Monitoring periodic and episodic flow events at Monterey Bay seeps using a new optical flow meter

Abstract

[1] To enable testing of the hypothesis that fluid flow from cold seeps responds to episodic tectonic events, we have developed a flow meter with a temporal resolution on the order of minutes. The Optical Tracer Injection System (OTIS) measures flow rates through the sediment-water interface (SWI) of 0.1 to >500 m/yr ± 10%, samples fluid for postrecovery chemical analysis, and is adaptable for use as a long-term real-time monitoring station. Laboratory and in situ testing demonstrate that the instrument meets temporal accuracy and resolution requirements necessary to detect transient flow events. In a deployment at an active seep site in Monterey Bay, the OTIS measured flow rates with tidal period variability. Time series analysis indicates flow rate and tide height records are in phase. This flow rate response to tides, together with seep fluid temperatures measured at the SWI of greater than 9°C, suggests the seep is connected via a highly permeable fracture or other conduit to an overpressured aquifer at a depth of at least 110 m. We infer the hydrogeology of this well-like system is further complicated by additional fluid sources since 3 He/ 4 He ratios are elevated, and there is a lack of correlation between flow rate and temperature records at frequencies lower than tidal frequencies. This short-duration deployment demonstrates the OTIS's potential to capture flow and chemistry transients associated with earthquakes and creep in the offshore environment.

A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems

“…Fluids in brine pools and mud volcanoes in the Gulf of Mexico [62], the SW Barents Sea [63] and the eastern Mediterranean Sea [64] all exhibit temperatures elevated above ambient, but the sediments are typically hypersaline, anoxic and/or physically disrupted, and are marked by an absence of metazoan fauna. Fluids advected at Extrovert Cliff (950 m) in Monterey Bay have temperatures up to 98C in contrast with an ambient 4.18C temperature, and elevated He concentrations and 3 He/ 4 He ratios associated with a deep aquifer and several fault zones [12,65]. However, the fauna does not appear vent-like; the sole presence of bacterial mats, vesicomyid clams and associated heterotrophic fauna may reflect low oxygen concentrations in overlying waters.…”

“…Seep sediments typically have higher methane and sulphide concentrations and seep carbonates are precipitated by anaerobic methane oxidation [3]. Microbial and metazoan composition distinctions and different metabolic, C fixation and trophic pathways are also recognized (See electronic supplemental material, table S1), yet both vent and seep fluids may contain methane [3], elevated temperatures [12] and similar geochemistry [9]. Vents and seeps exhibit a continuum of abiotic and biotic characteristics, however, biological observations along this continuum are often restricted to the large, symbiont-bearing megafauna.…”

Acoustic imaging of natural gas seepage in the North Sea: Sensing bubbles controlled by variable currents

“…Depending on the free gas distribution within the sediment and its permeability, the tidal load and unload on the seafloor may be propagating with a phase lag growing with depth (Wang et 167 LaBonte et al 2007). Because of high permeability and low methane concentrations of the topmost sandy layer, we do not expect such a phase delay to occur in the surficial clay layer.…”

A sediment flow‐through system to study the impact of shifting fluid and methane flow regimes on the efficiency of the benthic methane filter

“…Another long-term change of methane flux is postulated to be caused by global warming. Seafloor warming will decrease the stability of shallow submarine gas hydrates and increase methane fluxes (Kvenvolden 1993;LaBonte 2007). Because the Arctic Ocean is affected strongly by global warming and because gas hydrate stability is shallow compared with other oceans because of low temperatures, this region is likely to be one of the first regions to show temperature-related methane releases from gas hydrates (Buffett and Archer 2004;Biastoch et al 2011).…”

“…Aside from spatial variability, fluid flux, and associated methane supply with the attendant changes in the depth of the SMTZ, can also vary temporarily. Short-term fluid flow fluctuations can be created by blocked fluid paths, e.g., through spontaneous gas hydrate formation or carbonate precipitation (Luff et al 2004(Luff et al , 2005, which force the formation of new migration pathways, or by tidal pressure changes (Tryon et al 2002;LaBonte et al 2007;Røy et al 2008;Boles et al 2001).…”