Controls on Development and Diversity of Early Archean Stromatolites

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Controls on development and diversityof Early Archean stromatolitesAbigail C. Allwooda,b,1, John P. Grotzingerc,1, Andrew H. Knolld, Ian W. Burchb, Mark S. Andersona, Max L. Colemana,and Isik Kanika

aNational Aeronautics and Space Administration Astrobiology Institute at the Jet Propulsion Laboratory, California Institute of Technology, Pasadena,

CA 91109;b

Australian Centre for Astrobiology, University of New South Wales, Sydney 2052, Australia;c

Department of Geological and PlanetarySciences, California Institute of Technology, Pasadena, CA 91125; and dDepartment of Organismic and Evolutionary Biology, Harvard University,Cambridge, MA 02138

This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2002.

Contributed by John P. Grotzinger, April 14, 2009 (sent for review February 6, 2009)

The 3,450-million-year-old Strelley Pool Formation in WesternAustralia contains a reef-like assembly of laminated sedimentaryaccretion structures (stromatolites) that have macroscale charac-teristics suggestive of biological influence. However, direct mi-

croscale evidence of biologynamely, organic microbial remainsor biosedimentary fabricshas to date eluded discovery in theextensively-recrystallized rocks. Recently-identified outcrops withrelatively goodtextural preservation record microscale evidence of

primary sedimentary processes, including some that indicate prob-able microbial mat formation. Furthermore, we find relict fabricsand organic layers that covary with stromatolite morphology,linking morphologic diversity to changes in sedimentation, sea-

floor mineral precipitation, and inferred microbial mat develop-ment. Thus, the most direct andcompellingsignatures of life in theStrelley Pool Formation are those observed at the microscopic

scale. By examining spatiotemporal changes in microscale charac-teristics it is possible not only to recognize the presence ofprobable microbial mats during stromatolite development, butalso to infer aspects of the biological inputs to stromatolite

morphogenesis. The persistence of an inferred biological signalthrough changing environmental circumstances and stromatolitetypes indicates that benthic microbial populations adapted toshifting environmental conditions in early oceans.

microbe paleontology biosignature carbonate reef

Analysis of Earths earliest sedimentary record is crucialfor understanding the early evolution of life on Earth.Stromatolitesinternally-laminated, macroscopic sedimentarystructures, commonly of biological originform the dominant partof Earths early fossil record (1) and so provide a potentiallyimportant source of information about early life. However, stro-matolites are shaped by a complex interaction of physical, chemical,and biological processes, and identifyingunambiguous signatures oflife from the preserved morphology of the structures can beextremely difficult (24). Ideally, textural or microstructural evi-dence of microbial mats is needed in addition to morphological andcontextual clues to unravel processes of stromatolite formation and

gain direct evidence of the activities of benthic microbial commu-nities (5,6). To date, however, the search forsuch clues in theoldestknown stromatolites has been frustrated by diagenetic alteration,particularly recrystallization: a diagenetic process that commonlyaffects the chemical (precipitated) sediments with which stroma-tolites are often associated.

A possible biomediated origin was previously suggested for someof Earths oldest stromatolites, in the 3.43-Ga Strelley Pool For-mation, based on their morphology (1), morphological associationsand the spatiotemporal distribution of stromatolites in a reef-likepalaeoenvironment (79). However, microfossils, microbial sedi-mentary fabrics, and organic materials have not been identified todate. Putative microfossils and organic materials have been iden-tified in Early Archaean rocks (1014), but others have proposed

that those are abiotic structures shaped by hydrothermal processesand composed of mantle-derived carbon (e.g., ref. 15).

Recent mapping of the Strelley Pool Formation identifiedseveral well-preserved outcrops in which relict (bio)sedimentary fabricsand carbonaceous materials could be detected and mapped withinand among a variety of stromatolite forms. Here, we analyzesedimentary fabrics within the context of different stromatolitesand compare them with better-preserved Proterozoic examples togain detailed, direct microscale evidence of the physical, chemical,and biological processes that contributed to stromatolite growth inthe Strelley Pool Formation.

Geologic Setting

The Strelley Pool Formation is a 30 to 400-m-thick sedimentaryrock unit deposited on the Pilbara Craton between 3.43 billion and3.35 billion years ago. Outcrops of the formation extend across180 km (ref. 16 and references therein), but the reef-like carbon-ate platform buildup identified previously (7) is limited to 10 kmof outcrop in the southwestern Panorama Greenstone Belt (Fig.S1). In that area, the formation can be divided into 4 stratigraphicunits: a basal rocky coastline conglomerate (member 1), the stro-matolitic carbonate platform member (member 2; further subdi-

vided into 3 beds, each capped by a layer of large, acicular crystal

pseudomorphs), a stromatolitic chert member (member 3), and achertvolcaniclastic member (member 4). Member 2 is the focusof the present study. The principal facies of member 2 consist of: (i)6 morphologically-distinct types of stromatolites, (ii) acicular crystalpseudomorphs that were probably originally aragonite (1), (iii) flatlaminites, and (iv) f lat pebble intraclast conglomerates. The lithol-ogy of all member 2 facies consists of dolostone and chert (7, 8).

