Early-Middle Cambrian Palynomorph Microfossils and Related Geochemical Events in South China

0. INTRODUCTION

Data for Cambrian acritarch biostratigraphy in China are scarce, and most known acritarch assemblages are concentrated in the earliest Cambrian near the Neoproterozoic-Cambrian boundary. Since the 1970s, earliest Cambrian acritarch specimens and assemblages have been successively reported from South China (Fig. 1) and the Tarim Basin (Yao et al., 2005; Ding, 1992; Yin C., 1992, 1990; Yin L., 1987, 1986). To facilitate intercontinental biostratigraphic correlation, the earliest Cambrian Asteridium-Heliosphaeridium-Comasphaeridium (AHC) assemblage (Yao et al., 2005) is referred to herein, instead of the previously named Micrhystridium-Paracymatiosphaera-Megathrix assemblage (Yin, 1995). The earliest Cambrian acritarch AHC assemblage is generally synchronous with the Anabarites trisulcatus-Protohertzina anabarica and Siphogonuchites triangularis-Paragloborilus subglobosus small shelly fossil assemblages in South China (Yao et al., 2005). From the ²⁰⁶Pb/²³⁸U age of 535.2±1.7 Ma recently obtained from a tuff layer in the Meishucun Section, Yunnan (Zhu et al., 2009), the biostratigraphic range of the acritarch AHC assemblage can be estimated to fall in the time interval of approximately 542 to 530 Ma. Phosphorite and chert-bearing black shale at the Precambrian-Cambrian boundary (~542 Ma) constitute well-defined markers that serve as important points for correlation between sections in South China. Based on recent study of the Yangtiao Section of the Niutitang Formation in Kaili County, many specimens of Asteridium tornatum and Heliosphaeridium pliatum (Fig. 2) first occur in the chert bed approximately 5 cm above the base of the Niutitang Formation (Xie et al., 2015). In the overlying Early Cambrian'Chiungchussuan' interval that contains the trilobite Eoredlichia-Wudingaspis Zone (Lin, 2008), the characteristic acritarch form Skiagia, which is universally accepted as a cosmopolitan acritarch biostratigraphic marker, has been discovered in eastern Yunnan Province (Zang, 1992; Xing, 1982). Until the end of the last century, systematic investigation of acritarchs had been part of the work on the boundary between Cambrian Series 2 and Series 3 in eastern Guizhou Province (Yin and Yang, 1999). Subsequently, more acritarch studies have been carried out on the boundary biostratigraphy of Cambrian Series 2 to Series 3 in eastern Guizhou (Yin et al., 2010, 2009; Yang and Yin, 2001). At the same time, geochemical studies and biomarker evidence were carried out on Early-Middle Cambrian sedimentary rocks for chronostratigraphic correlation (Guo et al., 2014, 2007, 2005; Wang et al., 2014; Yamada et al., 2014; Zhu et al., 2004; Zhou et al., 1997). This review discusses the chronostratigraphic distribution of Early-Middle Cambrian acritarch assemblages and the relationships between microphytoplankton and geochemical changes during that interval of geological time in South China.

Figure  1.  First occurrences of small spinose acritarchs and the organic carbon isotope profile of Early Cambrian representative sections in South China. Stratigraphic columns and the organic carbon isotope profile are modified from Jiang et al. (2012).

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Figure  2.  Microfossils found from bottom cherts of the Lower Cambrian Niutitang Formation, Guizhou, South China. (a)-(c) Small spinose acritarchs. (a) Asteridium tornatum, (b) and (c) Heliosphaeridium ampliatum; (d) and (e) Sponge spicules; (f) and (h). Uncertain animal remains; (g) Megathrix longus. The scale bar equals 10 µm and the double bar equals 20 µm.

