- Email: [email protected]
Resolved measurements of 13CDH3 and 12CD2H2 from a mud volcano in Taiwan
Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Full length article
Resolved measurements of Taiwan
13
CDH3 and
12
CD2H2 from a mud volcano in
⁎
D. Rumblea, , J.L. Ashb, Pei-Ling Wangc, Li-Hung Lind, Yueh-Ting Lind, Tzu-Hsuan Tuc a
Geophysical Lab, Carnegie Institution of Washington, Washington, DC, USA Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, USA c Institute of Oceanography, National Taiwan University, Taipei, Taiwan d Department of Geosciences, National Taiwan University, Taipei, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Taiwan Mud volcano Methane Carbon and hydrogen isotopes Clumped isotopes
Resolved measurements of the relative abundances of 13CDH3 and 12CD2H2 from a Taiwan mud volcano confirm that thermogenic CH4 with a formation temperature of 150 °C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium.
1. Introduction The methane-emitting onshore mud volcanoes of Taiwan have been studied for their potential as signposts pointing towards hydrocarbon deposits (Sun et al., 2010). Interest in mud volcanoes extends globally not only as a resource landmark but also because surface emissions of CH4 threaten the atmosphere's increasing burden of greenhouse gases (Mazzini and Etiope, 2017). Establishing the conditions of methane generation in Earth's crust, whether abiotically, by thermal degradation of buried organic debris, or as metabolic products of methanogenic microbes, is of vital significance in identifying methane reservoirs and mapping their distribution and interconnections. We present fully resolved analyses of the doubly-substituted isotopologues of methane, 13 CDH3 and 12CD2H2, from a Taiwan mud volcano. Measuring two geothermometers provides mutual validation of estimated formation temperatures and demonstrates a close approach to isotopic exchange equilibrium in methane gas from a deep-seated thermogenic source. 2. Mud volcanoes of Taiwan The onshore mud volcanoes of southwestern Taiwan occur in interbedded Pliocene-Miocene mudstones and sands deposited rapidly in an accretionary prism resulting from active subduction of Eurasia beneath the Philippine Sea Plate (Sun et al., 2010). Mud diapirs and mud volcanoes are prevalent immediately offshore (Fig. 1), striking NE towards the mud volcanoes of southwestern Taiwan (Chen et al., 2014, 2017). Onshore mud volcanoes (Fig. 1) lie along tectonic structures ⁎
such as active thrust faults and anticlines (You et al. 2004). The Shinyannyuhu (SYNH) mud volcano of this study (22.804370N, 120.410088E) breaks through to the surface along the outcrop trace of the Chishan Fault Zone (Sun et al. 2010). The activity of surface venting varies hourly from noisy, violent bubbling, and spattering in the liquid mud pool to quiet, reduced flow. Over longer time periods, activity shifts between the bubbling liquid mud pool and nearby mud volcano craters. Changes in activity have been noted to correlate with earthquakes 3. Samples Samples of bubbling liquid mud were dipped directly from the liquid mud pool at SYNH and funneled into serum bottles over a 5minute interval and sealed with butyl-rubber septa. Push cores were driven into sediments proximal to the pond of bubbling liquid mud. Samples from specific depths were funneled into serum bottles and sealed with butyl-rubber septa. The serum bottles were inoculated with 70 ml of 1 N NaOH to desorb methane from clay minerals and to inhibit microbial activity. Methane was collected for analysis from serum bottles using a gas-tight syringe and injected on a GC line for purification. 4. Previous investigations of SYNH mud volcano Chemical, isotopic, and genomic analyses have been made of samples from the SYNH mud volcano. Bubbling liquid mud sampled at
Corresponding author. E-mail address: [email protected] (D. Rumble).
https://doi.org/10.1016/j.jseaes.2018.03.007 Received 6 December 2017; Received in revised form 8 March 2018; Accepted 9 March 2018 1367-9120/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Rumble, D., Journal of Asian Earth Sciences (2018), https://doi.org/10.1016/j.jseaes.2018.03.007
Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx
D. Rumble et al.
Fig. 1. Geologic/Tectonic map of Taiwan showing submarine mud diapirs and mud volcanoes in relation to on-land mud volcanoes (Chen et al., 2014; Chen, 2016; Chen et al., 2017). Label “SYNH” and star locate site of mud volcano from which methane was collected.
