Directory UMM :Data Elmu:jurnal:O:Organic Geochemistry:Vol31.Issue5.2000:
Organic Geochemistry 31 (2000) 475±487
www.elsevier.nl/locate/orggeochem
A ®eld study of the chemical weathering of ancient
sedimentary organic matter
S.T. Petsch a,*, R.A. Berner a, T.I. Eglinton b
a
Department of Geology and Geophysics, Yale University, New Haven, CT, 06520, USA
Department of Geochemistry and Marine Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
b
Abstract
Weathering pro®les developed on organic carbon-rich black shales were studied to examine the loss and degradation
of organic matter (OM) during weathering and its role in the geochemical carbon cycle. Analysis of weathered shales
reveals between 60 and nearly 100% total organic carbon (TOC) loss in highly weathered samples relative to initial,
unweathered TOC content. Pyrite loss coincides with or precedes organic carbon loss. Elemental analysis and ¯ash
pyrolysis±gas chromatography (Py±GC) of kerogen concentrates indicate that there is little or no selective enrichment
or depletion of Norg-containing, Sorg-containing, alkylaromatic, branched alkyl or long-chain n-alkyl moieties in most
pro®les during weathering. Kerogen O/C ratios consistently increase with TOC and pyrite loss. Infrared spectroscopy
(IR) reveals an increase in the relative abundance of CC and CO bonds relative to alkyl C±H bonds in progressively
weathered samples. These results suggest a two component model for kerogen weathering: largely non-selective oxidation and hydration, followed by cleavage/dissolution of oxidized kerogen fragments. The extent of weathering in a
given outcrop is likely limited by a combination of the rate of physical erosion and exposure of the rock to oxidizing
surface waters, with OM type/composition playing a lesser role. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Black shale; Organic matter; Weathering; Oxidation; Pyrolysis; IR spectroscopy
1. Introduction
As a major component of the geochemical carbon
cycle, weathering of ancient OM consumes oxygen and
releases CO2. This forms a balance with OM burial in
young sediments that, on geologic time scales, maintains
stability in the composition of the atmosphere, much
like respiration and photosynthesis do on shorter time
scales. The factors that control the overall global rate of
ancient OM weathering (and ultimately, the O2 content
of the atmosphere) remain, however, undetermined.
This has in part limited the successful development of
mathematical models to describe the evolution of earth's
* Corresponding author.
E-mail addresses: [email protected] (S.T. Petsch),
[email protected] (R.A. Berner), teglinton@whoi.
edu (T.I. Eglinton).
atmosphere (for reviews of geochemical carbon cycle
modeling and its relation to atmospheric O2, see Holland, 1978, or Berner, 1989).
There are strong indications that the eciency of OM
remineralization to CO2 during weathering is less than
100%. Ancient OM has been detected in modern sediments by compound identi®cation (Barrick et al., 1980;
Rowland and Maxwell, 1984) and isotopic signatures
(Sackett et al., 1974). Recently, anomalously old 14C
ages have been measured in certain OM fractions in
modern sediments (Eglinton et al., 1997, 1998). The fact
that unremineralized ancient OM may pass through
several oxidizing environments between the outcrop and
redeposition raises several questions, including: How
much OM ultimately escapes weathering and remineralization and is transported to downstream sediment
reservoirs? How well does the composition of this relict
material re¯ect the bulk OM from which it is derived?
And ultimately, what controls the rate of weathering at
a given shale exposure?
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00014-0
476
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Because weathering of ancient OM has not been studied exhaustively, the above questions remain unanswered. However, several previous studies have
documented signi®cant changes in shale and OM geochemistry during weathering. A 7 m core through a
weathering pro®le developed on the Mancos Shale
(Upper Cretaceous, Utah, USA) revealed TOC loss, total
solvent extractable organic matter loss (bitumen), and an
increase in bulk organic carbon d13C (LeythaÈuser, 1973).
Weathering of the Permian Phosphoria Formation of
Utah and Upper Cretaceous Pierre Shale of Colorado
showed TOC, bitumen and d13C trends similar to Mancos Shale weathering, along with preferential loss of
aromatic versus saturated fractions of bitumen and loss
of n-alkanes relative to branched and cyclic hydrocarbons (Clayton and Swetland, 1978). Lewan (1980)
examined weathering pro®les developed on several
black shales and found trends in TOC abundance and
carbon isotopic composition consistent with the above
studies. By contrast, this author observed that bitumen
yield normalized to TOC content was actually greatest
in weathered versus unweathered samples, and suggested that cleavage of small fragments from the kerogen (solvent-insoluble material that makes up the bulk
of OM in sedimentary rocks) may play an important
role in OM weathering. This hypothesis was also suggested by Littke et al. (1991) in a study of the Posidonia
Shale (Lower Toarcian, Germany); these authors also
estimated an OM loss rate (total weathering, not solely
remineralization) of 0.5 g C mÿ3 yearÿ1 from this shale
and a pyrite weathering rate four times greater. The
weathering environment these authors examined, however, was not at steady-state and revealed signi®cant
TOC in surface (weathered) samples. Weathering of
``paper coals'' from the Brazil Formation (Lower Pennsylvanian, Indiana, USA) was shown to alter the composition of isolated kerogens (Nip et al., 1989) by
selective removal of alkylaromatic moieties, leaving
behind a (more resistant?) highly aliphatic component
corresponding to the maceral cutinite. Accumulation of
oxidized reaction products has also been demonstrated
by studies of the aqueous oxidation kinetics of pyritefree coal (Chang, 1997).
These results suggest a rough model for weathering of
organic matter in black shales. Oxidizing sur®cial ¯uids
permeate down through the rock, attacking both
organic matter and reduced mineral phases such as
pyrite. Sul®de oxidation and consequent H2SO4 production may enhance rock permeability by swelling clay
minerals, both chemically and physically breaking apart
the rock fabric. Slow oxidation and hydration of a small
portion of the kerogen may be accompanied by cleavage of
this altered portion to release ``new'' bitumen (hypothetical, sparingly water-soluble polar organic compounds)
which in turn is advected away by ¯uid ¯ow. Increase in
d13C and enrichment of highly aliphatic material in both
kerogen and bitumen during weathering may indicate
selective degradation of speci®c OM components,
although in some respects these two observations are
contradictory. Aliphatic carbon is typically isotopically
depleted relative to bulk OM (because it derives from
lipid components), and an increase in aliphatic carbon
abundance is likely to result in 13C depletion. 13C
enrichment during weathering suggests selective degradation of aliphatic, isotopically depleted OM. However,
addition of modern soil OM (which is strongly 13C
enriched relative to the OM in these rocks) coupled with
selective enrichment of highly aliphatic ancient OM
could explain these observations.
This study seeks to re®ne the above model by capturing the progressive sequence of OM compositions from
within several black shale weathering pro®les. We have
determined bulk geochemical (%TOC, % pyrite, kerogen
O/C, N/C and S/C ratios) and structural characteristics
(via IR and Py±GC) from a series of depths within
weathering pro®les. By selecting pro®les from thermally
immature, geochemically distinct black shales, we
explore the role that organic matter type and associated
composition dierences may play in controlling the rate,
eciency and selectivity of weathering. Furthermore, we
compare TOC and pyrite content within single pro®les,
and between previously glaciated versus non-glaciated
outcrops, to constrain the overall rate of carbon release
from the weathering of black shales.
