Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue12.Dec2000:

Organic Geochemistry 31 (2000) 1387±1408
www.elsevier.nl/locate/orggeochem

Methane exchange between coal-bearing basins and
the atmosphere: the Ruhr Basin and the Lower Rhine
Embayment, Germany
Thomas Thielemann a,b, Andreas LuÈcke a, Gerhard H. Schleser a, Ralf Littke b,*
a
Institut fuÈr ErdoÈl und Organische Geochemie (ICG-4), Forschungszentrum JuÈlich, D-52425 JuÈlich, Germany
Lehrstuhl fuÈr Geologie, Geochemie und LagerstaÈtten des ErdoÈls und der Kohle, RWTH Aachen, D-52056 Aachen, Germany

b

Abstract
A precise knowledge of methane exchange processes is required to fully understand the recent rise of atmospheric
methane concentration. Three of these processes take place at the lithosphere/atmosphere boundary: bacterial consumption of methane and emission of bacterial or thermogenic methane. This study was initiated to quantify these
processes on a regional scale in the Ruhr Basin and the Lower Rhine Embayment. Since these areas are subject to
bituminous coal and lignite mining, natural and anthropogenically-induced methane exchange processes could be studied. The methane emission and consumption rates and their carbon isotope signal were measured at the lithosphere/
atmosphere boundary using ¯ux chambers. On most of the soils studied, methane consumption by bacteria was identi®ed. Thermogenic methane was released only at some of the natural faults examined. In active and abandoned bituminous coal mining areas methane emissions were restricted to small areas, where high emission rates were measured.
The carbon isotope composition of methane at natural faults and in mining subsidence troughs was typical of thermogenic methane (ÿ45 to ÿ32 % d13C). Methane exchange balancing revealed that natural methane emissions from
these two basins represent no source of atmospheric importance. However, methane release by upcast mining shafts

dominates the methane exchange processes and is by about two orders of magnitude greater than methane consumption by bacterial oxidation in the soils. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Methane balance; Coal bed methane; Methane emissions; Methane consumption; Stable carbon isotopes; Flux chamber;
Ruhr Basin; Lower Rhine embayment

1. Introduction
Methane, after carbon dioxide and water, is the third
most important atmospheric trace gas, contributing
about 23% to the anthropogenic part of the greenhouse
e€ect (Houghton, 1997). Methane concentration
reached 1.72 ppmV in 1994 (global average) with an
average annual increase of 0.6% (10.3 ppbV, Houghton
et al., 1995). Human activities such as rice farming, the
keeping of ruminants, biomass burning, land®ll installations, petroleum exploration and coal mining are

* Corresponding author. Tel.: +49-241-80-5748; fax: +49241-88-88152.
E-mail address: littke@lek.rwth- aachen.de (R. Littke).

important methane sources (Cicerone and Oremland,
1988). They are decisive for an average rise in atmospheric methane concentration with time, as indicated
by the positive, strong correlation between increase in

human world population and atmospheric methane
concentration within the last 300 years (Stau€er et al.,
1985; Blake and Rowland, 1988; Khalil et al., 1989).
The overall methane sources range between 400 and 640
Mt per year (Cicerone and Oremland, 1988), with 90%
of this gas being oxidized by photochemical and bacterial processes within the same year (Lelieveld et al.,
1993). The annual increase rate of methane is extremely
irregular but principally is slowing down (Houghton et
al., 1995). In 1992 for a few months this rate even
reached negative values (Khalil and Rasmussen, 1993),
which might correlate with the economic breakdown of

0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00104-2

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T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

the former Soviet Union and with less emissions from

natural gas production and distribution (Houghton,
1997). The precise extents of methane sources and sinks
still remain unknown. A better database is needed.
Methane emissions from coal mining on a global scale
are estimated to be 4 to 11% of the sources (Cicerone
and Oremland, 1988). In Germany emissions via upcast
mining shafts are known (SchoÈn et al., 1993), but across
the earth's surface have never been measured before.
They are balanced here.
German hard coal pits are obliged to measure
methane emissions via upcast mining shafts. The results
are published for single years (Treskow, 1978; 1985;
Fitzner, 1989; Zimmermeyer and Seeliger, 1989;
Bundesumweltministerium, 1997). As recent gas contents of hard coal vary between 0 and 15 m3/t (Freudenberg et al., 1996) only estimations exist about
methane emissions from stored coal and from abandoned
mines. Stored coals are assessed to emit 0.8 m3 methane/t
coal (Bundesumweltministerium, 1997). Eicker and HeûbruÈgge (1984) calculated that three quarters of the
abandoned mines release 98 Mill. m3 of methane per
year and project this number to 120 Mill. m3 for all
abandoned bituminous coal mines in the Ruhr Basin.

