Effects of weathering on organic matter: I. Changes in molecular composition of extractable organic compounds caused by paleoweathering of a Lower Carboniferous (Tournaisian) marine black shale

Chemical Geology 285 (2011) 144–156

Contents lists available at ScienceDirect

Chemical Geology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c h e m g e o

Effects of weathering on organic matter: I. Changes in molecular composition of extractable organic compounds caused by paleoweathering of a Lower Carboniferous (Tournaisian) marine black shale Leszek Marynowski a,⁎, Slawomir Kurkiewicz b, Michał Rakociński a, Bernd R.T. Simoneit c,d a

Faculty of Earth Sciences, University of Silesia, Będzińska 60, PL-41-200 Sosnowiec, Poland Department of Instrumental Analysis, Faculty of Pharmacy, Medical University of Silesia, PL-41-200 Sosnowiec, Poland c COGER, King Saud University, 11451 Riyadh, Saudi Arabia d Department of Chemistry, Oregon State University, Corvallis, OR 97331, USA b

a r t i c l e

i n f o

Article history: Received 2 September 2010 Received in revised form 31 March 2011 Accepted 1 April 2011 Available online 9 April 2011 Edited by: J. Fein Keywords: Paleoweathering Organic matter Pyrite framboids Black shale Biomarkers Maleimides PAHs

a b s t r a c t A detailed bulk and molecular study on paleoweathering of a Lower Carboniferous (Tournaisian) black shale from the Kowala quarry in the Holy Cross Mountains of Poland, revealed significant changes in total organic carbon (TOC), total sulfur (TS) and extract compositions. Paleoweathering resulted in a 97% decrease in TOC and total loss of sulfur, as well as changes in carbonate contents, extract yields and percentage yields of the organic fractions. Pyrite framboids, which are used extensively in paleoecological studies, decreased considerably in the partially weathered zone and totally vanished in the weathered zone. The decrease in TOC is accompanied by a pronounced reduction of organic compound concentrations, but the degradation range differs in the individual weathering zones. Here we show that less stable compounds such as low molecular weight aromatics (e.g. methylnaphthalenes, dibenzofuran, and dibenzothiophene), isorenieratane and its diagenetic products, or maleimides decrease significantly or disappear already in the partially weathered zone, while the more stable polycyclic aromatic hydrocarbons (PAHs) decrease (~ 90%) only in the weathered and highly weathered zones. Besides the organic matter (OM) content, the influence of paleoweathering on the distributions of organic compounds is important in the context of paleoenvironment, source and maturity interpretations. Almost all sterane and triterpane biomarker parameters change their values in the highly weathered zone, but some ratios, e.g. the 2-MeH index, are almost totally resistant to change. The aryl isoprenoid ratio (AIR) values decrease gradually with weathering. This modifies completely the potential interpretation of the nature of the photic zone anoxia. In addition to degradation of OM, some PAHs like benzo [b]fluoranthene increase in concentration in the partially weathered zone due to their formation from phenylderivatives. The correct recognition of paleoweathering in outcrop and drill core samples aids in the proper interpretation of biomarker parameters and contributes to a better understanding of the processes which took place during weathering. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Weathering of ancient organic matter (OM) is a major factor in the geochemical carbon cycle which controls the oxygen concentration in the atmosphere (Bolton et al., 2006; Chang and Berner, 1999; Hartnett et al., 1998; Wildman et al., 2004). Therefore, the understanding of how these processes took place during oxidative weathering and the estimation of all reaction rates is crucial for modeling atmospheric oxygen levels (Chang and Berner, 1999). One of the main factors controlling the rate of OM weathering, besides climate, is the OM source and maturity. For example, terrestrial OM is more resistant to oxidation

⁎ Corresponding author. E-mail address: [email protected] (L. Marynowski). 0009-2541/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.chemgeo.2011.04.001

processes than marine OM (e.g. Prahl et al., 1997; Sinninghe Damsté et al., 2002), and highly carbonized material like charcoal, black carbon or graphite is more resistant to such processes than immature sedimentary OM (e.g. Scott, 2010; Haberstroh et al., 2006). Moreover, a new report shows that charcoals formed at lower temperatures are much more susceptible to chemical oxidation than those from higher temperature conditions (Ascough et al., 2010). This may result in more resistant OM accumulating in the weathered zones of sedimentary rocks or being reworked and transported to sedimentary basins (e.g. Haberstroh et al., 2006). However, even in the case of highly metamorphosed OM in the anthracite stage of diagenesis, oxidative weathering is an effective and fast rock alteration process in view of geological time. For example, the total organic carbon content in low-grade metamorphic slates decreased from two- to seven-times during ~100 years of oxidation at ambient temperature (Fischer et al., 2007).

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

There are several reports documenting recent natural weathering of black shale which can result in inorganic (Peucker-Ehrenbrink and Hannigan, 2000; Jaffe et al., 2002; Wildman et al., 2004; Fischer and Gaupp, 2005; Fischer et al., 2009), bulk organic (Leythaeuser, 1973; Clayton and Swetland, 1978; Littke et al., 1989; Lo and Cardott, 1995; Petsch et al., 2000, 2005; Wildman et al., 2004; Fischer and Gaupp, 2005; Fischer et al., 2009) isotopic (Van Os et al., 1996) and molecular (Clayton and King, 1987; Petsch et al., 2000, 2005) changes. In contrast, there is a lack of studies on terrestrial paleoweathering, especially in the context of OM composition and transformation during gradual natural oxidation. Only some preliminary results concerning early diagenetic OM oxidation of Triassic terrestrial green and red shales from Poland were reported recently showing that oxidation processes resulted in a significant decrease in OM content and changes in the nalkane, triterpane, sterane and polycyclic aromatic hydrocarbon (PAH) distributions (Marynowski and Wyszomirski, 2008). Here we report for the first time a thorough description of paleoweathering of a Lower Carboniferous (Tournaisian) marine black shale in the context of general OM transformations and pyrite framboid diameter interpretations, as well as important changes in extractable organic matter compositions and effects on most of the commonly used geochemical parameters. This problem seems to be especially relevant to cases of paleoenvironmental, source and maturity interpretations based on molecular parameters for not only surface samples, but also potentially those from drill cores, where rock sequences exposed to aerial paleoweathering processes may have occurred. In addition, subsurface weathering, which occurs preferentially along fault joints or bedding planes, may affect OM over a considerable part of such formations (see Van Os et al., 1996). The Lower Carboniferous shale investigated here has a relatively low maturity of the OM and macroscopically visible alteration caused by weathering processes (Fig. S-A, Supplemental On-line Material), and is an excellent example where the gradual change from non-oxidized to highly oxidized OM is visible within a few meters of section. The study focuses on the characterization of organic compound groups like: hopanes, methylhopanes, steranes, isorenieratane, aryl isoprenoids and PAHs in terms of their stability during oxidative weathering. These compounds are commonly used in reconstruction of sedimentary environments and OM maturity (e.g. Peters et al., 2005 and citations therein), but secondary weathering processes and especially paleoweathering are rarely taken into account in the interpretation of such results. Here we show that oxidative paleoweathering processes can substantially change the primary composition of extractable OM, which as a consequence may lead to erroneous maturity and paleoenvironmental interpretations. The detailed discussion of individual compounds and molecular parameter changes during the gradual paleoweathering of this black shale may aid in reliable future interpretations and conclusions based on sedimentary biomarkers. 2. Materials and methods

145

Permian conglomerates (Fig. S-A). Although the conglomerates are generally organic-poor, some of the Devonian, mainly limestone pebbles are enriched in OM and there is no evidence for their weathering. At least 2 m of intensively weathered shales occur below the conglomerates (see Fig. S-A), which suggests that oxidative weathering took place between the Lower Carboniferous and Upper Permian. The most severely weathered sample, KQ 136-1 was collected ca 2 m below the recent surface. Above this sample, the correct determination of the marker horizon was problematic (see Fig. S-A) due to the complete or almost complete mineralization of the weathered kerogen (see Berner, 1989) and furthermore, access to upper samples was difficult. The detailed geological description of the study area was provided elsewhere (Berkowski, 2002; Marynowski and Filipiak, 2007; Rakociński, 2009; Marynowski et al., 2010). Previous analyses revealed that Upper Devonian samples from the same quarry are generally immature, with Rock-Eval Tmax values ranging from 421 to 425 °C (Marynowski and Filipiak, 2007) and an average vitrinite reflectance value of 0.53% Ro (Marynowski et al., 2001). This indicates that the horizons investigated here have also similarly low maturities. 2.2. Vitrinite reflectance For the vitrinite reflectance analysis, freshly polished rock fragments were used. Random reflectance was measured using an AXIOPLAN II microscope using 156 nm light and oil of 1.546 RI using a total magnification of 500×. The standards used were 0.42% and 0.898% reflectance (Ro). 2.3. Total organic carbon The total organic carbon (TOC) and total sulfur (TS) contents were determined using an Eltra Elemental Analyser (model CS530). For more details see Racka et al., (2010). 2.4. Pyrite framboid diameter analysis Six samples were selected for pyrite framboid analysis, namely two samples from the unweathered part of the analyzed bed (KQ1367 and KQ136-4), two samples from the partially weathered section (KQ136-3 and KQ136-2B) and two samples from the weathered region (KQ136-2A and KQ136-2) (cf. Fig. S-A). Samples in the form of small chips were polished, and framboid diameters were measured using a Philips Environmental Scanning Electron Microscope (ESEM) in the back-scattered electron (BSE) mode at the University of Silesia (Sosnowiec, Poland). Framboid diameters were measured using the ESEM internal measuring device (given in μm). For each sample where framboids occurred, minimum and maximum values, mean value and standard deviation were calculated, tabulated and shown as histograms (see Wignall and Newton, 1998).