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1. EARLY CAMBRIAN

Molecular biological studies indicate that the eukaryotic radiation occurred at about 800 Ma, while marine chemistry was in the process of transforming to resemble modern marine conditions (Parfrey et al., 2011; Douzery et al., 2004). On the basis of data including paleontological records, molecular diversification, comparative development, and ecological feeding strategies, Erwin et al. (2011) suggested that ecological success of metazoans took place during the Ediacaran (635 to 541 Ma) and the Cambrian (541 to 488 Ma).

Geochemical data of Neoproterozoic-Early Cambrian sediments in South China indicate that the ocean during this geological time interval was in transition from a reducing deep-sea setting rich in iron and sulfur to a shallow surface-water redox environment. This transition may have resulted from a remarkable increase of oxygen and reduction in the iron and sulfur levels, which may have been caused by cessation of deep ventilation and expansion of oxic conditions onto shelf areas (Feng et al., 2014; Jiang et al., 2009; Zhou and Jiang, 2009; Luo et al., 2003; Steiner et al., 2001). Such changes in marine chemistry and physical conditions must have directly affected the marine microbiota. Acritarchs are organic-walled preserved remains of phytoplankton that occupied an important role in the fundamental food chain in marine ecosystems. They can provide fundamental data on ecology and significant information for understanding the evolutionary history of phytoplankton and reconstructing ecological conditions during different geological periods.

Geochemical data reveal that the δ¹³C record of the Ediacaran Dengying Formation (~551 to ~542 Ma) is very stable, with δ¹³C values from +2‰ to +5‰, an average value close to +3‰ (Jiang et al., 2007), and a prominent negative δ¹³C excursion (−7.2‰) in the Lowest Cambrian (Li et al., 2009).

Organic-walled microfossils such as abundant coccoid algae, small spinose acritarchs and the tubular microfossil Megathrix appear abruptly in earliest Cambrian cherts and silicified phosphorites. The appearance of these fossils is before the first record of trilobites and immediately above dolostone of the Dengying Formation in South China and the Tarim Basin (Yao et al., 2005; Yin, 1986, 1987). Of these microfossils, some are morphological forms that are common in Neoproterozoic sediments, such as simple spherical vesicles (leiospherids) and hollow filamentous forms (normally named Siphonophycus)..The abrupt appearance of abundant small spinoseacritarchs in the earliest Cambrian could have been affected by the remarkable increase in oxygen levels and a reduction in the reproductive space of phytoplankton, since grazing animals appeared in late Neoproterozoic (Ediacaran) marine ecosystems (Butterfield, 2004; Banse, 1994; Brasier, 1992, 1990; Timms and Moss, 1984). The widely distributed earliest Cambrian AHC acritarch assemblage in South China could represent phytoplanktonic diversity in shallow water with a higher level of free oxygen, although the earliest Cambrian in the Yangtze Platform of South China might have been dominated by relatively reducing and euxinic conditions in deep water (Feng et al., 2014; Zhou and Jiang, 2009). Such subtle variations in the fundamental food chain through the Late Ediacaran and Early Cambrian in the Yangtze Platform are revealed by geological analysis (Fig. 1). For example, the organic carbon isotope gradient across the Late Ediacaran-Early Cambrian in the Yangtze Platform, which is consistent with the higher ¹³C_org values documented from shallow-water carbonates, may record the isotopic signature of organic matter from photosynthetic production (Wang et al., 2014; Jiang et al., 2012). Furthermore, analysis of molecular fossils extracted from earliest Cambrian sedimentary rocks of the Yangtze Gorge has indicated that an increase in straight-chain alkanesis correlated well with pristane and phytane, which are mainly derived from the phytyl groups contained in the chlorophylls of phototrophs (Yamada et al., 2014). Additionally, different regions and sections do not have the same microfossil composition. For example, the Early Cambrian Niutitang Formation of the Yangtiao Section in eastern Guizhou contains Asteridium, Heliosphaeridium and Megathrix (see Fig. 2), but no Comasphaeridium has yet been found there. Furthermore, in the Heishapo Section, which is several hundred kilometers northwest of the Yangtiao Section, only Asteridium and Heliosphaeridium have been recovered. However, specimens of Asteridium tornatum are commonly discovered in Lowest Cambrian cherts over almost the entire Yangtze Platform. Based on more recent acritarch studies of the Niutitang Formation in eastern Guizhou, the biostratigraphic range of the AHC acritarch assemblage is calculated to be about 1-2.5 m above the base of the Lower Cambrian and about 0.5-2 m below the overlying bed containing a Ni-Mo ore layer (Xie et al., 2015). In the global Cambrian chronostratigraphic correlation schemes (Landing et al., 2013; Babcock and Peng, 2007), the earliest Cambrian AHC acritarch assemblage in South China could correspond to Stage 1 and the lower part of Stage 2 (Babcock and Peng, 2007) or the Fortunian Stage (Landing et al., 2013; Moczydłowska and Yin, 2012).

