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Following the Ediacaran metazoan radiation, the “Cambrian Explosion” set up the major framework of todays' animal phyla as well as modern marine ecosystem ...

SCIENCE CHINA Earth Sciences • RESEARCH PAPER •

April 2014 Vol.57 No.4: 579–591 doi: 10.1007/s11430-013-4779-y

Spatiotemporal variability of ocean chemistry in the early Cambrian, South China JIN ChengSheng, LI Chao*, PENG XingFang, CUI Hao, SHI Wei, ZHANG ZiHu, LUO GenMing & XIE ShuCheng State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Wuhan 430074, China Received January 14, 2013; accepted June 14, 2013; published online January 15, 2014

Following the Ediacaran metazoan radiation, the “Cambrian Explosion” set up the major framework of todays’ animal phyla as well as modern marine ecosystem. Here, we present a preliminary investigation on the temporal and spatial (from shallow to deep waters) variations of the early Cambrian ocean chemistry in South China through analyzing a Fe-S-C systematic dataset integrated from literature. Our investigation indicates that the early Cambrian deep ocean in South China was still anoxic and Fe2+-enriched (i.e., ferruginous) although its surface was oxic, and in between a metastable euxinic (anoxic and sulfidic) water zone may have dynamically developed in anoxic shelf waters with an increasing weathering sulfate supply. Furthermore, accompanying marine transgression and regression cycles in the early Cambrian, such a “sandwich” structure in ocean redox chemistry demonstrates five evolutional stages, which can be well correlated to the spatiotemporal patterns of fossil records in South China. The good correlation between metazoan fossil occurrences and water chemistry in South China suggests that early animals possibly possessed ability to inhabit anoxic but generally not euxinic environments as free H2S was fatal to most eukaryotes. This view can well explain why those small shell fauna and sponges disappeared from shelf to slope areas where sulfidic Ni-Mo-rich shales were widely deposited. Thus, we conclude that the spatiotemporal variations of ocean chemistry and its biological effects probably played a key role in the phased animal radiations and “extinctions” in the early Cambrian. South China, early Cambrian, ocean chemistry, euxinic, Cambrian Explosion Citation:

Jin C S, Li C, Peng X F, et al. 2014. Spatiotemporal variability of ocean chemistry in the early Cambrian, South China. Science China: Earth Sciences, 57: 579–591, doi: 10.1007/s11430-013-4779-y

The late Neoproterozoic-early Cambrian transition is a critical period in evolution of life on the Earth. The Ediacaran biota disappeared before the Precambrian-Cambrian boundary (PCB, about 541 million years ago (Ma)), but significant life radiation occurred in stages in the early Cambrian (Knoll and Carroll, 1999; Marshall, 2006; Shu, 2008; Zhu et al., 2007; Zhu, 2010). The small shelly fauna in the Jinningian-Meishucunian demonstrated stages of radiation and “extinction” in fossil records (Steiner et al., 2007; Zhu et al., 2007; Zhu, 2010), marking the radiation of biomineraliza*Corresponding author (email: [email protected])

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tion and bilaterians. Most of modern important phyla have well developed in the Nangaoan, culminating in the appearance of Chengjiang biota (Knoll and Carroll, 1999; Marshall, 2006; Shu, 2008; Zhu et al., 2007; Zhu, 2010). Along with the appearance of preying animals in the late Neoproterozoic (Bengston and Zhao, 1992) and their following developments in the early Cambrian, modern biogeochemical cycle was gradually established (Logan et al., 1995). It has been assumed that a full oxygenation of the early Cambrian deep oceans caused the Cambrian Explosion (Knoll and Carroll, 1999; Logan et al., 1995; Scott et al., 2008). However, Fe-S-C systematics and trace metal data earth.scichina.com

