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  • 51. Mondav, Rhiannon
    et al.
    Woodcroft, Ben J.
    Kim, Eun-Hae
    McCalley, Carmody K.
    Hodgkins, Suzanne B.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Chanton, Jeffrey
    Hurst, Gregory B.
    VerBerkmoes, Nathan C.
    Saleska, Scott R.
    Hugenholtz, Philip
    Rich, Virginia I.
    Tyson, Gene W.
    Discovery of a novel methanogen prevalent in thawing permafrost2014In: Nature Communications, ISSN 2041-1723, E-ISSN 2041-1723, Vol. 5, p. 3212-Article in journal (Refereed)
    Abstract [en]

    Thawing permafrost promotes microbial degradation of cryo-sequestered and new carbon leading to the biogenic production of methane creating a positive feedback to climate change. Here we determine microbial community composition along a permafrost thaw gradient in northern Sweden. Partially thawed sites were frequently dominated by a single archaeal phylotype Candidatus 'Methanoflorens stordalenmirensis' gen. nov. sp. nov. belonging to the uncultivated lineage 'Rice Cluster II' (Candidatus 'Methanoflorentaceae' fam. nov.). Metagenomic sequencing led to the recovery of its near-complete genome revealing the genes necessary for hydrogenotrophic methanogenesis. These genes are highly expressed and methane carbon isotope data are consistent with hydrogenotrophic production of methane in the partially thawed site. In addition to permafrost wetlands 'Methanoflorentaceae' are widespread in high methane-flux habitats suggesting that this lineage is both prevalent and a major contributor to global methane production. In thawing permafrost Candidatus 'M. stordalenmirensis' appears to be a key mediator of methane-based positive feedback to climate warming.

  • 52. Natchimuthu, Sivakiruthika
    et al.
    Sundgren, Ingrid
    Gålfalk, Magnus
    Klemedtsson, Leif
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Danielsson, Åsa
    Bastviken, David
    Spatio-temporal variability of lake CH4 fluxes and its influence on annual whole lake emission estimates2016In: Limnology and Oceanography, ISSN 0024-3590, E-ISSN 1939-5590, Vol. 61, p. S13-S26Article in journal (Refereed)
    Abstract [en]

    Lakes are major sources of methane (CH4) to the atmosphere that contribute significantly to the global budget. Recent studies have shown that diffusive fluxes, ebullition and surface water CH4 concentrations can differ significantly within lakesspatially and temporally. CH4 fluxes may be affected at longer scales in response to seasons, temperature, lake mixing events, short term weather events like pressure variations, shifting winds and diel cycles. Frequent measurements of fluxes in the same system and integrated assessments of the impacts of the spatio-temporal variability are rare. Thereby, large scale assessments frequently lack information on this variability which can potentially lead to biased estimates. In this study, we analysed the variability of CH4 fluxes and surface water CH4 concentrations across open water areas of lakes in a small catchment in southwest Sweden over two annual cycles. Significant patterns in CH4 concentrations, diffusive fluxes, ebullition and total fluxes were observed in space (between and within lakes) and in time (over diel cycles to years). Differences observed among the lakes can be associated with lake characteristics. The spatial variability within lakes was linked to depth or distance to stream inlets. Temporal variability was observed at diel to seasonal scales and was influenced by weather events. The fluxes increased exponentially with temperature in all three lakes, with stronger temperature dependence with decreasing depth. By comparing subsets of our data with estimates using all data we show that considering the spatio-temporal variability in CH4 fluxes is critical when making whole lake or annual budgets.

  • 53.
    Neubeck, Anna
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Nguyen, Duc
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bastviken, David
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Holm, Nils
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Formation of H2 and CH4 by weathering of olivine at temperatures between 30 and 70°C2011In: Geochemical Transactions, ISSN 1467-4866, E-ISSN 1467-4866, Vol. 12, no 6Article in journal (Refereed)
    Abstract [en]

    Hydrocarbons such as CH4 are known to be formed through the Fischer-Tropsch or Sabatier type reactions in hydrothermal systems usually at temperatures   above 100°C. Weathering of olivine is sometimes suggested to account for abiotic formation of CH4 through its redox lowering and water splitting properties. Knowledge about the CH4 and H2 formation processes at low temperatures is important for the research about the origin and cause of early Earth and Martian   CH4 and for CO2 sequestration. We have conducted a series of low temperature, long-term weathering experiments in which we have tested the   CH4 and H2 formation potential of forsteritic olivine.

    The results show low temperature CH4 production that is probably influenced by chromite and magnetite as catalysts. Extensive analyses of a potential CH4 source trapped in the crystal structure of the olivine showed no signs of incorporated CH4. Also, the available sources of organic carbon were not enough to support the total amount of CH4 detected in our experiments. There was also a linear relationship between silica release into solution and the net CH4 accumulation into the incubation bottle headspaces suggesting that CH4 formation under these conditions could be a qualitative indicator of olivine dissolution.

    It is likely that minerals such as magnetite, chromite and other metal-rich minerals found on the olivine surface catalyze   the formation of CH4, because of the low temperature of the system. This may expand the range of environments plausible for abiotic CH4 formation both on Earth and on other terrestrial bodies.

  • 54.
    Nisbet, R.E.R.
    et al.
    Cambridge Univ.
    Fisher, R.
    Cambridge Univ.
    Nimmo, R.H.
    Cambridge Univ.
    Bendall, D.S.
    Cambridge Univ.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Emission of methane by plants.2009In: Proceedings of the Royal Society of London. Biological Sciences, ISSN 0962-8452, E-ISSN 1471-2954, Vol. 276, no 1660, p. 1347-1354Article in journal (Refereed)
    Abstract [en]

    It has been proposed that plants are capable of producing methane by a novel and unidentified biochemical pathway. Emission of methane with an apparently biological origin was recorded from both whole plants and detached leaves. This was the first report of methanogenesis in an aerobic setting, and was estimated to account for 10–45 per cent of the global methane source. Here, we show that plants do not contain a known biochemical pathway to synthesize methane. However, under high UV stress conditions, there may be spontaneous breakdown of plant material, which releases methane. In addition, plants take up and transpire water containing dissolved methane, leading to the observation that methane is released. Together with a new analysis of global methane levels from satellite retrievals, we conclude that plants are not a major source of the global methane production.

  • 55. Olefeldt, David
    et al.
    Roulet, Nigel T.
    Bergeron, Onil
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bäckstrand, Kristina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Christensen, Torben R.
    Net carbon accumulation of a high-latitude permafrost palsa mire similar to permafrost-free peatlands2012In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, p. L03501-Article in journal (Refereed)
    Abstract [en]

    Palsa mires, nutrient poor permafrost peatlands common in subarctic regions, store a significant amount of carbon (C) and it has been hypothesized their net ecosystem C balance (NECB) is sensitive to climate change. Over two years we measured the NECB for Stordalen palsa mire and found it to accumulate 46 g C m(-2) yr(-1). While Stordalen NECB is comparable to nutrient poor peatlands without permafrost, the component fluxes differ considerably in magnitude. Specifically, Stordalen had both lower growing season CO2 uptake and wintertime CO2 losses, but importantly also low dissolved organic carbon exports and hydrocarbon (mainly methane) emissions. Restricted C losses from palsa mires are likely to have facilitated C accumulation of unproductive subarctic permafrost peatlands. Continued climate change and permafrost thaw is likely to amplify several component fluxes, with an uncertain overall effect on NECB - highlighting the necessity for projections of high-latitude C storage to consider all C fluxes.

  • 56. Olefeldt, David
    et al.
    Turetsky, Merritt R.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    McGuire, A. David
    Environmental and physical controls on northern terrestrial methane emissions across permafrost zones2013In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 19, no 2, p. 589-603Article in journal (Refereed)
    Abstract [en]

    Methane (CH4) emissions from the northern high-latitude region represent potentially significant biogeochemical feedbacks to the climate system. We compiled a database of growing-season CH4 emissions from terrestrial ecosystems located across permafrost zones, including 303 sites described in 65 studies. Data on environmental and physical variables, including permafrost conditions, were used to assess controls on CH4 emissions. Water table position, soil temperature, and vegetation composition strongly influenced emissions and had interacting effects. Sites with a dense sedge cover had higher emissions than other sites at comparable water table positions, and this was an effect that was more pronounced at low soil temperatures. Sensitivity analysis suggested that CH4 emissions from ecosystems where the water table on average is at or above the soil surface (wet tundra, fen underlain by permafrost, and littoral ecosystems) are more sensitive to variability in soil temperature than drier ecosystems (palsa dry tundra, bog, and fen), whereas the latter ecosystems conversely are relatively more sensitive to changes of the water table position. Sites with near-surface permafrost had lower CH4 fluxes than sites without permafrost at comparable water table positions, a difference that was explained by lower soil temperatures. Neither the active layer depth nor the organic soil layer depth was related to CH4 emissions. Permafrost thaw in lowland regions is often associated with increased soil moisture, higher soil temperatures, and increased sedge cover. In our database, lowland thermokarst sites generally had higher emissions than adjacent sites with intact permafrost, but emissions from thermokarst sites were not statistically higher than emissions from permafrost-free sites with comparable environmental conditions. Overall, these results suggest that future changes to terrestrial high-latitude CH4 emissions will be more proximately related to changes in moisture, soil temperature, and vegetation composition than to increased availability of organic matter following permafrost thaw.