Sample Selection and Context

In the present study we examine fabrics of coniform and domicalstromatolites from the lower 2 beds of the platform carbonate(member 2). Two outcrops provide exceptional fabric preservationand exposure of those beds. The first is a 150-m-long section onsouthern Anchor Ridge (Fig. S1), exposing on-platform sections ofbed 1 (with encrusting/domical laminites) and bed 2 (with 3

coniform stromatolite types: cuspate swales, egg carton laminites,and small conical/crested laminites). The second outcrop is onsouthern Trendall Ridge (Fig. S1), where abundant large complexcone (LCC) stromatolites formed on a paleotopographic high,possibly a rimmed platform margin. The Trendall and Anchor

Author contributions: A.C.A., J.P.G., M.L.C., and I.K. designed research; A.C.A., J.P.G.,

A.H.K., I.W.B., and M.S.A. performed research; A.C.A., J.P.G., A.H.K., I.W.B., and M.S.A.

analyzed data; and A.C.A., J.P.G., and A.H.K. wrote the paper.

The authors declare no conflict of interest.

1To whom correspondence may be addressed. E-mail: [email protected] or

[email protected].

This article contains supporting information online at www.pnas.org/cgi/content/full/

0903323106/DCSupplemental.

95489555 PNAS June 16, 2009 vol. 106 no. 24 www.pnas.orgcgidoi10.1073pnas.0903323106
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Ridge outcrops were mapped in detail and samples were collectedfor slabbing, polishing,and thinsectioning.Thin sectionmicroscopyand Raman spectroscopy were used to identify and analyze thefabric components, and X-ray fluorescence element imaging (ac-quired with a Horiba XGT-5000 X-ray analytical microscope) wasused to assist in the detection of relict fabrics by mapping major andminor element distribution.

Encrusting/Domical Stromatolites

Encrusting/domical stromatolites are abundant in bed 1 of member2 throughout the study area (7, 8). They consist of adjacent(abutting) or laterally linked pseudocolumns of domical laminae(Fig. S2). The domical laminae typically initiate on a topographicfeature such as a boulder, intraclast, or small mound. In someplaces, the domes expand radially and coalesce with each successivelayer (as in Fig. S2), whereas in other places the domical laminaemaintain constant dimensions through successive layers.

Here, we focus on a particularly well-preserved outcrop ofdomical stromatolites that formed on the platform interior, wherebed 1 is 2 m thick and directly overlies altered volcanic rocks ofthe Mount Ada Basalt. Stromatolites initiated within 2050 cm ofthe lower contact, upon surfaces defined by minor topographicirregularities associated with a flat pebble intraclast conglomerate.Several additional surfaces of stromatolite initiation occur higher in

the bed. The bed thins rapidly to the north where it onlaps apaleo-high and becomes thicker toward the south before thinningagain because of underlying paleotopographic relief.

Sedimentary Fabrics. Sedimentary fabrics change systematicallyfrom base to top of the stromatolite pseudocolumns. The lowerparts consist of irregularly-laminated dolomite and chert withdiscontinuous layers of carbonaceous material and chert-filledlaminoid fenestrae (Figs. 1 and 2D). The dolomite and chert arerecrystallized, but variations in recrystallized texture hint at relictclastic and precipitated sedimentary fabrics. Although sedimentarygrains are not preserved, the dolomite crystal size variations inmany laminae resemble clastic sedimentary textures, and the lam-inae themselves have millimeter-scale irregularities and disconti-nuities like those associated with the trapping and binding of

fine-grainedparticles in younger and better-preserved stromatolites(17). Other laminae display faint palisade fabrics, consisting ofsubmillimeter acicular crystals arranged perpendicular to the lam-inae, that indicate in situ precipitation and growth of crystals at thesedimentwater interface (4). Chert-filled laminoid fenestrae (atype of primary or penecontemporaneous open space structure; ref.18) are a major part of the fabric in the lower strata. The fact thatthey were once open spaces is indicated by the void-fill pattern ofchalcedony and megaquartz within the structures (Fig. 1 B and C).Fenestrae are commonly associated with microbial mats in peritidalsettings, where they often form by degassing of decaying organicmaterial and/or drying out of the surface of microbial mats,resulting in shrinkage, lifting, and separation of the mats from thesediment surface (ref. 18 and references therein).

The fabric becomes more regularly laminated up section throughthe stromatolites, with fewer fenestrae (Fig. 2) and increasingpalisade fabrics. In the upper strata, the abundance of palisadefabrics and large acicular crystal pseudomorphs increases dramat-ically (Fig. 3 and Fig. S 3 c, e, and f). Approximately 2 m below theupper contact of the bed, the stromatolites give way to a thick (2m)bedof densely-packed acicular crystal pseudomorphs (Fig. S3a,b, and d).