In South China, specimens of the characteristic morphological genus Skiagia were first reported from gray shale and black shale interbedded with siltstone in the lower part of the Early Cambrian Chiunchussu (Qiongzhusi) Formation containing the trilobite Eoredlichia-Wutingaspis Zone, near Kunming, eastern Yunnan Province (Xing, 1982). At that time, many spinose acritarch specimens were identified as Baltisphaeridium, including B. cerinum, B. compressum, B. ornatum, B. dubium and one new species, B. multispinosum (Xing, 1982). Later, Zang (1992) used many well-preserved acritarch specimens, especially those obtained from the same horizon of the Chiunchussu Formation in the Maotianshan Drill Core, to determine that some specimens of Baltisphaeridium described by Xing (1982) can be referred to Skiagia orbiculare (Zang, 1992). Furthermore, from the morphological descriptions, some specimens of Baltisphaeridium multispinosum (see plate 1, figs. 10-12 in Xing, 1982) appear to coincide with the diagnosis of Skiagia ornata. The'Skiagiaornata-Fimbriaglomerella membranacea' acritarch assemblage (Zang, 1992) is known from the Early Cambrian'Chiungchussuan' interval that contains the trilobite Eoredlichia-Wudingaspis Zone, which is just above the trilobite Parabadiella Zone. Both zones are referred to the'Chiungchussuan Stage' (Lin, 2008).

The temporal distributions of both the Early Cambrian AHC acritarch assemblage and the'Skiagia ornate-Fimbriaglomerella membranacea' acritarch assemblage are recorded as being correlated with assemblages in contemporary strata in Norway, Sweden, Scotland, Newfoundland, Greenland, Poland and Russia (moczydłowska and Yin, 2012; moczydłowska and Zang, 2006; moczydłowska, 1998, 1991; Martin and Dean, 1983; Downie, 1982; Volkova et al., 1979).

2. EARLY-MIDDLE CAMBRIAN

Reports of Early-Middle Cambrian acritarch assemblages in China are still sparse. Since the 1990s, we have studied Early-Middle Cambrian acritarchs and other palynomorphs from the Jianhe, Taijiang, Majiang, and Danzai areas in southeastern Guizhou Province. For these areas, the Wuliu-Zenjiaya Section, the Jianshan Section, and the Danzai Section have been much more systematically sampled and studied in detail (Yin et al., 2010, 2009; Yang and Yin, 2001; Yin and Yang, 1999). The acritarch assemblages from Series 2 (Stage 4) rocks containing Bathynotus holopygus-Ovatoryctocara cf. granulata trilobite Zone are dominated by simple spherical'leiospherids' and an aggregated form, Bubomorpha, except for rare specimens bearing indistinct ornamentation. However, quite near to the Series 2-Series 3 boundary defined by the first appearance of the trilobite Oryctocephalus indicus, more diversified spinose acritarchs abruptly occur as part of a quite different acritarch assemblage from that of Series 2. As shown in Fig. 3, in the Wuliu-Zenjiaya Section, Series 2 is recognized as the Leiomarginata simplex-Fimbriaglomerella membranacea assemblage and Series 3 is characterized by the Cristalliniumcambriense-Heliosphaeridium nodosum-Globosphaeridium cerinum assemblage (Yin et al., 2010). Furthermore, many'Burgess Shale−type' small carbonaceous fossils including Wiwaxia sclerites, biomineralizing taxa such as chancelloriids, brachiopods and hyolithids, and various other remains of pterobranchs, priapulid−like scalidophorans and some specimens of cryptospores-like microfossils (Fig. 4), have been found from shale of Series 3 age in the Kaili Formation (Yin et al., 2013; Butterfield and Harvey, 2012; Thomas et al., 2012; Li and Lin, 2011).