2.1. Samples 2.5. Extraction and separation Nine samples were collected in August 2009 from the eastern part of the active Kowala Quarry, Holy Cross Mountains, Poland (Fig. S-A, Supplemental On-line Material). All samples belong to a 10 cm thick black shale horizon (Fig. S-A). This Tournaisian black shale is situated ca 40 cm above the Devonian/Carboniferous boundary and 2 m above the black Hangenberg shale (Fig. S-A, see Marynowski and Filipiak, 2007) and is the part of the Upper Devonian–Lower Carboniferous open marine sequence from the Holy Cross Mountains (Szulczewski, 1995). The sample horizon and other parts of the Lower Tournaisian section of the Kowala quarry are an excellent example of paleoweathering which took place after the Upper Carboniferous folding and before the Upper Permian deposition. This was affirmed by the observations on the unconformably overlying, non-weathered Upper

Cleaned and powdered samples were Soxhlet-extracted with dichloromethane in pre-extracted thimbles. The extractable organic matter (EOM) was further separated by thin layer chromatography (TLC) using pre-washed plates coated with silica gel (Merck, 20 × 20 × 0.25 cm). Prior to separation, the TLC plates were activated at 120 °C for 1 h. Plates were then loaded with the dichloromethane soluble fraction and developed with n-hexane. Aliphatic hydrocarbon (Rf 0.6–1.0), aromatic hydrocarbon (Rf 0.05–0.6), and polar compound (Rf 0.0–0.05) fractions were eluted and extracted from the silica with dichloromethane. The aliphatic and aromatic fractions of all samples were analysed in further detail by gas chromatography–mass spectrometry (GC–MS).

146

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

2.6. GC–MS The GC–MS analyses were performed with an Agilent 6890 Series Gas Chromatograph interfaced to an Agilent 5973 Network Mass Selective Detector and Agilent 7683 Series Injector (Agilent Technologies, Palo Alto, CA). A 0.5 μL sample was introduced into the cool oncolumn injector under electronic pressure control. Helium (6.0 Grade) was used as the carrier gas at a constant flow rate of 2.6 mL/min. The GC separation was on either of two fused-silica capillary columns: (1) J&W HP5-MS (60 m × 0.32 mm i.d., 0.25 μm film thickness) coated with a chemically bonded phase (95% polydimethylsiloxane and 5% diphenylsiloxane). The GC oven temperature was programmed from 40 °C (isothermal for 1 min) to 120 °C at a rate of 20 °C/min, then to 300 °C at a rate of 3 °C/min. The final temperature was held for 35 min. (2) J&W DB35-MS (60 m × 0.25 mm i.d., 0.25 μm film thickness) coated with a chemically bonded phase (35% phenyl-methylpolysiloxane). The GC oven temperature was programmed from 50 °C (isothermal for 1 min) to 120 °C at a rate of 20 °C/min, then to 300 °C at a rate of 3 °C/min. The final temperature was held for 45 min. The GC column outlet was connected directly to the ion source of the mass spectrometer. The GC–MS interface was kept at 280 °C, while the ion source and the quadrupole analyzer were at 230 and 150 °C, respectively. Mass spectra were recorded from 45–550 da (0–40 min) and 50–700 da (above 40 min). The mass spectrometer was operated in the electron impact mode (ionization energy: 70 eV). 2.7. GC–MS–MS Biomarkers in the aliphatic fractions were determined by GC–MS–MS using an Agilent Technologies 7890A gas chromatograph coupled with an Agilent Technologies 7000 GC–MS Triple Quad instrument. The GC was fitted with a HP1-MS column (60 m×0.32 mm i.d., × 0.25 μm film thickness) and operated using He carrier gas at constant flow 2.6 mL/min. The inlet was held constant at 250 °C, while the GC oven was held at 40 °C for 1 min, then heated at 20 °C/min to 200 °C and at 1 °C/min to 280 °C, with an isothermal hold for 2 min. The MS ion source was at 230 °C and the quadrupole at 150 °C for all analyses. The ionization energy under electron impact was 70 eV. The software used was MassHunter GC–MS Acquisition B.05.00.412 and MassHunter Workstation Software B.03.01 (Agilent Technologies). In the Multiple Reaction Monitoring (MRM) mode, the following reactions were monitored: For methylhopanes: 440.8 → 205.4; 426.8 → 205.4; 412.8 → 205.4; 384.8 → 205.4. For hopanes: 468.8 → 191.4; 454.8 → 191.4; 440.8 → 191.4; 426.8 → 191.4; 412.8 → 191.4; 398.8 → 191.4; 384.8 → 191.4; 370.8 → 191.4. For steranes: 441.8 → 217.4; 400.8 → 217.4; 286.8 → 217.4; 272.8 → 217.4. The optimum collision energy for these transition product ions was found to be at 5 V. N2 was used at a constant flow of 1.5 mL/min in the collision cell and He was the quench gas at a constant flow of 2.25 mL/min. 2.8. Quantification and identification An Agilent Technologies Enhanced ChemStation (G1701CA ver. C.00.00) and the Wiley Registry of Mass Spectral Data (8th ed.) software were used for data collection and mass spectra processing. The abundances of the selected aromatic compounds were calculated by comparisons of the peak area for an internal standard

(9-phenylindene) with the peak areas of the individual hydrocarbons obtained from the GC–MS ion chromatograms. Peak identification was carried out by comparison of retention times with standards and by interpretation of mass spectrum fragmentation patterns. 3. Results 3.1. General geochemical characteristics The gradual loss of organic carbon (OC) in black shales due to surface and subsurface weathering was previously shown and discussed in detail by Leythaeuser (1973), Clayton and Swetland (1978), Littke et al. (1989), Wildman et al. (2004), Petsch et al. (2000, 2005), and Fischer and Gaupp (2005). It is of interest that oxidative degradation of OC can occur not only in sedimentary rocks like black shales, but also in low-grade metamorphic black slates (Fischer et al., 2007, 2009). Similar to recent weathering (Wildman et al., 2004 and references therein), general features of paleoweathering of the black shale studied here are a gradual decrease in TOC and extractable compounds as reflected by lower EOM (Table 1) and decrease in concentrations of the individual compounds (Table 2). These changes are also connected with pyrite weathering observed here as a depletion of pyrite framboids (Fig. 1) and a decrease in total sulfur (TS) (Table 1, Fig. 2C). Taking into account the very low concentrations of organic sulfur compounds in the non-weathered part of the black shale, we assumed that essentially all sulfur in the samples is inorganic pyrite sulfur. Comparison of TOC and TS contents (Table 1, Fig. 2B and C) confirmed the observations by Wildman et al. (2004) that pyrite reacts faster with O2 than organic matter, which is displayed by the partially weathered samples (KQ136-3 and KQ136-2B) where TOC remains in the range of 3.8– 5.8% while TS decreased significantly to values between 0.02 and 0.06% (Table 1, Fig. 2B and C). Moreover, in samples KQ136-3 and KQ136-2B the number of pyrite framboids decreased significantly. We have found only 20 framboids in sample KQ136-3 and none in sample KQ136-2B (Fig. 1) even though macroscopically both samples are still black in color. Weathering did not change the general framboid diameter distribution (Fig. 1) but lowered the number of framboids, making interpretation much more difficult (for general information about pyrite framboid diameter analysis see Wilkin et al., 1996; Wignall and Newton, 1998; Bond and Wignall, 2010). Because pyrite is the most sensitive mineral to weathering processes (Wildman et al., 2004), we defined the three zones of weathering within the black shale based on TS concentration as well as the occurrence of pyrite framboids. Unweathered samples contain up to 0.5% TS and ubiquitous pyrite framboids. Partially weathered samples contain 0.01 to 0.1% TS and sparse pyrite framboids. Goethite is common as a pseudomorph after pyrite. The weathered zone contains b0.01% TS and no pyrite framboids. Goethite pseudomorphosis after pyrite is ubiquitous (Figs. S-B and 1). 3.2. Molecular composition 3.2.1. N-alkanes and isoprenoids The n-alkanes and isoprenoids, pristane (Pr) or phytane (Ph), are ubiquitous in sedimentary OM and, although not treated as typical biomarkers, they are routinely used as indicators of OM source, depositional environment and maturity (Didyk et al., 1978; Peters et al., 2005). The n-alkane distributions are more or less similar for all analyzed samples, with the short chain (n-C17–n-C19) over long chain (n-C27–n-C29) alkanes predominating. Surprisingly, the short to long chain alkane ratios (SCh/LCh) are lower in the highly weathered samples (Table 1, Fig. S-C Supplemental On-line Material) which is

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

147

Table 1 Bulk geochemical data, percentage yields of fractions and basic molecular parameters. Sample

