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

Oceanic methane emissions, a significant factor in the global methane budget, are poorly understood in terms of their dynamics and controlling processes. Gas emissions from seabed and sub-seabed features are triggered by geological processes such as tectonic stress and fracturing, and influenced by environmental changes. While seasonal temperature changes and long-term monitoring data have shown correlations between tides and gas emissions, the impact of future temperature and sea-level rise on methane emissions remains uncertain. This study hypothesizes that even small sea-level changes can significantly impact the intensity of deep-sea gas emissions, using tides as a proxy for daily variability. In-situ sediment pore-pressure and temperature measurements were taken over four days at two sites on the Vestnesa Ridge (NW Svalbard) to characterize short-term periodicity and the effect of tides on pressure-controlled emissions. The study sites were specifically chosen to avoid previously identified active geological structures, focusing on areas where gas plumes hadn't been observed in previous hydro-acoustic surveys. This approach allows examination of the subtle effects of tides and avoids confounding factors related to active geological structures.

Existing research highlights the widespread nature of ocean methane emissions but lacks a comprehensive understanding of their dynamics. Studies have linked gas emissions to geological processes like tectonic activity (e.g., North Anatolian Fault, Vestnesa Ridge) and environmental factors such as seasonal temperature changes (western Svalbard). Long-term monitoring near Vancouver Island has demonstrated tide-cycle control on gas emissions, a phenomenon also suggested for shallow-sea emissions on the west Svalbard shelf. While observations from formerly glaciated margins indicate significant greenhouse gas emissions after ice-sheet retreat, ice-core analyses suggest minimal post-LGM impact on global climate. A major challenge in quantifying present-day emissions is the potential for widespread, undetected micro-seepage. The lack of consensus on how these dynamic systems respond to climate change necessitates further investigation into the impact of future temperature and sea-level changes on methane emissions.

In-situ pore-pressure and temperature measurements were conducted using a cable-deployed piezometer equipped with differential pressure and temperature sensors. The piezometer, ballasted with lead weights (up to 1000 kg), was deployed at two stations (PZM1 and PZM2) on the Vestnesa Ridge. Station PZM1 was located near a seabed depression, within the gas hydrate stability zone, while PZM2 was located on the western segment of the ridge, approximately 80 km west of PZM1. Both sites lacked observable gas plumes in available hydro-acoustic data. The piezometer’s penetration into the sediment causes initial pressure and temperature changes, which dissipate over time, allowing for characterization of in-situ hydraulic and thermal regimes. Data were collected over 3 days at PZM1 and over 4 days at PZM2. Gas bubble velocities at PZM1 were calculated from pore-pressure fluctuations on the lower sensors. The height of potential gas columns was estimated using negative differential pore pressures and the unit weights of gas and pore water. A 1D transient diffusion-advection heat transfer model was used to examine the influence of pore-pressure variations and fluid advection on temperature fluctuations at PZM1. Tidal data were obtained from the TPXO 9.0 global tidal model to correlate with pressure and temperature changes. Capillary invasion and fracture opening mechanisms were considered to explain gas flow through the sediments.

Pore pressure and temperature data from PZM1 showed fluctuations, with negative pressures indicating upward free gas migration into the water column. Gas bubble velocities ranged from 0.3 cm/s to 5.7 cm/s. Negative pressure cycles suggested intermittent gas bubble plumes up to 25 meters high (in continuous gas-equivalent height). At PZM2, the pressure profile suggested partially gas-saturated sediments, with small pore-pressure fluctuations correlating with tidal cycles. These fluctuations decreased with depth, indicating reduced gas content at greater depths. The absence of temperature fluctuations accompanying the pressure fluctuations at PZM2 suggests no significant upward fluid advection. Analysis at PZM1 revealed a strong correlation between upward gas-charged fluid migration and tidal cycles. Three of four temperature peaks at 0.8 mbsf were reproduced by a model incorporating vertical fluid advection, with the duration of each thermal pulse matching tidal cycle periods. Lowest eastward tide velocities corresponded with temperature and upward velocity peaks. These findings suggest that even small (less than 1 meter) tidal height variations can significantly increase the height of gas bubble plumes. While capillary invasion is a possible explanation for gas flow, it is unlikely in the gas hydrate-rich environment of PZM1. Fracture opening, driven by a reduction in hydrostatic pressure during low tides, is a more likely mechanism, explaining intermittent gas emissions and the presence of near-surface gas hydrates and isolated free gas pockets. The contrast between PZM1 and PZM2 data suggests spatial variations in fluid distribution. The study highlights the significant impact of sea-level changes on deep-water gas emission systems. Low tides (sea-level drops) cause notable gas emissions, while higher sea-levels seem to inhibit emission.

The results demonstrate the remarkable sensitivity of deep-sea gas emission systems to subtle hydrostatic pressure changes caused by tides. The fracture opening model, favored over capillary invasion, effectively explains the observed intermittent gas emissions in a system characterized by both near-surface gas hydrates and free gas. The correlation between pressure and temperature fluctuations emphasizes spatial variations in gas advection. The study's findings show that even moderate sea-level rise could significantly reduce gas emissions, potentially offsetting the effects of warming ocean temperatures. The need for further investigation into the potential counterbalance between warming and sea-level effects on methane emissions is highlighted.

This study provides the first evidence of significant tide-induced variability in deep-sea methane emissions. The findings highlight the impact of even moderate sea-level changes on shallow gas accumulations. A fracture opening model is proposed as the dominant mechanism driving tide-related gas emissions. The results suggest a potential for sea-level rise to partially mitigate the increase in emissions expected from warming ocean temperatures. Future research should focus on long-term monitoring combining in-situ piezometry and hydro-acoustic surveys to enhance the understanding and prediction of seabed gas emissions in response to climate change.

The study's limitations stem from the fixed-point nature of the piezometer measurements. While providing high-resolution temporal data, this approach offers limited spatial coverage compared to hydro-acoustic surveys. Combining both methods is crucial for a more complete understanding of emission rates. The model used makes certain assumptions about parameters like hydraulic conductivity and thermal diffusivity, potentially impacting the accuracy of the predictions. The specific geological characteristics of the study sites may not be fully representative of all deep-sea gas hydrate systems, limiting the broad generalizability of some findings.