Controls on development and diversity of Early Archean stromatolites

Abstract

The approximately 3,450-million-year-old Strelley Pool Formation in Western Australia contains a reef-like assembly of laminated sedimentary accretion structures (stromatolites) that have macroscale characteristics suggestive of biological influence. However, direct microscale evidence of biology--namely, organic microbial remains or biosedimentary fabrics--has to date eluded discovery in the extensively-recrystallized rocks. Recently-identified outcrops with relatively good textural preservation record microscale evidence of primary sedimentary processes, including some that indicate probable microbial mat formation. Furthermore, we find relict fabrics and organic layers that covary with stromatolite morphology, linking morphologic diversity to changes in sedimentation, seafloor mineral precipitation, and inferred microbial mat development. Thus, the most direct and compelling signatures of life in the Strelley Pool Formation are those observed at the microscopic scale. By examining spatiotemporal changes in microscale characteristics it is possible not only to recognize the presence of probable microbial mats during stromatolite development, but also to infer aspects of the biological inputs to stromatolite morphogenesis. The persistence of an inferred biological signal through changing environmental circumstances and stromatolite types indicates that benthic microbial populations adapted to shifting environmental conditions in early oceans.

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 conglomerate (Cg, outlined in dotted red line) piled against upper right of stromatolite. (B) Chert-filled fenestra surrounded by D2 dolomite are shown. Transmitted plane polarized light. (C) Detail of cavity fill chert with chalcedony at margins and megaquartz at the center. Transmitted cross polarized light. (D) Composite X-ray fluorescence map of Ca (blue) and Fe (pink). The brighter pink areas highlight the greater Fe content of D3 dolomite compared with D1 and D2 in the surrounding dolomite (blue) laminae. The black areas in the rock fabric represent silica. (Scale bar: 0.5 cm.) (E) Detail of intraclast conglomerate from upper right corner of the sample shown in A. Red arrows point to white rims surrounding the clasts and organic material. The rims consist of D3 dolomite overgrowths (inner) and isopachous chert rims (outer). (F) Schematic illustration of the relationship between clasts, organic deposits, dolomite overgrowths and chert in the intraclast conglomerate shown in E.

Fig. 2.

Fabrics in lower to middle strata of encrusting/domical stromatolites at Anchor Ridge. (A) Polished slab showing edge of domical stromatolite with organic laminae and variable recrystallization. Note laminae become thinner toward margin, as shown by the dotted white lines and large arrows oriented normal to the paleosurface of the stromatolite. As a result, the cusp and dome 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 detail of inferred relict clastic texture with organic laminae (black layers) and dolomite laminae with minor chert (gray layers). (C) Polished slab showing detail of inferred clastic texture with 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.)

Fig. 3.

Images showing the vertical trend in fabrics and parallel change in laminar architecture through the encrusting/domical stromatolites. (A) Outcrop exposure showing part of 2 broad domical stromatolites and the cuspate depression in between. The lower strata show irregularly laminated fabrics with inferred clastic textures; laminae thin toward the margins and laminar geometry does not change significantly through successive layers. The abundance of precipitated textures (palisades, acicular crystal psedudomorphs) increases in the upper strata, where laminae maintain thickness laterally and the laminar geometry changes with each successive layer. Consequently, the cusp infills and the domes coalesce. Scale increments on card = 1 cm. (B) Polished slab showing acicular crystal pseudomorphs from strata ≈30 cm above the top of the photo in A. (C) Polished slab showing palisaded layer among irregular lamination. (D) Polished slab showing irregular lamination with organic layers (black laminae).

Fig. 4.

Stromatolites at the platform margin outcrop on southern Trendall Ridge. (A) Outcrop map showing cross-section view of stratigraphy from underlying altered volcanic rocks up through members 1–3 and part of member 4 of the Strelley Pool Formation. Note the paleotopographic feature on which the stromatolites were deposited: stromatolites only formed on the high side (Right). (B) Wavy laminites deposited in deeper water south of the paleohigh. (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. .

Fig. 5.

Sedimentary fabrics of an Early Archean coniform stromatolite and Mesoproterozoic Omachtenia 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. Numbers 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 laminae formed.

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.