The stratigraphy of cores is based primarily on patterns of variability in magnetic susceptibility (MS) values in combination with lithology, sediment color, grain size, δ¹⁸O, %TOC, %CaCO₃ and AMS ¹⁴C dates and by correlation to the magnetic susceptibility stack of Jessen et al. (2010) that are based on 11 cores from the western Svalbard margin. The pattern of variation in MS values is linked to characteristic sediment marker horizons: a dark grey-brown, coarse, unsorted layer of low magnetic susceptibility dating ∼24 ka (marked by a brown bar in Figures 4–6; Supplementary Figure S2–S5), a dark bluish-grey laminated clay deposit also of low magnetic susceptibility dating ∼15 ka (grey bar). A diatom layer deposited in the lower Holocene dates ∼10 ka (Jessen et al., 2010) (yellow bar).

The MS stratigraphy and age model of Jessen et al. (2010) cover the last 30 ka (Figure 4A). The stratigraphy has since been extended into MIS 4 (Rasmussen et al., 2014; Jessen and Rasmussen, 2015; El bani Altuna et al., 2021a) and through MIS 5 into MIS 6 (Michel et al., 1999; Risebrobakken et al., 2005). Most cores from western Vestnesa Ridge only reach into late MIS 3 (Figures 4I–L) while cores from off the crest and eastern Vestnesa Ridge reach into early or mid-MIS 3 or MIS 4 (Figures 4B–F; Supplementary Figure S2D). Only one record, the giant piston core KH19-09GPC reach into MIS 6 (Supplementary Figure S2E).

Deposits of MIS 6 (>130 ka) are predominantly of dark grey color, while MIS 5, MIS 4 and MIS 3 130–75 ka (75–60 ka, and 60–29 ka, respectively) are composed of layers of light greyish sediment and darker grey layers with a varying content of IRD and highly variable MS and δ¹⁸O values reflecting the DO millennial scale events (Michel et al., 1999; Rasmussen et al., 2014; Jessen and Rasmussen, 2015; 2019; Caricchi et al., 2019) (Figures 5A,C,E). The sediments of MIS 2 (28–12 ka) has layers of both light grey, and dark grey color. The last glacial maximum (LGM) is defined as the period of maximum oxygen isotope values dating 24–19 ka (Lisiecki and Raymo, 2005) (Figure 5; Supplementary Figure S4). The start of the LGM is marked by the coarse dark grey-brown layer.

The Heinrich stadial H1 of the early deglaciation on the western and northwestern Svalbard margin is generally marked by a thin light-grey horizon dating ∼19–15 ka and is characterized by very low sedimentation rates and a characteristic spike of low δ¹⁸O values (e.g., Birgel and Hass, 2004; Jessen et al., 2010; Jessen and Rasmussen, 2019) (Figure 5; Supplementary Figure S4). The Bølling interstadial of the deglaciation (∼15–14 ka) is marked by the dark-grey, bluish laminated layer of low MS seen in all records (Figures 4–6; Supplementary Figure S2–S5). The sediments of the following Allerød interstadial 14–13 ka contain a high concentration of IRD, while the Younger Dryas stadial (13–12 (11.7) ka) appears as a light grey layer with low δ¹⁸O (Figure 5A, C–G).

The Holocene interglacial is characterized by light brownish sediments. The cores show a typical Holocene MS signal of slowly increasing values to a peak dating ∼8.2 ka followed by gradually decreasing values (Jessen et al., 2010) (Figure 4A, H–K). The δ¹⁸O values are low characteristic for interglacial conditions (Figures 5A–C, E–G; Supplementary Figure S5C,F,G). The diatomite of whitish or yellowish color deposited at ∼10 ka in the early Holocene is present in most records from western Vestnesa Ridge areas, but has only been observed in a few cores from the eastern part as Holocene deposits are either thin or lacking here (Sztybor and Rasmussen, 2017a; b; Thomsen et al., 2019; El bani Altuna et al., 2021a) (Figures 4, 7; Supplementary Figure S2, S3).