23.6 °C had a pH of 8.36, Eh of −340 mV, chlorinity of 111–116 mM chloride, sulfate of 15–22 μM, and δ18OVSMOW of H2O = 6.3–7.5‰ (Cheng et al., 2012). The bulk isotopic compositions of CH4 and dissolved inorganic carbon (DIC) were δ13CVPDB = −34.5‰ and +3.4 to +5.4‰, respectively (Cheng et al., 2012). Analyses by Sun et al. (2010) gave values for SYNH of CH4 = 93.7–94.71 vol% and CO2 = 3.81–1.48 vol%. Bulk isotopic values were δ13CVPDB (CH4) = −32.3‰, δDVSMOW = −188‰, and δ13CVPDB (CO2) = −3.8‰. Following a recent earthquake, the temperature of the liquid mud pool increased to above 40 °C. Analysis for 16S rRNA gene sequences record the presence of Archaea and Bacteria in the pool of bubbling liquid mud and in sediments deposited adjacent to the pool (Cheng et al., 2012). Push cores of sediments were analyzed at depths of 1, 7, 11, 23, and 31 cm. Genomic
analysis gave evidence of stratified microbial communities with a variety of metabolisms including methanogenesis, anaerobic oxidation of methane, sulfate reduction, and sulfur oxidation. It was concluded that the methane venting as bubbles from the pool of liquid mud was thermogenic in origin and had migrated to the surface from deep sources. Local methanogenesis and methane oxidation in core samples, however, may have altered the isotopic composition and abundance of venting CH4 as it infiltrated into the sediments. The stratified microbial ecosystem at SYNH is fueled by mixing of upward moving, anoxic, methane-rich fluids with downward moving, oxic, sulfate-rich fluids (Cheng et al., 2012). Given the availability of comprehensive data on SYNH and its abundant methane, the site was sampled in 2017 for analysis of the clumped, doubly-substituted isotopologues 13CDH3 and 12CD2H2. We 2
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“Δ18” where both ratios 13CDH3/12CH4 and 12CD2H2/12CH4 were measured as the sum of unresolved ion beams at integer mass 18 (Stolper et al. 2015). 6. Clumped isotopologue geothermometry The abundances of 13CDH3 and 12CD2H2 in methane gas change as a function of temperature. The reactions 13
CH4 + 12CDH3 ⇆ 12CH4 + 13CDH3
(1)
and 2( CDH3) ⇆ 12CH4 + 12CD2H2 12
(2)
both shift to the right with decreasing temperature, yielding higher values of Δ13CDH3 (‰) and Δ12CD2H2 (‰) at lower temperature. The use of a stochastic reference frame for reporting clumped isotopologue abundances normalizes for changes in bulk 13C/12C and D/H composition and reveals temperature control. If the isotopologues of methane gas are in isotopic exchange equilibrium at a given temperature, the two geothermometers (1) and (2) should yield the same estimates of the temperature of thermal equilibrium. If the two geothermometers disagree, however, isotopic exchange equilibrium was not achieved and temperature estimates cannot be trusted (Young et al., 2017; see also Stolper et al., 2015, Tables 6 and 7 for examples of disequilibrium with the Δ18 measurement convention). For graphical display, values of Δ13CDH3 (‰) may be plotted on the x-axis vs. Δ12CD2H2 (‰) on the yaxis and the resulting curve contoured in temperature, facilitating the recognition of equilibrated vs. non-equilibrated gas samples (Young et al., 2017, Fig. 1).
Fig. 2. Plot of δ13CVPDB (‰) vs. δDVSMOW (‰) for SYNH compared to onshore and offshore mud volcanoes of Taiwan. (data: this study; Sun et al. 2010; Chen et al. 2017). Standard errors on Panorama analytical data give precision of 0.04‰ on δ13CVPDB and 0.05‰ on δDVSMOW.