Data of degassing from abandoned bituminous coal
mines in the Lower Rhine Embayment have not been
published. Lignite mining may release about 0.1 m3
methane/t into the atmosphere (SchoÈn et al., 1993).
Water-unsaturated soils are known to bear methaneconsuming (methanotrophic) bacteria (Kaserer, 1906;
Keller et al., 1983). These bacteria either oxidize atmospheric methane which di€uses into the soil, or methane
which is migrating from a source below (e.g. gas ®eld,
coal, land®ll, wetland) into the atmosphere (Whalen and
Reeburgh, 1990; Whalen et al., 1990). This self-cleaning
potential of soils towards methane emissions must
eventually be determined on a global scale. Estimations
vary from 1% of the global methane sinks (Born et al.,
1990) to 6% (DoÈrr et al., 1993) and 12.5% (Houghton
et al., 1995). The reason for such drastic deviations is
the small number of measurements on a global scale,
which in di€erent ecosystems present di€erent results.
Methane consumption rates on native soils in the temperate zone range from 0.08 mg/(m2d) (Koschorreck
and Conrad, 1993) to 6.79 mg/(m2d) (Tyler et al., 1994).
More precise estimations of the global role of methane
consumption by bacteria require a better database with

a high resolution in space and time. In this context, new
data are provided here for the Ruhr Basin and Lower
Rhine Embayment.
In order to balance methane exchange, areas with
bacterial and thermogenic methane emissions, as well as
methane consumption were considered. Their main
characteristics are listed in Table 1. In the Lower Rhine
Embayment (LRE) the localities vary in thickness of
post-Carboniferous rocks (0 to 900 m). They contain

either Late Carboniferous bituminous coal seams or
Tertiary lignite or both in the subsurface. Flux chamber
experiments were concentrated at locality LRE 2 (Table
1) to study seasonal variabilities in methane exchange
processes in detail. In the Ruhr Basin (RB) localities
have been chosen, where post-Carboniferous sediments
are 0±1260 m thick above bituminous coal bearing Carboniferous beds. Lignite deposits do not exist in the
Ruhr Basin. Underground mining activities were either
continuing or abandoned or absent, as indicated in Fig.
1. Locations RB 7, RB 13 and RB 14 were placed on

natural normal faults, to examine their gas exchange
characteristics. Locations RB 11 and RB 12 were sampled intensively as they showed the most variable
methane exchange patterns in the Ruhr Basin.

2. Sediment-, soil- and methane characteristics
In Germany, the Ruhr Basin and Lower Rhine
Embayment contain the most important bituminous
coal and lignite mines, respectively. They both contain
clastic Upper Carboniferous sediments of up to 3.5 km
thickness on top of older Paleozoic deposits. This thick
sequence contains more than 100 coal seams (Littke,
1987). The Variscan structural evolution of this area is
delineated by Drozdzewski (1993), Strack (1989) and
Zeller (1987). The subsidence history of both basins
from Carboniferous times to Present has been reconstructed by BuÈker et al. (1995) and Karg (1998). The
western part of the basin contains Permian, Cretaceous
and Cenozoic deposits, in some regions accompanied by
Triassic and Jurassic sediments. In the eastern part of
the Ruhr Basin, Cretaceous marine sediments unconformably overlie the Carboniferous and increase in
thickness northward. Quaternary glacial and ¯uvial

deposits overly the Cretaceous in some areas. In the
southern part of the Lower Rhine Embayment, a Miocene peat accumulation led to lignite seams of up to 100
m thickness (Hager and PruÈfert, 1988). Possible
methane source rocks are the Upper Carboniferous
bituminous coal-bearing strata of both the RB and LRE
as well as Tertiary lignite deposits in the LRE.
Exploration wells in both basins indicate that
methane is accumulated in the coal seams (99% of all
methane) and that the Carboniferous clastic rocks and
post-Carboniferous rocks are mainly gas-empty. The
only exceptions are Cretaceous sediments which cover
the Carboniferous coal-bearing strata in the eastern
Ruhr Basin. There, local gas shows have been reported
since the beginning of exploration activities (MuÈller,
1904; Wegner, 1924a,b). According to data by Colombo
et al. (1970) and TeichmuÈller et al. (1970), most of the
d13C-values for in situ coal bed methane range between
ÿ31 and ÿ49 %. This, according to a classi®cation by
Whiticar (1990, 1996), indicates early to late thermogenic

