The island of Sylt is located on the eastern seaboard of the North Sea (Germany) and is a wave-exposed, moraine-cored barrier island that borders on a shallow coastal shelf with a water depth of between 10–20 m. Coastal erosion of the sandy beaches along the western shoreline of Sylt led to the adoption of a beach nourishment strategy in 1972 that aims to compensate for the loss of sediment by replenishing aggregates extracted from an offshore resource (MEKUN.SH, 2022). Since the beginning of the measures, a total volume of 56.9 million m³ of sediment has been nourished to the beaches of Sylt (LKN.SH, 2022). The source area itself is located approximately 8 km offshore of the island, where unconsolidated sand and gravel deposits of Pleistocene age are found and have been extracted here since 1983 (Tremmler, 1994). The area is referred to as Westerland II and has a size of approximately 9 km² and is divided into subsections, each of which has been in use for several years until the targeted resource is exploited and the use is abandoned subsequently. The extraction of aggregates results in the formation of large depressions and large-scale features with diameters of several hundreds of meters across and depths of up to 20 m deep. The extraction is conducted by hopper-dredgers that pump sediment through a suction pipe, thereby creating a pattern of connecting craters on the seafloor. The infill of the depressions is a slow process, and the features remain detectable for decades after their use has been abandoned. Their presence entails a local change in the sedimentary regime and hence a shift in local habitat conditions (Mielck et al., 2019, 2021). A study by Figge et al. (2002) described that the sedimentary infill of the abandoned holes is mostly deposited from the suspension load of the water column. As a result, the seafloor in the depressions is mostly combined of marine muds, whereas the surrounding undisturbed seafloor offshore Sylt is primarily composed of medium and coarse sands (Zeiler et al., 2000; Mielck et al., 2015). Part of the relevance of this study stems from the fact that a much larger extraction site, Westerland III, was defined, which was taken in use in summer 2024 (LKN.SH, 2024). Figure 1 presents a map illustrating the location of the island, the ICOS station, and the dredging area.

Surface water was provided by the ship's water supply into an overflowing bucket, from which the water was pumped to the setup described below. Dissolved CH₄ concentrations were determined using a dissolved gas extraction unit coupled to a laser-based Greenhouse Gas Analyzer (GGA; Los Gatos Research, USA). Methane was extracted from the water via a hydrophobic membrane and hydrocarbon-free carrier gas on the other side of the membrane (water flow 1 L min⁻¹, nitrogen at 0.5 L min⁻¹). The carrier gas containing the extracted CH₄ was subsequently directed to the GGA for analysis. The time lag between water intake and corresponding signal detection was determined in the laboratory by switching the inlet between aerated freshwater and saltwater. All data were corrected accordingly. To convert the relative concentrations (ppm) reported by the GGA to absolute concentrations (nmol L⁻¹), discrete water samples were collected during the day (Bussmann et al., 2024). The CH₄ concentration in these samples was analyzed by gas chromatography following Magen et al. (2014). A correlation between the methane concentration of the water samples and the simultaneous GGA readings was used to convert all GGA measurements into concentrations of dissolved methane. To test the lower sensitivity of the setup, aerated freshwater with an equilibrium concentration of 2.9 nM was measured in the laboratory, and the instrument readings gave a concentration of 2.3 ± 0.3 nM.

Water samples were either taken from the ship's water supply for surface water or with a 5 L Niskin bottle from the bottom water. Attached to the Niskin bottle was a CTD (CTD60M, Sea & Sun Technology, Germany), a sensor to measure conductivity, temperature and depth, to measure salinity, temperature and dissolved oxygen profiles. The extraction of dissolved CH₄ was carried out using the headspace method (Magen et al., 2014). Surface water samples were collected at 11 locations along the transect, and depth profiles were obtained at 4 locations: location #1 at 3 depths and locations #2–#4 at 2 depths each. At all sampling locations and depths, three 1 L glass bottles (Schott, Germany) were filled to the brim with seawater. Each glass bottle was used to obtain one gas sample for the measurement of CH₄ concentration, δ13C-CH₄, and δ2H-CH₄ values (n=1). Surface samples were collected via a tap connected to the shipboard pump system (no filters or desalinators), while depth profile samples were filled directly from the Niskin bottle. After sampling, the glass bottles were sealed with plastic screw-on lids containing two septum ports. Care was taken to ensure that the bottles were overflown at least 3 times the bottle volume before capping to prevent the extraction of dissolved gases during filling. The remaining air was displaced by inserting a hypodermic needle (0.8 × 40 mm; BD MicrolanceTM, USA) into one port and another needle attached to a syringe filled with seawater into the other port. Then 40 mL of synthetic air (Air Liquide, Germany) from a 2.5 L stainless-steel canister was transferred into the glass bottles as headspace. An equilibrium between dissolved and gaseous CH₄ was established after the glass bottle was shaken vigorously for 2 min. The headspace gas was then extracted using a 60 mL plastic syringe (BD Luer-LokTM, USA) and transferred into 12 mL pre-evacuated Exetainers^® (Labco, UK).

