Australia: The Land Where Time Began

A biography of the Australian continent 

Ancient Ecosystem in the Dresser Formation Dated to About 3.48 Ga in the Pilbara, Western Australia

The response of microbial mats to physical sediments dynamics result in microbially induced sedimentary structures (MISS). These MISS are cosmopolitan, being found in many modern environments, that include shelves, tidal flats, lagoons, riverine shores, lakes, interdune areas, and sabkhas. Communities of microbial mats, that are highly diverse, are recorded by these structures and have been reported from many intervals in the geological record dating up to 3.2 Ga. A suite of MISS from some of the oldest sedimentary rocks in the geological record, the Early Archaean, about 3.48 Ga Dresser Formation, Western Australia, that are well preserved, is described by this contribution. Mapping of outcrops at scales of meter to mm defined 5 sub-environmental characteristics of an ancient coastal Sabkha. Associations of distinct macroscopic and microscopic MISS are contained by these sub-environments. Polygonal oscillation cracks and gas domes, erosional remnants and pockets, and mat chips are included in macroscopic MISS. Microscopic MISS is comprised of tufts, sinoidal structures, and laminae fabrics; the primary carbonaceous matter, pyrite, and haematite, plus trapped and bound grains, comprise the macroscopic laminae. Throughout the entire subsequent history of the Earth to the present there are identical suites of MISS. The geological record of MISS is extended by almost 300 My by this study. It is likely that microbial communities that form mats existed almost 3.5 Ga.

Sedimentary structures that are induced microbially and the early record of life on Earth

The reconstruction of the most ancient biota is challenging because of the sparse fossil record of the earliest life on Earth. Stromatolites are predominantly the basis for current interpretations of the diversity of the earlies life on Earth (e.g. Lowe, 1980; Walter et al., 1980; Byerly et al, 1986; Hoffman et al., 1999; Allwood et al., 2006, 2007, 2009, 2010; Hickman, 2012), organic microfossils of prokaryotes and biofilms (e.g. Awramik et al., 1983; Walsh & Lowe, 1985, 1999; Walsh, 1992; Hofmann, 2004; Schopf et al., 2007; Schopf & Bottjer, 2009; Sugitani et al., 2010, Wacey et al., 2011, 2012; Hickman, 2012) and carbon sulphur isotopic signatures (e.g. Shen et al., 2001, 2009; Ueno et al., 2006, 2008; Wacey et al., 2010). Microbially induced sedimentary structures (MISS) have provided another way to decode life in ancient sediments (review in Noffke, 2010).

Sedimentary structures that are microbially induced are formed by the microbial mats that colonise most aquatic environments, including tidal flats, lagoons, riverine shores, lakes, dune fields, and sabkhas (e.g. Gerdes & Krumbein, 1987; volume by Hagadorn et al., 1999; Eriksson et al., 2000; Prave, 2002; volume by Schieber et al., 2007; Beraldi-Campesi et al., 2009; volume by Noffke, 2009, Noffke, 2010; volume by Noffke & Chafetz, 2012). MISS arise from the response of microbiota exclusively to physical sediment dynamics. Within the organic matrix of the microbial mat the synergetic mineral production which was provided by the extracellular polymeric substances (EPS) are seen in stromatolites (e.g., Reid et al., 2000; Dupraz et al., 2009; Decho et al., 2011) does not commonly take place (Noffke & Awramik, 2013). Mineral precipitates do occur occasionally but they are of a temporary nature (e.g., Kremer et al., 2008). There are 2 steps involved in the formation of MISS:

1)    First, the primary shaping by the dynamics of physical sediment and,

2)    The diagenetic mineralisation of organic material.

(See Noffke, 2010, for details).

The microbial mats that form MISS react to sediment dynamics (erosion, deposition & latency) in 4 various ways (quantification of primary processes in Noffke & Krumbein, 1999; Noffke, 1999).

i)       Water currents passing over the sedimentary surface overgrown by mat cause erosive stress that triggers biostabilisation (Patterson et al., 1994). The microbial filaments arrange parallel to the depositional surface if they are affected by erosive stress; the sediment grains are interwoven by the network of filaments, and in a fraction of a second the EPS switch their biomolecular structure to be flexible and ductile (e.g. Stoodley et al., 2002). The removal of sediment gains by currents is prohibited by these microbial effects. The resistance of a sedimentary surface is increased by up to 12 magnitudes (epibenthic microbial mats), and around 0.02 magnitudes (biofilm-type overgrowth) (Noffke & Krumbein, 1999).