Dolomite Laminae. Under the microscope, 3 main types of dolomiteare observed (Fig. S4). D1, the most common type, consists ofapproximately equigranular, anhedral crystals in a sutured mosaic,consistent with moderate to advanced dolomite recrystallization.D2 consists of very fine-grained (2 m) dolomite and is locallyabundant around the margins of larger dolomite crystals and at the

contacts between chert and dolomite laminae. D3 consists ofequigranular, euhedral dolomite rhombs, or partial rhombs, withcrystal growth zones (Fig. S4b). D3 crystals typically occur asovergrowths at the margins of dolomite laminae where they pro-trude into chert-filled fenestrae or laminae. X-ray fluorescenceelement maps highlight the higher iron content of D3 compared

A

B C

D

E

F

Fig. 1. Encrusting/domical stromatolite fabrics. (A) Polished slab, showing

irregular wrinkly laminar fabric consisting of dolomite(D), chert(C), and organic

laminae (OM). Note organic layers on upper sides of flat pebble intraclast con-

glomerate(Cg, outlined in dottedred line) piled against upper right of stromat-

olite.(B) Chert-filledfenestra surrounded byD2 dolomiteare shown.Transmitted

plane polarizedlight.(C) Detail ofcavity fillchert withchalcedonyat marginsand

megaquartz at thecenter.Transmitted cross polarized light. (D) Composite X-ray

fluorescence mapof Ca (blue) andFe (pink). Thebrighterpinkareas highlight the

greater Fe content of D3 dolomitecompared with D1 andD2 in thesurrounding

dolomite(blue) laminae.The black areasin the rockfabric representsilica. (Scalebar:0.5 cm.)(E) Detailof intraclast conglomeratefrom upper right cornerof the

sample shown in A. Red arrows point to white rims surrounding the clasts and

organic material. The rimsconsistof D3 dolomiteovergrowths (inner) and isopa-

chous chert rims (outer). (F) Schematic illustration of the relationship between

clasts, organic deposits, dolomite overgrowths and chert in the intraclast con-

glomerate shown in E.

Allwood et al. PNAS June 16, 2009 vol. 106 no. 24 9549
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with the surrounding dolomite (Fig. 1D). The preservation ofgrowth zones indicate that D3 dolomite has not been overprintedby later diagenetic recrystallization, and there is no indication ofmajor replacement of a noncarbonate phase.

Chert Laminae. The chert laminae consist mainly of an equigranularmosaic of interlocking microcrystalline to mesocrystalline-quartz

crystals with undulose extinctionand crenulated crystalboundaries.Laminoid fenestrae in the lower parts of bed 1 have wall-coatingisopachous silica cements and radial-fibrous chalcedony, surround-ing a central infilling of anhedral megaquartz (Fig. 1 B and C). Afew inclusions of carbonaceous material, calcite, and dolomite arepresent.

The observed features in the chert are consistent with either: (i)selective replacement of a laminated, nonsilica precursor (mostlikely carbonate or organic material), or (ii) early diagenetic alter-ation of sedimentary silica laminae. It is worth noting that theobserved chert fabrics do not provide unequivocal evidence that anonsilica precursor has been replaced. Early burial diagenesis ofsedimentary (nonskeletal) silica involves dissolutionreprecipita-tion reactions that create porosity, preferentially alter some ele-

mentsof theoriginal fabric, and release H4SiO4 intosolution, whichcan then reprecipitate elsewhere in the sediment pile, resulting ina mixture of original sedimentary fabrics (e.g., laminae), dissolutionfabrics, and reprecipitation fabrics (cavity fill) (19). Thus, the

observed diagenetic chert fabrics are not necessarily evidence ofreplacement of a nonsilica precursor. Moreover, the chert laminaeare present in intraclast conglomerate clasts within beds 1, 2, and3, which supports a primary or very early diagenetic origin and is notconsistent with a late-stage (postmember 2 deposition) hydrother-mal replacement origin as proposed previously (20).

Organic Laminae. Raman spectra confirm that black laminae withinthe stromatolite fabric consist of disordered carbonaceous material(Fig. S5). The spectral characteristics of the organic matter aresimilar to those exhibited by the Strelley Pool Formation samplesexamined previously and are consistent with the regional lowergreenschistmetamorphism (21). This observation indicates that theorganic material has been subject to the thermal history of the hostrock and did not migrate recently into the bed. Significantly, the

organic laminae are also preserved in flat pebble intraclast con-glomerates that onlap the stromatolites (Fig. 1 A, E, and F). Theorganic matter is concentrated in a discrete layer on the upper sideof the intraclasts and is partially intermingled with the dolomite(D1/D2) that makes up the clasts. Furthermore, the organic layer

was then overgrown by D3 dolomite crystals that surround theclasts. The clasts, organics, and D2 dolomite overgrowths were thensurrounded by isopachous chert rims and chert matrix, both of

which contain no organic matter. Thus, it seems very unlikely thatthe organic matter migrated through the matrix and coated theclasts after burial. The weight of evidence indicates that the organiclayers were synsedimentary laminae formed at the stromatolite

water interface during deposition and are not younger contami-nants introduced to the rock during diagenesis.