sought to confirm or deny the thermogenic origin of the bubbling methane from a deeply buried source as well as to test for the impact of the local microbial community on the isotopic composition of CH4 by analyzing headspace gases released from core samples of sediments. 5. Mass spectrometric analysis of methane isotopologues
7. Results Purified CH4 was obtained for mass spectrometry by separating CH4 from H2, N2, Ar, O2, C2H6, C3H8, and higher hydrocarbons on two gas chromatograph columns connected in series, a 3 m, 1/8 in. OD 5A molecular sieve and a 2 m, 1/8 in. OD HayeSep D porous polymer column (Young et al., 2017). Samples of CH4 gas were analyzed on Panorama, Nu Instruments Ltd., a dual-inlet isotope ratio mass spectrometer capable of independently resolving the single-substituted isotopologue 13CH4 from 12CDH3 and also the doubly-substituted isotopologue 13CDH3 from 12CD2H2, free from interferences in the mass spectrum (Young et al., 2016). Each gas sample was analyzed twice, first, for the ratios 13CH4/12CH4 and 13CDH3/12CH4 and, second, for 12 CDH3/12CH4 and 12CD2H2/12CH4. A calibrated CH4 gas with a hightemperature, stochastic distribution of 13CDH3 and 12CD2H2 was loaded in the dual inlet system as reference. Values of isotopic abundances are reported in delta notation as δ13CVPDB (‰), and δDVSMOW (‰) and as Δ13CDH3 (‰) and Δ12CD2H2 (‰) where the reference for the clumped isotopologues is to a stochastic distribution of isotopologues calculated from the measured 13C/12C and D/H ratios of the unknown sample (Young et al., 2017). The reported values differ from the clumped isotope measurements of methane reported previously because 13CDH3 was not resolved mass spectrometrically from 12CD2H2 in the past. In earlier studies, measurements of clumping in methane were reported as
The SYNH site is one of dozens of onshore mud volcanoes in SW Taiwan. Mud volcanoes and mud diapirs are plentiful on the seafloor immediately to the SW, offshore in the adjacent waters of the South China Sea (Fig. 1). A comparison of measured δ13CVPDB (‰) and δDVSMOW (‰) values (Fig. 2; Table 1) shows that offshore methane is dominated by microbial production (Chen et al., 2017). Onshore mud volcanoes are characterized by thermogenic methane as is one nearby offshore site located on the upper slope of the active plate margin (Sun et al., 2010; Chen et al., 2017). Analyses of SYNH methane bubbles as well as methane from core samples at 2 and 8 cm depth are closely similar in δ13CVPDB (‰) and δDVSMOW (‰) and plot together with other onshore mud volcanoes in the thermogenic field. A SYNH core sample from 19 cm depth, however, is displaced from the other samples and lies on the lower boundary of the thermogenic field, approaching the field of values defined by methane production by fermentation (Fig. 2; Table 1). Measurements of Δ13CDH3 (‰) and Δ12CD2H2 (‰) in both analyzed aliquots of CH4 collected from the bubbling pool of liquid mud plot within error on the equilibrium curve at 150 °C (Fig. 3; Table 1). The pool of liquid mud had a temperature of 44 °C when sampled for this study. The samples of Cheng et al. (2012), described above, were taken
Table 1 Methane isotopic data from SYNH mud volcano.* Sample number
Description
δ13CVPDB (‰)
δDVSMOW (‰)
Δ13CDH3 (‰)
Δ2CD2H2 (‰)
SYNH02-EW-1 SYNH02-EW-3
Bubbling fluid Bubbling fluid
−35.66 (0.01) −35.60 (0.07)
−149.90 (0.02) −149.97 (0.04)
+3.22 (0.19) 2.90 (0.22)
+8.25 (1.37) 9.24 (2.00)
SYNH02-C5-01,C5-02,C6-01,C6-02
Core 2 cm
−35.91 (0.07)
−166.62 (0.04)
2.55 (0.24)
7.87 (2.33)
SYNH02-C5-04
Core 8 cm
−36.05 (0.04)
−182.83 (0.03)
2.07 (0.23)
9.80 (1.97)
SYNH02-C5-08/09/10/11
Core 19 cm
−40.33 (0.06)
−228.43 (0.03)
2.63 (0.35)
1.35 (2.27)
* Errors in parenthesis reported as one standard error of mass spectrometer runs measured for each sample.
