Figure 3: Two-dimensional carbon isotope analysis applied to define carbon sources in the ocean. This diagram is a composite of carbon isotope signatures referenced in Coffin et al. 1989, 1994; Bauer et al. 1992, 1998; Cherrier et al. 1999; and Peterson and Fry, 1985.
The combination of radio and stable carbon isotope analysis provides a useful tool to delineate carbon sources and cycles in complex environments (Fig. 3). Analysis of a series of carbon pools can be used to determine sources and fate of carbon. This approach is being used for a understanding the sources of methane in hydrates and the geochemical and biogeochemical cycles that control methane hydrate stability and formation.
Ocean floor carbon cycling is controlled by a complex mixture of biological, chemical and physical processes. Physical factors include the motion of the continental plates resulting in expulsion of fluids charged with reduced volatile compounds, flows of geothermal energy toward the sediment-ocean interface, cold pressure mediated seeps of reduced compounds, and transport of terrestrial carbon to the sea floor. These processes control chemical speciation through key elemental pools that affect biological diversity. For example ocean floor thermal seeps have a high flux of reduced compounds, e.g. Fe+2, CH4, H2S and NH4 that supports the chemoautotrophic population. Enzymatic oxidation of reduced compounds results in fixation of C02 into cellular biomass which support the foodweb.
In contrast petroleum seepage along the Texas-Louisiana Shelf fuels some portion of heterotrophic activity on the ocean floor. In many of these systems methane hydrates are present. Comparisons of different sites have shown that there is a wide range of reduced compounds which support both heterotrophic and chemosynthetic bacterial activity. The relative portion of heterotrophic and autotrophic carbon fixation and energy transport to higher trophic levels need to be determined. Nor has the relative portion of the biogeochemical and thermal input to the hydrate formation and stability been widely determined. The following text provides an overview of the variation of biogeochemical cycles in the ocean floor and known abundance and formation of methane hydrates in these regions.
Methane hydrates are present in sediments with carbon contents greater than 0.5% and methane concentrations that are above 10 ml L"1 (Kennicutt et al., 1993). Methane hydrate formation is associated with geothermal and biological sources, however, at the majority of sites that contain methane hydrates there are geothermal source of reduced compounds induce methane production. In addition to the direct conversion of thermal methane into the hydrate compounds such as H2S, NH4+, and Fe+2, that supports the autotrophic bacterial assemblage at a rate where C02 becomes the dominant electron acceptor and biological methane is incorporated into the hydrate. This must occur at a rate where all of the energetically more efficient electron acceptors, e.g. nitrate, iron, manganese, and sulfate are consumed. The formation of the hydrate through the vertical profile in the sediment depends on the presence of an upward advective or diffusive flux of geothermal compounds. Blake Ridge, in the Atlantic Ocean, off the coast of the Carolinas and Georgia is an exception where the dominant input of organic matter originates from sedimentation derived from coastal origin and water column primary production (Paull et al., 1994). For biogenic methane to support significant hydrate formation, organic input must occur at a rate where all of the energetically more efficient electron acceptors, e.g. nitrate, iron, manganese, and sulfate are consumed. A process that detracts from the formation of hydrates is anaerobic oxidation of methane using sulfate as the electron acceptor (Borowski et al., 1997).
The Texas-Louisiana shelf of the Gulf of Mexico is a region rich in gas hydrates. This region is a salt basin that formed during the Late Triassic rifting of the Pangea super continent, and was subsequently flooded by a thick salt deposit during the middle of the Jurassic marine incursions (Salvador, 1987). Thermogenic petroleum and gas deposits produced from Miocene to Pleistocene reservoirs in the region originate from deep buried marine sources of Mesozoic age (Kennicutt et al 1993; Sassen et al. 1994.). In this region are over pressured fracture zones that surround moving salt diapers and sheets, as well as dynamic faults that provide active conduits for vertical migration from deep reservoirs to shallow pools and the surface (Brooks et al., 1984, Sassen et al. 1993a, in Sassen et al. 1994). This physical structure and the availability of reduced compounds in the site support a wide range of chemosynthetic communities (Sassen et al. 1994). Carbon isotope analysis of the sediment and structural analysis of the hydrates demonstrates a high input of thermogenic methane in this region. For example methane 813C ranges between -36 to -40%c, indicating little or no biological recycling (Sassen and MacDonald, 1997). Structural analysis of the hydrates also supports this observation. In this region hydrate structure II is dominant as a result of the hydrate formation in the presence of a mixture of reduced thermogenic compounds (Sassen and MacDonald, 1997).
