Abstract
Abstract. We report a rare observation of a mini-fracture in near-surface sediments (30 cm below the seafloor) visualized using a rotational scanning X-ray of a core recovered from the Lomvi pockmark, Vestnesa Ridge, west of Svalbard (1200 m water depth). Porewater geochemistry and lipid biomarker signatures revealed clear differences in the geochemical and biogeochemical regimes of this core compared with two additional unfractured cores recovered from pockmark sites at Vestnesa Ridge, which we attribute to differential methane transport mechanisms. In the sediment core featuring the shallow mini-fracture at pockmark Lomvi, we observed high concentrations of both methane and sulfate throughout the core in tandem with moderately elevated values for total alkalinity, 13C-depleted dissolved inorganic carbon (DIC), and 13C-depleted lipid biomarkers (diagnostic for the slow-growing microbial communities mediating the anaerobic oxidation of methane with sulfate – AOM). In a separate unfractured core, recovered from the same pockmark about 80 m away from the fractured core, we observed complete sulfate depletion in the top centimeters of the sediment and much more pronounced signatures of AOM than in the fractured core. Our data indicate a gas advection-dominated transport mode in both cores, facilitating methane migration into sulfate-rich surface sediments. However, the moderate expression of AOM signals suggest a rather recent onset of gas migration at the site of the fractured core, while the geochemical evidence for a well-established AOM community at the second coring site suggest that gas migration has been going on for a longer period of time. A third core recovered from another pockmark along the Vestnesa Ridge Lunde pockmark was dominated by diffusive transport with only weak geochemical and biogeochemical evidence for AOM. Our study highlights that advective fluid and gas transport supported by mini-fractures can be important in modulating methane dynamics in surface sediments.
Highlights
Large-scale fractures are commonly observed on seismic profiles (Tobin et al, 2001; Weinberger and Brown, 2006; Plaza-Faverola et al, 2015) and can provide increased sediment permeability and conduits for fluid and gas transport
Our detailed X-ray imaging of cores retrieved from locations of known methane seepage in Vestnesa Ridge revealed a mini-fracture in the core Lomvi 893MC in the top 30 cm (Fig. 3) but not in any other core
The smooth porewater profiles of sulfate, sulfide, Total alkalinity (TA), and δ13C of dissolved inorganic carbon (DIC) in this core seem typical for locations with low methane input, as often found in settings characterized by diffusive transport regimes (Treude et al, 2003; Egger et al, 2018; Niemann et al, 2009)
Summary
Large-scale fractures are commonly observed on seismic profiles (Tobin et al, 2001; Weinberger and Brown, 2006; Plaza-Faverola et al, 2015) and can provide increased sediment permeability and conduits for fluid and gas transport. H. Yao et al.: Fracture-controlled fluid transport supports microbial methane-oxidizing communities and Brown, 2006; Briggs et al, 2011), Blake Ridge (Egeberg and Dickens, 1999), and the recently documented Storfjordrenna gas hydrate mounds in the Barents Sea (Hong et al, 2017b, 2018; Waage et al, 2019). Yao et al.: Fracture-controlled fluid transport supports microbial methane-oxidizing communities and Brown, 2006; Briggs et al, 2011), Blake Ridge (Egeberg and Dickens, 1999), and the recently documented Storfjordrenna gas hydrate mounds in the Barents Sea (Hong et al, 2017b, 2018; Waage et al, 2019) Seepage at these locations can sustain high biomass levels of chemosynthetic communities that either directly oxidize methane or metabolize products of methane oxidation, such as sulfide (Boetius and Suess, 2004; Niemann et al, 2013). Surface sediments may feature smaller-scale, branched fracture networks (hereafter referred to as mini-fractures) which propagate from macro-fractures as the fluid pressure increases (Friedman, 1975; Briggs et al, 2011; Anders et al, 2014)
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