Several cores from within pockmarks on Vestnesa Ridge has low, and sometimes nearly constant MS values (Figure 4; Supplementary Figure S1, S3). This is due to disturbance by strong seepage of methane or presence of gas hydrates (e.g., Novosel et al., 2005). However, in most cases the records display an undisturbed lithology (e.g., Myrvang, 2015; Sztybor and Rasmussen, 2017a; Åsheim, 2017; Matteis, 2018; Hansen et al., 2020) (Figure 4; Supplementary Figure S2, S3). Other cores from pockmarks can show the typical pattern and values of MS as found elsewhere on the western and northern Svalbard margin (e.g., Elverhøi et al., 1995; Knies et al., 1999; Birgel and Hass, 2004; Jessen et al., 2010; Chauhan et al., 2016a) (Figure 4E; Supplementary Figure S3B). Note that some records with low MS values but plotted on an expanded x-axis scale in fact do display the typical MS pattern (Figures 4I,J; Supplementary Figure S3A,C,F–H).

Cores from Vestnesa Ridge and outside of pockmark areas and published cores JM10-335GC, JM10-333GC (Sztybor, 2016), HH16-549GC (Åsheim, 2017), and cores from 2016 with a known stratigraphy from oxygen isotopes and/or AMS-¹⁴C dates, all show a clear correlation between %TOC and %CaCO₃ data with lithology (Figures 6A–C, F–H; Supplementary Figure S5C). The coarse unsorted dark layer and the laminated horizon show high %TOC and low %CaCO3, while the LGM have low %TOC and high %CaCO₃. Some cores show some deviating peaks in % CaCO₃ (Figures 6D,E; Supplementary Figure S5D,F).

The same pattern in distribution of %TOC and %CaCO₃ is seen in cores from the inactive western pockmark sites (Figures 6G,H; Supplementary Figure S5G, H). Here, the Holocene sediments show high %TOC >1%, and high %CaCO₃ >10%. Four cores sampled for pore water, %TOC and %CaCO₃ and presumably containing gas hydrates were not logged for magnetic susceptibility or opened for description. Based on the %TOC and %CaCO₃ records the stratigraphic marker horizons and Holocene deposits could be identified for three of the cores (Supplementary Figure S5A, B, F).

The δ¹³C records show generally high δ¹³C values in cores taken outside of pockmarks and highly variable values in records from inside of pockmarks. In environments unaffected by methane seepage δ¹³C values in the species measured in this study range −2 to 0‰ for the infaunal benthic foraminiferal species M. barleeanus and C. neoteretis on glacial and interglacial time scale (McCorkle et al., 1990; El bani Altuna et al., 2021a) and 0 to >+1‰ for the epifaunal species C. wuellerstorfi (e.g., McCorkle et al., 1990; 1997). In cores taken from pockmarks many have intervals of values <-10‰ (Figures 5A–C,E,G; Supplementary Figure S4C–E), a sign that the measured foraminiferal specimens have had coatings with authigenic carbonate, although not visible in the binocular microscope. Other cores have slightly lower δ¹³C than the typical range of the species (Figures 5D,F; Supplementary Figure S4A,B,E). The low δ¹³C intervals correlate with the glacial period and the deglaciation with peak low values in the Bølling and Allerød interstadials and lower Holocene until ∼10–9 ka (Figures 5A,B, D–G; Supplementary Figure S4A–C,E).