ii)    The filaments rearrange and orient themselves perpendicular to the sediment-mat surface if a microbial mat is exposed to sediment deposition. Microzones of turbulent disturbance of the water current are caused when they reach into the supernatant water. This effect, known as “baffling”, induces the fall-out of sediment grains that are suspended in the water. The grains are then built into the matrix of the mat, either by the migration of the filaments around them or by gluing the grains together by the sticky EPS and fixing them in their positions. Baffling and trapping are processes of active sediment accumulation. Depending on the type of mat, grain size selection or enrichment of heavy minerals either takes place or not.

iii)  The initial organisation of microbial cells that are distributed randomly and trichomes into a biofilm community that is highly structured is binding. There is communication among the microbes and they move into a positon within the sediment that allows both optimal access to light and/or nutrients and cooperative interaction with the metabolism of neighbouring microorganisms. This active arrangement occurs at times of quiet sediment dynamic conditions.

iv)  Growth is a process that is dependent on the availability of nutrients and/or light and not on the dynamic conditions of the sediment, and does not have a role in binding; it is biomass enrichment by the replication of cells and the production of EPS.

MISS show different morphologies from those of stromatolites as a result of their different modes of genesis (Noffke & Awramik, 2013). There are 17 main types of MISS that range on scales from cm² to km² that have been identified to date and their genesis and morphologies that resulted have been quantified (Noffke, 2010; see also volumes that have been edited by Hagadorn et al., 1999; Schieber et al., 2007; Noffke & Paterson, 2008; Noffke & Chafetz, 2012).

MISS are preserved by secondary processes while MISS are formed by these primary processes. MISS are lithified by rapid in situ mineralisation of the organic matter of the mats, including the cells of the microbes, trichomes, and filaments, as well as the EPS. The network of fossil microbes is commonly visible as laminae that are intertwined and bent in thin sections that are perpendicular to the layers of ancient mats. Included in the network are fossil EPS and (formerly) allochthonous sedimentary grains that were once bound by the mat during its lifetime. The appearance of fossil mat texture in MISS, however, differs fundamentally from that or organic microfossils in chert. This difference is a result of the preservation of the MISS. When mats in sedimentary siliciclastic rocks are preserved by the replacement of minerals, and not by impregnation, as occurs with organic microfossils or organic biofilms in primary chert (e.g. Walsh, 1992; Walsh & Lowe, 1999; Tice & Lowe, 2006; Schopf & Bottjer, 2009; Wacey, 2009). The mat laminae that form the texture in MISS have no discreet outlet, which contrasts with such organic material that is preserved so precisely in chert (Noffke, 2000; Noffke et al., 2002, 2003, 2006a, 2006b, 2008). The laminae appear “cloudy” with diffuse borders in MISS. According to Noffke et al. the explanation for this appearance is that during the diagenetic alteration of the mat the chemical compounds of the organic matter that is released by microbial decomposition diffuse away from their original microsite (e.g. Krumbein et al 1979; Knoll et al., 1988; Beveridge, 1989; Urrutia & Beveridge, 1994; Konhauser, et al., 1994; Schulze-Lam et al., 1996; review on these studies in Noffke, 2010). They react with chemical compounds that prevail in the surrounding water, and these result in initial “amorphous” mineral precipitates. These precipitates that are initially hydrated accumulate at nucleation sites where they gradually dehydrate and shrink over the course of diagenesis. Mineral phases of higher crystallinity result from the dehydration and crystallisation. On the microscopic scale, the minerals that replace microbial mats are arranged, however, as irregular clots that line the original shape of the laminae. This contrasts starkly with microfossils preserved in chert, where synsedimentary silicification and impregnation rapidly entombed organic matter, and detailed preservation of cells was possible (e.g Cady & Farmer, 1996). For this reason, analytical techniques of very high spatial resolution such as transmission electron microscopy (TEM) or secondary ion mass spectrometry (SIMS and Nano-SIME) are not particularly useful for resolving morphological characteristics of microbial cells that form MISS. The terms “laminae” and “filament-like texture” rather than “filament” or “trichome” are used in the MISS literature in order to acknowledge these differences in preservation. In the fossilised examples dating to the Archaean, Proterozoic, and Phanerozoic time periods, the fabrics of ancient mats may still include some original carbon, though pyrite, haematite, and goethite have largely replaced the original organic matter (review in Noffke, 2010).

A set of 7 biogenicity criteria for MISS has been developed and tested in many comparative studies (overview in Noffke, 2010). The first 4 criteria describe the depositional habitat of the occurrence of MISS as follows.

MISS occur:

i)                   in rocks of not more than low grade metamorphosis

ii)                at turning points regression-transgression, and

iii)              in the distributional “microbial mat facies.”

