SCIENCE CHINA Earth Sciences • RESEARCH PAPER •
January 2011 Vol.54 No.1: 84–92 doi: 10.1007/s11430-010-4062-4
Lower Carboniferous carbon isotope stratigraphy in South China: Implications for the Late Paleozoic glaciation QIE WenKun1, ZHANG XiongHua1*, DU YuanSheng2 & ZHANG Yang2 1
State Key Laboratory of Geological Process and Mineral Resources, China University of Geosciences, Wuhan 430074, China; 2 Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences, Wuhan 430074, China Received October 29, 2009; accepted March 10, 2010; published online October 20, 2010
Studies on carbon isotopes of bulk carbonates from Longan and Baping sections of Lower Carboniferous in Guangxi of China show that the stable carbon isotope compositions in carbonate rocks of the isolated platform and deep slope facies were resistant to the influence of early meteoric diagenesis and late burial diagenesis. Three major positive carbon isotope excursions have been recognized in Lower Carboniferous in South China. The first major positive δ13C shift of 4.19‰ occurred in the middle part of Siphonodella isosticha-upper Siphonodella crenulata zone (Tournaisian); the second with an amplitude of 4.65‰ occurred near the Tournaisian/Visean boundary; and the third of 2.23‰ in the lower part of Gnathodus bollandensis zone. The three positive shifts of δ13C can be correlated with global carbon isotope excursions and are consistent with the fall in global sea level, indicating that abundant organic carbon burial, lowering of atmospheric CO2, and glaciation may have occurred during these time intervals. Carboniferous, glaciation, carbon isotope, Guangxi, South China Citation:
Qie W K, Zhang X H, Du Y S, et al. Lower Carboniferous carbon isotope stratigraphy in South China: Implications for the Late Paleozoic glaciation. Sci China Earth Sci, 2011, 54: 84–92, doi: 10.1007/s11430-010-4062-4
The Early Carboniferous represents the time interval from Devonian greenhouse to Permo-Carboniferous icehouse in late Paleozoic. The Carboniferous glaciation was one of the longest and most important transitions of paleoclimate in earth history. A series of events occurred, such as the rise of vascular land plants [1], addition of organic carbon reservoir, drift of Gondwana [2], and the collision between northern Africa and Laurussia [3], which may produce the changes of global ocean currents circulation system and heat transport system [4]. These are thought to have brought about the lowering of atmospheric PCO2 and glaciation during the Carboniferous [5–10]. The first glacial deposits from late Paleozoic are discov*Corresponding author (email:
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© Science China Press and Springer-Verlag Berlin Heidelberg 2010
ered in the Amazon Basin of Brazil at the end of the Devonian [6]. Ice rafted sediments and tillites have been recognized extensively from the Visean, Serpukhovian, and Bashkirian in South America, Australia, and Tibet of China [7–10]. Because of a lack of precise paleontological control and the erosion of initial glacial deposits by subsequent glacial advance [11], the best methods of assessing glaciation during late Paleozoic may be based on the glacio-eustatic change and carbon and oxygen isotopic records in low latitudes, rather than on direct study of glacial sediments. Carbon isotopes of bulk carbonates, well-preserved brachiopod shells and oxygen isotopes of brachiopods, and conodonts have been widely used to reconstruct the global carbon cycle, paleotemperature, and glaciation during late Paleozoic [4, 5, 12–16]. Mii et al. [12] and Saltzman [4, 16] suggested that the major positive shifts of δ13C and δ18O in Tournaiearth.scichina.com
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sian reflect the abundant organic carbon burial, drawdown of atmospheric CO2 levels, and climatic cooling. Though the magnitude and timing of beginning and end of Gondwana glaciation are still discussed, through the analyses of carbon and oxygen isotopes and glacial deposits, it is generally accepted that there are three glaciation time intervals during late Paleozoic: Latest Devonian-Tournaisian, Late Visean-Serpukhovian-Bashkirian, and AsselianArtinskian [5, 17, 18]. The three glacial intervals in late Paleozoic and the glaciation during Late Ordovician and Cenozoic are consistent with the positive δ13C excursions [5, 12–16, 19–21]. The positive shift of δ13C could be regarded as one of the valid indications for glaciation when it is considered in a geological framework that involves evidence of glaciation. Currently, the sedimentary responses of Gondwana glacial events in South China during the late Paleozoic are investigated mainly by the analyses of glacio-eustatic change and sequence stratigraphy [22, 23]. However, it is difficult to clarify the relationships of glacial events among plates because of the differences of the vertical velocity of sedimentary basements. A few researchers also performed carbon isotopic analyses of carbonate from South China to examine carbon isotopic stratigraphy and global glacial events. However, the carbon isotopic records for South China were measured either from a short section spanning only 2–3 conodont biozones1) or from carbonate rocks de-
