic fractionation was larger as increasing amounts of gas hydrates that were calculated from both the decrease of gas pressure and increase of NaCl concentration in solution. The isotopic fractionation factors of oxygen and hydrogen in water between gas hydrate and liquid water were determined to be 1.0023-1.0032 and 1.014-1.022, respectively. The significant difference of the gas hydrate structures was not observed beyond the analytical errors. These factors are similar to those between ice and liquid water.

The interglacial–glacial cycle and geochemical evolution of Canadian and Fennoscandian Shield groundwaters

“…Craig and Hom, 1968;Hesse and Harrison, 1981;Souchez and Jouzel, 1984;Maekawa and Imai, 2000;Hesse, 2003;Maekawa, 2004). As observed in many different fluids in conservative systems (including precipitation, freshwater, seawater, and fluids of differing ionic compositions), the first ice that forms has substantially higher d 2 H and d 18 O values than the original fluid, but becomes progressively lighter as freezing progresses, until the final ice and residual fluid both have much lower d 2 H and d 18 O values than the original fluid.…”

Low-Temperature Clay Mineral Dehydration Contributes to Porewater Dilution in Bering Sea Slope Subseafloor

“…The maximum excursions of the discrete low Cl − spikes from the base values of vertical Cl − profiles are from ∼520 to 495 mM (4.8%) at around 350 mbsf at Site U1343 (Figure 2A) and from ∼530 to 473 mM (11%) at 278 mbsf at Site U1344 (Figure 2B), which indicates that the maximum contents of methane hydrate water in the porewater samples are only 4.8 and 11% at Sites U1343 and U1344, respectively. If the isotopic fractionation factor of hydrogen in water between gas hydrate and liquid water is 1.014-1.022 (Maekawa, 2004), then the δD-values at 350 mbsf at Site U1343 and 278 mbsf at Site U1344 showing the maximum excursion of low Cl − spikes should be 0.6-1.1 and 1.5-2.4‰ higher than the base value of the vertical δD profiles at Sites U1343 and Site U1344, respectively. These changes would be too small to be separately identified in a large δD variation.…”

“…Another possible source of freshwater is the dissociation of methane hydrate, if in the sampling process the porewater was diluted by water released from methane hydrate that was enriched in 18 O and D (Hesse and Harrison, 1981). The isotopic fractionation factors of oxygen and hydrogen in water between gas hydrate and liquid water were determined to be 1.0023-1.0032 and 1.014-1.022, respectively (Maekawa, 2004). The estimated end member (Cl − = 0 mM) δ 18 O values of freshwater for the discrete low Cl − spikes, which are 2.2-5‰ higher than the base values (Cl − ∼530 mM, δ 18 O ∼0‰ a), are similar to the previously reported oxygen isotopic fractionation.…”

Authigenic minerals from the Paola Ridge (southern Tyrrhenian Sea): Evidences of episodic methane seepage

“…Whilst general 13 C enrichment can be explained with the incorporation of heavy C derived from methanogenesis the enrichment of 18 O is more complicated. The 18 O enrichment in carbonates has previously been explained by different processes occurring in the sedimentary environment, as follows: 1) dissociation of gas hydrates that releases 18 O-rich water in the sediments (e.g., Matsumoto, 1989;Bohrmann et al, 1998;Aloisi et al, 2000;Maekawa, 2004;Hein et al, 2006); 2) interaction with hydrothermal fluids that may yield values as high as 6.5‰ (Clayton and Epstein, 1961); 3) dehydration of clay minerals at great burial depths (D€ ahlmann and de Lange, 2003); 4) crustal/igneous CO 2 circulation (Muehlenbachs and Hodges, 1978;Clayton and Epstein, 1961;Cocker et al, 1982); 5) precipitation of carbonates during glacial times when lower temperatures and isotopically heavier seawater caused a 3e4‰ d 18 O-shift of carbonates in the Mediterranean Sea (e.g., Vergnaud-Grazzini, 1971). Other sources of heavy oxygen can be due to the circulation of deep crustal water (e.g., Lecuyer and Allemand, 1999).…”