  • 57. Petrescu, A. M. R.
    et al.
    van Huissteden, J.
    Jackowicz-Korczynski, M.
    Yurova, A.
    Christensen, T. R.
    Crill, Patric M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bäckstrand, Kristina
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Maximov, T. C.
    Modelling CH4 emissions from arctic wetlands: effects of hydrological parameterization2008In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 5, no 1, p. 111-121Article in journal (Refereed)
    Abstract [en]

    This study compares the CH4 fluxes from two arctic wetland sites of different annual temperatures during 2004 to 2006. The PEATLAND-VU model was used to simulate the emissions. The CH4 module of PEATLAND-VU is based on the Walter-Heimann model. The first site is located in northeast Siberia, Indigirka lowlands, Kytalyk reserve (70 degrees N, 147 degrees E) in a continuous permafrost region with mean annual temperatures of -14.3 degrees C. The other site is Stordalen mire in the eastern part of Lake Tornetrask (68 degrees N, 19 degrees E) ten kilometres east of Abisko, northern Sweden. It is located in a discontinuous permafrost region. Stordalen has a sub arctic climate with a mean annual temperature of -0.7 degrees C. Model input consisted of observed temperature, precipitation and snow cover data. In all cases, modelled CH4 emissions show a direct correlation between variations in water table and soil temperature variations. The differences in CH4 emissions between the two sites are caused by different climate, hydrology, soil physical properties, vegetation type and NPP. For Kytalyk the simulated CH4 fluxes show similar trends during the growing season, having average values for 2004 to 2006 between 1.29-2.09 mg CH4 m(-2) hr(-1). At Stordalen the simulated fluxes show a slightly lower average value for the same years (3.52 mg CH4 m(-2) hr(-1)) than the observed 4.7 mg CH4 m(-2) hr(-1). The effect of the longer growing season at Stordalen is simulated correctly. Our study shows that modelling of arctic CH4 fluxes is improved by adding a relatively simple hydrological model that simulates the water table position from generic weather data. Our results support the generalization in literature that CH4 fluxes in northern wetland are regulated more tightly by water table than temperature. Furthermore, parameter uncertainty at site level in wetland CH4 process models is an important factor in large scale modelling of CH4 fluxes.

  • 58. Petrescu, A,
    et al.
    van Huissteden, J
    Jackowicz-Korczynski, M
    Yurova, A
    Christensen, T
    Crill, P
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Maximov, T
    Modelling CH4 emissions from arctic wetlands: effects of hydrological parameterization2007In: Biogeosciences Discussions, Vol. 4, p. 3195-3227Article in journal (Refereed)
  • 59. Philips, S. C.
    et al.
    Varner, R. K.
    Frolking, S.
    Munger, J. W.
    Bubier, J. L.
    Wofsy, S. C.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Interannual, seasonal, and diel variation in soil respiration relative to ecosystem respiration at a wetland to upland slope at Harvard Forest2010In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, p. G02019-Article in journal (Refereed)
    Abstract [en]

    Soil carbon dioxide efflux (soil respiration, SR) was measured with eight autochambers at two locations along a wetland to upland slope at Harvard Forest over a 4 year period, 2003–2007. SR was consistently higher in the upland plots than at the wetland margin during the late summer/early fall. Seasonal and diel hystereses with respect to soil temperatures were of sufficient magnitude to prevent quantification of the influence of soil moisture, although apparent short-term responses of SR to precipitation occurred. Calculations of annual cumulative SR illustrated a decreasing trend in SR over the 5 year period, which were correlated with decreasing springtime mean soil temperatures. Spring soil temperatures decreased despite rising air temperatures over the same period, possibly as an effect of earlier leaf expansion and shading. The synchronous decrease in spring soil temperatures and SR during regional warming of air temperatures may represent a negative feedback on a warming climate by reducing CO2 production from soils. SR reached a maximum later in the year than total ecosystem respiration (ER) measured at a nearby eddy covariance flux tower, and the seasonality of their temperature response patterns were roughly opposite. SR, particularly in the upland, exceeded ER in the late summer/early fall in each year, suggesting that areas of lower efflux such as the wetland may be significant in the flux tower footprint or that long-term bias in either estimate may create a mismatch. Annual estimates of ER decreased over the same period and were highly correlated with SR.

  • 60. Pirk, Norbert
    et al.
    Mastepanov, Mikhail
    Parmentier, Frans-Jan W.
    Lund, Magnus
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Christensen, Torben R.
    Calculations of automatic chamber flux measurements of methane and carbon dioxide using short time series of concentrations2016In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 13, no 4, p. 903-912Article in journal (Refereed)
    Abstract [en]

    The closed chamber technique is widely used to measure the exchange of methane (CH4) and carbon dioxide (CO2) from terrestrial ecosystems. There is, however, large uncertainty about which model should be used to calculate the gas flux from the measured gas concentrations. Due to experimental uncertainties the simple linear regression model (first-order polynomial) is often applied, even though theoretical considerations of the technique suggest the application of other, curvilinear models. High-resolution automatic chamber systems which sample gas concentrations several hundred times per flux measurement make it possible to resolve the curvilinear behavior and study the information imposed by the natural variability of the temporal concentration changes. We used more than 50 000 such flux measurements of CH4 and CO2 from five field sites located in peat-forming wetlands ranging from 56 to 78 degrees N to quantify the typical differences between flux estimates of different models. In addition, we aimed to assess the curvilinearity of the concentration time series and test the general applicability of curvilinear models. Despite significant episodic differences between the calculated flux estimates, the overall differences are generally found to be smaller than the local flux variability on the plot scale. The curvilinear behavior of the gas concentrations within the chamber is strongly influenced by wind-driven chamber leakage, and less so by changing gas concentration gradients in the soil during chamber closure. Such physical processes affect both gas species equally, which makes it possible to isolate biochemical processes affecting the gases differently, such as photosynthesis limitation by chamber headspace CO2 concentrations under high levels of incoming solar radiation. We assess the possibility to exploit this effect for a partitioning of the net CO2 flux into photosynthesis and ecosystem respiration as an example of how high-resolution automatic chamber measurements could be used for purposes beyond the estimation of the net gas flux. This shows that while linear and curvilinear calculation schemes can provide similar net fluxes, only curvilinear models open additional possibilities for high-resolution automatic chamber measurements.

  • 61.
    Prytherch, John
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Leeds, United Kingdom.
    Brooks, Ian
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Salisbury, Dominic
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anderson, Leif
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Air-sea CO2and CH4 gas transfer velocity in Arctic sea-ice regions fromeddy covariance flux measurements onboard Icebreaker Oden2017In: Geophysical Research Abstracts, ISSN 1029-7006, E-ISSN 1607-7962, Vol. 19, article id 697Article in journal (Refereed)
    Abstract [en]

    The Arctic Ocean is an important sink for atmospheric CO2, and there is ongoing debate on whether seafloor seeps in the Arctic are a large source of CH4 to the atmosphere. The impact of warming waters, decreasing sea-ice extent and expanding marginal ice zones on Arctic air-sea gas exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts as a near-impermeable lid to air-sea gas exchange, but is also hypothesised to enhance gas transfer rates through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. The dependence of the gas transfer rate on sea-ice concentration remains uncertain due to a lack of in situ measurements. Here we present the first direct estimates of gas transfer rate in a wide range of Arctic sea-ice conditions. The estimates were derived from eddy covariance CO2 and CH4 fluxes, measured from the Swedish Icebreaker Oden during two expeditions: the 3-month duration Arctic Clouds in Summer Experiment (ACSE) in 2014, a component of the Swedish-Russian-US Arctic Ocean Investigation on Climate-Cryosphere-Carbon Interactions (SWERUS-C3) in the eastern Arctic Ocean shelf region; and the Arctic Ocean 2016 expedition to the high latitude Arctic Ocean. Initial CO2 results from ACSE showed that the gas transfer rate has a near-linear dependence on sea-ice concentration, and that some previous indirect measurements and modelling estimates overestimate gas transfer rates in sea-ice regions. This supports a linear sea-ice scaling approach for assessments of polar ocean carbon fluxes. Air-sea gas transfer model assumptions (e.g. Schmidt number dependence) will be examined using simultaneous CO2 and CH4 measurements, and observations in different ice conditions (e.g. summer melt, autumn freeze up, central Arctic and marginal ice zones) will be compared.

  • 62.
    Prytherch, John
    et al.
    Stockholm University, Faculty of Science, Department of Meteorology . University of Leeds, UK.
    Brooks, Ian M.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Salisbury, Dominic J.
    Tjernström, Michael
    Stockholm University, Faculty of Science, Department of Meteorology .
    Anderson, Leif G.
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Stockholm University Baltic Sea Centre.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Direct determination of the air-sea CO2 gas transfer velocity in Arctic sea ice regions2017In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 44, no 8, p. 3770-3778Article in journal (Refereed)
    Abstract [en]

    The Arctic Ocean is an important sink for atmospheric CO2. The impact of decreasing sea ice extent and expanding marginal ice zones on Arctic air-sea CO2 exchange depends on the rate of gas transfer in the presence of sea ice. Sea ice acts to limit air-sea gas exchange by reducing contact between air and water but is also hypothesized to enhance gas transfer rates across surrounding open-water surfaces through physical processes such as increased surface-ocean turbulence from ice-water shear and ice-edge form drag. Here we present the first direct determination of the CO2 air-sea gas transfer velocity in a wide range of Arctic sea ice conditions. We show that the gas transfer velocity increases near linearly with decreasing sea ice concentration. We also show that previous modeling approaches overestimate gas transfer rates in sea ice regions.

