Deep-sea gas hydrate mounds and chemosynthetic fauna discovered at 3640 m on the Molloy Ridge, Greenland Sea

Abstract

Methane seepage at the seafloor can form gas hydrate and sustain chemosynthetic communities of deep-sea animals. Most known hydrate seeps occur shallower than 2000 m on continental slopes, whereas hydrothermal vents are found at greater depths along active spreading centres. Here we report the discovery of hydrate mounds with cold-seep fauna at 3640 m deep on the Molloy Ridge. The mounds display seafloor morphologies resulting from progressive stages of hydrate dissociation. Gas bubbles from the mounds rise to within 300 m of the ocean surface, and isotopic analysis shows the hydrates contain thermogenic gas. Crude oil sampled from the hydrate deposits indicates a young Miocene source rock formed in a fresh-brackish water paleo-environment. The hydrate mounds are inhabited by taxa including siboglinid and maldanid tubeworms, skeneid and rissoid snails, and melitid amphipods. Family-level composition of the fauna is similar to that of Arctic hydrothermal vents at similar depths, including the Jøtul vent field on the Knipovich Ridge, and less similar to nearby methane seeps at shallower depths. The overlap between seep and vent fauna in the Arctic has implications for understanding ecological connectivity across deep-sea habitats and assessing their vulnerability to future impacts from seafloor resource extraction in the region.

Fig. 1. Overview of the location of high-Arctic (>72 °N) cold seeps and hydrothermal vents.

a regional map of seeps (yellow) and vents (orange): yellow star = Freya gas hydrate mounds; orange star = Jøtul vent field 1 = Vestnesa Ridge seeps; 2 = Prins Karls Forland seeps; 3 = Storfjordrenna gas hydrate mounds; 4 = Bjørnøyrenna seeps; 5 = Leirdjupet Fault Complex seeps; 6 = Borealis Mud Volcano; 7 = Håkon Mosby Mud Volcano; 8 = Loki’s Castle; 9 = Aurora Vent Field. Seabed topography shown is from the Global Multi-Resolution Topography (GMRT) synthesis. b map of seafloor features observed during ROV dives at the Freya gas hydrate mounds (79.6 °N, depth 3640 m). Detailed bathymetry from MAREANO/Norwegian Mapping Authority.

Fig. 2. Water column characteristics at the Freya gas hydrate mounds.

a Depth profiles of temperature (red) and salinity (blue) measured by CTD, and b is topography as processed with Qimera and acoustic backscatter processed with FMMidwater using the shipboard multibeam echosounder (MBES) at the Freya gas hydrate mounds (79.6 °N, depth 3640 m). The dashed lines show the seafloor depth and the maximum depth of the top of the flare.

Fig. 3. Freya gas hydrate mounds showing different morphologies.

The mounds, made of hydrates, are covered by sediments and frenulate tubeworms forming a ‘Sclerolinum forest’ (a) with occasionally amphipods and caridean red shrimp (b, d). Sometimes, around and at the top of the mounds, there are centimetric carbonate crusts (b). c Shows the position where the sample of gas hydrate for geochemical analyses was taken (yellow star; Supplementary Fig. 1) and the sediment sample used for faunal identification, that on board also revealed the presence of oil. c, d The influence of hydrate buoyancy on mound morphology that leads to structural fractures and alterations in the integrity of the mounds, ultimately resulting in the formation of collapse-like features (e). f Background seafloor.

Fig. 4. Geochemistry of the gas emitted from Freya gas hydrate mounds.

Molecular and isotopic (δ¹³C, δD) composition of the gas contained in the gas hydrate. Sample data from Freya gas hydrate mounds are reported in yellow stars. For comparison, other high-latitudes cold seeps (location in Fig. 1) are reported: Borealis in ref. , Håkon Mosby Mud Volcano, Prins Karl Forland, Leirdjupet Fault Complex, Vestnesa Ridge, Storfjordrenna and Bjørnøyrenna. Genetic fields of hydrocarbons (CR-CO₂ reduction, F—methyl-type fermentation, EMT—early mature thermogenic gas, OA—oil-associated thermogenic gas, LMT—late mature thermogenic gas) after. a Isotopic composition of methane. b Plot of δ¹³C-CH₄ versus the composition of light hydrocarbon components (C₁/(C₂ + C₃) ratio). Grey arrows indicate the main processes affecting gases’ isotopic and molecular compositions. c Isotopic composition of CO₂ (δ¹³C-CH₄) versus methane δ¹³C-CH₄. The combination of the three plots indicates that the methane in the Freya gas hydrate mounds has a thermogenic origin.

Fig. 5. Fauna of the Freya gas hydrate mounds.

a In situ hydrate mound fauna, including Sclerolinum forest. b Tube-dwelling maldanid polychaete. c Melitid amphipod. d Ampharetid polychaete. e Stauromedusa Lucernaria cf. bathyphila. f Rissoid and skeneid gastropods on a maldanid polychaete tube. g Thyasirid bivalve.

Fig. 6. Family-level faunal similarity at high-Arctic (>72 °N) cold seeps and hydrothermal vents.

a Regional map of seeps (yellow) and vents (orange): yellow star = Freya gas hydrate mounds; orange star = Jøtul vent field, sites: 1 Vestnesa Ridge seeps; 2 = Prins Karls Forland seeps; 3 = Storfjordrenna gas hydrate mounds; 4 = Bjørnøyrenna seeps; 5 = Leirdjupet Fault Complex seeps; 6 = Borealis Mud Volcano; 7 = Håkon Mosby Mud Volcano; 8 = Loki’s Castle; 9 = Aurora Vent Field. Seabed topography shown is from the Global Multi-Resolution Topography (GMRT) synthesis. b Dendrogram of faunal similarity between sites from hierarchical single-linkage agglomerative clustering based on Sørensen Index. Data analysed from this study and published literature for sites (76 families at 8 sites; for data sources, please see Supplementary Table 1; data for Storfjordrenna and Bjørnøyrenna are combined because separate inventories are unavailable in the literature). c Two-dimensional ordination of faunal similarity between sites from non-metric multidimensional scaling (nMDS) based on Sørensen Index. Bubble diameters represent site depths (starred yellow bubble = Freya gas hydrate mounds; starred orange bubble = Jøtul vent field).