Clumped methane isotopologue-based temperature estimates for sources of
methane in marine gas hydrates and associated vent gases
Abstract
Gas hydrates stored in the continental margins of the world’s oceans
represent the largest global reservoirs of methane. Determining the
source and history of methane from gas hydrate deposits informs the
viability of sites as energy resources, and potential hazards from
hydrate dissociation or intense methane degassing from ocean warming.
Stable isotope ratios of methane (13C/12C, D/H) and the molecular ratio
of light hydrocarbons (C1/C2+3) have traditionally been applied to infer
methane source, but often yield ambiguous results when two or more
sources are mixed, or when compositions were altered by physical (e.g.,
diffusion) or microbial (e.g., methanotrophy) processes. We measured the
abundance of clumped methane isotopologue (13CH3D) alongside 13C/12C and
D/H of methane, and C1/C2+3 (the ratio of methane over ethane plus
propane) for 46 submarine gas hydrate specimens and associated vent
gases from 11 regions of the world’s oceans. These samples are
associated with different seafloor seepage features (cold seeps, oil
seeps, pockmarks, and mud volcanoes). The values of Δ13CH3D (the excess
abundance of 13CH3D relative to the stochastic distribution) and
apparent equilibration temperatures increase from cold seeps (15 to 65
℃) and pockmarks (36 to 54 ℃), to oil-associated gas hydrates (48 to 120
℃). These apparent temperatures are consistent with, or a few tens of
degrees higher than, the temperature expected for putative microbial
sources. Apparent methane generation depths were derived for cold seep,
pockmark, and oil seep methane from isotopologue-based temperatures and
the local geothermal gradients. Estimated methane generation depths
ranged from 0.2 to 5.3 kmbsf, and are largely consistent with source
rock information, and other chemical geothermometers based on clay
mineralogy and fluid chemistry (e.g., Cl, B, and Li). Methane associated
with mud volcanoes yielded a wide range of apparent temperatures (15 to
313℃). Gas hydrates from the Kumano Basin and Mediterranean Sea yielded
δ13C-CH4 values from -36.9 to -51.0‰, typical for thermogenic sources.
Δ13CH3D values (3.8 to 6.0‰) from these sites, however, are consistent
with prevailing microbial sources. These mud volcanoes (Kumano Basin,
Mediterranean Sea) are located at active convergent plate margins, where
hydrogen may be supplied from basement rocks, and fuel methanogenesis to
the point of substrate depletion. In contrast, gas hydrate from mud
volcanoes located in tectonically passive settings (Sorokin Trough,
Black Sea; Håkon Mosby Mud Volcano, Barents Sea slope) yielded
microbial-like δ13C-CH4 and C1/C2+3 values, and low Δ13CH3D values (1.6
to 3.3‰), which may be due to kinetic isotope effects. This study is the
first to document the link between methane isotopologue-based
temperature estimates and key submarine gas hydrate seepage features,
and validate previous models about their geologic driving forces.