Fracture-controlled fluid transport supports microbial methane-oxidizing communities at Vestnesa Ridge

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.

Benthic Foraminifera in Arctic Methane Hydrate Bearing Sediments

“…However, we cannot exclude that the pockmarks are also characterized by longer methane diffusive phases, since the area is covered by extensive microbial mats, which are usually absent in advective systems (Boetius and Suess, 2004). Sulfate and δ 13 C DIC profiles suggested current methane advection in cores 8 and 21 (Yao et al, 2019;Dessandier et al, under review) or lateral mechanic transport of methane due to strong advective seawater flow (Hong et al, 2016), while diffusion was likely dominant in the other cores. The two different methane transport mechanisms (advection vs. diffusion) have a different physical impact on the sediment geochemistry.…”

“…Core 20 was collected in sediment devoid of any chemosynthetic community (Figure 2A). Because of this, and considering core 20 pore water data (Yao et al, 2019), we considered this core our control core (core not influenced by methane seepage).…”

Biogeochemical Consequences of Nonvertical Methane Transport in Sediment Offshore Northwestern Svalbard

“…The most depleted δ 13 C value of archaeol, which can be considered as the AOM endmember signal, translates to a Δδ 13 C value of −54‰ when compared to the δ 13 C value of the dominantly microbial methane source (−56‰; H. Sahling et al, 2014). This magnitude of isotopic depletion of ANME lipids is relatively strong, in particular for ANME-1, when compared to reports from other settings including arctic cold seeps Niemann & Elvert, 2008;Niemann et al, 2006;Yao et al, 2019).…”

“…An alternative explanation for the formation of SMTZ‐2 could be injection of methane through recently formed nonvertical sediment fractures (Impey et al, 1997). It is notable that sediment fractures leading to nonvertical methane influx and buildup of AOM communities were found at three seep sites, two of which are at nearby artic locations, Gas Hydrate Pingo and Vestnesa Ridge (Briggs et al, 2011; Gründger et al, 2019; Yao et al, 2019). The observation that methane supply was much shallower at the peeper site compared to SMTZ‐1 in the gravity core further indicates a complex fluid advection system at the MASOX site.…”

Assessing the potential for non-turbulent methane escape from the East Siberian Arctic Shelf

“…Recent observations indicate that bottom water temperatures in the coastal and inner shelf regions of the ESAS (water depth < 30 m, Dmitrenko et al, 2011) are rising, while the central shelf sea may be subject to intense episodic warming (Janout et al, 2016). The increasing influx of warmer Atlantic water into the Arctic Ocean -the so-called Atlantification (Polyakov et al, 2017;Barton et al, 2018) -will not only further enhance this warming, but will also influence circulation and salinity patterns on the shelf (Carmack et al, 1995;Zhang et al, 1998;Biastoch et al, 2011). At the same time, it has been long recognized that the Arctic is a potential hotspot for methane emissions.…”