During our campaign (3–7 July 2023), we experienced a rather unusual weather situation for summer. Storm Poly, an extremely strong European storm, hit the United Kingdom, Benelux and Germany between 4 and 6 July 2023. With wind gusts of up to 146 km h⁻¹, it was one of the strongest summer storms ever recorded in the Netherlands (EUMETSAT, 2023). In Germany, a first peak was reached around noon (5 July 2023) in the area of East Frisia. In the evening, a second wind maximum developed from the coast of Lower Saxony towards the west coast of Schleswig-Holstein, with values sometimes slightly higher than at noon and in the afternoon (DWD, 2023), with wind gusts of up to 109 km h⁻¹ measured. At the ICOS stations Westerland and Helgoland, the highest wind speeds of 18 m s⁻¹ (64 km h⁻¹) and 31 m s⁻¹ (111 km h⁻¹) (see Fig. A1), respectively, were recorded in the evening and at noon on 5 July. Figure 2 shows the wind roses recorded at Westerland station during the storm on 5 July 2023 (two days before our measurements) and onboard of RV Mya II during the measurements in the North Sea on 7 July 2023. On 5 July, the wind direction changed from southeast to a westerly direction with a clearly higher wind speed of up to 18 m s⁻¹.

On 6 July, the weather situation gradually calmed down, allowing a cruise with the RV Mya II to be carried out on 7 July 2023 from 08:00 to 16:00 local summer time (06:00–14:00 UTC). The mean wind speed was 3.6 m s⁻¹ from southwesterly direction.

Since joining the ICOS atmospheric network, quality control of atmospheric measurements at the Westerland station is not only performed on the hourly averaged calibrated CH₄ and CO₂ values but also on the high-frequency raw data with a temporal resolution of a few seconds. Only through the inspection of these high-resolution data was it possible to identify episodic CH₄ enhancements. Both the raw data and the one-minute averaged CH₄ measurements at Westerland atmospheric station show CH₄ spikes of up to 300 ppb in individual episodes. As these CH₄ spikes occurred repeatedly during the summer of 2022, the data were analyzed in more detail, and possible correlations with other parameters were investigated. Figure 3 shows the minute-averaged CH₄ and CO₂ mole fractions over 4 d in July 2022, one year before the measurement campaign. The CH₄ mole fraction varies between 1950 and 2140 ppb with peaks of up to 160 ppb, while CO₂ varies between 415 and 421 ppm with no short-term peaks. In contrast to air masses transported from the mainland, where CH₄ and CO₂ are always well correlated, the observed spikes in Fig. 3 occur only in CH₄. The wind came from the marine sector between 240 and 350° (clean air sector) with high wind speeds between 6 and 14 m s⁻¹. Clearly visible are the periodic CH₄ peaks, which occur mainly at low tide (shaded in light yellow).

A systematic analysis of the high-frequency CH₄ data from 2022 revealed several consistent characteristics. CH₄ spikes occurred predominantly during the summer months, under winds from the marine clean air sector (240–350°), and during low tide. In addition, the CH₄ spikes showed no correlations with CO₂. Figure A2 presents a histogram of the number of days with CH₄ spikes per month in 2022, indicating that most spikes occurred in June, July, and August, some in May and October, and very few in January and March. However, it is also possible that no CH₄ peaks occurred under the above-mentioned conditions. To investigate the cause of the periodic CH₄ peaks, mobile measurements were carried out near the station and on the beach. As no elevated CH₄ concentrations were found, a very local source of CH₄ near the atmospheric station Westerland or on the beach could be ruled out. Subsequently, wind direction, distance from the station and peak shape were then used to identify dredging activities in the North Sea in the nearby Westerland II dredging area as a possible cause.

