Continued warming of the Arctic Ocean in coming decades is projected to trigger the release of teragrams (1 Tg = 10⁶ tons) of methane from thawing subsea permafrost on shallow continental shelves and ...dissociation of methane hydrate on upper continental slopes. On the shallow shelves (<100 m water depth), methane released from the seafloor may reach the atmosphere and potentially amplify global warming. On the other hand, biological uptake of carbon dioxide (CO₂) has the potential to offset the positive warming potential of emitted methane, a process that has not received detailed consideration for these settings. Continuous sea–air gas flux data collected over a shallow ebullitive methane seep field on the Svalbard margin reveal atmospheric CO₂ uptake rates (−33,300 ± 7,900 μmol m−2·d−1) twice that of surrounding waters and ∼1,900 times greater than the diffusive sea–air methane efflux (17.3 ± 4.8 μmol m−2·d−1). The negative radiative forcing expected from this CO₂ uptake is up to 231 times greater than the positive radiative forcing from the methane emissions. Surface water characteristics (e.g., high dissolved oxygen, high pH, and enrichment of 13C in CO₂) indicate that upwelling of cold, nutrient-rich water from near the seafloor accompanies methane emissions and stimulates CO₂ consumption by photosynthesizing phytoplankton. These findings challenge the widely held perception that areas characterized by shallow-water methane seeps and/or strongly elevated sea–air methane flux always increase the global atmospheric greenhouse gas burden.
Carbon dioxide (CO2) and methane (CH4) are natural and anthropogenic products that play a central role in the global carbon cycle and regulating Earth’s climate. Applications utilizing laser ...absorption spectroscopy, which continuously measure concentrations and stable isotope ratios of these greenhouse gases, are routinely employed to measure the source and magnitude of atmospheric inputs. We developed a discrete sample introduction module (DSIM) to enable measurements of methane and CO2 concentrations and δ13C values from limited volume (5–100 mL) gas samples when interfaced with a commercially available cavity ring-down spectroscopy (CRDS) analyzer. The analysis has a dynamic range that spans six orders of magnitude from 100% analyte to the lower limit of instrument detection (2 ppm). We demonstrate system performance for methane by comparing concentrations and δ13C results from the DSIM-CRDS system and traditional methods for a variety of sample types, including low concentration (nanomolar CH4) seawater and high concentration (>90% CH4) natural gas. The expansive concentration range of the field-portable DSIM-CRDS system can measure enhances analytical performance for investigating methane and CO2 dynamics and, potentially, other gases measured by laser absorption spectroscopy.
Methane is a key component in the global carbon cycle, with a wide range of anthropogenic and natural sources. Although isotopic compositions of methane have traditionally aided source ...identification, the abundance of its multiply substituted "clumped" isotopologues (for example, 13CH3D) has recently emerged as a proxy for determining methane-formation temperatures. However, the effect of biological processes on methane's clumped isotopologue signature is poorly constrained. We show that methanogenesis proceeding at relatively high rates in cattle, surface environments, and laboratory cultures exerts kinetic control on 13CH3D abundances and results in anomalously elevated formation-temperature estimates. We demonstrate quantitatively that H2 availability accounts for this effect. Clumped methane thermometry can therefore provide constraints on the generation of methane in diverse settings, including continental serpentinization sites and ancient, deep groundwaters.
Recent investigations of anchialine caves and sinkholes have identified complex food webs dependent on detrital and, in some cases, chemosynthetically produced organic matter. Chemosynthetic microbes ...in anchialine systems obtain energy from reduced compounds produced during organic matter degradation (e.g., sulfide, ammonium, and methane), similar to what occurs in deep ocean cold seeps and mud volcanoes, but distinct from dominant processes operating at hydrothermal vents and sulfurous mineral caves where the primary energy source is mantle derived. This review includes case studies from both anchialine and non-anchialine habitats, where evidence for in situ chemosynthetic production of organic matter and its subsequent transfer to higher trophic level metazoans is documented. The energy sources and pathways identified are synthesized to develop conceptual models for elemental cycles and energy cascades that occur within oligotrophic and eutrophic anchialine caves. Strategies and techniques for testing the hypothesis of chemosynthesis as an active process in anchialine caves are also suggested.
