secondary phosphatization of the earliest cambrian small...

Journal of Earth Science, Vol. 27, No. 2, p. 196–203, April 2016 ISSN 1674-487X Printed in China DOI: 10.1007/s12583-016-0691-7

Chen, Y. L., Chu, X. L., Zhang, X. L., et al., 2016. Secondary Phosphatization of the Earliest Cambrian Small Shelly Fossil Anabarites from Southern Shaanxi. Journal of Earth Science, 27(2): 196–203. doi:10.1007/s12583-016-0691-7. http://en.earth-science.net

Secondary Phosphatization of the Earliest Cambrian Small Shelly Fossil Anabarites from Southern Shaanxi

Yali Chen1, 2, Xuelei Chu*1, 3, Xingliang Zhang*3, Mingguo Zhai1, 3

1. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

2. University of Chinese Academy of Sciences, Beijing 100049, China 3. State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069, China

ABSTRACT: Biomineralization may have an extremely long evolutionary history since the Paleoarchean, while the widespread biomineralization among metazoan lineages started at the earliest Cambrian. However, the primary mineralogy of Anabarites shell remains controversial. Optical microscopic observations combined with the Back-Scattered Electron (BSE) and Energy-Dispersive X-ray Spectroscopy (EDS) analyses are used to study the shell of the fossil Anabarites from the Kuanchuanpu fauna in southern Shaanxi Province in China, which is correlated to the Cambrian Fortunian Stage. The EDS analysis shows that the phosphorus-rich layer closely adjacent to the calcified layer exhibits a Ca: P: C ratio compositionally similar to the mineral fluorapatite (Ca5(PO4,CO3)3(F,CO3). The result that the calcified layer and the phosphorus-rich layer have different chemical compositions is consistent with the optical observation that there is an obvious gap between these two layers and the phosphorus-rich layer can extend to the phosphatic material inside of the tube, suggesting the phosphorus-rich layer doesn’t belong to the original shell. We suggest that the phosphorous-rich layer is diagenetic in origin, precipitated as a result of phosphorus release during the decay of organic matter by microbes. Considering the outermost shell layer (OMS, biologically controlled carbonate shell layer) should display different isotopic information from the carbonate matrix (i.e., OMS is 12C concentrated due to the biogenic organic matter template is readily rich in 12C), NanoSIMS was used to map ion distributions of C and N in the shell of Anabarites and matrix. However, ion images show that the concentration differences of 12C, 13C and 26CN among the OMS and the matrix are unclear, while 12C and 26CN are supposed to be enriched in the OMS. Therefore, the minor isotopic differences between the shell and the matrix is hard to be detected by NanoSIMS, at least in our sample, probably due to alteration of the 12C-rich characteristic of the Anabarites OMS during the late diagenesis. KEY WORDS: biomineralization, Anabarites, Fortunian Stage, phosphatization, NanoSIMS, southern Shaanxi.

0 INTRODUCTION

Anabarites is one of the earliest appeared metazoans with mineralized skeletons and has a worldwide distribution at the base of the Cambrian System. However, its biological affinities are still uncertain largely because of the lack of informative soft parts, and the primary mineralogy of skeletal parts is highly controversial as well (Kouchinsky et al., 2009). In South China, the Anabarites trisulcatus–Protohertzina anabarica assemblage zone represents the first biozone of Fortunian Stage (Steiner et al., 2007), and its lowermost occurrence marks the beginning of

*Corresponding author: [email protected] [email protected] © China University of Geosciences and Springer-Verlag Berlin Heidelberg 2016 Manuscript received March 05, 2015. Manuscript accepted September 10, 2015.

the Cambrian. Anabarites trisulcatus Missarzhevsky (Voronova and Missarzhevsky, 1969) is abundant and well preserved in Kuanchuanpu fauna, which was recovered from the Fortunian bioclastic limestones in Ningqiang County, Shaanxi Province, tectonically located in the northwestern margin of the Yangtze Platform. Therefore, it would be the best material to investigate the mineralization of early metazoans.

Morphologically, Anabarites is characterized by calcareous tubes (thecae) or internal moulds with triradially symmetrical distribution of longitudinal elements of the sculpture (Kouchinsky et al., 2009). However, the phylogenetic placement of Anabarites within the Metazoa is still controversial, but mostly considered as either annelids (notably serpulids) or cnidarians (possibly close to the Scyphozoa) (see Kouchinsky et al. (2009) for a summary; Dzik, 1986; Fedonkin, 1986; Vol'kov, 1982; Glaessner, 1976; Missarzhevsky, 1974; Missarzhevsky, 1969).