Cores taken on the crest outside of pockmarks on eastern Vestnesa Ridge contain no Holocene sediments but show a coarse sandy, gravelly lag deposit at the top dating ∼14 ka right below the sediment surface (Sztybor and Rasmussen, 2017a) (Supplementary Figure S2C,D,F–I). The cores taken from inside of pockmarks at the very active pockmark field of seepage of methane at ∼1,200 m water depth (Figures 1B,D,E) cover MIS 4–2, ∼60–14 ka (except for some cores that may contain sediments of lower Holocene age dating <10 ka near the core top) (Figures 4B–G, Figure 5A–C). Sediments dating from the LGM (not including the coarse dark deposit dating ∼24 ka) are around 1–2 m thick, giving an average sedimentation rate for the period 24–19 ka of 23 cm/ka (range 19–34 cm/ka) (Table 2). Sediments from the deglaciation including the laminated horizon are thick with an average sedimentation rate of 91 cm/ka (range 38–140 cm/ka) (ignoring H1 of very low sedimentation rates of average ∼3 cm/ka (range 1.6–4.4 cm/ka)) (Table 2). Thus, in most cores from ∼1,200 m water depth, the laminated layer occurs close to the seafloor at about 20–100 cm down core depending on presence of sediments dating from the Early Holocene (Figures 4B,C,F). The longest record core HH15-1255PC reaches back to ∼60 ka and has an average depositional rate for MIS 3 of ∼10 cm/ka with variable MS values indicating millennial scale oscillations (Matteis, 2018) (Supplementary Figure S3F).

The southern ridge flank is relatively steep and affected by faulting and slumping, but not affected by seepage of gas and represented by cores 067GC and MSM5/5-712 (Figure 1A; Supplementary Table S1). These cores show the same characteristic MS signal for the time interval 30–14 ka (Howe et al., 2008; Zamelczyk et al., 2014). The shallow core 067 is very similar to the records from the eastern pockmark area with no Holocene and dating approximately 14 ka at the top. The record has similarly high sedimentation rates of 113 cm/ka for the deglaciation and 20 cm/ka for the LGM interval (Table 2). Core MSM5/5-712 (Müller et al., 2012; Zamelczyk et al., 2014; Falardeau et al., 2018; 2019) has 3.5 m of Holocene sediments (average Holocene sedimentation rates ∼30 cm/ka), deglaciation ∼87 cm/ka and 22 cm/ka for the LGM (Table 2).

We note that both the coarse dark, grey-brown layer and the laminated layer are thinner than in the records from eastern Vestnesa Ridge (Figures 4, 7; Supplementary Figure S3). The deposits dating from the LGM are also much thinner. The average sedimentation rates for the LGM are thus 10 cm/ka (range 2–22 cm/ka) at 1,300 m water depth compared to 23 cm/ka at ∼1,200 m water depth (Table 2). The sedimentation rates for the deglaciation (ignoring H1; see Table 2) are also lower than at eastern Vestnesa Ridge and is of 39 cm/ka (range 22–86 cm/ka) compared to almost 1 m/ka on the inner part at 1,200 m water depth.

In contrast to the records from the eastern part, western records show presence of >2.5 to >3 m thick Holocene deposits (Figures 4H–L, 7; Supplementary Figure S3G–J). The well-dated record of core JM10-330GC (Consolaro et al., 2015) confirms that the entire Holocene probably is preserved in all records except core HH12-940PC. The diatomite is prominent and located ∼2 m to >3 m below the seafloor. The Holocene sediments at western Vestnesa Ridge are generally deposited at high average sedimentation rates of 26 cm/ka (range 19–32 cm/ka) (Table 2).

The longest record HH12-940PC dates ∼50 ka at the bottom (calculated by extrapolation from the two lowermost dates in the core; Figure 4H). Thus, the average sedimentation rate for the mid-late MIS 3 at 28–50 ka is calculated to ∼13 cm/ka (Table 2).