The average hydraulic pattern in the depositional area is reflected in the distribution pattern of MISS. The last 3 criteria describe the MISS themselves as follows. The fossil MISS:

1)    Exhibit a strong resemblance to, or are identical with, geometries and dimensions to modern ones.

2)    Included in the MISS are macrotextures that represent, or were caused by, or are related to, ancient biofilms and microbial mats.

3)    The interpretation as MISS is supported by geochemical analyses.

More than 14 studies have explored systematically MISS from sties dating from ancient to modern, using this set of criteria, to compare structures in equivalent environmental settings from modern all the way back to the early Archaean (e.g. Noffke, 1999; 2000; Noffke & Krumbein, 1999; Noffke et al., 2001a, 2002, 2003, 2006a, 2006b, 2008). A dataset has been assembled by this suite of studies that enables the evolution of MISS-prokaryota to be monitored throughout the geological record of the Earth. The investigation of MISS is divided into 4 steps:

i)                   Detection, the visual reconnaissance during a geological survey of candidate sediments and sedimentary rocks.

ii)                Identification, a candidate structure (e.g. for erosional remnants and pockets) is measured with respect to geometry and dimension, as well as indices such as the MOD-I (modification index) are determined (see later in this article). The assembled data produced by these systematic studies now allow the quantitative comparison of any candidate structure with other MISS dating to other times in the history of the Earth, including modern times.

iii)              Confirmation, analyses on the mineralogy and geochemistry of the candidate structure are conducted (e.g. presence of carbon in laminae).

iv)              Differentiation (Noffke, 2010) A comparison is made with similar though abiotic phenomena, if there are any similar, abiotic phenomena existing.

As has been demonstrated, various lines of evidence have been compiled by this set of biogenicity criteria and they allow for the identification of fossil structures with a high probability. This methodological approach was also used for the study of possible MISS in the Dresser Formation.

Sedimentary structures that were microbially induced are listed for 1 target for the Mars Exploration Rover Program (Committee on an Astrobiology Strategy for the Exploration of Mars, 2997). Sabkha settings are well known on Mars, where the former existence of fluid water is recorded in sedimentary surface structures and rock beds (e.g. Grotzinger et al., 2005; Metz et al., 2009). The knowledge of the sabkha habitats of MISS and the criteria of biogenicity of MISS, as a result of the early history of Mars and the criteria and biogenicity of Earth may assist in the exploration of Mars.

The oldest mat communities that have formed MISS to date are from the Moodies Group, South Africa, that date to 3.2 Ga (Noffke et al., 2006b; Heubeck, 2009). In this study 2 wrinkle structures and 1 roll-up structure were detected in tidal deposits. The Ntombe Formation, Pongola Supergroup, South Africa, that dates to 2.9 Ga, includes 8 wrinkle structures, each of which records fossil microbial mats on shallow shelf settings (Noffke et al., 2003). There are 28 wrinkle structures, 2 sedimentary surfaces that yielded erosional remnants and pockets, and 1 bedding plane that has polygonal oscillation cracks, that are shown by the isochronous shelf of the Brixton Formation of the Witwatersrand Supergroup. Textures are visible resembling degraded microbial mat fabrics in thin sections from all study sites. It has been suggested that cyanobacteria were the constructing agents; though there is no unambiguous evidence that has been documented (see discussions in Noffke et al., 2003, 2006a, 2006b). MISS in the Pongola Supergroup in South Africa record highly diverse microbial mat ecosystems from ancient tidal and sabkha environments (Noffke et al., 2003, 2008). It is of significance that these ancient MISS strongly resemble the MISS in modern settings that are equivalent. As a result of quantification of the hydraulic and sediment-dynamic interaction with biofilms being made possible by modern MISS, the information that has been gained assists in the interpretation of ancient MISS, the environment they are situated in, and the evolution of prokaryotes. The similarity of MISS over 3 Gyr of the history of the Earth contrasts with the situation in stromatolites, where many of the early Archaean species differ from the modern ones (Noffke & Awramik, 2013).

The objective of this study was to advance the comparative investigation a step further and search for MISS in rocks older than 3.2 Ga. A series of distinct types of macrostructures and microstructures from sedimentary rocks dating to about 3.48 Ga of the Dresser Formation is described by this contribution.

