Sulphur isotope compositions of sedimentary phosphorites from the basal Cambrian of China: implications for Neoproterozoic-Cambrian biogeochemical cycling
Tóm tắt
The Meishucun Section (Yunnan Province, South China) is considered to be an important Precambrian–Cambrian boundary section, primarily because of its rich small shelly fossil record. In this article, we report the results of a sulphur isotope study of phosphate-bound sulphate from the Meishucun Section and several correlative sections in South China. Forty clastic, granular phosphorites from Meishucun yield tightly grouped δ 34 S values averaging 33‰ (CDT), which agree well with published evaporite data for the lower Cambrian of Siberia and elsewhere. We argue that these strongly positive values reflect the sulphur isotopic composition of ambient seawater, confirming further the existence of uniquely high δ 34 S values in the earliest Cambrian oceans. This novel use of trace-sulphate in phosphate to constrain seawater δ 34 S represents the first time that sulphate δ 34 S data for this period have been given precise biostratigraphic assignments. Superimposed on the overall trend are short-term, stratigraphic variations, which might reflect local variations in the sedimentary and early diagenetic environment. Our data, together with other published data, indicate that seawater sulphate δ 34 S rose from low values (15–20‰) during the pre-750 Ma Proterozoic to possibly all-time high values (>32‰) by the earliest Cambrian. We argue that this rise may, in part, relate to increases in the amount of sulphur isotopic discrimination during microbially mediated sulphate reduction as a result of increased sulphide reoxi-dation. On the other hand, the Neoproterozoic trend to high δ 34 S values appears to mirror a trend to decreasing seawater δ 13 C towards the Proterozoic–Phanerozoic transition, implying progressive increases in the efficiency of organic carbon recycling, which would normally be coupled with real increases in sulphate reduction on the global scale. We consider that both these changes in biogeochemical cycling derive ultimately from the introduction of macrofauna around this time and, in particular, from the influence of bioturbation on early diagenesis. Precise constraints on S-isotopic evolution during the Neoproterozoic require additional trace sulphate studies.
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Bliskovskiy V.Z., Grinenko V.A., Migidisov A.A., Savina L.I., Vernadskiy V.I.Sulphur isotopic compositon of the minerals of phosphoritesGeochemistry International19778148153
Bloch J., Krouse H.R.Sulfide diagenesis and sedimentation in the Albian Harmon Member, western CanadaJournal of Sedimentary Petrology199262235249
Brasier M.D., Cowie J.W., Brasier M.D.China and the Palaeotethyan Belt (India, Pakistan, Iran, Kazakhstan, and Mongolia)The Precambrian-Cambrian boundary1989OxfordClarendon Press117165
Cook P.J. & Shergold J.H. (eds) 1986. Proterozoic and Cambrian phosphorites. Phosphate deposits of the World 1 Cambridge University Press Cambridge.
Hall A.J., Boyce A.J., Fallick A.E., Hamilton P.J.Isotopic evidence of the depositional environment of Late Proterozoic stratiform barite mineralistion, Aberfeldy, ScotlandChemical Geology19918799114
Houghton M.L. 1980. Geochemistry of the Proterozoic Hormuz evaporites Southern Iran. MSc Thesis Oregon.
