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 methane over ethane plus propane (C1/C2+3) have traditionally been
applied to infer methane sources, 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 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 (oil seeps, pockmarks, mud volcanoes, and other cold
seeps). The average apparent equilibration temperatures of methane from
the Δ13CH3D (the excess abundance of 13CH3D relative to the stochastic
distribution) geothermometer 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
methane 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 mud volcanoes
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 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 on km-thick sediments in
tectonically less active or passive settings (Black Sea, North Atlantic)
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.