While thermal methane is present in the hydrates in the Gulf of Mexico this should not be assumed to be the only source of methane. Microbial productivity is known to be an important component of the carbon cycle at natural seeps where free-living bacteria consume hydrocarbons resulting in anoxic sediments and sulfate reduction (Sassen, 1980; Sassen et al., 1994; Sassen et al., 1993). Sediments in this region have been measured with mean concentrations of petroleum and C1-C5 gases at mean concentrations of 5650 ppm and 12,979 ppm, respectively (Sassen et al., 1994). In addition to the advection of methane in this system, petroleum migrates through the sediment and stimulates the heterotrophic biological activity which results in the reduction of electron acceptors and the production of biogenic methane. Evidence for biogenic methane at this site comes from carbon isotope ratios as light as -57%o in tube worms and mussels (Brooks et al., 1987). This combination of methane cycles (Figure 4) is likely to result in multiple sources of methane in the hydrates.
Preliminary data suggests that the Hakkon Mosby Mud Volcano in the Norwegian-Greenland Sea is also a complex system with a combination of biological and geologic cycles that control the methane hydrate formation. This location is one of the more recent sites where methane hydrate samples have been recovered. A recent issue of Geo-Marine Letters (Volume 19, Number 1/2, 1999) describes various aspects of this site in a number of papers. Unlike many other hydrate-rich regions, their formation here is associated with venting caused by past glaciations. Mud flow in the volcano is thought to be caused by a combination of 1) the rise of lower density preglacial biogenic silica oozes buried beneath higher density glacial marine sediments, and 2) the regions history of massive submarine slides (Vogt et al
1999). Associated fluid flows carry methane to the surface of the sea floor, allowing hydrates to form.
Heterotrophic I prv^ Carbon 1 rvjo Cycling
Heterotrophic I prv^ Carbon 1 rvjo Cycling
Figure 4: Contribution of thermogenic and biogenic methane to the formation of methane hydrates.
In earlier work on methane from Haakon-Mosby hydrates, 513C values around -60%o were reported, suggesting methane in these hydrates is a mixture of thermogenic and biogenic sources (Lein et al 1999). Vertical profiles of the microbial assemblage document active sulfate reduction, methane generation and methane oxidation (Pimenov et al. 1999). At this location both aerobic and anaerobic methane oxidation is observed (Pimenov et al., 1999). As noted in the Gulf of Mexico a diverse series of geochemical cycles results in the methane production. In the Hakkon Mosby Mud Volcano the hydrates were survey in cores taken across the mud volcano. Hydrate content through the transect range from 10-20% to 0%, by weight, over a 750 m span of the sediment (Ginsburg et al., 1999). The average through this region is 1.2% hydrate. Given the high methane content of hydrates this is a substantial methane loading.
In contrast, Blake Ridge methane gas is microbial in origin and does not have a significant thermogenic input (Paull et al., 1994.). At this site isotope analysis of the methane and C02 pools provide a thorough understanding of the sources. Here the 813C indicates that methane is produced by biological reduction of C02 and that there are diverse mechanisms for the transport of methane through the sediment (Borowski et al., 1997). Ranges in the methane 813C are -85 to -103%o. The 513C that is more depleted represents diffusive methane transport through the vertical gradient in this region. With the slow vertical transport there is cycling of the methane and C02 that results in isotope depletion (Figure 4). This conclusion is supported with an observation of light 813C, -30 to -51 %c, in the DIC pool in the same region.
Further understanding of the diffusive methane transport in this system is obtained with the analysis of sulfate and 834S analysis in the vertical profiles (Borowski et al., 1996). In the sediment, sulfate profiles provide an understanding of the carbon sources that is being cycled (Chanton et al., 1993). If the concentration of sulfate declines from the sediment-water column interface to depth, the source of organic matter is sedimentation from the surface ocean. At Blake Ridge sulfate concentrations are linear through the upper region of the sediment indicating the methane originates from deeper deposits of organic matter. Sulfate concentrations decline in the deep sediments as a result of anaerobic methane oxidation where the sulfate is the terminal electron acceptor. In this region there is a strong gradient in the 834S profile of sulfate with values that are approximately 60%o enriched. This gradient in the isotope signature results from the reduction of sulfate.
