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

Abstract

Sub-sea Arctic methane and gas hydrate reservoirs are expected to be severely impacted by ocean temperature increase and sea-level rise. Our understanding of the gas emission phenomenon in the Arctic is however partial, especially in deep environments where the access is difficult and hydro-acoustic surveys are sporadic. Here, we report on the first continuous pore-pressure and temperature measurements over 4 days in shallow sediments along the west-Svalbard margin. Our data from sites where gas emissions have not been previously identified in hydro-acoustic profiles show that tides significantly affect the intensity and periodicity of gas emissions. These observations imply that the quantification of present-day gas emissions in the Arctic may be underestimated. High tides, however, seem to influence gas emissions by reducing their height and volume. Hence, the question remains as to whether sea-level rise may partially counterbalance the potential threat of submarine gas emissions caused by a warmer Arctic Ocean.

Variability of Natural Methane Bubble Release at Southern Hydrate Ridge

“…Römer et al (2016) suggested that methane exsolution caused by tidal pressure variations in the Clayoquot Slope at 1,250 m may contribute to plume activation, but cannot explain the long duration increase in venting that were observed in response to hydrostatic pressure changes. Sultan et al (2020) found evidence that gas exsolution from pore fluids does occur during low tides, but that this is not sufficient alone to trigger the release of gas in the water column at Arctic seeps on the Vestnesa Ridge. Our results concur with this latter finding as we could not relate the reactivation of quiescent vents to a particular tidal phase.…”

“…Decreased hydrostatic pressure during low tides facilitates the opening, or dilatation, of fractures and makes it easier for pore gas pressure (P g ) to overcome the total stress (σ), leading to rapid gas discharge (Leifer & Boles, 2005;Liu & Flemings, 2009;Scandella et al, 2011;Tryon et al, 1999Tryon et al, , 2002. Recently, in-situ pore pressure measurements in gas-rich sediments on the Vestnesa Ridge (NW Svalbard) at water depths ranging from 910 and 1,330 m confirmed that tidally driven fluctuations of hydrostatic pressure generate local pore pressure gradients, which facilitate the release of gas into the water column during decreasing tide (Sultan et al, 2020). These mechanisms are generally well-supported by our results because the frequent increase of gas emissions we observe during tidal unloading is compatible with a pressure control on active gas emissions.…”

“…or shallow hydrate formation could cause blockages in the sediments and contribute to the variability of the gas release over short timescales of hours to days. The formation of hydrate from free gas can indeed be very rapid (Haeckel et al, 2004;Sultan et al, 2020;Torres et al, 1999), especially in shallow sediments (Santos et al, 2012;Sultan et al, 2014;Tryon et al, 2002) where the salinity is lower due to better seawater circulation (Colbert & Hammond, 2008).…”

“…Although the causes of temporal variations likely vary from seep to seep due to site-specific differences (e.g., source of methane, subsurface structure), some common external parameters are known to influence bubble fluxes across sites. Hydrostatic pressure variations, caused by the action of tides, swell, or storms can modulate gas ebullition in both shallow (Boles et al, 2001;Leifer & Boles, 2005;Mau et al, 2017;Schneider von Deimling et al, 2010) and deep seep areas (Römer et al, 2016;Sultan et al, 2020;Torres et al, 2002). Sultan et al (2020), further suggested that sea level rise and resultant rise in hydrostatic pressure, could durably reduce the rates of methane release from the seafloor.…”

“…Hydrostatic pressure variations, caused by the action of tides, swell, or storms can modulate gas ebullition in both shallow (Boles et al, 2001;Leifer & Boles, 2005;Mau et al, 2017;Schneider von Deimling et al, 2010) and deep seep areas (Römer et al, 2016;Sultan et al, 2020;Torres et al, 2002). Sultan et al (2020), further suggested that sea level rise and resultant rise in hydrostatic pressure, could durably reduce the rates of methane release from the seafloor. Additionally, increased hydrate dissociation linked to seasonal temperature variations (Berndt et al, 2014), ocean warming (Hautala et al, 2014), and even to isostatic rebound (Wallmann et al, 2018) have been linked to amplified methane gas release.…”