The Dresser Formation, Pilbara, Western Australia

The Dresser Formation is in the East Pilbara granite greenstone terrane, Western Australia. The Dresser Formation was chosen for this study because it contains some of the oldest and best-preserved volcanic and sedimentary rocks in the world (Barley et al., 1979; Hickman, 2012). It was determined that the age of this rock succession is 3.481 ± 3.5 Ga [Australian Stratigraphic Units Database (2012), Dresser Formation, Stratigraphic Number 36957]. The formation is restricted geographically to an area of about 25 km² of the North Pole Dome and is comprised of bedded chert, carbonate, and siliciclastics, as well as pillow basalts and dolerite (Van Kranendonk et al., 2008). Originally, the sedimentary rocks were micritic carbonates and evaporates that were deposited beneath shallow water, low energy (sabkha-type) conditions, that were interbedded with sandstone and conglomerate that had been deposited at times of growth faulting and tectonic activity (Lambert et al., 1978; Buick & Dunlop, 1990; Van Kranendonk, 2006; Van Kranendonk et al., 2008). Circulation of hydrothermal fluids that overprinted much of the original sedimentary mineralogy was permitted by repeated episodes of growth faulting that was associated with volcanic activity at times of carbonate-evaporite sedimentation. Pyrite, haematite, barite, and silica largely replaced gypsum (Van Kranendonk, 2006; Van Kranendonk et al., 2008).

Weathering is common in the Gibson Desert and the Great Victoria Desert in central Australia and in the Carnarvon Basin at the west coast in the Eocene to Oligocene (van der Graff, 1983). In the otherwise unconsolidated soils typical structures include irregular cones that are steep sided up to 35 cm in height, pisolites from 0.2 to 5 cm in diameter, and karst pipes that facilitate flow of subsurface water. However, in the North Pole study, none of these structures were observed. Instead, in the Dresser Formation the sedimentary rocks are highly consolidated. Stromatolites, ripple marks, ripple cross beds, as well as other primary sedimentary structures, that are well preserved, and do not display disturbance by any deep soil or silcrete formation.

Widespread evidence also remains for the primary mineralogy of the ancient sediments, as well as the macroscopic phenomena, including relic carbonate rhombs and rhombic voids in silica (Lambert et al., 1978), patches of dolomite chert in surface outcrop (Walter et al., 1980), peloidal and oncolytic grains (Buick & Dunlop, 1990), extensive carbonate in unweathered drill core material (Van Kranendonk et al., 2008), and observations in the current study of carbonate in many of our thin sections.

Sedimentary structures in ancient coastal sabkha settings of the Dresser Formation and discussion of their possible biogenicity

The subtidal zone

Wave ripple marks with crest to crest amplitudes of 8 cm, and occasional cross stratification on a small scale of climbing ripples records an ancient subtidal area. Ripple cross stratification lined by dark laminae were included in 2 rock beds. The slopes and valleys of ripple marks are draped by slightly crinkled, dark-coloured laminae were that observed in vertical thin sections. The laminae are spotted by tufts in close-up, that all have a similar height/base ratio of 10/50 to 25/75 μm that are arranged at regular distances of 100-125 μm from each other. The crinkled laminae and tufts were shown by Raman analysis to be mostly composed of pyrite plus small amounts of carbonaceous material within a matrix rich in silica. According to Noffke et al. this composition is consistent with syndepositional replacement of carbonaceous laminae by pyrite and later replacement of carbonate by silica.

Interpretation

Similar ripple structures and biofilm textures are known from modern subtidal zones. The structures, that are visible in vertical section in cores of fresh sediment, are known as “sinoidal structures” (Noffke et al., 2001b). Sinoidal structures are ripple marks that have been overgrown by microbial mats with the result that the ripple mark relief looks smoothed. The valleys of the ripple are filled in by the laminae of microbial mats, often alternating with sediment layers. The ripple valleys with their sediment infills that are biofilm covered are evident in the fossil example of the Dresser Formation. A cover of biofilm biostabilised the ripple slope before the sediment was deposited in the ripple valleys. The valley sediments were deposited in increments, interrupted by periods of no sedimentation during which biofilms would develop on the infill of the ripple valley (Gerdes & Krumbein, 1987). The proceeding biofilm was not eroded during the subsequent sediment deposition. According to Noffke et al. this preservation is observable along the slope that is not affected of the ripple mark, as well as along the individual surfaces that are covered by mat of the infilling sediment. The deposition of subsequent sediment would not have been resisted by mud layers and would also not have included tufts. The arrangement of tufts, as well as their sizes, is too regular to be a consequence of fine sediment that would have been pushed up locally by sediment grains that projected from the surface of the sediment. Also, any interpretation as being of abiotic sedimentary origin is contradicted by the mineralogical composition of the dark laminae.

Also included in the subtidal zone of the Dresser Formation are stromatolites, which earlier studies described (e.g., Buick & Dunlop, 1990; Van Kranendonk, 2006; Van Kranendonk et al., 2008). 