The fossilization of organic remains and shell material by


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calcium phosphate minerals provides an illuminating, but time-bounded, window into Ediacaran–Cambrian animal evolution (Creveling et al., 2014). The ocean chemistry from Ediacaran to Early Cambrian had been greatly changed after the Neoproterozoic Snowball Earth events. At the same time, sedimentation of marine phosphorites during this period was the first large-scale global phosphogensis event. So the evolution of metazoans, especially the appearance of mineralized skeletons (carbonate, phosphate and siliceous) for the first time in many groups, may be favored under this circumstance.

Many Small Shelly Fossils (SSFs) are preserved due to phosphatization. However, whether the phosphate part is pristine or the result of early diagenetic replacement of original calcitic, aragonitic or organic skeletons remains controversial, although many researchers favor the latter one (Kouchinsky, 2000; Zhu et al., 1996; Runnegar, 1985). As for the primary mineralogy of Anabarites shells, the debate also exists. Based on the fibrous structure sometimes visible on internal moulds, some researchers proposed that Anabarites had an originally aragonitic shell with the possibilities of calcite replacing the original aragonite and phosphate overgrowing on the interior of the tube or phosphate completely replacing the aragonite during early diagenesis (Feng, 2005; Conway Morris and Chen, 1989; Abaimova, 1978). However, Zhu et al. (1996) suggested that both single-layered phosphate shell and multilayered carbonate and phosphate shell coexisted according to the fossils preservation patterns and fine mineral structures in shell, which is supported by Chen et al. (2007). Chen et al. (2007) found that the δ18OVSMOW values of phosphate in the shell of Anabarites (16.7‰~17.0‰) sampled in Huize area, Yunnan Province are higher than those of the underlying Zhongyicun Formation phosphorites (13.1‰~15.5‰) and also much higher than those of the Early Cambrian Gezhongwu Formation phosphorites (9.6‰~13.3‰) in Zhijin area, Guizhou Province, suggesting the phosphatic parts in Anabarites shell are pristine because the 18O-enrichment can hardly be related to diagenesis or later hydrothermal processes which normally cause a shift towards a lighter oxygen isotopic composition.

Thus, the purpose of this study is to explore whether the phosphatic shell of the Anabarites from the Kuanchuanpu fauna in China is pristine.

1 LOCALITY AND STRATIGRAPHY

The Kuanchuanpu area, in Ningqiang County of southern Shaanxi Province, is located in the northwestern margin of the Yangtze Platform in South China (Fig. 1). The Ediacaran to the Lower Cambrian successions widely crop out in this region and yield abundant mineralized metazoan fossils. The studied sample was collected from the upper unit of the Kuanchuanpu Formation at the Shizhonggou Section in the Kuanchuanpu area (Fig. 1). The Kuanchuanpu Formation， disconformably overlying the Ediacaran dolostones of the Dengying Formation and underlying the Early Cambrian siltstones and shales of the Guojiaba Formation, can be subdivided into three rock units. The lower unit is composed of thin- to medium-bedded grayish white micritic limestones and cherts with no determinable small skeletal fossils; the mid unit consists of dark,

medium-thicked bedded cherts; the upper unit is dark-gray thin- to medium-bedded grainstones, containing abundant phosphatized SSFs and soft-bodied fossils (e.g. embryos, larvae and tiny adults of cnidarians) and phosphatic limestones with interbeds of cherts and clastic limestones. The studied Anabarites trisulcatus Missarzhevsky (Voronova and Missarzhevsky, 1969) occurs along with the SSFs (i.e. Paleosulcachites irregularis, Carinachites spinatus, Circotheca sp. and so on) stratigraphically below the first trilobite zone in South China, i.e. Parabadiella huoi zone, and above the Late Ediacaran Gaojiashan biota. Therefore, the sampled horizon is correlated to the Anabarites trisulcatus–Protohertzina anabarica Assemblage Zone in eastern Yunnan (Li et al., 2007), and is the Fortunian Stage in age. 2 METHODS 2.1 Optical Microscopy

The studied sample was cut into standard uncovered geological thin-sections and examined under the polarization microscope. The Anabarites fossils were found and then photographed with/without fluorescence under the optical microscope of Nikon Eclipse LV100POL equipped with a Nikon DXM 1200C digital camera. Various images of photographed Anabarites fossils were obtained by using the image-capturing software, such as the longitudinal section and cross section of the Anabarites tube shown in Fig. 2. In this study, we chose Square 1 and Square 2 in Fig. 2a as studying areas based on characteristics and clear structures of the shell.

