In the Qiangtang Basin, northern Tibet, the most complete and extensive marine sedimentary strata outcropped in the Shuanghu-Sewa-Amdo area during the Jurassic, especially the Early Jurassic. The organic-rich marine sediments – commonly referred to as black shales – were deposited in the Early Jurassic, therefore, many petroleum geologists have been focusing on them for many years. Although achievements in geological investigations and petroleum resource assessments during recent years have been remarkable, the environmental conditions, mechanics, and process that resulted in the deposition of high-organic sediments during the Early Toarcian (183–176.5 Ma, Early Jurassic) Oceanic Anoxic Event are still a matter of discussions. In this paper, we deal with the biomarker distributions of Lower Jurassic oil shales in the Biluo Co section, Shuanghu area of northern Tibet. The oil shales are characterized by a marked predominance of short chain n-alkanes with a maxiumun at C16 or C17, nC17/nC31 ratio values between 9.4 and 17.8, and low Pr/Ph ratios. Furthermore, a series of C27 and C29-C35 hopanes with minor amounts of gammacerane are present in all samples, as indicated by gammacerane/C30-17α-hopane values from 0.06 to 0.12 and the steranes C27/C29 ratios higher than 1 in the three samples. The above-mentioned parameters indicate that the organic matter source is attributed to an algal/bacterial contribution. According to maturity parameters, all the homohopane 22S/(22S+22R) values in this study are > 0.58 and the sterane 20S/(20S+20R) values are all between 0.48 and 0.59, which is consistent with a higher level of thermal maturity. Widespread anoxia led to the deposition of organic-rich sediments that removed isotopically light carbon from the oceans and drove carbon-isotope ratios to higher values with a positive excursion close to 2.17‰. From biomarker distributions, the differences in δ13Ckerogen are related to the difference in δ13C of CO2 in the photic zone rather than the organic matter compositions.
1. Jenkyns, H. C. Geochemistry of oceanic anoxic event. Geochem. Geophy. Geosy., 2010, 11(3), 1–30.
http://dx.doi.org/10.1029/2009GC002788
2. Schouten, S., van Kaam-Peters, H. M. E., Rijpstra, W. I. C., Schoell, M., Sinninghe Damste, J. S. Effects of an oceanic anoxic event on the stable carbon isotopic composition of early Toarcian carbon. Am. J. Sci., 2000, 300(1), 1–22.
http://dx.doi.org/10.2475/ajs.300.1.1
3. Barnard, P. C., Cooper, B. S. Oils and source rocks of the North Sea area. In: Petroleum Geology of the Continental Shelf of North-West Europe: proceedings of the second Conference on Petroleum Geology of the Continental Shelf of North-West Europe (Illing, L. V., Hobson, G. D, eds.). Institute of Petroleum (Great Britain), 1981, 169–175.
4. McArthur, J. M., Algeo, T. J., Van de Schootbrugge, B., Li, Q., Howarth, R. J. Basinal restriction, black shales, Re-Os dating, and the Early Toarcian (Jurassic) oceanic anoxic event. Paleoceanography, 2008, 23(4), PA4217.
http://dx.doi.org/10.1029/2008PA001607
5. Trabucho-Alexandre, J., Dirkx, R., Veld, H., Klaver, G., De Boer, P. L. Toarcian black shales in the Dutch Central Graben: record of energetic, variable depositional conditions during an oceanic anoxic event. J. Sediment. Res., 2012, 82(2), 104–120.
http://dx.doi.org/10.2110/jsr.2012.5
6. Jenkyns, H. C., Clayton, C. J. Lower Jurassic epicontinental carbonates and mudstones from England and Wales: chemostratigraphic signals and the early Toarcian anoxic event. Sedimentology, 1997, 44(4), 687–706.
http://dx.doi.org/10.1046/j.1365-3091.1997.d01-43.x
7. Röhl, H. J., Schmid-Röhl, A., Oschmann, W., Frimmel, A., Schwark, L. The Posidonia Shale (Lower Toarcian) of SW-Germany: an oxygen-depleted ecosystem controlled by sea level and palaeoclimate. Palaeogeogr. Palaeocl., 2001, 169(34), 273–299.