A comparison with sand dredger activity data showed that in June 2022, when dredging operations paused for two days, the CH₄ peaks continued to follow the usual tidal pattern (see Fig. A3), indicating that the emissions are not exclusively associated with the active extraction process but may also be influenced by the characteristics of the young seafloor depressions created by dredging. However, confirming this hypothesis required ship-based measurements.

Figure 4 shows the CH₄ and CO₂ mole fractions at Westerland station before and during the measurement campaign in July 2023. On 5 July, an air mass originating from the mainland (easterly wind) reached the station, leading to simultaneous increases in CH₄ and CO₂ mole fractions with values up to 2100 ppb and 445 ppm, respectively. The good correlation between CH₄ and CO₂ in air masses transported from the mainland can be seen. In the evening of 5th July and in the morning of the 6 July, one day before the shipboard measurement on RV Mya II, storm Poly hit the German Bight with hourly mean wind speeds of up to 17.5 m s⁻¹. In contrast, on 4 July, only one very small CH₄ peak of 15 ppb was detected from the clean air sector. However, this CH₄ spike was relatively small compared to other CH₄ spikes detected in July 2022 (Fig. 3). On the day following the storm, no distinct CH₄ spikes were observed from the clean air sector, as winds were predominantly from the south-southwest (SSW) rather than from the dredging area to the west, and wind speeds remained very low.

Figure 5 shows the shipboard transect of RV Mya II as a function of the atmospheric CH₄ mole fraction. The dark gray rectangle marks the Westerland II sand extraction area with sectors 1b and 1a (blue), which have been dredged since 2017 and from 2010 to 2017, respectively. During the campaign on 7 July, a mean wind speed of 3.6 m s⁻¹ from southwesterly direction was measured at the ICOS station Westerland. The two ship symbols indicate the coordinates of the dredging ship during the ship campaign, in the north during beach nourishment, and in the southwest during dredging. The hand-held sampling line was used once during the first transect from dredging area 1b to 1a (the northern direct transect in Fig. 5b) and a second time during the transect above the mud track from the actively dredging hopper dredger (south of the ship in Fig. 5b). An additional figure showing when the different sampling lines were used is included in the Appendix (Fig. A4).

The highest atmospheric CH₄ concentrations of up to 2450 ppb were measured directly in the areas where dredging is currently taking place or has taken place in recent years. The lowest CH₄ mole fraction of 1990 ppb was measured north-east of the dredging area. This shows a CH₄ excess of more than 450 ppb above the dredged holes. The highest CH₄ mole fractions were found directly with the hand inlet above a plume of advected suspended sediment from the hopper-dredger (Fig. 5b south of the hopper-dredger).

This suspended sediment was clearly visible from the RV Mya II and two transects were conducted along this track. During the first transect, the CH₄ mole fractions were measured with the inlet line attached to the mast. The mean mole fraction there was (2041 ± 4) ppb. The second transect contains mole fractions measured directly above the suspended sediment with the hand-held sampling line. Here, a mean CH₄ mole fraction of (2104 ± 7) ppb was measured. This shows that the spikes in CH₄ mole fractions at the Westerland station are most likely caused by offshore dredging activities.

Low surface water concentrations of dissolved CH₄ were observed during the transit from the port to the Westerland II area with 5.6 ± 3.7 nmol L⁻¹. In the Westerland II area higher concentrations were observed with 59.1 ± 21.3 nmol L⁻¹ on average and up to 90 nmol L⁻¹ (Fig. 6). At four stations bottom water was sampled (see Fig. 1). In the bottom water dissolved CH₄ was enriched, especially at locations #1 and #2 with 111 ± 12 nmol L⁻¹, while at the control station #4 only 24 ± 4 nmol L⁻¹ were observed. Due to the strong storm during the previous days, the CTD-profile revealed a completely mixed water column (Fig. A5).

