Martin J. Van Kranendonk

Questioning the evidence for Earth's oldest fossils.

Structures resembling remarkably preserved bacterial and cyanobacterial microfossils from ∼3,465-million-year-old Apex cherts of the Warrawoona Group in Western Australia currently provide the oldest morphological evidence for life on Earth and have been taken to support an early beginning for oxygen-producing photosynthesis. Eleven species of filamentous prokaryote, distinguished by shape and geometry, have been put forward as meeting the criteria required of authentic Archaean microfossils, and contrast with other microfossils dismissed as either unreliable or unreproducible. These structures are nearly a billion years older than putative cyanobacterial biomarkers, genomic arguments for cyanobacteria, an oxygenic atmosphere and any comparably diverse suite of microfossils. Here we report new research on the type and re-collected material, involving mapping, optical and electron microscopy, digital image analysis, micro-Raman spectroscopy and other geochemical techniques. We reinterpret the purported microfossil-like structure as secondary artefacts formed from amorphous graphite within multiple generations of metalliferous hydrothermal vein chert and volcanic glass. Although there is no support for primary biological morphology, a Fischer–Tropsch-type synthesis of carbon compounds and carbon isotopic fractionation is inferred for one of the oldest known hydrothermal systems on Earth.

Early Archaean microorganisms preferred elemental sulfur, not sulfate.

Microscopic sulfides with low 34S/32S ratios in marine sulfate deposits from the 3490-million-yearold Dresser Formation, Australia, have been interpreted as evidence for the presence of early sulfate-reducing organisms on Earth. We show that these microscopic sulfides have a mass-independently fractionated sulfur isotopic anomaly (Δ33S) that differs from that of their host sulfate (barite). These microscopic sulfides could not have been produced by sulfate-reducing microbes, nor by abiologic processes that involve reduction of sulfate. Instead, we interpret the combined negative δ34S and positive Δ33S signature of these microscopic sulfides as evidence for the early existence of organisms that disproportionate elemental sulfur.

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Biological activity is a major factor in Earth’s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth’s evolution is when did life arise and impact hydrosphere–atmosphere–lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800–3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals from the Isua supracrustal belt (ISB), southwest Greenland. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1–4-cm-high stromatolites—macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth’s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life’s origin in the Hadean eon (>4,000 Ma).

Geochemistry of metabasalts and hydrothermal alteration zones associated with c. 3.45 Ga chert and barite deposits: implications for the geological setting of the Warrawoona Group, Pilbara Craton, Australia

Relatively unaltered metabasalts of the Archaean Coonterunah and Warrawoona Groups, Pilbara Craton are compared with altered metabasalts from immediately beneath bedded cherts of these groups to provide evidence for the depositional environment and hydrothermal alteration processes of crust formation. The geochemistry of relatively unaltered basalt, stratigraphy, and inherited zircon data indicate that the lower Warrawoona Group (3.53–3.43 Ga) formed as an oceanic plateau complex built on a sialic basement to 3.724 Ga, following an analogue with the Phanerozoic Kerguelen oceanic plateau, and not as a mid-ocean ridge or convergent volcanic-arc complex as previously proposed. Advanced argillic, argillic, phyllic, and propylitic alteration zones in footwall basalts of this succession are products of repeated episodes of seafloor hydrothermal circulation, syngenetic with bedded chert deposition, in the distal parts of high-sulphidation epithermal systems. The upper part of the Warrawoona Group (3.350–3.315 Ga Euro Basalt) represents a continental flood basalt event, up to 8 km thick, that erupted onto the older succession across a regional unconformity on which the Strelley Pool Chert was previously deposited. Widespread silica–alunite alteration of dolomitic chert protoliths and phyllic and propylitic alteration of footwall basalts are interpreted as products of fluid circulation driven by heat from the overlying, newly erupted lavas.

Two types of Archean continental crust: Plume and plate tectonics on early Earth

Over 4.5 billion years, Earth has evolved from a molten ball to a cooler planet with large continental plates, but how and when continents grew and plate tectonics started remain poorly understood. In this paper, I review the evidence that 3.5 Ga continental nuclei in Australia formed as thick volcanic plateaux over hot, upwelling mantle and survived due to contemporaneous development of a thick, buoyant, unsubductable mantle root. This type of crust is distinct from, but complimentary to, high-grade gneiss terranes that formed through arc-accretion tectonics on what is envisaged as a vigorously convecting early Earth with small plates. Thus, it is proposed that two types of crust formed on early Earth, in much the same way as in modern Earth, but with distinct differences resulting from a hotter Archean mantle. A remaining question of how plate tectonics was initiated on Earth is investigated, using an analogy with Artemis Corona on Venus.Over 4.5 billion years, Earth has evolved from a molten ball to a cooler planet with large continental plates, but how and when continents grew and plate tectonics started remain poorly understood. In this paper, I review the evidence that 3.5 Ga continental nuclei in Australia formed as thick volcanic plateaux over hot, upwelling mantle and survived due to contemporaneous development of a thick, buoyant, unsubductable mantle root. This type of crust is distinct from, but complimentary to, high-grade gneiss terranes that formed through arc-accretion tectonics on what is envisaged as a vigorously convecting early Earth with small plates. Thus, it is proposed that two types of crust formed on early Earth, in much the same way as in modern Earth, but with distinct differences resulting from a hotter Archean mantle. A remaining question of how plate tectonics was initiated on Earth is investigated, using an analogy with Artemis Corona on Venus.