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Acknowledgements This article is dedicated to the memory of Professor Bor-Ming Jahn, late of the Institute of Earth Sciences, Academia Sinica, and Department of Geosciences, National Taiwan University, Taipei, Taiwan. He was a mentor, friend, and colleague to us. His life's work of research and teaching shines brightly as a beacon of excellence. We are grateful to P. Freedman, M. Mills, P. Li, and D. Rousell, Nu Instruments Ltd. for designing, building, and installing Panorama. Funding for Panorama was provided by the Deep Carbon Observatory (Sloan Foundation), National Science Foundation, US Department of Energy, University of California Los Angeles, Carnegie Institution of Washington, and Shell Projects and Technologies Emerging Technologies Group. E.D. Young and I.E. Kohl provide a productive and creative environment for research in the Panorama laboratory at UCLA. References Chen, S.-C., Hsu, S.-K., Wang, Y., Chung, S.-H., Chen, P.-C., Tsai, C.-H., Liu, C.-S., Lin, H.S., Lee, Y.-W., 2014. Distribution and characters of the mud diapirs and mud volcanoes off southwest Taiwan. J. Asian Earth Sci. 92, 201–214. Introduction to Taiwan Geology. Geological Society of Taiwan 204pp (in Chinese). Chen, N.-C., Yang, T.F., Hong, W.-L., Chen, H.-W., Chen, H.-C., Hu, C.-Y., Huang, Y.-C., Lin, S., Lin, L.-H., Su, C.-C., Liao, W.-Z., Sun, C.-H., Wang, P.-L., Yang, T., Jiang, S.-Y., Liu, C.-S., Wang, Y., Chung, S.-H., 2017. Production, consumption, and migration of methane in accretionary prism of southwestern Taiwan. Geochem. Geophys. Geosyst. 18, 1–20. Cheng, T.-W., Chang, Y.-H., Tang, S.-L., Tseng, C.-H., Chiang, P.-W., Chang, K.-T., Sun, C.H., Chen, Y.-G., Kuo, H.-C., Wang, C.-H., Chu, P.-H., Song, S.-R., Wang, P.-L., Lin, L.H., 2012. Metabolic stratification driven by surface and subsurface interactions in a terrestrial mud volcano. ISME J. 6, 2280–2290. Mazzini, A., Etiope, G., 2017. Mud volcanism: an updated review. Earth Sci. Rev. 168, 81–112. Stolper, D.A., Martini, A.M., Clog, M., Douglas, P.M., Shusta, S.S., Valentine, D.L., Sessions, A.L., Eiler, J.M., 2015. Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochim. Cosmochim. Acta 161, 219–247. Sun, C.-H., Chang, S.-C., Kuo, C.-L., Wu, J.-C., Shao, P.-H., Oung, J.-N., 2010. Origins of Taiwan’s mud volcanoes: evidence from geochemistry. J. Asian Earth Sci. 37, 105–116. You, C., Gieskes, J., Lee, T., Yui, T., Chen, H.-W., 2004. Geochemistry of mud volcano fluids in the Taiwan accretionary prism. Appl. Geochem. 19, 695–707. Young, E., Rumble, D., Freedman, P., Mills, M., 2016. A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases. Int. J. Mass Spectrom. 401, 1–10. Young, E.D., Kohl, I.E., Lollar, B.S., Etiope, G., Rumble, D., Li, S., (李姝宁), Haghnegahdar, M.A., Schauble, E.A., Mccain, K.A., Foustoukos, D.I., Sutclife, C., Warr, O., Ballentine, C.J., Onstott, T.C., Hosgormez, H., Neubeck, A., Marques, J.M., Pérez-Rodríguez, I., Rowe, A.R., Larowe, D.E., Magnabosco, C., Yeung, L.Y., Ash, J.L., Bryndzia, L.T., 2017. The relative abundances of resolved l2CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases. Geochim. Cosmochim. Acta 203, 235–264.
Fig. 3. Plot of Δ13CDH3 (‰) vs. Δ12CD2H2 (‰) for equilibrium methane and analyzed SYNH samples. The black equilibrium curve is contoured in T °C (Young et al. 2017, Fig. 1). Two aliquots of CH4 from bubbling liquid mud pool plot close to equilibrium curve. Core samples are increasingly displaced from equilibrium curve with increasing depth.
prior to the latest earthquake when the temperature of the mud pool was 23.6 °C. The agreement of two geothermometers analyzed in two separate aliquots of methane confirms that isotopic exchange equilibrium was approached closely in the gas sample. The estimate of 150 °C for the formation temperature of bubbling CH4 lies within the “gas window” for hydrocarbons generated by thermal degradation of buried organic matter. Displacement of methane from core samples both above and below the equilibrium curve may arise from mixing near-surface microbial CH4 with thermogenic methane produced at depth. The mixing trajectories of gases of different isotopic composition on a plot of Δ13CDH3 (‰) vs. Δ12CD2H2 (‰) typically loop above and below the equilibrium curve (see Young et al., 2017, Fig. 14). 8. Discussion The new measurements support the conclusion of Cheng et al. (2012) that methane bubbling from a pool of liquid mud is thermogenic in origin. The deviation of analyzed methane in core samples from the equilibrium curve suggests that mixing between thermogenic and microbial CH4 has taken place.