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from the early Cambrian black shales from South China indicated that anoxic and euxinic (containing free H2S) waters were still common (Goldberg et al., 2007; Guo et al., 2007a; Wille et al., 2008). In addition, Fe- speciation, trace metal and sulfur isotope data from the Laobao Formation, South China, suggested that the late Neoproterozoic ferruginous deep waters possibly had persisted into the early Cambrian (Canfield et al., 2008; Chang et al., 2010, 2012). The contradictions among geochemical records as well as contradictions between anoxic ocean chemistry and the Cambrian Explosion call for a re-examination of the ocean chemistry in the early Cambrian. Li and his colleagues (Li et al., 2010) recently proposed a “sandwich”-like ocean chemistry model for the Ediacaran ocean through studying the spatiotemporal variability of FeS-C systematic and trace metal data from the Doushantuo Formation (635–551 Ma) in South China. This “sandwich” model suggests that a metastable zone of euxinic (anoxic and sulfidic) waters dynamically impinged on the continental shelf and was sandwiched within ferruginous (Fe2+-enriched) deep waters in the Ediacaran oceans, and especially emphasizes that the near-shore euxinic waters dynamically developed in their sizes and locations along with related geochemical controlling factors. Recent studies into other geological periods indicated that this dynamic “sandwich”-like ocean chemistry structure might have widely existed in early Earth oceans (~2.7–0.58 billion years ago (Ga)) (Li et al., 2012; Jonston et al., 2010; Planavsky et al., 2011; Poulton et al., 2010; Poulton and Canfield, 2011), even possibly in the Ordovician-Silurian (Hammarlund et al., 2012) and the Permian-Triassic (Algeo et al., 2011) transitional oceans where biological mass extinctions occurred. The high spatial heterogeneity and the dynamic nature of the ocean chemistry proposed by the “sandwich” model thus may provide a key to resolving the contradictions described above in the early Cambrian, South China. The early Cambrian South China was tectonically and geographically inherited from the Neoproterozoic (Wang and Li, 2003), and consisted of the Yangtze and Cathaysia blocks (Figure 1(a)) with a wide range of water depths prevailed along a general present-day northwest-to-southeast axis (Figure 1(b)) (Steiner et al., 2001). In past decades, extensive studies have been carried out on the lithostratigraphy, biostratigraphy, chronostratigraphy, and chemostratigraphy of many sections with different depositional water depths in South China (Jiang et al., 2012; Steiner et al., 2007; Wang X Q, et al., 2012; Yang et al., 2003; Zhu et al., 2003), which not only provide the basis for stratigraphic division and correlation (Figure 1(c)), but also make it possible to explore the spatial variability of ocean chemistry in a basin scale. By integrating previously published Fe-S-C systematic data, in this study we look into the spatiotemporal variability of ocean chemistry in South China from the PCB to the Nangaoan. Furthermore, by correlating these spatiotemporal

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variations in ocean chemistry to those abundant fossil records in South China, we explore the relationship between the variability of ocean chemistry and the evolution of life in the early Cambrian.

