Methane (CH_4) is both a significant greenhouse gas and

resource. Its present and past cycling can be studied through

measurements of concentration and/or bulk isotopic ratios (^(13)C/^(12)C,

D/H, and ^(14)C/^(12)C). Currently, isotope ratios are measured by mass

spectrometric analysis of H_2 and CO_2 produced from CH_4, or by

spectroscopy of CH_4. However, the interpretation of bulk isotopic

variations of CH_4 are often equivocal, necessitating additional

tracers

Eiler, John M.

Sessions, Alex L.

Stolper, Daniel A.

Caltech Authors - Main

Goldschmidt 2012 Conference Abstracts
Mineralogical Magazine | www.minersoc.org 2413
Measurement of intact methane 
isotopologues, including 13CH3D 
DANIEL A STOLPER1, ALEX L SESSIONS1, JOHN M EILER1  
1California Institute of Technology, dstolper@caltech.edu  
 
Methane (CH4) is both a significant greenhouse gas and 
resource. Its present and past cycling can be studied through 
measurements of concentration and/or bulk isotopic ratios (13C/12C, 
D/H, and 14C/12C).  Currently, isotope ratios are measured by mass 
spectrometric analysis of H2 and CO2 produced from CH4, or by 
spectroscopy of CH4. However, the interpretation of bulk isotopic 
variations of CH4 are often equivocal, necessitating additional 
tracers.  
We have developed a technique for mass spectrometric analysis 
of several isotopologues of intact CH4, including 12CH4, 13CH4, 
12CH3D, and 13CH3D (and the method can be extended to others). 
Our most novel capability is the analysis of the multiply substituted 
isotopologue 13CH3D, which is expected to differ relative to a 
random isotopic distribution due to kinetic isotope effects, mixing 
processes, and as a function of temperature at equilibrium. 
Measurements of 13CH3D concentrations (along with the singly 
substituted species) could elucidate the formation temperatures of 
thermally generated CH4, discriminate between sources of CH4 to 
the atmosphere (e.g., microbial vs thermogenic gas), and help to 
characterize CH4 chemistry in the atmosphere where potentially 
large enrichments are expected  (e.g., [1], [2], [3]).  
Measurement of intact CH4 requires a mass spectrometer 
capable of separating CH4 species both from water and from internal 
isobars (CH4 adducts, fragments, and isotopologues). We show here 
the initial results and capabilities of such measurements using the 
MAT 253 Ultra prototype high-resolution gas-source mass 
spectrometer. We reproduced D values of known samples to within 
0.2 ‰ (1 s.e. = 0.1‰). In addition to demonstrating accuracy, this 
result reveals that the method may result in improved precision for 
D measurements of CH4 relative to conventional techniques. We 
have measured ratios of 13CH3D to mass 17 species (13CH4, 12CDH4, 
and 12CH5) to better than ±0.4 ‰ (1 s.e.); this value reflects counting 
statstics and should be improved with longer counting times. The 
critical enabling feature of our mass spectrometer is the ability to 
cleanly separate a portion of the 13CH3D+ peak from that of 13CH5+ 
(shown in Figure 1.) 
 
Figure 1:  Peak scan of mass 18 CH4 with a model fit through the 
data. Water is sufficiently resolved as to not appear at this scale. 
 
[1]Ma et al., (2008) GCA 72 5446-56. [2] Mroz et al., (1989) GRL 
16, 677-678. [3] Kaye and Jackman (1990) GRL 17, 659-60. 
The Unsaturated (Vadose) Zone— 
the Where of Weathering 
DAVID A. STONESTROM1*, MARJORIE S. SCHULZ2, AND 
ARTHUR F. WHITE3 
1US Geological Survey, Menlo Park, CA, USA, dastones@usgs.gov 
(*presenting author) 
2US Geological Survey, Menlo Park, CA, USA, mschulz@usgs.gov  
3US Geological Survey, Menlo Park, CA, USA, afwhite@usgs.gov 
(emeritus) 
 
