Based on detailed SEM observations, different types of carbonate microbodies were found in Zoophycos fillings characterized by four different colors. Microscopic analysis and testing by means of EDS demonstrate that CaCO3 is the main component of the microbodies, and other elements presented are Fe, Si, Mn or S, whose contents vary among the differently-colored fillings. Furthermore, carbonate microbodies are more abundant in brown (or red) and black fillings than in gray and gray-white fillings.
According to their morphology, the carbonate microbodies found in the fillings of Zoophycos can be divided into three groups: spheroids, framboids and rhabditiforms.
4.1 Spheroid microbodies
On the basis of different surface structures, the carbonate spheroids can be further divided into five types as follows: type 1: smooth spheroids; type 2: spheroids with tiny thorns; type 3: spheroids with a finely granulated surface; type 4: spheroids with a flocculent surface; and type 5: spheroids with a vermiform surface.
4.1.1 Type 1: Smooth spheroids (Fig. 4a–c)
These spheroids are well-preserved, with a relatively smooth surface, and their diameters range from 2 μm to 15 μm. The single spheroids are flattened (Fig. 4b, c), 29 μm in length and 21 μm in width. The surface of some spheroids exhibits a gap. The main elements of the spheroid surface (Fig. 4a, point 1) are C, O and Si, with some Ca, Al, and Fe.
4.1.2 Type 2: Spheroids with tiny thorns (Fig. 4d–f)
These spheroids, 6–9 μm in diameter, are characterized by regularly-distributed tiny thorns covering the surface. The main elements of the grains (Fig. 4d, point 1) are C, Ca, O, and Si, and the minor elements are S, Cl, K, Al and Na. A small fissure (0.1–2 μm in width) occurs locally on the surface displaying a smooth inner surface (Fig. 4e).
4.1.3 Type 3: Spheroids with a finely granulated surface (Fig. 4g–j)
These spheroids, 2–10 μm in diameter, are marked by a dense finely granulated surface. The fine granules are regularly arranged, but their shapes are irregular. Particularly, some spheroids touch each other and are arranged as a coenobium, involving two (Fig. 4g), three (Fig. 4h), or even four spheroids (Fig. 4i) linked together. Some spheroids have two relatively concave sides (Fig. 4h, i). The EDS analysis shows that the main elements of the spheroids and their surrounding rocks are C, Ca, O, and Si, but the specific elements on the surface of spheroids are S, Cl and K.
4.1.4 Type 4: Spheroids with a flocculent surface (Fig. 4k)
These spheroids are characterized by a complex flocculent surface; the surrounding matrix is composed of large, smooth calcite crystals (Fig. 4k, point 2). The elements of the spheroid are more complex and are commonly C, O, and Ca, and small amounts of Fe, Mn, and S (Fig. 4k, point 1). In appearance, this spheroid is similar to extracellular polymeric substances (EPS), nannofossils of sulfur bacteria, or analogous pyrite spheroids (Gong et al. 2008).
4.1.5 Type 5: Spheroids with a vermiform surface (Fig. 4l)
The spheroids, 2–3 μm in diameter, have a worm-like surface structure and form groups of about 30 closely-linked monomers (Fig. 4l). The spheroids are embedded in coarse-grained, smooth calcite crystals (Fig. 4l, point 2). The compositional elements of the spheroid surface (Fig. 4l, point 1) are O, Fe, C, Ca, Mn, and S, in which the main elements are O and Fe, with weight percentages of 61.64% and 14.42%, respectively.
4.2 Framboid microbodies
Based on the aggregation of tiny spheroids, the framboids found in the fillings of Zoophycos can be further divided into two types: type 1–framboid monomers, and type 2–framboid colonies.
4.2.1 Type 1: Framboid monomers (Fig. 5a
–
f)
Framboid monomers, commonly 3.5–7.5 μm in diameter, are an aggregate of tiny spheroids that exhibit many micrograins on the surface (Fig. 5a, b), embedded sporadically within calcite crystals or in the gap between two adjacent calcite crystals. The micrograins are distributed on the surface or inside the monomer densely and evenly. They also may display a flower-like arrangement. Six different kinds of framboids have been observed: (1) Three framboid monomers (3.5–7.5 μm in diameter) appear grouped (Fig. 5a), and their surfaces show numerous micrograins, 0.5–1 μm in diameter. The main elements of the micrograins are Fe and O, with weight percentages of 58.26% and 33.45%, respectively. Minor elements include C, Si, and Ca. (2) Framboid monomers (4.5 μm in diameter) are marked by many regular triangular and quadrilateral micrograins with edge-lengths of 0.4–0.5 μm (Fig. 5b). (3) Framboid monomers (7.5 μm in diameter) with about 70 closely adjacent micrograins (1 μm in diameter, Fig. 5c). (4) Framboid monomers (5.5 μm in diameter) preserved on the concave surface of calcite crystals and with micrograins, 0.5–0.75 μm in diameter (Fig. 5d). (5) Framboid monomers (5 μm in diameter) display many hexagonal micrograins, 0.65 μm in diameter; five adjoining monomers are closely squeezed to each other, showing concave pits after removal of the framboid monomers (Fig. 5e). (6) Framboid monomers (12 μm in diameter) embedded in Si-rich calcite crystals; the main elements of the micrograins (about 0.8 μm in diameter) are O, Fe, and C, along with a few minor elements (Ca, Si, Al) based on the SEM and EDS analyses (Fig. 5f, points 1, 2). These characteristics seem to indicate that there is a positive correlation, in diameter change, between the framboid monomers and their micrograins. This may suggest that the genesis of framboid monomers is related to microbial action.
