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Volume 32 Issue 3
Jun.  2021
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Mao Luo, Jitao Chen, Wenkun Qie, Jinyuan Huang, Qiyue Zhang, Changyong Zhou, Wen Wen. Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite. Journal of Earth Science, 2021, 32(3): 633-645. doi: 10.1007/s12583-020-1075-6
Citation: Mao Luo, Jitao Chen, Wenkun Qie, Jinyuan Huang, Qiyue Zhang, Changyong Zhou, Wen Wen. Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite. Journal of Earth Science, 2021, 32(3): 633-645. doi: 10.1007/s12583-020-1075-6

Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite

doi: 10.1007/s12583-020-1075-6
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  • Lime mud (i.e., micrite) is a major component of carbonate deposits. Various mechanisms (biotic versus abiotic) have been proposed for the formation of lime mud in Earth's history. However, the detailed role that microbes play in the nucleation and subsequent precipitation of micrites remains to be resolved. Herein we undertook a detailed geobiological characterization of laminated lime mudstone from the Middle Triassic Guanling Formation in Yunnan Province, southwestern China. Morphological features, together with previous geobiological investigations, suggest that the laminated lime mudstones represent the former presence of microbial mats. These lime mudstones consist mainly of calcite, dolomite and quartz, with clay minerals and pyrites as subordinate components. In particular, micro-analysis shows copious nano-globules (65-878 nm) and capsule-shaped nano-rods in laminations. These low-Mg calcite nano-globule aggregates are closely associated with mucilaginous biofilms resembling extracellular polymeric substances (EPS). Nano-sized globules coalesce to form semi-euhedral micrite crystals. We suggest that a decaying hydrolytic destruction of the EPS by microbial communities within microbial mat leads to the precipitation of the nano-globules by enhancing alkalinity in local micro-environment. As an intermediate, these nano-globules further aggregate to form micrite crystals possibly through a dissolution-reprecipitation process.
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Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite

doi: 10.1007/s12583-020-1075-6

Abstract: Lime mud (i.e., micrite) is a major component of carbonate deposits. Various mechanisms (biotic versus abiotic) have been proposed for the formation of lime mud in Earth's history. However, the detailed role that microbes play in the nucleation and subsequent precipitation of micrites remains to be resolved. Herein we undertook a detailed geobiological characterization of laminated lime mudstone from the Middle Triassic Guanling Formation in Yunnan Province, southwestern China. Morphological features, together with previous geobiological investigations, suggest that the laminated lime mudstones represent the former presence of microbial mats. These lime mudstones consist mainly of calcite, dolomite and quartz, with clay minerals and pyrites as subordinate components. In particular, micro-analysis shows copious nano-globules (65-878 nm) and capsule-shaped nano-rods in laminations. These low-Mg calcite nano-globule aggregates are closely associated with mucilaginous biofilms resembling extracellular polymeric substances (EPS). Nano-sized globules coalesce to form semi-euhedral micrite crystals. We suggest that a decaying hydrolytic destruction of the EPS by microbial communities within microbial mat leads to the precipitation of the nano-globules by enhancing alkalinity in local micro-environment. As an intermediate, these nano-globules further aggregate to form micrite crystals possibly through a dissolution-reprecipitation process.

