Carbon Isotope Records Indicative of Paleoceanographical Events at the Latest Permian Dalong Formation at Shangsi, Northeast Sichuan, China

INTRODUCTION

A series of environmental and biological changes were identified at Late Permian, including faunal mass extinction and cyanobacterial expansion (Xie et al., 2005; Erwin, 1994), volcanism (Kaiho et al., 2001), anoxia and photic zone euxinia (Kump et al., 2005), and elevated atmospheric CO₂ concentration (Fraiser and Bottjer, 2007). Several geochemistry records (e.g., C, O, S, Mg, Fe, Ca, Sr) have been widely investigated to explore the potential mechanisms and the processes of these events. In particular, the carbon isotope composition of well preserved marine carbonate was believed to relate to the changes in carbon cycle and the biological and geological processes within the ocean (e.g., Riccardi et al., 2007; Xie et al., 2007a, b; Berner, 2003; Cao et al., 2002; Kump, 1991). For example, δ¹³C value of bulk carbonate was demonstrated to be an important proxy of sea level changes (Li et al., 2007; Voigt et al., 2006). δ¹³C value of organic matter is supposed to be indicative of the changes of Pco₂, organic burial (Kump and Arthur, 1999), and even the vegetation cover (Goni et al., 1997). Moreover, the carbon isotope difference ∆¹³C (δ¹³C_org －δ¹³C_carb) between bulk organics and carbonate could reflect the metabolic pathways of CO₂ fixation and mechanisms of organic carbon evolution (Hayes et al., 1999; David, 1997). In conjunction with the carbon model, the carbon isotope difference was further used to calculate the fraction of organic carbon buried within the marine sediment (f_org) (Kump and Arthur, 1999), providing a quantitative means to investigate the organic burial (Huang et al., 2007).

This article aims to investigate the carbon isotope records in Late Permian strata at Shangsi Section, Sichuan Province, and to see whether they could provide some data related to the geological and paleoceanographic events. Our primary result shows the occurrence of a negative excursion in both the δ¹³C_carb and δ¹³C_org associated with the enhanced content of total organic carbon (TOC) and the anoxic condition, which is proposed to be indicative of the upwelling or volcanism activity occurred at the middle part of Dalong Formation of Shangsi area.

SAMPLING AND EXPERIMENTAL METHODS

In Late Permian, the South China craton was located in the lower latitude around the equator, forming the eastern margin of the Paleo-Tethys (Scotese and Langford, 1995). Shangsi Section is situated in Southeast Guangyuan, Sichuan Province, which was proposed to be one of the candidates for the GSSP of the Permian-Triassic boundary (Fig. 1). The sedimentary facies of the Late Permian Wujiaping Formation is a carbonate platform, and the Dalong Formation of the latest Permian is developed in the shelf-slope environment. The lithology of Dalong Formation is dominated by the siliciferous rocks and calcareous mudstone, which are intercalated by thin-bedded limestone. A detailed description of the lithology, sedimentary environmental conditions, and habitat types for Late Permian strata at Shangsi is presented by Yan et al. (2008) in this issue. The redox conditions of Dalong Formation are discussed by Ruan et al. (2008) and Zhou et al. (2008).

Figure  1.  Map showing the location of Shangsi Section (modified after Riccardi et al., 2007).

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Thirty-nine samples were collected from Dalong Formation and were crushed to less than 100 meshes before the carbon isotope measurement. The samples for the analysis of δ¹³C of bulk carbonate were allowed to react with 100% H₃PO₄ under vacuum to produce CO₂. For the determination of δ¹³C_org, the samples were treated with 10% HCl, rinsed with distilled water, dried at 60 ℃ overnight, and then combusted to get CO₂ (900 ℃). The CO₂ collected from both procedures was introduced into a Finigan MAT 251 mass spectrometer so that all the ratios of ¹³C/¹²C were measured. All the carbon-isotope ratios obtained are reported in the δ notion relative to the international PDB standard, where δ¹³C ‰ = [(¹³C/¹²C_sample － ¹³C/¹²C_standard)/¹³C/¹²C_standard] × 1 000, and the precision is better than ±0.1‰. The analysis was conducted at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan).

RESULTS

The positive relationship between δ¹³C_carb and δ¹⁸O_carb is commonly interpreted as a sign of the influence of meteoric diagenesis (Meyers and Lohmann, 1985). Most of the δ¹⁸O_carb values analyzed for Dalong Formation rocks range from -4.0‰ to -10.0‰, indicating a weak influence of meteoric diagenesis. This is further supported by the finding of the weak correlation (R²=0.18) between δ¹³C_carb and δ¹⁸O_carb (Fig. 2) in our samples analyzed.

Figure  2.  Crossplots showing the relationship between δ¹³C_org and δ¹⁸O_carb.

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The δ¹³C_org values range from -29.8 ‰ to -24.1‰ at Dalong Formation (Fig. 3), with two negative excursions being observed at the lower and middle parts, respectively. The δ¹³C_carb values, ranging from -3.2‰ to 4.9‰, exhibit a much more stable trend in comparison with the δ¹³C_org variation. However, a strong negative excursion is found in the middle part of Dalong Formation, coincident with the second excursion of δ¹³C_org. The ∆¹³C (δ¹³C_carb-δ¹³C_org) values range from 25.7‰ to 32.7‰, with two sharp drops occurring at the base and the middle of Dalong Formation. The first drop was associated with a remarkable increase in δ¹³C_org but a stable δ¹³C_carb value. However, it is notable that the second drop in the middle Dalong Formation is associated with the negative excursion of both the two carbon isotope records. The TOC content, with a variation ranging from 0.1% to 8.5%, remains low in the profile until an abrupt increase was present in the middle part of Dalong Formation.