1 The spatial variability of ocean chemistry Figure 1(c) summarizes the basic data on the lithostratigraphy, biostratigraphy, and chronostratigraphy of the early Cambrian Jinningian-Nangaoan in South China. Based on these data, the early Cambrian strata from the PCB to the Yu’anshan Formation (Nangaoan) can be divided into five intervals which can be roughly correlated in a basin scale (Figure 1(c)). Interval I ranges from the PCB to the middle of the Zhongyicun Member in the Zhujiajing Formation at the Xiaotan section, Yunnan Province. The basinal correlation of Interval I is based on (1) the widespread occurrences of the small shelly fossil assemblage 1 (SSF1) characterized by Anabarites trisulcantus-Protohertzina unguliformis in the continental shelf facies (Steiner et al., 2007), and (2) the close U-Pb ages for the tuff layers at the top of the Interval I at the inner-shelf Meishucun section ((539.4±2.5 Ma)) (Zhu et al., 2009) and at the shelf-margin Ganziping section ((536.3±5.5 Ma)) (Chen et al., 2009). Interval II is defined by the strata from the middle Zhongyicun Member to the top of the Dahai Member in the Zhujiajing Formation. The basinal correlation of Interval II is based on four lines of evidence: (1) clear lithologic transition and stratigraphic unconformity at the top of Interval II was observed widely in the basin; (2) characteristic phosphate nodules and Ni-Mo layers widely deposited just above the lithologic transition surface in the basin (Jiang et al., 2012; Wang X Q et al., 2012); (3) the SSF2 characterized by Paragloborilus subglobosus- Purella squamulosa and the SSF3 characterized by Watsonella crosbyi widely appear in the shelf facies (Steiner et al., 2007; Guo et al., 2008), but completely disappear in overlying strata (Steiner et al., 2007; Zhu et al., 2003, 2007); (4) the tuff layer at the top of the Interval II at the inner- shelf Xiaotan section and that at the slope Taoying section are reported to have similar U-Pb ages of 526.5±1.1 Ma (Compston et al., 2008) and 522.7±4.9 Ma (Wang X Q et al., 2012), respectively. Interval III is defined by the lower part of the Shiyantou Formation. This interval can be correlated in the basin by two facts: (1) the TOC contents drop down and other geochemical records (e.g., iron speciation and S-C isotopes) show a turning point at the top of the Interval III in a basinal scale (more details see Section 1.1–1.3); (2) fossils significantly disappear in this interval at a basinal scale. Interval IV is defined by the middle-upper part of the Shiyantou Formation, in which the SSF4 characterized by Sinosachites flabelliformis-Tannuolina zhangwentangi appears in the inner-shelf facies (Steiner et al., 2007), while numerous sponge fossils and other animal fossils, such as bivalved arthropods Sunella and tubular

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Figure 1 Geological background and stratigraphic correlations for the early Cambrian strata in South China. (a) Tectonic background (McFadden et al., 2008); (b) sedimentary lithofacies, paleogeographic map of South China in the early Cambrian (Steiner et al., 2001); (c) the stratigraphic correlation based on available lithology, fossil records (Chen, 1984; Ding et al., 1992; Guo et al., 2008; Guo et al., 2013; Li et al., 2004; Luo et al., 1984; Steiner et al., 2007; Yang et al., 2003; Zhao et al., 2001), and U-Pb dating data (Chen et al., 2009; Compston et al., 2008; Zhu et al., 2009). Lithology records for sections: 1-Xiaotan (Yunnan Province; Zhu et al., 2001); 2-Meishucun (Yunnan Province; Luo et al., 1984); 3-Shatan (Sichuan Province; Goldberg et al., 2007); 4-Zhongnan (Guizhou Province; Och et al., 2013); 5-Yangtze Gorges (Hubei Province; Jiang et al., 2012; Ishikawa et al., 2013); 6-Ganziping (Hunan Province; Chen et al., 2009); 7-Songtao (Guizhou Province; Goldberg et al., 2007); 8-Longbizui (Hunan Province; Wang et al., 2012); 9-Yuanjia (Hunan Province; Guo et al., 2013). All section numbers, lithology legends, and name abbreviations are the same throughout the text.

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fossils Sphenothallus were found in out-shelf to slope facies (Guo et al., 2007b, 2013; Steiner et al., 2001; Yang et al., 2003). Interval III and Interval IV represent a transition from marine transgression to regression in the basin. Interval V is defined by the strata (the Yu’anshan Formation in this study) overlying the boundary between Interval IV and V, which corresponds to the first appearance of trilobites in the early Cambrian. Interval V is featured by the Chengjiang biota and the rapid radiation of sponge animals (Zhu, 2010). Although uncertainties still remain, the basinal correlation framework outlined above provides an opportunity for us to look into the spatial variability of ocean chemistry in the early Cambrian, South China. 1.1