The role of bacterial sulfate metabolism in methane hydrate stability and formation has been investigated on the Cascadia Margin, in the northeastern Pacific Ocean. In this region bacterial biomass and activity are stimulated in specific regions of the hydrate zone; 74 and 225 mbsf (Cragg et al. 1996). Bacterial activity at these sites was analyzed in terms of total production and methane oxidation. Preliminary analysis of this region indicates that there is a fluid flux through the accretionary wedge that provides electron acceptors to these specific depths. Primary findings at these depths were that the production and activity of the bacterial population in vertical profiles was greater in the regions with methane hydrates. Also, there was lower bacterial production in regions with high H2S concentrations. The lower production may result from H2S toxicity. Subsequent analysis of bacterial activity and community structure showed a large diversity through vertical profiles where arechaea are dominantly responsible for methane oxidation in the deep anoxic sediments and aerobic methanotrophs oxidized methane in the shallow sediments (Elvert et al. inpress).
4. AUTOTROPHIC ACTIVITY AT THE SEDIMENT WATER COLUMN INTERFACE
Methane hydrates have also been observed in shallow sediments (Sassen et al., 1993; Vogt et al., 1999). In the Gulf of Mexico these surface sediment hydrates are observed to be present in a region where the stability as a function of pressure and temperature is not predicted. There appears to be a continuous formation and dissociation of these surface hydrates (MacDonald et al. 1993). Carbon isotope analysis suggests that there is a wide range of carbon sources that control the presence of the hydrates in this region. Radio carbon isotope analysis indicates that the flux of thermal methane and hydrocarbons supports a large part of the biological activity (Brooks et al., 1987). For clams, mussels and tube worms A14C ranged between -840 and -204%o.
While the more negative values indicate that the thermogenic carbon sources are supporting the surface organisms, the large range in values indicates a host of carbon sources and the more positive value demonstrates an alternate source. In these cases there is active biological cycling on the hydrate surface which may contribute to methane hydrate stability. In addition, the dissociation of the hydrates provides energy to the active benthic community. More recent carbon isotope analysis of organic sediments in hydrate rich regions supports the observation that there is a complex mixture of thermogenic and biogenic carbon sources that influence the methane hydrate formation and stability (Coffin and Grabowski, unpublished). These regions of the ocean floor-water column interface have been observed to have high biological activity.
Closer examination of the biology at these sites indicates that the higher trophic levels are supported with a diverse array of chemosynthetic carbon fixation cycles. A large number of the organisms obtain energy through symbiotic cycles with the chemosynthetic bacteria. Along the Texas-Louisiana Shelf there has been several years of analysis of the biota on the ocean floor. In this region active crude oil and gas seepage is widely distributed along the continental slope.
This energy supports a wide variety of organisms at the base of the food chain, the petroleum appears to drive the system to hypoxia which results in high availability of hydrogen sulfide and biogenic and thermogenic methane (Sassen et al. 1993). This activity in the reduced environment results in the formation of energy sources for the chemosynthetic bacterial community. The high microbiological activity at this site results in mounds of authigenic carbonate minerals. The formation of the carbonate base is a result of methane oxidation associated with sulfate reduction which increases alkalinity causing the carbonate formation (Paull et al., 1992). The activity by the bacterial consortium occurs independently in the sediments and in symbiotic association with a broad range of organisms. Sulfur oxidizing bacteria, Beggiatoa, are known to fix C02 in association with H2S oxidation (Zobell, 1963). Beggiatoa are abundant in mats through the regions that contain high loading of methane and oil in the sediments (Sassen et al., 1994).
In addition, chemosynthetic activity supports a large amount of the food chain through symbiotic associations. Tube worm bushes are abundant through this region of the ocean floor, on the crest of gas hydrate mounds, with bacterial symbionts that oxidize H2S (MacDonald et al., 1989). Symbiotic bacteria are also the basis of very high metazoan biomass at seeps (MacDonald et al., 1989) exhibited by methane-oxidizing mytilids (Childress et al., 1986; MacDonald et al., 1990) and sulfide-oxidizing vestimentiferans (Fisher, 1990; MacDonald et al., 1990). This symbiotic relationship is a complex balance of elemental cycling.