Cores HH15-1250GC, HH15-1251GC and HH15-1252PC are all taken on the northern flank of Vestnesa Ridge off the active pockmarks at the eastern part of the ridge (Figure 1A; Supplementary Figure S2C,D,H; Supplementary Table S1). They are taken at similar water depths (1,265 m, 1,273 m, and 1,273 m, respectively). All three cores possess the typical lithological features and patterns of MS and shows a lag deposit at the core top dating ∼14 ka and thus no Holocene deposits as for records on the eastern crest of the ridge. The average sedimentation rate for the LGM in the three records is 30 cm/ka (Table 2). In core HH15-1252PC which reach into MIS 4 (El bani Altuna et al., 2021a) (Figure 7L; Supplementary Figure S2D), the average sedimentation rate for MIS 3 is 19 cm/ka, the highest so far compared to sedimentation rates on the crest of the ridge.

Core HH15-1274GC taken on the northern flank of western Vestnesa Ridge at 1,514 m water depth is very comparable to the cores from the western crest of Vestnesa with ∼3 m of sediments of Holocene age (Figure 7E; Supplementary Figure S2B). For this site the average sedimentation rate is 24 cm/ka for the Holocene, 41 cm/ka for the deglaciation and 11 cm/ka for the LGM (Table 2). Core MSM5/5-723 from 1,350 m water depth shows >4 m of Holocene sediments with an average sedimentation rate exceeding 40 cm/ka (Werner et al., 2015) (Figure 1A).

Giant piston core KH19-09GPC of high resolution from 1767 m water depth north of Vestnesa Ridge and north of site HH15-1274GC shows an MS pattern with the characteristic intervals of low values marking the laminated and coarse grey-brown horizons and indicating that ∼1 m of Holocene sediments is present at this location (Supplementary Figure S2E). From comparison and correlation with the MS records of cores MD99-2303 and MD99-2304 from the western Svalbard margin (Michel et al., 1999) and age models by Risebrobakken et al. (2005), it becomes evident that core KH19-09GPC probably reach well into MIS 6 and possibly MIS 7 at the bottom dating >150 ka. In core KH19-09GPC, MIS 3 has an average sedimentation rate of 12 cm/ka; low rates deeper in the core are probably due to some level of compaction (Table 2).

The distribution of %TOC and %CaCO₃ appears very consistent between seep- and non-seep records and between the western and eastern pockmark fields regardless of time resolution (Figure 6; Supplementary Figure S5). Similar distribution patterns of %TOC and %CaCO₃ have also been recorded elsewhere from the Svalbard margin (e.g., Vogt et al., 2001). This indicates that these two parameters together form a good correlation tool for both seep- and non-seep sites. However, at the eastern active pockmark field at Vestnesa Ridge some calcium carbonate records show very high values and peaks in CaCO₃ that do not correlate with the general pattern (Figures 6D,E; Supplementary Figure S5A,F). This we attribute to precipitation of authigenic carbonate in these apparently most active core sites. For example, the carbonate peak in core HH12-929GC correlates with a layer of high concentrations of calcareous nodules of authigenic carbonate (Thomsen et al., 2019) (Figure 6D).

The high %TOC for the dark coarse layer (part debris flow, part IRD layer) and for the laminated layer (deposited by meltwater plumes) is probably mainly a cause of deposition of terrestrial carbon transported out to sea (e.g., Vogt et al., 2001) (Figure 6; Supplementary Figure S5). The Holocene sediments younger than ∼10 ka differs from the earlier periods by being characterized by both high %TOC and %CaCO₃ and deposition of fine muds. The high organic content during the Holocene is mainly of marine origin (e.g., Vogt et al., 2001).

Events of low δ¹³C are found in MIS 3, and MIS 2, including parts of the LGM (Figure 5; Supplementary Figure S4; note MIS 3 events are not highlighted). In pockmark cores, the debris flow/IRD event dating ∼24 ka generally show low values (Figures 5A,C–E; note no data points for this event in Figures 5B,G–I). The best resolved δ¹³C variations occurs during the deglaciation; therefore, we focus the discussion on these millennial scale events.