Iron chemistry

Iron (Fe) speciation proxies have been widely used in investigating ancient ocean redox chemistry, in particular, the Precambrian ocean (Canfield et al., 2008; Li et al., 2010; Poulton et al., 2010; Poulton and Canfield, 2011) due to their high sensitivity to water column redox conditions. Fe speciation analyses include total Fe (FeT) and highly reactive Fe (FeHR). FeHR consists of pyrite Fe (Fepy) and residual Fe phases reactive enough to form pyrite given sufficient exposure to H2S either in the water column or during early sediment diagenesis—specifically ferric oxides, magnetite, and Fe carbonates. Two Fe ratios, FeHR/FeT and Fepy/FeHR, are calculated to evaluate the water redox conditions. In most modern and ancient sediments deposited beneath anoxic bottom waters, FeHR/FeT exceeds 0.38, but this threshold value can be reduced to 0.15 (±0.10 (SD)) for thermally altered ancient sedimentary rocks (see Raiswell et al., 2008 and references therein), such as the early Cambrian rocks in South China, because of possible conversion of FeHR to unreactive Fe silicates during burial. Furthermore, when free sulfide generated by bacterial sulfate reduction (BSR) is able to accumulate in the anoxic seawaters (i.e., euxinia develops), and most of the FeHR in the system will be converted to FePy, such that the ratio of FePy/FeHR will be empirically greater than 0.7 (see Poulton and Canfield, 2011 and references therein). Thus we are able to distinguish water column euxinia readily from the lower FePy/FeHR ratios of anoxic (ferruginous) and oxic settings in the combination of FeHR/FeT>0.15 and FePy/FeHR>0.7. Because the validation of Fe speciation method is based on detrital iron scavenging into deep anoxic basin as proposed by the “iron shuttle” hypothesis (see Lyons and Severmann, 2006 for details), this method is more reliable for evaluating the depositional redox conditions for clastic rocks. In this study, we look into the spatial variability of the Early Cambrian ocean chemistry in South China through examining the spatial distribution of iron speciation data available for the black-rock sequence of Intervals III and IV. The black- rock sequence has been considered as marker

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strata for regionally stratigraphic correlation due to its wide distribution, and high organic carbon content as well as Ni-Mo-rich layer featuring its bottom (Jiang et al., 2012). Figure 2 summarizes the iron speciation data available for those black-rock sequences in sections with different water depths. Samples from the Shiyantou Formation including both the organic-rich (lower part) and the organic-low (upper part) siltstones at the inner-shelf Xiaotan section all have FeHR/FeT>0.38 and Fepy/FeHR0.7, indicating similar ferruginous conditions but intermittent euxinia developing in the water column during the deposition of the Ni-Mo-rich layer for the inner-outer shelf transitional area. Bottom water anoxia and euxinia are seemingly exaggerated in the upper-shelf Songtao area because the roughly time-equivalent lower Jiumengchong Formation at the Songtao section has even higher FeHR/FeT and Fepy/FeHR with many samples having Fepy/FeHR>0.7. However, this increasing water column euxinia switched back at the lower-slope Longbizui section: all samples from the roughly time-equivalent lower Niutitang Formation show very high FeHR/FeT ratios, but few samples have Fepy/ FeHR>0.7, suggesting a major ferruginous condition in the deepest waters. In conclusion, the spatial variations of iron speciation data from the black-rock sequence, particularly from its bottom part, suggest that the mid-depth euxinia may have developed within the ferruginous deep waters, at least during the deposition of the bottom black shales in the early Cambrian, South China. Thus, the “sandwich” ocean chemistry structure proposed for the late Neoproterozoic oceans (Li et al., 2010) possibly also existed in the early Cambrian oceans. 1.2 Sulfur isotopic composition of pyrite (34Spy) and total organic-matter (TOC) concentrations In a “sandwich” ocean, in term of H2S production, the development of euxinic waters in near shore is generally controlled by two factors related to BSR: the sulfate supply and primary organic production—their amounts determine the H2S production and their balance with highly reactive iron in an area. Normally, high productivity occurred mainly in the marginal shelf areas where terrestrial nutrient fluxes were relatively high, and accordingly low productivity in distal oceans theoretically would result in the disappearance of distal euxinic waters because of insufficient organic matter for BSR. In the case of ample sulfate supply, because the BSR weakens from shore to distal area, the development of near shore euxinic waters, which is controlled by organic