Mao Luo, Jitao Chen, Wenkun Qie, Jinyuan Huang, Qiyue Zhang, Changyong Zhou, Wen Wen. Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite. Journal of Earth Science, 2021, 32(3): 633-645. doi: 10.1007/s12583-020-1075-6
Citation: Mao Luo, Jitao Chen, Wenkun Qie, Jinyuan Huang, Qiyue Zhang, Changyong Zhou, Wen Wen. Microbially Induced Carbonate Precipitation in a Middle Triassic Microbial Mat Deposit from Southwestern China: New Implications for the Formational Process of Micrite. Journal of Earth Science, 2021, 32(3): 633-645. doi: 10.1007/s12583-020-1075-6
  • The Middle Triassic Guanling Formation crops out in a vast region of Guizhou and eastern Yunnan provinces (Feng et al., 2018; Luo et al., 2016b; Enos et al., 2006). In the study area, the Guanling Formation conformably overlies the Jialingjiang Formation of late Early Triassic, and is overlain by the Yangliujing Formation (Ladinian) of peritidal facies (Benton et al., 2013; Hu et al., 2011). Boundary between the Guanling and Jialingjiang formations is marked by a coarse-grained volcanic ash bed (green pisolite or 'green-bean rock') (Benton et al., 2013; Hu et al., 2011; Enos et al., 2006). In Luoping, the Guanling Formation consists of two members (I and II). The lithologic types and depositional environments of the two members have been described substantially and interpreted by previous authors (Luo et al., 2019, 2017, 2014, 2013; Feng et al., 2018, 2017; Benton et al., 2013; Bai et al., 2011; Hu et al., 2011; Zhang et al., 2009). Overall, the Member I represents deposition in an intertidal to subtidal environment while Member II accumulates in a shallow to deep subtidal setting with restricted circulation (Benton et al., 2013; Zhang et al., 2009; Enos et al., 2006; Hu et al., 1996; Figs. 12).

    Figure 1.  (a) Global paleogeographic map showing the location of South China in the Middle Triassic (240 Ma) (base map from Ron Blakeyʼs Mollweide map at http://www2.nau.edu/rcb7/mollglobe.html). (b) Paleogeography of the studied area during the early Middle Triassic (modified from Luo et al., 2019).

    Figure 2.  (a) Stratigraphic column showing the lithology, trace fossil distribution and sedimentary structures of the Middle Triassic Shangshikan Section (modified from Luo et al., 2019). Lower and upper fossil horizons denote the main strata in which abundant vertebrates and invertebrate fossils comprising the Luoping Biota were preserved. (b) Reticulated ridge structures on the upper bedding surfaces of laminated lime mudstones (bed 23). (c) Laminated lime mudstone (bed 25, horizon denoted by black stars) with reticulated ridge structures. These samples were taken for petrological and micro-analysis. M. Mudstone; W. wackestone; P. packstone; G. grainstone.

    The Luoping Biota, marking the final recovery of marine ecosystems following the end-Permian mass extinction (Chen and Benton, 2012), is recorded in Member II of the Guanling Formation (Hu et al., 2011). Abundant body fossils, including various clades of vertebrates and invertebrates, and various trace fossils (e.g., burrows and coprolites), are well preserved in laminated lime mudstones, which have been categorized as the lower and upper fossil horizons (Fig. 2a; Huang et al., 2019a, b, c; Luo et al., 2019, 2018, 2017; Wen et al., 2019, 2013, 2012; Feldmann et al., 2017, 2015, 2012; Hu et al., 2017, 2011, 2010; Benton et al., 2013). These fossils of the Luoping Biota were excavated from the three main quarries in the Luoping region, eastern Yunnan Province (Fig. 1), including the Dawazi (Daaozi) (104°19′52.4″E, 24°46′42.7″N), Shangshikan (104°19′40.7″E, 24°46′47.6″N), and Xiangdongpo (104°19′34.3″E, 24°47′13.7″N) (Fig. 2; Luo et al., 2019; Benton et al., 2013; Hu et al., 2011).

    The age of the Luoping Biota has been well constrained by conodont assemblages, which indicates the fossil-bearing strata (Member II) is Pelsonianin age (Middle Anisian) (Huang et al., 2009; Zhang et al., 2009). Pervasive RRSs are present on the upper bedding planes of laminated lime mudstones (Figs. 2b, 2c). These surface features, together with micro-analysis and petrological evidence, suggest that laminated lime mudstones bearing RRSs possibly represent the former presence of microbial mats in a carbonate setting (Luo et al., 2013). Laminated lime mudstone samples for present study were collected from the lower fossil horizon at Shangshikan Section (Fig. 2a).