2. Sample selection
Variability in weathering rates between dierent OM
types may play a strong role in controlling overall global OM remineralization. If particular OM types remineralize more rapidly than others, changes in the types
of OM exposed on the earth's surface through time may
then directly aect global carbon cycling and atmospheric composition. If the weathering characteristics of
a particular shale (rate, eciency, selectivity, oxidation
products) are in part related to OM composition,
weathering of a variety of shales may reveal information
about the reactivity of dierent types of OM within the
geosphere. Alternately, lithology and climate (expressed
through hydrology and erosion rate) may so strongly
control OM weathering that small dierences in weathering characteristics are overwhelmed. To examine
whether dierences exist in the loss and degradation of
OM of various types, three major classes of OM were
selected for this study (Table 1).
The Monterey formation was deposited in Mioceneage restricted marine basins o the coast of southern
California. Sediments are characterized by a mixture of
siliceous sediments and marine carbonates, with minor
contribution from clastic material eroded from the
emerging Coast Ranges. Rapid rates of sulfate reduction
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S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Table 1
Black shale weathering pro®les examined in this study
Formation name
Age
Exposure location
Unweathered
TOC content (%)
Organic matter
type
New Albany Shale
Late Devonian
9±12
II
Marcellus Shale
Woodford Shale
Monterey Shale
Green River Shale
Mid Devonian
Late Devonian
Miocene
Eocene
I. Deatsville, Nelson Co., Kentucky, USA
Clay City, Powell Co., Kentucky, USA
Warren, Herkimer Co., New York, USA
Arbuckle Mtns., Murray Co.,Oklahoma, USA
Gaviota Beach, Santa Barbara Co., California, USA
Nine Mile Canyon, Duchesne Co.,Utah, USA
8±10
15±25
8±20
15±18
II
II
II-S
I
coupled with low iron supply encouraged limited pyrite
formation and abundant OM sulfurization. OM sulfurization involves incorporation of sulfur into and
between otherwise labile molecules (which inhibits
degradation) and leads to generation of S-rich macromolecules (Russell et al., 1997; Sinninghe Damste et al.,
1998). OM in the Monterey is derived mainly from
marine phytoplankton and bacteria with some contribution from terrestrial plants, and is composed of
heavily S cross-linked n-alkyl chains and aromatic centers (Sinninghe Damste et al., 1989; Eglinton et al.,
1994; Schouten et al., 1995).
The Green River formation was deposited in shallow
Eocene-age lake basins formed by the rise of the Rocky
Mountains. The rock is dominated by carbonate-rich
shales. Lack of sulfate and detrital iron limited pyrite
formation and OM sulfurization. OM in the sediments
of the Green River is almost exclusively derived from
phytoplankton, and is composed mostly of long chain nalkyl fragments with minimal aromatic contribution
(Derenne et al., 1991; Eglinton, 1994). Within the
organic matrix of the Green River kerogen occur discrete particles (ultralaminae) derived from selective preservation of highly aliphatic biomacromolecules (termed
algaenan) which occur in some algal cell walls (Derenne
et al., 1991). Selective preservation of these highly aliphatic biopolymers has been documented in a variety of
lake and marine sediments (Goth et al., 1988; Derenne
et al., 1991, 1992; Flaviano et al., 1994).
The New Albany, Marcellus and Woodford formations were deposited in the Mid to Late Devonian, in
oxygen-de®cient, highly productive epeiric seas on the
craton of eastern North America. Deposited comparatively close to highlands in the east, the New Albany
and Marcellus contain abundant clay and detrital
quartz, while deeper water in the southwestern sections
of these seas far from the highlands led to lesser detrital
input and the greater silica content in the Woodford.
Marine phytoplankton provided the major source of
organic matter to these sediments, with minor contributions from bacteria and terrestrial plants (e.g.
Eglinton, 1994). Abundant detrital iron coupled with
sulfate reduction led to pyrite formation and limited
OM sulfurization during diagenesis. OM in these formations is characterized by alkyl chains of moderate
length cross-linked by C, O and S to each other and
relatively abundant aromatic centers.
Comparison of the weathering of OM between and
within these formations may provide some important
tests of susceptibility of OM to degradation. Aliphatic
biomacromolecules similar to those found in the Green
River appear to be selectively preserved during sediment
diagenesis (Goth et al., 1988; Derenne et al., 1991; Flaviano et al., 1994) and also during coal weathering (Nip
et al., 1989). Results of this study help test whether
selective preservation of highly aliphatic macromolecules is a ubiquitous feature of OM degradation.
Sulfurization of OM in recent sediments tends to
inhibit the degradation of S-containing OM relative to
S-free precursors (Sinninghe Damste et al., 1989; Russell
et al., 1997; Sinninghe Damste et al., 1998). These preserved S-rich macromolecules are interconnected by
varying degrees of S cross-linking. Results from this
study address whether the degree of cross-linking has
any eect on the relative rates of OM degradation
within a given rock. Type II kerogens, which are perhaps most representative of the bulk OM found in sedimentary rocks, are neither predominantly aliphatic nor
extensively S cross-linked. Weathering of these kerogens
may reveal the relative rates of degradation of aliphatic
versus aromatic moieties within a kerogen, as well as
determine which moieties are lost from the kerogen
during weathering, which accumulate oxidation products, and which simply remain unaltered.
3. Methods
Sampling sites were located where the transition from
unaltered rock to highly weathered shale is exposed,
shown schematically on Fig. 1. Samples were collected
along roadcuts or cli-faces. Visual signs of weathering
at the outcrops include lightening in color from black to
brown and an increase in rock ®ssility and friability,
typically over a distance of 4 m. Approximately 10±30
samples of 500 g each were obtained at intervals of
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S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
material into thin (100 mm) pellets. Transmission IR
spectra were obtained using a Bio-Rad (Digilab) FTS
175 infrared spectrometer at a resolution of 4 cmÿ1.
Samples were prepared for Py±GC by pressing 1±2 mg of
kerogen with a known mass of added internal standard
(poly-t-butylstyrene) onto an Fe±Ni wire. Flash pyrolysis (610 C, 5 s) was achieved using a FOM-3LX
Curie Point pyrolyzer/Horizons Instruments RF generator. On-line analysis of the pyrolysates was achieved
by interfacing the pyrolysis unit to an HP 5890-II gas
chromatograph equipped with a Restek Rtx-1 capillary
column (60 m0.32 mm i.d.; ®lm thickness 0.5 mm;
temperature program: 0 C for 5 min, ramp rate 3 C
minÿ1 to 320 C, hold for 20 min, He carrier gas, 1 ml
minÿ1 ¯ow rate; ¯ame ionization detector).
4. Results
Fig. 1. Schematic drawing of generalized sampling site. A
recent exposure (usually a roadcut) exposes a rind of weathered
material (lightly hatched in drawing) conforming to topography and overlying unaltered black shale (heavily hatched).
Shale samples are taken from along the face of the exposure
from unaltered shale at the center of the exposure to highly
weathered shale at the sides of the exposure (white circles trace
sampled pro®le), following a single stratigraphic horizon. By
measuring the slope of the hillside and dip of the strata, the
horizontal distance traced by the white circles is converted into
a depth of pro®le normal to the hillside.
20±50 cm along the exposed pro®les. Care was taken at
each site to collect samples from a single stratigraphic
horizon to avoid any bed-to-bed heterogeneity. Also, 5±
10 cm of thin coatings and detritus, resulting from
recent weathering on the exposed surface, were removed
from the face of each exposure prior to sampling.