The diffusive CH₄ flux from the sea into the atmosphere is shown in Fig. 7. As the wind was rather calm on 7 July 2023, the CH₄ fluxes during the transit course were also very low (3.0 ± 3.6 µmol m⁻² d⁻¹). In contrast, in the Westerland II dredging area the diffusive flux was with 45.4 ± 47.9 µmol m⁻² d⁻¹ much higher, but also very patchy. Only a few locations (4 %) had CH₄ fluxes larger than 100 µmol m⁻² d⁻¹ with maximal values of 340 µmol m⁻² d⁻¹. For the whole Westerland II dredging area this results in a diffusive CH₄ flux of 427 mol d⁻¹.

In addition to the dissolved CH₄ concentrations, the δ13C-CH₄ and δ2H-CH₄ values were determined for all samples. Figure 8 shows the δ13C-CH₄ and δ2H-CH₄ values from surface seawater samples for the North Sea transit area (samples #9–#11) and from the Westerland II area (samples #1–#8). The δ13C-CH₄ values of samples collected in the North Sea transit area varied between -49.4 ‰ and -51.3 ‰ with an average of -50.2 ± 0.9 ‰. The surface water samples within the Westerland II area showed more negative δ13C-CH₄ values between -65.6 ‰ and -67.6 ‰ with an average of -66.9 ± 1.2 ‰. Only sample #5 deviated from the other Westerland II samples with a δ13C-CH₄ of -42.4 ‰.

In comparison, δ2H-CH₄ values from the North Sea were -242 ‰ and -304 ‰ for samples #9 and #10, respectively. In the Westerland II area, δ2H-CH₄ values ranged between -199 ‰ and -227 ‰ with an average of -222 ± 9 ‰. Thus, the δ2H-CH₄ values from Westerland II were more positive compared to the North Sea samples.

At four different locations within the Westerland II area (#1–#4), bottom water was sampled alongside surface water. Figure 9 shows CH₄ concentrations as well as δ13C-CH₄ and δ2H-CH₄ values from surface and bottom water layers. A change in CH₄ concentrations and stable isotope values with depth was observed in three of the investigated profiles, except for location #3, where CH₄ concentrations and its stable isotope composition were similar in surface and bottom water at a depth of 27 m. At locations #1 and #2 an increase in CH₄ concentrations from 58 nmol L⁻¹ in the surface water layer to 122 ± 11 nmol L⁻¹ at 18 m depth as well as 102 ± 5 nmol at 18.7 m (location #1) and to 110 ± 9 nmol L⁻¹ at 24.2 m depth (location #2) was observed. At location #1, δ13C-CH₄ values slightly shifted towards more negative values with increasing depth from -67.3 ‰ to -69.4 ‰, while δ2H-CH₄ values slightly became more positive from -224 ‰ in the surface water layer to -215 ‰ at 18 m depth. In contrast, at location #3 the isotopic values did not show any major changes with increasing depth. A decrease in CH₄ concentration with increasing depth can be observed for location #4 with 51 nmol L⁻¹ in surface water layers and 24 ± 4 nmol L⁻¹ at 19.6 m depth. While the δ13C-CH₄ values showed no measurable changes with increasing depth, the δ2H-CH₄ values shifted towards more positive values from -227 ‰ in the surface water layer to -195 ‰ at 19.6 m depth.

To further investigate the specific characteristics and origin of the CH₄ peaks measured in the air above the sand dredging area, seven 3 L sample bags were filled with atmospheric air during the RV Mya II cruise. Figure 10 shows the in situ atmospheric CH₄ measurements (black) together with the colored data points of the bag samples. The vertical bars indicate the sampling periods. Bag HD3-52 was sampled during the transect from dredging area 1b to dredging area 1a using the hand-held line. Bags HD3-55 and HD3-54 were filled above the dredging area 1b using the mast-mounted sampling line. During the transect across the mud track generated by the actively dredging hopper dredge, two additional sampling bags were filled. The first sample (HD1-122) was collected using the mast-mounted sampling line. During a second transect, HD1-123 was taken directly over the mud track with the hand-held sampling tube. Bags HD3-56 and HD3-57 were sampled outside the dredging area and therefore represent background samples. The CH₄ mole fraction of the sampled air varied between 1978 and 2085 ppb, and the δ13C-CH₄ values varied between -47.5 ‰ for background samples and -48.7 ‰ above the dredging area.

