Sedimentary deformation relating to episodic seepage in the last 1.2 million years: a multi-scale seismic study from the Vestnesa Ridge, eastern Fram Strait

Abstract

Seafloor hydrocarbon seepage is a natural fluid release process that occurs worldwide on continental shelves, slopes, and in deep oceanic basins. The Vestnesa sedimentary ridge in the eastern Fram Strait hosts a deep-water gas hydrate system that became charged with hydrocarbons ∼2.7 Ma and has experienced episodic seepage along the entire ridge until a few thousand years ago, when seepage activity apparently ceased in the west but persisted in the east. Although it has been documented that faults and fractures play a key role in feeding the seeps with thermogenic gases, the mechanisms controlling seepage periodicity remain poorly understood. Here we integrate high-resolution P-cable 3D seismic and Chirp data to investigate the spatial and temporal evolution of high-resolution fractures and fluid flow features in the west of the Vestnesa Ridge. We characterize sediment deformation using a fracture density seismic attribute workflow revealing two highly deformed stratigraphic intervals and associated small-scale pockmarks (<20 m diameter). Chronostratigraphic constraints from the region show that these two highly deformed intervals are influenced by at least three major climatic and oceanic events during the last 1.2 million years: the Mid-Pleistocene Transition (∼1.25–0.7 Ma), the penultimate deglaciation (∼130 ka) and the last deglaciation (Heinrich Stadial 1: ∼16 ka). These periods of deformation appear associated with seismic anomalies potentially correlated with buried methane-derived authigenic carbonate and have been sensitive to shifts in the boundary of the free gas-gas hydrate interface. Our results show shifts (up to ∼30 m) in the depth of the base of the gas hydrate stability zone (GHSZ) associated with major changes in ocean bottom water temperatures. This ocean-driven effect on the base of the GHSZ since the Last Glacial Maximum coincides with the already highly deformed Mid-Pleistocene Transition sedimentary interval and likely enhanced deformation and gas leakage along the ridge. Our results have implications for understanding how glacial cycles impact fracture formation and associated seepage activity.

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

“…The dating of sedimentary cores points to a correlation between the glacial cycles and increased seepage directly after the glacial maxima; therefore, several authors proposed glacial tectonics as the dominant force behind the seepage activity cycles (Himmler et al., 2019; Schneider et al., 2018). These observations cannot however explain why a similar group of seafloor structures (pockmarks), further west on the crest of the ridge, connected to the recent (>8,000 years ago) seepage activity, is presently dormant (Consolaro et al., 2015; Cooke et al., 2023; Plaza‐Faverola et al., 2015). It was previously suggested that the spatial variation in seepage activity is driven predominantly by the tectonic forces from the seafloor spreading at the MR and KR (Plaza‐Faverola & Keiding, 2019; Plaza‐Faverola et al., 2015).…”

“…Gas hydrates with the underlying free gas accumulations are identified from seismic reflection data along the entire length of the Vestnesa Ridge (Goswami et al., 2015; Hustoft et al., 2009; Petersen et al., 2010; Plaza‐Faverola et al., 2017; Singhroha et al., 2016, 2019). Presently documented seepage is contained within the eastern part of the ridge crest, but past seepage activity on the western segment of the system is well documented in sediments, seismic data, and bathymetry (Cooke et al., 2023; Consolaro et al., 2015; Plaza‐Faverola et al., 2015; Vogt & Crane, 1994). Current seepage activity from pockmarks occurs at the termination of sub‐seafloor fluid flow fault systems (Plaza‐Faverola et al., 2015).…”

Glacial-interglacial sedimentation control on gas seepage exemplified by Vestnesa Ridge off NW Svalbard margin

“…The SMTZ at western Vestnesa Ridge is found >2 m in the pockmark of core site HH16-550GC and >10 m outside of the pockmark at site HH16-543GC (Laier et al, 2017) (Figure 1C; Figure 4K,L). Results from studies combining seismic data and pore pressure data from Vestnesa Ridge have indicated that the western part at present mostly experience gas seepage as slow and diffusive, while in the eastern part numerous faults and fractures from sediment deformation also allow gas to escape in the gas phase, i.e., termed advective seepage (Plaza-Faverola et al, 2015;Cooke et al, 2023). At Vestnesa Ridge the thick Holocene deposits in the western part effectively filled the pockmarks and slowed the seepage of methane.…”