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Figure 2 Spatiotemporal correlation of iron speciation data available for the early Cambrian strata in South China. Data source: 1-Xiaotan (Och et al., 2013); 2-Meishucun (Och et al., 2013; Wen et al., 2011) 4-Zhongnan: (Och et al., 2013); 7-Songtao (Canfield et al., 2008); 8-Longbizui (Wang et al., 2012). See text for stratigraphic correlations of five intervals in the basin.

matter supply, should be accompanied by a gradient of 34Spy, which decreases from shore to distal area. This gradient indeed has been observed in the 1.4–1.5 Ga-old Ropper basin, Australia (Shen et al., 2003) as well as in the ~1.8-Ga-old Animikie basin, America (Poulton et al., 2010). However, given the widespread low oceanic sulfate contents in the early Earth history, the development of euxinic waters mainly caused by increasing weathering sulfate supply might also occur in the near shore area if organic matter supply is enough. This situation might exist widely after the Neoproterozoic glaciation (Li et al., 2010). Owing to the global cover of ice during the Neoproterozoic glaciations, the weathering sulfate fluxes into the ocean are generally shut off. On the other hand, due to the persistent consumption of oceanic sulfate by the BSR and pyrite formation, the sulfate reservoir in deep oceans decreased, and even run out (Li et al., 2012). Once the weathering sulfate flux reset again after the global ice sheet melted down, a concentration and isotope gradient of the sulfate would have been formed because the persistent consumption of isotopically light sulfate by the BSR and pyrite formation when the weathering sulfate was diffused from the shore to the distal oceans. In such a case, one should expect to see higher 34Spy values in distal samples (Li et al., 2010, 2012).

For Intervals I and II (Figure 3), the 34Spy value from the shelf-margin Ganziping section ranges from 4.3‰ to 21.8‰ (mean=10.9‰), but the 34Spy value from the lowerslope Longbizui section varies between 16.6‰ and 31.6‰ (mean=23.2‰). For intervals III-IV (Figure 3), 34Spy values from the inner-shelf Shatan section, the shelf-margin Ganziping section, the upper-slope Songtao section, and the lower-slope Longbizui section range from 16.2‰ to 23.6‰ (mean=9.0‰), from 0.6‰ to 18.2‰ (mean=11.7‰), from 5.0‰ to 27.0‰ (mean=8.4‰), and from 10.7‰ to 33.0‰ (mean=21.0‰), respectively. Although the regional stratigraphic correlation is less than precise, the simple statistics above suggests that 34Spy values are generally higher for those deep-water samples during the Jinningian-Meishucunian, suggesting that the development of early Cambrian euxinic waters from inner shelf to slope areas during the deposition of Interval III as illustrated in section 1.1 was possibly controlled by the increasing weathering sulfate fluxes into the ocean in the early Cambrian, South China. Additional evidence comes from the TOC data. Persistent high TOC contents are found in the samples from Interval III in almost all sections with different water depths (up to 15% for Ganziping section and Songtao section, up to 9% for the Longbizui section, and even up to 4.9% in the Shatan

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Figure 3 Spatiotemporal correlation of 34Spy and TOC data available for the early Cambrian strata in South China. Data source: 3-Shatan (Goldberg et al., 2007); 6-Ganziping (Chen et al., 2009); 7-Songtao (Goldberg et al., 2007); 8-Longbizui (Wang et al., 2012).

