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1、GEOLOGY, March 2007 207 Geology, March 2007; v. 35; no. 3; p. 207210; doi: 10.1130/G23354A.1; 5 fi gures. 2007 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editinggeosociety.org. INTRODUCTION Microbial mats that generate stromatolites are a feature of
2、 many modern shallow-water peri- tidal environments. Several processes operate to generate these typically laminated deposits, including trapping and binding of sediment grains by fi lamentous cyanobacteria, calcifi - cation of autochthonous microorganisms, and precipitation of carbonates, either in
3、organi- cally or through microbiological mediation (Visscher and Stolz, 2005). Within some mod- ern microbial mats, sulfate reduction represents a bio-geochemical pathway that links bacterial metabolism and Mg-carbonate precipitation (Vasconcelos et al., 1995). Ancient stromatolites provide evidence
4、, through comparison with modern counter- parts, for the existence of life on Earth back to the early Precambrian (3.5 Ga), although the biogenicity of such structures is under debate (Allwood et al., 2006; Brasier et al., 2006). Even though stromatolites often lack any proof of the original microbe
5、s in the form of fossils, and rarely retain their primary geochemical signatures, stromatolites do preserve evidence of microbial biomineralization through the entire geological record (Riding, 2000). In some cases, stromatolitic structures have been interpreted as being largely abiotic CaCO3 pre- c
6、ipitates (Pope et al., 2000). Proving a biogenic microbial origin for ancient stromatolites can be very diffi cult. METHODS Stromatolites were collected along the Cor- vino Valley, northwest Calabria, Italy, from Norian peritidal meter-scale cycles. These were deposited within shallow and restricted
7、 marine environments that were locally hypersaline and dominated by microbial carbonates (Perri et al., 2003; Mastandrea et al., 2006). Scanning electron microscope (SEM) obser- vations of fresh surfaces of stromatolite were made on polished thin sections or freshly bro- ken surfaces, after ultrason
8、ic cleaning with alcohol, using a Cambridge Stereoscan 360 and a FEI-Philips ESEM-FEG Quanta 200F. All samples were carbon- or gold-coated, depend- ing on whether they were used for micro- analysis or textural study; some samples were fi rst etched with dilute HCl. X-ray diffraction (XRD) was used t
9、o determine mineral compo- sition (using a Philips PW1730), and Ca, Mg, and trace element content (Philips PW1480). Semiquantitative analyses of submicron-sized spots were performed with an EDAX Genesis 4000 energy dispersive spectrometer on the SEM, and measurements were confi rmed on a JEOL JXA-82
10、00 WD/ED Superprobe. RESULTS We document microbial fossils and origi- nal geochemical signatures from Triassic stro- matolitic dolomites, and propose a biological- geochemical model for their formation. Stromatolite lamination consists of an alter- nation of layers of light gray microspar (planar an
11、hedral to subhedral crystals, 560 m) and dark gray micrite (planar euhedral crystals, 5 m). Laminae are usually isopachous, even and smooth, with microsparitic layers varying from 200 to 500 m in thickness, and the dark-gray micritic laminae varying from 20 to 40 m (Fig. 1). Stromatolites have an al
12、most wholly dolomite mineralogy. Spot analyses show that the 5 m dolomite crystals have an average of 53 mol %Ca, whereas larger crystals have an excess of Mg up to 52 mol%. Stable isotope measure- ments give an average 18O of 0.3 (PDB) and 13C of 2.7 (PDB). SEM observations of fresh surfaces of str
13、o- matolite reveal the presence of spheroidal struc- tures, particularly within the dark micritic lami- nae. The majority of these structures consist of submicron-sized fragments of hollow spheroids with 100150-nm-thick walls, in some cases enclosed within dolomite crystals and protruding from their
14、 surfaces (Fig. 2A). Spheroids occur- ring between crystals within the intercrystalline pore space are commonly collapsed, or crushed between fl anking crystals. Rarely, the primary intercrystalline space has allowed the preserva- tion of whole spheroidal structures, which show a uniform diameter of
15、 1.0 m (Fig. 2B). Spheroid *E-mails: eperriunical.it; m.e.tuckerdurham. ac.uk. Bacterial fossils and microbial dolomite in Triassic stromatolites Edoardo Perri* Dipartimento di Scienze della Terra, Universit della Calabria, Via P. Bucci cubo 15b, 87036 Rende, Italy Maurice Tucker* Department of Eart
16、h Sciences, Durham University, Durham DH1 3LE, UK ABSTRACT Triassic stromatolitic dolomite from Italy preserves mineralized bacterial remains, one of the fi rst unequivocal identifi cations of such structures in the geological record. They consist of empty spheroids 1.0 m diameter resembling coccoid
17、 bacteria, and smaller, 150400 nm, objects interpreted as dwarf bacterial forms, occurring within and between syn-sedimentary dolomite crystals. Moreover, gently folded sheets, 100200 nm thick and several microme- ters long, form a sub-polygonal network reminiscent of EPS (extracellular polymeric su
