Negative carbon isotopic excursion on the Lower/Middle Cambrian ...

Vol. 46 No. 9

SCIENCE IN CHINA (Series D)

September 2003

Negative carbon isotopic excursion on the Lower/Middle Cambrian boundary of Kaili Formation, Taijiang County, Guizhou Province, China: Implications for mass extinction and stratigraphic division and correlation YANG Ruidong (ཷఫՊ)1,2, ZHU Lijun (ᅋोࢋ)2 & WANG Shijie (ฆಷࠍ)1 1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China; 2. Guizhou University of Technology, Guiyang 550003, China Correspondence should be addressed to Yang Ruidong (email: [email protected]) Received March 20, 2002; revised June 14, 2002

Abstract The stratigraphic division and correlation of the Lower/Middle Cambrian boundary is a global problem that has not yet been perfectly solved up to now. That is because there existed two global biogeographic regions during the period from Early Cambrian to Middle Cambrian. Although much work has been done from the angle of paleontology and great achievements have been acquired in this aspect, no biological assemblage has yet been established for global stratigraphic correlations due to the coexistence of the two global biogeographic regions üü the Atlantic biogeographic region and the Indian-Pacific biogeographic region during the Early-Middle Cambrian. So, to develop and establish other approaches to the stratigraphic division and correlation of the Lower/Middle Cambrian on a global scale is a possible way to solve the puzzling problem. This work systematically studied acritarch fossils from the Early-Middle Cambrian Kaili Formation at Taijiang County, Guizhou Province. The Lower/Middle Cambrian boundary was divided in terms of acritarch fossil assemblage. The divided boundary is generally consistent with what was divided by trilobite and can be correlated with the Lower/Middle Cambrian boundaries divided by acritarch assemblage in Siberia and Europe. On this basis, the Lower/Middle Cambrian boundary is divided in terms of an obvious carbon isotopic excursion on a global scale during the transitional period from Early Cambrian to Middle Cambrian boundaries in Siberia and North America. The method for the stratigraphic division and correlation of the Lower/Middle Cambrian boundary in terms of carbon isotopic oscillations is helpful to solving the global problem on the division and correlation of the Lower/Middle Cambrian boundary. It is also evidenced that the extinction of a lot of trilobites at the end of Early Cambrian is closely related with this event of carbon isotopic excursion. Keywords: negative carbon isotopic excursion, extinction, Lower/Middle Cambrian boundary, Guizhou Province. DOI: 10.1360/01yd0416

The stratigraphic division and correlation of the Lower/Middle Cambrian boundary is a global problem that has not yet been solved even up to now. That is because there existed two distinctly different biogeographic regions during the Early and Middle Cambrian[1] üüthe Indian-Pacific biogeographic region and the Atlantic biogeographic region. In the Atlantic biogeographic region the traditional Lower/Middle Cambrian boundary was marked by the extinction of Olenellids trilobite and the initial occurrence of Paradoxides[2]; in the Indian-Pacific bio-

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geographic region the Lower/Middle Cambrian boundary was marked by the extinction of trilobite genus Redlichia and the first appearance of trilobite Ptychopariids[3]. Although much more palaeontological work has been done, many achievements, including those of Yuan et al.[4], Sundberg and McCollum[5], Montanez et al.[6], Moczydlowska[7], and Geyer and Shergold[8], have been acquired, and a variety of new methods[4

ü8]

have been put forward, how to divide the Lower/Middle

Cambrian boundary is still a long-standing controversy due to the coexistence of the two biogeographic regions during the Early Cambrian in the world. For the above reasons, some geological workers began to look for other methods of stratigraphic division, for example, the geochemical methods. Among the geochemical methods, the one utilizing carbon isotopic excursion to divide and correlate the Lower/Middle Cambrian boundary is the best, and a great progress has been made in the stratigraphic division of Lower/ Middle Cambrian boundary. The typical studies in this aspect involve carbon and strontium isotopes in the strata at the Lower/Middle Cambrian boundaries in Siberia and North America[6,9]. Their research results revealed remarkable G 13C and 87Sr/86Sr excursions near the Lower/Middle Cambrian boundaries both in Siberia and in North America. Recent studies show that theG 13C and 87

Sr/86Sr excursions at the Lower/Middle Cambrian boundaries in Siberia are well consistent with those in North America[6,10,11]. Much research work has been done on the Lower/Middle Cambrian boundary in Taijiang, Guizhou Province, where the Lower and Middle Cambrian series are well developed as compared with those in other parts of the world at present time[4,12

ü17]