Total carbon, carbonate carbon and TOC were
determined following the method of Krom and Berner
(1983). TOC contents were also obtained on selected
acidi®ed (decarbonated) samples; good agreement with
the Krom and Berner method was observed. Pyrite sulfur content was determined by liberating H2S from the
shale during digestion in acidic CrCl2, trapping the sul®de with zinc acetate, and titrating with KIO3 (Can®eld
et al., 1986).
Powdered whole rock samples were ultrasonically
agitated for 15 min in (93:7 v/v) CH2Cl2:CH3OH to
extract bitumen. Extracted rock powders were then
demineralized under N2 in PTFE bombs at 40 C using
standard HCl/HCl:HF/HCl digestion procedure (Durand and Nicaise, 1980). Elemental analysis (CNS/O) of
kerogen concentrates was determined using a Carlo
Erba EA1108 elemental analyzer. Samples were prepared for IR analysis by adding 100 mg of vacuumdried kerogen to 900 mg IR grade KBr, storing the
mixture overnight in a dessicator, and pressing the
The TOC pro®les for the Monterey, Green River,
New Albany, Woodford and Marcellus shales are
shown in Fig. 2. Pro®les extend from zero meters (the
top of the soil surface) to ®ve meters into the hillside.
The New Albany shale (Fig. 2A) shows a smooth and
gradual carbon loss within the top 2±3 m of the pro®les.
A similar trend is observed for the available 2 m pro®le
of the Marcellus shale. Surface samples contain between
1 and 2.5% TOC, indicating that weathering is not
complete (OM is not completely released before erosion)
in the outcrop at these sites. The Green River (2B) and
Woodford (2C) pro®les indicate more than an order-ofmagnitude TOC loss during weathering. The two outlier
points (at 0.4 m depth) in Fig. 2B re¯ect samples
obtained from an observed rootmat in the soil and
record abundant modern soil carbon (Petsch and Eglinton, in prep.). Both the Green River and Woodford
TOC pro®les reveal less than 1% TOC at the pro®le
surface, indicating that weathering is more ecient at
these sites than the New Albany sites. The Monterey
TOC pro®le (2D) reveals much less TOC loss than
found in the other pro®les, possibly as a result of rapid
physical erosion at the sampling site (a rapidly eroding
sea-cli). Our measured TOC contents at depth
(unweathered samples) for all formations agree well
with published literature values for unaltered samples of
these shales (Eglinton, 1994, Tegelaar and Noble, 1994).
Inspection of TOC and pyrite pro®les (Fig. 3) reveals
that for all formations, pyrite loss coincides with or
precedes TOC loss during weathering. In all pro®les,
pyrite content approaches 0% at or below the top of the
pro®le, suggesting that pyrite weathering is 100% ecient at these sites while OM weathering is not.
Kerogen C, N, S and O contents are expressed on Fig. 4
as along-pro®le N/C, S/C and O/C atomic ratios. N/C and
S/C ratios are approximately constant for each formation
irrespective of the extent of weathering. This indicates that
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
479
Fig. 2. Variations in total organic carbon content (TOC) with depth into black shale weathering pro®les for the New Albany and
Marcellus shales (A), Green River Shale (B), Woodford Chert (C) and Monterey Shale (D).
kerogen from each formation has a distinct N/C and S/C
atomic ratio that is preserved during weathering. N/C
ratios do slightly increase in surface samples of two
pro®les (Green River and New Albany II), which may
indicate incorporation of modern soil OM. As expected,
S/C ratios are lowest in the lacustrine Green River
samples and highest in the highly sulfurized Monterey
samples. It is uncertain whether the slight decrease in
Monterey S/C ratios with weathering is signi®cant. The
large S/C ratio decrease observed at depth in the New
Albany I pro®le is not repeated in the New Albany II,
and may re¯ect incompletely removed pyrite sulfur. O/C
ratios increase with weathering of the New Albany pro®les, coincident with TOC loss beginning at 2 m depth.
With the exception of a single surface point (which
likely re¯ects modern lignin- or carbohydrate-rich OM)
Green River O/C ratios increase only modestly. The
trend in Monterey O/C ratios is less clear, but suggests
an O/C increase beginning relatively deep in the pro®le.
By comparing IR spectra for a suite of kerogens isolated from various depths within each pro®le, trends are
revealed in relative bond abundance with weathering.
For example, comparison of 6 kerogen concentrates
from the New Albany (Fig. 5A) reveals a marked
decrease during weathering in absorbance of bands
centered at 2930 and 2855 cmÿ1 (corresponding to
stretching of alkyl -CH2- and -CH3 groups) and at 1460
and 1375 cmÿ1 (corresponding to bending of alkyl -CH2and -CH3 groups) as well as ingrowth of absorbance of
bands near 1700 cmÿ1 (CO stretching) and near
1640 cmÿ1 (CC stretching). Similar trends are observed
in a suite of kerogens from the Green River weathering
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S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Fig. 3. Variations in pyrite sulfur content with depth into black shale weathering pro®les for the New Albany shales (A), Green River
Shale (B), Woodford Chert (C) and Monterey Shale (D).
pro®le (Fig. 5B) and Monterey pro®le (Fig. 5C), with
the exceptions that alkyl absorbance loss is less apparent
in the Monterey pro®le and is non-existent in the Green
River pro®le. While ingrowth of CO bonds is easy to
understand as a product of kerogen oxidation, the
increase in relative CC abundance is less intuitive.
However, the transition from alkane to alkene is a formal oxidation and perhaps CC bonds (newly formed
and/or pre-existing) are less soluble or less readily
cleaved during weathering and represent addition or
mild selective enrichment of aromatic or ole®nic moieties.
Kerogen compositions as revealed by Py±GC are
remarkably similar for all depths within the New
Albany and Monterey pro®les; variations are more distinct
within the Green River pro®le. In all cases, pyrograms are
dominated by a homologous series of n-alk-1-ene/nalkane doublets, with small contributions from alkylbenzenes, alkylthiophenes and branched hydrocarbons.
Very little variation is observed between weathered and
unweathered samples of New Albany kerogen (Fig. 6),
in spite of the nearly 5-fold loss in TOC content. Comparison of weathered and unweathered samples of
Green River kerogen (Fig. 7) reveals a marked loss of
isoprenoid and lower molecular weight alkyl fragments.
Dierences in pyrolysate composition with weathering
were more rigorously quanti®ed by calculating values
for 4 abundance ratio indices: methylthiophene/(toluene+oct-1-ene), alkylbenzenes/n-alk-1-enes, long chain/
short chain n-alk-1-enes, and isoprenoids/n-alk-1-enes
(Table 2). Values calculated for these indices reveal variations in kerogen composition within each weathering
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
pro®le (Fig. 8). Given the scatter of the data, the invariance of the thiophene index (Fig. 8A) agrees well with
constant kerogen S/C ratios for each pro®le. Interestingly, while Monterey S/C ratios revealed a slight
decrease with weathering, Monterey thiophene index
values are roughly constant. Values of the aromaticity
index consistently indicate a slight decrease in the
abundance of C1,2-alkylbenzenes relative to n-alkyl
moieties during weathering (Fig. 8B), with the exception
that New Albany II samples indicate a rather strong
increase in alkylbenzene abundance in surface samples.
