Microbialite development through the Ediacaran–Cambrian transition in China: Distribution, characteristics, and paleoceanographic implications

Highlights

• We present a high-resolution database with respect to the development of microbialites in China through the E-C transition.

• Detailed morphological and petrological characteristics of stromatolites and thrombolites are described.

• Microbialite development experienced two thriving intervals in China during the E-C transition.

• Thrombolites evolved to form large and complicated structures, likely indicating an improved environmental adaptation.

• The primary calcite mineralogical composition is one potential reason for the preservation of the calcified microfossils.

Abstract

Widespread development of microbialites harbors a series of clues about microbial activity, environmental condition, and aquatic chemistry. The Ediacaran-Cambrian transition draws extensive attention on the co-evolution of complex life and Earth's environment but the associated microorganism development has been largely ignored. In this study, we present a high-resolution database with respect to the spatial and temporal distributions of microbialites in China through the terminal Ediacaran to the early Cambrian Period and describe morphological and petrological characteristics of stromatolites and thrombolites in detail to shed light on the evolutionary process of microbial carbonates. Microbialite development experienced two thriving intervals during the Ediacaran-Cambrian transition: latest Ediacaran to early Fortunian, and Cambrian Age 3 to middle Age 4. The columnar and domical stromatolites show no marked morphological changes in the Ediacaran-Cambrian transition, but stratiform stromatolites exhibit a notable decline in Cambrian time, likely caused by increasing bioturbation in the Cambrian shelf environments. Meanwhile, thrombolites evolved to form large and complicated structures in the early Cambrian featured by meter-level mound morphology and columnar-branching microbial forms (fan-like/dendritic structures), likely indicating an improved environmental adaptation (e.g., photosynthesis efficiency and hydrodynamic conditions). Another remarkable change in microbialites is the emergence of large numbers of calcified microbial microfossils preserved within the laminated/clotted mesostructures in Cambrian facies, compared with the Ediacaran forms that lack such unique structural features. For the main control over the Cambrian microbial calcification event, this study stresses again the essential role of seawater chemistry (Mg/Ca molar ratios and Ca²⁺ concentrations) in the formation and preservation of calcified microorganisms based on previous insights and elaborate characteristics of their occurrence and microstructures in China. The transition of the Neoproterozoic “aragonite-dolomite sea” to the Cambrian “calcite sea” (likely widely distributed in Age 3) may have promoted to the generation of an original calcite mineralogy in microbial fossils, which has a stronger ability to resist diagenetic dissolution and substitution (e.g., phosphatization and silicification) than that of the aragonite precursor.

Introduction

As one of the oldest sedimentary records related to biological activity on Earth, microbialites are a component the evolution of the marine ecological system and seawater chemical composition and thus have been broadly used to restore paleoceanographic and paleoenvironmental information about the setting in which they formed (e.g., Burne and Moore, 1987; Grotzinger and Knoll, 1999; Riding, 2000). Microbialites, including stromatolites (laminated structure), thrombolites (clotted structure), and other minor types, largely represent a lithified benthic microbial community with the contribution of authigenic precipitation (generally carbonate) during diagenesis, and sometimes with the admixtures of skeletal organisms and terrigenous detritus (Burne and Moore, 1987). In terms of the formation mechanism, both microbially-induced and microbially-influenced processes likely played key roles in the mineralization of the microbial community, which contains a series of autotrophic (e.g., cyanobacteria) and heterotrophic (e.g., sulfate reducing bacteria) microorganisms (e.g., Visscher and Stolz, 2005; Dupraz et al., 2009; Riding, 2011b).

Cyanobacteria are seen as one of the most important producers of carbonate precipitation and organic matter, but their calcified microfossils are not ubiquitously preserved in rocks (Arp et al., 2001; Planavsky et al., 2009). The calcification of cyanobacteria refers to the sheath, which is one form of extracellular polymeric substance (EPS) affiliated with (and close to) microorganisms that has been internally crystallized by carbonate minerals, creating calcified microbial microfossils (Riding, 1977, Riding, 2011a). Calcified cyanobacteria are rare in modern marine environments, but they are present in several peaks in the geological record, e.g., Cambrian to Ordovician, Devonian to Carboniferous, and Late Triassic (Arp et al., 2001). There is still controversy concerning the development of large-scale cyanobacterial calcification, but its potential paleoceanographic and paleoenvironmental applications, e.g., seawater saturation state, atmospheric CO₂ level, and dissolved oxygen state, are attracting increasing amounts of attention (Knoll et al., 1993; Riding and Liang, 2005; Altermann et al., 2006; Kah and Riding, 2007; Riding et al., 2019).

