A ®eld study of the chemical weathering of ancient
sedimentary organic matter
S.T. Petsch
a,*, R.A. Berner
a, T.I. Eglinton
b aDepartment of Geology and Geophysics, Yale University, New Haven, CT, 06520, USAbDepartment of Geochemistry and Marine Chemistry, Woods Hole Oceanographic Institution, Woods Hole, MA, 02543, USA
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-chainn-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 oxi-dation 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 O2content
of the atmosphere) remain, however, undetermined. This has in part limited the successful development of mathematical models to describe the evolution of earth's
atmosphere (for reviews of geochemical carbon cycle modeling and its relation to atmospheric O2, see
Hol-land, 1978, or Berner, 1989).
There are strong indications that the eciency of OM remineralization to CO2during weathering is less than
100%. Ancient OM has been detected in modern sedi-ments 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 reminer-alization 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?
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* Corresponding author.
Because weathering of ancient OM has not been stu-died exhaustively, the above questions remain
unan-swered. However, several previous studies have
documented signi®cant changes in shale and OM geo-chemistry 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 carbond13C (LeythaÈuser, 1973).
Weathering of the Permian Phosphoria Formation of Utah and Upper Cretaceous Pierre Shale of Colorado showed TOC, bitumen andd13C trends similar to
Man-cos Shale weathering, along with preferential loss of aromatic versus saturated fractions of bitumen and loss of n-alkanes relative to branched and cyclic hydro-carbons (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 sug-gested that cleavage of small fragments from the kero-gen (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 sug-gested 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 mÿ3yearÿ1from this shale
and a pyrite weathering rate four times greater. The weathering environment these authors examined, how-ever, was not at steady-state and revealed signi®cant TOC in surface (weathered) samples. Weathering of ``paper coals'' from the Brazil Formation (Lower Penn-sylvanian, Indiana, USA) was shown to alter the com-position 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 pyrite-free 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
pro-duction 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 (hypothe-tical, 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 degra-dation 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 captur-ing 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 glo-bal OM remineralization. If particular OM types remi-neralize 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 atmo-spheric 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 weath-ering 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).
coupled with low iron supply encouraged limited pyrite formation and abundant OM sulfurization. OM sulfur-ization involves incorporation of sulfur into and between otherwise labile molecules (which inhibits degradation) and leads to generation of S-rich macro-molecules (Russell et al., 1997; Sinninghe Damste et al., 1998). OM in the Monterey is derived mainly from marine phytoplankton and bacteria with some con-tribution from terrestrial plants, and is composed of heavily S cross-linkedn-alkyl chains and aromatic cen-ters (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 chainn -alkyl fragments with minimal aromatic contribution (Derenne et al., 1991; Eglinton, 1994). Within the organic matrix of the Green River kerogen occur dis-crete particles (ultralaminae) derived from selective pre-servation of highly aliphatic biomacromolecules (termed algaenan) which occur in some algal cell walls (Derenne et al., 1991). Selective preservation of these highly ali-phatic 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 forma-tions were deposited in the Mid to Late Devonian, in oxygen-de®cient, highly productive epeiric seas on the craton of eastern North America. Deposited compara-tively 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 con-tributions 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 for-mations 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; Fla-viano et al., 1994) and also during coal weathering (Nip et al., 1989). Results of this study help test whether
selective preservation of highly aliphatic
macro-molecules 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 pre-served 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 per-haps most representative of the bulk OM found in sedi-mentary 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 pro-ducts, 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 of4 m. Approximately 10±30
samples of 500 g each were obtained at intervals of
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 I. Deatsville, Nelson Co., Kentucky, USA Clay City, Powell Co., Kentucky, USA
9±12 II
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 sul-fur 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 N2in PTFE bombs at 40C using
standard HCl/HCl:HF/HCl digestion procedure (Dur-and (Dur-and Nicaise, 1980). Elemental analysis (CNS/O) of kerogen concentrates was determined using a Carlo Erba EA1108 elemental analyzer. Samples were pre-pared for IR analysis by adding 100 mg of vacuum-dried kerogen to 900 mg IR grade KBr, storing the mixture overnight in a dessicator, and pressing the
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±2mg of kerogen with a known mass of added internal standard (poly-t-butylstyrene) onto an Fe±Ni wire. Flash
pyr-olysis (610C, 5 s) was achieved using a FOM-3LX
Curie Point pyrolyzer/Horizons Instruments RF gen-erator. 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: 0C for 5 min, ramp rate 3C
minÿ1to 320C, hold for 20 min, He carrier gas, 1 ml
minÿ1¯ow rate; ¯ame ionization detector).