2.2 Scanning Electron Microscope (SEM)

The area in Square 1 in Fig. 2a was further imaged in advance by SEM (LEO 1 450 VP), and the Back-Scattered Electron (BSE) images were also obtained. Figure 3 shows an optical photomicrograph and a BSE image of the shell in Square 1 in Fig. 2a, respectively. In addition, Energy-Dispersive X-ray Spectroscopy (EDS) was used to analyze in situ chemical composition for different layers seen in the shell in Square 2 in Fig. 2a.

2.3 NanoSIMS

NanoSIMS 50L is developed based on Ion Microprobe, which is a modern analytical instrument that has been optimized for sub-micron scale spatial resolution in determining elements and corresponding isotopes. It has already been widely used in cell and molecular biology studies. Nowadays, geologists also try to use NanoSIMS to explore early life on earth, considering its higher spatial resolution and sensitivity.

As for CAMECA NanoSIMS 50L, seven elements or isotopes can be mapped simultaneously using one fixed and six moveable detectors. And it is also equipped with two primary beams. Theoretically, sub-50 nm lateral resolution is achievable for elemental mapping of negative secondary ions using the Cs+ primary beam, and about 200 nm for positive secondary ions with the O– beam. Commonly, elements representative of organic material (i.e. C, N, O, and S) have relatively high negative secondary ion yields when sputtered with a Cs+ primary ion beam due to their high electron affinity. In this

Yali Chen, Xuelei Chu, Xingliang Zhang and Mingguo Zhai

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Figure 1. Paleogeographic map, geological map (modified from Cai et al., 2010) and stratigraphic column of Kuanchuanpu Formation, Shizhonggou section in

Ningqiang County, Shaanxi Province, China (modified from Ding et al., 1992).

study, all the ion mappings were performed using CAMECA NanoSIMS 50L at Institute of Geology and Geophysics, Chinese Academy of Sciences. Combined with BSE images, areas (e.g. fractures) filled with resin can be easily distinguished from the organic matter in sample. In this study, Cs+ primary beam was adopted for maximum spatial resolution. Images were acquired over a 30×30 μm field of view with an image size of 256×256 pixels, and each pixel measures 117 nm. To provide conductivity, thin section was coated with a thin layer of gold but not carbon because of carbon analysis. Before analysis, pre-sputtering is done for analyzed area to remove the conductive coating and surface contaminants, and to implant Cs+ ions into the sample matrix, and thus to reach a steady state of ion emission. Two NanoSIMS mappings for both C and N ions were obtained across the Anabarites shell in Square 1 in Fig. 2a.

3 RESULTS AND DISCUSSION The studied Anabarites trisulcatus is tube-like with a

triradial symmetry in cross section as shown in Fig. 2. It’s about 2 mm long, and both the aperture (wide part) and the bottom tip (narrow part) were broken during the taphonomic processes. In addition, phosphatic material is partly filled inside of the tube with diagenetic calcitic cements occupying the matrix. Chen and Peng (2005) proposed that the phosphatic infillings were brought into the tube through the aperture, considering the phosphatic material didn’t fully stuff the tube.

Under optical microscope, the studied shell of Anabarites is about 40 μm thick with at least six, layers (Fig. 2 and Fig. 3). In order to better describe the multiple-layered shell, we use OMS stands for the outermost shell layer and IS stands for the rest inner shell layers. It seems the IS can extend to the phosphatic material inside of the tube, suggesting IS is


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Figure 2. Both longitudinal sections and cross sections of Anabarites fossils under the optical microscope ((a)-(d)), with (b) and (d) processed with fluorescence

to see the shell clearly. Scale bar is 100 μm. Distributions of phosphate are shown as “P” in (a), i.e. inside the tube and part of the shell, and the rest is

dominated by carbonate. Two squares (Square 1 and Square 2) across shell of the Anabarites tube are chosen as studying areas.