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posited in nearshore carbonate platform where the isotopic data may be influenced by meteoric and excess evaporation and did not sustain primary nature [24, 25], and thus cannot be correlated with carbon isotopic studies abroad. In this paper, we perform conodont biostratigraphy and carbon isotope analyses of bulk carbonates from Longan section (shallow-water isolated carbonate platform facies) and Baping section (deep-water carbonate slope facies), combined with the data from the Pengchong section1) (Liuzhou, Guangxi) to investigate the variation in global carbon cycle and the glaciation during the Lower Carboniferous.
1 Geological setting and sections studied During the Early Carboniferous, South China was located in a subequatorial position on the east border of the Paleotethys ocean (Figure 1). In Tournaisian, nearshore siliciclastic sediments are restricted to the margin of the Yangtze and Cathaysia old lands; basin and slope facies were deposited in and along narrow intra-platform basins; and nearshore, offshore and isolated carbonate platform sediments cover extensive areas in South China [26]. For stable isotope analysis in Lower Carboniferous, we sampled sections from the DQGX carbonate platform and QG intra-platform basin in Guangxi where the most complete late Paleozoic succes-
Figure 1 Late Mississippian (340 Ma) global paleogeographic reconstruction (after Blakey, 2006, http://jan.ucc.nau.edu/~rcb7/mollglobe.html). Tournaisian paleogeography (after ref. [26]) and section localities of South China are discussed in text. 1. Longan section, Nanning; 2. Baping section, Hechi; 3. Pengchong section, Liuzhou. 1) Hou H F. Comparision of Tournaisian-Visean boundary strata in South China. New Progress Fairs of National Stratigraphic Work and Research, 2005
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sions in South China are exposed. 1.1
Longan section, western Guangxi
The Longan section (23°10′N, 107°27′E) is situated in Longan County, Guangxi, South China. It comprises the Longan Formation (Tournaisian), Du’an Formation (Visean-Serpukhovian), and Dapu Formation (Bashkirian) from bottom to top. The Longan Formation can be divided lithologically into three parts: the lower part comprising mainly thin- to middle-bedded mudstone, wackstone with intercalations of very thin (1–5 mm) calcareous mud and nodular limestone; the middle part mainly thin- or thickbedded wackstone, dolomitic limestone; and the upper part mainly thin- to thick-bedded wackstone with thin-bedded or nodular chert. The Du’an Formation can be divided into two parts: the lower part composed mainly of thick-bedded to massive skeletal and peloidal packstone and grain stone; the upper part of oncolite limestone, algal boundstone, peloidal and skeletal grain stone. The Dapu Formation is composed of thick-bedded dolomite and skeletal dolomitic limestone. Based on lithological and carbonate MF analyses, we conclude that the Longan Formation was deposited in deeper open platform or inner shelf, the Du’an Formation was deposited mainly in platform marginal shoal or open platform, and the Dapu Formation was deposited in platform marginal shoal [27, 28]. 1.2
Baping section, northern Guangxi
The Baping section (25°12′N, 107°27′22.7″E) is located in Nandan County of Guangxi (Figure 1). It comprises, from bottom to top, the Luzhai Formation (Tournaisian-Visean), Baping Formation (Late Visean-Serpukhovian), Huanglong Formation (Bashkirian). The Luzhai Formation is mainly composed of siliceous argillite and carbonaceous shale, deposited in intra-platform basin. The Baping Formation consists of thin- to middle-bedded mudstone with intercalation of thin calcareous mud and nodular chert or bedded chert; the lower part of Huanglong Formation is composed mainly of middle-bedded to massive wackstone and peloid packstone with thin-bedded and nodular chert. The observed features of the fine-grained limestone from Baping and Huanglong formations mark the deposition in ramp or outer shelf.