18、b- stance). Their granular-textured surfaces suggest bacterial degradation of original organic matter. These features confi rm a biological origin for the stromatolites, as in modern micro- bial mats, and the preserved original geochemical signatures indicate early precipitation of Mg-carbonates ind
19、uced through microbial sulfate-reducing metabolic activities. Keywords: bacterial fossils, microbialite, dolomite, stromatolite, Triassic. Figure 1. Field view of a stromatolite (left) and photomicrograph (right) of stromatolitic laminae formed of alternating dark micrite and lighter microsparite la
20、yers. 208 GEOLOGY, March 2007 walls have a fi ne granular texture formed of nano- crystal aggregates of Ca-rich dolomite (Fig. 2C) with an average Ca content of 55.4 mol%, and in some cases reaching nearly 70 mol%. Higher-magnifi cation observations of the same micritic laminae show a second type of
21、 spheroidal-ovoidal structure of sub-micron size. These are 150400 nm in diameter, isolated or in small clusters, and usually enclosed within the dolomite crystals (Fig. 3). Associated with the spheroidal structures are planar or sheet-like features. They consist of 100200-nm-thick gently folded she
22、ets, that occur within crystals and cross between crystals; they also envelop the larger spheroids (Fig. 4A, see also Fig. 2B). In some places, it is possible to observe that these sheets form a sub-polygonal network (Fig. 4B). Gener- ally, they are seen as even, isopachous ribbons extending for sev
23、eral microns, but they are also fragmented, showing an irregular granular- textured surface, enveloping even smaller, 100 nm, spheroidal bodies (Fig. 4C). The nanometer-sized spheroidal structures and the planar sheet-like structures have the same (Ca-rich) dolomite composition as the adjacent and e
24、nclosing crystals. All the micron-sized and submicron-sized spheroids and the planar sheet-like structures were observed both on fresh untreated and on gently acid-etched samples. Dark micritic dolomite laminae showed strong autofl uorescence, indicating the presence of organic matter. A draped arra
25、ngement was often seen, of a delicate folded layer between and upon the dolomite crystals that surround the submicron-sized spheroidal granular bod- ies (Fig. 5). The location, form, and dimensions of these organic matter relicts, inferred from their autofl uorescence, coincide exactly with the sphe
26、roidal and planar-sheet structures seen with the SEM. This confi rms a biological origin for the SEM structures; it could also indicate some role Figure 2. Scanning electron microscope (SEM) photomicrographs of micron-sized structures interpreted as mineralized bac- teria in a Triassic stromatolite.
27、 A: Fragments of spheroidal structures are enclosed within dolomite crystals and protrude from them (arrows). B: Well-preserved spheroids within the inter-crystalline space. Sheet-like struc- tures interpreted as mineralized EPS are also present. C: Close-up view of hollow spheroid showing the wall
28、formed of nano-crystal aggregates of dolomite. All fresh surfaces. Figure 3. Scanning electron microscope (SEM) photomicrographs of submicron-sized spheroidal structures interpreted as miner- alized dwarf bacteria. A: Spheroids (arrows) surrounded by an irregular bumpy-lumpy texture, interpreted as
29、mineralized degraded organic matter. Compare this texture with the smooth crystal surface and sharp crystal edges to the lower left and right. B: Close-up view of left part of Figure 2A showing some spheroidal bodies (arrows). Gently acid- etched surface. Figure 4. Scanning electron microscope (SEM)
30、 photomicrographs of planar sheet-like structures interpreted as mineralized EPS. A: Gently folded sheets occur within crystals and extend across them; they envelop the larger spheroids (arrows) (see also Fig. 2B). B: Sub-polygonal network (suggested by dashed lines) is defi ned by the planar struc-
31、 tures. C: Planar structures (arrows) connect- ing dolomite micro-crystals and showing a fragmented and irregular granular-textured surface consisting of sub-100-nm-sized spheroidal bodies. All fresh surfaces. GEOLOGY, March 2007 209 of the organic matter in their syn-depositional mineralization (Tr
32、ichet et al., 2001). The micritic laminae are nonluminescent, whereas microsparry and sparry dolomite have a dull luminescence. This suggests little or no later alteration of the dolomite crystals compris- ing the stromatolitic laminae. In the Triassic rocks we studied, later dia- genetic processes
33、generated coarse, near- stoichiometric to Mg-rich fabric-destructive dolomite crystals that contrast with the fi ne crystalline texture of the stromatolitic laminae containing the fossilized bacterial and EPS remains composed of Ca-rich dolomite. It is thus likely that the mineralized organic remain
