Living microbial ecosystems within the active zone of catagenesis: Implications for feeding the deep biosphere

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“…Methane concentrations were highest at 300-660 mbsf (~60°-100°C) and remained relatively high in the presence of sulfate deeper than 660 mbsf (Shipboard Scientific Party, 2001c). Ethane concentrations increased with depth and pointed to thermogenic sources deeper than 900 mbsf (Shipboard Scientific Party, 2001b, 2001cHorsfield et al, 2006). This observation agrees well with findings at adjacent Site 808, where the stable carbon and hydrogen isotopic composition of methane points to microbial CO 2 reduction as the major methane source in sediment between 20 and 1000 mbsf and to an increasing contribution of thermogenic methane at greater depth (Berner and Faber, 1993).…”

“…Carbon/nitrogen ratios <10 indicated predominantly marine organic matter sources. Pore water sulfate was consumed rapidly in surface sediments and a shallow sulfate-methane transition zone (SMTZ) was found at 3-6 mbsf, but sulfate concentrations increased again throughout the ~490 m thick lower Shikoku Basin sediment and linear concentration gradients were consistent with diffusive flux of sulfate from the oceanic basement into the overlying sediment (Shipboard Scientific Party, 2001b, 2001cHorsfield et al, 2006). Methane concentrations were highest at 300-660 mbsf (~60°-100°C) and remained relatively high in the presence of sulfate deeper than 660 mbsf (Shipboard Scientific Party, 2001c).…”

“…Site 1174 subsided at a moderate rate of ~50 m/my during the Miocene and the Pliocene, before rates changed dramatically in the Quaternary. The rate of subsidence rose 12-fold to ~600 m/my and resulted in a high heating rate of up to 130 K/my (Horsfield et al, 2006). Deformation bands have developed deeper than 218 mbsf, and at 808-840 mbsf a distinct 32 m thick décollement zone separates the protothrust domain with fractured and steepened bedding (<807.6 mbsf ) from a relatively little deformed underthrust section (>840.2 mbsf ) (Shipboard Scientific Party, 2001c).…”

Microbial abundance in lacustrine sediments: a case study from Lake Van, Turkey

“…The reasons can be diverse, e.g. rapid changes in sedimentary organic matter content due to changes in palaeoproductivity and/or preservation like in sapropel layers of the Mediterranean (Coolen et al 2002), due to exceptionally high heat flow in the Nankai Trough (Moore et al 2001), where cell concentration increases again at greater depth due to in situ thermogenic generation of microbial substrates (Horsfield et al 2006) or at the base of continental margins, where mass-wasting events alter sediment accumulation rates (Cragg et al 1995).…”

Data report: microbial abundance in subseafloor sediments of the equatorial Pacific Ocean, Expedition 320/321

“…Elevated temperature acting over geological time will lead to kinetically controlled abiotic organic maturation reactions converting recalcitrant organic matter such as kerogen into volatile products like fatty acids and petroleum that can migrate over long distances and supply microbial communities with organic substrates at cold sites like gas hydrates (Mangelsdorf et al, 2005) or cold seeps in the deep sea (Joye et al, 2004). Horsfield et al (2006) showed that in areas with high heat flow, like the Nankai Trough off Japan, in situ microbes utilize thermogenically produced substrates because temperatures become sufficiently high at relatively shallow depths. Locations with an in situ coupling will most probably remain rare because despite reports about microbes being able to survive at temperatures well over 100°C (Blöchl et al, 1997;Kashefi and Lovley, 2003;Takai et al, 2008), at temperatures above 80°C microbial activity is severely reduced and sediment becomes basically pasteurized (Wilhelms et al, 2001).…”