microfabrics in mesoproterozoic microdigitate...

PALAIOS, 2013, v. 28, p. 178–194

Research Article

DOI: 10.2110/palo.2012.p12-113r

MICROFABRICS IN MESOPROTEROZOIC MICRODIGITATE STROMATOLITES: EVIDENCE OFBIOGENICITY AND ORGANOMINERALIZATION AT MICRON AND NANOMETER SCALES

DONGJIE TANG,1 XIAOYING SHI,1,2* GANQING JIANG,3 and WENHAO ZHANG 1

1School of Geoscience and Resources, China University of Geosciences, Beijing 100083, China, [email protected], [email protected], [email protected];2State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences, Beijing 100083, China; 3Department of Geoscience,

University of Nevada, Las Vegas, Nevada 89154-4010, USA, [email protected]

ABSTRACT

Microdigitate stromatolites (MDS) are common in Neoarchean–Paleo-proterozoic successions but declined and gradually disappeared in Meso-and Neoproterozoic carbonates. The abundance of well-preserved fibrousfabrics and the absence of identifiable microbial fossils in MDS have beentaken as evidence of their abiotic origin in carbonate-supersaturated andanoxic Precambrian oceans. Micron- and nanometer-scale features ofMDS from the Mesoproterozoic Wumishan Formation (ca. 1.45–1.5 Ga)of the North China platform composed of alternating submillimetric darkand light laminasets are morphologically similar to those reported fromelsewhere in the geological record. The dark laminasets are micritic andcontain abundant fuzzy-edged micropeloids and filaments. The micro-peloids are commonly 10–70 mm in diameter and commonly surrounded bythin (,10 mm) rims composed of amorphous micrite or microsparite (somerims now replaced by silica). The filaments are morphologically similar tothe bacterial filaments in modern microbialites and contain kerogenouscomponents as indicated by Raman spectrometry analysis. All thelaminasets are characterized by fibrous fabrics that are expressed byalternating brown fibers (,10–25 mm) and light microsparitic strips ofapproximately equal width in transverse direction. Filaments, relics ofputative extracellular polymeric substances (EPS), micropeloids, andnanoglobules are closely associated with the brown fibers. Theseorganomineralization-related features suggest a biogenic origin for theMDS of the Wumishan Formation and may have an implication to otherMDS from the Neoarchean–Paleoproterozoic successions. Microbiallyinduced micro- and ultrastructures including fibrous fabrics, filaments,micropeloids, and nanoglobules are best preserved in silicified MDSsamples, implying that early silicification is critical for the preservationand recognition of organominerals.

INTRODUCTION

Microdigitate stromatolites (MDS) are an important group ofstromatolites in Neoarchean to Paleoproterozoic (.1.6 Ga) carbonatesuccessions, but they declined significantly in Meso- and Neoproterozoic(,1.6 Ga) carbonates (Grotzinger and Knoll, 1999). The abundance ofradial-fibrous texture indicative of an acicular aragonite precursor andthe general lack of microbial fossils in MDS have led most researchers tointerpret MDS as a special type of stromatolites formed mainly throughabiotic carbonate precipitation (e.g., Grotzinger and Reed, 1983;Hofmann and Jackson, 1987; Knoll et al., 1993; Sergeev et al., 1995;Grotzinger and Knoll, 1999). While heterotrophic bacterial metabolismmay have facilitated carbonate precipitation and nucleation during MDSformation (Knoll and Semikhatov, 1998), the ocean seawater chemistry,particularly the calcium carbonate saturation of surface seawater, mayhave been the major controlling factor behind the rise and declineof MDS in Proterozoic carbonate successions (e.g., Grotzinger and

Kasting, 1993; Grotzinger, 1994; Sumner and Grotzinger, 1996).Therefore, the decline and disappearance of MDS and other seafloorprecipitates (e.g., aragonite crystal fans) in Meso- and Neoproterozoiccarbonates are commonly taken as evidence for secular changes in oceanchemistry (e.g., Grotzinger, 1990; Grotzinger and Kasting, 1993; Sumnerand Grotzinger, 1996; Grotzinger and Knoll, 1999).

In contrast to the debatable abiotic origin of MDS, most otherstromatolites in modern and ancient environments are known to haveformed through repetitive accretion of microbial mats, their precipitates,and/or entrapped inorganic materials (Riding, 2000; Dupraz et al., 2009).Thus, by definition, stromatolites imply biogenesis, and their presence inArchean strata (#3.4 Ga) has been considered to be one type of the oldesttraces of life on Earth (e.g., Hofmann, 2000; Allwood et al., 2006;Altermann et al., 2006; Schopf et al., 2007; Benzerara et al., 2010).Questioning the biogenicity of some of the oldest stromatolites includingMDS is mainly based on the lack of fossil bacteria in these stromatolites(Lowe, 1994; Grotzinger and Knoll, 1999; Brasier et al., 2004) andmathematical modeling that demonstrates producibility of stromatolite-like structures by chemical precipitation and/or abiotic marine cementa-tion of detritus (Grotzinger and Rothman, 1996; McLoughlin et al., 2008).

Morphologically distinct bacterial fossil preservation in Archean–Paleoproterozoic successions could be difficult owing to the lack of invivo cyanobacterial calcification at potentially high pCO2 (Riding, 2006;Kah and Riding, 2007) and destruction of primary textures duringdiagenesis and metamorphism (Grotzinger and Reed, 1983; Hofmannand Jackson, 1987; Grotzinger and Knoll, 1999). Recent studiesindicate that micron- to nanometer-scale microstructures, such asnanoglobules (or nanospheres), micropeloids, fibrous fabrics, andmicritized extracellular polymeric substances (EPS), record organomi-neralization processes within a syndepositional microbial community,and they may be used as signatures for microbial activities instromatolite-forming microbial mats (e.g., Dupraz et al., 2004, 2009;Benzerara et al., 2006, 2010; Pacton et al., 2010; Perri et al., 2012).These micro- and ultrastructures may also be preserved in the rockrecord (e.g., Lepot et al., 2008) and can be used as evidence to supportbiogenicity of some of the oldest stromatolites such as MDS.

In this paper, we report the microfabrics and organomineral micro-and ultrastructures in MDS from the Mesoproterozoic WumishanFormation of the North China platform and discuss their biogenicity.The MDS are morphologically identical to many other MDS reportedfrom the Archean–Paleoproterozoic successions. Thus the organomi-neralization processes and products recorded in MDS of the WumishanFormation may help to understand the origin of microfabrics in otherMDS of early Earth history.