4
Contents lists available at ScienceDirect
Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Full length article
Resolved measurements of Taiwan
13
CDH3 and
12
CD2H2 from a mud volcano in
⁎
D. Rumblea, , J.L. Ashb, Pei-Ling Wangc, Li-Hung Lind, Yueh-Ting Lind, Tzu-Hsuan Tuc a
Geophysical Lab, Carnegie Institution of Washington, Washington, DC, USA Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, USA c Institute of Oceanography, National Taiwan University, Taipei, Taiwan d Department of Geosciences, National Taiwan University, Taipei, Taiwan b
A R T I C LE I N FO
A B S T R A C T
Keywords: Taiwan Mud volcano Methane Carbon and hydrogen isotopes Clumped isotopes
Resolved measurements of the relative abundances of 13CDH3 and 12CD2H2 from a Taiwan mud volcano confirm that thermogenic CH4 with a formation temperature of 150 °C is bubbling from a pool of liquid mud. Analysis for both doubly-substituted molecules provides confirmation that isotopic exchange equilibrium was achieved between methane isotopologues. Methane extracted from the headspace of core samples taken from sediments deposited proximal to the bubbling pool of mud shows small departures from isotopic equilibrium.
1. Introduction The methane-emitting onshore mud volcanoes of Taiwan have been studied for their potential as signposts pointing towards hydrocarbon deposits (Sun et al., 2010). Interest in mud volcanoes extends globally not only as a resource landmark but also because surface emissions of CH4 threaten the atmosphere's increasing burden of greenhouse gases (Mazzini and Etiope, 2017). Establishing the conditions of methane generation in Earth's crust, whether abiotically, by thermal degradation of buried organic debris, or as metabolic products of methanogenic microbes, is of vital significance in identifying methane reservoirs and mapping their distribution and interconnections. We present fully resolved analyses of the doubly-substituted isotopologues of methane, 13 CDH3 and 12CD2H2, from a Taiwan mud volcano. Measuring two geothermometers provides mutual validation of estimated formation temperatures and demonstrates a close approach to isotopic exchange equilibrium in methane gas from a deep-seated thermogenic source. 2. Mud volcanoes of Taiwan The onshore mud volcanoes of southwestern Taiwan occur in interbedded Pliocene-Miocene mudstones and sands deposited rapidly in an accretionary prism resulting from active subduction of Eurasia beneath the Philippine Sea Plate (Sun et al., 2010). Mud diapirs and mud volcanoes are prevalent immediately offshore (Fig. 1), striking NE towards the mud volcanoes of southwestern Taiwan (Chen et al., 2014, 2017). Onshore mud volcanoes (Fig. 1) lie along tectonic structures ⁎
such as active thrust faults and anticlines (You et al. 2004). The Shinyannyuhu (SYNH) mud volcano of this study (22.804370N, 120.410088E) breaks through to the surface along the outcrop trace of the Chishan Fault Zone (Sun et al. 2010). The activity of surface venting varies hourly from noisy, violent bubbling, and spattering in the liquid mud pool to quiet, reduced flow. Over longer time periods, activity shifts between the bubbling liquid mud pool and nearby mud volcano craters. Changes in activity have been noted to correlate with earthquakes 3. Samples Samples of bubbling liquid mud were dipped directly from the liquid mud pool at SYNH and funneled into serum bottles over a 5minute interval and sealed with butyl-rubber septa. Push cores were driven into sediments proximal to the pond of bubbling liquid mud. Samples from specific depths were funneled into serum bottles and sealed with butyl-rubber septa. The serum bottles were inoculated with 70 ml of 1 N NaOH to desorb methane from clay minerals and to inhibit microbial activity. Methane was collected for analysis from serum bottles using a gas-tight syringe and injected on a GC line for purification. 4. Previous investigations of SYNH mud volcano Chemical, isotopic, and genomic analyses have been made of samples from the SYNH mud volcano. Bubbling liquid mud sampled at
Corresponding author. E-mail address: [email protected] (D. Rumble).
https://doi.org/10.1016/j.jseaes.2018.03.007 Received 6 December 2017; Received in revised form 8 March 2018; Accepted 9 March 2018 1367-9120/ © 2018 Published by Elsevier Ltd.
Please cite this article as: Rumble, D., Journal of Asian Earth Sciences (2018), https://doi.org/10.1016/j.jseaes.2018.03.007
Journal of Asian Earth Sciences xxx (xxxx) xxx–xxx
D. Rumble et al.
Fig. 1. Geologic/Tectonic map of Taiwan showing submarine mud diapirs and mud volcanoes in relation to on-land mud volcanoes (Chen et al., 2014; Chen, 2016; Chen et al., 2017). Label “SYNH” and star locate site of mud volcano from which methane was collected.