  • 63. Routh, Joyanto
    et al.
    Hugelius, Gustaf
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Kuhry, Peter
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Filley, Timothy
    Tillman, Päivi Kaislahti
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology. Umea University.
    Becher, Marina
    Stockholm University, Faculty of Science, Department of Physical Geography and Quaternary Geology.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Multi-proxy study of soil organic matter dynamics in permafrost peat deposits reveal vulnerability to climate change in the European Russian Arctic2014In: Chemical Geology, ISSN 0009-2541, E-ISSN 1872-6836, Vol. 368, p. 104-117Article in journal (Refereed)
    Abstract [en]

    Soil organic carbon (SOC) in permafrost terrain is vulnerable to climate change. Perennially frozen peat deposits store large amounts of SOC, but we know little about its chemical composition and lability. We used plant macrofossil and biomarker analyses to reconstruct the Holocene paleovegetation and paleoenvironmental changes in two peat plateau profiles from the European Russian Arctic. Peat plateaus are the main stores of permafrost soil C in the region, but during most of the Holocene peats developed as permafrost-free rich fens with woody vegetation, sedges and mosses. Around 2200 cal BP, permafrost aggraded at the site resulting in frost heave and a drastic reduction in peat accumulation under the drier uplifted surface conditions. The permafrost dynamics (aggradation, frost-heave and thaw) ushered changes in plant assemblages and carbon accumulation, and consequently in the biomarker trends too. Detailed biomarker analyses indicate abundant neutral lipids, which follow the general pattern: n-alkanols > sterols >= n-alkanes >= triterpenols. The lignin monomers are not as abundant as the lipids and increase with depth. The selected aliphatic and phenolic compounds are source specific, and they have different degrees of lability, which is useful for tracing the impact of permafrost dynamics (peat accumulation and/or decay associated with thawing). However, common interpretation of biomarker patterns, and perceived hydrological and climate changes, must be applied carefully in permafrost regions. The increased proportion (selective preservation) of n-alkanes and lignin is a robust indicator of cumulative decomposition trajectories, which is mirrored by functional compounds (e. g. n-alkanol, triterpenol, and sterol concentrations) showing opposite trends. The distribution of these compounds follows first order decay kinetics, and concurs with the down core diagenetic changes. In particular, some of the biomarker ratios (e. g. stanol/sterol and higher plant alkane index) seem promising for tracing SOC decomposition despite changes in botanical imprint, and sites spanning across different soil types and locations. Carbon accumulation rate calculated at these sites varies from 18.1 to 31.1 gC m(-2) yr(-1), and it's evident selective preservation, molecular complexity of organic compounds, and freezing conditions enhance the long-term stability of SOC. Further, our results suggest that permafrost dynamics strongly impact the more undecomposed SOC that could be rapidly remobilized through ongoing thermokarst expansion.

  • 64. Sallstedt, T.
    et al.
    Bengtson, S.
    Broman, Curt
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Canfield, D. E.
    Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups, India2018In: Geobiology, ISSN 1472-4677, E-ISSN 1472-4669, Vol. 16, no 2, p. 139-159Article in journal (Refereed)
    Abstract [en]

    Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro-organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite-rich microbialites. Stromatolites from the 1.6Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine-grained in situ-precipitated apatite mats with average C-13(org) indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone-like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite-carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis-respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen-producing biotas in the formation of Paleoproterozoic shallow-basin phosphorites.

  • 65. Santoni, Gregory W.
    et al.
    Lee, Ben H.
    Goodrich, Jordan P.
    Varner, Ruth K.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    McManus, J. Barry
    Nelson, David D.
    Zahniser, Mark S.
    Wofsy, Steven C.
    Mass fluxes and isofluxes of methane (ch4) at a new hampshire fen measured by a continuous wave quantum cascade laser spectrometer2012In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 117, p. D10301-Article in journal (Refereed)
    Abstract [en]

    We have developed a mid-infrared continuous-wave quantum cascade laser direct-absorption spectrometer (QCLS) capable of high frequency (>= 1 Hz) measurements of (CH4)-C-12 and (CH4)-C-13 isotopologues of methane (CH4) with in situ 1-s RMS delta C-13(CH4) precision of 1.5 parts per thousand and Allan-minimum precision of 0.2 parts per thousand. We deployed this QCLS in a well-studied New Hampshire fen to compare measurements of CH4 isoflux by eddy covariance (EC) to Keeling regressions of data from automated flux chamber sampling. Mean CH4 fluxes of 6.5 +/- 0.7 mg CH4 m(-2) hr(-1) over two days of EC sampling in July, 2009 were indistinguishable from mean autochamber CH4 fluxes (6.6 +/- 0.8 mgCH(4) m(-2) hr(-1)) over the same period. Mean delta C-13(CH4) composition of emitted CH4 calculated using EC isoflux methods was -71 +/- 8 parts per thousand (95% C.I.) while Keeling regressions of 332 chamber closing events over 8 days yielded a corresponding value of -64.5 +/- 0.8 parts per thousand Ebullitive fluxes, representing similar to 10% of total CH4 fluxes at this site, were on average 1.2 parts per thousand enriched in C-13 compared to diffusive fluxes. CH4 isoflux time series have the potential to improve process-based understanding of methanogenesis, fully characterize source isotopic distributions, and serve as additional constraints for both regional and global CH4 modeling analysis.

  • 66. Saunois, M.
    et al.
    Bousquet, P.
    Poulter, B.
    Peregon, A.
    Ciais, P.
    Canadell, J. G.
    Dlugokencky, E. J.
    Etiope, G.
    Bastviken, D.
    Houweling, S.
    Janssens-Maenhout, G.
    Tubiello, F. N.
    Castaldi, S.
    Jackson, R. B.
    Alexe, M.
    Arora, V. K.
    Beerling, D. J.
    Bergamaschi, P.
    Blake, D. R.
    Brailsford, G.
    Brovkin, V.
    Bruhwiler, L.
    Crevoisier, C.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Covey, K.
    Curry, C.
    Frankenberg, C.
    Gedney, N.
    Höglund-Isaksson, L.
    Ishizawa, M.
    Ito, A.
    Joos, F.
    Kim, H. -S
    Kleinen, T.
    Krummel, P.
    Lamarque, J. -F
    Langenfelds, R.
    Locatelli, R.
    Machida, T.
    Maksyutov, S.
    McDonald, K. C.
    Marshall, J.
    Melton, J. R.
    Morino, I.
    Naik, V.
    O'Doherty, S.
    Parmentier, F. -JW.
    Patra, P. K.
    Peng, C.
    Peng, S.
    Peters, G. P.
    Pison, I.
    Prigent, C.
    Prinn, R.
    Ramonet, M.
    Riley, W. J.
    Saito, M.
    Santini, M.
    Schroeder, R.
    Simpson, I. J.
    Spahni, R.
    Steele, P.
    Takizawa, A.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tian, H.
    Tohjima, Y.
    Viovy, N.
    Voulgarakis, A.
    van Weele, M.
    van der Werf, G. R.
    Weiss, R.
    Wiedinmyer, C.
    Wilton, D. J.
    Wiltshire, A.
    Worthy, D.
    Wunch, D.
    Xu, X.
    Yoshida, Y.
    Zhang, B.
    Zhang, Z.
    Zhu, Q.
    The global methane budget 2000–20122016In: Earth System Science Data, ISSN 1866-3508, E-ISSN 1866-3516, Vol. 8, no 2, p. 697-751Article, review/survey (Refereed)
    Abstract [en]

    The global methane (CH4) budget is becoming an increasingly important component for managing realistic pathways to mitigate climate change. This relevance, due to a shorter atmospheric lifetime and a stronger warming potential than carbon dioxide, is challenged by the still unexplained changes of atmospheric CH4 over the past decade. Emissions and concentrations of CH4 are continuing to increase, making CH4 the second most important human-induced greenhouse gas after carbon dioxide. Two major difficulties in reducing uncertainties come from the large variety of diffusive CH4 sources that overlap geographically, and from the destruction of CH4 by the very short-lived hydroxyl radical (OH). To address these difficulties, we have established a consortium of multi-disciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate research on the methane cycle, and producing regular (∼ biennial) updates of the global methane budget. This consortium includes atmospheric physicists and chemists, biogeochemists of surface and marine emissions, and socio-economists who study anthropogenic emissions. Following Kirschke et al. (2013), we propose here the first version of a living review paper that integrates results of top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models, inventories and data-driven approaches (including process-based models for estimating land surface emissions and atmospheric chemistry, and inventories for anthropogenic emissions, data-driven extrapolations). For the 2003–2012 decade, global methane emissions are estimated by top-down inversions at 558 Tg CH4 yr−1, range 540–568. About 60 % of global emissions are anthropogenic (range 50–65 %). Since 2010, the bottom-up global emission inventories have been closer to methane emissions in the most carbon-intensive Representative Concentrations Pathway (RCP8.5) and higher than all other RCP scenarios. Bottom-up approaches suggest larger global emissions (736 Tg CH4 yr−1, range 596–884) mostly because of larger natural emissions from individual sources such as inland waters, natural wetlands and geological sources. Considering the atmospheric constraints on the top-down budget, it is likely that some of the individual emissions reported by the bottom-up approaches are overestimated, leading to too large global emissions. Latitudinal data from top-down emissions indicate a predominance of tropical emissions (∼ 64 % of the global budget, < 30° N) as compared to mid (∼ 32 %, 30–60° N) and high northern latitudes (∼ 4 %, 60–90° N). Top-down inversions consistently infer lower emissions in China (∼ 58 Tg CH4 yr−1, range 51–72, −14 %) and higher emissions in Africa (86 Tg CH4 yr−1, range 73–108, +19 %) than bottom-up values used as prior estimates. Overall, uncertainties for anthropogenic emissions appear smaller than those from natural sources, and the uncertainties on source categories appear larger for top-down inversions than for bottom-up inventories and models. The most important source of uncertainty on the methane budget is attributable to emissions from wetland and other inland waters. We show that the wetland extent could contribute 30–40 % on the estimated range for wetland emissions. Other priorities for improving the methane budget include the following: (i) the development of process-based models for inland-water emissions, (ii) the intensification of methane observations at local scale (flux measurements) to constrain bottom-up land surface models, and at regional scale (surface networks and satellites) to constrain top-down inversions, (iii) improvements in the estimation of atmospheric loss by OH, and (iv) improvements of the transport models integrated in top-down inversions. The data presented here can be downloaded from the Carbon Dioxide Information Analysis Center (http://doi.org/10.3334/CDIAC/GLOBAL_METHANE_BUDGET_2016_V1.1) and the Global Carbon Project.