The CH₄ and δ13C-CH₄ measurements for these seven sample bags were used to calculate the isotopic source signature using the Keeling plot method (Hoheisel et al., 2019; Hoheisel and Schmidt, 2024). In Fig. 10 (right panel), the linear regression between δ13C-CH₄ and 1/CH4 was calculated using a York fit (York et al., 2004). The isotopic source signature for δ13C-CH₄ was determined to be -66 ± 6 ‰. This fits well with CH₄ samples extracted from the sampled surface seawater above the dredging area with a mean isotopic signature of -66.9 ± 1.2 ‰ (see Sect. 3.5) and can be clearly distinguished from North Sea natural gas with a mean isotopic signature of -34 ‰ (Lowry et al., 2001).

Our starting point for the atmospheric and seawater measurements aboard RV Mya II are the periodically recurring CH₄ peaks at Westerland atmospheric monitoring station, which were particularly pronounced during the summer months of 2022, one year before our campaign. The occurrence of these peaks during marine clean air conditions (wind from the west) calls for a more precise understanding of the origin of these CH₄ peaks. Investigations in the vicinity of the station with mobile CH₄ measurements have ruled out the possibility of a local natural or anthropogenic CH₄ emitter between the station and the North Sea.

In addition, the tidal dependence of the peaks, only occurring at low tide, indicates that the origin of the CH₄ peaks is within the North Sea. The peak shape and strength of the CH₄ peaks also identify a locally concentrated emitter, i.e., a point source rather than a larger area source. The sand dredging activities in the Westerland II area, which take place in the summer months, are exactly in the wind direction that produces these CH₄ peaks in the atmospheric time series of the atmospheric monitoring station Westerland. In July 2023, it was also possible to determine whether the sand dredging activities alone were responsible for the CH₄ peaks or whether the beach nourishment also contributed. Atmospheric and soil chamber CH₄ measurements in the vicinity of the sand nourishment area at the beach in List (Sylt West Coast) did not show a significant CH₄ enhancement and are therefore not the origin of the CH₄ spikes at the Westerland atmospheric station.

During the cruise with RV Mya II in July 2023, the hopper dredger Thor R was extracting sand from the seabed and depositing the sand within its hull. Elevated atmospheric CH₄ concentrations were observed while circling around the vessel. The two highest atmospheric CH₄ concentrations of up to 2450 ppb were measured directly in the Westerland II dredging regions 1b and 1a, where dredging was currently taking place or has taken place in recent years. The highest CH₄ mole fractions were found directly at the hand inlet above a plume of suspended sediment from the hopper-dredger.

The atmospheric measurements on board RV Mya II already showed that the origin of the measured CH₄ peaks is located in the Westerland II area and originates from the sand dredging activities. In 2023, CH₄ spikes at the atmospheric station Westerland showed only modest enhancement, with up to 50 ppb during westerly winds, significantly lower than in 2022, when enhancements of up to 300 ppb were measured. In addition, storm Poly passed through the area just one day prior to the measurement campaign. Therefore, we did not expect very large CH₄ enhancements over the dredging area during our campaign. On the 4, 6 and 7 July, shortly before and after the storm Poly, only CH₄ spikes with enhancements of 11 ppb or even less than 5 ppb were monitored in the atmospheric air masses from the clean air sector. The effect of a storm on atmospheric and dissolved methane concentrations can be described in two ways. On the one hand, sediment resuspension caused by increased bottom stress from wind and waves could increase the release of methane from sediments. On the other hand, a storm can substantially increase the gas transfer velocity, thereby increasing the diffusive flux of methane out of the water. The overall effect on the atmospheric methane concentration varies depending on water depth and sediment structure. As we only observed minor enhancements, we assume that the effect of sediment resuspension was negligible.