GEOLOGIC SETTING

Stratigraphy and Age Constraints

The Wumishan Formation is the most widespread lithostratigraphicunit of the Mesoproterozoic North China platform (Chen et al., 1999;

* Corresponding author.

Published Online: April 2013

Copyright G 2013, SEPM (Society for Sedimentary Geology) 0883-1351/13/0028-0178/$3.00

Wang et al., 2000; Cheng et al., 2009). It consists of fourlithostratigraphic members dominated by dolostone facies (Fig. 1A).The Wumishan Formation has an average thickness of 3340 m in thecentral part of the platform and it thickens slightly toward the platformmargin in the north (Chen et al., 1999). Facies analyses indicate that themajority of this formation was deposited in peritidal environments of ashallow-water, epicontinental carbonate platform (e.g., Mei et al., 2001,2010; Zhou et al., 2006; Shi et al., 2008; Shi and Jiang, 2011; Tang et al.,2011). Cherty bands and nodules are locally abundant, especially inthe thinly laminated, peritidal dolostone facies. Microbialites, suchas stromatolites and thrombolites and various microbially inducedsedimentary structures (MISS), are well preserved in this formation (Shiet al., 2008; Shi and Jiang, 2011; Tang et al., 2011, 2012), along withsmall aragonite (now replaced by calcite or dolomite) crystal fans insome stratigraphic intervals. To date there has been no directradiometric age obtained from the Wumishan Formation. On the basisof U-Pb zircon ages obtained from its underlying and overlying strata(Fig. 1A), the Wumishan Formation is estimated to have beendeposited between 1.5 Ga and 1.45 Ga (Gao et al., 2007, 2008a,2008b; Li et al., 2010; Tang et al., 2011, 2012).

Occurrence of Microdigitate Stromatolites (MDS)

In the study area (Figs. 1B–C), MDS are best preserved in the firstand second members of the Wumishan Formation, with Pseudogymno-

solen as the most common group (Liang et al., 1984; Cao, 1991; Qiu andLiang, 1993; Medvedev et al., 2005; Mei et al., 2010). This type of MDShas been often taken as one of the typical examples for nonbiogenicstromatolites (Grotzinger and Knoll, 1999; Grotzinger and James,2000; Riding, 2008).

The MDS-bearing strata (Fig. 1A) in the study area are characterizedby well-developed, shallowing-upward parasequences that stack intodisconformity-bounded depositional sequences (Mei et al., 2001; Zhouet al., 2006). Most parasequences consist of three distinctive, subtidal tosupratidal units (Figs. 2A–C): thick-bedded to massive thromboliticdolomite (Unit A) in the lower part, laminated dolomite (Unit B) in themiddle, and thin-bedded micritic dolomite (Unit C) in the top. Therelative thickness of Units A–C may vary considerably among sectionsbut their vertical stacking pattern is persistent across the platform,consistent with parasequence development in peritidal environments ofan epicontinental sea (Chen et al., 1999; Mei et al., 2001; Zhou et al.,2006; Tang et al., 2011).

Most MDS occur in Unit A and lower part of Unit B (Figs. 2A–C),indicating that they have been formed mainly in upper subtidal to lowerintertidal environments. Overall, the depositional environments of theMesoproterozoic MDS in the study area are similar to those of theMDS documented from Neoarchean to Paleoproterozoic successions(Grotzinger, 1989, 1990), but the MDS reported here have morerestricted distribution in both water depth and facies types (Fig. 2D).

MATERIALS AND STUDY METHODS

In this study, macroscopic morphogenetic features of the MDS weredescribed from outcrop sections and polished slabs. Samples formicroanalyses were collected mainly from the Huailai and Yesanposections in Hebei Province (Fig. 1C). Most MDS samples are variablysilicified but they have relatively well-preserved microfabrics (Fig. 3A).In contrast, nonsilicified, pure carbonate samples have varying degreesof recrystallization and their primary microfabrics and textures are lesswell preserved (Fig. 3B). Thin sections were analyzed using a Zeiss AxioScope A1 petrographic microscope. Ultramicroscopic fabrics andmicrotextures were mainly examined under a Supra 55 Field EmissionScanning Electronic Microscopy (FESEM). In order to obtain betterresults, both thin sections and freshly broken fragments were used in

FESEM observation; some of them were coated with platinum prior toanalysis.

FESEM study of most samples was operated at 10–15 kV withworking distance of 3.0–9.4 mm. An in-lens detector for secondaryelectron imaging was used for nanotopography. Some freshly brokenfragments were analyzed using a Hitachi 3400N Scanning ElectronicMicroscopy (SEM) operating at 20 kV with a working distance of15.1 mm. Semiquantitative element concentrations of micron-sizedspots were analyzed by an Oxford energy dispersive X-ray spectrometer(EDS) connected to FESEM, operated at 10 kV with a workingdistance of 8.5 mm.

In order to ascertain the properties of organic remains in the MDS,some of the putative bacterial relics and micritized mucuslike filmsobserved in the sections were examined by a LabRAM HR800 Ramanspectrometer. The Raman microscope operating confocally at laserwavelength of 532 nm with a spectral footprint of 1 mm using a 1003

lens objective gave a spectral resolution better than 1.5 cm21. Eachspectrum required about 1 minute total scan time.

MACRO- AND MICROSCOPIC FEATURES OF THE MDS

Macrostructure and Morphology

The studied MDS are primarily preserved in laminated dolostonefacies that consist of alternating dark, relatively organic-rich micriticdolostone and light, organic-poor sparitic dolostone layers (Figs. 3A–C). The average thickness of each dark and light layer is 1–2 cm. Withinthese layers millimeter-scale couplets of dark-light laminasets are alsorecognizable (Fig. 3C). MDS are more clearly displayed in the lightlayers, showing as subelliptical to waterdrop-shaped columns 0.1–2 cmwide and 0.1–1.5 cm high (Figs. 3A–C). The stromatolitic columns arecommonly silicified and are darker than the background dolostone.Occasionally they display small divergent branches (Figs. 3C–D).Individual columns are composed of alternating dark and lightlaminasets that bend downward at the column margins (Fig. 3C),showing the features of syndepositional growth.

The morphology and structures of the MDS from the WumishanFormation are similar to those documented from Neoarchean–Paleoproterozoic successions (Hofmann and Jackson, 1987; Grotzingerand Knoll, 1999). In morphologic classification, these MDS belong tothe group of ministromatolites (Liang et al., 1984; Hofmann andJackson, 1987; Qiu and Liang, 1993).

