Abstract
Abstract. The Amon mud volcano (MV), located at 1250 m water depth on the Nile deep-sea fan, is known for its active emission of methane and non-methane hydrocarbons into the hydrosphere. Previous investigations showed a low efficiency of hydrocarbon-degrading anaerobic microbial communities inhabiting the Amon MV center in the presence of sulfate and hydrocarbons in the seeping subsurface fluids. By comparing spatial and temporal patterns of in situ biogeochemical fluxes, temperature gradients, pore water composition, and microbial activities over 3 yr, we investigated why the activity of anaerobic hydrocarbon degraders can be low despite high energy supplies. We found that the central dome of the Amon MV, as well as a lateral mud flow at its base, showed signs of recent exposure of hot subsurface muds lacking active hydrocarbon degrading communities. In these highly disturbed areas, anaerobic degradation of methane was less than 2% of the methane flux. Rather high oxygen consumption rates compared to low sulfide production suggest a faster development of more rapidly growing aerobic hydrocarbon degraders in highly disturbed areas. In contrast, the more stabilized muds surrounding the central gas and fluid conduits hosted active anaerobic hydrocarbon-degrading microbial communities. The low microbial activity in the hydrocarbon-vented areas of Amon MV is thus a consequence of kinetic limitations by heat and mud expulsion, whereas most of the outer MV area is limited by hydrocarbon transport.
Highlights
Hydrology andEarth System Sgruabvmitaatriionnealmfuodrcevsolicnadnuoceesvi(gMoSVro)ucsfioedrinmscchwearhsgeereoftemcutodnsi,cfluorids and methane gas from deep subsurface strata (Milkov, 2000; Kopf, 2002)
At the northern boundary of the central dome, we could not observe any indications of recent gas ebullition or bacterial mats at the seafloor (“transition zone” Ia). (II) Surrounding the central dome structure was an area of about 50 m width characterized by smooth ripples, marked by more stabilized sediments
The variable distribution and size of the mat patches in the central area suggest a spatially heterogeneous, but temporally persistent outflow of seep fluids in this habitat, as indicated by constant surficial temperature gradients in 2006 and 2009 (Fig. 5b, e). (III) The largest part of the Amon mud volcano (MV) was covered by biogenic mounds of 20–50 cm in height and width, formed by thalassinid mud shrimps
Summary
Earth System Sgruabvmitaatriionnealmfuodrcevsolicnadnuoceesvi(gMoSVro)ucsfioedrinmscchwearhsgeereoftemcutodnsi,cfluorids and methane gas from deep subsurface strata (Milkov, 2000; Kopf, 2002). Long-term degassing after eruption events via subsurface conduits can promote the formation of cold-seep ecosystemOscaet athne sSeacflioeonr, catetracting diverse and rich chemosynthetic communities (Sibuet and Olu, 1998; Werne et al, 2004; Niemann et al, 2006). In most cold-seep ecosystems, the anaerobic oxidation of methane (AOM) with sulfate is a key biogeochemical process, delivering sulfide to fuel chemosynthetic life (Boetius et al, 2000). Sulfide oxidation provides energy to thiSotrooplihdic Ebaactretrhia that are either free living or symbiotically associated with chemosynthetic animals (Jørgensen and Boetius, 2007). Due to the activity of methanotrophic microbial communities, the ocean is a relatively minor source of the greenhouse gas methane (Reeburgh, 2007)
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