Fig. 2. Fabrics in lower to middle strata of encrusting/domical stromatolites

atAnchor Ridge.(A) Polishedslab showing edgeof domical stromatolite with

organic laminae and variable recrystallization. Note laminae become thinner

towardmargin,as shownby thedottedwhitelines andlarge arrows oriented

normal tothe paleosurfaceof thestromatolite.As a result, thecuspanddome

geometry do not change significantly through successive layers. g, inferred

clastic or grainy fabric; p, relict palisades (precipitated) fabric; o, organic

laminae. (B) Polished slab showing detailof inferred relictclastic texture withorganic laminae (black layers) and dolomite laminae with minor chert (gray

layers).(C) Polishedslab showingdetail of inferredclastictexturewith organic

laminae (black layers) and incipient domical structures. (D) Polished slab

showing detail of irregular laminoid fabric with organic laminae, chert-filled

fenestrae and inferred relict grainy texture. (Scale bars: 1 cm.)

A B

C

D

Fig. 3. Images showing the vertical trend in fabrics and parallel change in

laminar architecture through the encrusting/domical stromatolites. (A) Outcrop

exposureshowingpart of 2 broad domical stromatolitesand the cuspatedepres-

sionin between.Thelowerstratashowirregularlylaminatedfabricswithinferredclastictextures; laminaethin toward themarginsand laminargeometrydoes not

change significantly through successive layers. The abundance of precipitated

textures(palisades,acicular crystal psedudomorphs) increasesin theupper strata,

where laminae maintain thickness laterally and the laminar geometry changes

witheach successive layer. Consequently,the cuspinfills and the domes coalesce.

Scale increments on card 1 cm. (B) Polished slab showing acicular crystal

pseudomorphs fromstrata30cm above the top ofthe photo inA. (C) Polished

slabshowing palisaded layer among irregularlamination.(D) Polishedslab show-

ing irregular lamination with organic layers (black laminae).

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Having determined with reasonable certainty that the organiclaminae in the encrusting/domical stromatolites are syngenetic,multiple hypotheses for the origin of the organic material remain tobe tested. On one hand, the laminae could represent allochthonousorganic detritus transported and settled onto the stromatolites, in

which case either biotic or abiotic (mantle or meteorite-derived)ultimate origins are possible. Alternatively, the organic laminaecould be the remains of microbial mats formed in situ at thesedimentwater interface.

Under transmitted light, the organic laminae are seen to consistof submillimeter-sized amorphous clots, specks, and wisps ofopaquematerial included in crystals or accumulated along dolomitegrain boundaries, where it appears to have been displaced by themargins of growing crystals during recrystallization. Insufficientmorphological detail has been preserved to determine whethermicrobial cells were once present as part of a microbial mat.Biogenic and abiogenic hypotheses must therefore be tested usingattributes at scales large enough to survive recrystallization. At thenaked eye and hand-lens scale, the polished slabs display sufficientattributes to test those hypotheses.

A hypothesis that proposes the laminae are formed of alloch-thonous organic detritus would predict that the organic particles

were subject to current reworking and gravity-driven settling, andtherefore tend to accumulate in lows. They would also potentially

mix with other grain types. Such fabrics are not observed. Rather,the stromatolites have discrete, organic-rich, wrinkly laminae be-tween sediment layers, indicating distinct episodes of organic layerformation (Figs. 1A and 2). Moreover, the organics formed mat-like layers that contoured the stromatolites from cusp to cusp, didnot thicken into lows, and possessed sufficient cohesive/adhesiveproperties to enable their establishment on stromatolite marginslopes greater than the angle of repose for particulate sediment[30o (22)] (Figs. 1A and 2A andB). Thus, the organic layers likelydo not consist of transported allochthonous organic detritus.Rather, their character and distribution are consistent with theirinterpretation as the remains of microbial mats formed at thesedimentwater interface, in many places on steep to near-verticalslopes, during stromatolite growth.

These observations lead to the question of the role of microbes

in stromatolite accretion, which requires an understanding of theroles of in situ chemical precipitation (i.e., precipitation at thesedimentwater interface) and clastic sedimentation (i.e., me-chanical deposition of sediment particles, including mineral parti-cles formed by precipitation in the water column). In the case ofclastic sedimentation, active microbial trapping and binding wouldbe the only plausible way to accumulate sediment on steep- to

vertical-sided domes. In the case of in situ precipitation, sedimentcan accumulate on any slope with or without the presence ofmicrobial mat. However, in a precipitative environment microbescould also have played 2 roles influencing the precipitation ofsediment: metabolically inducing mineral precipitation within themat microenvironment or forming an organic template forlocalization of mineral precipitation.

Determining the nature of the primary sediments (precipitated

in situ vs. clastic) is hampered by the absence of pristine primaryfabrics or interspace deposits between stromatolites, which mighthave offered clues in sedimentary fabrics not modified by stroma-tolite-forming processes. However, valuable insights are nonethe-less recorded within the stromatolites themselves: in the relictfabrics and geometry of the laminae, and their covariation throughtime. Many stromatolite laminae retain faint vestiges of clastic orpalisades/crystal fabrics. Laminae with probable clastic characterare more prevalent in the lower strata (Figs. 1 and 2) whereasprecipitated (palisades) fabrics become more abundant in the upperstrata (Fig. 3 B and C and Fig. S3). This vertical transition ismatched by a change in lamina geometry:most of the laminae in thelower strata become thinner toward the stromatolite margins (Fig.2 A and B), as would be expected if microbes were actively trapping