  • 67. Saunois, M.
    et al.
    Bousquet, P.
    Poulter, B.
    Peregon, A.
    Ciais, P.
    Canadell, J. G.
    Dlugokencky, E. J.
    Etiope, G.
    Bastviken, D.
    Houweling, S.
    Janssens-Maenhout, G.
    Tubiello, F. N.
    Castaldi, S.
    Jackson, R. B.
    Alexe, M.
    Arora, V. K.
    Beerling, D. J.
    Bergamaschi, P.
    Blake, D. R.
    Brailsford, G.
    Bruhwiler, L.
    Crevoisier, C.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Covey, K.
    Frankenberg, C.
    Gedney, N.
    Höglund-Isaksson, L.
    Ishizawa, M.
    Ito, A.
    Joos, F.
    Kim, H. -S
    Kleinen, T.
    Krummel, P.
    Lamarque, J. -F
    Langenfelds, R.
    Locatelli, R.
    Machida, T.
    Maksyutov, S.
    Melton, J. R.
    Morino, I.
    Naik, V.
    O'Doherty, S.
    Parmentier, F. -JW.
    Patra, P. K.
    Peng, C.
    Peng, S.
    Peters, G. P.
    Pison, I.
    Prinn, R.
    Ramonet, M.
    Riley, W. J.
    Saito, M.
    Santini, M.
    Schroeder, R.
    Simpson, I. J.
    Spahni, R.
    Takizawa, A.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Tian, H.
    Tohjima, Y.
    Viovy, N.
    Voulgarakis, A.
    Weiss, R.
    Wilton, D. J.
    Wiltshire, A.
    Worthy, D.
    Wunch, D.
    Xu, X.
    Yoshida, Y.
    Zhang, B.
    Zhang, Z.
    Zhu, Q.
    Variability and quasi-decadal changes in the methane budget over the period 2000–20122017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 18, p. 11135-11161Article in journal (Refereed)
    Abstract [en]

    Following the recent Global Carbon Project (GCP) synthesis of the decadal methane (CH4) budget over 2000–2012 (Saunois et al., 2016), we analyse here the same dataset with a focus on quasi-decadal and inter-annual variability in CH4 emissions. The GCP dataset integrates results from top-down studies (exploiting atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up models (including process-based models for estimating land surface emissions and atmospheric chemistry), inventories of anthropogenic emissions, and data-driven approaches. The annual global methane emissions from top-down studies, which by construction match the observed methane growth rate within their uncertainties, all show an increase in total methane emissions over the period 2000–2012, but this increase is not linear over the 13 years. Despite differences between individual studies, the mean emission anomaly of the top-down ensemble shows no significant trend in total methane emissions over the period 2000–2006, during the plateau of atmospheric methane mole fractions, and also over the period 2008–2012, during the renewed atmospheric methane increase. However, the top-down ensemble mean produces an emission shift between 2006 and 2008, leading to 22 [16–32] Tg CH4 yr−1 higher methane emissions over the period 2008–2012 compared to 2002–2006. This emission increase mostly originated from the tropics, with a smaller contribution from mid-latitudes and no significant change from boreal regions. The regional contributions remain uncertain in top-down studies. Tropical South America and South and East Asia seem to contribute the most to the emission increase in the tropics. However, these two regions have only limited atmospheric measurements and remain therefore poorly constrained. The sectorial partitioning of this emission increase between the periods 2002–2006 and 2008–2012 differs from one atmospheric inversion study to another. However, all top-down studies suggest smaller changes in fossil fuel emissions (from oil, gas, and coal industries) compared to the mean of the bottom-up inventories included in this study. This difference is partly driven by a smaller emission change in China from the top-down studies compared to the estimate in the Emission Database for Global Atmospheric Research (EDGARv4.2) inventory, which should be revised to smaller values in a near future. We apply isotopic signatures to the emission changes estimated for individual studies based on five emission sectors and find that for six individual top-down studies (out of eight) the average isotopic signature of the emission changes is not consistent with the observed change in atmospheric 13CH4. However, the partitioning in emission change derived from the ensemble mean is consistent with this isotopic constraint. At the global scale, the top-down ensemble mean suggests that the dominant contribution to the resumed atmospheric CH4 growth after 2006 comes from microbial sources (more from agriculture and waste sectors than from natural wetlands), with an uncertain but smaller contribution from fossil CH4 emissions. In addition, a decrease in biomass burning emissions (in agreement with the biomass burning emission databases) makes the balance of sources consistent with atmospheric 13CH4 observations. In most of the top-down studies included here, OH concentrations are considered constant over the years (seasonal variations but without any inter-annual variability). As a result, the methane loss (in particular through OH oxidation) varies mainly through the change in methane concentrations and not its oxidants. For these reasons, changes in the methane loss could not be properly investigated in this study, although it may play a significant role in the recent atmospheric methane changes as briefly discussed at the end of the paper.

  • 68.
    Silver, W.L.
    et al.
    Univ Cal - Berkeley.
    Thompson, A.W.
    Univ Cal - Berkeley.
    McGroddy, M.E.
    Univ Cal - Berkeley.
    Varner, R.K:
    Univ New Hampshire.
    Diaz, J.D.
    Univ Fed Sao Paulo.
    Silva, H.
    Univ New hampshire.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Keller, M.
    US Forest Service.
    Fine root dynamics and trace gas fluxes in two lowland tropical forest soils.2005In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 11, no 2, p. 290-306Article in journal (Refereed)
    Abstract [en]

    Fine root dynamics have the potential to contribute significantly to ecosystem-scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m<sup>−2</sup> yr<sup>−1</sup> for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=−0.96 year<sup>−1</sup>) than in the sandy loam soil (k=−0.61 year<sup>−1</sup>), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N<sub>2</sub>O) emissions were significantly greater in the clay soil (13±1 ng N cm<sup>−2</sup> h<sup>−1</sup>) than in the sandy loam (1.4±0.2 ng N cm<sup>−2</sup> h<sup>−1</sup>). Root mortality and decay following trenching doubled rates of N<sub>2</sub>O emissions in the clay and tripled them in sandy loam over a 1-year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m<sup>−2</sup> yr<sup>−1</sup>, accounting for 24% to 35% of the total soil CO<sub>2</sub> efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m<sup>2</sup> yr<sup>−1</sup>) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land-use change can contribute significantly to increased rates of nitrogen trace gas emissions.

  • 69. Singleton, Caitlin M.
    et al.
    McCalley, Carmody K.
    Woodcroft, Ben J.
    Boyd, Joel A.
    Evans, Paul N.
    Hodgkins, Suzanne B.
    Chanton, Jeffrey P.
    Frolking, Steve
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Saleska, Scott R.
    Rich, Virginia
    Tyson, Gene W.
    Methanotrophy across a natural permafrost thaw environment2018In: The ISME Journal, ISSN 1751-7362, E-ISSN 1751-7370, Vol. 12, no 10, p. 2544-2558Article in journal (Refereed)
    Abstract [en]

    The fate of carbon sequestered in permafrost is a key concern for future global warming as this large carbon stock is rapidly becoming a net methane source due to widespread thaw. Methane release from permafrost is moderated by methanotrophs, which oxidise 20-60% of this methane before emission to the atmosphere. Despite the importance of methanotrophs to carbon cycling, these microorganisms are under-characterised and have not been studied across a natural permafrost thaw gradient. Here, we examine methanotroph communities from the active layer of a permafrost thaw gradient in Stordalen Mire (Abisko, Sweden) spanning three years, analysing 188 metagenomes and 24 metatranscriptomes paired with in situ biogeochemical data. Methanotroph community composition and activity varied significantly as thaw progressed from intact permafrost palsa, to partially thawed bog and fully thawed fen. Thirteen methanotroph population genomes were recovered, including two novel genomes belonging to the uncultivated upland soil cluster alpha (USCa) group and a novel potentially methanotrophic Hyphomicrobiaceae. Combined analysis of porewater delta C-13-CH 4 isotopes and methanotroph abundances showed methane oxidation was greatest below the oxic-anoxic interface in the bog. These results detail the direct effect of thaw on autochthonous methanotroph communities, and their consequent changes in population structure, activity and methane moderation potential.

  • 70.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geokemi.
    Arghe, Fredrik
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry. Geologi.
    Whitmarsh, Bob
    Koyi, Hemin
    Quantification of the rate of methane production by serpentinization2008In: International Geological Congress, 33, 2008, p. CD-ROMConference paper (Other academic)
    Abstract [en]

    We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of the serpentinization front yields a front displacement (z) of 2197 ± 89 m. We ignored diffusive broadening of this front in the direction of fluid flow and used the shape of the front to constrain a Damköhler number (ND). This was scaled such that ND = t, where  is a rate constant for serpentinization in s-1 and t is time in s. We thus obtained ND = 6.0 ± 0.2. We then estimated values of  for (1) surface reaction as rate-limiting and (2) chemical transport as rate-limiting. Combining these values with our estimate of the Damköhler number yielded comparable timescales of 10,000 – 1,000,000 years for serpentinization. Combining this timescale with the front displacement yielded a time-averaged volumetric rate for serpentinization of 0.002-0.2 m3-serpentinite.m-2.yr-1.

    We then referred to the experimental results of Janecky and Seyfried (1986), which predicted that serpentinization by reaction with seawater probably occurs by a coupled set of reactions, with olivine, orthopyroxene and clinopyroxene dissolving independently and at different rates. Of this set of reactions, olivine dissolution is predicted to occur by the reaction:

    2(Mg,Fe)2SiO4 + 2H+ + H2O = (Mg,Fe)3Si2O5(OH)4 + (Mg,Fe)2+ (1)

    Because lizardite, which is the most abundant of the serpentine minerals typically contains no more than 5 wt. % FeO (Wicks & O’ Hanley 1988), excess Fe2+ will preferentially (with respect to Mg2+) enter the solution. Oxidation of Fe2+ by H2O and CO2 can then produce magnetite and CH4 by the reaction:

    12Fe2+ + 14H2O + CO2 = 4Fe3O4 + 24H+ + CH4 (2)

    Fe3O4 (magnetite) can be used as a proxy for CH4 produced by reactions (1) and (2) and exiting the mantle. The average mode of magnetite in 16 samples of >95% serpentinized peridotite recovered by scientific drilling within the region of exhumed mantle is 4.5 vol. %. This is equivalent to 1000 mol-Fe3O4.m-3. Combining this value with the time-averaged volumetric rate for serpentinization, we obtain a time-averaged annual production rate for magnetite of 2-200 mol.m-2.yr-1. Finally, based on the stoichiometry of reaction (2), we thus obtain an annual flux rate for CH4 production of 0.5-50 mol.m-2.yr-1.