The intertidal zone

Description

Daily micro-tides typically cause a narrow intertidal belt along a coast. Alternating bedding of coarse-fine layer couplets, record such tidal currents. A total outcrop of between 10 and 40 cm thick is made by couplets that are stacked together. There are 12 Bedding surfaces that are exposed well, that range from 10 cm² to 6 m² in size. Fragments of sediment 1-3.5 cm in diameter and up 0.4cm thick that have a distinct appearance compared to surrounding rock are littered on these ancient intertidal surfaces; some of these fragments  demonstrate flexible behaviour and appear to be rolled up.

Interpretation

It is suggested by Noffke et al. that such fragments on a centimetre scale may be interpreted as fragments of microbial mat (“chips”), which had been removed from their parent site, then transported, and finally deposited on the sedimentary surface (Noffke, 2010). Such chips from microbial mats frequently piled up in the current shadow behind current barriers. It is considered that some chips may role up as a result of currents or desiccation. There is a characteristic shape for microbial mat chips, which can be quantified by the morphology index of the chip from the microbial mat. This is described as the ratio between the greatest diameter of the microbial mat chip and its smallest diameter. For the microbial mat chips from the Dresser Formation (1.81; n = 41) compares closely to those of mat chips from the Pongola Supergroup, which dates to 2.9 Ga, (1.72; n = 55), and modern mat chips from Portsmouth Island, USA (1.75; n = 55), though simple mud clasts from Portsmouth Island have a much lower index of 1.41 (n = 50).

The lower supratidal zone

Description

A lower supratidal zone that is occasionally flooded during landwards storms record wash-over fans with internal lamination. In this study, 4 80 cm² to 1.30 m² sedimentary surfaces were found which displayed a peculiar surface morphology: the bedding surfaces are arranged into elevated portions of surface and deeper surfaces portions. According to Noffke et al. crinkled surface portions may occur. The topography change of the elevated surface areas compared to those of the deeper areas range from 1 to 3 cm; the slope angles connecting the elevated areas to the deeper surface areas are between 15 and 90^o.

Fragments were often observed in the depressed surface areas, which had sizes and shapes similar to those that had been described for the intertidal zone (above). Dark laminae forming a carpetlike network entangling grains of sand size are shown by thin sections of the fragments from this lower supratidal zone. The laminae appear diffuse, with n discrete outline being preserved. Therefore, it is difficult to determine the thickness of an individual filament. Noffke et al. estimated that filamentlike textures range from 5 to 20 μm in diameter. It is shown by Raman analysis that the filaments are composed of haematite and carbonaceous material that is finely clotted.

Interpretation

A large area of sabkha surface is occupied by the lower supratidal zone in modern coastal systems. In this study, a very typical sedimentary relief surface relief erosional remnants and pockets; 2 geometrical elements represent the relief, as follows:

i)                   Surface areas with flat tops that are overgrown and stabilised by microbial mat, and

ii)                Surface areas that are deeper lying where the sediment is exposed (Noffke, 1999, 2010).

Erosional remnants and pockets each have a size range of between 10s of cm² to many m², with the relief morphology being stabilised by the degree of biostabilisation by the microbial mat covering the remnants (Noffke & Krumbein, 1999) – the more pronounced the relief is, the higher was the degree of stabilisation by the mats and the higher the erosive force applied by the currents. In shaping the sedimentary surface (N) the degree of biostabilisation is expressed in the “modification index” (MOD-I) [MOD-I = I_A x I_S x I_N] (Noffke & Krumbein, 1999). The MOD-I for the description of erosional remnants and pockets is based on 3 subindices:

i)                   Area of depositional surface covered by mat to the total area of investigation (I_A­­ = A_m/A_i),

ii)                The angle of slopes of the erosional remnants I_S = sin α, and

iii)              The degree of planarity of the microbial mat cover (I_N = 1 – [(H_p – H_b)/H_pI).

Mat growth and baffling and trapping of grains results in planarity. No microbial influence in the formation of surface relief would be represented by MOD-I of 0, while a value of 1 would represent a maximum influence.

The erosional remnants and pockets that were described above are comparable directly to the eroded bedding surfaces that are eroded differentially than seen in the lower supratidal zone of the Dresser Formation; therefore the Dresser Formation can be interpreted as such MISS. The MOD-I of the 4 fossil erosional remnant and pocket-bearing sedimentary surfaces (0.18, 0.24, and 0.3) compare closely to similar erosional remnants and pockets from the Pongola Supergroup (0.35) and from modern settings of Mellum Island, Germany (0.25 and 0.3). Strong evidence for biological control is provided by this comparison, as sediment does not show such steep surface relief, and MOD-I approaches 0, if sediment is not consolidated and stabilised by biology. The erosional remnant and pocket relief will dissolve in areas where microbial mats diminish by the end of the growth season (Noffke & Krumbein, 1999). Therefore, it is known that abiotic erosional remnants and pockets exist. According to Noffke et al. it is likely that sediment grains in a network of laminae indicate syndepositional trapping and binding of the grains by the microbial mat. Modern examples, as well as younger Archaean examples, of microbial mat textures also show such a matrix.