The abundance of calcified cyanobacteria in Precambrian strata is very low when compared to that in Phanerozoic strata (Golubic and Seong-Joo, 1999; Arp et al., 2001; Schopf, 2006; Schirrmeister et al., 2016). Although numerous studies have reported the presence of microbial microfossils in Ediacaran records, most of the preservation types are post–mortem, e.g., kerogenization, phosphatization, pyritization, and silicification or (partially) replacement by Al-silicates during early diagenesis (Zhang et al., 1998; Brasier et al., 2011; Cai et al., 2012; Muscente et al., 2015). The following Cambrian period is regarded to have been an intense “cyanobacterial calcification episode” (Riding, 1992), which is represented by large quantities of calcified microbes in space and time (Riding, 1982, Riding, 1992; Rowland and Shapiro, 2002; Lee et al., 2015). Most of these sheath/wall calcified microbes are cyanobacteria (e.g., Girvanella and Botomaella groups) and suspected cyanobacteria (Epiphyton and Renalcis groups) (Riding, 2001), but they are generalized as calcified microbial microfossils in this study due to their similar preservation characteristics in microstructures and the ambiguous taxonomic assignment in specific types (e.g., Pratt, 1984; Stephens and Sumner, 2002; Ibarra and Sanon, 2019). Previous studies have provided a variety of information on the petrological and morphological characteristics of these calcified microbes (e.g., Rowland and Gangloff, 1988; Knoll et al., 1993; Shapiro, 2000; Turner et al., 2000; Zhuravlev, 2001; Rowland and Shapiro, 2002), but little is known about the evolutionary process of calcified microbes during the Ediacaran-Cambrian transition and about the similarities and differences of the characteristics of microbialites formed in the latest Precambrian and earliest Phanerozoic.

High seawater Ca²⁺ concentrations or CaCO₃ supersaturation states are interpreted to have driven the cyanobacterial calcification episodes, which provided sufficient Ca²⁺ for CaCO₃ nucleation on and within the sheath matrix (Arp et al., 2001; Riding and Liang, 2005). The CO₂-concentrating mechanisms (CCMs) likely played a role in the specific biologic processes that form an alkaline gradient by transporting bicarbonate into the cell and releasing OH⁻ to the sheaths during photosynthesis (Kaplan and Reinhold, 1999; Badger and Price, 2003; Riding, 2006). Even so, other potential effects, e.g., the Mg/Ca molar ratio, require systematic assessment in this cyanobacterial calcification episode to understand its possible synergic relationship with the seawater chemistry and environmental conditions. Seawater Mg/Ca molar ratio dominates the carbonate mineralogical compositions, and the transition from the aragonite-dolomite sea state in the Neoproterozoic to the calcite sea state in the early Cambrian may have favored the preservation of original calcite microfossils due to the thermodynamic differences between the primary carbonate minerals (Morse and Mackenzie, 1990; Brennan et al., 2004; Zhuravlev and Wood, 2008; Hood et al., 2011; Wood et al., 2017). In addition, cyanobacteria are one of the most important sources of atmospheric O₂ in Earth's early history as a result of oxygen-producing photosynthesis, and they possibly played a key role in the Proterozoic oxygenation event (Campbell and Allen, 2008; Lyons et al., 2014). The rise of eukaryotic algae evolved to more efficient phosphorus and organic carbon burial and potentially increased ocean and atmospheric O₂ contents in the Neoproterozoic time (Lenton et al., 2014; Lenton and Daines, 2018), but the roles of prokaryotes (e.g., cyanobacteria) have generally been ignored in the environmental evolution at that time. A case from modern oceans shows that the photosynthetic prokaryote Prochlorococcus, which is one species of cyanobacteria and the most abundant photosynthetic organism on Earth, may contribute about 20% O₂ to the atmosphere (Partensky et al., 1999; Munn, 2020); it implies that photosynthetic cyanobacteria are still of crucial value in O₂ production in the younger Earth. For these reasons, photosynthesis-dominated microbial communities with calcified functional microstructures, which were preserved in the laminated and clotted structures of microbialites during the early Cambrian, seem to provide vital atmospheric O₂ information about this time period, and this assumption needs to be clarified.

The aims of this study are to recover the evolution of microbialites during the transition from the Ediacaran to the Cambrian and to understand the paleoceanographic and paleoenvironmental significance of their morphological and functional changes during this crucial period. For this purpose, first we built a database (n = 226) on the development of microbialites from the uppermost Ediacaran to Cambrian Series 2 in China. Then, we generalized the exterior (growth forms) and interior (laminated/clotted structures) characteristics of the microbialites in different temporal intervals and focused on the occurrence, distribution, and microstructures of the calcified microbes in the early Cambrian. Next, we assessed the potential significance of the microbialite response to environmental changes in the early Cambrian. Finally, we attempted to interpret the startup mechanism of the Cambrian cyanobacterial calcification event from a paleoceanographic perspective.