This dierence remains unexplained, but may relate to
modern (higher plant) OM. Chain length index values
are constant for the New Albany pro®les, but progressively increase through the Green River pro®le (Fig.
8C). The chain length index trend for the Monterey
pro®le is unclear. The values for the isoprenoid index
are constant for the New Albany and Monterey pro®les,
but reveal that branched hydrocarbon moieties are preferentially removed during weathering of the Green
River Shale (Fig. 8D).
5. Discussion
Inspection of the TOC pro®les in Fig. 2 reveals that
OM degradation is not complete at any examined pro®le, because OM remains in surface samples to be eroded and transported to downstream sediments.
Furthermore, weathering has removed much more OM
from the Green River and Woodford outcrops than
from those of the New Albany, Marcellus or Monterey.
While tempting to assign this variable TOC loss to differences in OM composition, this is unlikely to be the
481
case. The Woodford and Green River are very dissimilar OM types (the Woodford being much more similar
to the New Albany). What the Woodford and Green
River do share is similar erosion rate and hydrology.
Because the Woodford is a siliceous shale and the Green
River is a carbonate shale, these two formations are
exposed as coherent, rather impermeable rocks in warm,
arid regions. In outcrop these rocks are less ®ssile and
friable than the New Albany, Marcellus or Monterey,
and hence may be more resistant to physical erosion. As
a result, we infer that a given volume of Woodford or
Green River rock remains in the outcrop and is exposed
to oxidizing surface ¯uids longer than the New Albany,
Marcellus or Monterey. Drier conditions (and deeper
water tables) facilitate greater penetration of surface
waters, expressed as greater depths of TOC loss in these
pro®les. The Monterey pro®le may represent the opposite extreme; this exposure is marked by very friable
rock and rapid physical erosion. At this site, insucient
time is aorded for signi®cant OM degradation because
the rock is not held in the outcrop for very long times,
and thus TOC loss is not as severe as in the other pro®les.
A common feature of these TOC pro®les is an ``S''
shape of constant (high) TOC at depth, a zone of
rapidly decreasing TOC content, and a zone of constant
(low) TOC towards the surface. This is particularly
apparent in the Green River and New Albany II pro®les. This shape (if inverted and reversed) resembles
TOC depth pro®les during early diagenesis, and can be
explained qualitatively in terms of a model that considers the reactants O2 and organic carbon using a
steady state 1-G diagenetic model (Berner, 1980) but
with the earthõÂs surface as the equivalent of the lower
Fig. 4. Variations in elemental composition of kerogens plotted against fractional TOC loss across black shale weathering pro®les.
Shown are Kerogen N/C ratios (A), S/C ratios (B) and O/C ratio (C).
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S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Fig. 5. Fourier transform infared spectra for kerogen concentrates from selected depths in the New Albany (A), Green
River (B) and Monterey (C) weathering pro®les.
boundary condition for diagenesis and sediment burial
rate replaced by erosion rate. Finer-scale features of the
pro®le may be accounted for by including pyrite oxidation and multiple pools of OM with dierent reactivities. In surface regions of the pro®les, the rate of OM
oxidation may be limited by the reactivity/abundance of
residual organic matter where TOC contents are low.
This is appropriate where the rock contains interstitial
space that is bathed in O2 or O2-containing water (or
some other oxidant such as organic peroxides). However, deeper into the shale permeability decreases considerably and access of OM to O2 (via diusion or
advection) becomes the limiting factor. At these depths
TOC will be much higher. It should be noted that TOC
content declines rapidly in each pro®le only above the
depth where pyrite content falls to zero, and thus where
O2 demand by pyrite oxidation is reduced. It is likely
that a combination of rate limitation by OM reactivity
in well-oxygenated depths of the pro®le and O2-limitation at greater depths, coupled with the regional erosion
rate, could be used to explain the S-shaped pro®les in all
of the examined shales. Larger TOC loss (i.e. Green
River and Woodford) corresponds to limited physical
erosion and greater contact time between OM and oxidizing surface waters; lesser TOC loss (i.e. Monterey)
corresponds to more rapid erosion and shorter OM-O2
contact time. Control of OM abundance by cumulative
oxygen exposure time has been documented in modern
sediments (Hartnett et al., 1998); it is not unlikely that
OM weathering may have similar controls.
It should be mentioned that the TOC gradient
observed in outcrop reveals little information about the
overall resistance of OM to weathering and remineralization for at least two reasons. One, it is unknown
whether TOC loss within a pro®le occurs as production
of CO2, generation of soluble oxidized OM, or cleavage
of otherwise unaltered, intact kerogen fragments, and
thus O2 consumption and CO2 production cannot be
inferred from TOC pro®les directly. Two, even minimal
TOC loss within a pro®le indicates nothing about subsequent OM oxidation during transport and storage in
downstream sediment reservoirs before anoxic reburial,
and the time spent within these reservoirs may have a
strong eect on dictating overall OM remineralization.
Although similar to the New Albany in age and OM
type, the outcrop region of the Marcellus Shale was
repeatedly scoured by advance and retreat of the Laurentide Ice Sheet, a process that resulted in removal of
weathered material and exposure of unaltered bedrock
after the ®nal ice sheet retreat some 15 k years ago
(Fleisher, 1986). If the New Albany pro®les are taken to
be at steady-state with respect to chemical weathering
and physical erosion, and given that the Marcellus and
New Albany outcrops exhibit roughly similar organic
matter types, lithology and hydrology, then the coincidence of the TOC pro®les for these two formations
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
483
Fig. 6. Pair of pyrograms comparing weathered and unweathered samples of kerogen isolated from the New Albany Shale weathering
pro®le. Labelled are carbon numbers of selected n-alkene/alkane doublets, C14 and C19 isoprenoids (star), internal standard (X) and
other components relating to pyrolysis indices.
suggests that the Marcellus is also at steady state and
that a maximum of 15 k year is required to develop a
complete weathering pro®le at these sites. Comparisons
between TOC and pyrite data within each pro®le provide another constraint on OM weathering rates. Pyrite
loss precedes or coincides with TOC loss from these
pro®les, suggesting that the kinetics of OM weathering
can be no faster than pyrite oxidation. A mass transfer
model using known pyrite oxidation kinetics and reasonable ¯uid ¯ow could be used to recreate TOC and
pyrite data and provide an estimate of bulk OM weathering rates at these sites.
While large dierences in OM composition do not
develop betweeen weathered and unweathered shales
(increases in O/C ratios excepted), each formation does
reveal individual characteristics during weathering that
re¯ect at least in part OM composition and reactivity.