2
Method
Conodont identification was conducted at Key Laboratory of Biogeology and Environmental Geology of Ministry of Education, China University of Geosciences. For conodont biostratigraphy studies, 101 and 62 samples were collected along Longan and Baping sections respectively. Each sample weighed 2–3 kg. In laboratory, carbonate rock samples
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were dissolved by 10% acetic acid. After being dissolved, the samples were washed by 20 and 160 mesh sieve. The grains between the two sizes were collected and dried. The conodonts were picked out under the binocular stereoscope, and then photographed under SEM. Carbon isotope values are likely to be preserved during the diagenetic processes that typically affect marine carbonates, and are widely used to investigate evolutionary trends of carbon isotopes of ancient ocean [5, 13, 16, 29]. Seventy-three and 32 carbonate samples for isotopic analyses were collected from Longan and Baping sections, respectively. Carbonate samples (1–5 g) were carefully collected to avoid cracks, calcite veins, and bioclasts. All of the samples were crushed to less than 100 mesh before carbon and oxygen isotope measurement. The samples for δ13C and δ18O analyses of bulk carbonates were allowed to react with 100% H3PO4 to produce CO2 under vacuum. The CO2 collected from above procedures was introduced into a Finigan MAT 251 mass spectrometer for the measurements of the ratios of 13C/12C and 18O/17O. A national standard was first calibrated by using GBW-04416 (δ13C = 1.61‰, δ18O= −11.59‰). The δ13C and δ18O values were reported in per mil relative to international V-PDB (Vienna Peedee belemnite), and the precision of the carbon and oxygen isotope measurements was better than ±0.1‰. The analyses were conducted at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan).
3 3.1
Results Biostratigraphy of Longan and Baping sections
The age of Longan Formation has been stated as Tournaisian on the basis of conodonts collected from Longan section. Fifty-three of 84 conodont samples from Longan Formation yield conodont elements. The conodonts from Longan Formation are allowed to discriminate the following four international standard conodont zones: Upper Siphonodella dupliata zone, Siphonodella sandibergi-Lower Siphonodella crenulata zone, Siphonodella isoticha-Upper Siphonodella crenulata zone, Gnathodus typicus-Scaliognathus anchoralis zone. The boundary between Longan Formation and Du’an Formation corresponds roughly to the T/V boundary determined by the first appearance of Gnathodus homopunctatus in the lowest part of Du’an Formation in Liuzhou, Guangxi [30]. The samples from Du’an and Dapu formations were barren of conodonts. The age of Du’an Formation by foraminifera has been stated as Visean-serpukhovian and the Dapu Formation was determined as Bashkirian [27]. The ages of Baping and Huanglong formations are stated as late Visean-Serpukhovian and Bashkirian separately by conodonts. Fifty-three of 62 carbonate samples from Baping
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section have yield abundant conodont elements that enable us to subdivide the deposits into four conodont zones: Gnathodus bilineatus bilineatus zone, Paragnathodus nodosus zone, G. bollandensis zone, and Declinognathodus noduliferous zone. 3.2
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pukhovian, the δ13C values are between 3.06‰ and 3.44‰, with an average of 3.24‰; the δ18O values are between −5.43‰ and −9.55‰, with an average of −7.80‰. In Bashkirian, the δ13C values rang from 1.66‰ to 4.74‰ and the δ18O values are between −4.40‰ and −10.298‰.