Microfabrics

Microscopically, the MDS columns consist of submillimeter-scale,alternating light and dark laminasets (Fig. 4A). Light laminasets aregenerally 30–145 mm (average 65 mm) thick and are composed ofvertically oriented, partially recrystallized fibrous fabrics (Figs. 4B–C).Dark laminasets are the major component of the MDS columns and arerelatively thick—230 to 1200 mm (average 680 mm)—and consist ofdense fibers and micropeloids (Figs. 4A, D). Fluorescence observationreveals that dark laminasets consist of well-developed finer laminae,which display as alternations of horizontally uneven dark (5–20 mmthick) and light (5–22 mm thick) finer laminae (Fig. 4E). According tothe density and continuity of the dark finer laminae, dark finerlaminasets can be divided into two categories: (1) type-1 darklaminasets with continuous and dense dark finer laminae; and (2)type-2 dark laminasets with sparse and discontinuous dark finerlaminae (Fig. 4E). The boundary between light and dark laminasets isnotched or sawtoothed, with light microcrystals protruding into thedark laminasets (Figs. 4D–E).

Under high magnification, the fibrous fabrics in light laminasets arecomposed of brown fibers, 15–25 mm in width, that alternate with light,microsparitic strips of approximately equal width in horizontal

PALAIOS MICROFABRICS IN MICRODIGITATE STROMATOLITES 179

direction (Figs. 4B–C). The brown fibers contain a considerableamount of dark-black filaments that are 0.5–0.8 mm in width but couldbe up to 10 mm in length. Filaments commonly show verticalorientation parallel to that of the fibers (Figs. 4B–C). Also presentwithin the brown fibers are scattered, dark, submicron particles; theirsizes are identical to the width of the filaments (Fig. 4C). Filaments andparticles are also present in the microsparitic strips between brownfibers, but they are much less abundant (Fig. 4C). The transition frombrown fibers to their adjacent microsparitic strips is commonly cryptic,with some filaments extending across the boundary (Fig. 4C).

Type-1 dark laminasets are 100–770 mm thick (mean 380 mm), and arecharacterized by faint fibers perpendicular to laminasets (Figs. 5A–B).Fibers contain abundant filaments which also show generally verticalorientation parallel to that of the fibers (Fig. 5B), while microspariticstrips, alternating with fibers, are poorly defined and less abundantthan those in light laminasets (Figs. 5A–B). Some micropeloids withblurry margins are also present in type-1 dark laminasets (Fig. 5B).Under fluorescence these micropeloids show weak orange-red auto-fluorescence (UV excitation, Fig. 5C). About 32 finer laminae, with amean spacing of 12 mm, are commonly visible in type-1 dark laminasets

FIGURE 1—Proterozoic stratigraphic succession of the North China platform and simplified geological map of the study area. A) Generalized stratigraphic column in the

study area. U-Pb zircon ages, from oldest to youngest, are from Gao et al. (2008a); Li et al. (2010); Su et al. (2008, 2010); Gao et al. (2007, 2008b). Ages with question mark are

inferred from existing age constraints. B) Locality of the study area. C) Simplified geologic map of the study area (after Tang et al., 2011).

180 TANG ET AL. PALAIOS

(Fig. 5D). The finer laminae are commonly thicker than those in type-2dark laminasets (Fig. 4E).

Type-2 dark laminasets range from 120 mm to 620 mm (mean 300 mm)in thickness. They are composed of distinct fibers (Figs. 5E–F) and

contain more abundant void-filling microspars (Figs. 4D–E), whichmake type-2 dark laminasets generally lighter in color than type-1 darklaminasets. The fibers, ,10 mm wide, contain vertical filament bundles,and alternate with light, microsparitic strips of approximately equal

FIGURE 2—Stratigraphic occurrence of microdigitate stromatolites (MDS) in parasequences of the Wumishan Formation and their suggested depositional environments. A–

B) The occurrence of MDS in the second member of the Wumishan Formation in Huailai, Hebei Province. C) The occurrence of MDS in the second member of the Wumishan

Formation at Yesanpo, Hebei Province. D) Suggested depositional environments for the distribution of MDS.


width (Figs. 5E–H). Compared with fibers, the strips have fewerfilaments. The boundary between fibers and alternating microsparstrips is also cryptic, and filaments can be interwoven between the two.Under fluorescence, some of the filaments and spherical particles withinfibers show weak orange-red autofluorescence (UV excitation,Fig. 5H). Type-2 dark laminasets generally contain 16 finer laminae,with a mean thickness of 19 mm (Fig. 5I). These finer laminae do notshow distinct nodular joints caused by lateral thickening.

Within type-2 dark laminasets, there are some light blocks unevenlydistributed along the faint fibers (Fig. 4D). These blocks are mainlycomposed of silica but have well-preserved micropeloids and filaments(Figs. 6A–D). The fibrous textures in those blocks are similar to thoseobserved in the light laminasets (Figs. 4B–C), but there are much moremicropeloids and filaments (Figs. 6A–D). Micropeloids are commonlysurrounded by thin rims (,10 mm) that are weakly defined with blurryor fuzzy margins. Micropeloids in type-2 dark laminasets are verysimilar to those observed in modern microbialites of marine (Duprazet al., 2004), hypersaline lake (Bontognali et al., 2010; Glunk et al.,2011), freshwater (Defarge et al., 1996; Power et al., 2011; Perri et al.,2012), and evaporative tidal flat environments (Sanchez-Roman et al.,2009, 2011). In modern calcified microbial mats, micropeloids arecomposed of micritic aggregates that are commonly surrounded bycoarser-grained, crystalline denticulate rims (Chafetz, 1986; Dupraz etal., 2004, 2009; Spadafora et al., 2010; Pacton et al., 2010; Perri et al.,2012). Most micropeloids observed in MDS have been silicified, andcontain recognizable filament remnants. However, the margins ofmicropeloids become blurry or fuzzy due to the replacement of possiblya calcite/aragonite precursor by aphanitic to amorphous silica (e.g.,Fig. 6B).

To better define the morphology of filaments in MDS, fourmorphological parameters (including filament/fiber widths, bending,tortuosity, and number of direction changes per unit length, assuggested by Hofmann et al., 2008) were measured under themicroscope. These morphometric data were summarized in Table 1,in comparison with those of known microbial filaments and naturalabiogenic fibers. The features of the parameters measured from theMDS can be characterized as follows.

1. Filament width: The median width of 100 morphologicallyinvestigated filaments in the MDS (Table 1) is 0.62 mm (quartiles Q.25

5 0.57 and Q.75 5 0.68 mm). Compared with abiogenic fibers, thefilaments in MDS show much smaller variabilities, which are similar tothose of the filaments with known microbial origin.