Weathering of the New Albany Shale is largely nonselective. OM is homogeneously degraded and lost during
weathering, without signi®cant relative loss/gain of Nor S-containing moieties or changes in the relative
abundance of the components measured by the four
pyrolysis indices. Oxidation products accumulate within
the kerogen during weathering of the New Albany. This
is seen both in the increase in O/C ratios with TOC loss
(Fig. 4C), and in the IR spectra (Fig. 5A). Infrared
spectroscopy also reveals a signi®cant loss in alkyl C±H
bonds relative to CO and CC bonds. It is surprising
that this loss is not observed in pyrolysis, but may be
explained if the alkyl bonds indicated as lost by IR
derive from mainly C1±C4 fragments, which are not
accounted for in the designed Py±GC experiment. Whether CC bonds are selective enriched during weathering
or form as weathering products must be resolved by
further study. Furthermore, the percent of total OM
accounted for by GC-amenable pyrolysis products is
small (
www.elsevier.nl/locate/orggeochem
A ®eld study of the chemical weathering of ancient
sedimentary organic matter
S.T. Petsch a,*, R.A. Berner a, T.I. Eglinton b
a
Department of Geology and Geophysics, Yale University, New Haven, CT, 06520, USA
Department of Geochemistry and Marine Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
b
Abstract
Weathering pro®les developed on organic carbon-rich black shales were studied to examine the loss and degradation
of organic matter (OM) during weathering and its role in the geochemical carbon cycle. Analysis of weathered shales
reveals between 60 and nearly 100% total organic carbon (TOC) loss in highly weathered samples relative to initial,
unweathered TOC content. Pyrite loss coincides with or precedes organic carbon loss. Elemental analysis and ¯ash
pyrolysis±gas chromatography (Py±GC) of kerogen concentrates indicate that there is little or no selective enrichment
or depletion of Norg-containing, Sorg-containing, alkylaromatic, branched alkyl or long-chain n-alkyl moieties in most
pro®les during weathering. Kerogen O/C ratios consistently increase with TOC and pyrite loss. Infrared spectroscopy
(IR) reveals an increase in the relative abundance of CC and CO bonds relative to alkyl C±H bonds in progressively
weathered samples. These results suggest a two component model for kerogen weathering: largely non-selective oxidation and hydration, followed by cleavage/dissolution of oxidized kerogen fragments. The extent of weathering in a
given outcrop is likely limited by a combination of the rate of physical erosion and exposure of the rock to oxidizing
surface waters, with OM type/composition playing a lesser role. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: Black shale; Organic matter; Weathering; Oxidation; Pyrolysis; IR spectroscopy
1. Introduction
As a major component of the geochemical carbon
cycle, weathering of ancient OM consumes oxygen and
releases CO2. This forms a balance with OM burial in
young sediments that, on geologic time scales, maintains
stability in the composition of the atmosphere, much
like respiration and photosynthesis do on shorter time
scales. The factors that control the overall global rate of
ancient OM weathering (and ultimately, the O2 content
of the atmosphere) remain, however, undetermined.
This has in part limited the successful development of
mathematical models to describe the evolution of earth's
* Corresponding author.
E-mail addresses: [email protected] (S.T. Petsch),
[email protected] (R.A. Berner), teglinton@whoi.
edu (T.I. Eglinton).
atmosphere (for reviews of geochemical carbon cycle
modeling and its relation to atmospheric O2, see Holland, 1978, or Berner, 1989).
There are strong indications that the eciency of OM
remineralization to CO2 during weathering is less than
100%. Ancient OM has been detected in modern sediments by compound identi®cation (Barrick et al., 1980;
Rowland and Maxwell, 1984) and isotopic signatures
(Sackett et al., 1974). Recently, anomalously old 14C
ages have been measured in certain OM fractions in
modern sediments (Eglinton et al., 1997, 1998). The fact
that unremineralized ancient OM may pass through
several oxidizing environments between the outcrop and
redeposition raises several questions, including: How
much OM ultimately escapes weathering and remineralization and is transported to downstream sediment
reservoirs? How well does the composition of this relict
material re¯ect the bulk OM from which it is derived?
And ultimately, what controls the rate of weathering at
a given shale exposure?
0146-6380/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0146-6380(00)00014-0
476
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Because weathering of ancient OM has not been studied exhaustively, the above questions remain unanswered. However, several previous studies have
documented signi®cant changes in shale and OM geochemistry during weathering. A 7 m core through a
weathering pro®le developed on the Mancos Shale
(Upper Cretaceous, Utah, USA) revealed TOC loss, total
solvent extractable organic matter loss (bitumen), and an
increase in bulk organic carbon d13C (LeythaÈuser, 1973).
Weathering of the Permian Phosphoria Formation of
Utah and Upper Cretaceous Pierre Shale of Colorado
showed TOC, bitumen and d13C trends similar to Mancos Shale weathering, along with preferential loss of
aromatic versus saturated fractions of bitumen and loss
of n-alkanes relative to branched and cyclic hydrocarbons (Clayton and Swetland, 1978). Lewan (1980)
examined weathering pro®les developed on several
black shales and found trends in TOC abundance and
carbon isotopic composition consistent with the above
studies. By contrast, this author observed that bitumen
yield normalized to TOC content was actually greatest
in weathered versus unweathered samples, and suggested that cleavage of small fragments from the kerogen (solvent-insoluble material that makes up the bulk
of OM in sedimentary rocks) may play an important
role in OM weathering. This hypothesis was also suggested by Littke et al. (1991) in a study of the Posidonia
Shale (Lower Toarcian, Germany); these authors also
estimated an OM loss rate (total weathering, not solely
remineralization) of 0.5 g C mÿ3 yearÿ1 from this shale
and a pyrite weathering rate four times greater. The
weathering environment these authors examined, however, was not at steady-state and revealed signi®cant
TOC in surface (weathered) samples. Weathering of
``paper coals'' from the Brazil Formation (Lower Pennsylvanian, Indiana, USA) was shown to alter the composition of isolated kerogens (Nip et al., 1989) by
selective removal of alkylaromatic moieties, leaving
behind a (more resistant?) highly aliphatic component
corresponding to the maceral cutinite. Accumulation of
oxidized reaction products has also been demonstrated
by studies of the aqueous oxidation kinetics of pyritefree coal (Chang, 1997).
These results suggest a rough model for weathering of
organic matter in black shales. Oxidizing sur®cial ¯uids
permeate down through the rock, attacking both
organic matter and reduced mineral phases such as
pyrite. Sul®de oxidation and consequent H2SO4 production may enhance rock permeability by swelling clay
minerals, both chemically and physically breaking apart
the rock fabric. Slow oxidation and hydration of a small
portion of the kerogen may be accompanied by cleavage of
this altered portion to release ``new'' bitumen (hypothetical, sparingly water-soluble polar organic compounds)
which in turn is advected away by ¯uid ¯ow. Increase in
d13C and enrichment of highly aliphatic material in both
kerogen and bitumen during weathering may indicate
selective degradation of speci®c OM components,
although in some respects these two observations are
contradictory. Aliphatic carbon is typically isotopically
depleted relative to bulk OM (because it derives from
lipid components), and an increase in aliphatic carbon
abundance is likely to result in 13C depletion. 13C
enrichment during weathering suggests selective degradation of aliphatic, isotopically depleted OM. However,
addition of modern soil OM (which is strongly 13C
enriched relative to the OM in these rocks) coupled with
selective enrichment of highly aliphatic ancient OM
could explain these observations.
This study seeks to re®ne the above model by capturing the progressive sequence of OM compositions from
within several black shale weathering pro®les. We have
determined bulk geochemical (%TOC, % pyrite, kerogen
O/C, N/C and S/C ratios) and structural characteristics
(via IR and Py±GC) from a series of depths within
weathering pro®les. By selecting pro®les from thermally
immature, geochemically distinct black shales, we
explore the role that organic matter type and associated
composition dierences may play in controlling the rate,
eciency and selectivity of weathering. Furthermore, we
compare TOC and pyrite content within single pro®les,
and between previously glaciated versus non-glaciated
outcrops, to constrain the overall rate of carbon release
from the weathering of black shales.
