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Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary
Journal of China University of Geosciences , V a l . 1 7 , No. 1 , p. 49 - 54 , March 2006 Printed in China
I S S N 1002-0705
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary Huang Xianyu ($&j!?,fi) Jiao Dan (gB) State K e y Laboratory o f Geological Processes and Mineral Resources, China University o f Geosciences , Wuhan 430074, China Lu Liqiang (%$$$) Faculty o f Material Science and Chemical Engineering, China University o f Geosciences , W u h a n 430074, China Huang Junhua (g@q) State K e y Laboratory o f Geological Processes and Mineral Resources, China University o f Geosciences , Wuhan 430074, China Xie Shucheng" (d$#H&) State K e y Laboratory o f Geological Processes and Mineral Resources, T h e K e y Laboratory of' Biogeology and Environmental Geology o f the Ministry of Education, China University o f Geosciences , W u h a n 430074, China ABSTRACT: Aromatic compounds extracted from sedimentary rocks can reflect environmental conditions, organic sources and maturity. The aromatics, identified in association with mass extinction in particular, would provide a signature assisting our understanding of the causes of the biotic crisis. Aromatic hydrocarbons were fractionated from the total lipid extracts of 37 samples taken from the PermianProvince in South Triassic boundary (beds 23 to 34) of section B at Meishan (%!$&),Zhejiang (?%?I) China. These aromatics were analyzed by using gas chromatography-mass spectrometry (GC-MS). Main compounds identified include naphthalene, phenanthrenes , fluorene , dibenzothiophene , dibenzofuran , fluoranthene, pyrene and some of their methyl homologues. The indices of methyl phenanthrene distribution fraction indicate the comparable maturity (within the oil window, 0.7% - 1.0% of the mean vitrinite reflectance) of the organic matter throughout the whole profile analyzed. The ratio of dibenzothiophene to phenanthrene (DBW PHN) varies generally at a comparable pace with lithology. Significantly, a gradual decrease of this ratio was observed within bed 24 limestone, which is probably due to the variation of sedimentary environment. This change is in line with the drop in the carbon isotope composition of carbonate, the loss of the Changhsingian reef ecosystem, and the decrease of cyanobacteria abundance within the bacteria population. The coincidence of these records suggests a close relation between the biotic crisis and marine environmental conditions, and these records clearly show the onset of the biotic crisis prior to event bed 25. KEY WORDS: Meishan , Permian-Triassic boundary, aromatic hydrocarbons, molecular fossil, dibenzothiophene , fluorene series, paleoenvironment. This paper is supported by the National Natural Science Foundation of China (No. 40232025) and the Program for New Century Excellent Talent in University of the Ministry of Education of China (NCET-04-0729).
* Corresponding
author: xiecugBl63. com
Manuscript received August 2 2 , 2005. Manuscript accepted December 15, 2005.
INTRODUCTION T h e Permian-Triassic ( TI/ P ) boundary, dated to be (251. 4 5 0 . 3 ) Ma ago, is marked by the most drastic mass extinction of organisms in the Phanerozoic. In order to explore the pattern and the cause of
50
H u a n g Xianyu, Jiao Dan, I,u Liqiang, H u a n g Junhua and Xie Shucheng
the mass extinction, extensive research has been focused on biostratigraphy , isotopic chronostratigraphy , event stratigraphy, ecostratigraphy and sequence stratigraphy (Payne et al. , 2004; Reichow et al. , 2002; Yin et al. , 2001; Jin et al. , 2000; Zhang et al. , 1996, 1995; Yang et al. , 1991). Recently, along with the advance of molecular stratigraphy, a great number of lipid biomarkers, including normal alkanes, isoprenoids (e. g. pristane, phytane) , terpanes ( mainly hopanes ) , steranes and aryl isoprenoids (Grice et al. , 2005; Xie et al. , 2005; Schwab and Spangenberg, 2004; IA and T o n g , 2002; Pancost et al. , 2002; Silliman et al. , 2002) have been identified in the samples from the Td P boundary worldwide. These organic geochemical data have provided significant information related to organic sources and maturity, biological change and sedimentary environmental conditions in association with the mass extinction. With the exception of aryl isoprenoids, few reports have focused on the aromatic compounds extracted from samples across the T d P boundary. In general, free aromatic compounds do not occur in live organisms. However, aromatic compounds are widespread in recent and ancient sediments, oil and coal, owing to postdepositional geochemical reactions. Aromatic compounds, of varied structures or alkyl substitutes, could reflect environmental conditions, sources and maturity, and abnormal thermal events in particular. For example, researchers assigned the particular distribution of polycyclic aromatic hydrocarbons from the T/ K (Tertiary/ Cretaceous) boundary samples to the global forest paleo-fires caused by an ex-terrestrial impact ( Hajime and Akira, 1999 ; Venkatesan and Dahl, 1989). In addition, the distribution of aryl isoprenoids derived from green sulfur bacteria sheds light on the relation between anoxic event and mass extinction across the T d P boundary (Grice et al. , 2005). In conclusion, it can be stated that aromatic compounds could provide us with organic records, assisting our understanding of the drastic mass extinction. This article presents the distributions of aromatic compounds identified in the samples taken from the Tr/P boundary at section B of Meishan near the GSSP (Global Stratotype Section and Point).