Using a simple Gaussian plume model READY (Rolph et al., 2017), the dispersion of different CH₄ fluxes in the Westerland II sand dredging area was used to calculate possible CH₄ peaks at the Westerland monitoring station. With an averaged CH₄ flux of 427 mol d⁻¹ over the dredging area and the wind speed and direction measured during the campaign on 7 July 2023, only CH₄ peaks of 0.15 ppb would be expected at the atmospheric station Westerland at a distance of 7 km from the dredging area. This is consistent with our atmospheric measurements at the Westerland station, which show no major peaks on the 4, 6 and 7 July 2023. For larger CH₄ peaks with an enhancement of more than 15 ppb, the CH₄ flux in the Westerland sand dredging area would have to be up to 45 000 mol CH₄ d⁻¹ during low tide. These temporal differences should be analyzed in more detail in a future study.

The dredging of marine aggregates is a common practice across Europe and in the North Sea, but the distribution of active extraction sites and concession areas is far from uniform (EMODnet, 2024; Staudt et al., 2021). In the context of the German EEZ (exclusive economic zone) of the North Sea, the investigated site offshore Sylt is fairly unique and presents the only major and currently active site in the area (Schultze and Nehls, 2017). In other countries, such as Denmark or the Netherlands, similar activities do occur and are more widespread but differ in spatial distribution and intensity and require in all cases an assessment of source properties and environmental considerations (e.g., Nørgaard-Pedersen et al., 2022; van Dalfsen et al., 2000).

Studies on the influence of dredging on greenhouse gas (GHG) emissions, especially CH₄ are rather ambiguous: dredging activity can reduce GHG emissions, such as in shallow lakes and watercourses (Slamet et al., 2020; Nijman et al., 2022), probably due to the removal of organic material from these systems. However, fine-grained sediments with high organic matter contents are known to be an emitter of GHG (Slamet et al., 2020; Müller et al., 2025). Currently, not much is known about the influence of dredging on GHG emissions from marine or coastal environments.

Another source of the observed atmospheric peaks of CH₄ could be related to processes within the water column. Direct investigation of CH₄ concentrations within the water column revealed elevated bottom water concentrations of up to 111 nmol L⁻¹, 2 times higher than surface water CH₄ concentrations. These elevated values were restricted to the locations within the Westerland II area. Higher CH₄ concentrations in bottom water were mirrored by elevated surface water concentrations of 59.1 ± 21.3 nmol L⁻¹ (versus 5.6 ± 3.7 nmol L⁻¹ in the transit area). With the same method dissolved CH₄ concentrations have been measured in the area south of our study area. For a better comparison, only marine water samples with a salinity > 30 were used, and the riverine influence was excluded from these studies. Average dissolved CH₄ concentrations ranged from 12 nmol L⁻¹ in June 2019, 26 ± 33 nmol L⁻¹ in September 2019 and 42 ± 63 nmol L¹ in September 2020 (Bussmann et al., 2024, 2022). Thus, our CH₄ concentration data from the dredging area Westerland II are within the range of the more southern areas of the North Sea, while our background concentrations were much lower, close to the equilibrium concentration of dissolved CH₄ at atmospheric mixing ratios. This may be due to strong outgassing from the storm of the previous days.

In the dredging area, Westerland II, the diffusive flux from the sea into the atmosphere was approximately 15 times higher than in the surrounding area. However, the distribution of the diffusive flux was also very patchy. A general pattern between the diffusive flux and the atmospheric concentration with similarly high values in the dredging area can be found (Fig. A3). Nevertheless, a direct correlation between the diffusive flux and the atmospheric concentration is not possible on the small scale of meters due to different velocities of mixing in the atmosphere and surface water. In addition, after the heavy storm the previous days, the CH₄ inventory in the water column has to be build up again.

At the moment it is not clear what the origin of the increased dissolved CH₄ is. It could be released from the sediment by resuspension through the sucking activities of the dredging ship. Another possibility is that the created depressions at the bottom are filled up with fine sediment characterized by a higher organic matter ratio. The degradation of organic material of this sediment could lead to increased methanogenesis in these areas. High CH₄ concentrations in these deposited sediments could then be released into the overlying water, resulting in the observed enrichment of CH₄ in the bottom water. If very high CH₄ concentrations are reached, the oversaturation may trigger ebullition, allowing CH₄ to escape directly into the atmosphere with minimal interaction, such as consumption by methanotrophic bacteria in the water column (DelSontro et al., 2015; McGinnis et al., 2006). Further investigations on the CH₄ distribution within the sediments could clarify this discussion.