2. Sample selection
Variability in weathering rates between dierent OM
types may play a strong role in controlling overall global OM remineralization. If particular OM types remineralize more rapidly than others, changes in the types
of OM exposed on the earth's surface through time may
then directly aect global carbon cycling and atmospheric composition. If the weathering characteristics of
a particular shale (rate, eciency, selectivity, oxidation
products) are in part related to OM composition,
weathering of a variety of shales may reveal information
about the reactivity of dierent types of OM within the
geosphere. Alternately, lithology and climate (expressed
through hydrology and erosion rate) may so strongly
control OM weathering that small dierences in weathering characteristics are overwhelmed. To examine
whether dierences exist in the loss and degradation of
OM of various types, three major classes of OM were
selected for this study (Table 1).
The Monterey formation was deposited in Mioceneage restricted marine basins o the coast of southern
California. Sediments are characterized by a mixture of
siliceous sediments and marine carbonates, with minor
contribution from clastic material eroded from the
emerging Coast Ranges. Rapid rates of sulfate reduction
477
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Table 1
Black shale weathering pro®les examined in this study
Formation name
Age
Exposure location
Unweathered
TOC content (%)
Organic matter
type
New Albany Shale
Late Devonian
9±12
II
Marcellus Shale
Woodford Shale
Monterey Shale
Green River Shale
Mid Devonian
Late Devonian
Miocene
Eocene
I. Deatsville, Nelson Co., Kentucky, USA
Clay City, Powell Co., Kentucky, USA
Warren, Herkimer Co., New York, USA
Arbuckle Mtns., Murray Co.,Oklahoma, USA
Gaviota Beach, Santa Barbara Co., California, USA
Nine Mile Canyon, Duchesne Co.,Utah, USA
8±10
15±25
8±20
15±18
II
II
II-S
I
coupled with low iron supply encouraged limited pyrite
formation and abundant OM sulfurization. OM sulfurization involves incorporation of sulfur into and
between otherwise labile molecules (which inhibits
degradation) and leads to generation of S-rich macromolecules (Russell et al., 1997; Sinninghe Damste et al.,
1998). OM in the Monterey is derived mainly from
marine phytoplankton and bacteria with some contribution from terrestrial plants, and is composed of
heavily S cross-linked n-alkyl chains and aromatic centers (Sinninghe Damste et al., 1989; Eglinton et al.,
1994; Schouten et al., 1995).
The Green River formation was deposited in shallow
Eocene-age lake basins formed by the rise of the Rocky
Mountains. The rock is dominated by carbonate-rich
shales. Lack of sulfate and detrital iron limited pyrite
formation and OM sulfurization. OM in the sediments
of the Green River is almost exclusively derived from
phytoplankton, and is composed mostly of long chain nalkyl fragments with minimal aromatic contribution
(Derenne et al., 1991; Eglinton, 1994). Within the
organic matrix of the Green River kerogen occur discrete particles (ultralaminae) derived from selective preservation of highly aliphatic biomacromolecules (termed
algaenan) which occur in some algal cell walls (Derenne
et al., 1991). Selective preservation of these highly aliphatic biopolymers has been documented in a variety of
lake and marine sediments (Goth et al., 1988; Derenne
et al., 1991, 1992; Flaviano et al., 1994).
The New Albany, Marcellus and Woodford formations were deposited in the Mid to Late Devonian, in
oxygen-de®cient, highly productive epeiric seas on the
craton of eastern North America. Deposited comparatively close to highlands in the east, the New Albany
and Marcellus contain abundant clay and detrital
quartz, while deeper water in the southwestern sections
of these seas far from the highlands led to lesser detrital
input and the greater silica content in the Woodford.
Marine phytoplankton provided the major source of
organic matter to these sediments, with minor contributions from bacteria and terrestrial plants (e.g.
Eglinton, 1994). Abundant detrital iron coupled with
sulfate reduction led to pyrite formation and limited
OM sulfurization during diagenesis. OM in these formations is characterized by alkyl chains of moderate
length cross-linked by C, O and S to each other and
relatively abundant aromatic centers.
Comparison of the weathering of OM between and
within these formations may provide some important
tests of susceptibility of OM to degradation. Aliphatic
biomacromolecules similar to those found in the Green
River appear to be selectively preserved during sediment
diagenesis (Goth et al., 1988; Derenne et al., 1991; Flaviano et al., 1994) and also during coal weathering (Nip
et al., 1989). Results of this study help test whether
selective preservation of highly aliphatic macromolecules is a ubiquitous feature of OM degradation.
Sulfurization of OM in recent sediments tends to
inhibit the degradation of S-containing OM relative to
S-free precursors (Sinninghe Damste et al., 1989; Russell
et al., 1997; Sinninghe Damste et al., 1998). These preserved S-rich macromolecules are interconnected by
varying degrees of S cross-linking. Results from this
study address whether the degree of cross-linking has
any eect on the relative rates of OM degradation
within a given rock. Type II kerogens, which are perhaps most representative of the bulk OM found in sedimentary rocks, are neither predominantly aliphatic nor
extensively S cross-linked. Weathering of these kerogens
may reveal the relative rates of degradation of aliphatic
versus aromatic moieties within a kerogen, as well as
determine which moieties are lost from the kerogen
during weathering, which accumulate oxidation products, and which simply remain unaltered.
3. Methods
Sampling sites were located where the transition from
unaltered rock to highly weathered shale is exposed,
shown schematically on Fig. 1. Samples were collected
along roadcuts or cli-faces. Visual signs of weathering
at the outcrops include lightening in color from black to
brown and an increase in rock ®ssility and friability,
typically over a distance of 4 m. Approximately 10±30
samples of 500 g each were obtained at intervals of
478
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
material into thin (100 mm) pellets. Transmission IR
spectra were obtained using a Bio-Rad (Digilab) FTS
175 infrared spectrometer at a resolution of 4 cmÿ1.
Samples were prepared for Py±GC by pressing 1±2 mg of
kerogen with a known mass of added internal standard
(poly-t-butylstyrene) onto an Fe±Ni wire. Flash pyrolysis (610 C, 5 s) was achieved using a FOM-3LX
Curie Point pyrolyzer/Horizons Instruments RF generator. On-line analysis of the pyrolysates was achieved
by interfacing the pyrolysis unit to an HP 5890-II gas
chromatograph equipped with a Restek Rtx-1 capillary
column (60 m0.32 mm i.d.; ®lm thickness 0.5 mm;
temperature program: 0 C for 5 min, ramp rate 3 C
minÿ1 to 320 C, hold for 20 min, He carrier gas, 1 ml
minÿ1 ¯ow rate; ¯ame ionization detector).
4. Results
Fig. 1. Schematic drawing of generalized sampling site. A
recent exposure (usually a roadcut) exposes a rind of weathered
material (lightly hatched in drawing) conforming to topography and overlying unaltered black shale (heavily hatched).
Shale samples are taken from along the face of the exposure
from unaltered shale at the center of the exposure to highly
weathered shale at the sides of the exposure (white circles trace
sampled pro®le), following a single stratigraphic horizon. By
measuring the slope of the hillside and dip of the strata, the
horizontal distance traced by the white circles is converted into
a depth of pro®le normal to the hillside.
20±50 cm along the exposed pro®les. Care was taken at
each site to collect samples from a single stratigraphic
horizon to avoid any bed-to-bed heterogeneity. Also, 5±
10 cm of thin coatings and detritus, resulting from
recent weathering on the exposed surface, were removed
from the face of each exposure prior to sampling.