2. Bending (degrees/micron): For abiogenic fibers, bending valuesare low, with a median of 0.15 (Q.25 5 0.04, Q.75 5 0.38). Filaments inMDS and microbial filaments have higher median values of 8.19 and0.87, respectively, and broader distributions.

3. Tortuosity: Median values of tortuosity for abiogenic fibers,microbial filaments, and filaments in MDS are similar (,1.1), while therange for microbial filaments and filaments in MDS are somewhatlarger.

4. Direction changes (n/mm): Most abiogenic fibers show very lowdirection-change values (median 0.0). Though some abiogenic fibersshow slightly elevated values, they are still lower than those ofmicrobial filaments (Q.5 5 25) and filaments in MDS (Q.5 5 396).

As it can be seen in Table 1 and the above statement, themorphological parameters measured from the filaments in the MDS

FIGURE 3—Macroscopic features of MDS from the Wumishan Formation. A) Dark silicified MDS in carbonate layers. B) Dark, nonsilicified MDS in gray matrix without

clear preservation of their internal fabrics. C) Polished slabs showing the morphology of silicified MDS; arrows pointing to divergent MDS. D) Divergent MDS (arrow).


FIGURE 4—Microfabrics in MDS from the Wumishan Formation. A) MDS column at low magnification, showing alternating light and dark laminasets. B) Fibrous fabrics of

light laminasets. C) Enlargement of the boxed area in 4B, showing brownish fibers (marked by red double-headed arrows) with mineralized filaments (possible bacterial relics,

marked by arrows) and microsparitic strips (marked by yellow double-headed arrows) with less filament relics. D–E) Plane-polarized light and fluorescence photomicrographs,

showing three distinct laminasets: the dark laminasets with dense and continuous finer laminae (5DL1, type-1 dark laminasets), the dark laminasets with sparse and

discontinuous finer laminae (5DL2, type-2 dark laminasets), and light laminasets (5LL).


FIGURE 5—Microscopic fabrics of dark laminasets in MDS. A) Type-1 dark laminasets with suberect fibers and horizontal finer laminae. B) Close view of the boxed part in

5A, showing putative filamentous and coccoid bacteria relics, and irregular micropeloids with vague boundaries (in yellow circles). C) Fluorescence photomicrograph of the

same view as in 5B, showing faint orange-red autofluorescence stimulated from possible bacterial relics (arrows). D) Fluorescence photomicrograph of type-1 dark laminasets,

showing dense and continuous finer laminae. E) Type-2 dark laminasets with brown fibers. F) Close view of the boxed part in 5E, showing clear fibers and irregular

micropeloids with vague boundaries (in yellow circles). G) Close view of the boxed part in 5F, showing suberect filaments and spherical particles (oblique cross sections of the

filaments or putative coccoid bacteria relics). H) Fluorescence photomicrograph showing faint orange-red autofluorescence stimulated from possible bacterial relics in partial

fibers (arrows). I) Fluorescence photomicrograph showing sparse and discontinuous finer laminae in type-2 dark laminasets.


FIGURE 6—Micropeloids and fibrous fabrics in the type-2 dark laminasets, and Raman spectrums of a filament, a micropeloid, and its surrounding rim. A) Irregular

micropeloids (arrows) in type-2 dark laminaset. B) Subspherical micritic micropeloid (marked by yellow circle), surrounded by microsparitic or amorphous rim that is partially

silicified (arrows). C) Vertically arranged micropeloids (arrows). D) Enlargement of the boxed area in 6C, showing putative bacterial filaments aligning fibers. E) Raman

spectrum of the micropeloid in 5B (in white circle), showing Raman bands at 463, 1362, 1606, and ,2800 cm21, and an indistinct shoulder on low-frequency side of the D band.

F) Raman spectrum of filaments in 5E (in white circle), showing Raman bands at 204, 462, 1352, 1601, and ,2800 cm21. G) Raman spectrum of the micropeloid in 6B (red

circle), showing Raman bands at 208, 463, 1359, 1603, and ,2800 cm21. H) Raman spectrum of the rim surrounding micropeloid in 6B (in black circle), showing Raman bands

at 205 and 463 cm21. A–D are photomicrographs with plane-polarized light.


are similar to those of known microbial filaments, but distinct fromthose of the abiological filaments described by Hofmann et al. (2008).

In order to ascertain the properties of potential organic relics inmicropeloids and filaments in the MDS, Raman spectrometry analysiswas conducted. A micropeloid in Figure 5B shows Raman bands at463, 1362, 1606, and ,2800 cm21, and presents an indistinct shoulderon the low-frequency side of the band at 1362 cm21 (Fig. 6E).Filaments in Figure 5G show Raman bands at 204, 462, 1352, 1601,and ,2800 cm21 (Fig. 6F). The nucleus of a micropeloid in Fig. 6Bshows Raman bands at 208, 463, 1359, 1603, and ,2800 cm21

(Fig. 6G), while the microsparitic rim of the micropeloid shows Ramanbands at 205 and 463 cm21 (Fig. 6H).

In summary, five kinds of microfabrics are recognizable in the MDScolumns: (1) light microsparitic laminasets; (2) dark laminasets withdense and laterally continuous finer laminae (type-1 dark laminasets);(3) dark laminasets with sparse and laterally discontinuous finerlaminae (type-2 dark laminasets); (4) fibers containing vertical filamentbundles; and (5) micropeloids composed of filament relics and micrites.Vertically, MDS laminasets display as repetitive cycles in order withinthe light laminasets, type-1 dark laminasets and type-2 dark laminasets.Fibers are widespread in all of the three laminaset types; micropeloidsare abundant in dark laminasets, particularly in type-2 dark laminasets.Filaments in the MDS are similar to microbial filaments but distinctfrom abiogenic fibers statistically for the four morphological param-eters, including filament/fiber widths, bending, tortuosity, and numberof direction changes per unit length.

ULTRASTRUCTURE

Under FESEM, type-1 dark laminasets in MDS columns (Figs. 4D–E) were observed to contain a variety of possible organic remainsincluding filaments with constant diameter, their fragments (Fig. 7A–C), and mucuslike films or filaments with inconstant diameter (putativemicritized EPS) (Figs. 7D–F). Nanoglobules and polyhedrons are seento associate closely with all of these putative organic components(Figs. 7C–F). The mucuslike films or filaments are partially replaced bysmall nanoglobules (,45 nm) (Figs. 7E–F). Small nanoglobules cancoalesce into larger (60–100 nm) nanoglobules (Fig. 7F), which in turnaggregate into submicron-scale polyhedrons (Figs. 7B–F). Energyspectrum analysis (EDS) indicates that the major elements inpolyhedrons (Fig. 7G) and mucuslike filaments (Fig. 7H) are similar;both enriched in Si and O, with subordinate amounts of C, Mg, and Ca.