  • 71.
    Skelton, Alasdair
    et al.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Whitmarsh, R.
    Edinburgh Univ.
    Arghe, F.
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Koyi, H.
    Uppsala Univ.
    Constraining the rate and extent of mantle serpentization from seismic and petrological data: Implications for chemosynthesis and tectonic processes.2005In: Geofluids, ISSN 1468-8115, E-ISSN 1468-8123, Vol. 5, no 3, p. 153-164Article in journal (Refereed)
    Abstract [en]

    We used seismic velocity as a proxy for serpentinization of the mantle, which occurred beneath thinned but laterally continuous continental crust during continental break up, prior to opening of the Atlantic Ocean. The serpentinized sub-continental mantle is now exhumed, beneath the Iberia Abyssal Plain and was accessed by scientific drilling on Ocean Drilling Program legs 149 and 173. Chromatographic modelling of kinetically limited transport of the serpentinization front yields a front displacement of 2197 ± 89 m, a time-integrated fluid flux of 1098 ± 45 m<sup>3</sup> m<sup>−2</sup> and a Damköhler number of 6.0 ± 0.2. Whether either surface reaction or chemical transport limit the rate of reaction, we calculate timescales for serpentinization of approximately 10<sup>5</sup>–10<sup>6</sup> years. This yields time-average fluid flux rates for H<sub>2</sub>O, entering and reacting with the mantle, of 60–600 mol m<sup>−2</sup> a<sup>−1</sup> and for CH<sub>4</sub>, produced as a by-product of oxidation of Fe<sup>++</sup> to magnetite and exiting the mantle, of 0.55–5.5 mol m<sup>−2</sup> a<sup>−1</sup>. This equates to a CH<sub>4</sub>-flux of 0.18–1.8 Tg a<sup>−1</sup> for coeval serpentinization of the mantle that was exhumed west of Iberia. This represents 0.03–0.3% of the present-day annual CH<sub>4</sub>-flux from all sources and a higher fraction of pre-anthropogenic (lower) CH<sub>4</sub> levels. CH<sub>4</sub> released by serpentinization at or beneath the seafloor could provide substrate for biological chemosynthesis and/or promote gas-hydrate formation. Finally, noting its volumetric extent and rapidity (<10<sup>6</sup> years), we interpret serpentinization to be a reckonable component of tectonic processes, contributing both diapiric and expansional forces and helping to ‘lubricate’ extensional processes. Given its anisotropic permeability, actively deforming serpentinite might impede melt migration which may be of interest, given the apparent lack of melt in some rifted margins.

  • 72.
    Stranne, Christian
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Center for Coastal & Ocean Mapping/Joint Hydrographic Center, USA.
    O'Regan, Matthew
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dickens, Gerald R.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Rice University, USA.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Miller, C.
    Preto, Pedro
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Jakobsson, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Dynamic simulations of potential methane release from East Siberian continental slope sediments2016In: Geochemistry Geophysics Geosystems, ISSN 1525-2027, E-ISSN 1525-2027, Vol. 17, no 3, p. 872-886Article in journal (Refereed)
    Abstract [en]

    Sediments deposited along continental margins of the Arctic Ocean presumably host large amounts of methane (CH4) in gas hydrates. Here we apply numerical simulations to assess the potential of gas hydrate dissociation and methane release from the East Siberian slope over the next 100 years. Simulations are based on a hypothesized bottom water warming of 3 degrees C, and an assumed starting distribution of gas hydrate. The simulation results show that gas hydrate dissociation in these sediments is relatively slow, and that CH4 fluxes toward the seafloor are limited by low sediment permeability. The latter is true even when sediment fractures are permitted to form in response to overpressure in pore space. With an initial gas hydrate distribution dictated by present-day pressure and temperature conditions, nominally 0.35 Gt of CH4 are released from the East Siberian slope during the first 100 years of the simulation. However, this CH4 discharge becomes significantly smaller (approximate to 0.05 Gt) if glacial sea level changes in the Arctic Ocean are considered. This is because a lower sea level during the last glacial maximum (LGM) must result in depleted gas hydrate abundance within the most sensitive region of the modern gas hydrate stability zone. Even if all released CH4 reached the atmosphere, the amount coming from East Siberian slopes would be trivial compared to present-day atmospheric CH4 inputs from other sources.

  • 73.
    Sun, Xiaole
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Andersson, Per
    Naturhistoriska riksmuseet, Laboratoriet för isotopgeologi.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM). Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Gustafsson, Bo
    Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Conley, Daniel J.
    Lunds universitet.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Stockholm Resilience Centre, Baltic Nest Institute.
    Climate Dependent Diatom Production is Preserved in Biogenic Si Isotope Signatures2011In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 8, no 11, p. 3491-3499Article in journal (Refereed)
    Abstract [en]

    The aim of this study was to reconstruct diatom production in the subarctic northern tip of the Baltic Sea, Bothnian Bay, based on down-core analysis of Si isotopes in biogenic silica (BSi). Dating of the sediment showed that the samples covered the period 1820 to 2000. The sediment core record can be divided into two periods, an unperturbed period from 1820 to 1950 and a second period affected by human activities (from 1950 to 2000). This has been observed elsewhere in the Baltic Sea. The shift in the sediment core record after 1950 is likely caused by large scale damming of rivers. Diatom production was inferred from the Si isotope composition which ranged between δ30Si −0.18‰ and +0.58‰ in BSi, and assuming fractionation patterns due to the Raleigh distillation, the production was shown to be correlated with air and water temperature, which in turn were correlated with the mixed layer (ML) depth. The sedimentary record showed that the deeper ML depth observed in colder years resulted in less production of diatoms. Pelagic investigations in the 1990's have clearly shown that diatom production in the Baltic Sea is controlled by the ML depth. Especially after cold winters and deep water mixing, diatom production was limited and dissolved silicate (DSi) concentrations were not depleted in the water column after the spring bloom. Our method corroborates these findings and offers a new method to estimate diatom production over much longer periods of time in diatom dominated aquatic systems, i.e. a large part of the world's ocean and coastal seas.

  • 74. Sundqvist, E.
    et al.
    Vestin, P.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Persson, T.
    Lindroth, A.
    Short-term effects of thinning, clear-cutting and stump harvesting on methane exchange in a boreal forest2014In: Biogeosciences, ISSN 1726-4170, E-ISSN 1726-4189, Vol. 11, no 21, p. 6095-6105Article in journal (Refereed)
    Abstract [en]

    Forest management practices can alter soil conditions, affecting the consumption and production processes that control soil methane (CH4) exchange. We studied the short-term effects of thinning, clear-cutting and stump harvesting on the CH4 exchange between soil and atmosphere at a boreal forest site in central Sweden, using an undisturbed plot as the control. Chambers in combination with a high-precision laser gas analyser were used for continuous measurements. Both the undisturbed plot and the thinned plot were net sinks of CH4, whereas the clear-cut plot and the stump harvested plot were net CH4 sources. The CH4 uptake at the thinned plot was reduced in comparison to the undisturbed plot. The shift from sink to source at the clear-cut and stump harvested plots was probably due to a rise in the water table and an increase in soil moisture, leading to lower gas diffusivity and more reduced conditions, which favour CH4 production by archea. Reduced evapotranspiration after harvesting leads to wetter soils, decreased CH4 consumption and increased CH4 production, and should be accounted for in the CH4 budget of managed forests.

  • 75. Sundqvist, Elin
    et al.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Molder, Meelis
    Vestin, Patrik
    Lindroth, Anders
    Atmospheric methane removal by boreal plants2012In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 39, p. L21806-Article in journal (Refereed)
    Abstract [en]

    Several studies have proposed aerobic methane (CH4) emissions by plants. If confirmed, these findings would further increase the imbalance in the global CH4 budget which today underestimates CH4 sinks. Oxidation by OH-radicals in the troposphere is the major identified sink followed by smaller contribution from stratospheric loss and oxidation by methano-and methylotrophic bacteria in soils. This study directly investigated CH4 exchange by plants in their natural environment. At a forest site in central Sweden, in situ branch chamber measurements were used to study plant ambient CH4 exchange by spruce (Picea abies), birch (Betula pubescens), rowan (Sorbus aucuparia) and pine (Pinus sylvestris). The results show a net uptake of CH4 by all the studied plants, which might be of importance for the methane budget. Citation: Sundqvist, E., P. Crill, M. Molder, P. Vestin, and A. Lindroth (2012), Atmospheric methane removal by boreal plants, Geophys. Res. Lett., 39, L21806, doi:10.1029/2012GL053592.

  • 76. Sundqvist, Elin
    et al.
    Molder, Meelis
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Kljun, Natascha
    Lindroth, Anders
    Methane exchange in a boreal forest estimated by gradient method2015In: Tellus. Series B, Chemical and physical meteorology, ISSN 0280-6509, E-ISSN 1600-0889, Vol. 67, article id 26688Article in journal (Refereed)
    Abstract [en]

    Forests are generally considered to be net sinks of atmospheric methane (CH4) because of oxidation by methanotrophic bacteria in well-aerated forests soils. However, emissions from wet forest soils, and sometimes canopy fluxes, are often neglected when quantifying the CH4 budget of a forest. We used a modified Bowen ratio method and combined eddy covariance and gradient methods to estimate net CH4 exchange at a boreal forest site in central Sweden. Results indicate that the site is a net source of CH4. This is in contrast to soil, branch and leaf chamber measurements of uptake of CH4. Wetter soils within the footprint of the canopy are thought to be responsible for the discrepancy. We found no evidence for canopy emissions per se. However, the diel pattern of the CH4 exchange with minimum emissions at daytime correlated well with gross primary production, which supports an uptake in the canopy. More distant source areas could also contribute to the diel pattern; their contribution might be greater at night during stable boundary layer conditions.

