Impact of tides and sea-level on deep-sea Arctic methane emissions

Ocean methane emissions are widespread but their dynamics and governing physical processes remain poorly constrained, creating uncertainty in the global atmospheric methane budget. Gas emissions arise from geological features and processes (faults, fractures, pockmarks, mud volcanoes, hydrothermal systems), migration of sub-seafloor gas, and hydrate dissociation under changing thermodynamic conditions. Emission periodicity is controlled by stress accumulation and fracture pathways and can correlate with tectonic stress and micro-seismicity. Environmental factors in the Arctic, including seasonal bottom-water temperature changes and tides, have been implicated in controlling seepage distribution and timing. Despite decades of research, there is no consensus on how present-day emissions will respond to climate change, and hydro-acoustic surveys may underestimate seepage because of temporal variability. This study hypothesizes that small sea-level changes (tides) can significantly modulate deep-sea gas emission intensity and periodicity. To test this, the authors conducted continuous in situ pore-pressure and temperature measurements over several days at two deep sites on Vestnesa Ridge (NW Svalbard) to characterize short-term seepage periodicity and tidal effects on near-surface pressure fields.

Prior work documents methane seepage along the western Svalbard margin at distinct depth ranges (~80 m, ~300–400 m near the shelf break, and ~1000–1200 m on Vestnesa Ridge). At 300–400 m, seepage has been linked to hydrate dissociation driven by seasonal to millennial temperature changes and post-LGM unloading. On Vestnesa Ridge, active plumes concentrate on the eastern segment, while western pockmarks indicate past seepage despite lack of present water-column plumes, suggesting potential micro-seepage undetected by acoustics. Multi-year hydro-acoustic studies indicate seasonal ocean temperature controls seepage migration; long-term cabled observatory data off Vancouver Island show tidal control of emissions. Conflicting interpretations exist regarding the magnitude of deglacial methane release and its climatic impact. Reviews highlight uncertainties in future Arctic methane emissions under warming. These studies motivate higher-resolution, continuous observations to resolve short-term forcing (tides) and potential underestimation by snapshot acoustic surveys.

Field measurements: A cable-deployed piezometer with a 60 mm diameter lance (length 8.42 m at PZM1 and 9.92 m at PZM2) carrying nine differential pore-pressure ports and co-located temperature sensors was deployed from R/V Kronprins Haakon. The system was ballasted (up to 1000 kg) and connected to a surface buoy. Differential pore pressures were measured relative to ambient seawater hydrostatic pressure; sensor accuracies were ±0.5 kPa (pressure) and ±0.05 °C (temperature). Study area and sites: Two stations on the western Svalbard margin/Vestnesa Ridge were selected where no hydro-acoustic plumes had been previously observed: PZM1 at the southeastern onset of Vestnesa Ridge on the continental slope, adjacent to an elongated depression with known free gas and hydrates in cores; and PZM2 ~80 km west on the western ridge segment. Both are within the gas hydrate stability zone (GHSZ). Data acquisition: Continuous pore-pressure and temperature time series were recorded for ~3 days at PZM1 and >4 days at PZM2. Sensor depths ranged from ~0.8 to ~7.9 mbsf (PZM1) and ~0.8 to ~9.4 mbsf (PZM2). Penetration-induced excess pressures and heat dissipated to equilibrium before monitoring fluctuations. Ancillary data: Tidal heights and currents at the sites were obtained from the TPXO 9.0 global tidal model. Bubble rising velocity estimation: At PZM1, time lags between successive pressure maxima recorded on vertically separated sensors were used to compute bubble front rising velocities (velocity = sensor spacing / time lag), focusing on the five deepest sensors. Gas plume height inference: Negative differential pressures at the shallowest sensor (0.8 mbsf) during emission pulses were converted to equivalent continuous gas column heights using h = ΔP / (ρw − ρg), with methane unit weight as low as 0.66 kN/m³, providing a lower bound on plume height. Thermal-advection modeling: A 1D transient diffusion–advection heat transfer model ∂T/∂t = k ∂²T/∂z² − v ∂T/∂z was solved using an explicit finite-difference scheme to test whether measured temperature pulses could be explained by upward advection. Boundary conditions: fixed seabed temperature of −0.64 °C and basal temperature equal to measured at 2.3 mbsf. Thermal diffusivity k = 3.5×10⁻⁵ m²/s (derived from post-insertion thermal decay). Upward fluid velocity ve was derived from the hydraulic gradient between 0.8 and 2.3 mbsf and a hydraulic conductivity of 2.5×10⁻⁵ m/s, tuned to match observed bubble velocities at 4.7 mbsf. Modeled temperature at 0.8 mbsf was compared against observations to assess timing and duration of thermal pulses relative to tides.