4.2.2 Type 2: Framboid colonies
The framboid colonies (Fig. 5g–l) identified in three samples are characterized by adjoining of many micrograins or microspherules and are similar to the pyrite framboids found in light-gray spreiten of Zoophycos. They are interpreted as microbial colonies (Gong et al. 2007, 2008).
In sample 1 (Fig. 5g), the framboid colony, appearing inside calcite crystals, is about 12 μm in width and 15 μm in length, and includes nearly 200 closely-gathered microspherules (single, about 0.8 μm in diameter).
In sample 2 (Fig. 5h), the framboid colony is more than 10 μm in diameter and consists of many microspherules, along with some residual pits observed on the framboid colony surface.
In sample 3 (Fig. 5i), the framboid colony is about 40 μm in width and 50 μm in length, with more microspherules than in the previous two samples, and shows many concave pits which are the sites of former microspherules. The EDS-data show that the main elements of the microspherules are Fe and O, with weight percentages of 61.07% and 30.13%, respectively. A higher value of Fe is present in the area of framboid colony, and other minor elements include C, Ca, O, Si and Al.
In sample 4 (Fig. 5j), many framboid monomers (more than 5 μm in diameter) with an uneven surface adjoin, and are embedded or wrapped in calcite crystals, with O and Fe as main elements.
In sample 5 (Fig. 5k, l), larger framboid colonies are visible. In the enlarged view (Fig. 5l), the cauliflower-like framboid colony (about 17.3 × 14.7 μm2) consists of numerous micrograins that cover the surrounding calcite crystals. These micrograins, about 1.33 μm in diameter, are evenly distributed and regularly arranged. The EDS data show that the main elements of the micrograin surface are Fe and O, with other minor elements (C, Ca, and Si).
4.3 Rhabditiform microbodies
Based on surface features and shapes, rhabditiform microbodies can be further divided into six types as follows: type 1: linear smooth rhabditiform bodies; type 2: smooth rhabditiform bodies with expanding ends; type 3: biserial rhabditiform bodies; type 4: spiral rhabditiform bodies; type 5: thorny rhabditiform bodies; and type 6: branched rhabditiform microbodies.
4.3.1 Type 1: Linear smooth rhabditiform bodies (Fig. 6a)
These rhabditiform bodies are about 20 μm long and about 1 μm in diameter, appearing as a straight line with a smooth surface and round cross-section. It occurs dispersed in the black fillings of Zoophycos.
4.3.2 Type 2: Smooth rhabditiform bodies with expanding ends (Fig. 6b)
This rhabditiform body, 15.38 μm in visible length and 0.77 μm in width, also forms a straight line and has a smooth surface. In addition, it has an expanding end that is about 1.15 μm in diameter. It occurs dispersed in the black fillings of Zoophycos.
4.3.3 Type 3: Biserial rhabditiform bodies (Fig. 6c)
The rhabditiform body forms a biserial straight line and is smooth. Its length is 10 μm and its single-row diameter is 0.4 μm. It is preserved in the gap between calcite crystals in the black fillings of Zoophycos.
4.3.4 Type 4: Spiral rhabditiform bodies (Fig. 6d)
This rhabditiform body is 9 μm in length and about 1 μm in diameter, and has the shape of a inverted “L-shaped spiral”. It occurs in the gap between calcite crystals in the black fillings of Zoophycos.
4.3.5 Type 5: Thorny rhabditiform bodies (Fig. 6e, f)
The prominent features of these rhabditiform bodies are a straight course and dense needle-like thorns on the surface. The main elements of the thorns (Fig. 6f, point 1) are O, C, Ca, Mn, Fe, Si and S. The weight percentages of Mn and Fe are 17.75% and 6.87%, respectively; Si and S are minor elements. As shown in Fig. 6e, three rhabditiforms bodies are preserved together and arranged parallel to each other, with a visible length of 50 μm and a diameter of 1.5 μm. The thorns on the surface are 5–7 μm in length and less than 1 μm in diameter.
4.3.6 Type 6: Branched rhabditiform microbodies (Fig. 6g, h)
These microbodies have a branched rhabditiform shape (about 4.0 μm in diameter) (Fig. 6g). The enlarged view (Fig. 6h) shows a granular surface. The microscopic analysis revealed their chemical composition of CaCO3.