and binding sediment or causing intramat precipitation, leading tothicker accretion on horizontal surfaces. Laminae in the upperstrata, however, are isopachous, as expected if the layers formeddominantly by surface-normal crystal growth (4). These parallelchanges in fabric and morphology suggest that stromatolite accre-tion was initially dominated by microbial trapping and binding ofsediment and/or intramat precipitation, but became increasinglydominated by in situ precipitation through time. Importantly, thestacking of marginal-thinning laminae in the lower strata resulted

in minimal change in laminae morphology during early stromatolitegrowth, whereas stacking of isopachous, precipitated laminae dur-ing later growth resulted in cusp infill and coalescence of the domes(Fig. 3). The correlation between changes in stromatolite morphol-ogy and sedimentary process, as inferred from fabrics and laminararchitecture, conforms well to expectationsfrom theoretical modelsof stromatolite morphogenesis (4, 23, 24) for a system evolving frommicrobially-dominated to precipitation-dominated ac cretion.Therefore, it is reasonable to conclude that benthic microbialcommunities or colonies contributed to formation of the encrust-ing/domical stromatolites, but their morphogenetic influence de-creased over time as chemical precipitation increased.

Coniform Stromatolites

The coniform stromatolites of member 2/bed 2 contrast markedly

to the encrusting/domical stromatolites of bed 1 in terms of fabricsand morphology. Here, we examine 2 types of coniform stroma-tolites to analyze these differences: the LCC stromatolites, whichoccur mainly near the platform margin, and the pseudoconicalcuspate swale stromatolites, which dominate the platforminterior.

Well-preserved LCC specimens were collected from an outcropon southern Trendall Ridge, where abundant stromatolites clus-tered on the high side of a rim-like topographic feature at theplatform margin (Fig. 4). Cuspate swale stromatolite samples werecollected at southern Anchor Ridge. Both stromatolite types consistof alternating dolomite and chert laminae. The LCC structures havedistinctive coniform morphology (Fig. 4) that is inherited throughstacks of laminae that form pseudocolumns up to 2 m high (i.e., 2-mstratigraphic thickness) (7). Individual stromatolites are laterally

separated by a few centimeters up to several decimeters of flat-lyinglaminated dolomite/chert. The cuspate swale stromatolites consistof pseudoconical structures with concave-upward slopes that gradeinto trough-shaped interspaces (Fig. S6). The apices of adjacentstructures are connected by saddle-shaped ridges) (7, 9).

One notable aspect of the fabrics in both LCC and cuspate swalestromatolites is the high degree of lateral correlation betweenstromatolite and intercolumn laminae, which attests to uniformrates of accumulation and some commonality of depositionalprocess between stromatolite and intercolumn areas. However,another notable feature is the contrast between the type of fabricsseen in the stromatolite and intercolumn areas (Fig. 5A), whichattests to differences in the processes occurring in the 2 areas.

The intercolumn fabric is characterized by slightly to moderatelyundulose laminae that thicken and thin laterally. Current scours,

topographic infill geometry (e.g., onlap, drape), tangential trunca-tions, low-angle cross laminations, and graded fabrics are wellexpressed and abundant (Fig. 5A). Together, these features indicatedeposition of clastic sediment from current and wave-agitated

water. However, rather than being defined by changes in size andcomposition of clastic grains (none are visible because of recrys-tallization), they are defined by variations in size and type (i.e.,chert, D1, D2, or D3 dolomite) of the neomorphic crystals. Here,

we infer that crystal size, in a relative sense, approximates sedimentgrain size. Although it cannot be demonstrated because of thecomplete absence of primary grains, the distinct relationship ofcrystal size to stratification style supports this inference. In contrastto the intercolumn fabrics, the stromatolite interior fabrics consistof thin, continuous, nearly isopachous laminae reminiscent of

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precipitated layers. In the case of the cuspate swales there is agradual lateral transition between the 2 fabrics across the concaveslope (Fig. S6), whereas in the LCC stromatolites the transition is

very sharp across an abrupt slope change. This contrast betweenstromatolite and intercolumn fabrics persists through many hun-dreds of laminae, attesting to long-lived differentiation of sedimen-tary processes on the stromatolites compared with the intercolumn

area.Although the presence of a benthic microbial community couldcause such highly-localized and sustained modification of sedimen-tary process, the textural hallmarks typically associated with mi-crobial mat buildup are absent: namely, wrinkled or crinkly lami-noid fabrics, fenestrae and organic laminae (organic matter is rare,occurring only as faint, lamina-parallel wisps of organic material insome samples). However, it is also true that neither precipitationnor clastic sedimentation alone satisfactorily explains the combi-nation of stromatolite and intercolumn area fabrics. It also seemsunlikely that in situprecipitation on the stromatolite was juxtaposedagainst clastic sedimentation in the intercolumn areas, because, asalready noted, the lateral correlation of intercolumn and stromat-olite laminae indicates that material both on and off the stroma-tolites accumulated at a uniform rate, signifying commonality ofdepositional process.