Under FESEM, the micropeloids are vertically aligned to form fiberswithin parts of type-2 dark laminasets (Fig. 8A). The micropeloids aresubspheroidal in shape, composed of submicron particles or polyhe-drons, and commonly surrounded by microsparitic rims (Ri inFig. 8A). In comparison with those found in modern stromatolitesand lithified microbial mats (e.g., Perri and Spadafora, 2011; Perri

et al., 2012), the micropeloids in MDS have much narrower andinconspicuous rims, and the crystal morphology is less well defined, buttheir textures are similar. In some of the micropeloids, mucuslike films(Fig. 8B) and aggregates of nanoglobules can be recognized (Fig. 8C).The mucuslike films are replaced or entombed by smaller nanoglobules(,45 nm) (Fig. 8C) that in many cases coalesce into larger (150–300 nm)polyhedrons (Fig. 8B). In addition, relatively well-preserved filaments,,0.6 mm in width and 10–15 mm in length, line the margins of the fibers(Fi in Fig. 8A). In some cases, the filaments are also seen to be replacedor entombed by nanoglobules, which are similar to the organomineralultrastructures observed in experimental cultures (e.g., Chafetz andBuczynski, 1992; van Lith et al., 2003) and in modern mineralizedmicrobial mats (e.g., Defarge et al., 1996; Glunk et al., 2011). Ingeneral, micrite (now replaced by silica) constitutes the majorcomposition of fibrous fabrics, accounting for ,80% of the volume,except for the microsparitic rims surrounding micropeloids.

The microsparitic strips adjacent to the brown fibers (Figs. 4C, 8A)are largely composed of euhedral crystals. These crystals, 0.5–3 mm insize, are variable in shape and often appear as clusters that arerandomly distributed within the strips.

In summary, three types of ultrastructures have been identified inMDS columns: (1) putative organic relics, such as filaments, theirfragments, and mucuslike films or filaments (purported EPS); (2)organominerals, including nanoglobules, polyhedrons, and theiraggregated micropeloids; and (3) euhedral crystals mainly constitutingmicrosparitic strips and the rims surrounding micropeloids. These threeultrastructures are commonly seen in dark laminasets, but are difficultto identify in light laminasets due to recrystallization.

DISCUSSION

Potential Organic Relics in MDS

The value of morphological information in the interpretation of fossilstructures has recently been debated. Some scholars take morphologyas evidence of biogenic origin for microfossil-like features in the case ofthe earliest fossils on the Earth (e.g., Brasier et al., 2002; Schopf et al.,2002) and even in Mars meteorite ALH84001 (e.g., McKay et al., 1996;Buseck et al., 2001), while some others dismiss the value of morphologyin detecting fossil microbes (e.g., Garcia-Ruiz and Amoros, 1998;Garcia-Ruiz et al., 2002). On the other hand, however, it has beenargued that microbes can produce distinctive fabrics structurallyorganized differently from those of abiogenic origin (Hofmann et al.,2008), and microbial morphology can be preserved for much longerthan detectable organic relics of microorganisms (Guidry and Chafetz,2003). Filaments in the MDS, similar to microbial filaments, havesmaller variabilities in filament width, larger bending, more directionalchanges per unit length, and larger variabilities of tortuosity than those

TABLE 1—Morphometric data for filament features in the MDS and comparison with related structures.

Bending Tortuosity Direction changes n Width n References

Microbial filaments

Q.25 0.48 1.04 10.7 158 1.5 471 Hofmann et al.,

2008Q.50 0.87 1.09 24.8 158 2.0 471

Q.75 1.71 1.35 57.7 158 2.6 471

Inorganic filament

Q.25 0.04 1.00 0.0 162 0.7 851 Hofmann et al.,

2008Q.50 0.15 1.01 0.0 162 2.1 851

Q.75 0.38 1.10 4.5 162 5.0 851

Filaments in MDS

Q.25 5.57 1.05 275 35 0.57 100 Herein

Q.50 8.19 1.08 396 35 0.62 100

Q.75 12.3 1.13 591 35 0.68 100


FIGURE 7—Ultrafabrics of a type-1 dark laminaset in MDS column. A–B) Filament relics (5Fi, putative filamentous bacteria) in type-1 dark laminasets. C) Enlargement of

the boxed part in 7B, showing filaments and polyhedrons. D) Mucuslike films (purported EPS) and associated polyhedron (5Po). E) Mucuslike filaments with inconstant

diameter and closely associated polyhedrons. F) Enlargement of 7E, showing nanoglobules (5Np) and polyhedrons (5Po) formed by Np aggregates. G) EDS spectrum of the

polyhedron marked by black + in 7D; H) EDS spectrum of the boxed area in 7E (Pt derived from coating).


of abiogenic fibers (Table 1). In addition, the sizes and morphology offilaments in the MDS share a general similarity with the mineralizedfilamentous bacteria identified in modern microbial mats (e.g., Pactonet al., 2010) or modern tufa (e.g., Perri et al., 2012). Thus, it seems thatthe filamentous bundles in the MDS are most likely derived from thecalcification of filamentous bacterial relics, while the spherical particlespossibly either resulted from oblique cross sections of the mineralizedfilamentous bacteria or coccoid bacteria (e.g., Perri et al., 2012).

Although autofluorescence could be caused by trace elements (e.g.,Neuweiler et al., 2000) and/or lattice failure (e.g., Neuweiler et al.,1999), most works on carbonates have shown that the presence of

organic compounds is the main cause (e.g., Bezouska et al., 1998;Neuweiler et al., 2000, 2003). In the silicified MDS, autofluorescence ismost likely caused by organic relics, rather than by trace elements and/or carbonate crystal lattice failure, as the trace element contents ofsilicified MDS (data of three MDS samples are not provided) are oneorder less than those of average post-Archean shales (Taylor andMcLennan, 1985). As well, the areas showing autofluorescence in thethin sections are closely associated with putative organic relics withdistinctive morphologies, whereas areas devoid of those textures arenot. Thus, the orange-red autofluorescence stimulated from thefilaments and spherical features within the fibers of MDS (UV

FIGURE 8—Ultrastructure of micropeloids and fibrous fabrics in type-2 dark laminasets under FESEM. A) Micritic micropeloids (5P) and microsparitic rims (5Ri) align to

form a fiber (between dotted lines). Microsparitic strips adjacent to the fiber contain euhedral quartz (5Q) and dolomite (5Dol) crystals. Filament relics (5Fi) are present along

the margin of the fiber. B) Enlargement of a micropeloid, showing mucuslike films and closely associated polyhedrons (5Po) and micropores. C) Enlargement of the

micropeloid center, showing aggregates of nanoglobules.


excitation, Figs. 5C, H), more likely indicates the presence of organicmatter.

