Abstract
Deep-sea fans have been proposed to act as carbon sinks, rapid deposition driving shallow methanogenesis to favor net storage within the gas hydrate stability zone (GHSZ). Here, we present new evidence of widespread gas venting from the GHSZ on the upper Amazon deep-sea fan, together with analyses of the first samples of gas hydrates recovered offshore NE Brazil. Multibeam water column and seafloor imagery over an 18,000-km2 area of the upper Amazon fan reveal 53 water column gas plumes, rising from venting features in water depths of 650–2600 m. Most gas vents (60%) are located along seafloor faults that record the ongoing gravitational collapse of the fan above deep décollements, while others (40%) are located in water depths of 650–715 m within the upper edge of the GHSZ. Gas compositions from hydrates recovered in vents at three locations on and north of the fan indicate biogenic sources (dominantly methane with 2–15% of CO2; δ13C from − 81.1 to − 77.3‰), whereas samples from vents adjacent to the fan proper include possible thermogenic contributions (methane 95%, CO2 4%, and ethane 1%; δ13C – 59.2‰). These results concur with previous findings that the upper edge of the GHSZ may be sensitive to temporal changes in water temperatures, but further point to the importance of gas escape from within areas of gas hydrate stability. Our results suggest the role of fluid migration along pathways created by faulting within rapidly deposited passive margin depocenters, which are increasingly recognized to undergo gravitational collapse above décollements. Our findings add to evidence that gas can escape from sediments to the sea in areas where gas hydrates are stable on passive margins, and suggest the need of further studies of the dynamics of deep-sea depocenters in relation to carbon cycling.
Highlights
Sediment burial in submarine depocenters drives biogeochemical processes that sequester organic carbon over geological timescales, and convert part of it into mobile hydrocarbons, principally methane (Dickens et al 2004)
We present the first evidence of gas release to the water column from the Amazon fan, using multibeam echosounder (MBES) imagery acquired across an area of 18,000 km2, in water depths of 250–3500 m (Fig. 1), which allows us to map the distribution of gas flares in relation to seafloor vents and structures
Type II seeps show a clear spatial relationship with seafloor lineaments observed on MBES imagery (Fig. 2(B and C)), which seismic profiles show correspond to structures related to the compressional thrust-fold belt of the upper Amazon fan (Fig. 2(D))
Summary
Sediment burial in submarine depocenters drives biogeochemical processes that sequester organic carbon over geological timescales, and convert part of it into mobile hydrocarbons, principally methane (Dickens et al 2004). Methane that migrates toward the seabed may be stored in gas hydrates and/or authigenic carbonates (Arning et al 2013), or released to the oceans by seafloor venting (Römer et al 2014). While most vented gas may dissolve in the water column (McGinnis et al 2006), a portion may reach the atmosphere (Solomon et al 2009). The latter possibility is of interest in relation to carbon stable isotopic evidence that methane emissions are 60–110% greater than current global estimates (Schwietzke et al 2016). An improved understanding of the occurrence and mechanisms of deep-sea gas venting is of importance for our understanding of global carbon cycling in
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