While the host receives carbon in the form of methane from the chemotrophic bacterial assemblage, the bacteria require a mix of elements to account for the nutritional balance. In studies of the bivalve, Solemya reidi Bernard, it was observed that ammonium uptake and assimilation by the chemotrophic bacteria depletes nitrogen from the host and promotes the passive membrane transport of exogenous ammonium (Lee et al. 1992a). However, further analysis of deep-sea bivalves (Bathy modiolus sp., undescribed) indicated that a broad combination of nitrogen sources may be used to support the nutrient requirements of the host and chemoautotrophic organisms (Lee et al., 1992b). Depending on the availability of nitrogen at the site sources could include ammonium and particulate and dissolved organic nitrogen.
Current ocean modeling focuses on surface ocean primary production, transmission of sun light through the ocean and transport of land based carbon sources through the coastal margin. In most open ocean environments the dominant carbon cycle is thought to be a balance of primary production and heterotrophic cycling with subsequent key element mineralization and transition of energy through the food chain. Over the last twenty years refined analysis of chemical and biological cycles suggest that other factors may contribute to the ocean water column carbon cycling (Fig. 5). A wide variety of observations promote the need to reorganize modeling of ocean carbon cycling. While the early analysis of methane cycling in the ocean was believed to be dominated by activity in anoxic microniches the more recent work strongly suggests that the sediments also contributes to this cycle.
For example, pockmarks in sediments on the ocean floor along eastern Skageerak between the coast of Denmark and Norway suggesting that there is a high flux of methane from the ocean floor into the water column (Hovland, 1992). Methane fluxes have been measured from the sediment, into the water column in the Cariaco Trench with values ranging from 12.5 to 17.5 mmol cm"2 yr"1 (Scranton, 1998). High concentrations of methane that are 30-70% supersaturated relative the concentration at atmospheric equilibrium (Lamontagne et al., 1973). Studies demonstrate the presence of methane oxidation in the open ocean water column (Sieburth 1993), leading to speculation that the methane oxidation resulted from microscale anoxic cycles in floating organic rich marine seston. Bacterial species have been observed in the seston which are obligatory anaerobic. Such observations are supported by a wide range of other microbiological and biogeochemical surveys. Water column 16s rRNA surveys demonstrate that in deep ocean waters a significant fraction of the bacterial assemblage are archaea bacteria (Fuhrman et al., 1992; Massana et al., 1997). A large segment of this phylum is autotrophic.
Further support for autotrophic carbon cycling in the water column comes from 513C analysis of bacteria. An examination of 813C of particulate organic matter and bacterioplankton in transects from the Mississippi River outflow to the high salinity end member indicated that besides carbon from the Mississippi river and primary production a significant, alternate source of carbon support microbial production (Kelley et al., 1998). The range of 513C found in bacteria and seston was from -25%o to -32%c. Future investigation will determine if petroleum or methane are the primary carbon source. While the early analysis of methane cycling in the ocean was believed to be dominated by activity in anoxic microniches the more recent studies demonstrate that the sediments also contributes to this cycle.
In addition to methane, ammonium oxidation has been shown to be a substantial fraction of carbon cycling through the microbial assemblage in the Mississippi River Plume in the Gulf of Mexico (Pakulski et al., 1995). For this study 20-60 % of the total oxygen demand was attributed to ammonium oxidation, resulting in a substantial portion of the total bacterial production. When present in sufficient concentrations, methane will be cycled before ammonium (Jones and Morita, 1983).
Figure 5: Carbon sources that contribute to the water column biological activity.
Figure 5: Carbon sources that contribute to the water column biological activity.
Comparisons of Nitrosomonas oceanus selecting an energy source demonstrated that the presence of methane inhibited ammonium oxidation (Ward, 1987). However, subsequent studies showed that both methane and ammonium oxidation could occur simultaneously (Ward and Kilpatrick, 1990). These studies initiate the need for further analysis of ocean carbon cycling. In addition to methane other geothermal energy sources need to be examined as an energy source in the water column. High active petroleum seeps have been observed in the Gulf of Mexico surface ocean (MacDonald et al., 1993 Mitchell et al., 1999). In addition, ethene to butene (C2-C4) have been reported to be in concentrations that range from 134 to 37 pM, respectively (Plass-Diilmer et al. 1995).
The turnover time of these carbon pools need to be measured to quantify the support to the bacterial assemblage relative to other forms. Understanding the role of methane hydrates in the transport of carbon to the water column is an interesting topic. Methane hydrate regions are highly populated with diverse biological species. There is likely to be high concentrations of carbon that seeps from these regions and into the water column. Recent estimates of the methane flux to the water column stated that 1-10% of the total methane hydrate was transported to the water column (Ginsburg et al., 1999).