Analytical challenges in obtaining high quality measurements of rare earth elements (REEs) from small pore fluid volumes have limited the application of REEs as deep fluid geochemical tracers. Using ...a recently developed analytical technique, we analyzed REEs from pore fluids collected from Sites U1325 and U1329, drilled on the northern Cascadia margin during the Integrated Ocean Drilling Program (IODP) Expedition 311, to investigate the REE behavior during diagenesis and their utility as tracers of deep fluid migration. These sites were selected because they represent contrasting settings on an accretionary margin: a ponded basin at the toe of the margin, and the landward Tofino Basin near the shelf's edge. REE concentrations of pore fluid in the methanogenic zone at Sites U1325 and U1329 correlate positively with concentrations of dissolved organic carbon (DOC) and alkalinity. Fractionations across the REE series are driven by preferential complexation of the heavy REEs. Simultaneous enrichment of diagenetic indicators (DOC and alkalinity) and of REEs (in particular the heavy elements Ho to Lu), suggests that the heavy REEs are released during particulate organic carbon (POC) degradation and are subsequently chelated by DOC. REE concentrations are greater at Site U1325, a site where shorter residence times of POC in sulfate-bearing redox zones may enhance REE burial efficiency within sulfidic and methanogenic sediment zones where REE release ensues.
Cross-plots of La concentrations
versus Cl, Li and Sr delineate a distinct field for the deep fluids (z
>
75
mbsf) at Site U1329, and indicate the presence of a fluid not observed at the other sites drilled on the Cascadia margin. Changes in REE patterns, the presence of a positive Eu anomaly, and other available geochemical data for this site suggest a complex hydrology and possible interaction with the igneous Crescent Terrane, located east of the drilled transect.
► We measured REE in pore water of highly reduction sediment in northern Cascadia. ► REE content is related to residence times of organic carbon in SO
4-baering zone. ► HREE enrichment is due to organic matter diagenesis and chelation with metabolites. ► The presence of a distinct deep fluid is apparent in REE and other geochemical data.
Methane (CH
) emissions from Arctic lakes are a large and growing source of greenhouse gas to the atmosphere with critical implications for global climate. Because Arctic lakes are ice covered for ...much of the year, understanding the metabolic flexibility of methanotrophs under anoxic conditions would aid in characterizing the mechanisms responsible for limiting CH
emissions from high-latitude regions. Using sediments from an active CH
seep in Lake Qalluuraq, Alaska, we conducted DNA-based stable isotope probing (SIP) in anoxic mesocosms and found that aerobic Gammaproteobacterial methanotrophs dominated in assimilating CH
. Aerobic methanotrophs were also detected down to 70 cm deep in sediments at the seep site, where anoxic conditions persist. Metagenomic analyses of the heavy DNA from
CH
-SIP incubations showed that these aerobic methanotrophs had the capacity to generate intermediates such as methanol, formaldehyde, and formate from CH
oxidation and to oxidize formaldehyde in the tetrahydromethanopterin (H
MPT)-dependent pathway under anoxic conditions. The high levels of Fe present in sediments, combined with Fe and CH
profiles in the persistent CH
seep site, suggested that oxidation of CH
, or, more specifically, its intermediates such as methanol and formaldehyde might be coupled to iron reduction. Aerobic methanotrophs also possessed genes associated with nitrogen and hydrogen metabolism, which might provide potentially alternative energy conservation options under anoxic conditions. These results expand the known metabolic spectrum of aerobic methanotrophs under anoxic conditions and necessitate the re-assessment of the mechanisms underlying CH
oxidation in the Arctic, especially under lakes that experience extended O
limitations during ice cover.
Methane (CH(4)) emitted from high-latitude lakes accounts for 2-6% of the global atmospheric CH(4) budget. Methanotrophs in lake sediments and water columns mitigate the amount of CH(4) that enters ...the atmosphere, yet their identity and activity in arctic and subarctic lakes are poorly understood. We used stable isotope probing (SIP), quantitative PCR (Q-PCR), pyrosequencing and enrichment cultures to determine the identity and diversity of active aerobic methanotrophs in the water columns and sediments (0-25 cm) from an arctic tundra lake (Lake Qalluuraq) on the north slope of Alaska and a subarctic taiga lake (Lake Killarney) in Alaska's interior. The water column CH(4) oxidation potential for these shallow (∼2 m deep) lakes was greatest in hypoxic bottom water from the subarctic lake. The type II methanotroph, Methylocystis, was prevalent in enrichment cultures of planktonic methanotrophs from the water columns. In the sediments, type I methanotrophs (Methylobacter, Methylosoma and Methylomonas) at the sediment-water interface (0-1 cm) were most active in assimilating CH(4), whereas the type I methanotroph Methylobacter and/or type II methanotroph Methylocystis contributed substantially to carbon acquisition in the deeper (15-20 cm) sediments. In addition to methanotrophs, an unexpectedly high abundance of methylotrophs also actively utilized CH(4)-derived carbon. This study provides new insight into the identity and activity of methanotrophs in the sediments and water from high-latitude lakes.