KQ136_1 KQ136_2 KQ136_2A KQ136_2B KQ136_3 KQ136_4 KQ136_5 KQ136_6 KQ136_7

CC [%]

TC [%]

TOC [%]

TS [%]

EOM [mg/g TOC]

4.99 4.00 6.42 2.74 5.67 9.82 5.87 6.73 8.23

0.79 1.01 2.90 4.16 6.48 6.38 4.98 5.01 6.42

0.19 0.53 2.13 3.84 5.80 5.20 4.28 4.20 5.43

0.00 0.00 0.002 0.02 0.06 0.58 0.77 0.60 1.08

0.01 0.01 0.01 0.01 0.02 0.03 0.02 0.03 0.04

Fractions AL [%]

AR [%]

POL [%]

29 17 20 20 14 18 19 13 14

14 10 4 14 34 38 38 46 50

57 73 76 66 52 44 43 41 36

CPI(Total)

CPI(25

1.13 1.18 1.01 1.08 1.12 1.14 1.11 1.09 1.13

1.73 2.16 1.51 1.31 1.34 1.36 1.36 1.33 1.40

– 31)

Pr/Ph

Pr/nC17

Ph/nC18

SCh/LCh

0.61 0.66 2.45 3.52 3.51 4.05 5.30 5.02 3.75

0.94 0.92 2.14 3.23 4.36 5.69 5.78 5.69 5.27

1.17 1.02 0.83 1.02 1.52 1.72 1.30 1.26 1.62

2.64 2.55 12.61 7.67 4.45 3.37 4.28 4.00 3.07

CC = carbonate content; TC = total carbon; TOC = total organic carbon; TS = total sulfur. Al = aliphatic fraction; Ar = aromatic fraction; POL = polar fraction. CPI(Total) = Carbon Preference Index: 0.5 [Σ(C25–C33) odd + Σ(C23–C31) odd]/Σ (C24–C32)even. CPI(25–31) = Carbon Preference Index: (C25 + C27 + C29) + (C27 + C29 + C31)/2(C26 + C28 + C30). Pr = Pristane; Ph = Phytane; SCh/LCh = short chain to long chain n-alkanes ratio: (nC17 + nC18 + nC19)/(nC27 + nC28 + nC29).

opposite to previously reported data (Faure et al., 1999; Marynowski and Wyszomirski, 2008), where the evolution of the n-alkane distributions was marked by a gradual short chain alkane increase and long chain compound degradation. This may suggest that beside oxidation, more weathered samples from the upper part of the horizon (Fig. S-A) were affected by water washing, which removed

the more water-soluble short-chain compounds including alkanes (Palmer, 1993; Skręt et al., 2010). Both Pr/nC17 and Ph/nC18 ratios, as well as the Pr/Ph ratio, decrease with degree of oxidation (Table 1, Figs. S-B, and 2D and E). Lower values start with the partially weathered sample KQ136-3, gradually decrease to sample KQ136-2A (less obvious for Ph/n-C18)

Table 2 Concentrations of individual compounds and molecular parameters based on biomarker and PAHs distributions. Biomarker ratios

Samples Unweathered

Isorenieratane and aryl isoprenoids Isorenieratane [μg/g TOC] Isorenieratane + 2,3,6-/3,4,5- [μg/g TOC] C13–C22 aryl isoprenoids [μg/g TOC] AIR Hopanes Ster/17α-hop. 22 S/(22 S + 22R) 2-MeH index% 30 M/(M + H) 33(S + R)/31(S + R) (32 + 33)/(29 + 30) C29/C30H Steranes and diasteranes 20 S/(20 S + 20R) ββ/(ββ + αα) Sum DIA/C27R Sum DIA/C29R 20 S/(20 S + 20R) DIA C27 [%] C28 [%] C29 [%] C30 [%] Aromatic hydrocarbons MPI1 MPI3 Ph/∑MP

Transitional

Weathered

KQ136_7

KQ136_6

KQ136_5

KQ136_4

KQ136_3

KQ136_2B

KQ136_2A

KQ136_2

KQ136_1

0.35 0.67 58.02 3.26

0.11 0.22 59.66 4.58

0.15 0.31 57.67 4.00

0.42 0.88 57.67 2.57

0.09 0.20 17.86 1.93

0.0 0.0 2.58 0.75

0.0 0.0 0.35 0.28

0.0 0.0 0.16 0.14

0.0 0.0 0.23 0.07

0.33 0.53 5.06 0.26 0.13 0.32 0.78

0.35 0.52 4.38 0.27 0.13 0.26 0.83

0.32 0.53 4.22 0.28 0.12 0.26 0.84

0.29 0.52 5.11 0.27 0.13 0.26 0.86

0.24 0.52 5.46 0.29 0.11 0.22 0.94

0.19 0.54 5.86 0.24 0.06 0.07 1.37

0.15 0.57 5.68 0.25 0.06 0.05 1.76

0.14 0.58 5.61 0.24 0.24 0.33 1.60

0.17 0.59 6.89 0.08 0.39 0.46 1.03

0.22 0.23 0.42 0.18 0.55 26 11 60 4

0.21 0.21 0.42 0.15 0.57 23 9 63 5

0.22 0.23 0.43 0.16 0.58 24 10 62 5

0.21 0.23 0.42 0.18 0.59 25 10 60 4

0.23 0.22 0.36 0.16 0.58 27 11 59 4

0.23 0.21 0.37 0.19 0.59 29 12 56 3

0.25 0.22 0.35 0.19 0.61 30 12 55 3

0.39 0.47 7.07 6.48 0.60 38 21 41 0

0.41 0.50 8.72 7.50 0.59 36 22 42 0

0.21 0.49 1.68

0.22 0.49 1.58

0.21 0.51 1.72

0.21 0.48 1.58

0.15 0.43 2.32

0.15 0.51 2.64

0.24 1.08 2.84

0.32 1.38 2.32

0.44 1.30 1.47

AIR = Aryl isoprenoid ratio: (C13–C17)/(C18–C22) (Schwark and Frimmel, 2004), Ster/17α-hop = regular steranes consist of the C27, C28, C29 ααα(20S + 20R) and αββ(20S + 20R), 17α-hopanes consist of the C29 to C33 pseudohomologue (including 22S and 22R epimers), 2MeHI = 2α-methylhopane/(C30 17α(H)-hopane + 2α-methylhopane) index (in percents) (Summons et al., 1999), 30M/(M + H) = C30-17α-hopane to(C30-17α-hopane + 17β-moretane) ratio, 33(S + R)/31(S + R) = peak areas of C33 22R and 22S-homohopanes to C31 22R and 22S-homohopanes, (32 + 33)/(29 + 30) = peak areas of C32 + C33 22R and 22S-homohopanes to sum of C29 + C3017α(H)-hopanes, C29/C30H = C29-17α(H)norhopane to C30-17α(H)-hopane ratio, 20S/(20S + 20R) = C29 5α,14α,17α, 20S-sterane vs. C29 5α,14α,17α, 20S-sterane + C29 5α,14α,17α, 20R-sterane ratio, ββ/(ββ + αα) = C29 5α,14β,17β, 20S and 20R-steranes vs. sum of C29 5α,14β,17β(H) 20S and 20R and C29 5α,14α,17α (H) 20S and 20R steranes ratio, Sum DIA/C27R = Sum of C27 13β,17α (H) 20S and 20R-diasteranes vs. C27 5α,14α,17α, 20R-sterane, Sum DIA/C29R = Sum of C27 13β,17α, 20S and 20R-diasteranes vs. C29 5α,14α,17α, 20R-sterane, 20S/(20S + 20R) DIA = C27 13β,17α, 20S-diasteranes vs. C27 13β,17α, 20S and 20R-diasteranes ratio,C27[%] = content of C27 ααα 20R sterane, C28[%]—content of C28 ααα 20R sterane, C29[%] = content of C29 ααα 20R sterane, C30[%] = content of C30 ααα 20R sterane, MPI1 = 1.5(2-MP + 3-MP)/(Ph + 1-MP + 4-MP + 9-MP) (Radke and Welte, 1983), MPI3 = (3-MP + 2-MP)/(9-MP + 4MP + 1-MP) (Radke, 1987), Ph/∑MP = Ph/(1-MP + 4-MP + 9-MP + 2-MP + 3-MP).

148

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Fig. 1. Pyrite framboid size distributions in selected samples and schematic diagram showing organic compound stabilities on the background of the weathering zones. HMW PAHs = high molecular weight polycyclic aromatic hydrocarbons.

and then decrease significantly for samples KQ136-2 and KQ136-1 (Table 1, Fig. 2). Similar trends were reported for low-temperature air oxidation experiments on low-organic carbon content Callovian shales (Faure et al., 1999; Elie et al., 2000, 2004).

3.2.2. Steranes and hopanes Steranes and hopanes are the most common biomarkers occurring since the Archean in different types of sedimentary environments (Brocks et al., 1999). They are widely used as paleoenvironmental, source and maturity indicators and their diagenetic and catagenetic transformations in sedimentary OM are well recognized (Peters et al., 2005; Killops and Killops, 2005).