section; Figure 3), suggesting a sulfate control rather than organic carbon on the H2S production by BSR in the early Cambrian, South China. This conclusion indicates that the oceanic sulfate concentrations in the early Cambrian, South China were as low as those in their Ediacaran ones. This inference is consistent with the results of the contemporary sulfur isotopes from Mexico and California, USA, reported by Loyd et al. (2012). It is worthy noting that for those samples in Interval V (i.e., the Nangaoan; Figure 3), their 34Spy values in the upper-slope Songtao section are higher than those from the lower-slope Longbizui section, which is reverse in their spatial pattern relative to those observed for younger intervals, implying that sulfate may have substantially accumulated in the deep-waters in the early Nangaoan (Figure 3). 1.3 Carbon isotopic composition of sedimentary organic matter (13Corg) Chemoautotrophic organisms (e.g., sulfur oxidizing bacteria) can synthesize isotopically lighter (up to 15‰) biomass by using recycled carbon relative to photosynthetic organic matter (Conway et al., 1994). Methanotrophs can synthesize biomass which is isotopically lighter by 15‰ to 40‰ than photosynthetic organic matter through assimilating extremely 13C-depleted methane (Conway et al., 1994; Summons et al., 1994, 1998). Figure 4 summarizes the spatial variation of 13Corg data available for the latest Neoprote-

rozoic to early Cambrian transitional strata in South China. For a more reliable stratigraphic correlation, we look into the spatial variations of 13Corg data around the boundary between Intervals II and III (Figure 4) in order to explore the spatial variability of ocean chemistry in the early Cambrian South China. The lithologic transition from limestone to organic-rich siltstone at the boundary at the inner-shelf Xiaotan section is accompanied by a huge drop of 13Corg from 21.5‰ to 35.8‰. A similar but smaller drop of 13Corg (from 30.12‰ to 33.57‰) was observed at the shelf-margin Ganziping section where the lithology changes from dolostone/chert to black shale. In contrast, the 13Corg values remain unchanged around 32‰ at the outer-shelf Yangtze Gorges section where the lithology changes from black-shale-interbedded mudstone to black shale. However, a positive shift of 13Corg was found at the upper-slope Songtao section (34.9‰ to 32.4‰), the lower-slope Longbizui section (36.29‰ to 33.37‰) and the basinal Yuanjia section (34.8‰ to 30.6‰), where lithology changes from siliceous rock to black shale. The most negative 13Corg values at the boundary were found at the Longbizui section (Figure 4). The complex spatial variations of 13Corg values observed above can be explained by the spatial heterogeneity of water chemistry and the lateral sulfate gradient proposed by the “sandwich” model. (1) At the inner-shelf Xiaotan section, the heavier 13Corg for the upper Interval II could be related to the terrigenous input of 13C-rich aged carbon, which is

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Figure 4 Spatiotemporal variations of 13Corg data available for the latest Neoproterozic-Early Cambrian successions in South China. Data source: 1-Xiaotan (Cremonese et al., 2013); 5- Yangtze Gorges (Jiang et al., 2012; Ishikawa et al., 2013); 6-Ganziping (Chen et al., 2009); 7-Songtao (Goldberg et al., 2007; Guo et al., 2007b); 8-Longbizui (Wang et al., 2012); 9-Yuanjia (Guo et al., 2013).

dominant organic source to the low-TOC limestone (Jiang et al., 2012); in contrast, due to the rapid transgression, the Xiaotan area could be covered by anoxic and non-euxinic waters (i.e., the ferruginous zone around the chemocline, see Figure 2 in Li et al. (2010)) during the deposition of lower Interval III. Fed by ample oxidants (e.g., SO42) from weathering fluxes during the transgression, massive 13Cdepleted chemoautotrophic biomass was synthesized, and subsequently added into the sediments, finally resulting in the large negative shift in 13Corg. (2) In the outer-shelf Yangtze Gorges area, however, there was generally no change in water chemistry—they were all anoxic (sometimes euxinic) before and after the transgression. Accordingly, the source of the sedimentary organic matter remained the same with a significant input from chemoautotrophic biomass, causing light but generally unchanged 13Corg through the boundary between Intervals II and III at the Yangtze Gorges section. (3) At the shelf-margin Ganziping section, the dolostone at the Interval II was possibly deposited on a shallow highland (Chen et al., 2009). Slightly heavy 13Corg in the dolostone reflects increasing proportion of photosynthetic versus chemoautotrophic biomass into the sediments because of the depression of chemoautotrophic activity in shallow waters where dissolved O2 contents were relatively high. When the Ganziping area was covered by anoxic, even euxinic waters during the transgression rec-