http://dx.doi.org/10.1016/S0031-0182(01)00201-2
8. Littler, K., Hesselbo, S. P., Jenkyns, H. C. A carbon-isotope perturbation at the Pliensbachian–Toarcian boundary: evidence from the Lias Group, NE England. Geol. Mag., 2010, 147(2), 181–192.
http://dx.doi.org/10.1017/S0016756809990458
9. Hermoso, M., Minoletti, F., Rickaby, R. E. M., Hesselbo, S. P., Baudin, F., Jenkyns, H. C. Dynamics of a stepped carbon-isotope excursion: Ultra high-resolution study of Early Toarcian environmental change. Earth. Planet. Sc. Lett., 2012, 319–320, 45–54.
http://dx.doi.org/10.1016/j.epsl.2011.12.021
10. Jenkyns, H. C., Clayton, C. J. Black shales and carbon isotopes in pelagic sediments from the Tethyan Lower Jurassic. Sedimentology, 1986, 33(1), 87–106.
http://dx.doi.org/10.1111/j.1365-3091.1986.tb00746.x
11. McArthur, J. M., Donovan, D. T., Thirlwall, M. F., Fouke, B. W., Mattey, D. Strontium isotope profile of the early Toarcian (Jurassic) oceanic anoxic event, the duration of ammonite biozones, and belemnite palaeotemperatures. Earth. Planet. Sc. Lett., 2000, 179(2), 269–285.
http://dx.doi.org/10.1016/S0012-821X(00)00111-4
12. Jenkyns, H. C., Jones, C. E., Gröcke, D. R., Hesselbo, S. P., Parkinson, D. N. Chemostratigraphy of the Jurassic System: applications, limitations and implications for palaeoceanography. J. Geol. Soc. London, 2002, 159, 351–378.
http://dx.doi.org/10.1144/0016-764901-130
13. Hesselbo, S. P., Jenkyns, H. C., Duarte, L. V., Oliveira, L. C. V. Carbon-isotope record of the Early Jurassic (Toarcian) Oceanic Anoxic Event from fossil wood and marine carbonate (Lusitanian Basin, Portugal). Earth. Planet. Sc. Lett., 2007, 253(3–4), 455–470.
http://dx.doi.org/10.1016/j.epsl.2006.11.009
14. Gómez, J. J., Arias, C. Rapid warming and ostracods mass extinction at the Lower Toarcian (Jurassic) of central Spain. Mar. Micropaleontol., 2010, 74(3–4), 119–135.
http://dx.doi.org/10.1016/j.marmicro.2010.02.001
15. Pancost, R. D., Crawford, N., Magness, S., Turner, A., Jenkyns, H. C., Maxwell, J. R. Further evidence for the development of photic-zone euxinic conditions during Mesozoic oceanic anoxic events. J. Geol. Soc. London, 2004, 161(3), 353–364.
http://dx.doi.org/10.1144/0016764903-059
16. Bowden, S. A., Farrimond, P., Snape, C. E., Love, G. D. Compositional differences in biomarker constituents of the hydrocarbon, resin, asphaltene and kerogen fractions: An example from the Jet Rock (Yorkshire, UK). Org. Geochem., 2006, 37(3), 369–383.
http://dx.doi.org/10.1016/j.orggeochem.2005.08.024
17. Chen, L., Yi, H. S., Hu, R. Z., Zhong, H., Zou, Y. R. Organic Geochemistry of the Early Jurassic Oil Shale from the Shuanghu Area in Northern Tibet and the Early Toarcian Oceanic Anoxic Event. Acta Geol. Sin-Engl., 2005, 79(3), 392–397.
18. Li, C., He, Z. H., Yang, D. M. The problems of geological tectonics in the Qiangtang Area, Tibet. Global Geology, 1996, 15(3), 18–23 (in Chinese with English abstract).