34、s, together with the smallest dolomite crystals, have retained their original composition. Microbial Dolomites The microbial dolomites discussed here formed in a peritidal, slightly hypersaline envi- ronment (Mastandrea et al., 2006). The 18O and 13 C values for the fi ne-grained dolomite (average 1
35、8O = 0.3 and 13C = 2.7 ) are con sistent with a marine origin, and so are inter- preted as original values. Normal-marine 13C values are not unusual for microbialites and are not an argument against a microbial origin (Andres et al., 2006). Upper Triassic marine cal- cite has a low negative 18O sign
36、ature (Veizer et al., 1997), so that, with the characteristic frac- tionation effect of dolomite over co-precipitating CaCO3, low positive 18O values for the dolo- mite are consistent with precipitation from sea- water. The 18O values also indicate that there has been little resetting of the isotope
37、 signatures through later recrystallization. The nonlumines- cent nature of the micritic dolomite supports this interpretation. Micron-sized Spheroidal Bodies The micron-sized spheroidal bodies are inter- preted as the fossilized remains of original syn- sedimentary bacterial forms, based on the fol
38、- lowing evidence: 1) Fragmented and whole spheroids are enclosed within dolomite crystals or have been crushed between crystals; these features clearly indicate their syn-sedimentary origin. Thus, later diagenetic origins are ruled out, as is con- tamination from modern soils or water, and poor pre
39、paration techniques. 2) The composition of the spheroids, a Ca-rich dolomite, is identical to that of the host micritic dolomite crystals. This also indicates very early dolomite precipitation and rules out a later dia- genetic origin, because later dolomite typically consists of much larger crystal
40、s, with a fabric- destructive mosaic, and a Mg-rich composition. 3) The spheroidal structures occur within even and uniform micritic laminae showing no evidence of erosion, borings, or subaerial expo- sure, during which time syn-sedimentary con- tamination could have occurred. 4) The spheroidal shap
41、e, uniform dimen- sions, and hollow structure exclude chemical precipitation of some mineral form, but sup- port a biological origin (Southam and Donald, 1999). The latter interpretation is supported also by the rarity of these structures, suggesting that they are the result of some particularly fav
42、orable fossilization process. The shape and dimension of the 1.0-m- sized spheroidal forms strongly resemble those of living coccoid bacteria that are present in modern microbial mat communities (Vascon- celos et al., 2006). The appearance of these spheroids, along with their likely paleoenvi- ronme
43、nt, suggests comparison with some mod- ern groups of sulfate-reducing bacteria, such as those illustrated in van Lith et al. (2003b). It is likely that the remarkable preservation of the bacterial bodies here is a result of their previ- ous transformation into spores, probably as a consequence of ad
44、verse environmental condi- tions. Spores have a hard protective coating and several layers of protective membrane that encase the bacterium. Thus, bacteria are able to survive high temperatures, pressures, and chemical attack for considerable periods of time (Vreeland et al., 2000). Planar Sheet-lik
45、e Structures The planar sheet-like structures can be inter- preted as mineralized extracellular polymeric substance (EPS). They are likely to have had an organic origin, because they do not have a crystalline shape; they crosscut the dolomite crystals and have a similar dolomite compo- sition to the
46、 bacterial relics. Moreover, their arrangement, which often envelops bacterial remains, closely resembles the sub-polygonal honeycomb structure of modern EPS (Dfarge et al., 1996) and mineralized Holocene EPS (Camoin et al., 1999). Nanospheres The nanospheres in the range of 150400 nm are interprete
47、d as fossilized dwarf bacteria- like life forms, similar to those described in Dupraz et al. (2004). Living dwarf bacteria are recognized as signifi cant components of stromatolite-producing microbial mat commu- nities (Stolz et al., 2001). However, the identi- fi cation and even existence of living
48、 and fossil subbacteria-sized (300 nm) microorganisms (known as nannobacteria, nanobacteria, or nanobes) remain controversial. Nevertheless, spheroidal mineralized objects 300 nm in size have often been interpreted as fossilized microbes in sulfi des, oxides, and clays, and particularly in microbial
49、 carbonates (Vascon- celos et al., 1995; Camoin et al., 1999; Folk, 1999; Sprachta et al., 2001). Moreover, liv- ing nanobacteria, in the range of 100300 nm, have been found in several organic tissues, from which they have been isolated and cul- tured (Sommer et al., 2004). An additional bio- logical explanation for the nanospheres is that they could be interpreted as mineralized ultra- microbacteria (Southam and Donald, 1999). These are dwarf bacterial living forms that include several types of bacteria with the abil- ity to reduce their cell volume