The isotope values of CH₄ in surface water between the North Sea transit and the Westerland II area differ significantly. δ13C-CH₄-values from the Westerland II area were 17 ‰ more depleted in ¹³C-CH₄ compared to the North Sea transit, while δ2H-CH₄-values showed a shift towards more ²H enriched values of 20 ‰ and 83 ‰, respectively. This difference indicates CH₄ contributions from different CH₄ sources in the two areas. Dual isotope plots according to Whiticar (2020) were used to further constrain possible sources or pathways of CH₄ formation (Fig. 11). In the marine environment, the most common sources of CH₄ are either thermogenic CH₄ formation or microbial methanogenesis by methanogenic archaea (Judd, 2004). The isotopic composition of microbial CH₄ depends on the active methanogenic population forming CH₄ either by hydrogenotrophic, acetoclastic or methylotrophic methanogenesis (Whiticar, 1999; Conrad, 2009). The δ13C- and δ2H-CH₄-values of thermogenic CH₄ are generally less negative than those of microbial CH₄ depending on the source (e.g., Whiticar, 1999).

Our data, based on stable isotope composition (Fig. 11), indicates that dissolved CH₄ in the Westerland II area is of microbial origin, likely formed by hydrogenotrophic methanogenesis. Site #5, located at the edge of the area, is an exception with more positive δ13C-CH₄ values, though δ2H-CH₄ values remain unaffected. In contrast, stable isotope values from the North Sea transit samples do not clearly indicate a specific CH₄ source, possibly due to aerobic CH₄ oxidation and/or a mix of different sources.

Mau et al. (2017) observed δ13C-CH₄ values ranging from -60.1 ‰ to -23.4 ‰ in seepage along the Svalbard continental margin, attributing the variability to mixing with background CH₄ or aerobic microbial oxidation. In our study, ocean currents likely have little effect due to the small-scale observation area. The δ13C-CH₄ enrichment in the North Sea may reflect microbial CH₄ consumption, preferentially degrading ¹²C-CH₄. CH₄ oxidation in the North Sea is limited by concentration and nutrients (Hackbusch et al., 2019). Further studies should explore if dredging or benthic methanotroph inoculation (Schmale et al., 2015) could increase CH₄ oxidation rates.

The more positive δ13C-CH₄ values in the North Sea could also be due to another source. Klintzsch et al. (2023) found that phytoplankton, including marine algae and cyanobacteria, produce CH₄ with δ13C-CH₄ values ranging from -50 ‰ to -20 ‰, differing from classical methanogenesis by methanogens. However, algal blooms were observed only on the surface in the Westerland II area, excluding phytoplankton as a substantial CH₄ source. Therefore, CH₄ oxidation is the most probable cause for the North Sea sites. The δ2H-CH₄ values generally exhibit greater variability due to kinetic fractionation effects associated with CH₄ production and oxidation. This could explain the large discrepancy between the δ2H-CH₄ values of site #9 and #10 outside the Westerland II area compared to the less negative δ2H-CH₄ values within this area, though a larger sample size should be targeted in future studies for more robust comparisons between the two areas.

The surface and bottom water profiles of dissolved CH₄ concentrations and stable isotopes (Fig. 9) provide key insights into the origin of increased CH₄ concentrations in the Westerland II area. At location #4, CH₄ concentrations decreased with depth while δ13C-CH₄ and δ2H-CH₄ values changed towards more positive values. This pattern may reflect several processes. While CH₄ oxidation could contribute to the isotopic enrichment of the residual CH₄ pool, mixing or exchange with water masses influenced by different CH₄ sources and thus characterized by different isotopic values of CH₄ or a higher degree of prior CH₄ oxidation may equally explain the observed concentration and isotopic gradients. At locations #1 and #2, CH₄ concentrations nearly doubled at the bottom, but stable isotopes remained unchanged, indicating methanogenic CH₄, likely from dredging or diffusion at the sediment-water interface. A similar pattern was observed at location #3, though recent dredging and the storm Poly may have evenly distributed the CH₄ throughout the water column.