The upper supratidal zone

Description

There are 4 well preserved bedding surfaces, 90 cm² to 3.6 m² in areal extent, that display patterns of polygonal cracks. Several perennial shallow ponds within an ancient supratidal zone that underwent periods of seasonal desiccation are recorded in the cracks. Cracks that are 3-10 cm wide separated the polygons from each other. There is a hole that is close to the centre of many polygons.

The surfaces of the rock beds also display fine, reticulate “honeycomb” patterns that have about 1 mm high ridges up to about ⁓4 mm high tufts, at 2 of these sites in the Dresser Formation. Several generations of honeycomb-like cm-scale compartments characterise the reticulate pattern. Each subsequent generation of compartments is smaller than the previous. The compartments have a maximum dimension ratio of approximately 1:2 and a surface area ration of 1:4.

Interpretation

Seasonal tidal ponds develop in modern coastal sabkhas that may reach up to 15 cm in depth. Epibenthic mats grow in these ponds. According to Noffke et al., such structures are directly comparable to the cracks and polygons with central holes and marginal ridges that are observed in the upper supratidal zone of the Dresser Formation, therefore the Dresser structures can be interpreted as microbially induced polygonal oscillation cracks and gas domes. Polygon shaped patches of microbial mat that are separated from each other are known as polygonal oscillation cracks. The surface of microbial mats crack into patches that are polygon shaped as it dries out, these patches being up to 50 cm in diameter, in seasons when aridity is high (Noffke, 2010; Carmona et al., 2011). Cracks up to 10 cm wide separate each polygon from neighbouring polygonal patches. When the humidity and/or rainfall in the following season returns the patches of mat expand which closes the gaps that resulted from the cracking, when there is sometimes overgrowth of neighbouring patches by a new layer of mat, though the polygons of mat remain, however, clearly visible. There may be slight thickening of margins of the individual polygons of mat as a result of the shrinking and expanding of the polygons of mat (hence oscillating) over time. The margins of the polygons tend to curl up as a result of this oscillation. The production of gas by the colonising microorganisms in the deeper portions of the microbial mat may cause the centre of each polygon to bend up. As a result of the increasing pressure these gas domes erupt at some point. The roof of the gas dome collapses as the gas is released. The hole that results from this eruption remains visible in the microbial mat. Other exceptionally well preserved fossil examples of such polygonal oscillation cracks are present in the Pongola Supergroup in South Africa, which dates to 2.9 Ga (Noffke et al., 2008).

Such structures are comparable directly to cracks and polygons with central holes and marginal ridges that are observable in the supratidal zone of the Dresser Formation; Therefore, according to Noffke et al. the Dresser structures can be interpreted as polygonal oscillation cracks and gas domes that were induced by microorganisms. In the rocks of the Dresser Formation some polygons that have been exposed display circular patterns of wrinkled folds, which suggests a matrix that was formerly ductile, was present, a microbial mat. It has been found that the frequency distribution of the diameters of polygons divided by the diameter of gas escape holes of the Dresser Formation match those from younger fossil and modern examples. The relation of polygon diameters to gas escape hole diameters resemble all examples that are fossil and modern. (I.e., all ductile material (microbial mats) reacted in the same manner.

A reticulate pattern of ridges and tufts is shown at the surface of the modern microbial mats in ponds. The surface of the mat appears from above to be covered by a net of “cells” that are honeycomb-shaped and that are on a centimetre scale in diameter. Ridges and tufts up to 3 mm in height define the compartments. Such a compartment pattern is a consequence of active arrangement of microorganism filaments (Shepard & Sumner, 2010). Communication (signal transfer) is possibly aided by this arrangement according to Noffke et al. within the microbial mat (Stoodley et al., 2002; Noffke et al., 2013). In the Dresser Formation there are closely comparable tufted and honeycomb patterns. They are believed to represent similar arrangement of filaments in ancient microbial mats. If a circular expression of the microbial compartments is assumed, the surface area ratio between 3 generations of compartments is approximately 4:1 for each of the 3 modal peaks across all examples that were studied.

