Tidally controlled gas bubble emissions: A comprehensive study using long‐term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 17; no. 9; pp. 3797 - 3814
• Main Authors: Römer, Miriam, Riedel, Michael, Scherwath, Martin, Heesemann, Martin, Spence, George D.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.09.2016
• Subjects: Bubbles; Earthquakes; Emissions; Methane; methane seepage; multibeam rotary sonar; Northern Cascadia Margin; Observatories; Oceans; Sediment load; Seismic activity; Solubility; temporal variability; Tides; time‐series analyses; Water column; Water depth; IE, East Pacific; INE, Canada, British Columbia, Vancouver I
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1002/2016GC006528

Long‐term monitoring over 1 year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity‐temperature‐depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day‐long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant subsurface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans. Key Points: Continuous monitoring of transient gas bubble emissions over 1 year was enabled by the NEPTUNE cabled observatory Flare onsets were triggered and controlled by tidally induced bottom pressure changes Decreasing pressure allowing gas to migrate and shifting the CH4 solubility in pore waters leads to ebullition