Ocean drilling has revealed the existence of vast microbial populations in the deep subseafloor, but to date little is known about their metabolic activities. To better understand the biogeochemical ...processes in the deep biosphere, we investigate the stable carbon isotope chemistry of acetate and other carbon-bearing metabolites in sediment pore-waters. Acetate is a key metabolite in the cycling of carbon in anoxic sediments. Its stable carbon isotopic composition provides information on the metabolic processes dominating acetate turnover in situ. This study reports our findings for a methane-rich site at the northern Cascadia Margin (NE Pacific) where Expedition 311 of the Integrated Ocean Drilling Program (IODP) sampled the upper 190
m of sediment. At Site U1329,
δ
13C values of acetate span a wide range from −46.0‰ to −11.0‰ vs. VPDB and change systematically with sediment depth. In contrast,
δ
13C values of both the bulk dissolved organic carbon (DOC) (−21.6
±
1.3‰ vs. VPDB) and the low-molecular-weight compound lactate (−20.9
±
1.8‰ vs. VPDB) show little variability. These species are interpreted to represent the carbon isotopic composition of fermentation products. Relative to DOC, acetate is up to 23.1‰ depleted and up to 9.1‰ enriched in
13C. Broadly,
13C-depletions of acetate relative to DOC indicate flux of carbon from acetogenesis into the acetate pool while
13C-enrichments of pore-water acetate relative to DOC suggest consumption of acetate by acetoclastic methanogenesis. Isotopic relationships between acetate and lactate or DOC provide new information on the carbon flow and the presence and activity of specific functional microbial communities in distinct biogeochemical horizons of the sediment. In particular, they suggest that acetogenic CO
2-reduction can coexist with methanogenic CO
2-reduction, a notion contrary to the hypothesis that hydrogen levels are controlled by the thermodynamically most favorable electron-accepting process. Further, the isotopic relationship suggests a relative increase in acetate flow to acetoclastic methanogenesis with depth although its contribution to total methanogenesis is probably small. Our study demonstrates how the stable carbon isotope biogeochemistry of acetate can be used to identify pathways of microbial carbon turnover in subsurface environments. Our observations also raise new questions regarding the factors controlling acetate turnover in marine sediments.
Summary
Aerobic methane (CH4) oxidation mitigates CH4 release and is a significant pathway for carbon and energy flow into aquatic food webs. Arctic lakes are responsible for an increasing proportion ...of global CH4 emissions, but CH4 assimilation into the aquatic food web in arctic lakes is poorly understood. Using stable isotope probing (SIP) based on phospholipid fatty acids (PLFA‐SIP) and DNA (DNA‐SIP), we tracked carbon flow quantitatively from CH4 into sediment microorganisms from an arctic lake with an active CH4 seepage. When 0.025 mmol CH4 g−1 wet sediment was oxidized, approximately 15.8–32.8% of the CH4‐derived carbon had been incorporated into microorganisms. This CH4‐derived carbon equated to up to 5.7% of total primary production estimates for Alaskan arctic lakes. Type I methanotrophs, including Methylomonas, Methylobacter and unclassified Methylococcaceae, were most active at CH4 oxidation in this arctic lake. With increasing distance from the active CH4 seepage, a greater diversity of bacteria incorporated CH4‐derived carbon. Actinomycetes were the most quantitatively important microorganisms involved in secondary feeding on CH4‐derived carbon. These results showed that CH4 flows through methanotrophs into the broader microbial community and that type I methanotrophs, methylotrophs and actinomycetes are important organisms involved in using CH4‐derived carbon in arctic freshwater ecosystems.
Dissolved methane concentrations in the ocean are close to equilibrium with the atmosphere. Because methane is only sparingly soluble in seawater, measuring it without contamination is challenging ...for samples collected and processed in the presence of air. Several methods for analyzing dissolved methane are described in the literature, yet none has conducted a thorough assessment of the method yield, contamination issues during collection, transport and storage, and the effect of temperature changes and preservative. Previous extraction methods transfer methane from water to gas by either a "sparge and trap" or a "headspace equilibration" technique. The gas is then analyzed for methane by gas chromatography. Here, we revisit the headspace equilibration technique and describe a simple, inexpensive, and reliable method to measure methane in fresh and seawater, regardless of concentration. Within the range of concentrations typically found in surface seawaters (2‐1000 nmol L‐1), the yield of the method nears 100% of what is expected from solubility calculation following the addition of known amount of methane. In addition to being sensitive (detection limit of 0.1 ppmv, or 0.74 nmol L‐1), this method requires less than 10 min per sample, and does not use highly toxic chemicals. It can be conducted with minimum materials and does not require the use of a gas chromatograph at the collection site. It can therefore be used in various remote working environments and conditions.
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