The unsaturated zone (UZ), that portion of the Earth’s crust 
between land surface and the regional water table—the latter being 
defined as the surface below which pore-water pressure is 
persistently above atmospheric—is the vital bio-geochemical 
reactor that sustains the planetary critical zone. Weathering 
reactions in the UZ sustain life by releasing bioessential elements 
such as potassium, calcium, and magnesium that are mostly locked 
up in solid mineral phases during the bioaccessible parts of the rock 
cycle. The UZ sits at the intersection of the lithosphere, atmosphere, 
and terrestrial biosphere, acting as a highly non-linear regulator of 
water, carbon, and nutrient dynamics. It is the key interface 
controlling hydrospheric influences on land based life. UZ water is 
the mobile solvent that delivers aqueous reactants and removes 
aqueous products from the sites of weathering reactions. It also 
partitions the pore space within the evolving geometry of soil 
structure, thereby controlling crucial pore-geometric relations 
including proximal and long-range connectedness (topology) of gas 
and liquid phases. The resulting dynamics mediate residence times, 
redox states, complexation, and chemical activity, along with fluid 
permeabilities and solute dispersivities. Hysteresis in the relations 
between water content θ and matric pressure ψ acts to partially 
constrain fluxes of water (and heat) to the uppermost UZ. This 
provides plant-root microcosms more opportunity for water and 
nutrient extraction and moderates weathering reactions by 
controlling equilibrium versus kinetic control of chemical 
processes. UZs may be classified according to water-table depth, 
water balance, and water-flux variability in time and space. These 
determine whether UZs act as (1) rapid, disequilibrium pass-through 
conduits with little opportunity for weathering reactions, (2) 
equilibrium controlled static reservoirs of water and solutes where 
not much is happening, or (3) totally happening places essential for 
oxygen-intensive life. 
The ideas herein stem from many fruitful discussions and 
collaborations with Art White, Jorie Schulz, Alex Blum, Jennifer 
Harden, Sue Brantley, Kate Maher, Michelle Walvood, Bridget 
Scanlon, Jim Constantz, John Nimmo, Dave Prudic, Amanda Garcia, 
Brian Andraski, Jacob Rubin, Laurie Flint, Alan Flint, and others. 
Vive la Weathering. Vive l’UZ Science. Vive l’Art! 


English

Measurement of intact methane isotopologues, including ^(13)CH_3D

Goldschmidt 2012 Conference Abstracts
Mineralogical Magazine | www.minersoc.org 2413
Measurement of intact methane 
isotopologues, including 13CH3D 
DANIEL A STOLPER1, ALEX L SESSIONS1, JOHN M EILER1  
1California Institute of Technology, dstolper@caltech.edu  
 
Methane (CH4) is both a significant greenhouse gas and 
resource. Its present and past cycling can be studied through 
measurements of concentration and/or bulk isotopic ratios (13C/12C, 
D/H, and 14C/12C).  Currently, isotope ratios are measured by mass 
spectrometric analysis of H2 and CO2 produced from CH4, or by 
spectroscopy of CH4. However, the interpretation of bulk isotopic 
variations of CH4 are often equivocal, necessitating additional 
tracers.  
We have developed a technique for mass spectrometric analysis 
of several isotopologues of intact CH4, including 12CH4, 13CH4, 
12CH3D, and 13CH3D (and the method can be extended to others). 
Our most novel capability is the analysis of the multiply substituted 
isotopologue 13CH3D, which is expected to differ relative to a 
random isotopic distribution due to kinetic isotope effects, mixing 
processes, and as a function of temperature at equilibrium. 
Measurements of 13CH3D concentrations (along with the singly 
substituted species) could elucidate the formation temperatures of 
thermally generated CH4, discriminate between sources of CH4 to 
the atmosphere (e.g., microbial vs thermogenic gas), and help to 
characterize CH4 chemistry in the atmosphere where potentially 
large enrichments are expected  (e.g., [1], [2], [3]).  
Measurement of intact CH4 requires a mass spectrometer 
capable of separating CH4 species both from water and from internal 
isobars (CH4 adducts, fragments, and isotopologues). We show here 
the initial results and capabilities of such measurements using the 
MAT 253 Ultra prototype high-resolution gas-source mass 
spectrometer. We reproduced ?D values of known samples to within 
0.2 ‰ (1 s.e. = 0.1‰). In addition to demonstrating accuracy, this 
result reveals that the method may result in improved precision for 
?D measurements of CH4 relative to conventional techniques. We 
have measured ratios of 13CH3D to mass 17 species (13CH4, 12CDH4, 
and 12CH5) to better than ±0.4 ‰ (1 s.e.); this value reflects counting 
statstics and should be improved with longer counting times. The 
critical enabling feature of our mass spectrometer is the ability to 
cleanly separate a portion of the 13CH3D+ peak from that of 13CH5+ 
(shown in Figure 1.) 
 