SAMPLING AND EXPERIMENTAL METHODS Thirty-seven samples were collected from section
I3 of Meishan (beds 23 to 3 4 ) . After cleaning the surface, the air-dried samples were ground to less
than 80 meshes. As much as 150 g of each sample was Soxhlet-extracted with chloroform for 72 h with native copper added t o remove sulfur. T h e extract was concentrated on a rotary evaporator under reduced pressure and transferred to a small vial. After weighing the total extract lipids and eliminating asphaltenes, the hexane-solute organics were fractionated by column chromatography into saturated hydrocarbons, aromatics and non-hydrocarbons. T h e aromatics were then analyzed by G U MS using a Hewlett-Packard 5973A MS, interfaced directly with a Hewlett-Packard 6890 GC equipped with an HP-5MS capillary column (30 m X 0. 25 mm XO. 25 p m ) . T h e operating conditions were as follows: temperature ramped from 70 to 280 "Cat 3 'C/ min, finally held at 280 "Cfor 20 min, H e as carrier gas; the ionization energy of the mass spectrometer was set at 70 e V ; the scan range was from 50 to 550 amu. T h e identification of compounds was based on published literature ( Wang, 1993) and the NIST chemical data library. T h e quantification was conducted by using the peak area of the corresponding ion chromatograms.
RESULTS AND DISCUSSION
A typical total ion chromatogram ( T I C ) of aromatic fraction is shown in Fig. 1. T h e aromatic compounds identified in the samples inciude naphthalene, phenanthrene ( P ) , fluorene ( F ) , dibenzofuran (DBF ) , dibenzothiophene ( DBT ) , benzonaphthiophene, fluoranthene (Fluo) , pyrene ( P y r ) and their alkyl substituent homologues. Here we mainly present and discuss the aromatic indices of geological significance, such as the phenanthrene series, thiophene series and fluorene series.
Phenanthrene Series The phenanthrene series, both the parent phenanthrene and its alkyl homologues, could be found in all samples. Anthracene, an isomer of phenanthrene, was absent in all samples, probably due to the active chemistry. Phenanthrene and its alkyl homologues might be the pyrolytic products of steranes and terpenoids, kerogen or non-hydrocarbons (Mackenzie, 1984). In general, the methyl phenanthrenes ( M P ) have five kinds of isomers, that is, 1-, 2-, 3-, 4- and 9-methyl phenanthrenes. T h e 4- and 9-methyl isomers would co-elute in ordinary capillary columns. However, Radkle et al. (1982) suggested that the influence of
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary
P I
Bed 32 TIC
m/z 178+192+206
I
9-MP
mlz 166+168+184
Bed26 TIC
10
20
30
40 50 Scadtime
60
70
80
20
DMP
40 Scadtime
30
Scadtime
DBF
51
30 Scadtime
50
P Y ~ mi2202
,,;!L_
40 30
40 50 Scadtime
60
Figure 1. Selective TIC and the mass chromatograms of the detected aromatic hydrocarbons. the 4-methyl isomer could be omitted because its content was very low or even disappeared owing to its unstable structure. Enhanced maturity will lead to the rearrangement of methyl phenanthrenes on the basis of thermodynamic stability, with the 2- and 3methyl p isomers being more stable than 1- and 9methyl a isomers. As a result, the methyl phenanthrenes could be used as a tracer of thermal history based on their isomer abundance. One such application is the index of the methyl phenanthrene distribution fraction ( M P D F ) (Kvalheim et al. , 1987 ) , which was used to calculate the mean vitrinite reflectance (R,) and is defined as follows yoR,--O. 112+3.739F2 ~
2-MP Fz=2-MP+3-MP+1-MP+9-MP where F2 is the methyl phenanthrene index, and MP is methyl phenanthrene. On the basis of the above equation, t h e R,, values of most samples at Meishan range between 0. 7 % and 1. O%, falling within the oil window ( 0 . 5 % 1. 2 % ) . T h i s maturity does not contradict the RockEva1 analysis of the same samples from Meishan (T,,, 426 - 466 "C, Xie et al. , 2005). These data suggest the possibility for the investigation of biomarkers at Meishan. T h e standard deviation of the calculated maturity of all samples is around 0. 094, which means a similar maturity of the whole section.