Records from both the presently apparently inactive pockmarks at western Vestnesa and the eastern active pockmarks tend to show recurrent patterns and timings in low δ¹³C values, while records unaffected by seepage show comparatively high δ¹³C throughout and within the typical range of the measured species (Figure 5; Supplementary Figure S4). The records affected by seepage thus show low values in the laminated horizon dating ∼15 ka correlating with the Bølling interstadial, in the Allerød interstadial and early Holocene indicating strong seepage of gas (note that H1 and Younger Dryas intervals generally have comparatively higher δ¹³C values of −3 to −2‰ just below the typical range of the measured species and indicating low to moderate seepage during these events). In addition, records from the SW Svalbard shelf at 76 °N also show this pattern with the same timing as on Vestnesa Ridge indicating that a large geographical area was affected by seepage at the same time (El bani Altuna et al., 2021b and references therein). Three records with the typical configuration of high MS values all show low δ¹³C in the laminated horizon, one core being from outside of pockmarks, the other two from inside the ‘Lomvi’ pockmark (Figure 5D; Supplementary Figure S4A,B). This indicates a particularly strong seepage event at that time. Holocene deposits younger than ∼9–10 ka BP in all studied cores affected by seepage have typical δ¹³C values for the measured benthic foraminiferal species indicating a reduction in gas seepage (Figures 5A–C,E–G).

Even if these low δ¹³C events (often <-10‰) are due to coating of the foraminiferal shells with authigenic carbonate, it would be difficult to reconcile that these recurrent patterns in low δ¹³C with similar timing and over a large geographical area, in particular in MIS 2 to lower MIS 1, is a random distribution (Figure 5; Supplementary Figure S4). The four lithological units of low δ¹³C are sedimentologically very different (the unsorted dark layer deposited from debris flow evens, the laminated horizon from turbid meltwater plumes, the Allerød period from intense ice rafting, and the early Holocene, with decreasing ice rafting). Therefore, similarity in timing indicates that the events must have a forcing in the pace of the DO millennial scale events. Bottom water temperature increased by up to 6 °C during Heinrich stadial H1 as shown by nearby core record HH15-1252PC from the northern flank of Vestnesa Ridge. The BWT reached a maximum of 5.5 °C before the start of the Bølling interstadial (El bani Altuna et al., 2021a) (Figure 1A; Figure 2E). The temperature rise is probably the likely cause of dissociation of gas hydrates (e.g., Buffet and Archer, 2004; Ketzer et al., 2020) as also shown by modeling of the thickness of the GHSZ at the shelf of southwestern Svalbard (El bani Altuna et al., 2021b). The presence of chemosymbiotic bivalves in H1 in many pockmark records from both the western and eastern part of Vestnesa Ridge points to low to moderate seepage during most of the event (Figures 4A–C,H; Supplementary Figure S3A). Seep-associated- or chemosymbiotic macrofaunas tend to avoid areas of very strong seepage that causes toxic levels of H₂S (e.g., Sahling et al., 2002; Levin et al., 2003). We note that seepage in older Heinrich stadials also was low to moderate where many also are marked by layers of chemosymbiotic bivalves (Figures 5B,H).