. The Lower/Middle Cambrian bound-

ary at Taijiang Country, Guizhou Province has already been taken as a typical boundary section, and also accepted by both Chinese and overseas scholars. In 2001, members of the International Cambrian Subdivision Committee made an investigation-tour of the section. Although the Balang Lower/Middle Cambrian boundary at Taijiang, Guizhou, has been well documented from the angle of paleontology, it is still necessary to carry out a geochemical investigation (G 13C) on the Lower/Middle Cambrian boundary at the Balang section to facilitate its global stratigraphic correlation. 1 The Lower/Middle Cambrian boundary at Balang Village, Taijiang County, Guizhou Province The Kaili Formation at the Balang section, Taijiang County, bears abundant well-preserved fossils and within the Lower/Middle Cambrian boundary also occur the so-called Taijiang biota and Kaili biota, both of which are characterized by abundant trilobites[18]. The Lower/Middle Cambrian boundary at the Balang section has been studied in detail in terms of trilobite fossils by Yuan et al.[4], and it is suggested that the beginning of the Middle Cambrian is marked by the first appearance of Oryctocephalus (O. indicus) or Oryctocephalina within the evolutionary lineage from Lancastria to Oryctocephalus. Because of the first appearance of Oryctocephalus indicus

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(Reed) in layer No. 9-2, and the occurrence of Early Cambrian key fossils, such as Redlichia, Bonnia, Bathynotus, Ovatoryctocara, Oryctocephalops, Protoryctocephalus, Olenoides abnormis, O. octaspinus, Nangaops and Paraantagmus in layer No. 8, it is considered that the Lower/Middle Cambrian boundary at the Balang section should be located at the base of layer No. 9 (fig. 1). It was also proposed by Sundberg et al. [5] that fossil Oryctocephalus indicus (Reed), which has also been found in North America, Siberia, India and Australia, could be taken as a standard fossil marking the beginning of the Middle Cambrian. Sundberg and McCollum made an on-the-spot investigation of the Balang section at Taijiang, Guizhou two years ago and inferred that the Balang section is a potential candidate for global stratotype section point (GSSP) of the Lower/Middle Cambrian boundary. They also suggested that the Balang section at Taijiang be a perfect one for the stratigraphic division of Lower/Middle Cambrian boundary and it can be correlated with that in North America (fig. 1).

Fig. 1. Correlation chart of key trilobites near the Lower/Middle Cambrian boundary of Guizhou Province, China and North America (Taijiang data from Yuan Jinliang et al., 1997; North America data after Sundberg et al., 1999) (L/M = Lower/Middle).

As the Lower/Middle Cambrian boundary and related strata have been well documented and the distribution and assemblage characteristics of acritarch fossils are also described in detail, the typical acritarch fossil assemblages or standard fossils have been defined for the division of the Lower/Middle Cambrian boundary. It is no doubt that this method is of great significance for the stratigraphic division of the Lower/Middle Cambrian boundary in those trilobite-free areas. On the basis of the distribution characteristics of acritarchs from the Lower and Middle Cambrian series of Europe and North America, the Lower/Middle Cambrian boundary divided by acritarch assemblages at the Balang section coincides with that by trilobites (fig. 2).

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Fig. 2. The map showing the distribution of acritarchs and the Early/Middle Cambrian boundary in the Kaili Fm. at Balang, Taijiang County, Guizhou Province (L/M =Lower/Middle).

2 Carbon isotopic excursion near the Lower/Middle Cambrian boundary at the Balang section, Taijiang County, Guizhou Province The Lower-Middle Cambrian Kaili Formation at Balang Village, Taijiang County consists of dark-gray calcareous mudstones containing 15%ü30% CaCO3. But the lower and upper parts of the Kaili Formation are composed of thin-bedded limestones and dolomites, with thin-bedded gray calcareous mudstone intercalations near the Lower/Middle Cambrian boundary (fig. 3).

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Fig. 3.

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The map showing the relationship between secular variation in G 13C values and the Lower/ Middle Cam-

brian boundary from the Kaili Formation of Taijiang County, Guizhou Province, China.

The sections prepared from the calcareous mudstones were examined under microscope. It was found that calcium carbonate is present as microcrystalline calcite, and the calcareous mudstones are almost free from the influence of diagenesis at later stages because mudstones are so capable of blocking groundwater that the calcareous material (calcite) could not be replaced. So the carbon isotope data of carbonates in the calcareous mudstones obtained in this study generally reflect the geochemical features of palaeo-ocean water at the time of sedimentation. The carbon isotopic (PDB) analysis of the samples was conducted on a MAT251 mass spectrometer at the Institute of Geochemistry, Chinese Academy of Sciences. The samples were first pretreated with orthophosphoric acid and then analyzed on the MAT251 mass spectrometer. The results of carbon isotopic analysis showed that there is an obvious carbon isotopic excursion at the Lower/Middle Cambrian boundary and G 13C values at the top of the Lower Cambrian tend to decrease from positive values, i.e. from 1.331‰ ü0.740‰ for the dolomite at the top of the Qingxudong Formation to 0.352‰ for layer No. 7 of the Kaili Formation. But strongly negative G 13C values appear at layer No. 8 of the Kaili Formation and decline drastically to –3.028‰ at Lower/Middle Cambrian boundary I marked by trilobite Redlichia extinction. To the basement of the Middle Cambrian (sublayer No. 2 in layer No. 9 of the Kaili Formation, i.e. Lower/Middle Cambrian boundary II marked by the first appearance of Oryctocephalus indicus