Table 1
List of seven sampling locations in the Lower Rhine Embayment, 15 locations in the Ruhr Basin and their characteristics. Geological information was taken from Drozdzewski et al.
(1982) and Hager and PruÈfert (1988), mining situation from Wrede et al. (1983), Wrede and Zeller (1988) and Hilden et al. (1995)
Post-Carboniferous
covering rock
thickness (m)

Coal type
below

Mining
activity

Number of
¯ux chamber
experiments

Methane
exchange
process

Exchange
range
[mg/(m2d)]

Remarks

LRE1
LRE2

0
800±900

Lignite
Lignite

Abandoned
None

10

402

Bacterial consumption
Bacterial consumption

0 to ÿ0.31
ÿ0.15 to ÿ3.32

LRE3
LRE4
LRE5
LRE6
LRE7
RB1
RB2
RB3
RB4
RB5
RB6
RB7

RB8
RB9
RB10
RB11
RB12
RB13
RB14
RB15

300-320
500±550
340±360
200±220
180±220
0
0
0
3
60
450±470
600±620
940±960
480±490
450±470
500±600
550±650
820
870
1250±1300

Lignite/Hard coal
Lignite/Hard coal
Hard coal
Lignite/Hard coal
Lignite/Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal
Hard coal

Abandoned
None
Abandoned
None
None
None
Abandoned
Abandoned
Abandoned
Abandoned
None
Active
None
None
Abandoned
Active
Abandoned
None
None
None

30
8
5
24
6
4
10
6
22
8
8
23
34
5
7
84
115
12
12
15

Bacterial consumption
Bacterial consumption
Bacterial consumption
Bacterial consumption
Bacterial consumption
Bacterial consumption
Thermogenic emission
Bacterial consumption
Bact. consum./thermo.emiss.
Bact. consum./thermo.emiss.
Bacterial consumption
Bacterial consumption
Bacterial consumption
Bacterial consumption
Bacterial consumption
Bact. consum./thermo.emiss.
Bact. consum./thermo.emiss.
Thermogenic emission
Thermogenic emission
Bacterial emission

ÿ0.4
ÿ0.46
ÿ0.25
ÿ0.12
ÿ0.67
ÿ0.30
354
ÿ0.24
ÿ0.32
ÿ0.40
ÿ0.46
ÿ0.25
ÿ0.36
ÿ0.21
ÿ0.43
ÿ0.28
ÿ0.54
6.57
482
0.5

Lignite outcrop
Farmland meadow, forest,
natural fault
Meadow, farmland
Farmland
Farmland
Farmland
Meadow (fen)
Forest, hard coal outcrop
Day fall, water
Quarry, hard coal outcrop
Farmland
Meadow
Meadow, farmland
Farmland, natural fault
Farmland, forest
Farmland
Meadow
Farmland
Meadow, water
Farmland, natural fault
Farmland, natural fault
Moor (bog), wetland

to
to
to
to
to
to
to
to
to
to
to
to
to
to
to
to
to
to
to
to

ÿ1.02
ÿ1.6
ÿ1.26
ÿ1.27
ÿ1.82
ÿ0.66
28,607
ÿ0.72
ÿ1.64/1.10 to 13.62
ÿ1.25/0.69 to 3.82
ÿ1.85
ÿ1.79
ÿ3.67
ÿ0.90
ÿ1.35
ÿ3.45/16.0 to 78,640
ÿ4.96/4.3 to 99,830
274
8,803
98.6

T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

No.