The hopane and sterane distributions and molecular parameters for the unweathered, partially weathered and weathered zones are presented in Figs. 3–5, S-D and Table 2. The steranes are detectable using GC–MS but in two highly weathered samples (KQ136-2 and KQ136-1) they are only present as traces (Fig. S-D). However, they are readily detected using the more sensitive GC–MS–MS method (Fig. S-D). Also, in the case of the hopanes, they are detected best by the GC–MS– MS method, especially for the most weathered samples. Essentially all parameters calculated from the hopane and sterane distributions changed their values due to paleoweathering (Table 2), but the alteration usually occurs in the highly weathered zone (Figs. 3 and 4). More detailed interpretations of the results are presented below.

Fig. 2. Composite depth plot of the black shales showing the (A) contents of bulk carbonate (CC), (B) total organic carbon (TOC) and (C) total sulfur (TS), as well as (D) pristane to phytane (Pr/Ph) and (E) pristane to n-heptadecane (Pr/n-C17) ratios.

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

149

Fig. 3. Composite depth plot of the black shales showing the molecular parameters based on sterane distributions. For abbreviation definitions see Table 2.

3.2.3. Isorenieratane and aryl isoprenoids Isorenieratane and its diagenetic products are biomarkers of green sulfur bacteria and are used as important paleoindicators of photic zone euxinia in the water column (Summons and Powell, 1987; Sinninghe Damsté and Schouten, 2006). They are thermally less stable than n-alkanes or PAHs and are almost totally degraded during late catagenesis (Requejo et al., 1992). The diaryl isoprenoids, isorenieratane and 2,3,6-/3,4,5-trimethylsubstituted diaryl isoprenoids, are detectable but occur at very low concentrations in the unweathered black shale (Table 2, Fig. 6). For example, the concentrations of isorenieratane ranged from 22 to 33 μg/g

TOC in the Famennian Dasberg black shale of the Kowala section (Marynowski et al., 2010), and in the Hangenberg black shale its concentrations are N3 μg/g TOC (Marynowski and Filipiak, 2007). In these samples it ranged from 0.11 to 0.42 μg/g TOC in the unweathered part of the bed (Table 2). Moreover, both diaryl isoprenoids are sensitive to weathering as indicated by their disappearance in the partially weathered zone and absence in the weathered zone (Fig. 7A). A similar pattern is observed for the aryl isoprenoids (Fig. 7B). Their concentrations also decrease sharply with weathering and are near 0 μg/g TOC in the weathered part of the section. Especially noteworthy is that their distribution also changed significantly. As weathering progresses the

Fig. 4. Composite depth plot of the black shales showing the molecular parameters based on hopane distributions. For abbreviation definitions see Table 2.

150

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Fig. 5. Comparison of methylhopane distributions from the different weathering zones of the black shale using the GC–MS–MS method. Compound abbreviations: 2-MeH = 2αmethyl-17α(H)-hopane, 3-MeH = 2β-methyl-17α(H)-hopane. The 22S and 22R epimers are shown for C31-homohopanes. An HP-1 column was used.

concentrations of the low molecular weight (C13–C15) aryl isoprenoids (AI) decrease in relation to high molecular weight AI (Figs. 6 and 7C). Other diagenetic products of isorenieratane (see Koopmans et al., 1996; Clifford et al., 1998) also decrease in conjunction with oxidative weathering (Fig. 6).

3.2.4. Polycyclic aromatic compounds Polycyclic aromatic compounds (PACs) are major constituents of rock extracts, coals and crude oils, but their distributions strongly depend on OM source, depositional conditions and possible occurrence of pyrolytic and/or diagenetic processes (Radke, 1987; Simoneit, 1998; Finkelstein et al., 2005; Marynowski and Simoneit, 2009). PAHs are usually utilized as maturity indicators of source rocks and coals (Radke, 1988; Szczerba and Rospondek, 2010). Besides the abundant PAHs the other aromatic compounds common in sedimentary OM are oxygen, sulfur and nitrogen bearing PACs, often formed as diagenetic products (e.g. Asif et al., 2009). The concentrations of most of the PAHs decrease with the degree of weathering as is obvious in Fig. 7D and E (Table 3). The gradual decrease starts from the boundary between the partially-weathered and weathered zones and decreases to very low levels in the highly weathered zone (Fig. 7D and E, Table 3). It is interesting that some PAHs, like benzo[b]fluoranthene, increase in the boundary zone and then decrease again in the highly oxidized samples (Fig. 7F, see discussion below). Furthermore, the concentrations the PAHs also change significantly (Fig. 8). Some PAHs, e.g. benzo[a]pyrene, disappear in the early stage of weathering, while others, e.g. benzo [e]pyrene or benzo[b]fluoranthene, are more resistant to the oxidation processes (Fig. 8). On the other hand, low molecular weight aromatic compounds, as for example naphthalene, dibenzofuran and dibenzothiophene, decrease sharply already in the partially weath-

ered zone (Table 3). The detailed information about changes in PAH concentrations with reference to weathering zones is given in Table 3. 3.2.5. Maleimides Maleimides (1H-pyrrole-2,5-diones), which are polar degradation products of tetrapyrrole pigments such as chlorophyll a or bacteriochlorophylls c, d, and e (Grice et al., 1996), are present in the unweathered samples and detectable as traces in the partially weathered sample KQ136-3. They are totally absent in the more weathered samples. The typical distribution of maleimides in the non-altered black shale is similar to that previously described from the Hangenberg shale (Marynowski and Filipiak, 2007), where methylethylmaleimide is the most abundant component, and the second most abundant is dimethylmaleimide. Other 1H-pyrrole-2,5-dione derivatives are present only in trace amounts. 4. Discussion 4.1. Time of weathering An important aspect for a discussion of black shale weathering is the geological time of weathering and the range of OM maturity during the alteration. This is based mainly on a better understanding of the origin in the distribution changes of organic compounds. Because the whole Devonian–Carboniferous sequence in the Kowala area is covered by unweathered Permian (Zechsteinian or Rotliegendes) conglomerates, the paleoweathering should be younger than and most probably connected with the last, Upper Carboniferous– Early Permian phases of the Variscan folding. According to the modeling study of Narkiewicz et al. (2010) and previous presumptions (e.g. Belka, 1990) the maximum heat flow (significantly higher than the recent one, see Belka, 1990) caused by the Upper Palaeozoic

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

151

Fig. 6. Partial mass fragmentograms for m/z 133 showing the changes in the distribution of isorenieratane (filled circle), 2,3,6-/3,4,5-TM substituted diaryl isoprenoids (open circle) and their derivatives including aryl isoprenoids (numbers identify the individual carbon number of pseudohomologues) in the three black shale samples from the unweathered and partially weathered zones. An HP-5 column was used.

Fig. 7. Composite depth plot of the black shales showing: (A) isorenieratane and 2,3,6-/3,4,5-trimethyl-substituted diaryl isoprenoid concentrations (μg/g TOC), (B) C13–C22 aryl isoprenoid concentrations (μg/g TOC), (C) AIR, the aryl isoprenoids ratio, (D) pyrolytic PAH concentrations (μg/g TOC), (E) benzo[a]pyrene concentrations (μg/g TOC), and (F) benzo [b]fluoranthene concentrations (μg/g TOC). For abbreviation definitions see Tables 2 and 3.

152

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Table 3 Concentrations of the selected polycyclic aromatic compounds (in μg/g TOC). Sample

Ph

3-MP

3-MP

9-MP

1-MP

2-MN

1-MN

DBF

DBT

Fl

Py

BaA

Chr

Triph

BeP

BbFl

BaP

B[ghi]Pe

Cor

9-PhP

1-PhP

PAHs

KQ136_1 KQ136_2 KQ136_2A KQ136_2B KQ136_3 KQ136_4 KQ136_5 KQ136_6 KQ136_7

1.51 1.18 0.46 1.54 7.55 11.55 15.57 12.54 9.70

0.23 0.12 0.03 0.07 0.35 1.01 1.35 1.19 0.87

0.35 0.18 0.05 0.13 0.63 1.35 1.71 1.43 1.05

0.24 0.11 0.04 0.19 1.13 2.53 3.12 2.73 1.99

0.21 0.10 0.04 0.19 1.14 2.42 2.87 2.57 1.89

0.00 0.00 0.00 0.00 0.09 0.33 1.57 1.33 2.32

0.00 0.00 0.00 0.00 0.10 0.39 1.87 1.50 2.32

0.00 0.02 0.02 0.07 0.92 2.42 3.69 3.41 2.64

0.06 0.06 0.02 0.03 0.83 3.66 4.79 4.00 2.95

0.85 0.39 0.51 1.44 3.78 4.19 4.46 3.59 2.39

0.55 0.23 0.33 1.06 3.16 3.74 4.06 3.56 2.48

0.03 0.04 0.10 0.37 1.10 1.32 1.33 1.20 0.76

0.09 0.12 0.67 1.15 1.81 1.33 1.30 1.09 0.71

0.05 0.08 0.79 1.24 1.73 1.14 0.99 0.75 0.48

0.14 0.37 1.64 1.99 2.92 1.50 1.34 1.19 0.71

0.27 0.88 3.84 4.07 5.48 2.61 2.56 1.93 1.32

0.00 0.01 0.02 0.07 0.40 0.51 0.55 0.58 0.34

0.15 0.30 0.91 0.63 0.77 1.08 1.13 1.29 0.68

0.38 0.14 0.11 0.22 0.22 0.37 0.20

0.00 0.01 0.13 0.15 0.18 0.13 0.13 0.11 0.08

0.00 0.01 0.05 0.07 0.08 0.05 0.04 0.04 0.03

2.16 2.50 5.98 7.41 13.69 12.34 12.98 11.33 7.41

Ph = phenanthrene, MP = methylphenanthrene, MN = methylnaphthalene, DBF = dibenzofuran, DBT = dibenzothiophene, Fl = fluoranthene, Py = pyrene, BaA = benz[a] anthracene, Chr = chrysene, Triph = triphenylene, BeP = benzo[e]pyrene, BbFl = benzo[b]fluoranthene, BaP = benzo[a]pyrene, B[ghi]Pe = benzo[ghi]perylene, Cor = coronene, PhP = phenylphenanthrene, PAHs = Fl + Py + BbFl + BaP + B[ghi]P + Cor (Finkelstein et al., 2005).