  • Petrologic thin sections of laminated lime mudstones were made to observe microscopic fabrics, composition, and diagenetic features. Freshly broken chips of lime mudstone were prepared to examine surface mineral textures under scanning electron microscopy (SEM). Samples were made along the vertical profile of laminated lime mudstone and coated with Pt to get a high-resolution SEM imaging result. Additional samples were acid etched to examine the grain size of carbonate minerals, with treating procedure following Luo et al. (2014). Acid etched samples were also Pt coated for SEM imaging. Sample chips were examined using Zeiss VP FESEM 1555 SEM at the Centre of Microscopy, Characterization and Analysis, University of Western Australia. Further microanalysis and energy dispersive X-ray spectrometry (EDS) analyses were conducted using TESCAN MAIA3 TriglavTM SEM equipped with an Oxford ULTIM MAX 170 detector at the State Key Laboratory of Palaeobiology and Stratigraphy (LPS), Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences.

  • The surface features of these reticulated ridge structures have been described elsewhere (Luo et al., 2013), and thus will not be included here. Paper-thin laminations are commonly developed in lime mudstone (Fig. 3a). Laminated lime mudstone is composed of micrite and microspar that are randomly distributed (Figs. 3a–3d). SEM images of etched samples show that sizes of micrite range from 1 to 4 μm, with sutured micrite crystal boundaries. Fossil skeletons are rare, with only a few ostracods and un-identified shell fragments (Fig. 3c). Pyrite grains are common, and are patchily distributed on some occasions (Fig. 3d).

    Figure 3.  Photomicrographs showing laminations in lime mudstone with reticulated ridge structures on bedding surfaces. (a) Lime mudstone with dark ultrathin laminations (white arrows); (b) close-up of boxed area in (a) showing the laminations, note lime mud composed of micrites (white arrows) and microspars (red arrows); (c) a few ostracods (white arrows) in lime mudstone; (d) back scattered image showing pyrite crystals in lime mudstone; Py. pyrite; Cal. calcite; Dol. dolomite.

    Mineralogical composition of this fine-grained sediment was determined qualitatively through the association of relative element in element mapping and back scattered image by SEM. There is close association of Ca, and C and O (denoting calcite), in the upper area examined, and also Ca, Mg, C, and O (denoting dolomite) in the lower area (Figs. 4b–4c, 4e–4f). Presence of clay mineral is represented by the association of K, Al, and Si (Figs. 4g–4i). Element mapping of Fe shows pyrite, which has also been observed in back scattered image (Fig. 3d). On some occasions, organic matter remnants also occur (Figs. 4b–4c). It is supported that the carbon sourced signal is coupled with O signal but decoupled with Ca and Mg (Figs. 4b–4c, 4e–4f), suggesting that the carbon-source signal might come from organic matter. In summary, the fine grained sediment consists mainly of calcite, dolomite, and quartz, with clay minerals and pyrite as subordinate mineral components. Calcite and dolomite crystals are randomly distributed.

    Figure 4.  Back scattered electron (BSE) image (a) and images of element mapping showing the distribution of C, Ca, Fe, O, Mg, K, Al, and Si (b)–(i) of the same area. Lime mudstone thin section sample is from bed 25. The white circle in (b), (c) and (e) denotes the same area, which suggests the presence of organic matter remnants in thin section.

    In freshly broken sample chips (coated with Pt), abundant sub-globular to globular granules preserved mostly as aggregated forms, are observed in laminations under SEM. The surfaces of these nano-sized globules are mostly grainy, showing no crystallographic morphologies (Figs. 5, 6a–6d, 7, 8a–8b). These amorphous globules often coalesce together to form semi-euhedral micrite crystals (Figs. 5b, 5d, 5e, 6a5d) or larger amorphous complexes (Figs. 6e–6f). The formed semi- euhedral micrite crystals, 1–4 μm in size, are characterized by rough crystal surfaces (Figs. 5b, 5e, 6d), smoothly curved to jagged boundaries (Figs. 5a, 6a–6b) and defects (Fig. 5b). Nano-globules composing the semi-euhedral micrite crystals are still discernable (Figs. 5b, 5e, 6a–6d). Size measurement of these nano-globules shows a diameter ranging from 65 to 878 nm (based on a measurement of 204 individuals). The average value is 216 nm, with standard deviation of 107 (Fig. 8c). On some occasions, two euhedral micrite crystals stacked together (Figs. 6a–6b). In addition, nano-sized (less than 200 nm in length), capsule-shaped rods were also found in association with either nano-globule aggregates or semi-euhedral micrite crystals (Figs. 5e–5f, 6d). The EDS result suggests the semi- euhedral micrite crystals are composed of low-magnesium calcite (Fig. 9b). Usually clay minerals are found in association with nano-globule aggregates (Figs. 5c, 6a, 7a). In addition, mucilaginous materials were found to attach nano-globule aggregates or semi-euhedral micrite surfaces (Fig. 7b).