1389

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T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

Fig. 1. Map of the coal-bearing basins with seven locations of methane exchange measurements in the Lower Rhine Embayment
(LRE 1±7) and with 15 locations in the Ruhr Basin (RB 1±15). Also indicated are areas of former, recent and future bituminous coal
and lignite mining, after Wrede et al. (1983), Wrede and Zeller (1988), Hilden et al. (1995) and informations of the mining industry
(Rheinbraun AG).

methane. The isotopic data show a distinct depth correlation, with methane becoming lighter towards the top
of the Carboniferous and towards fault zones, independent of coali®cation patterns (TeichmuÈller et al., 1970).
The coal gas is characterized by low concentration
ratios of methane over higher-molecular-weight hydrocarbons (< 1000, mainly < 300), which are typically
thermogenic features (Bernard, 1978). The isotopic
composition of methane in overlying Cretaceous rocks
ranges from ÿ30 to ÿ65 %. Lommerzheim (1994)
interprets this methane as thermogenic gas plus bacterial
admixture. Hence, the isotopic composition of coal bed
methane indicates the majority of gas to be of thermogenic origin. The 13C-depleted values at the top of the
Carboniferous as well as in the overlying Cretaceous
rocks are either due to adsorption/desorption processes
or indicate bacterial methanogenesis (Wingerning, 1975;
Freudenberg et al., 1996).

In general, soils in the Lower Rhine Embayment are
silt-dominated, whereas they are more sandy in the
Ruhr Basin. In the Lower Rhine Embayment the sediments at the ground's surface predominantly are Quaternary glacial, eolian and ¯uvial deposits. About 74%
of the area is covered by luvisols (loess, loess loam) and
over 13.5% by gleyic luvisols. Cambisols account for
about 5% and podzols for 7.5% of the area (Heide,
1988). Sediments at the surface of the Ruhr Basin are
mainly Cretaceous marine sands and marls and Quaternary glacial and ¯uvial sands. Forty four percent of
the area is covered by podzols on sands, 18% by luvisols
on loess, 17% by gleyic luvisols on top of marls and
moraines, 15% by cambisols on calcareous rocks and
6% by gleysols in riverbeds (Dahm-Ahrens, 1995).
Wetlands cover less than 0.05% of the area in both
basins. Undeveloped entisols on outcropping coal seams
can be neglected.

T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

3. Methods
To determine methane exchange rates between lithosphere and atmosphere, ¯ux chambers were used. They
consist of a metal ring which was driven 4±8 cm into the
soil. A plastic, translucent cap on top of the ring sealed
the ¯ux chamber from the surrounding atmosphere, as
described by Cramer (1997). The volume of ¯ux chambers varied between 15 and 20 l. Gas samples were taken
from ¯ux chambers at variable time intervals by ®lling
them into evacuated metal containers of 69.2 ml and
were analysed gas chromatographically in the laboratory. A Hewlett Packard GC (HP5890, Series II) was
used to perform the measurements. Gas separation
proceeded on a 10 m Poraplot Q-capillary column. Six
methane standards (2.146 vpm, 9.81 vpm, 200 vpm, 490
vpm, 2.01%, 99.5%) were used to calibrate the GC
data, balanced against the standard no. 1658 of the
National Bureau of Standards (USA). For calculations
see Appendix A and B. To ensure comparability of the
methane data, all concentrations were converted into
values at standard temperature and pressure conditions
(STP: 0 C, 101.325 kPa). Methane exchange rates are
given as negative values if they represent a sink of
atmospheric methane and as positive values if they
represent a source.
Soil temperatures were measured with a Hg-thermometer, which was pushed into the soil to a depth of 3
cm. The depth was controlled with a yard-stick. The
temperature was read about 5 min after the Hg-thermometer had been inserted into the soil to ensure equilibration of the mercury temperature with soil
temperature.
Undisturbed soil samples of 300 and 600 cm3 volume
were cored and the moist soil samples weighed to calculate the bulk soil density (see Appendix C). Afterwards, the samples were dried for three days at 105 C
and weighed again. The di€erence between dry and
moist weight gave the gravimetric and volumetric water
content of the samples. 100 to 150 g of dried soil were
ground and its grain density measured with a Quantachrome helium-pyknometer. With the bulk soil density
and its grain density, the bulk porosity was determined
(Appendix C). From this and the volumetric water content, the gas-®lled portion of soil volume was calculated
according to Appendix C.
The stable carbon isotope measurements of methane
in atmospheric concentrations (1.8 vpm and lower) have
been conducted with a Precon-C-IRMS unit (Micromass, UK). The preconcentration interface (Precon)
contains a combustion furnace and is connected to an
isotope ratio mass spectrometer (IRMS) described by
Brand (1995). The central part in the Precon-unit is a
450 mm ceramic tube ®lled with two catalytic wires (Pt,
Ni) and an oxidation wire (Cu, 0.1 mm each). The tube
is placed in a combustion furnace. The latter was heated