  • 77. Tang, Jing
    et al.
    Miller, Paul A.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Olin, Stefan
    Pilesjo, Petter
    Investigating the influence of two different flow routing algorithms on soil-water-vegetation interactions using the dynamic ecosystem model LPJ-GUESS2015In: Ecohydrology, ISSN 1936-0584, E-ISSN 1936-0592, Vol. 8, no 4, p. 570-583Article in journal (Refereed)
    Abstract [en]

    This paper compares two flow routing algorithms' influences on ecohydrological estimations in a northern peatland catchment, within the framework of an arctic-enabled version of the dynamic ecosystem model LPJ-GUESS. Accurate hydrological estimations are needed to fully capture vegetation dynamics and carbon fluxes in the subarctic peatland enviroment. A previously proposed distributed hydrological method based on the single flow (SF) algorithm extracted topographic indices has shown to improve runoff estimations in LPJ-GUESS. This paper investigates model performance differences caused by two flow routing algorithms, and importantly both permafrost processes and peatland hydrology are included in the model. The newly developed triangular form-based multiple flow (TFM) is selected due to its improved consideration of flow continuity and more realistic flow estimation over flat surfaces. A variety of measured data is included to assess both hydrological and ecological accuracy, and the results demonstrate that the choice of flow algorithm does matter for mesoscale ecohydrology applications. The allowance of flow convergence and consideration of flow partition differences from different terrain forms in the TFM algorithm yield better correspondence with the observed hydrological processes and also carbon fluxes. By directing flow to only one downslope cell together with its poorer depiction of flow over flat areas, the SF algorithm can result in too high runoff estimations for low-flat regions and overestimate carbon uptake and release in the peatland. The results of this study also highlight the need for care when selecting flow routing algorithms for biogeochemical estimations, especially within hydrologically and climatically sensitive environments.

  • 78. Thonat, T.
    et al.
    Saunois, M.
    Bousquet, P.
    Pison, I.
    Tan, Z.
    Zhuang, Q.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bastviken, D.
    Dlugokencky, E. J.
    Zimov, N.
    Laurila, T.
    Hatakka, J.
    Hermansen, O.
    Worthy, D. E. J.
    Detectability of Arctic methane sources at six sites performing continuous atmospheric measurements2017In: Atmospheric Chemistry And Physics, ISSN 1680-7316, E-ISSN 1680-7324, Vol. 17, no 13, p. 8371-8394Article in journal (Refereed)
    Abstract [en]

    Understanding the recent evolution of methane emissions in the Arctic is necessary to interpret the global methane cycle. Emissions are affected by significant uncertainties and are sensitive to climate change, leading to potential feedbacks. A polar version of the CHIMERE chemistry-transport model is used to simulate the evolution of tropospheric methane in the Arctic during 2012, including all known regional anthropogenic and natural sources, in particular freshwater emissions which are often overlooked in methane modelling. CHIMERE simulations are compared to atmospheric continuous observations at six measurement sites in the Arctic region. In winter, the Arctic is dominated by anthropogenic emissions; emissions from continental seepages and oceans, including from the East Siberian Arctic Shelf, can contribute significantly in more limited areas. In summer, emissions from wetland and freshwater sources dominate across the whole region. The model is able to reproduce the seasonality and synoptic variations of methane measured at the different sites. We find that all methane sources significantly affect the measurements at all stations at least at the synoptic scale, except for biomass burning. In particular, freshwater systems play a decisive part in summer, representing on average between 11 and 26 % of the simulated Arctic methane signal at the sites. This indicates the relevance of continuous observations to gain a mechanistic understanding of Arctic methane sources. Sensitivity tests reveal that the choice of the land-surface model used to prescribe wetland emissions can be critical in correctly representing methane mixing ratios. The closest agreement with the observations is reached when using the two wetland models which have emissions peaking in August–September, while all others reach their maximum in June–July. Such phasing provides an interesting constraint on wetland models which still have large uncertainties at present. Also testing different freshwater emission inventories leads to large differences in modelled methane. Attempts to include methane sinks (OH oxidation and soil uptake) reduced the model bias relative to observed atmospheric methane. The study illustrates how multiple sources, having different spatiotemporal dynamics and magnitudes, jointly influence the overall Arctic methane budget, and highlights ways towards further improved assessments.

  • 79.
    Thornton, Brett
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Microbial lid on subsea methane2015In: Nature Climate Change, ISSN 1758-678X, E-ISSN 1758-6798, Vol. 5, p. 723-724Article in journal (Refereed)
    Abstract [en]

    Submarine permafrost thaw in the Arctic has been suggested as a trigger for the release of large quantities of methane to the water column, and subsequently the atmosphere - with important implications for global warming. Now research shows that microbial oxidation of methane at the thaw front can effectively prevent its release.

  • 80.
    Thornton, Brett F.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Geibel, Marc C.
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Humborg, Christoph
    Stockholm University, Faculty of Science, Department of Environmental Science and Analytical Chemistry.
    Mörth, Carl-Magnus
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Methane fluxes from the sea to the atmosphere across the Siberian shelf seas2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 11, p. 5869-5877Article in journal (Refereed)
    Abstract [en]

    The Laptev and East Siberian Seas have been proposed as a substantial source of methane (CH4) to the atmosphere. During summer 2014, we made unique high-resolution simultaneous measurements of CH4 in the atmosphere above, and surface waters of, the Laptev and East Siberian Seas. Turbulence-driven sea-air fluxes along the ship's track were derived from these observations; an average diffusive flux of 2.99mgm(-2) d(-1) was calculated for the Laptev Sea and for the ice-free portions of the western East Siberian Sea, 3.80mgm(-2)d(-1). Although seafloor bubble plumes were observed at two locations in the study area, our calculations suggest that regionally, turbulence-driven diffusive flux alone accounts for the observed atmospheric CH4 enhancements, with only a local, limited role for bubble fluxes, in contrast to earlier reports. CH4 in subice seawater in certain areas suggests that a short-lived flux also occurs annually at ice-out.

  • 81.
    Thornton, Brett F.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences. Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Horst, Axel
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Carrizo, Daniel
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Holmstrand, Henry
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    Andersson, Per
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Gustafsson, Örjan
    Stockholm University, Faculty of Science, Department of Applied Environmental Science (ITM).
    A High-Volume Cryosampler and Sample Purification System for Bromine Isotope Studies of Methyl Bromide2013In: Journal of Atmospheric and Oceanic Technology, ISSN 0739-0572, E-ISSN 1520-0426, Vol. 30, no 9, p. 2095-2107Article in journal (Refereed)
    Abstract [en]

    A system was developed for collecting from the ambient atmosphere the methyl halides CH3Cl and CH3Br in quantities sufficient for chlorine and bromine isotope analysis. The construction and operation of the novel cryogenic collection system (cryosampler) and sample purification system developed for this task are described. This study demonstrates the capability of the cryosampler by quantifying the CH3Cl and CH3Br collected from atmospheric samples and the nonfractionating bromine isotope fingerprint of CH3Br from synthetic air samples of controlled composition. An optimized cryosampler operation time of 4 h at a flow rate of 15 L min(-1) is applied to yield the nearly 40 ng required for subsequent Br-81-CH3Br analyses. The sample purification system is designed around a packed column gas chromatography-quadropole-mass spectrometry (GCqMS) system with three additional cryotraps and backflushing capacity. The system's suitability was tested by observing both the mass recovery and the lack of Br-81 isotope fractionation induced during sample purification under varying flow rates and loading scenarios. To demonstrate that the entire system samples and dependably delivers CH3Br to the isotope analysis system without inducing isotope fractionation, diluted synthetic air mixtures prepared from standard gases were processed through the entire system, yielding a Br-81-CH3Br of +0.03 parts per thousand +/- 0.10 parts per thousand relative to their starting composition. Finally, the combined cryosampler-purification and analysis system was applied to demonstrate the first-ever Br-81-CH3Br in the ambient atmosphere with two samples collected in the autumn of 2011, yielding -0.08 parts per thousand +/- 0.43 parts per thousand and +1.75 parts per thousand +/- 0.13 parts per thousand versus standard mean ocean bromide for samples collected at a suburban Stockholm, Sweden, site.

  • 82.
    Thornton, Brett F.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wik, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Climate-forced changes in available energy and methane bubbling from subarctic lakes2015In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 42, no 6, p. 1936-1942Article in journal (Refereed)
    Abstract [en]

    Strong correlations between seasonal energy input and methane (CH4) bubbling (ebullition) in northern lakes suggest that energy proxies might provide a constraint on the magnitude of future CH4 emissions. Ebullition is a major pathway for transporting anaerobically produced CH4 from lake sediments to the atmosphere and represents a large unquantified CH4 source. In high-latitude, postglacial lakes during the ice-free season, solar shortwave energy input can constrain CH4 productivity via control of sediment temperature. Utilizing long-term climatic predictors, we calculate CH4 ebullition from three subarctic lakes in northern Sweden over the period of 1916-2079. Using observed energy trends, the seasonal average lake CH4 ebullition is predicted to increase by 80% between the 1916-1926 decade and the 2040-2079 period. Present-day seasonal average methane ebullition is estimated to have already increased 24% since the 1916-1926 decade.