23.6 °C had a pH of 8.36, Eh of −340 mV, chlorinity of 111–116 mM chloride, sulfate of 15–22 μM, and δ18OVSMOW of H2O = 6.3–7.5‰ (Cheng et al., 2012). The bulk isotopic compositions of CH4 and dissolved inorganic carbon (DIC) were δ13CVPDB = −34.5‰ and +3.4 to +5.4‰, respectively (Cheng et al., 2012). Analyses by Sun et al. (2010) gave values for SYNH of CH4 = 93.7–94.71 vol% and CO2 = 3.81–1.48 vol%. Bulk isotopic values were δ13CVPDB (CH4) = −32.3‰, δDVSMOW = −188‰, and δ13CVPDB (CO2) = −3.8‰. Following a recent earthquake, the temperature of the liquid mud pool increased to above 40 °C. Analysis for 16S rRNA gene sequences record the presence of Archaea and Bacteria in the pool of bubbling liquid mud and in sediments deposited adjacent to the pool (Cheng et al., 2012). Push cores of sediments were analyzed at depths of 1, 7, 11, 23, and 31 cm. Genomic
analysis gave evidence of stratified microbial communities with a variety of metabolisms including methanogenesis, anaerobic oxidation of methane, sulfate reduction, and sulfur oxidation. It was concluded that the methane venting as bubbles from the pool of liquid mud was thermogenic in origin and had migrated to the surface from deep sources. Local methanogenesis and methane oxidation in core samples, however, may have altered the isotopic composition and abundance of venting CH4 as it infiltrated into the sediments. The stratified microbial ecosystem at SYNH is fueled by mixing of upward moving, anoxic, methane-rich fluids with downward moving, oxic, sulfate-rich fluids (Cheng et al., 2012). Given the availability of comprehensive data on SYNH and its abundant methane, the site was sampled in 2017 for analysis of the clumped, doubly-substituted isotopologues 13CDH3 and 12CD2H2. We 2
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“Δ18” where both ratios 13CDH3/12CH4 and 12CD2H2/12CH4 were measured as the sum of unresolved ion beams at integer mass 18 (Stolper et al. 2015). 6. Clumped isotopologue geothermometry The abundances of 13CDH3 and 12CD2H2 in methane gas change as a function of temperature. The reactions 13
CH4 + 12CDH3 ⇆ 12CH4 + 13CDH3
(1)
and 2( CDH3) ⇆ 12CH4 + 12CD2H2 12
(2)
both shift to the right with decreasing temperature, yielding higher values of Δ13CDH3 (‰) and Δ12CD2H2 (‰) at lower temperature. The use of a stochastic reference frame for reporting clumped isotopologue abundances normalizes for changes in bulk 13C/12C and D/H composition and reveals temperature control. If the isotopologues of methane gas are in isotopic exchange equilibrium at a given temperature, the two geothermometers (1) and (2) should yield the same estimates of the temperature of thermal equilibrium. If the two geothermometers disagree, however, isotopic exchange equilibrium was not achieved and temperature estimates cannot be trusted (Young et al., 2017; see also Stolper et al., 2015, Tables 6 and 7 for examples of disequilibrium with the Δ18 measurement convention). For graphical display, values of Δ13CDH3 (‰) may be plotted on the x-axis vs. Δ12CD2H2 (‰) on the yaxis and the resulting curve contoured in temperature, facilitating the recognition of equilibrated vs. non-equilibrated gas samples (Young et al., 2017, Fig. 1).
Fig. 2. Plot of δ13CVPDB (‰) vs. δDVSMOW (‰) for SYNH compared to onshore and offshore mud volcanoes of Taiwan. (data: this study; Sun et al. 2010; Chen et al. 2017). Standard errors on Panorama analytical data give precision of 0.04‰ on δ13CVPDB and 0.05‰ on δDVSMOW.