Lagoon

A stack up to 2.80 m thick of black-white coloured, laminated beds formed the top stratigraphic section that is recorded in the Dresser Formation. Noffke et al., consider it likely that this rock unit records the flooding of the sabkha and the reestablishment of a lagoon. The gentle currents in the lagoon would be represented by the planar lamination. Fragments of 1 mm to 3.5 cm in size were found within the laminated stack. That the material of the fragments was originally soft and ductile is documented by the fragments being flat, wavy, or rolled up. The fragments have the same composition as the laminated host rock. They are composed of dark-coloured goethite plus carbon that alternate with laminae of translucent quartz layers. It is indicated by the carbon Raman signal that there was a synergetic origin for the carbon.

Interpretation

Biolaminites are often present in modern lagoons (Gerdes & Krumbein, 1987). These structures are comprised of stacks of microbial mats, which are sometimes intercalated by sediment laminae that are visible in vertical section through thick microbial mats. When developing at sites where there has been long periods of quiet sedimentary conditions, such as in deepening lagoons or coastal sabkha lakes such as Solar Lake, Res Sea, which are the best examples (Gerdes & Krumbein, 1987). Such Biolaminites, or if fossils are present, Stratifera, may be the origin of thick laminated rock beds that are present in the Dresser Formation. It is likely that the fossil fragments present in these rock beds are microbial mat chips. The wavy appearance documents the ductile nature of the organic layers. There is a single fragment that is rolled up, that Noffke et al. suggest is probably a result of the ductile response to bottom currents. Hydrothermal overprint alone is not likely to have caused a laminated pattern of rock, as chips and roll-ups of the same composition in between the laminae have been preserved. Also, it is not likely that the migration path of fluids would be in such a regular pattern of planes within the rock, let alone exclusively in this portion of the stratigraphic profile, even if hydrothermal water circulated through the rock.

Barrier shoal

It appears that the zones described above could have been sheltered by barrier shoals composed on oncoids until eventual inundation by the ocean.

Ancient microbial mat-forming biota of the Dresser Formation

A great consistency of MISS over geological time was shown by a series of studies that compared systematically modern MISS with ancient MISS (Noffke, 2000, 2010; Noffke et al., 2001a, 2002, 2003, 2006a, 2006b, 2008). Diverse microbial mat ecosystems, as early as the Mesozoic, are recorded by the MISS assemblages, as has been demonstrated by Noffke et al., (2001, 2008) and (Noffke, 2010).

The sedimentary structures from the Dresser Formation that have been described here have been interpreted as MISS based on several lines of evidence.

1)    They have morphologies that are very similar to those of various fossils that are more recent and modern MISS. Noffke et al. based their interpretation of these MISS examples, that have been studied extensively, qualitative morphological characteristics and on numerical data that quantify the morphologies of the structures, which allows quantitative comparison among MISS, as well as between MISS and non-biological sedimentary structures. Transient forms between MISS and surrounding sedimentary features are not, in general, observed, neither in fossil nor modern sequences; i.e., all MISS are distinct. Within 1 morphotype there are variations of MISS that occur and are considered by quantitative data; transitional forms which display morphological characteristics that are intermediate do not, however, exist between MISS and any abiotic types of sedimentary structure.

2)    The close association of sedimentary structures that were microbiologically induced in the Dresser Formation provide an important second line of evidence. The same associations that are displayed by modern as well as by fossil sabkhas are displayed by these MISS.

3)    Microscopic biotextures that strongly resemble the textures that are known from the younger (Archaean and Proterozoic) fossil record of MISS are included in many of these sedimentary structures.

4)    According to Noffke et al. geochemical and petrological analyses are consistent with the typical mineral associations that are found in fossil microbial mats from other Archaean sites that have been described previously. Carbon in particular is bound to the microtextures. The interpretation of biogenicity of MISS in the Dresser Formation is supported by the 4 complementary lines of evidence, individually and collectively.

Weathering of rock surfaces that occurred during the Eocene (Lower Tertiary) has been described in detail from other parts of Australia (van der Graff, 1983). Noffke et al. ask the question is it possible that surface weathering might mimic some of the MISS features that have been proposed for the Dresser Formation? Unconsolidated soils were produced by weathering in the Tertiary, that include cones that are irregularly shaped and steep-sloped, pisolites of pebble size, and vertical karst pipes. Noffke et al. found none of these phenomena in the stratigraphic sections they studied in the Pilbara. The morphologies of the weathering structures, moreover, differ significantly from those of the sedimentary structures that are present in the Dresser Formation that were described as MISS. Also, the distribution of the MISS in the Dresser Formation correlates with specific sections of the stratigraphic profiles. According to Noffke et al. they are not related to the modern morphological topography of the study area, which is also the case with the weathering structures in other parts of Australia (van der Graff, 1983). Also, the MISS form specific associations related to adjacent tidal zones differ significantly from any weathering or erosive origin, as does the identification of carbon that is thermally mature that is intimately associated with many of the Dresser Structures. Finally, the question would be why identical MISS and MISS associations are present in modern environments that, obviously, have not been subjected to weathering in the Eocene, if the sedimentary structures that were interpreted by Noffke et al. as MISS were actually of weathering origin in the Eocene.