Thiophene Series Several sulfur-bearing organic compounds were also detected in the aromatic fraction, including dibenzothiophene, benzonaphthothiophene , and their alkyl homologues, but without thiophene and its homologues. In general, free sulfur compounds do not exist in living organisms, and are formed through the chemical reactions of organic molecules with the sulfur species (such a s H2S, HS- ) in sediment during the early stage of digenesis. In a lower temperature, thiophene, which has a low molecular weight, is produced. T h e products are benzothiophene, dibenzothiophene or higher molecular-weight compounds. Along with the temperature effect, sedimentary environmental conditions also greatly affect the distribution of these sulfur-bearing compounds, and they are often concentrated in saline sediments. T h e ratio of dibenzothiophene to phenanthrene (DBT/PHN) has usually been used as a good index of lithology, with the carbonate rocks greater than 1 and mudstone (shale) below 1 ( H u g e s et al. , 1995). This ratio is further driven by the sedimentary environments (e. g . the redox conditions) , especially the sulfur-rich reductive environment. Figure 2 clearly shows that the ratio of DBT/PHN varies in a comparable pace with the lithology across the profile. T h e ratio is above or near 1 in the Permian carbonate rocks but less than 1 in the Triassic clastics. This phenomenon does not contradict with previous con-
52
Huang Xianyi
clusions on the molecular ratio from other sedimentary rocks. However, the Permian carbonate of bed 27 has a low D B T I P H N value (about 0.20) , showing a characteristic similar to the Triassic clastics. T h i s may be due to the specific sedimentary environment and is discussed later. Significantly, a gradual decrease of DBT/PHN could be observed within bed 2 4 limestone. T h e thermal effect could be ruled out for this observed decrease owing t o the comparable maturity of the whole profile a s discussed earlier. Furthermore, the gradual decrease of DBT/PHN is in line with the drop of the carbon isotope composition of carbonate ( C a o et al. , 2002), the loss of the Changhsingian reef ecosystem near the equator in the Tethys ( Weidlich, 2002) , and the decrease of the relative abundance of cyanobacteria ( Xie et al. , 2005). T h e coincidence of these biological and geochemical records suggests the close relation between the biotic crisis and marine environmental conditions. These records jointly revealed that the onset of biotic crisis was clearly prior to event bed 25 ( Y i n and T o n g , 1998). It is notable that the DBT/PHN ratio displays a large fluctuation within the Triassic sediments. T h i s fluctuation might reflect the instability of sedimentary environment during the Early Triassic. Whether this unstable environment caused the protracted recovery of Triassic organisms needs, and merits, further investigation.
Fluorene Series T h e fluorene series, containing fluorene, dibenzothiophene, and dibenzofuran, have similar five-membered ring structures, and may come from the same precursors ( L i n et al. , 1987). Because of the relatively active chemistry of the 9-a-carbon ato m , these compounds could be easily substituted by other functional groups. Philip et al. (1988) pointed out that dibenzofuran and fluorene were more easily enriched in a fresh water environment because of the absence of hydrogen sulfide. In contrast, dibenzothiophene was more easily produced in a strongly reductive environment and under strong bacteria activity. O n these points, abundant dibenzofuran often occurs in a slightly reductive or oxic environment, and fluorene is concentrated in normal conditions, whilst dibenzothiophene is often found in high content in a strongly reductive environment (Lin et al. , 1987). All 37 samples analyzed were assigned into three groups on the basis of the relative abundance of the
ao Dan,
IA I,iqiang, H u a n g Junhua and Xie Shucheng DBT/P 0
0.5
I
I
1.o I
I
1.5 I
,
i
Figure 2. Profile showing the trend of the ratio of dibenzothiophene to phenanthrene. The simple lithology of the studied profile near the P/Tr boundary includes lime mudstone ( beds 23, 24 ) , palecolored, altered ash clay beds (beds 25, 28, 31, 33), a laminated organierich calcareous claystone (bed 26) , a lime mud bed ( bed 27 ) , and grey organic-rich shale, pale marl or muddy limestone (beds 29, 30, 32, 34). three fluorene series (Fig. 3). Group A is characterized by abundant dibenzothiophene ( > 8 0 % ) , and contains beds 34-1, 33-2, 33-1, 32-3, 32-1, 31, 301 , 27d, 27c, 27b, 25-3, and 25-2. I n group C , dibenzofuran takes a percentage of 50% - 6 7 % , and dibenzothiophene 20% - 40%, and the samples include 29-2, 26-3, 26-2, 26-1, 24e-3, 24e-2, 24e-1, 24d-2, 24d-1, 24c-2, 24c-1, 24b-2, 24b-1, 23-3, and 23-2. T h e other samples are classified into group €3, with a moderate concentration of both dibenzothiophene and dibenzofuran. In all samples, the percentage of fluorene was quite low.