sought to confirm or deny the thermogenic origin of the bubbling methane from a deeply buried source as well as to test for the impact of the local microbial community on the isotopic composition of CH4 by analyzing headspace gases released from core samples of sediments. 5. Mass spectrometric analysis of methane isotopologues
7. Results Purified CH4 was obtained for mass spectrometry by separating CH4 from H2, N2, Ar, O2, C2H6, C3H8, and higher hydrocarbons on two gas chromatograph columns connected in series, a 3 m, 1/8 in. OD 5A molecular sieve and a 2 m, 1/8 in. OD HayeSep D porous polymer column (Young et al., 2017). Samples of CH4 gas were analyzed on Panorama, Nu Instruments Ltd., a dual-inlet isotope ratio mass spectrometer capable of independently resolving the single-substituted isotopologue 13CH4 from 12CDH3 and also the doubly-substituted isotopologue 13CDH3 from 12CD2H2, free from interferences in the mass spectrum (Young et al., 2016). Each gas sample was analyzed twice, first, for the ratios 13CH4/12CH4 and 13CDH3/12CH4 and, second, for 12 CDH3/12CH4 and 12CD2H2/12CH4. A calibrated CH4 gas with a hightemperature, stochastic distribution of 13CDH3 and 12CD2H2 was loaded in the dual inlet system as reference. Values of isotopic abundances are reported in delta notation as δ13CVPDB (‰), and δDVSMOW (‰) and as Δ13CDH3 (‰) and Δ12CD2H2 (‰) where the reference for the clumped isotopologues is to a stochastic distribution of isotopologues calculated from the measured 13C/12C and D/H ratios of the unknown sample (Young et al., 2017). The reported values differ from the clumped isotope measurements of methane reported previously because 13CDH3 was not resolved mass spectrometrically from 12CD2H2 in the past. In earlier studies, measurements of clumping in methane were reported as
The SYNH site is one of dozens of onshore mud volcanoes in SW Taiwan. Mud volcanoes and mud diapirs are plentiful on the seafloor immediately to the SW, offshore in the adjacent waters of the South China Sea (Fig. 1). A comparison of measured δ13CVPDB (‰) and δDVSMOW (‰) values (Fig. 2; Table 1) shows that offshore methane is dominated by microbial production (Chen et al., 2017). Onshore mud volcanoes are characterized by thermogenic methane as is one nearby offshore site located on the upper slope of the active plate margin (Sun et al., 2010; Chen et al., 2017). Analyses of SYNH methane bubbles as well as methane from core samples at 2 and 8 cm depth are closely similar in δ13CVPDB (‰) and δDVSMOW (‰) and plot together with other onshore mud volcanoes in the thermogenic field. A SYNH core sample from 19 cm depth, however, is displaced from the other samples and lies on the lower boundary of the thermogenic field, approaching the field of values defined by methane production by fermentation (Fig. 2; Table 1). Measurements of Δ13CDH3 (‰) and Δ12CD2H2 (‰) in both analyzed aliquots of CH4 collected from the bubbling pool of liquid mud plot within error on the equilibrium curve at 150 °C (Fig. 3; Table 1). The pool of liquid mud had a temperature of 44 °C when sampled for this study. The samples of Cheng et al. (2012), described above, were taken
Table 1 Methane isotopic data from SYNH mud volcano.* Sample number
Description
δ13CVPDB (‰)
δDVSMOW (‰)
Δ13CDH3 (‰)
Δ2CD2H2 (‰)
SYNH02-EW-1 SYNH02-EW-3
Bubbling fluid Bubbling fluid
−35.66 (0.01) −35.60 (0.07)
−149.90 (0.02) −149.97 (0.04)
+3.22 (0.19) 2.90 (0.22)
+8.25 (1.37) 9.24 (2.00)
SYNH02-C5-01,C5-02,C6-01,C6-02
Core 2 cm
−35.91 (0.07)
−166.62 (0.04)
2.55 (0.24)
7.87 (2.33)
SYNH02-C5-04
Core 8 cm
−36.05 (0.04)
−182.83 (0.03)
2.07 (0.23)
9.80 (1.97)
SYNH02-C5-08/09/10/11
Core 19 cm
−40.33 (0.06)
−228.43 (0.03)
2.63 (0.35)
1.35 (2.27)
* Errors in parenthesis reported as one standard error of mass spectrometer runs measured for each sample.