19. Matte, P., Tapponnier, P., Arnaud, N., Bourjot, J., Avouac, J. P., Vidal, P., Liu, Q., Pan, Y. S., Wang, Y. Tectonics of Western Tibet, between the Tarim and the Indus. Earth. Planet. Sc. Lett., 1996, 142(3–4), 311–330.
http://dx.doi.org/10.1016/0012-821X(96)00086-6
20. Chen, L., Lin, A. T.-S., Da, X. J., Yi, H. S., Tsai, L. L.-Y., Xu, G. W. Sea-level changes recorded by cerium anomalies in the Late Jurassic (Tithonian) black rock series of Qiangtang Basin, north-central Tibet. Oil Shale, 2012, 29(1), 18–35.
http://dx.doi.org/10.3176/oil.2012.1.03
21. Lin, J. H., Yi, H. S., Li, Y., Wang, C. S., Peng, P. A. Characteristics of biomarker compounds and its implication of Middle Jurassic oil shale sequence in Shuanghu Area, Northern Tibet Plateau. Acta Sedimentol. Sinica, 2001, 19(2), 287–292 (in Chinese with English abstract).
22. Farrimond, P., Eglinton, G., Brassell, S. C., Jenkyns, H. C. The Toarcian black shale event in northern Italy. Org. Geochem., 1988, 13(4–6), 823–832.
http://dx.doi.org/10.1016/0146-6380(88)90106-4
23. Farrimond, P., Eglinton, G., Brassell, S. C., Jenkyns, H. C. Toarcian anoxic event in Europe: an organic geochemical study. Mar. Petrol. Geol., 1989, 6(2), 136–147.
http://dx.doi.org/10.1016/0264-8172(89)90017-2
24. Farrimond, P., Talbot, H. M., Watson, D. F., Schulz, L. K., Wilhelms, A. Methylhopanoids: Molecular indicators of ancient bacteria and a petroleum correlation tool. Geochim. Cosmochim. Ac., 2004, 68(19), 3873–3882.
http://dx.doi.org/10.1016/j.gca.2004.04.011
25. Blanc, P., Connan, J. Origin and occurrence of 25-norhopanes: a statistical study. Org. Geochem., 1992, 18(6), 813–828.
http://dx.doi.org/10.1016/0146-6380(92)90050-8
26. Sabatino, N., Neri, R., Bellanca, A., Jenkyns, H. C., Baudin, F., Parisi, G., Masetti, D. Carbon-isotope records of the Early Jurassic (Toarcian) oceanic anoxic event from the Valdorbia (Umbria–Marche Apennines) and Monte Mangart (Julian Alps) sections: palaeoceanographic and stratigraphic implications. Sedimentology, 2009, 56(5), 1307–1328.
http://dx.doi.org/10.1111/j.1365-3091.2008.01035.x
27. Palliani, R. B, Mattioli, E., Riding, J. B. The response of marine phytoplankton and sedimentary organic matter to the early Toarcian (Lower Jurassic) oceanic anoxic event in northern England. Mar. Micropaleontol., 2002, 46(3–4), 223–245.
http://dx.doi.org/10.1016/S0377-8398(02)00064-6
28. Al-Suwaidi, A., Damborenea, S., Hesselbo, S., Jenkyns, H., Manceñido, M., Riccardi, A. Evidence for the Toarcian oceanic anoxic event in the Southern hemisphere (Los Molles Formation, Neuquén Basin, Argentina). Geochim. Cosmochim. Ac., Goldschmidt Conference Abstracts, 2009, 73(13), Suppl. 1, A33.
29. Peters, K. E., Walters, C. W., Moldowan, J. M. The Biomarker Guide: II. Biomarkers and Isotopes in Petroleum Exploration and Earth History, second ed. Cambridge University Press, Cambridge, 2005.
30. Romero-Sarmiento, M.-F., Riboulleau, A., Vecoli, M., Versteegh, G. J.-M. Aliphatic and aromatic biomarkers from Gondwanan sediments of Late Ordovician to Early Devonian age: An early terrestrialization approach. Org. Geochem., 2011, 42(6), 605–617.
http://dx.doi.org/10.1016/j.orggeochem.2011.04.005
31. Wang, T. G. A contribution to some sedimentary environmental biomarkers in crude oils and source rocks in China. Geochemica, 1990, 19, 256–263 (in Chinese).
