A 160,000-year-old history of tectonically controlled methane seepage in the Arctic

Abstract

The geological factors controlling gas release from Arctic deep-water gas reservoirs through seabed methane seeps are poorly constrained. This is partly due to limited data on the precise chronology of past methane emission episodes. Here, we use uranium-thorium dating of seep carbonates sampled from the seabed and from cores drilled at the Vestnesa Ridge, off West Svalbard (79°N, ~1200 m water depth). The carbonate ages reveal three emission episodes during the Penultimate Glacial Maximum (~160,000 to 133,000 years ago), during an interstadial in the last glacial (~50,000 to 40,000 years ago), and in the aftermath of the Last Glacial Maximum (~20,000 to 5,000 years ago), respectively. This chronology suggests that glacial tectonics induced by ice sheet fluctuations on Svalbard mainly controlled methane release from Vestnesa Ridge. Data corroborate past methane release in response to Northern Hemisphere cryosphere variations and suggest that Arctic deep-water gas reservoirs are sensitive to temperature variations over Quaternary time scales.

Fig. 1. Location and bathymetry of the sampling area.

(A) Vestnesa Ridge (VR; white box) located in eastern Fram Strait, North Atlantic, northeast of the Molloy transform fault (MTF); KR, Knipovich Ridge; Gr, Greenland. Bathymetry from (54). (B) High-resolution bathymetry maps of the two sampled pockmarks over a low-resolution map, revealing multiple depressions and mounds within Lunde and Lomvi pockmarks. Stars indicate seabed samples (P1606001, P1606002, P1606005, P1606010, P1606011, P1606012, and P1606023). Triangles indicate drill sites of MeBo cores 127 (core GeoB21616-1) and 138 (core GeoB21637-1). The dashed line indicates seismic profile shown in (C). (C) Seismic profile across the SW sector of Lunde. Triangles indicate MeBo cores. The white vertical bar represents the maximum drilling depth of 23.95 m below seafloor, modified from (21). Note that the high-amplitude reflectors correspond to seep carbonates.

Fig. 2. Core lithology and representative seep carbonate samples with detailed U-Th ages (in ka ± 2σ).

(A) Splice of adjacent MeBo cores 127 (core GeoB21616-1) and 138 (core GeoB21637-1) showing seabed and core samples and respective range of U-Th ages. Carbonate vertical scale is exaggerated for better visibility. (B) Scan of cut surface of seabed sample P1606001. (C and D) Scans of representative core samples. White arrows point to bivalve shells. Mic, microcrystalline carbonate cemented sediment; Vfc, void-filling cement.

Fig. 3. Timing of seep carbonate formation (U-Th ages ± 2σ) relative to regional paleoclimate (vertical shaded bars) and glacial tectonics.

U-Th ages obtained from seabed samples indicate multiple post-LGM methane seepage and associated carbonate formation. Ages obtained from core samples [samples 127 (GeoB21616-1) and 138 (GeoB21637-1)] reveal two pre-LGM seepage periods between ~160 to 133 ka and ~50 to 40 ka. Timing of seep carbonate formation and paleoclimate variations suggest glacial tectonics, i.e., forebulge movement and post-glacial isostatic adjustment as main geological drivers of episodic methane release from Vestnesa Ridge. H4 and H5, Heinrich stadials (cold); HE1, Heinrich event 1 (cold); B-A, Bølling-Allerød interstadial (warm); YD, Younger Dryas (cold).

Fig. 4. Sketch illustrating the impact of glacial tectonics resulting from the waxing and waning of the Svalbard ice sheet on the subsurface fluid system of Vestnesa Ridge adapted from (20).

(A) During ice sheet growth, horizontal mass transfer within the viscous asthenosphere (open arrows) facilitates migration of a glacial forebulge underneath Vestnesa Ridge. Sediment compaction due to vertical crustal adjustment (black arrows) causes reservoir overpressure. Reactivation of subvertical faults, connecting the gas reservoir to the seabed, provides migration pathways for deep-sourced fluids with methane (CH₄) through the GHSZ. (B) Post-glacial relaxation and associated mass transfer in the asthenosphere (open arrows) result in isostatic adjustment (black arrows) and fault reactivation, providing fluid migration pathways (not to scale). BSR, bottom simulating reflector, indicating the lower boundary of the GHSZ; MTF, Molly transform fault.