Total carbon, carbonate carbon and TOC were
determined following the method of Krom and Berner
(1983). TOC contents were also obtained on selected
acidi®ed (decarbonated) samples; good agreement with
the Krom and Berner method was observed. Pyrite sulfur content was determined by liberating H2S from the
shale during digestion in acidic CrCl2, trapping the sul®de with zinc acetate, and titrating with KIO3 (Can®eld
et al., 1986).
Powdered whole rock samples were ultrasonically
agitated for 15 min in (93:7 v/v) CH2Cl2:CH3OH to
extract bitumen. Extracted rock powders were then
demineralized under N2 in PTFE bombs at 40 C using
standard HCl/HCl:HF/HCl digestion procedure (Durand and Nicaise, 1980). Elemental analysis (CNS/O) of
kerogen concentrates was determined using a Carlo
Erba EA1108 elemental analyzer. Samples were prepared for IR analysis by adding 100 mg of vacuumdried kerogen to 900 mg IR grade KBr, storing the
mixture overnight in a dessicator, and pressing the
The TOC pro®les for the Monterey, Green River,
New Albany, Woodford and Marcellus shales are
shown in Fig. 2. Pro®les extend from zero meters (the
top of the soil surface) to ®ve meters into the hillside.
The New Albany shale (Fig. 2A) shows a smooth and
gradual carbon loss within the top 2±3 m of the pro®les.
A similar trend is observed for the available 2 m pro®le
of the Marcellus shale. Surface samples contain between
1 and 2.5% TOC, indicating that weathering is not
complete (OM is not completely released before erosion)
in the outcrop at these sites. The Green River (2B) and
Woodford (2C) pro®les indicate more than an order-ofmagnitude TOC loss during weathering. The two outlier
points (at 0.4 m depth) in Fig. 2B re¯ect samples
obtained from an observed rootmat in the soil and
record abundant modern soil carbon (Petsch and Eglinton, in prep.). Both the Green River and Woodford
TOC pro®les reveal less than 1% TOC at the pro®le
surface, indicating that weathering is more ecient at
these sites than the New Albany sites. The Monterey
TOC pro®le (2D) reveals much less TOC loss than
found in the other pro®les, possibly as a result of rapid
physical erosion at the sampling site (a rapidly eroding
sea-cli). Our measured TOC contents at depth
(unweathered samples) for all formations agree well
with published literature values for unaltered samples of
these shales (Eglinton, 1994, Tegelaar and Noble, 1994).
Inspection of TOC and pyrite pro®les (Fig. 3) reveals
that for all formations, pyrite loss coincides with or
precedes TOC loss during weathering. In all pro®les,
pyrite content approaches 0% at or below the top of the
pro®le, suggesting that pyrite weathering is 100% ecient at these sites while OM weathering is not.
Kerogen C, N, S and O contents are expressed on Fig. 4
as along-pro®le N/C, S/C and O/C atomic ratios. N/C and
S/C ratios are approximately constant for each formation
irrespective of the extent of weathering. This indicates that
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
479
Fig. 2. Variations in total organic carbon content (TOC) with depth into black shale weathering pro®les for the New Albany and
Marcellus shales (A), Green River Shale (B), Woodford Chert (C) and Monterey Shale (D).
kerogen from each formation has a distinct N/C and S/C
atomic ratio that is preserved during weathering. N/C
ratios do slightly increase in surface samples of two
pro®les (Green River and New Albany II), which may
indicate incorporation of modern soil OM. As expected,
S/C ratios are lowest in the lacustrine Green River
samples and highest in the highly sulfurized Monterey
samples. It is uncertain whether the slight decrease in
Monterey S/C ratios with weathering is signi®cant. The
large S/C ratio decrease observed at depth in the New
Albany I pro®le is not repeated in the New Albany II,
and may re¯ect incompletely removed pyrite sulfur. O/C
ratios increase with weathering of the New Albany pro®les, coincident with TOC loss beginning at 2 m depth.
With the exception of a single surface point (which
likely re¯ects modern lignin- or carbohydrate-rich OM)
Green River O/C ratios increase only modestly. The
trend in Monterey O/C ratios is less clear, but suggests
an O/C increase beginning relatively deep in the pro®le.
By comparing IR spectra for a suite of kerogens isolated from various depths within each pro®le, trends are
revealed in relative bond abundance with weathering.
For example, comparison of 6 kerogen concentrates
from the New Albany (Fig. 5A) reveals a marked
decrease during weathering in absorbance of bands
centered at 2930 and 2855 cmÿ1 (corresponding to
stretching of alkyl -CH2- and -CH3 groups) and at 1460
and 1375 cmÿ1 (corresponding to bending of alkyl -CH2and -CH3 groups) as well as ingrowth of absorbance of
bands near 1700 cmÿ1 (CO stretching) and near
1640 cmÿ1 (CC stretching). Similar trends are observed
in a suite of kerogens from the Green River weathering
480
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Fig. 3. Variations in pyrite sulfur content with depth into black shale weathering pro®les for the New Albany shales (A), Green River
Shale (B), Woodford Chert (C) and Monterey Shale (D).
pro®le (Fig. 5B) and Monterey pro®le (Fig. 5C), with
the exceptions that alkyl absorbance loss is less apparent
in the Monterey pro®le and is non-existent in the Green
River pro®le. While ingrowth of CO bonds is easy to
understand as a product of kerogen oxidation, the
increase in relative CC abundance is less intuitive.
However, the transition from alkane to alkene is a formal oxidation and perhaps CC bonds (newly formed
and/or pre-existing) are less soluble or less readily
cleaved during weathering and represent addition or
mild selective enrichment of aromatic or ole®nic moieties.
Kerogen compositions as revealed by Py±GC are
remarkably similar for all depths within the New
Albany and Monterey pro®les; variations are more distinct
within the Green River pro®le. In all cases, pyrograms are
dominated by a homologous series of n-alk-1-ene/nalkane doublets, with small contributions from alkylbenzenes, alkylthiophenes and branched hydrocarbons.
Very little variation is observed between weathered and
unweathered samples of New Albany kerogen (Fig. 6),
in spite of the nearly 5-fold loss in TOC content. Comparison of weathered and unweathered samples of
Green River kerogen (Fig. 7) reveals a marked loss of
isoprenoid and lower molecular weight alkyl fragments.
Dierences in pyrolysate composition with weathering
were more rigorously quanti®ed by calculating values
for 4 abundance ratio indices: methylthiophene/(toluene+oct-1-ene), alkylbenzenes/n-alk-1-enes, long chain/
short chain n-alk-1-enes, and isoprenoids/n-alk-1-enes
(Table 2). Values calculated for these indices reveal variations in kerogen composition within each weathering
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
pro®le (Fig. 8). Given the scatter of the data, the invariance of the thiophene index (Fig. 8A) agrees well with
constant kerogen S/C ratios for each pro®le. Interestingly, while Monterey S/C ratios revealed a slight
decrease with weathering, Monterey thiophene index
values are roughly constant. Values of the aromaticity
index consistently indicate a slight decrease in the
abundance of C1,2-alkylbenzenes relative to n-alkyl
moieties during weathering (Fig. 8B), with the exception
that New Albany II samples indicate a rather strong
increase in alkylbenzene abundance in surface samples.
This dierence remains unexplained, but may relate to
modern (higher plant) OM. Chain length index values
are constant for the New Albany pro®les, but progressively increase through the Green River pro®le (Fig.