It has been suggested that Raman spectroscopy can be effectivelyused for the detection of bond types, functional groups, and/orcompound classes (Kudryavtsev et al., 2001; Schopf et al., 2002,2005; Heim et al., 2012), and thus the analytical technique has beenwidely used in detecting organic compounds in ancient terrestrial (e.g.,Schopf et al., 2002, 2007; Edwards et al., 2007; Schopf andKudryavtsev, 2012) and purported extraterrestrial fossils (e.g., Elleryet al., 2004; Villar and Edwards, 2006). In our study, two lines ofevidence prove that the purported bacterial filaments (Fig. 5G) andmicropeloids (Figs. 5B, 6B) in the MDS contain biogenic carbonaceouskerogen: (1) the appearance of both a first-order disordered (D) Ramanband at ,1350 and graphitic (G) band at ,1600 cm21, and a second-order, lower and broader band around ,2800 cm21 (Figs. 6E–G); and(2) the occurrence of a shoulder on the low-frequency side of the Dband (Fig. 6E) (Kudryavtsev et al., 2001; Schopf et al., 2002, 2005,2007; Edwards et al., 2005; Villar and Edwards, 2006; Chen et al.,2007). In contrast, the rims encasing the micropeloids (Fig. 6B) do notshow signatures of carbonaceous kerogen (Fig. 6H), probably suggest-ing abiogenic origin. All analyzed spots show vibrational bands at ,206and ,463 cm21, which are corroborative bands of quartz (Villar andEdwards, 2006), indicating that the MDS have been extensivelysilicified.

Mineralized EPS relics in ancient microbialites are commonlyidentified by their mucuslike morphology and the bearing of organiccarbon. In modern and recent microbialites, mucuslike films orfilaments are usually interpreted as EPS relics (e.g., Perri et al., 2012;Sanz-Montero and Rodrıguez-Aranda, 2012; Tang et al., 2012).Although it is difficult to distinguish potential organic carbon in themucuslike films or filaments in the MDS from inorganic carbon due tothe coexistence of C with Ca, Mg, and O, and low content of C in thepresent samples (Figs. 7G–H), the mucuslike films or filaments in theMDS could also be interpreted as micritized EPS according to theirsimilarity with those recognized in modern and recent stromatolites(e.g., Spadafora et al., 2010; Perri and Spadafora, 2011; Perri et al.,2012), as well as their close association with the kerogen-bearingfilaments and micropeloids. In addition, nanoglobules and polyhedronsare commonly associated with or entombed in the mucuslike films,which are similar to the mineralized EPS relics and associatednanoglobules observed in modern lithified microbial mats (e.g., Duprazet al., 2004; Aloisi et al., 2006; Spadafora et al., 2010; Glunk et al., 2011;Perri and Spadafora, 2011; Perri et al., 2012).

Possible Genesis of Fibrous Fabrics

The morphology and fibrous fabric texture of the MDS from theWumishan Formation are similar to those of other Neoarchean toPaleoproterozoic successions. The presence of possible bacterialfilament relics and purported EPS, along with the closely associatednanoglobules, polyhedrons, and micropeloids, provide evidence of abiogenic origin. Filaments preserved in the brown fibers are generallyvertically oriented, possibly representing their primary upward growth(Figs. 4C, 5B, F, 6D), suggesting in situ mineralization of bacterialfilaments either syndepositionally or during very early burial. Theorganomineralization may have first started in the EPS surroundingfilamentous bacteria (Fig. 9A). Instead of in vivo mineralization ofbacterial sheaths, partial mineralization of EPS formed nanoglobules,micropeloids, and microsparites that may have served as the mineralmatrix maintaining the growth orientation of living and postmortembacteria (Fig. 9A). Subsequent organic matter decay and mineralizationof bacterial filaments happened after their death formed verticallyoriented fine fibers (Fig. 9). Thus, the brown fibers in MDS (Figs. 4A–C) are likely the result of the mineralization of bacterial filaments orfilament bundles, while microsparitic strips between the fibers are likely

derived from the mineralization of gelatinous EPS surrounding thefilaments (Fig. 9).

Fibrous fabrics formed by mineralization of bacterial filaments arecommon in modern and ancient microbialites, such as in tufa (Freytetand Plet, 1996; Janssen et al., 1999; de Wet and Davis, 2010), travertine(Chafetz and Guidry, 1999), and sinter (Jones et al., 1998, 2001;Berelson et al., 2011). For example, abundant fibrous fabrics resultedfrom mineralization of Phormidium are reported from fresh-water tufasin France (Freytet and Plet, 1996) and Belgium (Janssen et al., 1999). Inmodern marine microbialites associated with coral reefs in the SouthChina Sea, abundant vertically growing, hairy hyphae have beenobserved in gelatinous EPS (Shen and Wang, 2008). Dense fibersformed by mineralization of filamentous cyanobacteria Ortonella oralgae Solenopora are also known from Late Devonian microbialites(e.g., Shen and Webb, 2004). More intriguing examples include siliceousstromatolites from Yellowstone National Park, United States (Berelsonet al., 2011) and the Waimangu geothermal field, New Zealand (Joneset al., 2005). Those siliceous stromatolites consist of alternating lightand dark laminae, similar to light and dark laminasets observed in theMDS of the Wumishan Formation (Fig. 4A). In those siliceousstromatolites, however, the light laminae have clear fibrous fabricsthat are likely the result of mineralization of vertically grownfilamentous bacteria and the dark laminae resulted from lithificationof horizontally grown bacteria.

FIGURE 9—Proposed filament-mineralization and lamination-forming processes in

MDS. A) Degradation and mineralization processes of a filament and encasing EPS.

B–E) characteristics of MDS-forming microbial mats in winter, early spring, late

autumn and early winter, respectively. Microbial mats mineralized in winter are thin;

fibers in them are sparse; filaments in fibers are poorly preserved. Microbial mats

mineralized in spring to early summer are thick, with dense fibers; micropeloids are

relatively small and lack microsparitic rims commonly; filaments in fibers are

relatively well preserved. Microbial mats mineralized in later summer to autumn are

similar to those in spring to early summer; but organic relics are less, and

micropeloids are larger, commonly surrounded by microsparitic rims.