It was concluded by Noffke et al. that a complex system of microbial mats were recorded by the Dresser Formation. The question is then which prokaryotes might have formed these microbial mats in the Early Archaean. Noffke et al. underlined that modern MISS and microbial mats that formed MISS may only serve as analogue models for the examples from the Dresser Formation. The genetic information of individual microbial groups such as cyanobacteria of the present is highly variable, which differs at the present even within metres of the same setting. Any conclusions on the existence of certain groups in the fossil record, let alone in the fossil record at 3.48 Ga, must therefore be speculative. Also, microorganisms are found in biofilms, not as individual cells or groups. A biofilm is a microbial community that has grown attached to a solid substrate, in which all the members of the community interact in such a way that they foster the harvest of light and nutrients, EPS production, etc. (Stoodley et al., 2002; Noffke et al., 2013). It can therefore be stated with confidence that the fossil MISS are the  expression of biofilms that were formed by microbes interacting in a similar manner with the shallow, photic zone sedimentary habitat as do younger fossil and modern microbial mats (Noffke, 2010). The main message of the MISS in the Dresser Formation is that microbenthos existed and had the ability to construct coherent, carpet-like microbial mats. The mats were capable of withstanding erosion, respond to deposition, and to withstand semi-arid climate conditions.

Noffke et al. concluded, by using modern MISS and microbial mats strictly as models, which the ancient microbial mats, which formed MISS, in the Dresser Formation were dominated by microbes that mimicked the behaviour of cyanobacteria of the present. According to Noffke et al. it is important to note that the cyanobacteria are 1 major group of microbes that are capable of producing the large amounts of EPS that are necessary to make possible high biostabilisation effects (Linda L. Jahnke, frdl. Pers. Comm. 2013; Paterson et al., 1994; Noffke & Paterson, 2008; Noffke, 2010). Such high amounts of EPS are recorded by wrinkle structures that are non-transparent (Noffke et al., 2002). The presence of cyanobacteria that were already in the Pongola Supergroup, South Africa, is suggested by detailed studies on the interaction of stromatolites (Beukes & Lowe, 1989) and of MISS (Noffke et al., 2008) with their hydraulically affected sedimentary environment. It is known that the cyanobacteria are the first organisms to produce oxygen in the fossil record. It is, however, important to bear in mind that there are 2 types of photosynthesis, oxygenic and anoxygenic. The existence of early cyanobacteria of anoxygenic photosynthesis is not excluded, though the abundant evidence indicating an anoxic atmosphere in the Early Archaean (Farquhar et al., 2000; Hazen et al., 2008; Sverjensky & Lee, 2010) suggests oxygenic photosynthesis was not established until possibly the Neoarchaean Era. If the stromatolite-forming or MISS–forming microbial mats in the Pongola Supergroup were indeed cyanobacteria that had the capacity to produce oxygen, then it would support the latest findings of palaeosols in the Pongola Supergroup that indicate an atmosphere that was rich in oxygen around that time (Crowe et al., 2013). It was noted by Noffke et al. that with regard to the microbiota of the Dresser Formation, that while there are a number of filamentous cyanobacteria that are capable of anoxygenic photosynthesis, by making use of H₂S in the place of H₂O as the electron donor, Chloroflexus or sulphur-oxidising or iron-oxidising bacteria, such as Beggiatoa would also be able to use this pathway (e.g. Bailey et al., 2009). Both groups are capable of forming substantial microbial mats (e.g. Bailey et al., 2009). When all these thoughts are drawn together a conservative interpretation of ancient microbenthos from the Dresser Formation would be that the biofilms (microbial mats) of the Dresser sabkha behaved in a similar manner to the communities of modern microbenthic biofilms that are present in sabkha settings at the present.

Summary

In this paper the sedimentary structures that are preserved in the coastal sabkha palaeoenvironment of the Dresser Formation, which dates to about 3.48 Ga, are interpreted as MISS. Assemblages of the MISS form, that are shown to be typical for sub-environments of sabkhas through geological time. By using modern MISS in the equivalent sabkha settings as analogue models, Noffke et al. concluded that the MISS in the Dresser Formation have recorded a complex microbial ecosystem, which was previously unknown, and represent one of the most ancient signs of life on Earth.

Sources & Further reading

1. Noffke, Nora, N., et al. (2013). "Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ca. 3.48 Billion-Year-Old Dresser Formation, Pilbara, Western Australia." Astrobiology 13(12): 1103-1124.