Natural gas can exist in solid form of crystalline ice-like structures known as gas hydrates that are stable within the subsurface under high-pressure and low-temperature conditions bounded by the gas hydrate stability zone (GHSZ). The kinetics of hydrate formation and dissociation also critically depends on the supply and composition of gas and liquid water within available pore space of sediments, hence even under an appropriate envelope of GHSZ pressure and temperature conditions, gas hydrates are not, per se, guaranteed1. However, wherever persistent subsurface methane (or heavier fractions of natural gas) and water coexist within available pore space, then the GHSZ is a robust indication of the conditions under which gas hydrate is likely to form. The present distribution and stability of gas hydrates beneath oceans and permafrost, along with their potential to release large fluxes of methane and other potent greenhouse gases, are fundamental to determining long-term atmospheric composition and its impact on climate change. Previous research reveals that subglacial soils, lakes, peatlands and marine sediments can store significant reserves of carbon within a GHSZ beneath the palaeo-ice sheets that covered North America and also beneath the Antarctic ice sheet today2,3. It has been argued that as a result of active methanogenesis, this significant carbon pool could provide a major contribution to global atmospheric methane emissions and composition following deglaciation of Antarctica4. However, to date, few studies have investigated how gas hydrates responded to past climate change—specifically—the impact of extensive ice-sheet expansion on gas hydrate stability and dissociation during the last glaciation. In particular, three leading research questions warrant attention: how did the subglacial footprint of the former ice-sheet affect the GHSZ? How could this GHSZ govern methane storage and release across the glaciated margin? How could post-glacial ice-sheet retreat impact on this former subglacial gas hydrate reservoir?

Persistent and extensive discharge of methane gas flares into the water column offshore of Prins Karls Forland (PKF), western Svalbard has been reported since they were first observed in 2008 (refs 5, 6). More than 1,000 individual gas flares, predominantly ejecting methane (C 1 ∼98.9–99.9%) from known hydrocarbon sources7 cluster across a broad zone of the seabed between 80 and 420 m depth8 (Fig. 1). Gas flares can be grouped in two distinct sets based on depth: a deep zone that spans 380–420 m below sea level (m.b.s.l.) and a shallow zone between 80 and 130 m.b.s.l. Deeper gas flares can theoretically be associated with the base of the present-day GHSZ, which pinches out on the seabed at 396 m water depth6. It has been argued that a temperature increase of 1 °C within the West Spitsbergen Current bottom water and/or annual fluctuations of 0.6–4.9 °C has been sufficient to force a downslope migration of the GHSZ in this area from 360 to 396 m.b.s.l. over the past three decades5,6. Despite a lack of records of vertical fluid flow in the study area, U/Th isotope analyses on authigenic carbonate records reveal that there has been significant methane flow-induced precipitation at the deeper gas flares sites since at least 3 ka (ref. 5). In contrast, shallow gas flares cluster across the main ridge of the Forlandet moraine complex9 (Fig. 1). The region is characterized by a sediment blanket that diminishes from several hundred metres over the continental slope to a minimum of a few tens of metres on the shelf10,11. The tectonically induced Forlandsundet graben (Fig. 1) is infilled with several kilometres thick sediment section12,13, and hence has major potential to host free gas. Sediment thickness to the southeast at Isfjorden varies up to ∼100 m (ref. 14). Hydrocarbon source rocks are extensive in the Svalbard region, including Triassic and Early Jurassic formations7, as well as organic-rich Miocene deposits15. The major hydrocarbon reservoirs within our study area are concentrated within Early and Middle Triassic source sequences7. Natural gas including thermogenic methane—the lightest hydrocarbon fraction—is generated under high pressure and temperatures up to 200 °C at depth within sedimentary basins from a mixture of insoluble organic compounds known as kerogen16. In 1992, during exploratory drilling in Svalbard, gas blow outs from depths of 630 m beneath today’s sediment surface brought operations to a complete halt17.

Figure 1: The West Svalbard shelf at present. The western Svalbard margin (IBCAO v.3 (ref. 51) in grey and high-resolution multibeam data in the blue scale—see Methods) showing the observational compilation used in the modelling experiments along the transect a to b (semi-transparent vertical curtain). Yellow and red triangles show geothermal temperature gradients (19–112 °C km−1)5,35,36 and average long-term bottom water temperatures (−0.8 to 2.2 °C)32, respectively, which were used to constrain the LGM boundary conditions along the transect. Also indicated are the LGM marine limits25,28, used to estimate isostatic loading (25 and 48 m in white rounds), minimum ice-surface elevation21 (green flags), location of modern GHSZ, approximate LGM ice-sheet limit and modern gas flare locations (see Methods). Inset is the location of the study area in respect to pockmark fields (red ovals) and major tectonic lineaments of Hornsund fracture zone (dashed brown lines) across the western Svalbard margin. Full size image

It is well established that the ultimate stable ice-sheet stand across the West Spitsbergen shelf was concurrent with maximum thickness and horizontal extent (representing the Last Glacial Maximum (LGM) stage) and persisted for at least 5 kyr (20–15 ka)18,19. Onshore and offshore radiocarbon dating20, cosmogenic 10Be surface exposure dating20,21, numerical modelling22, offshore high-resolution multibeam23 and seismic surveys9 reveal a LGM sequence that extended across the continental margin to the shelf break. By inference, the ice sheet in this sector was cold based and covered the Spitsbergen continental shelf to a distance of ∼45 km offshore (∼150 m.b.s.l.; Fig. 1). Field evidence further reveals that the ice surface was at least ≥473 m above the present sea level (m.a.s.l.) as determined from the 10Be exposure age of the boulders over the PKF, and ∼700–900 m.a.s.l. over the west Spitsbergen margin21,22 (Fig. 1). Wet-based, fast-flowing ice streams discharged across the shelf from Kongsfjorden and Isfjorden, and bounded the cold-based ice lobe that flowed across our study area and west of PKF24.

Radiocarbon (14C) dating of whale bones and mollusk shells sampled on the raised beaches define the earliest post-LGM marine limits on the northern PKF (25 m.a.s.l.) and western Spitsbergen (45–48 m.a.s.l.)25,26 (Fig. 1). The resulting sea level curves define a local glacio-isostatic loading scenario that was at least 100 m at PKF and western Spitsbergen. The time scale for ice-sheet stabilization at its LGM stand and subsequent retreat was 20–15 ka in this region of western Svalbard, followed by complete deglaciation of the continental margin that was complete by 12–10 ka (ref. 24). This glacial chronology is consistent with an initial phase of relatively slow post-glacial isostatic rebound of 1.5–5 m ka−1 for West and North Svalbard that commenced at ∼13–12 ka, followed by an episode of accelerated uplift (15–30 m ka−1) between 10.5 and 9 ka (ref. 27).

In this study, we model the impact of the paleo-ice sheet on the GHSZ offshore of western Svalbard by integrating geophysical mapping with the glacial geology. Our results reveal a potentially large subglacial gas hydrate reservoir that accumulated under high-pressure/low-temperature LGM conditions and that would have subsequently dissociated, releasing methane during deglaciation and marine incursion.