Abstract
Detailed knowledge of the extent of post-genetic modifications affecting shallow submarine hydrocarbons fueled from the deep subsurface is fundamental for evaluating source and reservoir properties. We investigated gases from a submarine high-flux seepage site in the anoxic Eastern Black Sea in order to elucidate molecular and isotopic alterations of low-molecular-weight hydrocarbons (LMWHC) associated with upward migration through the sediment and precipitation of shallow gas hydrates. For this, near-surface sediment pressure cores and free gas venting from the seafloor were collected using autoclave technology at the Batumi seep area at 845 m water depth within the gas hydrate stability zone. Vent gas, gas from pressure core degassing, and from hydrate dissociation were strongly dominated by methane (> 99.85 mol.% of ∑[C1–C4, CO2]). Molecular ratios of LMWHC (C1/[C2 + C3] > 1000) and stable isotopic compositions of methane (δ13C = − 53.5‰ V-PDB; D/H around − 175‰ SMOW) indicated predominant microbial methane formation. C1/C2+ ratios and stable isotopic compositions of LMWHC distinguished three gas types prevailing in the seepage area. Vent gas discharged into bottom waters was depleted in methane by > 0.03 mol.% (∑[C1–C4, CO2]) relative to the other gas types and the virtual lack of 14C–CH4 indicated a negligible input of methane from degradation of fresh organic matter. Of all gas types analyzed, vent gas was least affected by molecular fractionation, thus, its origin from the deep subsurface rather than from decomposing hydrates in near-surface sediments is likely. As a result of the anaerobic oxidation of methane, LMWHC in pressure cores in top sediments included smaller methane fractions [0.03 mol.% ∑(C1–C4, CO2)] than gas released from pressure cores of more deeply buried sediments, where the fraction of methane was maximal due to its preferential incorporation in hydrate lattices. No indications for stable carbon isotopic fractionations of methane during hydrate crystallization from vent gas were found. Enrichments of 14C–CH4 (1.4 pMC) in short cores relative to lower abundances (max. 0.6 pMC) in gas from long cores and gas hydrates substantiates recent methanogenesis utilizing modern organic matter deposited in top sediments of this high-flux hydrocarbon seep area.
Highlights
Submarine gas hydrate accumulations are widely distributed on continental margins (Judd et al, 2002) and owing to their hydrocarbon storage capacity are of great interest as future energy source
Gravity cores and pressure cores from the Batumi seep area generally comprised hemipelagic sediments consisting of finely laminated coccolith ooze (Unit 1; Ross and Degens, 1974) and mid-Holocene sapropel (Unit 2) underlain by late Pleistocene-early Holocene lacustrine mud (Unit 3; Fig. 3)
Shallowest hydrates were unambiguously present in GC 18 in about 90 cm bsf (Fig. 3) taken in an area of focused gas expulsion (Fig. 1b), but might have been recovered in the core catcher of GC 14, which was bent during sediment penetration
Summary
Submarine gas hydrate accumulations are widely distributed on continental margins (Judd et al, 2002) and owing to their hydrocarbon storage capacity are of great interest as future energy source. The relative methane partitioning into its major sedimentary pools (dissolved in interstitial waters, as free gas, adsorbed to mineral surfaces, or bound in hydrates) is crucial for an assessment of the methane bioavailability and for calculations of methane budgets and fluxes. Various parameters, such as concentration of LMWHC, water availability, and temperatures in bottom water and sediment, control the partitioning into subsurface methane pools, which in turn respond highly dynamic to changes in these parameters. Pape et al / Chemical Geology 269 (2010) 350–363 volcano that the intensity of advective gas and heat transport from greater depth strongly affects the extent of hydrate formation and decomposition and the amount of hydrate-bound LMWHC
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