    Figure 5.  SEM images showing the nano-sized globule aggregates on surfaces of freshly broken specimen from bed 25. (a) Nano-globule (white arrows) forming aggregates, note a semi-euhedral micrite crystal (dashed arrow) in association with nano-granules; (b) close-up of the micrite crystal in (a), which is composed of aggregated nano-globules, note the semi-euhedral, micrite-sized crystals exhibit defined crystal boundaries (white dashed lines) and defects (white arrow); (c) scattered nanosized globules (arrows) associated with clay mineral (CM); (d) close-up of boxed area in (c) showing the details of nano-globules; (e) close-up of arrowed portion in (d) showing another semi-euhedral crystals composed of nano-globules, note a capsule-shaped granule (white arrow) attached to the semi-euhedral crystal; (f) close-up of boxed area in (d) showing the possible capsule-shaped granules (white arrows) in association with amorphous nano-globules (dashed arrows).

    Figure 6.  SEM images showing the nano-sized globule aggregates and semi-euhedral micrite crystals. Photos were taken from freshly broken sample from bed 25 at Shangshikan Section. (a) Nano-globules coalescing to form semi-euhedral cuboid individuals (white arrows); (b) another view showing two semi-euhderal micrite crystals stacking together, dashed lines denote nearly formed smooth crystal surfaces, note the some of the nano-globules comprising the semi-euhedral crystals are still discernable (white arrows); (c) dispersed and aggregated nano-globules forming amorphous complexes and semi-euhedral micrite crystals; (d) close-up of small boxed area in C showing a nearly euhedral micrite crystal with two defined faces (dashed lines); note the capsule-shaped granule on crystal surface (white arrow); (e) close-up of larger boxed area in C showing nano-globule aggregates forming large amorphous complex and micrite crystal (dashed lines denote poorly defined crystal faces); (f) close-up of boxed area in (e) showing the larger amorphous complex composed of stacked nano-globules (white arrows).

    Figure 7.  (a) Nano-globules of various sizes form aggregates. The upper part of image (bowed area) shows clay mineral (possibly chlorite); (b) close-up of boxed area in (a) showing the mucus-like extracellular polymeric substances (EPS) in association with coalescent nano-globules.

    Figure 8.  Close-up of nano-globules (a)–(b) from the laminated lime mudstone bearing reticulated ridge structures, rock chip is from bed 25, note the grainy surfaces of nano-globules; (c) size distribution of nano-globules. SD is abbreviation of standard deviation.

    Figure 9.  SEM images and EDS results showing different mineral phases in laminated lime mudstone samples bearing RRSs structures. (a) SEM image showing a euhedral dolomite crystal (boundary indicated by dashed lines); (b) SEM image showing the low magnesium-calcite, dolomite and clay minerals co-exist, note the central area (Mg-calcite) of photo corresponds to Fig. 5d; (c) EDS result of white cross (1) in (a) confirming the dolomite (Dol) composition; (d) and (e) corresponds to the EDS results analyzed on the white cross (2) and (3) in (b), respectively. Note the clay mineral is a Fe-bearing silicate.

    Acid etched samples (coated with Pt) were also examined under SEM, which did not show the presence of these nano- globular aggregates or semi-euhedral micrite crystals composed of nano-globule aggregates.

  • Carbonate precipitates have been reported in both natural depositional systems and laboratory benchmark experiments under either purely inorganic solutes or microbially induced systems, suggesting that carbonate minerals can be precipitated through an either biotic or abiotic process (Blue et al., 2017; Fang et al., 2017; Han et al., 2017; Littlewood et al., 2017; Rodriguez-Blanco et al., 2017; Luo et al., 2016a; Chen et al., 2014; González-Muñoz et al., 2010; Spadafora et al., 2010; Bontognali et al., 2008; Rodriguez-Navarro et al., 2007; Braissant et al., 2003; Chafetz and Buczynski, 1992). Further, there are extreme similarities in sizes and morphologies (crystal polymorph) between calcium carbonates precipitated through inorganic synthesis (abiotic) and those formed through a microbially induced (biotic) process, making it difficult to explain whether those various lime muds (i.e., micrites) encountered in sedimentary systems have a microbial origin or not.