These events can be difficult to survey. The flux of compounds from the sediment is not always constant. An earthquake along the Klamath River Delta in Northern California was observed to produce active venting, then reduced substantially after one year and totally subsided with in five years (Field and Jennings, 1987). The observation promotes the need for long term understanding of the flux of reduced compounds into the watercolumn for a more thorough understanding.
Methane hydrates, a potentially harvestable energy, form at low temperature and high pressure from methane which has resulted from the thermal or microbiological decomposition of organic material. Prior to the capture and utilization of this resource the biogeochemical factors which affect hydrate stability and formation must be determined. Methane is a high energy product of the result of a terminal electron accepting process; its migration towards aerobic waters can fuel chemoautotrophic production and growth. Natural abundance radiocarbon and stable isotopic composition serve as effective tools to trace the sum of the processes which have affected methane formation and consumption (Fig. 3).
The isotope analysis of bacterial biomarkers, dissolved inorganic carbon, organic carbon and methane will assist in analysis the relative contribution thermogenic and biogenic methane to hydrates (Fig. 4). A combination of analyses of microbial cycling rates and community diversity, coupled with the two-dimensional carbon isotope analysis provide the potential to access the rate of methane hydrate formation and stability and the biogeochemical processes that control these cycles. These isotopes also allow the tracing of sea floor autotrophically fixed carbon to higher trophic levels. Research following these tracers into the water column has led to the suggestion that high energy componds derived from the benthos may fuel heterotrophic processes in the water column. These data suggest future development of ocean modeling requires addition of geothermal energy that influences the carbon cycling.
Deep Biosphere: Source of Methane for Oceanic Hydrate
Peter Wellsbury and R. John Parkes
Department of Earth Sciences University of Bristol, UK
Methane is an important product of anaerobic bacterial metabolism. Bacterial methane makes a substantial contribution to global methane reserves. Methanogenesis is the final step in the anaerobic degradation of organic matter, and can continue in deeply buried sediments. Methane can also be produced abiologically at elevated temperatures and pressures e.g., thermal breakdown of organic matter, crustal and hydrothermal processes. The boundary between biological and abiological processes is not always clear. Bacteria can be active at temperatures up to 113°C and pressures in excess of 1000 atm, and abiological processes can produce energy sources for bacterial methanogenesis. In addition, deep sourced thermogenic methane can diffuse to the surface, and under certain conditions, biogenic methane can have a chemical and stable isotope signature indicating an abiological origin.
Oceanic gas hydrates contain predominantly methane with a bacterial origin (Kvenvolden 1995). Recent studies have demonstrated the presence of bacteria to depths of over eight hundred metres below the sea floor (mbsf) in marine sediments (Parkes et al. 2000). Bacterial populations and their activity are stimulated in oceanic hydrate deposits to such an extent that some processes are more intense at depth than in near-surface sediment (Cragg et al. 1996, Wellsbury et al. 1997), demonstrating that gas hydrates are a unique deep subsurface habitat.
The role of bacteria in methane production, and their activity in sediments containing hydrate, is reviewed in detail below.
2. BACTERIAL METHANOGENESIS
Bacteria which produce methane are called methanogens. They produce methane to generate energy for survival and growth. Methanogens are obligate anaerobes, killed by traces of oxygen, and thrive only in anoxic conditions. Methanogens can only use a limited range of small molecules for metabolism. These small molecules are supplied as the end product of the metabolism of
other types of bacteria. Thus methanogenic bacteria form only a sub-set of the total microbial population in their environment. In these circumstances, one bacterium's metabolic waste product is another's food; thus it is important to understand the interactions of different types of bacteria in effecting the breakdown of organic matter to methane.
In oxic conditions, organic material can be degraded fully to carbon dioxide by a single bacterium. Conversely, under anoxic conditions, the degradation of organic matter to carbon dioxide or methane is carried out by a sequence of interacting bacterial types (Fig. 1).
soluble monomers fatty acids, amino acids, sugars, phenol-substituted acids
( hydrolysis/fermentation J
( hydrolysis/fermentation J
^---low-rr soluble monomers fatty acids, amino acids, sugars, phenol-substituted acids z>
^---low-rr low-molecular-wt. Volatile Fatty Acids propionate, butyrate etc. hydroxy fatty acids lactate, pyruvate etc.
alcohols ethanol, propanol etc.
f methane | ^ oxidation J, f methane | ^ oxidation J,
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