Jarvis I., Burnett W., Nathan Y., Almbaydin F.S.M., Attia A.K.M., Castro L.N., Flicoteaux R., Hilmy M.E., Husain V., Qutawnah A.A., Serjani A., Zanin Y.N.Phosphorite geochemistry: State-of-the-art and environmental concernsEclogae Geologicae Helveticae 8719943643700
Jensen S., Grant S.W., Kaufman A.J., Corsetti F.A.Afterthoughts on chemostratigraphy of Neoproterozoic-Cambrian units, White-Inyo Region, E. California and W. Nevada: Implications for global correlation and faunal distribution: discussion and replyPalaios 1996507786
Kampschulte A., Strauss H.The Sulfur Isotopic Composition of Jurassic and Cretaceous Seawater as Deduced From Trace Sulfate in Belemnites Goldschmidt-Conference, Heidelberg, 313–4496 Journal of Conference Abstracts19961303
Kirschvink J.L., Margaritz M., Ripperdan R.L., Zhuravlev A.Y., Rozanov A.Y.The Precambrian-Cambrian boundary problem 2: Magnetostratigraphic and carbon isotope correlations for Tommotian and Atdabanian time between Siberia, Morocco and South ChinaGSA Today199116991
Knoll A.H., Kaufman A.J., Semikhatov M.A., Grotzinger J.P.Sizing up the sub-Tommotian unconformity in SiberiaGeology 2319951211391143
Li R.Trace fossils and the Sinian-Cambrian boundary in the Meishucun sections, eastern Yunnan, China and their relations to small shelly fossilsGeological Review199137214220
Luo H.On the Sinian-Cambrian boundary of Meishucun and Wanjiawan, Jinning County, YunnanActa Geologica Sinica1980295111[in Chinese]
Luo H., Jiang Z., Wu X., Lin O., Song X., Xue X.A further researchon the Precambrian-Cambrian boundary at Meishucun Section of Jinning, Yunnan, ChinaActa Geologica Sinica (English edition)19925197206
Luo H., Jiang Z., Wu X., Song X., Lin O., Xing Y., Liu G., Zhang S., Tao Y.Sinian–Cambrian boundary stratotype section at Meishucun, Jinning, Yunnan, China1984Yunnan, ChinaPeople’s Publishing House
Mattes B.W., Conway Morris S., Robertson A.H.F., Searle M.P., Ries A.C.Carbonate/evaporite deposition in the Late Precambrian-Early Cambrian Ara Formation of Southern OmanThe Geology and Tectonics of the Oman Mountains Geological Socety, London, Special Publication199049617636
Misi A., Kyle J.R.Upper Proterozoic carbonate stratigraphy, diagenesis and stromatolitic phosphorite formation, Irece Basin, Bahia, BrazilJournal of Sedimentary Research1994A642299310
Pisarchik Y.K., Golubchina M.N.Sulfur isotope composition for Cambrian calcium sulfates from the Siberian PlatformGeochemistry International19758227230
Pisarchik Y.K., Golubchina M. N., Toksubayev A.I.Isotopic composition of sulfur in calcium sulfates from the Upper Lena Formation (Siberian Platform)Geochemistry International19774182185
Shields G.A. 1996. Event stratigraphy around the PrecambrianlCambrian boundary with special emphasis on western Mongolian and South China. Doctoral thesis ETH Zurich.
Shields G.A., Stille P., Brasier M.D., Glenn C., Lucas J., Prévot-Lucas L.Isotopic records across two phosphorite giant episodes compared: the Precambrian-Cambrian and the late Cretaceous-recentMarine authigenesis: From Global to Microbial1999Tulsa, Oklahoma, USASEPM special volumein press
Siegmund H. 1995. Fazies und Genese unterkambrischer Phosphorite und mariner Sedimente der Yangtze Plattform SüdChina. Berliner geowissenschaftliche Abhandlungen A173 Berlin.
Siegmund H. & Erdtmann B.-D. 1995. Lower Cambrian phosphorites of south China: Microfacies and genetic interpretation. Zentralblatt für Geologie und Paläontologie. Teil 1 1/2 257–270.
Smirnov A.I., Ivnitzkaya R.B., Zalavina T.P.Experimental data on the possibility of the chemical precipitation of phosphate from seawaterGeology of Phosphorite deposits State Scientific Research Institute for the chemistry of raw materials19627289302
Strauss H.The Sulfur Isotopic Composition of Seawater as deduced From Trace Sulfates in Carbonates Goldschmidt-Conference, Heidelberg, 313–4496 Journal of Conference Abstracts19961601
Thode H.G., Monster J.Sulfur-isotope geochemistry of petroleum, evaporites and ancient seasFluids in subsurface environments—a symposium American Association of Petroleum Geologists Memoirs19654367377
Xing Y., Luo H., Jiang Z., Zhang S.A candidate global stratotype section and point for the Precambrian-Cambrian boundary at Meishucun, Yunnan, ChinaJournal of China University of Geosciences1991214757