Accurately attributing atmospheric fossil methane to either anthropogenic or natural (geological) sources remains a significant challenge due to a lack of distinct chemical markers. Understanding the distribution and contribution of geological methane sources is crucial for refining global methane budgets and climate change predictions. Current estimates of atmospheric methane increase (9.3 ± 2.4 ppb yr⁻¹ from 2014-2019) underscore the urgency to understand its sources. A substantial discrepancy exists between top-down (inverse modeling) and bottom-up (empirical upscaling) estimates of global methane budgets, highlighting the incomplete understanding of sources and sinks. Discriminating between fossil methane sources (e.g., hydrocarbon extraction, coal mining, natural seepage) is particularly difficult due to their similar isotopic signatures (δ¹³C and δD). Traditional approaches often attribute marine methane sources to microbial processes in shallow sediments, overlooking the potential contribution of thermogenic methane from deeper reservoirs. The Arctic, with its vast hydrocarbon reserves and history of glacial erosion, presents a particularly important yet understudied region. This study focuses on the Barents Sea, a region with a complex history of uplift and erosion, significant hydrocarbon resources, and a potential for extensive thermogenic hydrocarbon leakage. The aim is to investigate the extent of this leakage, its characteristics, and its implications for global methane budgets.

Existing literature highlights the challenges in quantifying global methane emissions, particularly distinguishing between anthropogenic and natural sources. Studies have predominantly focused on microbial methane production in shallow sediments, with limited empirical data on thermogenic methane seepage in the Arctic. While some studies have identified microbial methane seeps in the Arctic, thermogenic methane emissions are often underrepresented in global emission maps. The Barents Sea, with its complex geological history and extensive hydrocarbon reserves, provides an ideal location to investigate the interplay between geological factors, glacial erosion, and methane leakage. Previous research has indicated significant hydrocarbon potential in the Barents Sea but also noted evidence of past hydrocarbon leakage. Basin modeling studies have suggested compromised reservoir sealing potential due to uplift and erosion. This study builds upon these findings by providing empirical evidence of extensive hydrocarbon leakage and evaluating its implications for Arctic methane budgets.

This study integrated multiple datasets to investigate hydrocarbon leakage in the northern Norwegian Barents Sea. These included: (1) Multibeam water column data from four CAGE research cruises (using Kongsberg EM302 echosounder) and one MAREANO seabed mapping project (using Kongsberg EM710 multibeam system) to identify gas emission sites (seeps). Gas flares were classified as weak, medium, or strong based on backscatter strength. (2) 2D seismic lines and seismic interpretation results from the Norwegian Petroleum Directorate (NPD) to characterize subsurface geology and identify potential reservoir formations and faults. (3) Sentinel-1 Synthetic Aperture Radar (SAR) satellite images to detect oil slicks. (4) In-situ measurements of dissolved methane concentrations in seawater and hydrocarbon gas composition in bottom sediments to quantify methane fluxes. The sea-air methane flux was calculated using a bulk flux equation, considering gas transfer velocity, methane concentration in surface water, and wind speed. Statistical analysis of gas flare distribution, density, and strength was conducted. Two additional less diverse surveys were used to test the hypothesis whether similar geological structures elsewhere emit hydrocarbons. The overall approach combined remote sensing, geophysical, and geochemical techniques to comprehensively characterize the extent and nature of hydrocarbon leakage.

The study identified 7380 hydrocarbon seeps across three study sites (Sentralbanken high, Storbanken high, and Kong Karls platform) in the northern Norwegian Barents Sea, covering a surveyed area of 3730 km². At Sentralbanken high alone, 4137 acoustic flares were identified, clustered in distinctive patches and not consistently associated with fault lineaments. The seepage zones showed a NE orientation, similar to regional structural elements. Around 89 flares appeared to reach close to the sea surface. Oil slicks were observed on SAR images and oil droplets were found in surface water samples. Water column samples consistently showed methane supersaturation, suggesting that methane plumes expand across the actively seeping area. Surface mixed layer CH₄ concentrations indicated sea-to-air fluxes ranging from 1.2 to 3 µmol m⁻² d⁻¹, lower than fluxes reported in shallower seep regions. The simultaneous release of methane and oil at Sentralbanken may decrease mass transfer between bubbles and water column. The density of seepage in the Barents Sea significantly exceeds that reported in other seep regions (Western Svalbard and Northern US Atlantic margin). These findings demonstrate one of the largest cold seep regions in the Arctic and highlight natural oil seepage in the Barents Sea. The strong hydrocarbon discharge is attributed to the exhumed structural highs and potentially ongoing leakage for at least 15,000 years since the last deglaciation.

The findings demonstrate extensive and persistent hydrocarbon leakage in the Barents Sea, highlighting the significance of geologically controlled, natural hydrocarbon release in formerly glaciated regions. The high density of seeps and the presence of both methane and oil suggest a significant contribution to both water column and potentially atmospheric methane budgets. The lack of a strong correlation between seep intensity and faulting in the apex area of Sentralbanken suggests that erosion and exhumation of reservoirs might play a dominant role in promoting leakage. The observation of oil slicks and oil-coated bubbles indicates that the oil may aid in transporting methane to the surface layer. While the sea-to-air flux is lower than in shallower seep regions, the extensive area of seepage and the potentially long duration of leakage suggest a considerable overall methane contribution. The study emphasizes the importance of considering geologically controlled methane releases when estimating Arctic marine methane sources and the potential for similar phenomena in other formerly glaciated hydrocarbon basins.

This study provides compelling evidence for extensive natural methane and oil leakage from sub-marine Arctic reservoirs, highlighting a previously underestimated source of thermogenic methane. The high density of seeps in the Barents Sea and the potential for similar processes in other glaciated regions suggest a need for re-evaluation of global methane budgets. Future research should focus on expanding seep mapping to other high-latitude glaciated shelves and improving our understanding of the long-term dynamics of these geological methane sources. Further investigation into the impacts of oil on methane transport and atmospheric fluxes is warranted.

The study primarily focuses on the northern Norwegian Barents Sea, limiting the generalizability of findings to other Arctic regions. The accuracy of methane flux calculations relies on assumptions about gas transfer velocities and wind speeds. Manual interpretation of acoustic flares and oil slicks introduces subjectivity. The vertical resolution of seismic data limited the detailed characterization of shallow sediments. Limited water column data coverage might underestimate the total number of seeps.