There is an apparent time lag from the peak warm bottom water conditions during H1 until gas seepage intensified during the Bølling and Allerød interstadials. Thus, downward diffusion of heat took time to reach substantial layers of gas hydrates. Several modeling studies have shown that in the deep sea, the time from bottom water warming to increased emission of gas can take a thousand to several thousands of years (e.g., Phrampus and Hornbach, 2012; Karstens et al., 2018). Other studies have invoked active faulting from tectonism due to the proximity of Vestnesa Ridge to active spreading zones, the Molloy Transform Fault, and/or offshore glacial isostatic rebound (e.g., Plaza-Faverola et al., 2015; Dazcinnies et al., 2021; Vachon et al., 2022). However, these slower processes alone cannot explain the rapid millennial DO-scale fluctuations in gas emissions that aligns with climatic and oceanographic changes. Another possible forcing factor is the high sediment loading during the deglaciation (Bølling and Allerød interstadials and lower Holocene) (Figure 7; Table 2). High sedimentation rates at the Vøring Plateau caused an increase in temperature in the sediments eventually destabilizing gas hydrates with a time delay of 1,000 years (e.g., Karstens et al., 2018). However, the high sedimentation rates we record at Vestnesa Ridge is an order of magnitude lower than at the Vøring Plateau (∼1 m/ka versus >10 m/ka) (Table 2) but could have added to the effect from the bottom water warming. Hydrostatic pressure would have counteracted the effect of temperature rise due to the rapidly increasing sea level during the deglaciation (e.g., Fairbanks, 1989). Then again, the effects of sea level changes are small in deep water and much smaller than the effect of a 3 °C temperature increase that would thin the GHSZ by 85% (e.g., Buffett and Archer, 2004).

Core HH12-930GC differs by having only slightly decreased δ¹³C values in the Bølling-Allerød interstadials and early Holocene compared to nearby core HH16-549GC just 11 m away. In older sediments of mid-late MIS 3 and early MIS 2, HH12-930GC shows a pattern in δ¹³C similar to the other cores (Myrvang, 2015; Chu et al., 2023) (Figures 5C,E). The reason for this deviation for the deglaciation interval is unknown but taken as a signal of heterogeneity in the ‘Lunde’ pockmark.

Since the δ¹³C patterns and timing of low values are similar between western and eastern records there is clear evidence of periodical intensification of seepage in the western part of the ridge and that the area was as active as the eastern part during MIS 3, 2 and lower MIS 1 until ∼10–9 ka and with high δ¹³C from that time onwards (Figures 4, 5; Supplementary Figure S3, S4). The time of decrease in gas emission correlates approximately with the time when ice rafting ceased or decreased over the Svalbard margin and the Svalbard-Barents Sea Ice Sheet had retreated into fjord heads or further inland (e.g., Ślubowska et al., 2005; Ślubowska-Woldengen et al., 2007; Forwick and Vorren, 2009). The rapidly settled fine-grained Holocene mud deposited from the contour currents and with its high content of TOC and CaCO₃ (Figures 5H,I; Supplementary Figure S5F,G) probably reduced seepage to a slow diffusion of gas through the sediments. We suggest that this slow diffusion formed a basis for efficient oxidation by the microbial consortia (and that little if any gas would escape because of the ‘microbial filter’ or ‘methanotrophic sink’; e.g., James et al., 2016). 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; 2023; 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. According to our results the change from advective to diffusive seepage probably occurred at ∼10 ka. Nevertheless, active tectonism and onshore and offshore isostatic rebound continued after ∼10 ka and until today (e.g., Forman et al., 2004; Vachon et al., 2022; Domel et al., 2023). Fine-grained drift sediments can often seal fractured or coarse, porous glacial sediments in contourites exampled by studies of the very seismically active Hikurangi Margin, New Zealand (Bailey et al., 2021).