4. Results
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-of-magnitude 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 Eglin-ton, 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% e-cient 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 Fig. 1. Schematic drawing of generalized sampling site. A
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 at2 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 iso-lated 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 -CH3groups) and at 1460
and 1375 cmÿ1(corresponding to bending of alkyl -CH 2
-and -CH3groups) as well as ingrowth of absorbance of
bands near 1700 cmÿ1 (CO stretching) and near
1640 cmÿ1(C
C stretching). Similar trends are observed
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
for-mal 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 moi-eties.
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
pro®le (Fig. 8). Given the scatter of the data, the invar-iance of the thiophene index (Fig. 8A) agrees well with constant kerogen S/C ratios for each pro®le. Interest-ingly, 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 progres-sively 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 pre-ferentially 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
ero-ded 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 dif-ferences in OM composition, this is unlikely to be the
case. The Woodford and Green River are very dissim-ilar OM types (the Woodford being much more simdissim-ilar 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 oppo-site 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
con-siders 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
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 oxida-tion and multiple pools of OM with dierent reactiv-ities. 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). How-ever, deeper into the shale permeability decreases
con-siderably 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
-limita-tion 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 oxi-dizing 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 reminer-alization 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 sub-sequent 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 Laur-entide 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 coin-cidence of the TOC pro®les for these two formations Fig. 5. Fourier transform infared spectra for kerogen
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 pro-vide 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 rea-sonable ¯uid ¯ow could be used to recreate TOC and pyrite data and provide an estimate of bulk OM weath-ering 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 non-selective. OM is homogeneously degraded and lost during weathering, without signi®cant relative loss/gain of N-or S-containing moieties N-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. Whe-ther 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 (<10%). Although these products are believed to be structurally and isotopically representative (Larter and Hors®eld, 1993; Eglinton, 1994), more polar
pro-ducts are not well resolved (and hence
under-represented) on the apolar stationary phases typically used in Py±GC.
Weathering of the Green River kerogen is less uni-form. N/C and S/C ratios remain low and constant within the weathering pro®le (indicating little selective loss/gain of N- or S-containing moieties), and O/C ratios increase only modestly with TOC loss (suggesting little accumulation of oxidation products within the 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 selectedn-alkene/alkane doublets, C14and C19isoprenoids (star), internal standard (X) and
kerogen). Infrared spectroscopy reveals ingrowth of CO relative to alkyl C-H bonds across the whole pro®le,
and a sudden ingrowth of CC relative to alkyl C±H
bonds beginning at 228cm (Fig. 5B). Pyrolysis likewise indicates non-uniformity in kerogen composition with weathering, most clearly seen in the relative loss of iso-prenoids (Fig. 8D) and selective enrichment of long n -alkyl chains. The increase in long n-alkyl chain abun-dance and the relative lack of alkyl C±H bond loss shown by IR (compared with the New Albany pro®les)
suggests that a fraction of Green River OM comprised of long alkyl chains (algaenan?) is relatively preserved during OM weathering. However, given the nearly 20-fold loss of TOC content during weathering of Green River kerogen and modest relative enrichment of this highly aliphatic component, it is not likely that selective rates of weathering strongly control OM degradation in the Green River Shale.
The Monterey Shale is the least extensively weath-ered of all examined pro®les, and appears to weather Table 2
Indices of kerogen composition variation, calculated using relative peak area of pyrolysis products listed in column 2
Index name Compound peak area ratio Indication
Thiophene 2-Methyl thiophene to toluene+oct-1-ene (nC8:1) Enrichment/depletion of S-containing moieties
Aromaticity Toluene+ethyl benzene+o,m,p-xylene tonC7:1+
nC8:1+nC9:1
Enrichment/depletion of alkyl-benzene moieties
Chain length nC21:1+nC22:1+nC23:1+nC24:1tonC6:1+nC7:1+
nC8:1+nC9:1
Enrichment/depletion of long-chain alkyl fragments; loss/ gain of bridging atoms
Isoprenoid prist-1-ene+C14isoprenoid tonC13:1+nC15:1+nC17:1 Enrichment/depletion of branched hydrocarbon fragments
Fig. 7. Pair of pyrograms comparing weathered and unweathered samples of kerogen isolated from the Green River Shale weathering pro®le. Labelled are carbon numbers of selectedn-alkene/alkane doublets, C14and C19isoprenoids (star), internal standard (X) and
non-selectively. N/C and S/C ratios are approximately constant with TOC loss, and O/C ratios variations are inconclusive. Infrared spectroscopy reveals patterns similar to weathering of the New Albany, but with relative greater ingrowth of CC and CO bonds and
lesser loss of alkyl C±H bonds. Lack of S/C and thio-phene index variations suggests that there is no expres-sed preference for weathering kerogen components with diering degrees of S cross-linking. Thus, while sulfur-ization signi®cantly aects the preservation of OM in sediments, it appears to have less importance in con-trolling OM weathering.