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and abundant acritarchs), G 13C values rise rapidly from –3.028‰ to –0.846‰ (fig. 3), and from –0.846‰ to +2.797‰ at sublayer No. 5 in layer No. 9 of the Kaili Formation. Such a G 13C sharp shift on the Early/Middle Cambrian boundary at the Balang section, Taijiang County coincides with the carbon isotopic characteristics for the Early/Middle Cambrian boundaries in Siberia and North America (fig. 4). This kind of rapid (100 ka), large-magnitude shift of G 13C values (ı4‰) on the Early/Middle Cambrian boundary is of wide occurrence throughout the world.

Fig. 4.

The map showing correlations between G 13C value curve and bio-zone of the Kaili Formation and those at

the Lower / Middle Cambrian boundary in North America (North America data from Montanez et al., 2000).

On the basis of the biostratigraphy and geochemo-stratigraphy data available, the Balang section is a typical candidate for GSSP of Lower/Middle Cambrian boundary. It possesses typical biostratigraphic features, but also shows an obvious variation trend in carbon isotopic composition of palaeo-ocean water. However, whether the Lower/Middle Cambrian boundary is defined at the horizon of trilobite Redlichia extinction, or at the horizon where G 13C values were suddenly decreased to –3.028‰ (Lower/Middle Cambrian boundary I), or at the horizon where trilobite Oryctocephalus indicus first appeared and abundant acritarchs appeared, andG 13C values varied from –3.028‰ to –0.846‰ (Lower/Middle Cambrian boundary II) still needs to be further studied under a global correlation program.

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Carbon isotopic excursion and its implication for mass extinction and stratigraphy

The appearance of strongly negative carbon isotopic excursion at the Lower/Middle Cambrian boundary is a ubiquitous phenomenon in the world. The negative carbon isotopic excursion indicates that major paleogeographic changes and climatic fluctuations all predated the trilobites extinction at the end of the Early Cambrian[19]. Not only a negative G 13C excursion, but also a rapid increase in 87Sr/86Sr values were observed in the Lower/Middle Cambrian boundary, which have been documented by numerous geochemo-stratigraphic studies[6,10,11]. Extensive erosion of the sequences caused by an early Middle Cambrian regression event on a global scale can explain why this negative G 13C isotopic excursion has not been recognized before, though it is among the largest carbon isotopic excursion during the Phanerozoic. The strata near the Lower/Middle Cambrian boundary in the North America Basin are composed of shales intercalated with carbonate rocks. The stratigraphic relationship between the shales and the carbonate rocks is a sedimentation record of the rapid rise of sea level, which were recognized previously in the Laurentian and Siberian continental sections[9,20]. The prolonged environmental stress during the negative carbon isotopic excursion is indicated by the lack of bio-perturbation in shales and the occurrence of carbonate shell beds formed during the episodic of trilobite remain assemblages[16]. The negative carbon isotopic excursion implies major paleooceanic geographical changes, and probably climatic changes, which led to the mass extinction of trilobites at the end of Early Cambrian. This negative G 13C excursion is considered to have been caused by: (i) the introduction of G 13C-depleted, anoxic water into shallow-water carbonate platforms during the latest Early Cambrian transgression, and (ii) the associated decrease in organic carbon burial amount due to the major biomass reduction. An anoxic water column below the surface mixed layer may have prevailed in Early Cambrian oceans during the transgression, accompanied by the sluggish circulation and strong stratification. These oceanic conditions would be favored by the low-latitude continentality and depressed meridional temperature gradients that would be the characteristics of the Early Cambrian greenhouse period[21]. The G 13C values of carbonates at the peak of negative isotopic excursion (< 4.0‰) suggest that the ocean’s carbon isotopic composition is approximate to that of the terrestrial weathering input to the ocean[22]. This suggests the influence of river flux on the ocean’s isotopic composition, and that the primary productivity was greatly reduced and the rates of organic carbon burial were diminished as well in response to the reconstruction of the environmental stress after the mass extinction. The subsequent shift to more positive G 13C values during the earliest Middle Cambrian seems to be the result of the increase of oxygen amount in the ocean due to sea level fall and oceanic circulation rise in the early Middle Cambrian. In addition, when the oceanic environmental conditions were fully improved, the productivity of surface ocean water would be recovered. As can be seen from fig. 2, starting from sublayer No. 2 in layer No. 9, abundant acritarchs (reflecting surface ocean water productivity levels) began to appear, indicating the recovery of surface ocean