    Several lines of evidence support that the nano-globules observed within these laminated lime mudstones in the Middle Triassic Guanling Formation, southwestern China were formed through a microbially induced organomineralization. Firstly, there is a close association of capsule-shaped granules with those nano-globules or semi-euhedral micrite crystals from our observation. These nano-sized capsule-shaped rods have sizes less than 200 nm (Figs. 5e–5f, 6d), with morphologies resembling those described by Folk (1993). Folk (1993) asserted that these nano-sized objects might represent nanobacteria in ancient sediments. More recent studies suggest, however, that those so-called nanobacterial forms might not be truly fossilized bacteria as their sizes surpass the lower size limit of bacteria, which are normally larger than 200 nm (Nealson, 1999; Nealson and Stahl, 1997). More recent studies have shown that these so-called nanobacterial forms possibly represent decayed organic debris of microbial communities (e.g., Pacton et al., 2010; Schieber and Arnott, 2003). This is evidenced by the extreme morphological similarities between nano-sized objects from the rock record and those produced in various decaying organic matters (Schieber and Arnott, 2003). We agree with the latter interpretation and interpret that the capsule-shaped nano-rods observed herein might represent remnants of organic matter during microbial degradation. In this respect, the close association between these capsule-shaped nano-rods with nano- globule aggregates supports that the latter could have a microbial origin.

    Secondly, the morphology, size range and mineral composition of these nano-globules resemble those mineral globules produced by bacterial nucleation in culture experiments (Bontognali et al., 2008; Aloisi et al., 2006). The semi-euhedral micrite crystal, composed of fused nano-globlue aggregates, consists of low-Mg calcite through EDS analysis (Fig. 9d). Such low-Mg calcite nano-globules have been observed in an EPS (extracellular polymeric substances) aggregates surrounding cultured sulfate reducing bacteria cell (Bontognali et al., 2008). In addition, mucilaginous materials were found to attach those globular aggregates or semi-euhedral micrite surfaces (Fig. 7b). Their morphologies are quite similar to EPS observed in both ancient and modern microbial mats (e.g., Luo et al., 2014). This further supports that formation of nano-globules is most likely linked to an EPS matrix surrounding microbes.

    It has been suggested that nano-sized granules can be artifacts related to sample preparation, which are introduced through sputter coating and etching of samples (e.g., Kirkland et al., 1999; Bradley et al., 1997). In our microanalysis, freshly broken samples and etched samples were both prepared to examine the surficial mineral textures under SEM. Nano-globules were only observed in freshly broken samples, excluding its possibility as artifacts brought about by sample etching. In addition, both etched and freshly broken samples were coated with Au/Pt. The absence of these nano-globules in etched samples thus precludes them as artifacts during sputter coating.

    Previous investigations have showed copious reticulated ridge structures (RRSs) on upper bedding planes of these laminated lime mudstones (Figs. 2b–2c; Luo et al., 2013; Hu et al., 2011). A comparison of these macro-features with those from modern microbial mats, together with evidence from petrological investigation and micro-analysis under SEM, suggests that these laminated lime mudstones bearing RRSs possibly represent the former presence of microbial mats (Cuadrado and Pan, 2018; Luo et al., 2013; Hagadorn and Bottjer, 1997). From evidence mentioned above, it is inferred that these nano-globules were possibly precipitated within an EPS matrix excreted by microbial communities in a microbial mat microenvironment. Microbial metabolism during microbial mat growth could lead to the precipitation of these nano-globules and their further crystallization into micrite crystals.