Figure  3.  Distribution of acritarch taxa and relevant geochemical profiles in the Early-Middle Cambrian Kaili Formation of the Wuliu-Zengjiaya Section, Jianhe, Guizhou Province, China. Acritarch taxa are after Yin et al. (2010); δ¹³C_carb(‰, V-PDB) and δ³⁴SCRS (‰, V-CDT) profiles are from Guo et al.(2010, 2014); and Pr+Ph/(nC₁₇ + nC₁₈) and Pr+Ph(µg/g TOC) profiles are from Wang et al. (2014).

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Figure  4.  Microfossils collected from the Middle Cambrian Kaili Formationof the Wuliu-Zengjiaya Section, Jianhe, Guizhou Province, China. (a)-(f) Varied spinose acritarchs (cited from Yin et al., 2010); (g), (h) and (j) cryptospore-like microfossils (cited from Yin et al., 2013); (i) and (k) Wiwaxia sclerites (cited from Li et al., 2012). The scale bar equals 10 µm.

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Obviously, the remarkable biotic change represented by both acritarch assemblages and trilobite zones indicates that a major geological and biologic event had taken place during the transition from Series 2 to Series 3. The existence of this event is strongly supported by geochemical studies of δ¹³C_carb and δ³⁴S_CRS (Guo et al., 2014, 2010) and biomarker evidence (Wang et al., 2014) from the Wuliu-Zengjiaya Section. A remarkable negative shift of δ¹³C_carb occurs at the Series 2-Series 3 boundary defined by the first appearance of Oryctocephalus indicus (Guo et al., 2010), and an excursion towards less ³⁴S enriched values immediately precedes the proposed level of the Cambrian Series 2 to Series 3 transition (Guo et al., 2014). In addition, extensive development of a shallow marine carbonate ramp in the Early Cambrian and early Middle Cambrian (Wotte et al., 2012) could also have led to relevant variations in oxygen content, temperature, and nutrient source. All these changes could have affected propagation of phytoplankton and even variation in the biota. Such variation is typically represented by the differences between the acritarch assemblages of Series 2 and Series 3 in the Wuliu-Zengjiaya Section. As part of this remarkable change, the negative δ¹³C_org values and higher TOC content in Series 2 changes to positive δ¹³C_org values and lower TOC content in Series 3. Biomarker evidence alters from less abundant isoprenoid hydrocarbons and relative enrichment of midchain monomethyl branched alkanes (mmb-alkanes) in Series 2 to relative enrichment of isoprenoid hydrocarbons and a lower abundance of mmb-alkanes in Series 3 (Fig. 3) (Wang et al., 2014). Changes in the geochemistry and biomarkers occurred concurrently with variations in the organically preserved microfossils in the Kaili Formation of the Wuliu-Zengjiaya Section, and the depositional environment distinctly transformed from a relatively reducing environment in Series 2 to an environment with enhanced oxygen content and increased nutrients in Series 3 (see Fig. 3).The Wuliu-Zengjiaya Section of the Kaili Formation is an international candidate stratotype-section for the boundary between Cambrian Series 2 and Series 3: this section records the major geological and biotic events that occurred during this interval of geological time.