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary
53
oranthene (greater than 1 of the ratio of fluoranthene t o pyrene) , whilst the petrogenic source features an enhanced pyrene (less than 1 of the ratio) (Sicre et al. , 1987). Most samples have a value around 0. 5 in the ratio of fluoranthene to pyrene. T h e relatively high value of the ratio occurs in the volcanic clay beds (beds 25, 31, 3 3 ) , which may be related to volcanic activity. Fluoranthene, containing a five-membered ring, is more stable than pyrene in the high temperature pyrogenic process. O n this point, this diagnostic index could be used to distinguish volcanic clay. Notably, bed 27 was characterized by a minimum of 0.03.
Figure 3. Ternary plot distribution of fluorene serial compounds.
CONCLUSIONS A variety of aromatic compounds have been detected in the samples collected from section B at MeiDibenzothiophene is enriched in the volcanic clay shan, Zhejiang Province and they include naphthabeds and muddy limestones (beds 25, 31, 3 2 , 33, 34 lene, phenanthrene, fluorene series ( fluorene, in group A , and beds 28, 29, 30, 34 in group B ) , dibenzothiophene, and dibenzofuran) , fluoranthene, whilst dibenzofuran is concentrated in carbonate pyrene and their methyl homologues. Based on the rocks. This discrimination indicates that the relative distribution of the aromatics identified, the following amount of dibenzofuran and dibenzothiophene is reconclusions can be made. lated to lithology, and the sulfur-containing organic (1) T h e MPDF of all samples has a quite low compounds were more easily formed in muddy lime- standard derivation ( 0. 094 ) , which indicates the stones. Significantly, bed 27 limestone displays a comparable maturity of the organic matter throughdistribution of the three fluorene series similar to out the whole profile analyzed. T h e Eo calculated Triassic clastics, but different from Permian lime- from the MPDF falls within the oil window range, stone. This characteristic of bed 27 is consistent with indicating that extractable organic matter is still at a the result of DBT/PHN values. Bed 27, intercalated relatively low maturity and could well record the by two volcanic clay beds (beds 25 and 2 8 ) , has re- original information preserved in biomarkers. ceived quite a lot of attention. Bed 27 is the horizon ( 2 ) T h e ratio of DBT/PHN shows a variation in where the T r / P boundary was defined, and this thin the profile related t o lithology. T h e Permian limehorizon is widely distributed in South China. Other stone features a value greater than or near 1. In conmolecular fossil records also reflect its distinguish- trast, the Triassic clastics show a value less than 1. able character. On the basis of the abundant aryl iso- Significantly, the gradual decrease of DBT/PHN obprenoids and biomarkers of green sulfur bacteria, the served within bed 24 limestone is in agreement with photic zone was believed to be penetrated by H 2 S at the drop of the carbon isotope composition of carbonthe time when bed 27 was deposited (Grice et a l . , ate, the loss of the Changhsingian reef ecosystem and 2005). T h e distinguishable fluorene distribution in the decrease of the relative abundance of cyanobactebed 27 may indicate this kind of strongly reductive ria. T h e agreement of different records suggests the environment. existence of a close relation between the biotic crisis and marine environment, and these records clearly Other Aromatic Compounds indicate the onset of the biotic crisis prior to event In addition to t h e aromatic compounds discussed bed 25. earlier, other aromatics identified include pyrene and ( 3 ) T h e volcanic clay and Triassic clastics are fluoranthene. T h e relative abundance of the two characterized by a high relative concentration of compounds has been used as a diagnostic of organic dibenzothiophene whilst the Permian carbonate rocks sources in modern environmental organic geochemis- have a high amount of dibenzofuran. This reflects try; t h e pyrolytic process favors the formation of flu- that the relative amount of dibenzofuran and dibenzo-
Huang Xianyu, Jiao Dan, IA I-iqiang, Huang Junhua and Xie Shucheng
54
thiophene was driven by lithology. Bed 27 limestone displays a distribution in the three fluorene series and a low DBT/PHN ratio similar t o Triassic clastics, but different from the Permian limestone. T h e difference probably suggests that some other factors might also affect these molecular parameters. REFERENCES CITED Cao, C. Q. , Wang, W . , Jin, Y. G. , 2002. T h e Carbon Isotope Change in the Meishan Permian-Triassic Transition, Zhejiang Province. Chinese Science Bulletin, 47( 4 ) : 302
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Zhejiang Province. Journul of China University of Geosciences, 1 3 ( 2 ) : 177-181 Mackenzie, A. S. , 1984. Applications of Biological Markers in Petroleum Geochemistry. In: Brooks, J. , Welte, D. ,
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eds. , Advances in Petroleum Geochemistry. Academic Press, London. 114-214 Pancost, R. D. , Crawford, N. , Maxwell, J. R. , 2002. Molecular Evidence for Basin-Scale Photic Zone Euxinia in the Permian Zechstein Sea. Chrmicul Geology, 188: 217 -227 Payne, J. I-. , Lehrmann, D. J. , Wei, J. , et al. , 2004. Large Perturbations of the Carbon Cycle during Recovery from the End-Permian Extinction. Science, 305: 506 509 Philip, R. P. , Bakel, A. , Galvez-Sinibald, A. , et al. , 1988.