In the eastern part of Vestnesa Ridge, Holocene sediments are generally absent but occurs locally within some pockmarks (Figures 5A–D; Supplementary Figure S4C). Here, even a thin cover of fine Holocene sediments apparently caused reduced seepage at ∼10–9 ka as seen in high δ¹³C values as in for example, core JM10-335GC. Nearby core HH12-928PC, has no Holocene sediments and low δ¹³C throughout and contain large amounts of authigenic carbonates indicate strong seepage (Figures 6A,E; Supplementary Figure S4D). The two cores were taken about 40 m apart in the elevated area at the rim of ‘Lomvi’ pockmark (Figure 1E). Likely the upward migrating gas found other escape routes (e.g., Treude et al., 2020), which may have focused seepage to sites with no Holocene sediments younger than ∼10 ka, or strong seepage could have prevented settling at some of these sites. In the deep depression in ‘Lomvi’ pockmark core HH12-929GC and nearby core HH15-1293PC contained gas hydrates from ∼125 cm below the seafloor. Both shows the low and constant MS patterns. Core HH16-551GC taken 16 m away shows the typical MS signal for the western Svalbard margin, and contained no gas hydrate (Figure 1E; Figure 4E,F; Supplementary Figure S1I), The SMTZ was located >4 m in core 551GC (Laier, 2017). We have no porewater data for core HH12-929GC, but likely the SMTZ here was much shallower. A hiatus occurs from the top of the layer of authigenic nodules at 110 cm downcore covering the time interval 20–8 ka in the early Holocene probably indicating strong seepage in combination with bottom current activity and that nodules formed at the sediment surface (Thomsen et al., 2019) (Figure 6D). In core HH16-549GC from ‘Lunde’ pockmark (Figure 1D), the SMTZ was located ∼1.5 m below the seafloor (Laier, 2017; Åsheim, 2017). In the central part of the pockmark in bacterial mats the SMTZ was at the seafloor (Melaniuk et al., 2022a). Thus, the sedimentary and seep environments and the depth of the SMTZ are very variable within both the ‘Lomvi’ and ‘Lunde’ pockmarks and within short distances. At western Vestnesa Ridge in the southern pockmark, core JM10-331GC show low magnetic susceptibility while core HH17-839GC shows typical MS values indicating some heterogeneity here (Figure 1C; Supplementary Figure S3H,I) and likely in other pockmarks as well.

The pockmarks and gas seepage on Vestnesa Ridge have primarily been investigated by 2D reflection seismic data (e.g., Hustoft et al., 2009; Bünz et al., 2012). Using high-resolution Chirp data as in the present study provides means for more detailed observations of gas seepages and free gas in the shallow subsurface.

Several of gas-related acoustic phenomena have been observed on Chirp lines passing along the crest of Vestnesa Ridge, i.e., partly or completely wipeout of the signal (known as acoustic blanking), chaotic reflection patterns and lack of penetration, downwards bending seismic reflectors caused by reduced seismic velocity, and high-amplitude reflections (known as bright spots) caused by high gas and/or hydrate content.

Chirp lines crossing active pockmarks in the eastern active pockmark area mostly shows a completely chaotic seismic signal with no reflections from neither the mapped MIS 5–2 units or any older depositional units (Figure 9A). This seismic pattern confirms that sediment deformation caused by active seepage of gas occurs on the eastern Vestnesa Ridge. Further, regular occurrences of acoustic blanking signify that free gas also exists outside the pockmarks in this area, as also evidenced by sediment cores (Figure 6G; Supplementary Figure S4A, S5C).

Chirp lines from the western Vestnesa Ridge also display existence of pockmarks and several acoustic phenomena evidencing presence of free gas and gas hydrates in the sediment column despite the lack of acoustic flare observations in this part of the ridge area (Figure 9B). This is taken as evidence that gas seepages in the western ridge area have either slowed down or are prevented in reaching to the seafloor.

A notable difference between the eastern and western pockmark areas are the presence of the MIS 1 unit in the latter area, while this unit is missing in the eastern area (Figures 9A,B). This could hint that presence or lack of MIS 1 sediments may play a role in gas seepage activity at Vestnesa Ridge. This is also supported by core data from both eastern and western pockmarks (Figures 1C–E; Figures 5A–C, E–G; Supplementary Figure S3H,I). In addition, Chirp data crossing some of the pockmarks in the western area show that a disturbed and chaotic pattern characterize the MIS 5–3 units and deeper layers, while the overlaying MIS 1 and partly MIS 2 units appear relatively undisturbed. This suggest that similar to that of the present eastern pockmark area, some heterogeneity in the seepage patterns existed in the western pockmark area, as also evidenced by the core study.