sequences occurred during the Upper Carboniferous–Early Permian. This suggests that maximum thermal stress of this black shale happened during the time before that alteration, and therefore no other significant diagenetic changes took place after the weathering period. This implies that the current maturity and molecular composition of the unweathered black shale, at the stage of late diagenesis–early catagenesis (Ro = 0.5–0.65%, max temp = ~ 50– 80 °C, see Marynowski, 1999; Belka 1990), is more or less the same as that during the Upper Carboniferous–Lower Permian oxidation/ weathering period. Also symptomatic are karst processes that were especially intense during the Permian–Triassic Variscan tectonic movements and up-lift in the Holy Cross Mountains (Urban, 2007). According to Marynowski et al. (2002a) the geothermal gradients in the Mesozoic and Cenozoic were similar as today (17.2–29.6 °C km− 1), which, together with the thickness of the sedimentary rocks (not exceeding 2000 m in the Kowala area; Kutek and Głazek, 1972;

Narkiewicz et al., 2010) younger than Permian, implies maximum temperatures lower than 60 °C. 4.2. Molecular changes caused by paleoweathering The stability of extractable organic compounds from sedimentary rocks and petroleum varies significantly due to differences in thermodynamic properties of individual hydrocarbons (e.g. Laurent and Helgeson, 1998). Some of them, especially those with biological configurations, are unstable and are preferentially cracked or transformed due to isomerization and aromatization processes. Other groups of compounds, like many unsubstituted PAHs or n-alkanes, are thermally stable and may occur in hydrothermal petroleum (Simoneit and Fetzer, 1996; McCollom et al., 1999) or metamorphic rocks (Price, 1993; Schwab et al., 2005). In contrast, much less is known about the stability of organic compounds under oxidative weathering conditions. Clayton and King

Fig. 8. Summed mass fragmentograms for m/z 228 + 252 showing the changes in the distributions of four and five ring PAHs in the unweathered, partially weathered and weathered zones. A DB-35MS column was used.

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

(1987) showed for the first time changes due to weathering in the distributions of regular steranes, triaromatic steroid hydrocarbons and methylphenanthrenes. Experiments of natural and artificial oxidation of organic-poor sediments revealed changes in n-alkane distributions and evolution of several geochemical parameters like: 20S/(20S + 20R) for C29 steranes, 22S/(22S + 22R) for C31 hopanes, sum of methylphenanthrenes to phenanthrene ratio, methylphenanthrene index or pristane to n-heptadecane ratio (Faure et al., 1999; Elie et al., 2000). All changes noted for the parameters above were similar to those occurring during thermal maturation. Also Püttmann et al. (1989), Sun (1998) and Sun and Püttmann (2001) reported that secondary, hydrothermal oxidation of OM influenced the maturity parameters which are based on the PAH distributions. Moreover, Bennett and Larter (2000) and Charrié-Duhaut et al. (2000) proposed that oxygenated compounds like fluorenones, steroid ketones, benzothiophenic acids and sulfones are formed as a result of post-sampling oxidation of OM. In most cases the results above are based on simulation experiments, while data from natural, ambient OM oxidation are still sporadic. Generally, in our samples the aliphatic hydrocarbons, including the biomarkers, show different variations in their distributions caused by weathering effects, but in many cases these differences appear only in the zone of highly advanced weathering. For example, almost all sterane ratios change significantly in the late stage of weathering (samples KQ136-1 and KQ136-2; see Fig. 3, Table 2), but besides weathering we cannot exclude the water washing effect (Palmer, 1993). Interestingly, our data do not confirm the previous report on sterane changes caused by weathering (Clayton and King, 1987). Those authors described the relative loss of the 20S diastereomer of the C29 sterane due to weathering with a gradual decrease of the 20S/ (20S + 20R) ratio. Here we observe the opposite for the low maturity samples where the 20S/(20S + 20R) ratios increase with degree of paleoweathering (Fig. 3B). These results are in accordance with laboratory simulation experiments, where under artificial oxidation of an immature shale the 20S/(20S + 20R) ratio values also drastically increased (Elie et al., 2000). The same or similar pattern of alteration is observed for other sterane ratios such as the ββ/(ββ + αα) ratio (Fig. 3C), the diasteranes vs. steranes ratio (Fig. 3D), or the less characteristic 20S/(20S + 20R) 13β,17α-diasterane ratio (Fig. 3E). It is therefore possible that the sterane weathering transformations proceed in different ways, depending on their distribution in the non-weathered samples which resulted from the thermal maturation of the source rock. Certain hopane ratios, e.g. the 2-MeHI, are almost totally resistant to changes (Fig. 4C) which suggests that the rate of degradation of some pentacyclic triterpanes is more or less similar and does not change in values of particular compound ratios. However, the other biomarker parameters based on hopane distributions changed due to paleoweathering (Table 2, Fig. 4). It is noteworthy that Clayton and King (1987) mention source and maturity interpretations based on pentacyclic triterpenoids were not affected by surface weathering. Here we show that almost all biomarker parameters changed during paleoweathering, especially in the advanced range of alteration (Fig. 4). Besides changes in sterane and hopane distributions, the variation in the relationship between these two biomarker groups is also noted. The ratio of 17α(H)-hopane to C29-ααα 20R sterane (Fig. 3A) and the total sterane to total hopane ratio (Table 2) gradually increase and decrease, respectively. This suggests that steranes are less resistant to weathering than hopanes and they disappear already in the partially weathered zone. The differences between the aromatic hydrocarbon compositions of the weathered and unweathered samples are even more significant than for the aliphatic hydrocarbons (Fig. 9). It is interesting that changes in individual compound groups occur at different weathering stages. For example, when the aryl isoprenoid concentrations decreased almost to 0 μg/TOC (Fig. 7B), the pyrolytic PAHs still

153

survive at concentrations similar to those for the unaltered samples (Fig. 7D; Table 3). This indicates that the degradation rate of isorenieratane and its derivatives is much greater than that of the PAHs. Furthermore, the degradation rate of isorenieratane and aryl isoprenoids is similar to pyrite degradation, which suggests that some less stable parts of the OM react with O2 as fast as pyrite (see Wildman et al., 2004 and compare with Tables 1 and 2; Figs. 2 and 7). The occurrence of PAHs in black shale is probably connected with wildfires, as is also detectable in the slightly older part of the section (Marynowski and Filipiak, 2007). We find PAHs in both the oxidized and unoxidized parts of the black shale, but in the latter case the PAHs are overwhelmed by the quantitatively more important isorenieratane derivatives as well as other, less stable compounds (Fig. 9). However, some PAHs detected in this black shale are not only connected with wildfires, but may also have formed by other processes. For example benzo[b]fluoranthene increased in the boundary zone and decreased again in the highly oxidized samples (Fig. 7F). This may indicate limited oxidation of OM caused by geosynthesis (sensu Alexander et al., 1995) of this compound under partial oxic conditions (see also Marynowski et al., 2002b; Rospondek et al., 2007, 2009). The same is noted here for the less stable phenyl-derivatives of PAHs like 9- and 1-phenylphenanthrene (PhP; Table 3) which suggests that they are products of diagenetic oxidation (usually hydrothermal—see Marynowski et al., 2002b; Rospondek et al., 2009). We propose that benzo[b] fluoranthene is the oxidation product from the aromatization of 9and 1-PhP, which occurred during weathering. PhP formation also proceeds during thermal maturation. Thus, these processes explain the increase of benzo[b]fluoranthene in the partially oxidized zone compared to the other PAHs (Fig. 7, Table 3). The concentrations of phenanthrene, as well as other PAHs and their methyl-derivatives, decreased byN90% in the weathered zone (Table 3). It is known that phenanthrene and other unsubstituted PAHs are characterized by a greater stability than their methylderivatives under the influence of oxidation processes (Clayton and King, 1987; Püttmann et al., 1989; Gieskes et al., 1990; Speczik et al., 1995; Bechtel et al., 2000, 2001; Marynowski and Wyszomirski, 2008). Therefore, the ratio of phenanthrene to the sum of the methylphenanthrenes (Ph/ΣMP) increased with weathering (Table 2). But surprisingly, the reverse trend is observed in the most oxidized sample KQ136-1 (Table 2). The methylphenanthrene indices MPI1 and MPI3, both increase with weathering (Table 2) in accordance with the studies on natural oxidation of Kupferschiefer (Püttmann et al., 1989; Speczik et al., 1995), but opposite to the report that the MPI values did not change significantly during weathering by Clayton and King (1987). The important differences in PAH distributions between individual weathering zones (Figs. 8 and 9) suggest that in some cases the distribution differences ascribed to evidence for wildfires may be connected with weathering processes (e.g. Finkelstein et al., 2005; Marynowski and Simoneit, 2009; Marynowski and Zatoń, 2010; Nabbefeld et al., 2010). Another characteristic manifestation due to weathering is a significant depletion of low-molecular-weight aromatic compounds (Gieskes et al., 1990) such as the methylnaphthalenes (Table 3). They are very sensitive to weathering coupled with water washing (Skręt et al., 2010) and therefore their concentrations decrease even in the upper part of the non-altered black shale (Table 3). A similar situation is the case for low-molecular-weight oxygen and sulfur aromatic compounds, i.e. dibenzofuran and dibenzothiophene (Table 3), although their degradation or loss occurs a bit later. In Fig. 1 we show the stability of selected organic compounds in relation to the paleoweathering zones influencing the black shale. The diagram is based on the occurrence and lack of particular organic compound groups in these samples. Interestingly, some of these compounds are less resistant to weathering than pyrite framboids,