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water productivity levels. In fig. 1, it can be seen that the extinction of a lot of trilobites occurred at the top of layer No. 8. This is the mass extinction of trilobites that occurred at the end of the Early Cambrian. It is this extinction event that led to strongly negative G 13C value (3.028‰). The difference in stratigraphic thickness from the horizon of trilobite extinction at the end of Early Cambrian to the horizon where acritarchs extensively appeared in the earliest Middle Cambrian is only 0.8 m at the Balang section. According to Hesselbo et al., if a lamina, measuring 50 Pm in thickness, was deposited at a rate of 1 Pm per year, the deposition of 1 mm-thick shale would take about 10 ka[23]. Then, the deposition of a 0.8 m-thick stratum will take about 800 ka. This result is generally in coincidence with the time (1000 ka) required for the survival of biomass after a large or small mass extinction in the geological history[24]. This is also in coincidence with the mass extinction at the end of Ordovician and during the Late Devonian[25,26]. From trilobite extinction to the appearance of acritarchs in large amounts during the Early Cambrian at the Balang section, Taijiang County, the whole process would take about 800 ka, shorter than what was estimated by Kirchner et al. That is probably because the horizon of trilobite extinction and the appearance of abundant acritarchs has not been defined accurately. Moreover, the errors involved in the estimation may be attributed to the erosion of shale lamina. Therefore, the time required for the mass extinction and survival near the Lower/Middle Cambrian boundary at the Balang section, Taijiang County, Guizhou Province, China, is in consistency with that for a whole process from a large or small mass extinction to the sequent mass survival in the geological history. This is also a piece of strong evidence that the Balang section is a complete, ideal stratigraphic section. The section can reflect the rule of normal bio-evolution, but that is not because the change of regional environment led to the local bio-extinction. So it is suggested that the bio-zones (trilobite zones and acritarch assemblage zones) divided at the Balang section are of great significance for the stratigraphic division and correlation on a global scale. In response to a rapid decrease in G 13C near the Lower/Middle Cambrian boundary, a shortterm increase occurred in 87Sr/86Sr at the same time. The short-term increase in seawater 87Sr/86Sr could reflect the extensive flux of terrestrial material into the ocean. The phenomenon is very recognizable at the Cordilleran passive margin, North America[6]. In addition, the short-term rapid increase of G 13C reflects an overall increase in the nutrient level, primary productivity and organic matter burial rate in the ocean. On the contrary, the short-term rapid decrease of G 13C may reflect the decreasing flux of surface water into the ocean, and the lowering of global oceanic sedimentation rates, which resulted in a decrease in seawater G 13C values. These short-term oscillations in seawater G 13C and 87Sr/86Sr values may be ascribed to the influence of short-term sea level fluctuations and short-term tectonic events during the Cambrian on the palaeooceanic geographic conditions and the burial of organic matter[27]. The high-resolution Sr and C isotopic curves presented in this paper can take us closer to the understanding of the isotopic evolution of Cambrian seawater. On the other hand, the strongly

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negative G 13C isotopic excursion near the Lower/Middle Cambrian boundary, which had not been reported anywhere, has been found recently in North America[6] and Siberia[9], and it also has been found on the Lower/Middle Cambrian boundary at the Balang section, Taijiang County, Guizhou Province, China in our study. So, it is believed that the strongly negative G 13C isotopic excursion near the Lower/Middle Cambrian boundary is closely related to the mass trilobite extinction at the end of Early Cambrian. Additionally, such a strongly negative G 13C isotopic excursion near the Lower/Middle Cambrian boundary at the Balang section, Taijiang County, Guizhou, will probably open up a new approach to the stratigraphic division and global correlation of Lower/Middle Cambrian boundary stratigraphic division and global correlation of Lower and Middle Cambrian strata throughout the world. Nevertheless, whether the G 13C and

87

Sr/86Sr oscillations near the Lower/Middle Cambrian

boundary are connected with tectonic events, ocean evolution, palaeoclimatic conditions, etc. is still a problem worthy to be further studied. Acknowledgements The authors wish to thank Prof. Ouyuang Ziyuan and Yin Leiming for their help in this study. This work was jointly supported by the National Natural Science Foundation of China (Grant No. 40062001), the Governor Foundation of Guizhou Province, the Post-doctoral Foundation of Guizhou University of Technology and the Excellent Youth Scientists and Technicians Foundation of Guizhou Province, and a special program by MSTC (Grant No. 2002CCC02600).

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