Figure 1:  Peak scan of mass 18 CH4 with a model fit through the 
data. Water is sufficiently resolved as to not appear at this scale. 
 
[1]Ma et al., (2008) GCA 72 5446-56. [2] Mroz et al., (1989) GRL 
16, 677-678. [3] Kaye and Jackman (1990) GRL 17, 659-60. 
The Unsaturated (Vadose) Zone— 
the Where of Weathering 
DAVID A. STONESTROM1*, MARJORIE S. SCHULZ2, AND 
ARTHUR F. WHITE3 
1US Geological Survey, Menlo Park, CA, USA, dastones@usgs.gov 
(*presenting author) 
2US Geological Survey, Menlo Park, CA, USA, mschulz@usgs.gov  
3US Geological Survey, Menlo Park, CA, USA, afwhite@usgs.gov 
(emeritus) 
 
The unsaturated zone (UZ), that portion of the Earth’s crust 
between land surface and the regional water table—the latter being 
defined as the surface below which pore-water pressure is 
persistently above atmospheric—is the vital bio-geochemical 
reactor that sustains the planetary critical zone. Weathering 
reactions in the UZ sustain life by releasing bioessential elements 
such as potassium, calcium, and magnesium that are mostly locked 
up in solid mineral phases during the bioaccessible parts of the rock 
cycle. The UZ sits at the intersection of the lithosphere, atmosphere, 
and terrestrial biosphere, acting as a highly non-linear regulator of 
water, carbon, and nutrient dynamics. It is the key interface 
controlling hydrospheric influences on land based life. UZ water is 
the mobile solvent that delivers aqueous reactants and removes 
aqueous products from the sites of weathering reactions. It also 
partitions the pore space within the evolving geometry of soil 
structure, thereby controlling crucial pore-geometric relations 
including proximal and long-range connectedness (topology) of gas 
and liquid phases. The resulting dynamics mediate residence times, 
redox states, complexation, and chemical activity, along with fluid 
permeabilities and solute dispersivities. Hysteresis in the relations 
between water content θ and matric pressure ψ acts to partially 
constrain fluxes of water (and heat) to the uppermost UZ. This 
provides plant-root microcosms more opportunity for water and 
nutrient extraction and moderates weathering reactions by 
controlling equilibrium versus kinetic control of chemical 
processes. UZs may be classified according to water-table depth, 
water balance, and water-flux variability in time and space. These 
determine whether UZs act as (1) rapid, disequilibrium pass-through 
conduits with little opportunity for weathering reactions, (2) 
equilibrium controlled static reservoirs of water and solutes where 
not much is happening, or (3) totally happening places essential for 
oxygen-intensive life. 
The ideas herein stem from many fruitful discussions and 
collaborations with Art White, Jorie Schulz, Alex Blum, Jennifer 
Harden, Sue Brantley, Kate Maher, Michelle Walvood, Bridget 
Scanlon, Jim Constantz, John Nimmo, Dave Prudic, Amanda Garcia, 
Brian Andraski, Jacob Rubin, Laurie Flint, Alan Flint, and others. 
Vive la Weathering. Vive l’UZ Science. Vive l’Art! 


https://authors.library.caltech.edu/39811/1/Eiler_2012p2413.pdf

Measurement of intact methane isotopologues, including ^(13)CH_3D

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