1391

overnight to 1000 C and conditioned with clean oxygen
(2 ml/min) to burn o€ residual carbon and to oxidize
the Cu-wire. The oxygen ¯ow was stopped before measuring. Gas samples in 150 ml-containers were ¯ushed
with a continuous ¯ow of helium across three chemical
traps to purify the gas by removing water, carbon dioxide and carbon monoxide. This gas was then passed into
the ceramic tube, where methane was converted into
carbon dioxide completely. This carbon dioxide was
cryofocussed, sent across 25 m of a Poraplot Q-capillary
column to focus the carbon dioxide peak and measured
in the IRMS (continuous ¯ow mode). Isotope values are
given in the d-notation relative to the internationally
adopted PDB standard. Minimum amount of methane
needed was 5 nmol. The measurements showed a standard deviation of 0.15±0.4 % (n=5).

4. Results
4.1. Bacterial methane emissions
Bacterial methane emissions are a typical feature of
wetlands (fens, bogs), where anaerobic conditions permit methane generation from organic matter or by
reduction of carbon dioxide (Alber et al., 1993; Rice,
1993). As both sedimentary basins are densely populated (523 persons/km2 in 1994), the soils are either
widely sealed or of agricultural use and dewatered.
Therefore, remaining bogs are today restricted to 29
km2 and fens to 174 km2 in the Ruhr Basin (DahmArens, 1995). These areas cover only about 2 to 3% of
the total area. In the Lower Rhine Embayment bogs
cover an area of 5 km2 and fens 32 km2 or about 0.5%
of the area (Heide, 1988). To test their methane
exchange characteristics, one bog (RB 15) and one fen
(LRE 7, see Fig. 1) have been examined. The bog was
situated on Quaternary glacial sands, above the
groundwater table at 10 to 40 cm depth. Predominantly
(Sphagnum) moss grew in the bog, while reed and
swamp grass covered up to 30% of the area. The ¯ux
chamber was always placed at about the same position,
covering a plant association which was representative of
this location. No plants had to be cut for chamber
emplacements, so that natural gas emission characteristics were guaranteed. The methane emission rates for
the bog are presented for a period of 24 months in Fig.
2. The results vary by two orders of magnitude and
show a positive correlation with soil temperature. Bacterial methane production rises with about a one month
delay after temperatures increased in spring time (Fig.
2). The correlation coecient between soil temperature
and methane emission was 0.94. Between May and
October, bacterial methane emissions were higher than
30 mg/(m2d). The minimum values were reached in
January and Febuary with less than 3 mg/(m2d). The 15

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T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

measured methane emission rates average 42.3 mg/(m2d).
Data spread unequally across the year. The warmer period
was slightly overrepresented. The non-weighted annual
average emission rate was 35.7 mg/(m2d).
Fig. 3 presents the results of a ¯ux chamber experiment
at locality RB 15. A linear rise in methane concentration

indicates a methane emission rate of 44.1 mg/(m2d). The
isotopic composition of methane changed exponentially
from data typical of atmospheric methane (ÿ47 %) to
about ÿ60 %, which proves the bacterial methane emission to be the reason for this isotope trend. The isotopic
shift became faster with increasing emission rate.

Fig. 2. Emissions of bacterial methane and soil temperature in a bog, northern Ruhr Basin (RB 15). The dashed line traces the most
likely seasonal variation of methane emission. The inlet presents the temperature dependance of this process.

Fig. 3. Flux chamber experiment on a bog at locality RB 15. The shift in carbon isotopic composition of methane indicates a constant
admixture of bacterial methane into the ¯ux chamber.

T. Thielemann et al. / Organic Geochemistry 31 (2000) 1387±1408

Fens were developed on less than 0.03% of the area
of both basins. One fen was examined near the river
Niers in MoÈnchengladbach (LRE 7, Fig. 1). Grass was
growing on Quaternary point bar deposits of this water.
Due to water drainage systems installed for agriculture,
the groundwater table today does not reach the surface
and varies between 0.3 and 1.0 m below the surface. The
results for the fen di€er signi®cantly from the bog.
Across the fen, ¯ux chamber experiments revealed a
methane consumption instead of emission within the
soil. Exchange rates of