154

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Fig. 9. TIC traces of the aromatic fractions of: (A) sample KQ 136-2B from the partially weathered zone with the predominance of high molecular weight PAHs and high amounts of oxygen-containing aromatic compounds and (B) sample KQ 136–7 from the unweathered zone with a predominance of aryl isoprenoids and alkyl derivatives of PAHs. DNF = dinaphthofuran isomers, B[b]NF = benzo[b]naphthofuran isomers, BBF = benzobisbenzofuran isomers, PhDBF = phenyldibenzofuran isomers, PhPh = phenylphenanthrene isomers, MP = methylphenanthrenes, DMP = dimethylphenanthrenes, MN = methylnaphthalenes, TMN = trimethylnaphthalenes, TeMN = tetramethylnaphthalenes, BH = benzohopanes, TAS = triaromatic steroids, UCM = unresolved complex mixture, numbers identify the individual carbon number of pseudohomologues of aryl isoprenoids. IS = internal standard, a DB-35MS column was used.

while others are partially resistant even in the highly weathered zone. This indicates that OM loss is a consequence of complex decomposition and degradation processes. 4.3. Paleoenvironmental and OM source implications Paleoenvironmental reconstruction studies based on biomarker data usually do not take into account paleoweathering processes which can significantly change the molecular composition of the extractable organic matter. This is particularly important in the case of partial weathering which may be macroscopically unrecognizable. One of the best examples of potentially misleading biomarker interpretations is based on the aryl isoprenoid distributions. In our black shale horizon the AIR values are high in the non-weathered part of the bed which corresponds to the interpretation by Schwark and Frimmel (2004) of intermittent photic zone anoxia, and very low values in the weathered section as also reported by those authors to indicate persistent anoxic conditions (Table 2, Fig. 7C). It is also interesting that our paleoweathering data indicate the AIR, with the AI concentrations, are very sensitive weathering parameters due to their gradual decrease with progressive oxidation (Fig. 6 and Fig. 7B and C). This data explains the inconsistency in AIR values with other biomarker parameters reported recently from the Famennian Dasberg section of Kowala (Marynowski et al., 2010; see also Racka et al., 2010), which is probably connected with weathering/oxidation processes of the more susceptible organic-poor samples. It confirms that when an interpretation of photic zone anoxia is made based on characteristic aryl isoprenoid distributions, the potential weathering processes must be taken into account. Most of the molecular parameters for the hopane and sterane groups change their values in the highly weathered zone, but some,

like 22S/(22S + 22R) (Fig. 4) or C30H/C29αααR (Fig. 3A), only change gradually. This is noteworthy in considerations of paleoweathering processes also in drill core materials when prospecting for hydrocarbons and using those biomarker parameters as paleoenvironmental, OM source and maturity indicators. The maleimides, degradation products of the photosynthetic tetrapyrrole pigments, are used as indicators of anoxygenic photosynthesis (Grice et al., 1996), especially the most characteristic methyl-isobutyl-maleimide (Pancost et al., 2002). Here we have shown that the maleimides are less resistant to weathering processes (Fig. 1) and thus are absent in weathered or partially weathered surface and core samples. They should be considered as indicators for the effects of oxidative weathering. On the other hand, as noted above, some molecular parameters seem to be quite resistant to oxidation processes. For example changes in the 2-MeHI are not significant even in highly weathered samples (Table 2, Figs. 4c and 5). Because this parameter is commonly used in assessing trace input from cyanobacteria (e.g. Summons et al., 1999; Brocks et al., 1999) and episodes of microbial changes during mass extinction events (Xie et al., 2005, 2010; Jiao et al., 2009) its stability is very important for the interpretation above. 5. Conclusions The study of paleoweathering of a Lower Carboniferous (Tournaisian) black shale revealed a significant (up to 97%) decrease of TOC, total loss of sulfur, as well as changes in carbonate content, extract yields and percentage yields of organic fractions. During the paleoweathering progression the study of pyrite framboid diameters is difficult or impossible even in the seemingly non-weathered shale due to the rapid decay of pyrite.

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Paleoweathering processes could significantly influence paleoenvironmental, source and maturity interpretations when based on biomarker parameters. This affects not only surface samples in which case weathering is usually recognizable, but potentially also drill cores where rock sequences exposed to aerial paleoweathering processes may occur. The concentrations of all organic compounds decrease with degree of weathering alteration, but the rate of depletion depends on the stability of a particular compound group. For example low molecular weight aromatic compounds like methylnaphthalenes, dibenzofuran, dibenzothiophene decrease significantly or disappear already in the partially weathered zone. The same is the case for the diaryl isoprenoids, i.e. isorenieratane and 2,3,6-/3,4,5-trimethylsubstituted diaryl isoprenoids and their diagenetic products, as well as the maleimides. Rapid degradation during oxidative weathering of these commonly used biomarkers is particularly important, because as they are preferentially degraded, the paleoenvironmental interpretations based on their distributions need to be applied with caution. On the other hand, the more stable PAHs exhibit a ~90% decrease only in the weathered and highly weathered zones. Interestingly, certain PAHs like benzo[b]fluoranthene increase in concentration in the partially weathered zone due to their formation from phenyl-derivatives and subsequently decrease in the highly weathered zone. Almost all biomarker parameters, i.e. ratios based on the sterane and hopane distributions, change in the highly weathered zone, but some methylhopane ratios, e.g. the 2-MeH index, are almost totally resistant to change. The changes in sterane distributions caused by weathering differ from those reported previously by Clayton and King (1987), which implies that such changes may depend on type of weathering and/or OM maturity. The aryl isoprenoid ratio (AIR) values decrease gradually with weathering, starting in the partially weathered zone, which completely modifies its potential for interpreting the nature of photic zone anoxia. The results presented here may aid in the correct recognition of the signs of paleoweathering in surface and drill core samples and improve the understanding of the processes that occur during weathering.

Acknowledgements This work was supported by MNISW grants: NN307 4272 34 (to MR) and NN307 2379 33 (to LM). MR wishes to acknowledge the European Social Fund (UPGOW scholarship) for financial support. The authors thank the manager of the Kowala quarry, Mr. Kazimierz Kwiecień, for his kindness and long-term support during the field work.

Appendix A. Supplementary materials Supplementary data to this article can be found online at doi:10.1016/j.chemgeo.2011.04.001.

References Alexander, R., Bastow, T.P., Fisher, S.J., Kagi, R.I., 1995. Geosynthesis of organic compounds: II. Methylation of phenanthrene and alkylphenanthrenes. Geochimica et Cosmochimica Acta 59, 4259–4266. Ascough, P.L., Bird, M.I., Scott, A.C., Collinson, M.E., Cohen-Ofri, I., Snape, C.E., Le Manquais, K., 2010. Charcoal reflectance measurements: implications for structural characterization and assessment of diagenetic alteration. Journal of Archaeological Science 37, 1590–1599. Asif, M., Alexander, R., Fazeelat, T., Pierce, K., 2009. Geosynthesis of dibenzothiophene and alkyl dibenzothiophenes in crude oils and sediments by carbon catalysis. Organic Geochemistry 40, 895–901. Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S., 2000. Geochemical and isotopic composition of organic matter in the Kupferschiefer of the Polish Zechstein basin: relation to maturity and base metal mineralization. International Journal of Earth Sciences 89, 72–89.