32. Summons, R. E., Jahnke, L. L., Hope, J. M., Logan, G. A. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature, 1999, 400, 554–557.
http://dx.doi.org/10.1038/23005
33. Brocks, J. J., Love, G. D., Summons, R. E., Knoll, A. H., Logan, G. A., Bowden, S. A. Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea. Nature, 2005, 437, 866–870.
http://dx.doi.org/10.1038/nature04068
34. Hermann, E., Hochuli, P. A., Méhay, S., Bucher, H., Brühwiler, T., Ware, D., Hautmann, M., Roohi, G., Ur-Rehman, K., Yaseen, A. Organic matter and palaeoenvironmental signals during the Early Triassic biotic recovery: The Salt Range and Surghar Range records. Sediment. Geol., 2011, 234(1–4), 19–41.
http://dx.doi.org/10.1016/j.sedgeo.2010.11.003
35. Seifert, W. K., Moldowan, J. M. Applications of steranes, terpanes and monoaromatics to the maturation, migration and source of crude oils. Geochim. Cosmochim. Ac., 1978, 42(1), 77–95.
http://dx.doi.org/10.1016/0016-7037(78)90219-3
36. Pye, K., Krinsley, D. H. Microfabric, mineralogy and early diagenetic history of the Whitby Mudstone Formation (Toarcian), Cleveland Basin, U.K. Geol. Mag., 1986, 123(3), 191–203.
http://dx.doi.org/10.1017/S0016756800034695
37. Hesselbo, S. P., Gröcke, D. R., Jenkyns, H. C., Bjerrum, C. J., Farrimond, P., Bell, H. S. M., Green, O. R. Massive dissociation of gas hydrate during a Jurassic oceanic anoxic event. Nature, 2000, 406, 392–395.
http://dx.doi.org/10.1038/35019044
38. Pearce, C. R., Cohen, A. S., Coe, A. L., Burton, K. W. Changes in the extent of marine anoxia during the Early Jurassic: Evidence from molybdenum isotopes. Geochim. Cosmochim. Ac., Goldschmidt Conference Abstracts, 2006, 70(18), Suppl., A476.
39. Tyson, R. V. The ‘‘productivity versus preservation’’ controversy: cause, flaws, and resolution. In: The Deposition of Organic-Carbon-Rich Sediments: Models, Mechanisms, and Consequences (Harris, N. B., ed.), SEPM Special Publications, 2005, 82, 17–33.
40. Kauffman, E. G. Benthic environments and paleoecology of the Posidonienschiefer (Toarcian). In: Neues Jahrbuch für Geologie und Paläontologie Abhandlungen, 1978, 157, 18–36.
41. Morris, K. A. A classification of Jurassic marine shale sequences: An example from the Toarcian (Lower Jurassic) of Great Britain. Palaeogeogr. Palaeocl., 1979, 26, 117–126.
http://dx.doi.org/10.1016/0031-0182(79)90144-5
42. Wignall, P. B. Black Shales. Oxford University Press, Oxford, England, 1994, 127.
43. O’Brien, N. R. Significance of lamination in Toarcian (Lower Jurassic) shales from Yorkshire, Great Britain. Sediment. Geol., 1990, 67(1–2), 25–34.
http://dx.doi.org/10.1016/0037-0738(90)90025-O
44. Ten Haven, H. L., De Leeuw, J. W., Rullkötter, J., Sininnghe Damsté, J. S. Restricted utility of the pristane/phytane ratio as a palaeoenvironmental indicator. Nature, 1987, 330, 641–643.
http://dx.doi.org/10.1038/330641a0
45. Van Breugel, Y., Baas, M., Schouten, S., Mattioli, E., Sinninghe Damsté, J. S. Isorenieratane record in black shales from the Paris Basin, France: Constraints on recycling of respired CO2 as a mechanism for negative carbon isotope shifts during the Toarcian oceanic anoxic event. Paleoceanography, 2006, 21(4), PA4220.
http://dx.doi.org/10.1029/2006PA001305
http://dx.doi.org/10.1111/j.1365-3091.2010.01202.x