An alternative hypothesis that incorporates alternating clasticsedimentation and biofilm-nucleated precipitation arises fromcomparison with Mesoproterozoic Omachtenia omachtensis andGongylina differenciata stromatolites (Uchuro-Maya region, Sibe-ria) (25). The microstructure of those stromatolites formed byemplacement of mm-scale sediment-rich laminae during deposi-tional events such as storms or high tides, alternating with devel-opment of thin, laterally continuous micritic laminae, preserved bypenecontemporaneous mineral precipitation within thin organicsheets (Fig. 5B). Silicified structures show that the organic sheetsformed through decay of microbial mats that inhabited and stabi-lized the sediment surface during intervals of nondeposition (25).

In the Strelley Pool Formation, the thin, near-isopachous lami-nae that span the coniform stromatolites and the intercolumn areas(e.g., location 1; Fig. 5A) resemble Omachtenias organic-sheet-nucleated precipitates, whereas the undulating intercolumn depos-its resemble the sedimentary event beds. Thus, the processesinferred for formation ofOmachtenia and Gongylina stromatolitesmay also explain coniform stromatolite development in the StrelleyPool Formation (Fig. 6). However, closer examination of LCC andcuspate swale stromatolites suggests some minor differences.

The Omachtenia style of stromatolite morphogenesis involvestemporal (rather than spatial) variations in sedimentary regime,and implies that contemporaneous sedimentary layers shouldtherefore exhibit the same fabric, having formed under the sameregime. In the LCC stromatolites this is true at some stratigraphiclevels; some stromatolite laminae correlate with similarly thin,isopachous laminae in the intercolumn area, whereas clastic de-posits lie above and below those, reflecting temporal alternationbetween 2 regimes. However, there are many places (e.g., locations2 and 3; Fig. 5A) where sets of stromatolite laminae (with precip-itate-like fabric) transit directly across the stromatolite margins intoa set of correlative, and therefore contemporaneous, clastic lami-nae. Evidently, there were spatial variations in the way those

laminae formed.These relationships indicate that at least some organic films

influencing formation of laminae on the stromatolite terminated atthe stromatolite margins. In addition, the direct correlation ofstromatolite laminae with clastic intercolumn deposits may indicatethat the stromatolite laminae formed by microbial adhesion ofclastic sediment, rather than by in situ precipitation on the organicfilms. We have argued against microbial trapping and binding ofsediment particles on the stromatolite because of the lack of texturalevidence in the stromatolitic laminae. However, a possible expla-nation for this apparent paradox involves settling of micrometer-scale, water column-nucleated crystals onto both the stromatolite(whereupon they adhered to an organic layer) and the intercolumnsurfaces (where no organic layer existed and the particles could be

A

B C

Fig. 4. Stromatolitesat the platformmargin outcrop on southern Trendall Ridge. (A) Outcrop map showing cross-section viewof stratigraphy fromunderlying

altered volcanic rocks up through members 13 and part of member 4 of the Strelley Pool Formation. Note the paleotopographic feature on which the

stromatoliteswere deposited: stromatolitesonly formed on the high side (Right). (B) Wavy laminitesdeposited in deeperwatersouthof thepaleohigh.(C) LCC

(conical) stromatolites formed on the paleohigh. The dotted white line traces a single lamina across 2 coniform stromatolites. The sample indicated is shown in

Fig. 5. (Scale rule in B and C: 15 cm.) Modified from ref. 32.

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moved around by currents and waves), thereby creating a singlelayer with lateral variation in cohesiveness. Deposition of watercolumn-nucleated precipitates is a well-documented phenomenonin precipitative environments such as evaporite basins (ref. 26 andreferences therein). Such extremely-fine crystals settling from the

water column may not require a well-developed, trapping andbinding mat community to adhere to stromatolite slopes. A thinorganic film could have sufficed, either a thin biofilm or decayedmat remnants like those observed in Omachtenia and Gongylina,and inferred here to have been templates for precipitation. Exper-iments have shown that the presence of a thin, low profile butmucilaginous organic film on a submerged surface greatly enhancessediment particle adhesion, even under flow conditions.* Thus, the

presence of a either a low-profile biofilm or the decayed remnantsof a maton the stromatolite could have facilitated particle adhesion.

The surviving textural evidence cannot be used to discriminatebetween this latter hypothesis and the hypothesis that the stromat-olite laminae formed by localized in situ precipitation. Potentially,a combination of grain adhesion and mat-nucleated precipitationcould have contributed to stromatolite accretion. However, thegrain adhesion hypothesis better explains the consistent verticalthickness of laminae throughout stromatolite and intercolumnareas and the resulting consistency of laminar geometry through the

pseudocolumn (7). Such geometry is consistent with vertical settlingof particles onto the surface.

In summary, formation of the Strelley Pool Formation coniformstromatolites likely involved a combination of the following sed-imentation/accretion modes (illustrated in Figs. 5A and 6):

1. Formation of laterally extensive laminae by precipitationwithin thin organic layers (location 1; Fig. 5A).

2. Rare deposition of laterally restricted sedimentary eventlayers in the low areas between stromatolites (location 4;Fig. 5A).