(a) (b)

Figure 3. An optical photomicrograph (a) and a BSE image (b) of Anabarites shell. Photo (a) is the enlargement of Square 1 in Fig. 2 after rotating 90 degrees

anticlockwise. IS and OMS stand for inner shell layer and outermost shell layers respectively. Scale bar is 20 μm.

probably the late diagenetic origin. In Fig. 4b, a particulate appears closely to the OMS, even though the composition of this particulate is unclear. However, the IS，being originally parallel to the OMS, starts to warp around this particulate, which indicates that the OMS is the original shell layer of the Anabarites with IS precipitated after Anabarites died.

Also, in the BSE image, an obvious black linear structure indicates a gap exists between OMS and IS and the mineral compositions in the OMS and the IS are different (Fig. 3). In situ mineral compositions of OMS and IS by EDS were analyzed and the results are shown in Fig. 5. In EDS spectra, OMS exhibits a Ca : C ratio compositionally identical to the calcium carbonate (CaCO3), while IS is phosphorus-rich and shows a Ca: P: C ratio compositionally identical to the mineral

fluorapatite (Ca5(PO4, CO3)3(F,CO3). At the Ediacaran-Cambrian transition, many skeletonized

metazoans occurred, which means these metazoans already have the ability to produce the biominerals (Brasier, 1992). And almost all of the mineralized products formed under biologically controlled process are composite materials comprised of both mineral and organic components (Weiner and Dove, 2003). These organic matters, including proteins, hydrocarbons, lipids and so on, serve as a template in the nucleation and orient growth of biominerals. However, the amount of organic matter varied. In some mollusc shells, the value can be as low as 0.01%, while it can reach as high as 20%–30% in vertebrate bones or teeth (Degens, 1976). Thus, considering the biologically controlled carbonate shell layer


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Figure 4. Optical observation of the Anabarites shell. The IS warps around

the particulate in (b). Scale bar in (a) is 100 μm and in (b) is 50 μm.

Table 1 Element compositions of the OMS (Spectrum 1) and IS

(Spectrum 2)

Element Spectrum 1 (wt%) Spectrum 2 (wt%)

C 15.32 9.8

O 46.25 34.96

Ca 38.43 35.38

F - 5.05

P - 14.81

(OMS) should displays different isotopic information from the carbonate matrix (i.e., OMS is 12C concentrated due to the biogenic organic template is readily rich in 12C), NanoSIMS was used to map ion distributions of C and N in the multilayered shell of Anabarites and matrix, and the results are shown in Fig. 6. Two mappings were obtained to gain the whole image across the Anabarites shell in Square 1 in Fig. 2a. And the BSE images were also used to compare with those ion mapping images in Fig. 6.

In Fig. 6, the areas where the highest ion intensity in each ion image corresponds with fractures (the black pits and irregular patches) produced during thin section preparation and gaps (linear structures) in Fig. 3b, which are filled with resin, are excluded in the following analysis. Those ion images clearly show that the concentration differences of 12C, 13C and 26CN between the OMS and the matrix are unclear, while 12C and 26CN are supposed to be enriched in the OMS. Thus, the 12C-rich characteristic of the biologically controlled Anabarites shell may be altered during the late diagenesis. Therefore, the minor isotopic differences between the biologically controlled carbonate shell and the carbonate matrix, at least in our sample; do not show up in the ion mappings by NanoSIMS. However, if the OMS and carbonate matrix formed in different time and different solutions, isotopic differences between the OMS and the carbonate matrix may be influenced not only by biologically controlled process.

As Chen and Peng (2005) mentioned, the phosphate inside of Anabarites was brought in, so the question is why the mineral outside the tube is carbonate dominated? Given that seawater is supersaturated with respect to apatite, however, the precipitation of calcium phosphate is still inhibited by kinetic