    Preservation of the metastable nano-globule aggregates in geological samples is an enigma. Only a few examples have showed the preservation of Mg-calcite nano-globules from ancient microbial mat deposits (e.g., Tang et al., 2013; You et al., 2013). We suggest the special micro-environmental conditions (i.e., presence of organic polymers and high alkalinity) within the Luoping microbial mat deposits might have helped stabilize the preservation of these amorphous nano-globules (Rodriguez-Blanco et al., 2017; Dimasi et al., 2006). In addition, the high Mg concentration and high pH value within the microbial mat micro-environment might have also played a critical role on stabilizing the metastable nano-globules observed herein. Under such conditions, the overall solubility of those nano-globules could be decreased, thereby enhancing their stabilization (Rodriguez-Blanco et al., 2017).

  • In modern microbial mats, microbes of different functional groups act together to fuel the energy flow and element cycling within the mats (Dupraz et al., 2009, 2004). These microbes in microbial mats, which belong to six main functional groups, form a semi-closed ecosystem governing the growth of microbial mat and its lithification (Dupraz et al., 2009, 2004; Dupraz and Visscher, 2005; Visscher et al., 1998). EPS, as a mucus material surrounding the cells of microorganisms of various functional groups, plays a key role in the precipitation of CaCO3 in microbial mats (Dupraz et al., 2009, 2004). EPS consists of chemical reactive macromolecules that have complex chemical compositions including polysaccharides, protein, peptides, phospholipids, carboxylic acids, and amino groups (Dupraz et al., 2009, 2004; Braissant et al., 2007, 2003; Ercole et al., 2007; Branda et al., 2005; Kawaguchi and Decho, 2002; Sutherland, 2001). Some of these chemicals are negatively charged (e.g., carboxylic acids, hydroxyl groups, amino groups), binding large amounts of free Ca2+ from solution (Dupraz et al., 2009, 2004). When EPS is active and maintains its cation binding capacity, it acts as an inhibitor for CaCO3 precipitation as the concentration of Ca2+ is largely depleted in the surrounding aqueous environment. Precipitation of CaCO3 is promoted by degradation of EPS, which can be performed by heterotrophic metabolism (e.g., bacterial sulfate reduction) as this process consumes organic matter including EPS (Baumgartner et al., 2006; Dupraz et al., 2004). Low-Mg or high-Mg calcite is the usual precipitates within an EPS matrix after its degradation (e.g., Kremer et al., 2008; Dupraz et al., 2004). Alternatively, increasing the ion concentration by evaporation or arranging the EPS into an acidic template may also lead to precipitation of CaCO3 (e.g., Arp et al., 2003; Trichet and Défarge, 1995).

    Such a process explains the formation of those pervasive nano-globules observed within the Luoping microbial mat deposit. Specifically, the decay of EPS matrix within microbial mat by heterotrophs (e.g., sulfate reducing bacteria) possibly resulted in an increase in alkalinity. The bound Ca2+ ions were also released after the decay of EPS, leading to a super saturating state with respect of CaCO3 and subsequent nucleation of nano- globule aggregates (Fig. 10). The associated EPS biofilm, and nano-rods representing decay of organic matter, further support such an EPS decay process. More importantly, our findings suggest that the microbially induced micrite precipitation seems to be a multi-step process, with the formation of amorphous, nano- globules within an EPS matrix as incipient CaCO3 precipitates (Fig. 10). These amorphous, low-Mg calcite nano-globules further fused together, forming semi-euhedral to euhedral micrite crystals through a possible dissolution-reprecipitation process. There is a possibility that these amorphous, nano-globules were formed by a smaller, nano-sized amorphous calcium carbonate (ACC) precursor, similar to the formation of vaterites by ACC (e.g., Rodriguez-Navarro et al., 2007). However, whether these globules are truly vaterites or ACC remain to be resolved through further characterization utilizing Raman spectrometry, X-ray diffraction (XRD) and transmitted electron microscopy (TEM) analyses (e.g., Li et al., 2019; Rodriguez-Navarro et al., 2007). ACC and vaterites would produce different Raman and XRD spectrums distinguishing one from another.