- PZM2 (western Vestnesa): Temperatures at shallow sensors were nearly constant; pore pressures were positive overall. The upper four sensors (0.8–4.7 mbsf) displayed small pressure fluctuations synchronized with low tides, indicating exsolution of dissolved gas under reduced hydrostatic pressure and re-dissolution under high tides. Fluctuation amplitude decreased with depth, implying decreasing gas content with depth. Lack of corresponding temperature anomalies suggests no significant upward advective heat transport. Under high tides, pore pressure fluctuations vanished, consistent with full dissolution of free gas. - PZM1 (southeastern onset of Vestnesa): Pore pressures fluctuated with notable negative excursions at the shallowest sensor (as low as −240 kPa), and the uppermost temperature sensor recorded peaks above seawater temperature (−0.64 °C), evidencing upward fluid flow and free gas migration into the water column. Pressure fronts across deeper sensors indicated upward passage of gas bubbles: calculated rising velocities ranged from 0.3 to 5.7 cm/s (five deepest sensors). Near the seabed, pressure fronts were diffusive, precluding velocity estimates. - Equivalent continuous gas plume heights inferred from negative pressures at 0.8 mbsf reached up to ~25 m. Temperature peaks coincided with plume height peaks. Despite plume heights sufficient for sonar detectability, concurrent hydro-acoustic surveys did not show plumes, implying strong temporal variability and the importance of survey timing. - Tidal forcing: Temperature pulses and upward velocities at PZM1 occurred during low tides, with pulse durations of ~12 h (semidiurnal), and corresponded to periods of lowest eastward tidal velocity (<3 cm/s). Tidal height variations were <1 m, yet they strongly modulated emission intensity and timing. - Mechanism: Capillary invasion threshold pressure was estimated at ~144 kPa (γ ~72 mN/m, grain radius ~5 µm). A 10 kPa pore pressure decrease (≈1 m tide) is ~7% of this threshold. Given the presence of near-surface gas hydrates and the GHSZ bases (∼145 m at PZM1, ∼170 m at PZM2), capillary invasion alone is unlikely. A fracture opening/dilation mechanism, where small hydrostatic pressure drops reduce effective stress and allow gas in pre-existing fractures/reservoirs to migrate, best explains observations. - Implication: Even moderate sea-level variations (<1 m) significantly influence deep-water gas emissions; high tides reduce emission height/volume, suggesting that future sea-level rise could partially counteract warming-induced degassing.

The observations confirm the hypothesis that small sea-level changes modulate deep-sea methane emissions. At PZM1, negative pore pressures, temperature spikes, and upward-propagating pressure fronts indicate intermittent gas release events tied to low tides, while at PZM2 only small, reversible pressure fluctuations occur without advective heat signatures. The semidiurnal periodicity and alignment with tidal minima and weak currents suggest tidal hydrostatic pressure drops trigger gas exsolution and/or migration. Mechanistically, the presence of gas hydrates within the GHSZ disfavors capillary invasion as the dominant process. Instead, fracture dilation/opening during low tides likely reduces effective stress and allows gas in pre-existing fractures or connected reservoirs below/near the GHSZ to migrate upward; at high tide, increased hydrostatic pressure and hydrate strength counteract gas pressure, suppressing seepage. This dynamic can prevent formation of massive, continuous shallow hydrate layers, consistent with observed chaotic near-seafloor seismic facies and isolated hydrate/free-gas nodules. The contrast between sites, despite similar tidal forcing, likely reflects spatial differences in background versus focused fluid flow systems and possibly variations in the local tectonic stress field along the margin. The findings imply that present-day hydro-acoustic snapshot surveys may underestimate seepage due to timing; integrating continuous pore-pressure monitoring with recurring acoustics can better quantify emission variability. Importantly, the results suggest that sea-level rise could dampen gas emission intensity, partially offsetting expected increases from ocean warming, introducing a nuanced feedback in Arctic methane system projections.

Continuous in situ pore-pressure and temperature monitoring at two Vestnesa Ridge sites demonstrates that subtle hydrostatic pressure changes associated with tides (<1 m) strongly influence shallow gas dynamics and methane bubble emissions at deep-water depths. At one site, low tides induced intermittent gas release with bubble rising velocities of 0.3–5.7 cm/s and inferred plume heights up to ~25 m; high tides suppressed emissions. A fracture opening/dilation mechanism best explains the observations in hydrate-bearing fine-grained sediments. These results indicate that sea-level rises of similar magnitude could significantly reduce emission intensity and partially counterbalance warming-driven degassing. The study highlights that fixed-point pore-pressure monitoring can reveal seepage processes invisible to sporadic hydro-acoustic surveys. Future work should establish long-term piezometer observatories combined with recurrent hydro-acoustic mapping to upscale these findings across the Arctic and improve predictive models of seabed gas emissions under changing sea level and temperature.

- Observations are limited to two fixed-point sites and short monitoring periods (~3–4 days), which may not capture longer-term variability (seasonal, interannual) or spatial heterogeneity. - Hydro-acoustic surveys did not coincide with all inferred emission pulses, preventing direct concurrent validation of plume presence in the water column. - Parameters required for quantitative fracture mechanics (e.g., fracture toughness, geometry, in situ stress anisotropy) were not measured, limiting mechanistic quantification. - Site selection avoided visibly active structures, which may bias results toward background seepage behavior. - Model assumptions (1D heat transport, constant thermal diffusivity, prescribed hydraulic conductivity and velocity) simplify complex 3D processes and may not capture all dynamics.