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199-212 Zhang, K. X . , Lai, X. I-. , Ding, M. H . , et al. , 1995. Conodont Sequence and Its Global Correlation of PermianTriassic Boundary in Meishan Section, Changxing, Zhejiang Province. Earth Science-Journal of’ Chinu University o / Geosriences , 20 ( 6 ) : 669 - 678 ( in Chinese with English Abstract) Zhang, K. X . , T o n g , J. N.,Yin, H. F . , et a l . , 1996. Sequence Stratigraphy of the Permian-Triassic Boundary Section of Changxing, Zhejiang. Actu Geologicu Sinica , 7 0 ( 3 ) : 270-283
I S S N 1002-0705
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary Huang Xianyu ($&j!?,fi) Jiao Dan (gB) State K e y Laboratory o f Geological Processes and Mineral Resources, China University o f Geosciences , Wuhan 430074, China Lu Liqiang (%$$$) Faculty o f Material Science and Chemical Engineering, China University o f Geosciences , W u h a n 430074, China Huang Junhua (g@q) State K e y Laboratory o f Geological Processes and Mineral Resources, China University o f Geosciences , Wuhan 430074, China Xie Shucheng" (d$#H&) State K e y Laboratory o f Geological Processes and Mineral Resources, T h e K e y Laboratory of' Biogeology and Environmental Geology o f the Ministry of Education, China University o f Geosciences , W u h a n 430074, China ABSTRACT: Aromatic compounds extracted from sedimentary rocks can reflect environmental conditions, organic sources and maturity. The aromatics, identified in association with mass extinction in particular, would provide a signature assisting our understanding of the causes of the biotic crisis. Aromatic hydrocarbons were fractionated from the total lipid extracts of 37 samples taken from the PermianProvince in South Triassic boundary (beds 23 to 34) of section B at Meishan (%!$&),Zhejiang (?%?I) China. These aromatics were analyzed by using gas chromatography-mass spectrometry (GC-MS). Main compounds identified include naphthalene, phenanthrenes , fluorene , dibenzothiophene , dibenzofuran , fluoranthene, pyrene and some of their methyl homologues. The indices of methyl phenanthrene distribution fraction indicate the comparable maturity (within the oil window, 0.7% - 1.0% of the mean vitrinite reflectance) of the organic matter throughout the whole profile analyzed. The ratio of dibenzothiophene to phenanthrene (DBW PHN) varies generally at a comparable pace with lithology. Significantly, a gradual decrease of this ratio was observed within bed 24 limestone, which is probably due to the variation of sedimentary environment. This change is in line with the drop in the carbon isotope composition of carbonate, the loss of the Changhsingian reef ecosystem, and the decrease of cyanobacteria abundance within the bacteria population. The coincidence of these records suggests a close relation between the biotic crisis and marine environmental conditions, and these records clearly show the onset of the biotic crisis prior to event bed 25. KEY WORDS: Meishan , Permian-Triassic boundary, aromatic hydrocarbons, molecular fossil, dibenzothiophene , fluorene series, paleoenvironment. This paper is supported by the National Natural Science Foundation of China (No. 40232025) and the Program for New Century Excellent Talent in University of the Ministry of Education of China (NCET-04-0729).
* Corresponding
author: xiecugBl63. com
Manuscript received August 2 2 , 2005. Manuscript accepted December 15, 2005.