The absence of bacterial fossils in most Precambrian MDS(Grotzinger and Reed, 1983; Hofmann and Jackson, 1987; Knollet al., 1993; Sergeev et al., 1995) could be caused by the destruction oforganic components during long-lasting complex diagenesis andmetamorphism (Altermann, 2008; de Wet and Davis, 2010). Additionalcauses may include high atmosphere-ocean CO2 concentration in earlyEarth history that may have prevented calcification of bacterial sheaths(Arp et al., 2001a; Pratt, 2001; Riding, 2006; Kah and Riding, 2007). Inthe MDS from the Wumishan Formation, silicified samples andsilicified portion of the samples best preserve the organomineralcomponents including filaments, micropeloids, EPS relics, and nano-globules. Thus, we believe that early silicification is critical for thepreservation of microfabrics and organominerals in the MDS. Theamorphous silica surrounding the micropeloids (Fig. 8A) suggests thatthe silica replacement was earlier than complete degradation of organicmatrix or EPS (Bartley et al., 2000; Berelson et al., 2011).

Possible Genesis of Alternating Laminae in MDS

As one of the most prominent features in stromatolites, laminae canbe formed by a variety of physical, biological, and biochemicalprocesses. In the Mesoproterozoic MDS, laminae seem to have beengenerated by in situ organomineralization and carbonate precipitation,which differ from those in modern marine stromatolites. In modernmarine stromatolites, laminae are formed by alternations of micriticcrust layers of mineralized microbial mats and ooid-fused layers (e.g., inBahamas; Visscher et al., 1998; Reid et al., 2000; Stolz et al., 2001, 2009)or carbonate grain-bound layers (e.g., in Shark Bay; Reid et al., 2003;Burns et al., 2009; Jahnert and Collins, 2012) resulting from microbialtrapping and binding of inorganic particles and mineral precipitation(Andres and Reid, 2006; Stolz et al., 2009). These characteristics makethe MDS more similar to modern freshwater tufas (Freytet and Plet,1996; Janssen et al., 1999; Arp et al., 2001b; Brasier et al., 2010) andsinters (Jones et al., 1998; Berelson et al., 2011), which are commonlydevoid of sediment-bound layers.

In modern tufa deposits from the Paris Basin, clear alternations ofmillimeter-scale light and dark laminae have been documented (Freytetand Plet, 1996). The light laminae (a- laminae) are formed by calcifiedfilament bundles of Phormidium incrustatum, separated by widerinterstices filled with microsparitic precipitates, whereas the darklaminae (b-laminae) consist of densely packed suberect filaments ofthe same species, with little microsparitic cement (Freytet and Plet,1996; Freytet and Verrichia, 1998). Similar growth patterns offilamentous bacteria have been also observed in modern tufas fromBelgium (Janssen et al., 1999), Germany (Arp et al., 2001b, 2010),Greece (Brasier et al., 2010), Poland (Szulc and Smyk, 1994), and Spain(Arenas et al., 2010), commonly with Phormidium as the predominantfilamentous bacteria.

Alternations of distinct laminae in modern tufas are usually believedto record annual seasonal variations (Kano et al., 2003; Kawai et al.,2009; Arp et al., 2010; Gradzinski, 2010; Jones and Renaut, 2010). Theycan be the result of variations in microbial groups (Freytet and Plet,1996; Freytet and Verrichia, 1998; Janssen et al., 1999), growth patterns(Janssen et al., 1999; Arp et al., 2001b; Jones et al., 1998, 2005) and/orcalcification (Kano et al., 2003) in response to seasonal changes intemperature, light intensity, carbonate precipitation rate and saturation(Andrews and Brasier, 2005; Kawai et al., 2009). Though it is by nomeans easy to determine the periodicity of laminar cycles in tufas, someof them have been successfully confirmed as seasonal cycles byvariations in d18O (Kano et al., 2003; Shiraishi et al., 2008) or changesin plant pollen (Freytet and Plet, 1996; Janssen et al., 1999) and larvaetubes (Janssen et al., 1999; Brasier et al., 2010), which usually occur inthe laminae precipitated in the spring. In the European studies the lightporous laminae are primarily interpreted to have formed in the summerto autumn seasons when carbonate precipitation is high, whereas the

dark, dense laminae are formed in the spring when biomass productionis high (Freytet and Plet, 1996; Freytet and Verrichia, 1998; Janssenet al., 1999). However, the reverse has been observed in tufas in Japan,which are attributed to influence from local climate and otherenvironmental conditions (Kano et al., 2003). Some of the cycliclaminae in modern aragonite travertines even record daily variations inmicrobial metabolism, EPS production and degradation, and mineral-ization rates (Takashima and Kano, 2008; Okumura et al., 2011, 2012).

In some well-studied modern sinters and siliceous stromatolites,however, the alternations between dark and light laminae seem to bemore closely related to growth patterns of bacterial filaments or distinctmicrobial groups. For example, in the Waimangu geothermal field,New Zealand (Jones et al., 2005), siliceous lilypad stromatolites arecomposed of three distinct laminae: flat-lying Phormidium filaments (P-laminae), upright filaments of Phormidium commonly associated withFischerella (U-laminae), and mucus, diatoms, and pyrite framboids (M-laminae). The clear tripartite cycles of the laminae in these stromatolitesare interpreted to record seasonal or a 40-day hydrologic variation(Jones et al., 1998, 2005). In the siliceous stromatolites of theYellowstone National Park, USA, the alternation of laminated mainbody and well-cemented drape is thought to record cycles on a scale ofyears, whereas the alternations of dark and light laminae in the mainbody reflect approximately diurnal cycles (Berelson et al., 2011; Mataet al., 2012). The dense dark laminae composed of reclining filamentousbacteria likely correspond to nighttime, while the porous light laminaecharacterized by suberect filamentous bacteria are possibly formed indaytime, in which larger subcircular pores may derive from gas bubblesproduced through photosynthesis (Mata et al., 2012).