Of course, the unanswered question remains: why does not all OM remineralize at the outcrop? The results of this study so far suggest that oxidation/dissolution kinetics may be limiting, erosive transport of particulate
matter may strongly in¯uence overall weathering rates, and selective reaction of speci®c kerogen moieties does not predominate. This conclusion has intriguing impli-cations for the geochemical carbon cycle; if weathering is controlled mainly by uplift and erosion on the con-tinents (Holland, 1978), perhaps ancient OM reminer-alization may be approximated by some cumulative oxygen exposure time (analogous with Hartnett et al., 1998) and not severely constrained by inherent varia-tions in weathering rate based on OM type.
6. Conclusions
contained in the unaltered rock. While little to no selec-tive enhancement of speci®c OM components is observed, accumulation of oxidation products does occur. The rate of OM weathering at the exposures used in this study is likely dominated more by factors aecting physical erosion of the rock than any inherent dier-ences in reactivity between dierent organic matter
types and compositions. The formation of an 3 m
thick steady-state weathered zone in the rock appears to be rapid relative to erosion. It is likely that models of ¯uid ¯ow and mass transfer that accurately capture the texture of observed TOC and pyrite pro®les can be developed to better constrain the rate of organic carbon release from these systems. The organic matter in black shales, despite undergoing protracted diagenesis and mild thermal alteration as a result of sediment burial and consolidation, is extremely labile when exposed to O2-rich surface waters. OM is rapidly removed from the
outcrop by an as yet undetermined mechanism likely associated with slow oxidation followed by rapid clea-vage, dissolution and advection [as either CO2(aq) or
dissolved OM]. Future studies will include models of ¯uid ¯ow, erosion and chemical reaction to constrain OM weathering rates, petrography of OM, analysis of bitumen geochemistry, and further investigation of
kerogen composition using Py±GC±MS and13C NMR.
Acknowledgements
The authors wish to acknowledge Mike Lewan (USGS) for guidance with sample selection, S. Chang and E. Morrow at Yale for pyrite analyses and helpful discussion, and R. Nelson, C. Reddy and B. Benitez-Nelson for assistance at WHOI. S. Derenne and R.
Littke are thanked for providing insightful and
thoughtful reviews. This project is supported by the (US) National Science Foundation (EAR-9804781) and the (US) Department of Energy (DE-FG02-95ER14522). Graduate student support is provided by the Yale Uni-versity Institute for Biospheric Studies. This study is Woods Hole Oceanographic Institution No. 9933.
References
Barrick, R.C., Hedges, J.I., Peterson, M.L., 1980. Hydrocarbon geochemistry of the Puget Sound region I: sedimentary acyclic hydrocarbons. Geochimica et Cosmochimica Acta 44, 1349±1362.
Berner, R.A., 1980. Early Diagenesis: A Theoretical Approach. Princeton University Press, Princeton, N.J.
Berner, R.A., 1989. Biogeochemical cycles of carbon and sulfur and their eect on atmospheric oxygen over Phanerozoic time. Paleogeography, Paleoclimatology, Paleoecology 75, 97±122.
Can®eld, D.E., Raiswell, R., Westrich, J.T., Reaves, C.M., Berner, R.A., 1986. The use of chromium reduction in the analysis of reduced inorganic sulfur in sediments and shales. Chemical Geology 54, 149±155.
Chang, S., 1997. Coal Weathering and the Geochemical Carbon Cycle. Ph.D. dissertation, Yale University.
Clayton, J.L., Swetland, P.J., 1978. Subaerial weathering of sedimentary organic matter. Geochimica et Cosmochimica Acta 42, 305±312.
Derenne, S., Largeau, C., Casadevall, E., Berkalo, C., Rous-seau, B., 1991. Chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant out walls of microalgae: origin of ultralaminae. Geochimica et Cosmochimica Acta 55, 1041±1050.
Derenne, S., Le Berre, F., Largeau, C., Hatcher, P., Connan, J., Raynaud, J.F., 1992. Formation of ultralaminae in marine kerogens via selective preservation of thin resistant out walls of microalgae. Organic Geochemistry 19, 345±350.
Eglinton, T.I., 1994. Carbon isotopic evidence for the origin of macromolecular aliphatic structures in kerogen. Organic Geochemistry 21, 721±735.
Eglinton, T.I., Benitez-Nelson, B.C., Pearson, A., McNichol, A.P., Bauer, J.E., Druel, E.R.M., 1997. Variability in radiocarbon ages of individual organic compounds from marine sediments. Science 277, 796±799.