155

Bechtel, A., Gratzer, R., Püttmann, W., Oszczepalski, S., 2001. Variable alteration of organic matter in relation to metal zoning at the Rote Fäule front (LubinSieroszowice mining district, SW Poland). Organic Geochemistry 32, 377–395. Belka, Z., 1990. Thermal maturation and burial history from conodont colour alteration data. Holy Cross Mountains, Poland. Courier Forschungs-Institut Senckenberg 118, 241–251. Bennett, B., Larter, S.R., 2000. The isolation, occurrence and origin of fluorenones in crude oils and rock extracts. Organic Geochemistry 31, 117–126. Berkowski, B., 2002. Famennian Rugosa and Heterocorallia from Southern Poland. Palaeontologia Polonica 61, 3–88. Berner, R.A., 1989. Biogeochemical cycles of carbon and sulphur and their effect on atmospheric oxygen over Phanerozoic time. Palaeogeography, Palaeoclimatology, Palaeoecology 75, 97–122. Bolton, E.W., Berner, R.A., Petsch, S.T., 2006. The weathering of sedimentary organic matter as a control on atmospheric O2: II. Theoretical modeling. American Journal of Science 306, 575–615. Bond, D.P.G., Wignall, P.B., 2010. Pyrite framboid study of marine Permian–Triassic boundary sections: a complex anoxic event and its relationship to contemporaneous mass extinction. Geological Society of America Bulletin 122, 1265–1279. Brocks, J.J., Logan, G.A., Buick, R., Summons, R.E., 1999. Archean molecular fossils and the early rise of eukaryotes. Science 285, 1033–1036. Chang, S., Berner, R.A., 1999. Coal weathering and the geochemical carbon cycle. Geochimica et Cosmochimica Acta 63, 3301–3310. Charrié-Duhaut, A., Lemoine, S., Adam, P., Connan, J., Albrecht, P., 2000. Abiotic oxidation of petroleum bitumens under natural conditions. Organic Geochemistry 31, 977–1003. Clayton, J.L., Swetland, P.J., 1978. Subaerial weathering of sedimentary organic matter. Geochimica et Cosmochimica Acta 42, 305–312. Clayton, J.L., King, J.D., 1987. Effects of weathering on biological marker and aromatic hydrocarbon composition of organic matter in Phosphoria shale outcrop. Geochimica et Cosmochimica Acta 51, 2153–2157. Clifford, D.J., Clayton, J.L., Sinninghe Damsté, J.S., 1998. 2,3,6-/3,4,5-Trimethyl substituted diaryl carotenoid derivatives (Chlorobiaceae) in petroleums of the Belarussian Pripyat River Basin. Organic Geochemistry 29, 1253–1267. Didyk, B.M., Simoneit, B.R.T., Brassell, S.C., Eglinton, G., 1978. Organic geochemical indicators of palaeoenvironmental conditions of sedimentation. Nature 272, 216–222. Elie, M., Faure, P., Michels, R., Landais, P., Griffault, L., 2000. Natural and laboratory oxidation of low-organic-carbon-content sediments: comparison of chemical changes in hydrocarbons. Energy & Fuels 14, 854–861. Elie, M., Faure, P., Michels, R., Landais, P., Griffault, L., Mansuy, L., Martinez, L., 2004. Effects of water-cement solutions on the composition of organic compounds leached from oxidized Callovo-Oxfordian argillaceous sediment. Applied Clay Science 26, 309–323. Faure, P., Landais, P., Griffault, L., 1999. Behavior of organic matter from Callovian shales during low-temperature air oxidation. Fuel 78, 1515–1525. Finkelstein, D.B., Pratt, L.M., Curtin, T.M., Brassell, S.C., 2005. Wildfires and seasonal aridity recorded in Late Cretaceous strata from south-eastern Arizona, USA. Sedimentology 52, 587–599. Fischer, C., Gaupp, R., 2005. Change of black shale organic material surface area during oxidative weathering: implications for rock–water surface evolution. Geochimica et Cosmochimica Acta 69, 1213–1224. Fischer, C., Karius, V., Thiel, V., 2007. Organic matter in black slate shows oxidative degradation within only a few decades. Journal of Sedimentary Research 77, 355–365. Fischer, C., Schmidt, C., Bauer, A., Gaupp, R., Heide, K., 2009. Mineralogical and geochemical alteration of low-grade metamorphic black slates due to oxidative weathering. Chemie der Erde—Geochemistry 69, 127–142. Gieskes, J.M., Simoneit, B.R.T., Magenheim, A.J., Leif, R.N., 1990. Retrograde oxidation of hydrothermal precipitates and petroleum in Escanaba Trough sediments. Applied Geochemistry 5, 93–101. Grice, K., Gibbson, R., Atkinson, J.E., Schwark, L., Eckardt, C.B., Maxwell, J.R., 1996. Maleimides (1H-pyrrole-2,5-diones) as molecular indicators of anoxygenic photosynthesis in ancient water column. Geochimica et Cosmochimica Acta 60, 3913–3924. Haberstroh, P.R., Brandes, J.A., Gélinas, Y., Dickens, A.F., Wirick, S., Cody, G., 2006. Chemical composition of the graphitic black carbon fraction in riverine and marine sediments at sub-micron scales using carbon X-ray spectromicroscopy. Geochimica et Cosmochimica Acta 70, 1483–1494. Hartnett, H.E., Keil, R.G., Hedges, J.I., Devon, A.H., 1998. Influence of oxygen exposure time on organic carbon preservation in continental margin sediments. Nature 391, 572–574. Jaffe, L.A., Peucker-Ehrenbrink, B., Petsch, S.T., 2002. Mobility of rhenium, platinum group elements and organic carbon during black shale weathering. Earth and Planetary Science Letters 198, 339–353. Jiao, D., Perry, R.S., Engel, M.H., Sephton, M.A., 2009. Biomarker indicators of bacterial activity and organic fluxes during end Triassic mass extinction event. Proceedings of SPIE. The International Society for Optical Engineering 7097 doi:10.1117/12.796160. Killops, S.D., Killops, V.J., 2005. An Introduction to Organic Geochemistry. WileyBlackwell. 408 pp. Koopmans, M.P., Köster, J., van Kaam-Peters, H.M.E., Kenig, F., Schouten, S., Hartgers, W.A., de Leeuw, J.W., Sinninghe Damsté, J.S., 1996. Diagenetic and catagenetic products of isorenieratene: molecular indicators for photic zone anoxia. Geochimica et Cosmochimica Acta 60, 4467–4496. Kutek, J., Głazek, J., 1972. The Holy Cross area, central Poland, in the Alpine cycle. Acta Geologica Polonica 22, 604–653.

156

L. Marynowski et al. / Chemical Geology 285 (2011) 144–156

Laurent, R., Helgeson, H.C., 1998. Calculation of the thermodynamic properties at elevated temperatures and pressures of saturated and aromatic high molecular weight solid and liquid hydrocarbons in kerogen, bitumen, petroleum, and other organic matter of biogeochemical interest. Geochimica et Cosmochimica Acta 62, 3591–3636. Leythaeuser, D., 1973. Effects of weathering on organic matter in shales. Geochimica et Cosmochimica Acta 37, 113–120. Littke, R., Klussmann, U., Krooss, B., Leythaeuser, D., 1989. Quantification of loss of calcite, pyrite, and organic matter due to weathering of Toarcian black shales and effects on kerogen and bitumen characteristics. Geochimica et Cosmochimica Acta 55, 3369–3378. Lo, H.B., Cardott, B.J., 1995. Detection of natural weathering of Upper McAlester coal and Woodford Shale, Oklahoma, U.S.A. Organic Geochemistry 22, 73–83. Marynowski, L., 1999. Thermal maturity of organic matter in Devonian rocks of the Holy Cross Mountains. Przegląd Geologiczny 47, 1125–1129 (in Polish with English summary). Marynowski, L., Filipiak, P., 2007. Water column euxinia and wildfire evidence during deposition of the Upper Famennian Hangenberg event horizon from the Holy Cross Mountains (central Poland). Geological Magazine 144, 569–595. Marynowski, L., Simoneit, B.R.T., 2009. Widespread Upper Triassic to Lower Jurassic wildfire records from Poland: evidence from charcoal and pyrolytic polycyclic aromatic hydrocarbons. Palaios 24, 785–798. Marynowski, L., Wyszomirski, P., 2008. Organic geochemical evidences of early diagenetic oxidation of the terrestrial organic matter during the Triassic arid and semi arid climatic conditions. Applied Geochemistry 23, 2612–2618. Marynowski, L., Zatoń, M., 2010. Organic matter from the Callovian (Middle Jurassic) deposits of Lithuania: compositions, sources and depositional environments. Applied Geochemistry 25, 933–946. Marynowski, L., Czechowski, F., Simoneit, B.R.T., 2001. Phenylnaphthalenes and polyphenyls in Palaeozoic source rocks of the Holy Cross Mountains, Poland. Organic Geochemistry 32, 69–85. Marynowski, L., Salamon, M., Narkiewicz, M., 2002a. Thermal maturity and depositional environments of organic matter in the post-Variscan succession of the Holy Cross Mountains. Geological Quarterly 46, 25–36. Marynowski, L., Rospondek, M.J., Meyer zu Reckendorf, R., Simoneit, B.R.T., 2002b. Phenyldibenzofurans and phenyldibenzothiophenes in marine sedimentary rocks and hydrothermal petroleum. Organic Geochemistry 33, 701–714. Marynowski, L., Filipiak, P., Zatoń, M., 2010. Geochemical and palynological study of the Upper Famennian Dasberg event horizon from the Holy Cross Mountains (central Poland). Geological Magazine 147, 527–550. McCollom, T.M., Simoneit, B.R.T., Shock, E.L., 1999. Hydrous pyrolysis of polycyclic aromatic hydrocarbons and implications for the origin of PAH in hydrothermal petroleum. Energy & Fuels 13, 401–410. Nabbefeld, B., Grice, K., Summons, R.E., Hays, L.E., Cao, C., 2010. Significance of polycyclic aromatic hydrocarbons (PAHs) in Permian/Triassic boundary sections. Applied Geochemistry 25, 1374–1382. Narkiewicz, M., Resak, M., Littke, R., Marynowski, L., 2010. New constraints on the Middle Palaeozoic to Cenozoic burial and thermal history of the Holy Cross Mts. (Central Poland): results of numerical modeling. Geologica Acta 8, 189–205. Pancost, R.D., Crawford, N., Maxwell, J.R., 2002. Molecular evidence for basin-scale photic zone euxinia in the Permian Zechstein Sea. Chemical Geology 188, 217–227. Palmer, S., 1993. Effect of biodegradation and water washing on crude oil composition. In: Engel, M.H., Macko, S.A. (Eds.), Organic Geochemistry. Plenum Press, New York, pp. 511–533. Peters, K.E., Walters, C.C., Moldowan, J.M., 2005. The Biomarker Guide, Biomarkers and Isotopes in Petroleum Exploration and Earth History2nd ed. Cambridge University Press, New York, NY. Petsch, S.T., Berner, R.A., Eglinton, T.I., 2000. A field study of the chemical weathering of ancient sedimentary organic matter. Organic Geochemistry 31, 475–487. Petsch, S.T., Edwards, K.J., Eglinton, T.I., 2005. Microbial transformations of organic matter in black shales and implications for global biogeochemical cycles. Palaeogeography, Palaeoclimatology, Palaeoecology 219, 157–170. Prahl, F.G., De Lange, G.J., Scholten, S., Cowie, G.L., 1997. A case of post-depositional aerobic degradation of terrestrial organic matter in turbidite deposits from the Madeira Abyssal Plain. Organic Geochemistry 27, 141–152. Price, L.C., 1993. Thermal stability of hydrocarbons in nature: limits, evidence, characteristics and possible controls. Geochimica et Cosmochimica Acta 57, 3261–3280. Peucker-Ehrenbrink, B., Hannigan, R.E., 2000. Effects of black shale weathering on the mobility of rhenium and platinum group elements. Geology 28, 475–478. Püttmann, W., Merz, C., Speczik, S., 1989. The secondary oxidation of organic material and its influence on Kupferschiefer mineralization of southwest Poland. Applied Geochemistry 4, 151–161. Racka, M., Marynowski, L., Filipiak, P., Sobstel, M., Pisarzowska, A., Bond, D.P.J., 2010. Anoxic Annulata Events in the Late Famennian of the Holy Cross Mountains (Southern Poland): Geochemical and palaeontological record. Palaeogeography, Palaeoclimatology, Palaeoecology 297, 549–575. Radke, M., 1987. Organic geochemistry of aromatic hydrocarbons. In: Brooks, J., Welte, D. (Eds.), Advances in Petroleum Geochemistry, 2. Academic Press, New York, pp. 141–207.