  • 83.
    Thornton, Brett F.
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Wik, Martin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Double-counting challenges the accuracy of high-latitude methane inventories2016In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 43, no 24, p. 12,569-12,577Article in journal (Refereed)
    Abstract [en]

    Quantification of the present and future contribution to atmospheric methane (CH4) from lakes, wetlands, fluvial systems, and, potentially, coastal waters remains an important unfinished task for balancing the global CH4 budget. Discriminating between these sources is crucial, especially across climate-sensitive Arctic and subarctic landscapes and waters. Yet basic underlying uncertainties remain, in such areas as total wetland area and definitions of wetlands, which can lead to conflation of wetlands and small ponds in regional studies. We discuss how in situ sampling choices, remote sensing limitations, and isotopic signature overlaps can lead to unintentional double-counting of CH4 emissions and proposethat this double-counting can explain a pan-Arctic bottom-up estimate from published sources, 59.7 Tg yr-1(range 36.9–89.4 Tg yr-1) greatly exceeding the most recent top-down inverse modeled estimate of thepan-Arctic CH4 budget (235 Tg yr-1).

  • 84. Torbick, Nathan
    et al.
    Persson, Andreas
    Olefeldt, David
    Frolking, Steve
    Salas, William
    Hagen, Stephen
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Li, Changsheng
    High Resolution Mapping of Peatland Hydroperiod at a High-Latitude Swedish Mire2012In: Remote Sensing, ISSN 2072-4292, E-ISSN 2072-4292, Vol. 4, no 7, p. 1974-1994Article in journal (Refereed)
    Abstract [en]

    Monitoring high latitude wetlands is required to understand feedbacks between terrestrial carbon pools and climate change. Hydrological variability is a key factor driving biogeochemical processes in these ecosystems and effective assessment tools are critical for accurate characterization of surface hydrology, soil moisture, and water table fluctuations. Operational satellite platforms provide opportunities to systematically monitor hydrological variability in high latitude wetlands. The objective of this research application was to integrate high temporal frequency Synthetic Aperture Radar (SAR) and high spatial resolution Light Detection and Ranging (LiDAR) observations to assess hydroperiod at a mire in northern Sweden. Geostatistical and polarimetric (PLR) techniques were applied to determine spatial structure of the wetland and imagery at respective scales (0.5 m to 25 m). Variogram, spatial regression, and decomposition approaches characterized the sensitivity of the two platforms (SAR and LiDAR) to wetland hydrogeomorphology, scattering mechanisms, and data interrelationships. A Classification and Regression Tree (CART), based on random forest, fused multi-mode (fine-beam single, dual, quad pol) Phased Array L-band Synthetic Aperture Radar (PALSAR) and LiDAR-derived elevation to effectively map hydroperiod attributes at the Swedish mire across an aggregated warm season (May-September, 2006-2010). Image derived estimates of water and peat moisture were sensitive (R-2 = 0.86) to field measurements of water table depth (cm). Peat areas that are underlain by permafrost were observed as areas with fluctuating soil moisture and water table changes.

  • 85.
    Treat, C.
    et al.
    Mount Holyoke College.
    Bubier, J.L.
    Mount Holyoke College.
    Varner, R.K.
    Univ New Hampshire.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Timescale dependence of environmental and plant-mediated controls on CH4 flux in a temperate fen.2007In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 112, no G01014Article in journal (Refereed)
    Abstract [en]

    This study examined daily, seasonal, and interannual variations in CH4 emissions at a temperate peatland over a 5-year period. We measured net ecosystem CO2 exchange (NEE), CH4 flux, water table depth, peat temperature, and meteorological parameters weekly from the summers (1 May to 31 August) of 2000 through 2004 at Sallie's Fen in southeastern New Hampshire, United States. Significant interannual differences, driven by high variability of large individual CH4 fluxes (ranging from 8.7 to 3833.1 mg CH4 m−2 d−1) occurring in the late summer, corresponded with a decline in water table level and an increase in air and peat temperature. Monthly timescale yielded the strongest correlations between CH4 fluxes and peat and air temperature (r2 = 0.78 and 0.74, respectively) and water table depth (WTD) (r2 = 0.53). Compared to daily and seasonal timescales, the monthly timescale was the best timescale to predict CH4 fluxes using a stepwise multiple regression (r2 = 0.81). Species composition affected relationships between CH4 fluxes and measures of plant productivity, with sedge collars showing the strongest relationships between CH4 flux, water table, and temperature. Air temperature was the only variable that was strongly correlated with CH4 flux at all timescales, while WTD had either a positive or negative correlation depending on timescale and vegetation type. The timescale dependence of controls on CH4 fluxes has important implications for modeling.

  • 86. Turetsky, Merritt R.
    et al.
    Kotowska, Agnieszka
    Bubier, Jill
    Dise, Nancy B.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Hornibrook, Ed R. C.
    Minkkinen, Kari
    Moore, Tim R.
    Myers-Smith, Isla H.
    Nykanen, Hannu
    Olefeldt, David
    Rinne, Janne
    Saarnio, Sanna
    Shurpali, Narasinha
    Tuittila, Eeva-Stiina
    Waddington, J. Michael
    White, Jeffrey R.
    Wickland, Kimberly P.
    Wilmking, Martin
    A synthesis of methane emissions from 71 northern, temperate, and subtropical wetlands2014In: Global Change Biology, ISSN 1354-1013, E-ISSN 1365-2486, Vol. 20, no 7, p. 2183-2197Article in journal (Refereed)
    Abstract [en]

    Wetlands are the largest natural source of atmospheric methane. Here, we assess controls on methane flux using a database of approximately 19 000 instantaneous measurements from 71 wetland sites located across subtropical, temperate, and northern high latitude regions. Our analyses confirm general controls on wetland methane emissions from soil temperature, water table, and vegetation, but also show that these relationships are modified depending on wetland type (bog, fen, or swamp), region (subarctic to temperate), and disturbance. Fen methane flux was more sensitive to vegetation and less sensitive to temperature than bog or swamp fluxes. The optimal water table for methane flux was consistently below the peat surface in bogs, close to the peat surface in poor fens, and above the peat surface in rich fens. However, the largest flux in bogs occurred when dry 30-day averaged antecedent conditions were followed by wet conditions, while in fens and swamps, the largest flux occurred when both 30-day averaged antecedent and current conditions were wet. Drained wetlands exhibited distinct characteristics, e. g. the absence of large flux following wet and warm conditions, suggesting that the same functional relationships between methane flux and environmental conditions cannot be used across pristine and disturbed wetlands. Together, our results suggest that water table and temperature are dominant controls on methane flux in pristine bogs and swamps, while other processes, such as vascular transport in pristine fens, have the potential to partially override the effect of these controls in other wetland types. Because wetland types vary in methane emissions and have distinct controls, these ecosystems need to be considered separately to yield reliable estimates of global wetland methane release.

  • 87.
    White, M.L.
    et al.
    Univ New Hampshire.
    Varner, R.K.
    Univ New Hampshire.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geology and Geochemistry.
    Mosedale, C.H.
    Univ New Hampshire.
    Controls on the Seasonal Exchange of CH3Br in Temperate Peatlands.2005In: Global Biogeochemical Cycles, ISSN 0886-6236, E-ISSN 1944-9224, Vol. 19, no GB4009Article in journal (Refereed)
    Abstract [en]

    Measurements of CH3Br exchange at two New Hampshire peatlands (Sallie's Fen and Angie's Bog) indicate that net flux from these ecosystems is the sum of competing production and consumption processes. Net CH3Br fluxes were highly variable and ranged from net emission to net uptake between locations within a single peatland. At Sallie's Fen, net CH3Br flux exhibited positive correlations with peat temperature and air temperature during all seasons sampled, but these relationships were not observed at Angie's Bog where flux varied according to microtopography. The major CH3Br production process at Sallie's Fen appeared dependent on aerobic conditions within the peat, while CH3Br production at Angie's Bog was favored by anaerobic conditions. There was evidence of aerobic microbial consumption of CH3Br within the peat at both sites. In a vegetation removal experiment conducted at Sallie's Fen with dynamic chambers, all collars exhibited net consumption of CH3Br. Net CH3Br flux had a negative correlation with surface temperature and a positive correlation with water level in collars with all vegetation clipped consistent with aerobic microbial consumption. Vegetated collars showed positive correlations between net CH3Br flux and air temperature. A positive correlation between net CH3Br flux and surface temperature was also observed in collars in which all vegetation except Sphagnum spp. were clipped. These correlations are consistent with seasonal relationships observed in 1998, 1999, and 2000 and suggest that plants and/or fungi are possible sources of CH3Br in peatlands. Estimates of production and consumption made on two occasions at Sallie's Fen suggest that peatlands have lower rates of CH3Br consumption compared to upland ecosystems, but a close balance between production and consumption rates may allow these wetlands to act as either a net source or sink for this gas.

  • 88.
    Wik, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bastviken, David
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Danielsson, Åsa
    Norback, Elin
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bubbles trapped in arctic lake ice: Potential implications for methane emissions2011In: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 116, p. G03044-Article in journal (Refereed)
    Abstract [en]

    The amount of methane (CH(4)) emitted from northern lakes to the atmosphere is uncertain but is expected to increase as a result of arctic warming. A majority of CH4 is thought to be released through ebullition (bubbling), a pathway with extreme spatial variability that limits the accuracy of measurements. We assessed ebullition during early and late winter by quantifying bubbles trapped in the ice cover of two lakes in a landscape with degrading permafrost in arctic Sweden using random transect sampling and a digital image processing technique. Bubbles covered up to similar to 8% of the lake area and were largely dominated by point source emissions with spatial variabilities of up to 1056%. Bubble occurrence differed significantly between early and late season ice, between the two lakes and among different zones within each lake (p < 0.001). Using a common method, we calculated winter fluxes of up to 129 +/- 486 mg CH(4) m(-2) d(-1). These calculations are, on average, two times higher than estimates from North Siberian and Alaskan lakes and four times higher than emissions measured from the same lakes during summer. Therefore, the calculations are likely overestimates and point to the likelihood that estimating CH(4) fluxes from ice bubble distributions may be more difficult than believed. This study also shows that bubbles quantified using few transects will most likely be unsuitable in making large-scale flux estimates. At least 19 transects covering similar to 1% of the lake area were required to examine ebullition with high precision in our studied lakes.