INTRODUCTION T h e Permian-Triassic ( TI/ P ) boundary, dated to be (251. 4 5 0 . 3 ) Ma ago, is marked by the most drastic mass extinction of organisms in the Phanerozoic. In order to explore the pattern and the cause of
50
H u a n g Xianyu, Jiao Dan, I,u Liqiang, H u a n g Junhua and Xie Shucheng
the mass extinction, extensive research has been focused on biostratigraphy , isotopic chronostratigraphy , event stratigraphy, ecostratigraphy and sequence stratigraphy (Payne et al. , 2004; Reichow et al. , 2002; Yin et al. , 2001; Jin et al. , 2000; Zhang et al. , 1996, 1995; Yang et al. , 1991). Recently, along with the advance of molecular stratigraphy, a great number of lipid biomarkers, including normal alkanes, isoprenoids (e. g. pristane, phytane) , terpanes ( mainly hopanes ) , steranes and aryl isoprenoids (Grice et al. , 2005; Xie et al. , 2005; Schwab and Spangenberg, 2004; IA and T o n g , 2002; Pancost et al. , 2002; Silliman et al. , 2002) have been identified in the samples from the Td P boundary worldwide. These organic geochemical data have provided significant information related to organic sources and maturity, biological change and sedimentary environmental conditions in association with the mass extinction. With the exception of aryl isoprenoids, few reports have focused on the aromatic compounds extracted from samples across the T d P boundary. In general, free aromatic compounds do not occur in live organisms. However, aromatic compounds are widespread in recent and ancient sediments, oil and coal, owing to postdepositional geochemical reactions. Aromatic compounds, of varied structures or alkyl substitutes, could reflect environmental conditions, sources and maturity, and abnormal thermal events in particular. For example, researchers assigned the particular distribution of polycyclic aromatic hydrocarbons from the T/ K (Tertiary/ Cretaceous) boundary samples to the global forest paleo-fires caused by an ex-terrestrial impact ( Hajime and Akira, 1999 ; Venkatesan and Dahl, 1989). In addition, the distribution of aryl isoprenoids derived from green sulfur bacteria sheds light on the relation between anoxic event and mass extinction across the T d P boundary (Grice et al. , 2005). In conclusion, it can be stated that aromatic compounds could provide us with organic records, assisting our understanding of the drastic mass extinction. This article presents the distributions of aromatic compounds identified in the samples taken from the Tr/P boundary at section B of Meishan near the GSSP (Global Stratotype Section and Point).
SAMPLING AND EXPERIMENTAL METHODS Thirty-seven samples were collected from section
I3 of Meishan (beds 23 to 3 4 ) . After cleaning the surface, the air-dried samples were ground to less
than 80 meshes. As much as 150 g of each sample was Soxhlet-extracted with chloroform for 72 h with native copper added t o remove sulfur. T h e extract was concentrated on a rotary evaporator under reduced pressure and transferred to a small vial. After weighing the total extract lipids and eliminating asphaltenes, the hexane-solute organics were fractionated by column chromatography into saturated hydrocarbons, aromatics and non-hydrocarbons. T h e aromatics were then analyzed by G U MS using a Hewlett-Packard 5973A MS, interfaced directly with a Hewlett-Packard 6890 GC equipped with an HP-5MS capillary column (30 m X 0. 25 mm XO. 25 p m ) . T h e operating conditions were as follows: temperature ramped from 70 to 280 "Cat 3 'C/ min, finally held at 280 "Cfor 20 min, H e as carrier gas; the ionization energy of the mass spectrometer was set at 70 e V ; the scan range was from 50 to 550 amu. T h e identification of compounds was based on published literature ( Wang, 1993) and the NIST chemical data library. T h e quantification was conducted by using the peak area of the corresponding ion chromatograms.
RESULTS AND DISCUSSION
A typical total ion chromatogram ( T I C ) of aromatic fraction is shown in Fig. 1. T h e aromatic compounds identified in the samples inciude naphthalene, phenanthrene ( P ) , fluorene ( F ) , dibenzofuran (DBF ) , dibenzothiophene ( DBT ) , benzonaphthiophene, fluoranthene (Fluo) , pyrene ( P y r ) and their alkyl substituent homologues. Here we mainly present and discuss the aromatic indices of geological significance, such as the phenanthrene series, thiophene series and fluorene series.
Phenanthrene Series The phenanthrene series, both the parent phenanthrene and its alkyl homologues, could be found in all samples. Anthracene, an isomer of phenanthrene, was absent in all samples, probably due to the active chemistry. Phenanthrene and its alkyl homologues might be the pyrolytic products of steranes and terpenoids, kerogen or non-hydrocarbons (Mackenzie, 1984). In general, the methyl phenanthrenes ( M P ) have five kinds of isomers, that is, 1-, 2-, 3-, 4- and 9-methyl phenanthrenes. T h e 4- and 9-methyl isomers would co-elute in ordinary capillary columns. However, Radkle et al. (1982) suggested that the influence of
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary
P I
Bed 32 TIC
m/z 178+192+206
I
9-MP
mlz 166+168+184
Bed26 TIC
10
20
30
40 50 Scadtime
60
70
80
20
DMP
40 Scadtime
30
Scadtime
DBF
51
30 Scadtime
50
P Y ~ mi2202
,,;!L_
40 30
40 50 Scadtime
60
Figure 1. Selective TIC and the mass chromatograms of the detected aromatic hydrocarbons. the 4-methyl isomer could be omitted because its content was very low or even disappeared owing to its unstable structure. Enhanced maturity will lead to the rearrangement of methyl phenanthrenes on the basis of thermodynamic stability, with the 2- and 3methyl p isomers being more stable than 1- and 9methyl a isomers. As a result, the methyl phenanthrenes could be used as a tracer of thermal history based on their isomer abundance. One such application is the index of the methyl phenanthrene distribution fraction ( M P D F ) (Kvalheim et al. , 1987 ) , which was used to calculate the mean vitrinite reflectance (R,) and is defined as follows yoR,--O. 112+3.739F2 ~
2-MP Fz=2-MP+3-MP+1-MP+9-MP where F2 is the methyl phenanthrene index, and MP is methyl phenanthrene. On the basis of the above equation, t h e R,, values of most samples at Meishan range between 0. 7 % and 1. O%, falling within the oil window ( 0 . 5 % 1. 2 % ) . T h i s maturity does not contradict the RockEva1 analysis of the same samples from Meishan (T,,, 426 - 466 "C, Xie et al. , 2005). These data suggest the possibility for the investigation of biomarkers at Meishan. T h e standard deviation of the calculated maturity of all samples is around 0. 094, which means a similar maturity of the whole section.