Radke, M., 1988. Application of aromatic compounds as maturity indicators in source rocks and crude oils. Marine and Petroleum Geology 5, 224–236. Radke, M., Welte, D.H., 1983. The Methylphenanthrene Index (MPI). A maturity parameter based on aromatic hydrocarbons. In: Bjorøy, M., et al. (Ed.), Advances in Organic Geochemistry 1981. J. Wiley and Sons, New York, pp. 504–512. Rakociński, M., 2009. Record of Lower Carboniferous anoxic events and volcanic activity in the Kowala Quarry near Kielce (central Poland). Przegląd Geologiczny 57, 1046–1047 (in Polish). Requejo, A.G., Allan, J., Creaney, S., Gray, N.R., Cole, K.S., 1992. Aryl isoprenoids and diaromatic carotenoids in Paleozoic source rocks and oils from the Western Canada and Williston Basins. Organic Geochemistry 19, 245–264. Rospondek, M.J., Marynowski, L., Góra, M., 2007. Novel arylated polyaromatic thiophenes: phenylnaphtho[b]thiophenes and naphthylbenzo[b]thiophenes as markers of organic matter diagenesis buffered by oxidising solutions. Organic Geochemistry 38, 1729–1756. Rospondek, M.J., Marynowski, L., Chachaj, A., Góra, M., 2009. Novel aryl polycyclic aromatic hydrocarbons: phenylphenanthrene and phenylanthracene identification, occurrence and distribution in sedimentary rocks. Organic Geochemistry 40, 986–1004. Schwab, V., Spangenberg, J.E., Grimalt, J.O., 2005. Chemical and carbon isotopic evolution of hydrocarbons during prograde metamorphism from 100 °C to 550 °C: case study in the Liassic black shale formation of the Central Swiss Alps. Geochimica et Cosmochimica Acta 69, 1825–1840. Schwark, L., Frimmel, A., 2004. Chemostratigraphy of the Posidonia Black Shale, SWGermany II. Assessment of extent and persistence of photic-zone anoxia using aryl isoprenoid distribution. Chemical Geology 206, 231–248. Scott, A.C., 2010. Charcoal recognition, taphonomy and uses in palaeoenvironmental analysis. Palaeogeography, Palaeoclimatology, Palaeoecology 291, 11–39. Simoneit, B.R.T., 1998. Biomarker PAHs in the environment. In: Neilson, A.H. (Ed.), Part I, PAHs and Related Compounds. The Handbook of Environmental Chemistry, vol. 3. Springer-Verlag, Berlin, pp. 176–221. Simoneit, B.R.T., Fetzer, J.C., 1996. High molecular weight polycyclic aromatic hydrocarbons in hydrotermal petroleums from the Gulf of California and Northeast Pacific Ocean. Organic Geochemistry 24, 1065–1077. Sinninghe Damsté, J.S., Rijpstra, W.I.C., Reichart, G.-J., 2002. The influence of oxic degradation on the sedimentary biomarker record II. Evidence from Arabian Sea sediments. Geochimica et Cosmochimica Acta 66, 2737–2754. Sinninghe Damsté, J.S., Schouten, S., 2006. Biological markers for anoxia in the photic zone of the water column. In: Volkman, J. (Ed.), Marine Organic Matter: Biomarkers, Isotopes and DNA. The Handbook of Environmental Chemistry, vol. 2. Springer Verlag, Berlin, pp. 127–163. part N. Summons, R.E., Powell, T.G., 1987. Identification of aryl isoprenoids in source rocks and crude oils: biological markers for the green sulfur bacteria. Geochimica et Cosmochimica Acta 51, 557–566. Skręt, U., Fabiańska, M.J., Misz-Kennan, M., 2010. Simulated water-washing of organic compounds from self-heated coal wastes of the Rymer Cones Dump (Upper Silesia Coal Region, Poland). Organic Geochemistry 41, 1009–1012. Speczik, S., Bechtel, A., Sun, Y.Z., Püttmann, W., 1995. A stable isotope and organic geochemical study of the relationship between the Antracosia shale and Kupferschiefer mineralization (SE Poland). Chemical Geology 123, 133–151. Summons, R.E., Jahnke, L.L., Hope, J.M., Logan, G.A., 1999. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400, 554–557. Sun, Y.-Z., 1998. Influence of secondary oxidation and sulfide formation on several maturity parameters in Kupferschiefer. Organic Geochemistry 29, 1419–1429. Sun, Y.-Z., Püttmann, W., 2001. Oxidation of organic matter in the transition zone of the Zechstein Kupferschiefer from the Sangerhausen Basin, Germany. Energy & Fuels 15, 817–829. Szczerba, M., Rospondek, M.J., 2010. Controls on distributions of methylphenanthrenes in sedimentary rock extracts: critical evaluation of existing geochemical data from molecular modeling. Organic Geochemistry 41, 1297–1311. Szulczewski, M., 1995. Depositional evolution of the Holy Cross Mts. (Poland) in the Devonian and Carboniferous—a review. Geological Quarterly 39, 471–488. Urban, J., 2007. Permian to Triassic paleokarst of the Świętokrzyskie (Holy Cross) Mts, central Poland. Geologia AGH 33, 5–50. Van Os, B., Middelburg, J.J., De Lange, G.J., 1996. Extensive degradation and fractionation of organic matter during subsurface weathering. Aquatic Geochemistry 1, 303–312. Wignall, P.B., Newton, R., 1998. Pyrite framboid diameter as a measure of oxygendeficiency in ancient mudrocks. American Journal of Science 298, 537–552. Wildman, R.A., Berner, R.A., Petsch, S.T., Bolton, E.W., Eckert, J.O., Mok, U., Evans, J.B., 2004. The weathering of sedimentary organic matter as a control on atmospheric O2: I. Analysis of black shale. American Journal of Science 304, 234–249. Wilkin, R.T., Barnes, H.L., Brantley, S.L., 1996. The size distribution of framboidal pyrite in modern sediments: an indicator of redox conditions. Geochimica et Cosmochimica Acta 60, 3897–3912. Xie, S., Pancost, R.D., Yin, H., Wang, H., Evershed, R.P., 2005. Two episodes of microbial change coupled with Permo/Triassic faunal mass extinction. Nature 434, 494–497. Xie, S.C., Pancost, R.D., Wang, Y.B., Yang, H., Wignall, P.B., Luo, G.M., Jia, C.L., Chen, L., 2010. Cyanobacterial blooms tied to volcanism during the 5 m.y. Permo-Triassic biotic crisis. Geology 38, 447–450.