In comparison with the laminar microfabrics and their patterns inmodern tufas and sinters, it seems reasonable to infer that the cyclicalternations of three distinct laminasets (light laminasets, type-1 darklaminasets, and type-2 dark laminasets) in the Mesoproterozoic MDSmay have recorded seasonal variations in microbial biomass andcarbonate precipitation rate. Differences in orientation of bacterialfilaments and microbial groups have not been observed in the MDS.The light laminasets were likely generated in winter seasons (Figs. 9B–C). Due to relatively lower temperature in the winter, microbial growthmay have been slowed down or largely ceased, and degradation rate oforganic matter was larger than biomass production. Active heterotro-phic degradation of organic matter generated HCO3

2, and EPSreleased previously absorbed bivalent cations (e.g., Ca2+, Mg2+),providing preferential sites for carbonate nucleation (Spadafora et al.,2010) and thus facilitating carbonate precipitation (Dupraz et al., 2004,2009). The resultant light laminasets are mainly composed ofmicrosparites. Due to the efficient degradation of organic matter,microbial remnants are rare in the light laminasets (Figs. 4B–C, 9B–C).The type-1 dark laminasets with dense and laterally continuous finerlaminae (Fig. 5D) in the Mesoproterozoic MDS are similar to the b-laminae (Freytet and Plet, 1996; Janssen et al., 1999) or the micrite-impregnated biofilms (Arp et al., 2001b) in modern tufa. They mayhave formed during the spring to early summer seasons (Figs. 9C–D).As the temperature rose in the spring, microbes grew and bloomed,forming densely packed bacterial filaments and having high biomassproduction. Due to active photosynthesis and rapid microbial growth,EPS secreted by microbes absorbed bivalent cations (e.g., Ca2+, Mg2+)(Braissant et al., 2007), thus reducing the alkalinity in the microenvi-ronment and inhibiting active carbonate precipitation (Aloisi et al.,2006; Dupraz et al., 2004, 2009). Therefore, type-1 dark laminasetsconsist mainly of micrite derived from in situ organomineralization andrare microsparitic cement (Figs. 9C–D). The type-2 dark laminasetswith sparse and laterally discontinuous finer laminae (Fig. 5I) aresimilar to the a-laminae (Freytet and Plet, 1996; Janssen et al., 1999) orthe porous microsparitic biofilms (Arp et al., 2001b) in modern tufa.They may have formed during late summer to autumn seasons(Figs. 9D–E). In these seasons, microbial growth rate was slowed


down and biomass production rate approximately equals the degrada-tion rate of organic matter, thus forming relatively sparser filamentbundles with more interstices. In this condition concurrent photosyn-thesis and EPS degradation would facilitate carbonate precipitation(Visscher et al., 2000; Braissant et al., 2007; Okumura et al., 2011). Themicrosparitic blocks (Figs. 4D–E, 6A) in type-2 dark laminasets mayrepresent the primary voids that were later filled by carbonateprecipitates (Figs. 9D–E).

It is worth noting that the finer laminae preserved within the type-1and type-2 dark laminasets of the Mesoproterozoic MDS, expressed bydistinct patterns (Figs. 4E, 5D, I), are microtexturally similar to thedaily sublaminae (we use the term finer laminae) observed from themodern aragonite travertine in southwestern Japan (Takashima andKano, 2008; Okumura et al., 2011). There is a possibility that the finerlaminae in the dark laminasets of the MDS record the diurnal cycles. Inthe modern aragonite travertine, however, the sublaminae aresignificantly thicker; both individual daytime and nighttime sublaminaeattain 100 mm in thickness. The textural differences between the twotypes of sublaminae are ascribed to changes in production of EPS,heterotrophic activities, and the rate of carbonate precipitation(Okumura et al., 2011, 2012). In the Mesoproterozoic MDS, theindividual finer laminae have an average thickness of ,15 mm, which ismuch thinner than those in the modern aragonite travertine. In additionto the potential reduction of thickness due to compaction, thinner finerlaminae in the MDS could also imply that the carbonate saturationindex (SIcalc.), and thus carbonate precipitation rate (R), in theMesoproterozoic seawater might be much lower than those in modernaragonite travertine (SIcalc. 5 1.4, Mg/Ca 5 3, R 5 200 mm/day;Okumura et al., 2011). Differences in finer laminae between the type-1and type-2 dark laminasets of the MDS could also be caused by dailytemperature variations. The denser and unevenly thickened finerlaminae in type-1 dark laminasets (Figs. 4E, 5D) might have formedin environments with a high day-night difference in temperature duringthe spring to early summer, whereas the less clear finer laminae in type-2 dark laminasets might have formed in response to a low day-nightdifference in temperature in late summer to autumn seasons.

CONCLUSIONS

Microdigitate stromatolites (MDS) are widespread in peritidaldolomite facies of the Mesoproterozoic Wumishan Formation (ca.1.50–1.45 Ga) in North China. The MDS columns are composed ofalternating dark and light laminasets; both of which are characterizedby vertically oriented, fibrous fabrics. The fibrous fabrics in MDS werepreviously interpreted as abiotic carbonate precipitates in carbonate-supersaturated Proterozoic oceans. Our study using micron- andnanometer-scale observations and Raman spectroscopic analysisreveals the presence of filamentous bacteria and EPS relics in thefibers and other closely associated organomineral components such asnanoglobules, polyhedrons, and micropeloids. These features provideevidence that active microbial activity was involved in the formation ofthe Mesoproterozoic MDS and may also shed light on understandingthe origin of other MDS in Neoarchean–Paleoproterozoic successions.

Vertically oriented fine fibers in both light and dark laminasets of theMDS often align partially preserved possible bacterial filaments. Thismay suggest that the fibrous fabrics were formed through in situmineralization of filamentous bacteria either syndepositionally or veryearly during burial. Instead of in vivo mineralization of bacterialsheaths, organomineralization may have initiated in EPS, formingnanoglobules, micropeloids, and microsparites surrounding bacterialfilaments. Subsequent postmortem organic matter decay and mineral-ization of bacterial filaments formed the fibrous fabrics that likelyreflected the growth orientation of filamentous bacteria. The alternat-ing dark and light laminasets in MDS may record variations in thedensity rather than the growth habitat of filamentous bacteria. Changes

in the density of putative filamentous bacteria during MDS depositioncould have derived from seasonal environmental changes—an inferencethat requires independent evidence in the future.

Laminasets in the Mesoproterozoic MDS likely result from in situmineralization, and they share similarity with laminae revealed inmodern stromatolitic tufas. The cyclic alternations of the three distinctlaminaset types (light laminasets, type-1 and type-2 dark laminasets) inthe MDS columns likely recorded seasonal variations in temperature,which might have exerted a great influence on temperature-dependentmicrobial activities (such as growth patterns and metabolism) andmicroenvironmental changes (such as alkalinity and carbonate satura-tion). Temperature has controls on biomass production and EPSdegradation, and microenvironment has great influences on carbonateprecipitation rate.

ACKNOWLEDGMENTS

The study was supported by the Ministry of Science and Technology(2011CB808806) and the National Natural Science Foundation ofChina (40972022 and 41272039). We are grateful to Dr. Henry Chafetz,Dr. Russell Shapiro, Dr. Jorn Peckmann, two anonymous reviewers,and co-editor John-Paul Zonneveld for constructive suggestions andcomments that have improved the paper greatly. Thanks are given toPei Yunpeng, Zhao Guisheng, and Li Danqiu for their assistance infield work, and to Luo Jun for his kind help in FESEM observation.

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