8C). The chain length index trend for the Monterey
pro®le is unclear. The values for the isoprenoid index
are constant for the New Albany and Monterey pro®les,
but reveal that branched hydrocarbon moieties are preferentially removed during weathering of the Green
River Shale (Fig. 8D).
5. Discussion
Inspection of the TOC pro®les in Fig. 2 reveals that
OM degradation is not complete at any examined pro®le, because OM remains in surface samples to be eroded and transported to downstream sediments.
Furthermore, weathering has removed much more OM
from the Green River and Woodford outcrops than
from those of the New Albany, Marcellus or Monterey.
While tempting to assign this variable TOC loss to differences in OM composition, this is unlikely to be the
481
case. The Woodford and Green River are very dissimilar OM types (the Woodford being much more similar
to the New Albany). What the Woodford and Green
River do share is similar erosion rate and hydrology.
Because the Woodford is a siliceous shale and the Green
River is a carbonate shale, these two formations are
exposed as coherent, rather impermeable rocks in warm,
arid regions. In outcrop these rocks are less ®ssile and
friable than the New Albany, Marcellus or Monterey,
and hence may be more resistant to physical erosion. As
a result, we infer that a given volume of Woodford or
Green River rock remains in the outcrop and is exposed
to oxidizing surface ¯uids longer than the New Albany,
Marcellus or Monterey. Drier conditions (and deeper
water tables) facilitate greater penetration of surface
waters, expressed as greater depths of TOC loss in these
pro®les. The Monterey pro®le may represent the opposite extreme; this exposure is marked by very friable
rock and rapid physical erosion. At this site, insucient
time is aorded for signi®cant OM degradation because
the rock is not held in the outcrop for very long times,
and thus TOC loss is not as severe as in the other pro®les.
A common feature of these TOC pro®les is an ``S''
shape of constant (high) TOC at depth, a zone of
rapidly decreasing TOC content, and a zone of constant
(low) TOC towards the surface. This is particularly
apparent in the Green River and New Albany II pro®les. This shape (if inverted and reversed) resembles
TOC depth pro®les during early diagenesis, and can be
explained qualitatively in terms of a model that considers the reactants O2 and organic carbon using a
steady state 1-G diagenetic model (Berner, 1980) but
with the earthõÂs surface as the equivalent of the lower
Fig. 4. Variations in elemental composition of kerogens plotted against fractional TOC loss across black shale weathering pro®les.
Shown are Kerogen N/C ratios (A), S/C ratios (B) and O/C ratio (C).
482
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
Fig. 5. Fourier transform infared spectra for kerogen concentrates from selected depths in the New Albany (A), Green
River (B) and Monterey (C) weathering pro®les.
boundary condition for diagenesis and sediment burial
rate replaced by erosion rate. Finer-scale features of the
pro®le may be accounted for by including pyrite oxidation and multiple pools of OM with dierent reactivities. In surface regions of the pro®les, the rate of OM
oxidation may be limited by the reactivity/abundance of
residual organic matter where TOC contents are low.
This is appropriate where the rock contains interstitial
space that is bathed in O2 or O2-containing water (or
some other oxidant such as organic peroxides). However, deeper into the shale permeability decreases considerably and access of OM to O2 (via diusion or
advection) becomes the limiting factor. At these depths
TOC will be much higher. It should be noted that TOC
content declines rapidly in each pro®le only above the
depth where pyrite content falls to zero, and thus where
O2 demand by pyrite oxidation is reduced. It is likely
that a combination of rate limitation by OM reactivity
in well-oxygenated depths of the pro®le and O2-limitation at greater depths, coupled with the regional erosion
rate, could be used to explain the S-shaped pro®les in all
of the examined shales. Larger TOC loss (i.e. Green
River and Woodford) corresponds to limited physical
erosion and greater contact time between OM and oxidizing surface waters; lesser TOC loss (i.e. Monterey)
corresponds to more rapid erosion and shorter OM-O2
contact time. Control of OM abundance by cumulative
oxygen exposure time has been documented in modern
sediments (Hartnett et al., 1998); it is not unlikely that
OM weathering may have similar controls.
It should be mentioned that the TOC gradient
observed in outcrop reveals little information about the
overall resistance of OM to weathering and remineralization for at least two reasons. One, it is unknown
whether TOC loss within a pro®le occurs as production
of CO2, generation of soluble oxidized OM, or cleavage
of otherwise unaltered, intact kerogen fragments, and
thus O2 consumption and CO2 production cannot be
inferred from TOC pro®les directly. Two, even minimal
TOC loss within a pro®le indicates nothing about subsequent OM oxidation during transport and storage in
downstream sediment reservoirs before anoxic reburial,
and the time spent within these reservoirs may have a
strong eect on dictating overall OM remineralization.
Although similar to the New Albany in age and OM
type, the outcrop region of the Marcellus Shale was
repeatedly scoured by advance and retreat of the Laurentide Ice Sheet, a process that resulted in removal of
weathered material and exposure of unaltered bedrock
after the ®nal ice sheet retreat some 15 k years ago
(Fleisher, 1986). If the New Albany pro®les are taken to
be at steady-state with respect to chemical weathering
and physical erosion, and given that the Marcellus and
New Albany outcrops exhibit roughly similar organic
matter types, lithology and hydrology, then the coincidence of the TOC pro®les for these two formations
S.T. Petsch et al. / Organic Geochemistry 31 (2000) 475±487
483
Fig. 6. Pair of pyrograms comparing weathered and unweathered samples of kerogen isolated from the New Albany Shale weathering
pro®le. Labelled are carbon numbers of selected n-alkene/alkane doublets, C14 and C19 isoprenoids (star), internal standard (X) and
other components relating to pyrolysis indices.
suggests that the Marcellus is also at steady state and
that a maximum of 15 k year is required to develop a
complete weathering pro®le at these sites. Comparisons
between TOC and pyrite data within each pro®le provide another constraint on OM weathering rates. Pyrite
loss precedes or coincides with TOC loss from these
pro®les, suggesting that the kinetics of OM weathering
can be no faster than pyrite oxidation. A mass transfer
model using known pyrite oxidation kinetics and reasonable ¯uid ¯ow could be used to recreate TOC and
pyrite data and provide an estimate of bulk OM weathering rates at these sites.
While large dierences in OM composition do not
develop betweeen weathered and unweathered shales
(increases in O/C ratios excepted), each formation does
reveal individual characteristics during weathering that
re¯ect at least in part OM composition and reactivity.
Weathering of the New Albany Shale is largely nonselective. OM is homogeneously degraded and lost during
weathering, without signi®cant relative loss/gain of Nor S-containing moieties or changes in the relative
abundance of the components measured by the four
pyrolysis indices. Oxidation products accumulate within
the kerogen during weathering of the New Albany. This
is seen both in the increase in O/C ratios with TOC loss
(Fig. 4C), and in the IR spectra (Fig. 5A). Infrared
spectroscopy also reveals a signi®cant loss in alkyl C±H
bonds relative to CO and CC bonds. It is surprising
that this loss is not observed in pyrolysis, but may be
explained if the alkyl bonds indicated as lost by IR
derive from mainly C1±C4 fragments, which are not
accounted for in the designed Py±GC experiment. Whether CC bonds are selective enriched during weathering
or form as weathering products must be resolved by
further study. Furthermore, the percent of total OM
accounted for by GC-amenable pyrolysis products is
small (