factors (Atlas, 1975) or/and by the high ambient concentration of microbially generated bicarbonate ions (Briggs and Wilby, 1996). It seems the precipitation of calcium carbonate or calcium phosphate has been switched to the default set for the precipitation of CaCO3 (Allison, 1988). Consequently, calcium carbonate precipitation is usually favored. In the shrimp experiments that Briggs and Kear (1994) designed to investigate the influence of decaying matter, they found that where the initial acidic byproducts of microbial metabolism (such as CO2 and H2S) were allowed to escape by diffusion (under open conditions), pH remained predominantly alkaline and crystal bundles of CaCO3 precipitated. In contrast, where the experiment was completely closed and anoxic, a pronounced and persistent fall in pH resulted in a shift of the equilibrium in favor of the precipitation of apatite. Thus, the relative concentrations of dissolved calcium carbonate and phosphate were controlled by the decay-induced pH values. As for Anabarites, the skeleton tube obviously created a compartment for accommodating the soft tissues. And the ambient pore water together with the degradation of massive soft tissues inside of the tube provided enough phosphorus for the inner layer and tube interior. After Anabarites died, soft tissues started to decay accompanying with the decreasing pH caused by metabolic byproducts such as CO2 and H2S, and then CaCO3 began to dissolve, and calcium phosphate may simultaneously replace carbonate shells and soft tissues inside the tube or overgrow on the inner wall of the tube. As Briggs and Kear (1994) pointed out, the influence of pH is very localized, different minerals may form in different parts of the same decaying carcass and the mineralization may varied in different individuals considering the varied preservation conditions. This is able to explain, at least partly, so many preservation patterns of Anabarites shell (i.e. totally carbonated, totally phosphated or co-occurrence of carbonate and phosphate in different positions) and the varied amount of phosphatic content within different Anabarites tubes. These phosphatic precipitates are commonly cryptocrystalline and their formation are not controlled by organisms themselves but by the pore water pH resulted from the metabolic byproducts, which are characteristic for the biominerals produced by biologically induced mineralization (Zhang, 2012; Frankel and Bazylinski, 2003; Fortin et al., 1997; Tebo et al., 1997). Recently, Creveling et al. (2014) also proved that the phosphorus within the interiors of SFFs was derived from phosphate remobilized by organic decay and bacterial iron reduction, when the younger Cambrian strata from the Thorntonia Limestone, Australia, to display exceptional phosphatic preservation of SFFs. Thus, we suggest that the primary mineralized wall of the Anabarites tube is very thin (the OMS is less than 10 μm in thickness) and the phosphorus-rich IS precipitated as a result of microbial induced phosphatization during the organic decay after Anabarites died. 4 CONCLUSION The optical microscopic observations combined with the BSE and EDS analyses, indicate that the primary mineralized wall of the Anabarites tube is very thin (the OMS is less than


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Figure 5. An optical photomicrograph (a) and a BSE image (b) of Anabarites shell analyzed by EDS and the EDS spectra of OMS (c) and IS (d). Photo (a) is

the enlargement of Square 2 in Fig. 2. Scale bar is 20 μm.

Figure 6. NanoSIMS ion images of carbon (12C-, 13C-) and nitrogen (26CN-), plus a BSE image from the Anabarites shell. Calibration bars showing variations in

ion intensity, namely brighter colors indicating higher intensity. Scale bar is 5 μm.


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10 μm in thickness) and is calcified, but not phosphatic in mineralogy, albeit its frequent occurrence in phosphorous deposits. The thicker phosphatic layer (the IS is about 30 μm in thickness) lining the inner wall of the Anabarites tube, which is Ca5(PO4, CO3)3(F,CO3) in composition, was diagenetic in origin, and its precipitation was controlled by the pore water chemistry during the decaying process. Although the biologically controlled carbonate shell layer (OMS) should display different isotopic information from the carbonate matrix, ion mappings by NanoSIMS did not detect the minor differences in our sample. During the Early Cambrian, although the ocean chemistry favored sedimentation of marine phosphorites and phosphatization of skeletal remains, Anabarites still selected calcium carbonate during skeletongenesis. This may imply that the ocean chemistry may have enhanced phosphatized preservation of SSFs but may have not influence the mineral selection of skeletongenesis.

ACKNOWLEDGMENTS

This research is funded by MOST Special Fund from the State Key Laboratory of Continental Dynamics, Northwest University, MOST (No. 2011CB808805), and NSFC (No. 41172029). We thank Liu Wei from the Northwest University, and Hao Jialong and Hu Sen at the NanoSIMS Laboratory, Institute of Geology and Geophysics, Chinese Academy of Sciences, for the technical assistance. REFERENCES CITED Abaimova, G. P., 1978. Anabaritids–Ancient Fossils with

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