Eglinton, T.I., Benitez-Nelson, B.C., Pearson, A., McNichol, A.P., GonÄi, M.A., Ertel, J.R., 1998. A coupled molecular isotopic approach to trace sources of organic carbon pre-served in the sedimentary record. V. M. Goldschmidt Con-ference, Mineralogical Magazine 62A, 413±414.
Eglinton, T.I., Irvine, J.E., Vairavamurthy, A., Zhou, W., Manowitz, B., 1994. Formation and diagenesis of macro-molecular organic sulfur in Peru margin sediments. Organic Geochemistry 22, 781±799.
Flaviano, C., Le Berre, F., Derenne, S., Largeau, C., Connan, J., 1994. First indications of the formation of kerogen amorphous fractions by selective preservation; role of non-hydrolysable macromolecular constituents of eubacterial cell walls. Organic Geochemistry 22, 711±759.
Fleischer, P.J., 1986. Glacial geology and Late Wisconsinan stratigraphy, Upper Susquehanna drainage basin, New York. New York State Museum, Bulletin 455, 121±142. Goth, K., de Leeuw, J.W., Puttmann, W., Tegelaar, E.W.,
1988. Origin of Messel oil shale kerogen. Nature 376, 7359± 7361.
Hartnett, H.E., Keil, R.G., Hedges, J.I., Devol, A.H., 1998. In¯uence of oxygen exposure time on organic carbon pre-servation in continental margin sediments. Nature 391, 572± 574.
Holland, H.D., 1978. The Chemistry of the Atmosphere and Oceans. Wiley Interscience, New York.
Krom, M.D., Berner, R.A., 1983. A rapid method for the determination of organic and carbonate carbon in geological samples. Journal of Sedimentary Petrology 53, 660±663. Larter, S.R., Hors®eld, B., 1993. Determination of structural
components of kerogens by the use of analytical pyrolysis methods. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 271±287. Lewan, M. D., 1980. Geochemistry of Vanadium and Nickel in
LeythaÈuser, D., 1973. Eects of weathering on organic matter in shales. Geochimica et Cosmochimica Acta 37, 120±133. Littke, R., Klussman, U., Krooss, B., LeythaÈuser, D., 1991.
Quanti®cation of loss of calcite, pyrite and organic matter during weathering of Toarcian black shales and eects on kerogen and bitumen characteristics. Geochimica et Cosmo-chimica Acta 55, 3369±3378.
Nip, M., de Leeuw, J.W., Schenck, P.A., Windig, W., Meuze-laar, H.L.C., Crelling, J.C., 1989. A ¯ash pyrolysis and pet-rographic study of cutinite from the Indiana Paper Coal. Geochimica et Cosmochimica Acta 53, 671±683.
Petsch, S.T., Eglinton, T.I., in preparation. Radiocarbon age determination and addition of modern organic matter to weathering pro®les developed on black shales.
Rowland, S.J., Maxwell, J.R., 1984. Reworked triterpenoids and steroid hydrocarbons in a recent sediment. Geochimica et Cosmochimica Acta 48, 617±624.
Russell, M., Grimalt, J.O., Hartgers, W.A., Taberner, C., Rouchy, J.M., 1997. Bacterial and algal markers in sedi-mentary organic matter deposited under natural sulphuriza-tion condisulphuriza-tions (Lorca Basin, Murcia, Spain). Organic Geochemistry 26, 605±625.
Sackett, W.M., Poag, C.W., Eadie, B.J., 1974. Kerogen recy-cling in the Ross Sea. Antarctica. Science 185, 1045±1047. Schouten, S., Eglinton, T.I., Sinninghe DamsteÂ, J.S., de Leeuw,
J.W., 1995. In¯uence of sulfur cross-linking on the molecular size distribution of sulfur-rich macromolecules in bitumen. In: Vairavamurthy, M.A., Schoonen, M.A.A. (Eds.), Geo-chemical Transformations of Sedimentary Sulfur. ACS Sym-posium series 612, American Chemical Society, pp. 80±92. Sinninghe DamsteÂ, J.S., Eglinton, T.I., de Leeuw, J.W.,
Schenck, P.A., 1989. Organic sulfur in macromolecular sedi-mentary organic matter. I: Structure and origin of sulfur-containing moieties in kerogen, asphaltenes and coal as revealed by ¯ash pyrolysis. Geochimica et Cosmochimica Acta 53, 873±889.
Sinninghe DamsteÂ, J.S., Kok, M.S., Koester, J., Schouten, S., 1998. Sulfurized carbohydrates; an important sedimentary sink for organic carbon. Earth and Planetary Science Letters 164, 7±13.