3. Formation of laterally variable laminae from accumulation ofclastic layers between stromatolites (consisting of water-columnprecipitated particles) coupled with either: (i) adhesion of watercolumn-nucleated particles to thin organic films on stromato-lites, or (ii) localized precipitation of laminae on stromatolites,nucleated on thin organic layers. (locations 2 and 3; Fig. 5A).

In reality there were probably infinitely variable intermediatemodes, combining aspects of these 3 end member modes. How-ever, there also appear to be relatively distinct examples of thefabrics associated with each end member mode. The changingrelative importance of each of these modes of accretion/depositionthrough time can be traced through the changing arrangement of

the different fabric suites.In this view, the role of microbes in coniform stromatoliteaccretion was largely passive. Microbes simply provided a layerof organic material that formed a template for cr ystal nucleationand/or particle adhesion. Accepting this interpretation, the mostdirect evidence for microbial involvement in stromatolite mor-phogenesis comes not from stromatolite morphology or specifictextural observations in isolation, but from the spatiotemporalarrangement of textures and fabrics within the context of stro-matolite morphology. If morphology does not of itself encapsu-late the microbial influence, then it is unlikely that a morphotypecan be definitively linked to a specific type of organism, or evena metabolic strategy such as photosynthesis (27).

Genesis and Variability of Stromatolites

The existence of microbial mats during formation of stromatolitesin the Strelley Pool Formation can be deduced from different setsof evidence in multiple stromatolite types. In domical stromatolites,evidence of microbial mat formation lies in the observation thatcohesive layers of organic material formed at discrete, regularintervals at the surface of stromatolites, coupled with the fact thatthose laminae adhered to the steep stromatolite margins and didnot preferentially thicken into topographic lows. In the coniformstromatolites, microbial activity is inferred from the juxtaposition ofcontemporaneous but contrasting sedimentary fabrics and theirarrangement within the context of stromatolite morphology. Inboth instances the interpretation benefits from comparisons withmicrobially-influenced microstructure in well-preserved Protero-zoic stromatolites (25). Unfortunately, microfossils are not pre-

served because of redistribution of the organic material by neo-morphic crystal growth during recrystallization. Biomarkerpreservation is possible but perhaps unlikely because of the thermalmaturity of the organic matter (28).

In addition to preserving different types of biosignatures, thestromatolites preserve evidence that microbes played a variable rolein accretion. In coniform stromatolites, microbes may simply haveprovided a template, perhaps postmortem, for chemical precipita-tion or adhesion of fine crystalline sediment nucleated in the watercolumn. In the encrusting/domical stromatolites, thinning of lam-inae at the margins suggests a component of active microbialtrapping and binding and/or intramat biomediated precipitation,

which gave way through time to in situ precipitation during laterstages of accretion. Effectively, the proportion of microbial mat

*Salant NL, Hassan MA, Fall meeting of the American Geophysical Union 10 14 Dec, 2007,

San Francisco, CA, abstract H13D-1534.

A

B

Fig. 5. Sedimentary fabrics of an Early Archean coniform stromatolite and

MesoproterozoicOmachtenia stromatolite. (A) Coniform LCC stromatolite from

the Early Archean Strelley Pool Formation. The stromatolite is on the right, and

flat-lying intercolumn laminae are on the left. Polished slab showing a cross-

section view. Dark laminae are chert-rich, light laminae are dolomite-rich. Dark

cross-cutting fractures filled with hematite are the result of recent weathering.

Dotted white lines highlight bundles of laminae with different character. Num-

bers refer to locations discussed in text. (B) Mesoproterozoic O. omachtensis

stromatolite from the Uchuro-Maya region, Siberia, showing precipitated and

clastic textures with thin organic laminae. Thin section, plane polarized light.

Colored arrows on laminae indicate the inferred processes by which those lam-

inae formed.



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formation, relative to other processes of deposition, played a rolein determining stromatolite morphogenetic variability.

Whether biological factors were the principal control on stro-matolite initiation is unclear from either fabrics or morphology.However, some contextual features provide insight to alternatepossibilities. A major clue is the fact that most stromatolites appearto initiate on a preexisting topographic feature such as an intraclast,cobble, boulder, ripple crest, or mound. Microbial colonization and

biofilm formation at thebenthic boundary layer can hingeon subtlelateral topographic variations that affect fluid circulation andchemical gradients in pore spaces of the upper millimeters ofsediment (29). Thus, slightly-elevated locations such as ripple crestsare differentiated from their surroundings, from an ecological orbiochemical point of view, in terms of pore space chemistry, andcould become preferred substrates for local mat formation andstromatolite initiation. Precipitation could also occur preferentiallyon highs, because elevated sites would be more likely to remain freeof sediment, enabling uninterrupted crystal growth. However,evidence from Proterozoic rocks suggests that seafloor carbonateprecipitation can be facilitated by or nucleated within mats (e.g. ref.14), a process that is also inferred here for established stroma-tolites in the Strelley Pool Formation. Thus, if the better-

understood, younger geologic record is the key to the deep past,then microbial colonization and biofilm formation may have beenthe initializing factor, leading to subsequent mat-nucleated precip-itation or particle adhesion. Experimental work may determine

whether this latter hypothesis is likely or whether seafloor precip-itation could equally have provided rapid initial stabilization of thesediment, before microbial colonization on the highs.