    Figure 10.  Cartoon model showing the formation process of nano-globules subsequent micrite crystals within the Luoping microbial mat deposit. (a) Growth of microbial mat (green) at sediment-water interface. Lithified (fossilized) mat structures were represented by grey laminations with reticulated ridges in sediments. (b) Close-up of growing mat showing copious filamentous cyanobacteria within extracellular polymeric substances (EPS). (c) Decomposition of organic matter and EPS and bacteria induces the alkalinity and supersaturating microenvironment, leading to the formation of various sized nano-globules. (d) Further decomposition of EPS leads to the further formation of semi-euhedral micrites and organic remnants represented by nano-rods.

  • Carbonate sedimentation is a common phenomenon in the Precambrian, with a majority of carbonate deposits consisting of lime muds (e.g., Cantine et al., 2019; Kaźmierczak et al., 1996; Knoll et al., 1993). The origin of these fine grained carbonates, however, is cryptic in comparison with those in the Phanerozoic. It is most probable that lime mud production has originated from various mechanisms during the Phanerozoic, with the governing process still remaining to be resolved as continued studies suggest different competing mechanisms (Trower et al., 2019; Enríquez and Schubert, 2014). For the lime mud production during the Precambrian, two possible mechanisms have been proposed. For instance, Knoll et al. (1993) has proposed that the supersaturated seawater with respect to CaCO3 during the Proterozoic facilitates abundant micrite precipitation in the water column. Such a process has also been invoked to interpret the rare occurrence of calcified cyanobacteria in the Precambrian as these fine grained micrite particles are served as competing nucleating site for carbonate precipitation, thereby inhibiting the calcification of cyanobacteria (Knoll et al., 1993). In addition, the whiting events (formation of fine-grained carbonates in the open water column) may also be ascribed as a possible mechanism contributing part, if not all, of the lime mud production in the Precambrian (Tosti and Riding, 2017; Seong-Joo and Golubic, 1999; Robbins and Blackwelder, 1992). Geochemical and microanalyses on Bahama whitings suggest that a significant fraction of the lime mud in whitings is linked to the microbial metabolism of picoplankton cyanobacteria (e.g., Synechococcus) as their cells can act as nucleation sites for CaCO3 crystallization (Robbins et al., 1997; Robbins and Blackwelder, 1992). However, it remains to be answered whether such whitings are formed through a similar nucleation process as what we observed in the Luoping microbial mat.

    Regardless of what mechanisms contribute to the lime mud production in the Precambrian, what seems to be interesting is that a great number of lime mud deposits in the Precambrian are closely associated with stromatolites and thrombolites (e.g., Tosti and Riding, 2017; Tang et al., 2013; Seong-Joo and Golubic, 1999). Furthermore, nano-sized globues have also been observed in certain Precambiran microbial deposits (e.g., Tang et al., 2013). We thus hypothesize that these fine grained lime muds might have been formed through a microbially induced process similar to what we have observed in the Luoping microbial mat deposit. In particular, the nucleation and formation of Precambrian micrite may be initiated through the formation of nano-globules as an intermediate forms within an EPS matrix in stromatolites, which requires further investigations on the Precambrian microbial deposits.

  • We undertook a detailed geobiological characterization of laminated lime mudstones from the Middle Triassic Guanling Formation in Yunnan Province, southwestern China, in an attempt to understand the formational process of micrite induced by microbial metabolism. Previous investigations have suggested that the laminate lime mudstone bearing pervasive reticulated ridge structures record the former presence of microbial mats. The laminated lime mudstone consists mainly of calcite, dolomite and quartz, with clay mineral and pyrites as subordinate components. Remarkably, copious nano-globules (65–858 nm) and capsule-shaped nano-rods are found in the lime mudstone. The low-Mg calcite nano-globules are closely associated with mucilaginous biofilms resembling extracellular polymeric substances (EPS). Nano-globules coalesce and form semi-euhedral micrite crystals. We suggests that a decaying hydrolytic destruction of the EPS by microbial communities within microbial mats might lead to the precipitation of these nano-globules through enhancing the alkalinity of the micro- environments. These intermediate nano-sized granules further coalesce to form semi-euhedral micrite possibly through a dissolution-reprecipitation process. The high alkalinity fueled by the EPS degradation might have promoted such transformation from nano-globules to micrite crystals. Our study provides evidence for the formational process of micrite induced by microbial metabolism, and highlights the microbial metabolism within microbial mat in contributing to the lime mud production during the Precambrian.

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