Thiophene Series Several sulfur-bearing organic compounds were also detected in the aromatic fraction, including dibenzothiophene, benzonaphthothiophene , and their alkyl homologues, but without thiophene and its homologues. In general, free sulfur compounds do not exist in living organisms, and are formed through the chemical reactions of organic molecules with the sulfur species (such a s H2S, HS- ) in sediment during the early stage of digenesis. In a lower temperature, thiophene, which has a low molecular weight, is produced. T h e products are benzothiophene, dibenzothiophene or higher molecular-weight compounds. Along with the temperature effect, sedimentary environmental conditions also greatly affect the distribution of these sulfur-bearing compounds, and they are often concentrated in saline sediments. T h e ratio of dibenzothiophene to phenanthrene (DBT/PHN) has usually been used as a good index of lithology, with the carbonate rocks greater than 1 and mudstone (shale) below 1 ( H u g e s et al. , 1995). This ratio is further driven by the sedimentary environments (e. g . the redox conditions) , especially the sulfur-rich reductive environment. Figure 2 clearly shows that the ratio of DBT/PHN varies in a comparable pace with the lithology across the profile. T h e ratio is above or near 1 in the Permian carbonate rocks but less than 1 in the Triassic clastics. This phenomenon does not contradict with previous con-
52
Huang Xianyi
clusions on the molecular ratio from other sedimentary rocks. However, the Permian carbonate of bed 27 has a low D B T I P H N value (about 0.20) , showing a characteristic similar to the Triassic clastics. T h i s may be due to the specific sedimentary environment and is discussed later. Significantly, a gradual decrease of DBT/PHN could be observed within bed 2 4 limestone. T h e thermal effect could be ruled out for this observed decrease owing t o the comparable maturity of the whole profile a s discussed earlier. Furthermore, the gradual decrease of DBT/PHN is in line with the drop of the carbon isotope composition of carbonate ( C a o et al. , 2002), the loss of the Changhsingian reef ecosystem near the equator in the Tethys ( Weidlich, 2002) , and the decrease of the relative abundance of cyanobacteria ( Xie et al. , 2005). T h e coincidence of these biological and geochemical records suggests the close relation between the biotic crisis and marine environmental conditions. These records jointly revealed that the onset of biotic crisis was clearly prior to event bed 25 ( Y i n and T o n g , 1998). It is notable that the DBT/PHN ratio displays a large fluctuation within the Triassic sediments. T h i s fluctuation might reflect the instability of sedimentary environment during the Early Triassic. Whether this unstable environment caused the protracted recovery of Triassic organisms needs, and merits, further investigation.
Fluorene Series T h e fluorene series, containing fluorene, dibenzothiophene, and dibenzofuran, have similar five-membered ring structures, and may come from the same precursors ( L i n et al. , 1987). Because of the relatively active chemistry of the 9-a-carbon ato m , these compounds could be easily substituted by other functional groups. Philip et al. (1988) pointed out that dibenzofuran and fluorene were more easily enriched in a fresh water environment because of the absence of hydrogen sulfide. In contrast, dibenzothiophene was more easily produced in a strongly reductive environment and under strong bacteria activity. O n these points, abundant dibenzofuran often occurs in a slightly reductive or oxic environment, and fluorene is concentrated in normal conditions, whilst dibenzothiophene is often found in high content in a strongly reductive environment (Lin et al. , 1987). All 37 samples analyzed were assigned into three groups on the basis of the relative abundance of the
ao Dan,
IA I,iqiang, H u a n g Junhua and Xie Shucheng DBT/P 0
0.5
I
I
1.o I
I
1.5 I
,
i
Figure 2. Profile showing the trend of the ratio of dibenzothiophene to phenanthrene. The simple lithology of the studied profile near the P/Tr boundary includes lime mudstone ( beds 23, 24 ) , palecolored, altered ash clay beds (beds 25, 28, 31, 33), a laminated organierich calcareous claystone (bed 26) , a lime mud bed ( bed 27 ) , and grey organic-rich shale, pale marl or muddy limestone (beds 29, 30, 32, 34). three fluorene series (Fig. 3). Group A is characterized by abundant dibenzothiophene ( > 8 0 % ) , and contains beds 34-1, 33-2, 33-1, 32-3, 32-1, 31, 301 , 27d, 27c, 27b, 25-3, and 25-2. I n group C , dibenzofuran takes a percentage of 50% - 6 7 % , and dibenzothiophene 20% - 40%, and the samples include 29-2, 26-3, 26-2, 26-1, 24e-3, 24e-2, 24e-1, 24d-2, 24d-1, 24c-2, 24c-1, 24b-2, 24b-1, 23-3, and 23-2. T h e other samples are classified into group €3, with a moderate concentration of both dibenzothiophene and dibenzofuran. In all samples, the percentage of fluorene was quite low.