orded in the lower Interval III, an increase of chemoautotrophic biomass to sedimentary organic matter occurred similarly as suggested for the inner-shelf Xiaotan section at this period, leading to the negative shift in 13Corg. (4) In slope to deep basin areas, anoxic and ferruginous water conditions have been suggested for the deposition of siliceous rocks in Interval II (Chang et al., 2010, 2012). Such a ferruginous deep ocean with an extremely low sulfate (in turn, other oxidants) concentration in seawater as proposed by the “sandwich” model, would not favor the BSR and sulfur oxidizing, which might use sulfate and other oxidants as electron acceptor, but would favor the methane production and anaerobic methane oxidizing, which transferred extremely 13C-depleted carbon from methane into sedimentary organic matter due to the contribution from methanotrophic biomass, thus in turn very light 13Corg observed in the siliceous rocks. In contrast, sulfate as well as other oxidant concentrations in the deep waters would be substantially elevated due to a significant increase of weathering inputs during the following transgression. Increased availability of sulfate and other oxidants would favor the development of euxinic waters in deep waters as well as chemoautotrophic biomass formation by sulfur-oxidizing bacterial and other chemoautotrophic activities, but not methanotrophic activity. Therefore, 13Corg became heavier during the deposition of the transgressive black shales at these deep-water

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sections. During the deposition of black shales in Interval III, relatively increased primary production and sulfate concentrations in the upper-slope Songtao area would facilitate BSR as well as chemoautotrophic activities, in particular, sulfur oxidizing, which significantly contributed to sedimentary organic matter. Because of the persistent consumption of weathering sulfate by BSR and pyrite formation during its transportation from the shelf margin to upper slope, the trace amount of sulfate left in lower-slope Longbizui area might again relatively favor the methane production and anaerobic methane oxidation, providing extremely 13C-depleted biomass for sedimentary organic matter. Terrestrial weathering sulfate may have been nearly run out when it arrived in the basinal Yuanjia area, thus anaerobic methane oxidations as well as chemoautotrophic activities were all depressed. Under such a condition, the majority of sedimentary organic matter would derive mainly from the photosynthetic organisms in surface waters, which offered relatively 13C-rich biomass, resulting in heavier 13Corg up to 30.6‰. Therefore, it will be reasonable to see the most negative 13Corg values at the Longbizui section. In summary, the spatial variations of 13Corg data around the boundary of Intervals II and III in South China support the existence of “sandwich” ocean chemistry structure as well as the sulfate gradient in the early Cambrian oceans.

2 The temporal evolution of ocean chemistry The spatial variations of the Fe-S-C systematic data as described above suggest that the “sandwich” ocean chemistry structure might have evolved into the early Cambrian oceans in South China. However, because of significant changes in controlling geochemical factors along with the transgressionregression cycles, the five stratigraphic intervals defined in this study from the PCB to the Yu’anshan Formation in Nangaoan recorded different ocean chemistry states. Interval I is from the PCB to the middle Zhongyicun Member of the Zhujiajing Formation. Samples from the inner-shelf Meishucun, outer-shelf Zhongnan, upper-slope Songtao, and the lower-slope Longbizui sections all have FeHR/FeT>0.38 and FePy/FeHR0.38 and FePy/FeHR0.38, and FePy/FeHR0.38 and FePy/FeHR>0.7 (Figure 2), indicating that the euxinic waters beneath the chemocline had expanded from inner shelf into to upper slope areas. In contrast, most samples from the corresponding lower-slope Longbizui section have FeHR/FeT>0.38 and FePy/FeHR0.38 but FePy/FeHR

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