Carbon and oxygen isotopic compositions
The Lower Carboniferous δ13C and δ18O values from the Longan section are presented in Figure 2, ranging from −1.42‰ to 3.58‰ and from −8.87‰ to −2.00‰, respectively. During the Tournaisian, the δ13C values are between −1.42‰ and 3.58‰, with an average of 1.13‰; δ18O values are between −8.87‰ and −2.17‰, with an average of −4.29‰. From the Visean to Serpukhovian, the δ13C values range from 0.37‰ to 3.32‰, with an average of 2.01‰; δ18O values range from −6.02‰ to −2.00‰, with an average of −4.19‰ (Figure 3). Isotope profiles for Baping section are shown in Figure 4. The late Visean-Early Bashikirian δ13C values of the Baping section generally range from 1.29‰ to 4.74‰ and the δ18O values fluctuate between −10.298‰ and −2.57‰. In late Visean, the δ13C values are between 1.29‰ and 2.89‰, with an average of 2.19‰; the δ18O values are between −2.57‰ and −9.56‰, with an average of −6.53‰. In Ser-
Figure 2 Cross-plot of carbon and oxygen stable isotope values in Lower Carboniferous, Guangxi Zhuang Autonomous Region. Early meteoric calcite, updip Fe-calcite and downdip Fe-calcite cements are from ref. [31]; Carboniferous brachiopod shells calcite from refs. [12, 32], excluding 10 anomalous data.
Figure 3 Lithologic column, conodont biozones, and δ13C and δ18O results of bulk carbonate from Longan section. Shadow area shows δ13C values range of normal sea water in modern marine settings; division of G. homopunctatus zone is from ref. [30].
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Figure 4 Lithologic column, conodont biozones, and δ13C and δ18O results of bulk carbonate from Baping section. Shadow area shows δ13C values range of normal sea water in modern marine settings.
3.3
Analyses of carbon and oxygen isotopes
δ13C background values are intensively controlled by paleogeographical settings [29]. Isotopic data for epicontinental seas may be influenced by meteoric diagenesis and excess evaporation. The δ13C and δ18O values of ancient limestone have been shifted toward very low values in close proximity to exposure surfaces [33] and have been shifted toward higher values caused by excess evaporation [15]. Thus, it is important to perform analyses of sedimentary environment and diagenesis to evaluate the validity of δ13C in marine carbonates. As noted above, the Longan section was deposited in deeper isolated carbonate platform, which was far away from Yangtze Oldland and Cathaysia Oldland (Figure 1). The isotopic data could not be altered by river influx. Even the shallowest water facies in the Longan section do not bear evidence of karstic weathering, mud crack, and evaporate rocks [27, 28], and minimize the likelihood of being altered of isotopic data by meteoric diagenesis. From Visean to Bashkirian, the δ13C values of Du’an and Dapu formations range from 0.37‰ to 3.32‰, suggesting a very weak influence of meteoric diagenesis alteration. The δ13C and δ18O values of dolomites precipitated by replacement in
Longan and Du’an formations are consistent with those from adjacent beds and are likely to preserve the primary nature. Baping and Huanglong formations in Baping section deposited in deeper slope and outer shelf separately, where the carbon isotopes were not significantly reset isotopically by meteoric diagenesis [31]. Two criteria are usually used to judge diagenesis of carbonate rocks. 1) The positive relationship between δ13Ccarb and δ18Ocarb is commonly interpreted as a sign of the influence of meteoric diagenesis [34]. The carbon and oxygen values from Longan and Baping sections show weak and no covariation (R12=0.0561, R22=0.0158), respectively. Carbon and oxygen isotope values of Longan and Baping sections are cross-plotted on Figure 2. The characteristics of the cross-plot and the very weak covariation of carbon and oxygen stable isotope indicate a very weak influence of meteoric diagenesis. 2) δ18Ocarb
