Gas flow offshore Oregon and California, how much is coming out: Results from MBES, SBES, GasQuant, BubbleBox and ROV-based studies during FK190612
Abstract
Several studies are developing methods for quantitative gas flow and flux measurements from bubble releasing seep sites. During cruise FK190612 with RV FALKOR from Schmidt Ocean Institute (June-July 2019), at 13 seep sites between 180 and 1300m water depth (Oregon and California) massive chemoherm complexes with living tubeworms, clams, bacteria patches, gas rich/mousse sediment and gas hydrates near the seafloor surface were found. Water column imaging (EM302 and EK60) confirmed previously detected seeps, but also discovered new sites, some of which were investigated in more detail. Two areas off the coast of Oregon, in the Astoria Canyon at 850m and 500m depth (AC850 and AC500) were investigated in detail, using two small landers for visually measuring bubble sizes and rising rates (BubbleBox; 80Hz frame rate) and determining bubble release activity over a larger area hydroacoustically (GasQuant-II; 4Hz ping interval).
Site AC850m shows a hummocky seafloor (1.5 - 2m height differences) composed of dark, mousse-like sediments, with bacteria-covered peaks and valleys within a 900m2 area. Continuous GasQuant measurements over three days confirmed EM302 measurements that had shown this site was not constantly active. The previously studied AC500 site represents a 400m x 100m seep area with a massive chemoherm complex at the western end. The site also has an E-W striking 10m deep valley structure of 100m length ("Bubble Alley") with substantial gas release from more than 30 locations at the seafloor. At 500m depth, the site is above the gas hydrate stability zone; clams or clam shells occurred in close proximity to gas release vents, however, other typical seep biota, such as tubeworms or substantial bacteria patches, were absent. "Bubble Alley" was monitored twice using GasQuant and the BubbleBox for several days. Still ongoing data analyses show the coupling between gas release and tidal pressure changes, as well as short term cyclic changes. Parallel measurements with the EM302 and EK60 systems will be used to calibrate ship-based data with accurate bubble size distributions and seep-area calculated flow rates. Applying such a calibration to the entire hydroacoustic data set along the margin will provide a good estimate of the regional methane release and a data set for comparisons with previous and future measurements.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMOS51A..03G
- Keywords:
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- 3002 Continental shelf and slope processes;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3045 Seafloor morphology;
- geology;
- and geophysics;
- MARINE GEOLOGY AND GEOPHYSICS;
- 3050 Ocean observatories and experiments;
- MARINE GEOLOGY AND GEOPHYSICS