Figure  3.  Plots showing the lithology, habitat type, and the variation trends of δ¹³C_org, δ¹³C_carb, ∆¹³C, and TOC content at Shangsi Section. The data of TOC content are from Xie et al. (2008) and the subdivision of habitat type is from Yan et al. (2008).

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DISCUSSION

It was reported that the sea level change could be reflected by the variation of δ¹³C_carb (Li et al., 2007; Voigt et al., 2006). The upper unit of Wujiaping Formation was deposited in the outer shelf. The depositional facies of Dalong Formation was inner shelf and continental slope. Coincident with the sea level change shown by the depositional environment, a declined value of δ¹³C_carb was observed at Wujiaping Formation and enhanced values at the lower Dalong Formation.

However, the most noticeable feature in carbon isotope records in the interval analyzed here is the remarkable and coincident drop in δ¹³C_org, δ¹³C_carb, and ∆¹³C in the middle part of Dalong Formation. This isotope drop is coincident with the presence of the maxima in TOC content. These geochemical data might suggest the occurrence of some geological/paleoceanographic events in the relatively deep water at Dalong Formation. The eastern Sichuan was located at the western margin of the Permian Yangtze platform (Scotese and Langford, 1995), facing to the eastern Paleo-Tethys, where the upwelling was frequently observed (Lü et al., 2004). The upwelling might be responsible for the carbon isotope excursion recorded in this section.

The marine carbon cycle could be divided into two subcycles: organic and inorganic variations (Berner, 2003; Kump, 1991). Figure 4 shows the simple carbon cycle model proposed by Berner (2003). CO₂ dissolved in water is fixed in the ocean via phytoplankton photosynthesis. The preferential takeup of ¹²C by phytoplankton photosynthesis leaves the accumulation of ¹³C in seawater, which in turn causes a positive δ¹³C of bulk carbonate. Consequently, the phytoplankton bloom will cause a positive δ¹³C values in bulk carbonate if the carbon pool in seawater keeps unchanged. However, the carbon pool in surface seawater will change due to the occurrence of geological/paleoceanographic events, such as the upwelling.

Figure  4.  Diagram showing the carbon cycle pathways (modified from Berner, 2003).

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In general, the photosynthesis in surface water will lead to a ¹²C-depleted CO₂ in seawater. In contrast, the oxidation of organic matter will produce a ¹²C-enriched CO₂ in bottom water. The upwelling will thus overturn the ¹²C-enriched CO₂ in bottom water into the surface, producing a ¹²C-enriched surface water. As a consequence, the carbon isotope composition of both organic and inorganic materials will be enriched in ¹²C. The strong negative excursion of δ¹³C_org and δ¹³C_carb could be a record of the upwelling in the middle part of Dalong Formation.

The upwelling explanation is not contradictory with the maxima content of TOC found in the middle part of Dalong Formation. The richness and accumulation of organic matter within marine sediments depend on primary productivity, depositional rate, redox condition, and hot fluid activity (Chen et al., 2006). The upwelling will transport the nutrition in bottom water into the surface water, leading to the bloom of some microorganisms. This causes the association of great burial of organic matter with the upwelling observed in modern environments, as well as in geological rocks. Great Mo abundance is further suggestive of the great organic burial in this abnormal interval (Zhou et al., 2008).

The occurrence of upwelling is also in line with the appearance of anoxia in this interval. Microorganism blooms due to the enhanced nutrition induced by the upwelling will cause the deposition of a large amount of organics. This deposition process of organic matter will further diminish the dissolved oxygen and thus an anoxic condition. The molecular records indicative of redox conditions as shown by the ratios of pristane to phytane, and dibenzothiophene to phenanthrene are consistent with the ratio of Mo/U. Both the organic and inorganic records are suggestive of the existence of anoxia in this interval throughout the whole Wujiaping and Dalong formations (Ruan et al., 2008; Zhou et al., 2008).

In addition to the upwelling, volcanism activity is widely observed at the Dalong Formation in South China (Isozaki et al., 2007; Yin and Lu, 2006; Liang and Ding, 2004). The volcanism will release a great amount of ¹²C-enriched CO₂ into the atmosphere and further to the seawater. As a result, the carbon isotope composition of both the organics and carbonate will decrease in response to the ¹²C-enriched CO₂ dissolved in seawater. However, volcanic contribution to the enhanced preservation of organic matter and the anoxic condition requires further investigation.

CONCLUSIONS

An abrupt negative excursion in the carbon isotope records of both bulk organic matter and carbonate was observed in the middle part of Dalong Formation, in association with a drop in the carbon isotope difference of the two records and the maxima of TOC content. The negative drop of the paired carbon isotope records is suggestive of the input of ¹²C-enriched CO₂. The molecular ratios of pristane to phytane and dibenzothiophene to phenanthrene indicate the presence of the anoxic condition in this interval. The enhanced TOC content supports the elevated burial of organic matter due to the anoxia. These isotopic and organic geochemical data suggest the occurrence of the upwelling in this abnormal interval. The contribution of volcanism activity observed in South China cannot be excluded to the input of ¹²C-enriched CO₂ and the negative shifts in carbon isotope composition of bulk organic matter and carbonate.