Although the location and relative amount of microbial mat

formation played a role in determining stromatolite initiation,distribution, andmorphogenesis,this does notnecessarily imply thatstromatolite morphologic changes equate to biodiversity. Changesin stromatolite morphology described herein are evidently linked toshifts in environmental processes through the Strelley Pool For-mation. For example, in the case of domical stromatolites, thechange from inherited to coalescing morphology through time isclearly linked to increasing precipitation, as evidenced in the

vertical increase in crystals and palisades fabrics (Fig. 3). Further-more, the change from domical to coniform morphology is accom-panied by a relative decrease in evidence for precipitation. Theselatter observations conform well to model predictions of shiftinginteraction between the amount of surface-normal growth drivenby in situ precipitation and the amount of vertical growth driven by

Fig. 6. A simplified schematic representation of 2 inferred modes of formation of coniform stromatolites in the Strelley Pool Formation, incorporating spatial

and temporal variations in process sequences, and the resulting sedimentary fabrics in relation to morphology. The first mode (Left) is similar to the sequence

of processes that formed Proterozoic Omachtenia stromatolites and involves temporal variations in laterally-uniform processes (25). The second mode (Right)

also involves lateral variations in process because of the formation of microbial films only on stromatolites.



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microbial processes (23). There is no biodiversity implicit in thisrelationship, simply a change in the relative influence of microbialinput compared with other processes. The textural and morpho-logical evidence described herein does not provide direct evidenceof biodiversity, and without microfossils it is impossible to test thenull hypothesis that the entire stromatolite assemblage involved just1 type of microorganism. Nonetheless, the environmental changesthat accompanied stromatolite morphologic changes imply thatmicrobial communities at the stromatolite surfaces had to adapt to

those changes.It is clear from sedimentological evidence that any microbialcommunities present during deposition of the Strelley Pool For-mation would have been subject to significant environmental shifts,including changes in water depth, sedimentation rate, precipitationrate, and wave or current energy. Extant microbial systems respondto such changes by altering the survival strategies or geneexpressionof individual species and the composition of multispecies commu-nities. Therefore, to the extent that modern analogs guide inter-pretation of ancient processes, then ecologically-diverse microbialmat communities were probably involved in stromatolite formationand changed their community composition and survival strategiesin response to changing environmental conditions. Those changesmay be reflected in the diverse array of stromatolite morphologiesand textures that formed during deposition of the Strelley Pool

Formation.One important question involves the possible role of photosyn-

thetic organisms in Early Archean mat communities (e.g., ref. 30).Early studies of modern tufted mat stromatolites suggested coni-form morphology was caused by the phototactic aggregation offilamentous cyanobacteria (31). However, recent experimentalresults on tuft formation under variable illumination conditionsindicate that coniform morphology can develop independently ofphotosynthesis (27). The morphology of Strelley Pool Formationconiform stromatolites suggests a tendency for vertical growth (7),

which has been attributed to the vertical migration of photosyn-thetic microbial communities (23). However, chemotaxis and thesettling of sediment could, in principal, play the same role in causing

vertical growth in stromatolites. That is, continuing sediment dep-osition could prompt migration of microbes toward the sediment

water interface, along a vertical chemical gradient in the uppermillimeters of sediment. Thus, whether or not photoautotrophs

were included within Strelley Pool Formation microbial commu-nities is unclear from either textural or morphological evidence.Perhaps the best evidence for possible photoautotrophs in formerStrelley Pool microbial communities lies in the effect of waterdepth, and inferred seafloor illumination, on stromatolite distribu-tion. Previous studies documented a regional trend wherein stro-matolites occur only in the shallow water parts of the Strelley PoolFormation carbonate platform and are absent in laterally equiva-lent deeper water deposits (7). The present study documents evenmore compelling evidence for this relationship along southernTrendall Ridge, where stromatolite distribution across relict topog-raphy at the platform margin was very tightly controlled by waterdepth. (Fig. 4A).

In conclusion, evidence preserved in the Strelley Pool Formationsuggests that microbial mat communities probably existed 3.43billion years ago in the Pilbara sea, flourishing under shiftingenvironmental conditions, resulting in a morphologically-diverseassemblage of stromatolites. These stromatolites may have beeninhabited by diverse microbial c ommunities, possibly includingphotoautotrophs.

ACKNOWLEDGMENTS. Wethank theGeologicalSurveyof WesternAustralia forgenerous field support, Malcolm Walter andArthur Hickman forhelpful discus-sions,George Rossman foruse of Caltech Raman Laboratory, andDawnSumnerand Ian Fairchild for helpful reviews. Fieldwork was partly supported by theAgouron Institute. A.C.A. was supported by the National Aeronautics and SpaceAdministration(NASA) PostdoctoralProgram. Thework of A.C.A.,M.S.A.,M.L.C.,and I.K. was conducted at the Jet Propulsion Laboratory under contract to theNational Aeronautics and Space Administration (with Research and TechnologyProgram/Astrobiology Science and Technology for Exploring Planets funding).

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Controls on Development and Diversity of Early Archean Stromatolites