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Acknowledgements This article is dedicated to the memory of Professor Bor-Ming Jahn, late of the Institute of Earth Sciences, Academia Sinica, and Department of Geosciences, National Taiwan University, Taipei, Taiwan. He was a mentor, friend, and colleague to us. His life's work of research and teaching shines brightly as a beacon of excellence. We are grateful to P. Freedman, M. Mills, P. Li, and D. Rousell, Nu Instruments Ltd. for designing, building, and installing Panorama. Funding for Panorama was provided by the Deep Carbon Observatory (Sloan Foundation), National Science Foundation, US Department of Energy, University of California Los Angeles, Carnegie Institution of Washington, and Shell Projects and Technologies Emerging Technologies Group. E.D. Young and I.E. Kohl provide a productive and creative environment for research in the Panorama laboratory at UCLA. References Chen, S.-C., Hsu, S.-K., Wang, Y., Chung, S.-H., Chen, P.-C., Tsai, C.-H., Liu, C.-S., Lin, H.S., Lee, Y.-W., 2014. Distribution and characters of the mud diapirs and mud volcanoes off southwest Taiwan. J. Asian Earth Sci. 92, 201–214. Introduction to Taiwan Geology. Geological Society of Taiwan 204pp (in Chinese). Chen, N.-C., Yang, T.F., Hong, W.-L., Chen, H.-W., Chen, H.-C., Hu, C.-Y., Huang, Y.-C., Lin, S., Lin, L.-H., Su, C.-C., Liao, W.-Z., Sun, C.-H., Wang, P.-L., Yang, T., Jiang, S.-Y., Liu, C.-S., Wang, Y., Chung, S.-H., 2017. Production, consumption, and migration of methane in accretionary prism of southwestern Taiwan. Geochem. Geophys. Geosyst. 18, 1–20. Cheng, T.-W., Chang, Y.-H., Tang, S.-L., Tseng, C.-H., Chiang, P.-W., Chang, K.-T., Sun, C.H., Chen, Y.-G., Kuo, H.-C., Wang, C.-H., Chu, P.-H., Song, S.-R., Wang, P.-L., Lin, L.H., 2012. Metabolic stratification driven by surface and subsurface interactions in a terrestrial mud volcano. ISME J. 6, 2280–2290. Mazzini, A., Etiope, G., 2017. Mud volcanism: an updated review. Earth Sci. Rev. 168, 81–112. Stolper, D.A., Martini, A.M., Clog, M., Douglas, P.M., Shusta, S.S., Valentine, D.L., Sessions, A.L., Eiler, J.M., 2015. Distinguishing and understanding thermogenic and biogenic sources of methane using multiply substituted isotopologues. Geochim. Cosmochim. Acta 161, 219–247. Sun, C.-H., Chang, S.-C., Kuo, C.-L., Wu, J.-C., Shao, P.-H., Oung, J.-N., 2010. Origins of Taiwan’s mud volcanoes: evidence from geochemistry. J. Asian Earth Sci. 37, 105–116. You, C., Gieskes, J., Lee, T., Yui, T., Chen, H.-W., 2004. Geochemistry of mud volcano fluids in the Taiwan accretionary prism. Appl. Geochem. 19, 695–707. Young, E., Rumble, D., Freedman, P., Mills, M., 2016. A large-radius high-mass-resolution multiple-collector isotope ratio mass spectrometer for analysis of rare isotopologues of O2, N2, CH4 and other gases. Int. J. Mass Spectrom. 401, 1–10. Young, E.D., Kohl, I.E., Lollar, B.S., Etiope, G., Rumble, D., Li, S., (李姝宁), Haghnegahdar, M.A., Schauble, E.A., Mccain, K.A., Foustoukos, D.I., Sutclife, C., Warr, O., Ballentine, C.J., Onstott, T.C., Hosgormez, H., Neubeck, A., Marques, J.M., Pérez-Rodríguez, I., Rowe, A.R., Larowe, D.E., Magnabosco, C., Yeung, L.Y., Ash, J.L., Bryndzia, L.T., 2017. The relative abundances of resolved l2CH2D2 and 13CH3D and mechanisms controlling isotopic bond ordering in abiotic and biotic methane gases. Geochim. Cosmochim. Acta 203, 235–264.
Fig. 3. Plot of Δ13CDH3 (‰) vs. Δ12CD2H2 (‰) for equilibrium methane and analyzed SYNH samples. The black equilibrium curve is contoured in T °C (Young et al. 2017, Fig. 1). Two aliquots of CH4 from bubbling liquid mud pool plot close to equilibrium curve. Core samples are increasingly displaced from equilibrium curve with increasing depth.
prior to the latest earthquake when the temperature of the mud pool was 23.6 °C. The agreement of two geothermometers analyzed in two separate aliquots of methane confirms that isotopic exchange equilibrium was approached closely in the gas sample. The estimate of 150 °C for the formation temperature of bubbling CH4 lies within the “gas window” for hydrocarbons generated by thermal degradation of buried organic matter. Displacement of methane from core samples both above and below the equilibrium curve may arise from mixing near-surface microbial CH4 with thermogenic methane produced at depth. The mixing trajectories of gases of different isotopic composition on a plot of Δ13CDH3 (‰) vs. Δ12CD2H2 (‰) typically loop above and below the equilibrium curve (see Young et al., 2017, Fig. 14). 8. Discussion The new measurements support the conclusion of Cheng et al. (2012) that methane bubbling from a pool of liquid mud is thermogenic in origin. The deviation of analyzed methane in core samples from the equilibrium curve suggests that mixing between thermogenic and microbial CH4 has taken place.
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