  • 89.
    Wik, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Varner, Ruth K.
    Bastviken, David
    Stockholm University, Faculty of Science, Department of Geological Sciences. Linköping University, Sweden.
    Multiyear measurements of ebullitive methane flux from three subarctic lakes2013In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 118, no 3, p. 1307-1321Article in journal (Refereed)
    Abstract [en]

    Ebullition (bubbling) from small lakes and ponds at high latitudes is an important yet unconstrained source of atmospheric methane (CH4). Small water bodies are most abundant in permanently frozen peatlands, and it is speculated that their emissions will increase as the permafrost thaws. We made 6806 measurements of CH4 ebullition during four consecutive summers using a total of 40 bubble traps that were systematically distributed across the depth zones of three lakes in a sporadic permafrost landscape in northernmost Sweden. We identified significant spatial and temporal variations in ebullition and observed a large spread in the bubbles' CH4 concentration, ranging from 0.04% to 98.6%. Ebullition followed lake temperatures, and releases were significantly larger during periods with decreasing atmospheric pressure. Although shallow zone ebullition dominated the seasonal bubble CH4 flux, we found a shift in the depth dependency towards higher fluxes from intermediate and deep zones in early fall. The average daily flux of 13.4mg CH4 m(-2) was lower than those measured in most other high-latitude lakes. Locally, however, our study lakes are a substantial CH4 source; we estimate that 350kg of CH4 is released via ebullition during summer (June-September), which is approximately 40% of total whole year emissions from the nearby peatland. In order to capture the large variability and to accurately scale lake CH4 ebullition temporally and spatially, frequent measurements over long time periods are critical.

  • 90.
    Wik, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Johnson, Joel E.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    DeStasio, Joel P.
    Erickson, Lance
    Halloran, Madison J.
    Fahnestock, M. Florencia
    Crawford, Maurice K.
    Phillips, Stephen C.
    Varner, Ruth K.
    Sediment Characteristics and Methane Ebullition in Three Subarctic Lakes2018In: Journal of Geophysical Research - Biogeosciences, ISSN 2169-8953, E-ISSN 2169-8961, Vol. 123, no 8, p. 2399-2411Article in journal (Refereed)
    Abstract [en]

    Ebullition (bubbling) from climate-sensitive northern lakes remains an unconstrained source of atmospheric methane (CH4). Although the focus of many recent studies, ebullition is rarely linked to the physical characteristics of lakes. In this study we analyze the sediments of subarctic postglacial lakes and investigate how sediment properties relate to the large spatial variation in CH4 bubble flux, quantified over multiple years using bubble traps. The results show that the sediments from our lakes are rich in total organic carbon, containing 37 kg/m(3) on average. This number is roughly 40% higher than the average for yedoma deposits, which have been identified as high CH4 emitters. However, the quantity of total organic carbon is not a useful indicator of high emissions from the study lakes. Neither is the amount of CH4 in the sediment a reliable measure of ebullition potential. Instead, our data point to coarse detritus, partly from buried submerged aquatic vegetation and redeposited peat as spatial controls on fluxes, often in combination with previously established effects of incoming solar radiation and water depth. The results once again highlight the climate sensitivity of northern lakes, indicating that biological responses to warmer waters and increased energy input and heating of organic sediments during longer ice-free seasons can substantially alter future CH4 emissions.

  • 91.
    Wik, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Thornton, Brett F.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Bastviken, David
    MacIntyre, Sally
    Varner, Ruth K.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Energy input is primary controller of methane bubbling in subarctic lakes2014In: Geophysical Research Letters, ISSN 0094-8276, E-ISSN 1944-8007, Vol. 41, no 2, p. 555-560Article in journal (Refereed)
    Abstract [en]

    Emission of methane (CH4) from surface waters is often dominated by ebullition (bubbling), a transport mode with high-spatiotemporal variability. Based on new and extensive CH4 ebullition data, we demonstrate striking correlations (r(2) between 0.92 and 0.997) when comparing seasonal bubble CH4 flux from three shallow subarctic lakes to four readily measurable proxies of incoming energy flux and daily flux magnitudes to surface sediment temperature (r(2) between 0.86 and 0.94). Our results after continuous multiyear sampling suggest that CH4 ebullition is a predictable process, and that heat flux into the lakes is the dominant driver of gas production and release. Future changes in the energy received by lakes and ponds due to shorter ice-covered seasons will predictably alter the ebullitive CH4 flux from freshwater systems across northern landscapes. This finding is critical for our understanding of the dynamics of radiatively important trace gas sources and associated climate feedback.

  • 92.
    Wik, Martin
    et al.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Varner, Ruth K.
    Thornton, Brett
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    McCalley, Carmody
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Large isotopic variations and similarities in methane ebullition from northern lakesManuscript (preprint) (Other academic)
    Abstract [en]

    Lakes are abundant in northern, high latitude landscapes and considered a substantial source of atmospheric methane (CH4). In spite of this, little is known about how CH4 release mechanisms relate to underlying organic sources and biogenic production pathways in different types of water bodies. Here, we present a substantial, multiyear dataset of the stable isotopes of CH4 ebullition from three interconnected, subarctic post-glacial lakes. The δ13C-CH4 and δD-CH4 vary from -78.4 to -53.1‰ and from -369.8 to -218.8‰, respectively. Overall, these observations suggest predominantly acetoclastic methanogenesis in the shallow zones, possibly fueled by in-situ plant production, and a shift towards a mix with hydrogenotrophy at depth. The bubbles’ δ13C-CH4 are similar to most of those reported from other northern natural systems, but we found differences in δD-CH4, possibly due to evaporation-driven fractionation over summer. Stable isotopes provide valuable information about underlying organic sources and production pathways, however, due to large overlaps they may not be effective in reducing uncertainties in emissions potentials among different water body types, and in general between lakes and wetlands.

  • 93. Wilson, Rachel M.
    et al.
    Tfaily, Malak M.
    Rich, Virginia I.
    Keller, Jason K.
    Bridgham, Scott D.
    Medvedeff Zalman, Cassandra
    Meredith, Laura
    Hanson, Paul J.
    Hines, Mark
    Pfeifer-Meister, Laurel
    Saleska, Scott R.
    Crill, Patrick
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Cooper, William T.
    Chanton, Jeff P.
    Kostka, Joel E.
    Hydrogenation of organic matter as a terminal electron sink sustains high CO2: CH4 production ratios during anaerobic decomposition2017In: Organic Geochemistry, ISSN 0146-6380, E-ISSN 1873-5290, Vol. 112, p. 22-32Article in journal (Refereed)
    Abstract [en]

    Once inorganic electron acceptors are depleted, organic matter in anoxic environments decomposes by hydrolysis, fermentation, and methanogenesis, requiring syntrophic interactions between microorganisms to achieve energetic favorability. In this classic anaerobic food chain, methanogenesis represents the terminal electron accepting (TEA) process, ultimately producing equimolar CO2 and CH4 for each molecule of organic matter degraded. However, CO2:CH4 production in Sphagnum-derived, mineral-poor, cellulosic peat often substantially exceeds this 1:1 ratio, even in the absence of measureable inorganic TEAs. Since the oxidation state of C in both cellulose-derived organic matter and acetate is 0, and CO2 has an oxidation state of +4, if CH4 (oxidation state -4) is not produced in equal ratio, then some other compound(s) must balance CO2 production by receiving 4 electrons. Here we present evidence for ubiquitous hydrogenation of diverse unsaturated compounds that appear to serve as organic TEAs in peat, thereby providing the necessary electron balance to sustain CO2:CH4 > 1. While organic electron acceptors have previously been proposed to drive microbial respiration of organic matter through the reversible reduction of quinone moieties, the hydrogenation mechanism that we propose, by contrast, reduces CAC double bonds in organic matter thereby serving as (1) a terminal electron sink, (2) a mechanism for degrading complex unsaturated organic molecules, (3) a potential mechanism to regenerate electron-accepting quinones, and, in some cases, (4) a means to alleviate the toxicity of unsaturated aromatic acids. This mechanism for CO2 generation without concomitant CH4 production has the potential to regulate the global warming potential of peatlands by elevating CO2:CH4 production ratios.

  • 94. Woodcroft, Ben J.
    et al.
    Singleton, Caitlin M.
    Boyd, Joel A.
    Evans, Paul N.
    Emerson, Joanne B.
    Zayed, Ahmed A. F.
    Hoelzle, Robert D.
    Lamberton, Timothy O.
    McCalley, Carmody K.
    Hodgkins, Suzanne B.
    Wilson, Rachel M.
    Purvine, Samuel O.
    Nicora, Carrie D.
    Li, Changsheng
    Frolking, Steve
    Chanton, Jeffrey P.
    Crill, Patrick M.
    Stockholm University, Faculty of Science, Department of Geological Sciences.
    Saleska, Scott R.
    Rich, Virginia
    Tyson, Gene W.
    Genome-centric view of carbon processing in thawing permafrost2018In: Nature, ISSN 0028-0836, E-ISSN 1476-4687, Vol. 560, no 7716, p. 49-+Article in journal (Refereed)
    Abstract [en]

    As global temperatures rise, large amounts of carbon sequestered in permafrost are becoming available for microbial degradation. Accurate prediction of carbon gas emissions from thawing permafrost is limited by our understanding of these microbial communities. Here we use metagenomic sequencing of 214 samples from a permafrost thaw gradient to recover 1,529 metagenome-assembled genomes, including many from phyla with poor genomic representation. These genomes reflect the diversity of this complex ecosystem, with genus-level representatives for more than sixty per cent of the community. Meta-omic analysis revealed key populations involved in the degradation of organic matter, including bacteria whose genomes encode a previously undescribed fungal pathway for xylose degradation. Microbial and geochemical data highlight lineages that correlate with the production of greenhouse gases and indicate novel syntrophic relationships. Our findings link changing biogeochemistry to specific microbial lineages involved in carbon processing, and provide key information for predicting the effects of climate change on permafrost systems.

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