Distribution and Geochemical Implication of Aromatic Hydrocarbons across the Meishan Permian-Triassic Boundary
53
oranthene (greater than 1 of the ratio of fluoranthene t o pyrene) , whilst the petrogenic source features an enhanced pyrene (less than 1 of the ratio) (Sicre et al. , 1987). Most samples have a value around 0. 5 in the ratio of fluoranthene to pyrene. T h e relatively high value of the ratio occurs in the volcanic clay beds (beds 25, 31, 3 3 ) , which may be related to volcanic activity. Fluoranthene, containing a five-membered ring, is more stable than pyrene in the high temperature pyrogenic process. O n this point, this diagnostic index could be used to distinguish volcanic clay. Notably, bed 27 was characterized by a minimum of 0.03.
Figure 3. Ternary plot distribution of fluorene serial compounds.
CONCLUSIONS A variety of aromatic compounds have been detected in the samples collected from section B at MeiDibenzothiophene is enriched in the volcanic clay shan, Zhejiang Province and they include naphthabeds and muddy limestones (beds 25, 31, 3 2 , 33, 34 lene, phenanthrene, fluorene series ( fluorene, in group A , and beds 28, 29, 30, 34 in group B ) , dibenzothiophene, and dibenzofuran) , fluoranthene, whilst dibenzofuran is concentrated in carbonate pyrene and their methyl homologues. Based on the rocks. This discrimination indicates that the relative distribution of the aromatics identified, the following amount of dibenzofuran and dibenzothiophene is reconclusions can be made. lated to lithology, and the sulfur-containing organic (1) T h e MPDF of all samples has a quite low compounds were more easily formed in muddy lime- standard derivation ( 0. 094 ) , which indicates the stones. Significantly, bed 27 limestone displays a comparable maturity of the organic matter throughdistribution of the three fluorene series similar to out the whole profile analyzed. T h e Eo calculated Triassic clastics, but different from Permian lime- from the MPDF falls within the oil window range, stone. This characteristic of bed 27 is consistent with indicating that extractable organic matter is still at a the result of DBT/PHN values. Bed 27, intercalated relatively low maturity and could well record the by two volcanic clay beds (beds 25 and 2 8 ) , has re- original information preserved in biomarkers. ceived quite a lot of attention. Bed 27 is the horizon ( 2 ) T h e ratio of DBT/PHN shows a variation in where the T r / P boundary was defined, and this thin the profile related t o lithology. T h e Permian limehorizon is widely distributed in South China. Other stone features a value greater than or near 1. In conmolecular fossil records also reflect its distinguish- trast, the Triassic clastics show a value less than 1. able character. On the basis of the abundant aryl iso- Significantly, the gradual decrease of DBT/PHN obprenoids and biomarkers of green sulfur bacteria, the served within bed 24 limestone is in agreement with photic zone was believed to be penetrated by H 2 S at the drop of the carbon isotope composition of carbonthe time when bed 27 was deposited (Grice et a l . , ate, the loss of the Changhsingian reef ecosystem and 2005). T h e distinguishable fluorene distribution in the decrease of the relative abundance of cyanobactebed 27 may indicate this kind of strongly reductive ria. T h e agreement of different records suggests the environment. existence of a close relation between the biotic crisis and marine environment, and these records clearly Other Aromatic Compounds indicate the onset of the biotic crisis prior to event In addition to t h e aromatic compounds discussed bed 25. earlier, other aromatics identified include pyrene and ( 3 ) T h e volcanic clay and Triassic clastics are fluoranthene. T h e relative abundance of the two characterized by a high relative concentration of compounds has been used as a diagnostic of organic dibenzothiophene whilst the Permian carbonate rocks sources in modern environmental organic geochemis- have a high amount of dibenzofuran. This reflects try; t h e pyrolytic process favors the formation of flu- that the relative amount of dibenzofuran and dibenzo-
Huang Xianyu, Jiao Dan, IA I-iqiang, Huang Junhua and Xie Shucheng
54
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