Direct dating of Archean microbial ichnofossils

Well-preserved Archean pillow lavas from the ca. 3.35 Ga Euro Basalt of the Pilbara Craton, Western Australia, contain micron-sized tubular structures mineralized by titanite (CaTiSiO 4 ) with residual organic carbon preserved along their margins. Direct U-Pb dating of titanite in the tubular structures demonstrates an Archean age. These tubular microstruc- tures are identical to microbial ichnofossils in modern basalts, ophiolites, and greenstone belts, and are interpreted as a biogenic signature in these ancient rocks. Microbial colonization of basaltic glass thus appears to have been part of a deep subsurface biosphere established early in Earths history.

On the Geologic Time Scale 2008

Abstract This report summarizes the international divisions of the geologic time scale and ages. Over 35 chronostratigraphic units have been formalized since 2000, with about one third of the almost 100 geologic stages of the Phanerozoic still awaiting international definition. The same numerical time scale is used as in Geologic Time Scale 2004 for the majority of stage boundaries. Exceptions are made if the definitions for stage boundaries are at a different level than the previous “working” versions (e.g., base of Serravallian, base of Coniacian, and bases of Ghzelian, Kasimovian and Serpukhovian). In most cases, numerical changes in ages are within GTS2004 age error envelopes. On-screen display and production of user-tailored time-scale charts is provided by the TimeScale Creator, a public JAVA package available from the ICS website (www.stratigraphy.org) and www.tscreator.com.

Chapter 4.1 Paleoarchean Development of a Continental Nucleus: the East Pilbara Terrane of the Pilbara Craton, Western Australia

Publisher Summary This chapter describes the lithostratigraphy, geochemistry, and structural and metamorphic geology of the ancient, eastern nucleus of the Pilbara Craton. The 3.53–2.83 Ga Pilbara Craton of Western Australia is one of only two areas on the Earth that contain large, well-exposed areas of little deformed, low-grade Paleoarchean rocks and the other being the Kaapvaal Craton in southern Africa—and as such is important for understanding the early Earth and the processes involved in the formation of continental crust. The Pilbara Craton is famous for its well preserved Paleoarchean volcano-sedimentary succession that includes evidence of the oldest life on the Earth, and for its classic dome-and-keel geometry of ovoid, domical granitic complexes, and flanking arcuate, synclinal greenstone belts. The 3.53–2.83 Ga Pilbara Craton is a nearly circular piece of crust in the northwestern part of Western Australia whose boundaries are defined by aeromagnetic and gravity anomalies, and by orogenic belts. A significant result of the extensive SHRIMP geochronology of volcanic and sedimentary rocks from the Pilbara Supergroup is the discovery of abundant inherited zircons that are older than the oldest dated supracrustal rocks.

Biogenicity of Morphologically Diverse Carbonaceous Microstructures from the ca. 3400 Ma Strelley Pool Formation, in the Pilbara Craton, Western Australia

Morphologically diverse structures that may constitute organic microfossils are reported from three remote and widely separated localities assigned to the ca. 3400 Ma Strelley Pool Formation in the Pilbara Craton, Western Australia. These localities include the Panorama, Warralong, and Goldsworthy greenstone belts. From the Panorama greenstone belt, large (> 40 μm) lenticular to spindle-like structures, spheroidal structures, and mat-forming thread-like structures are found. Similar assemblages of carbonaceous structures have been identified from the Warralong and Goldsworthy greenstone belts, though these assemblages lack the thread-like structures but contain film-like structures. All structures are syngenetic with their host sedimentary black chert, which is associated with stromatolites and evaporites. The host chert is considered to have been deposited in a shallow water environment. Rigorous assessment of biogenicity (considering composition, size range, abundance, taphonomic features, and spatial distributions) suggests that cluster-forming small (<15 μm) spheroids, lenticular to spindle-like structures, and film-like structures with small spheroids are probable microfossils. Thread-like structures are more likely fossilized fibrils of biofilm, rather than microfossils. The biogenicity of solitary large (>15 μm) spheroids and simple film-like structures is less certain. Although further investigations are required to confirm the biogenicity of carbonaceous structures from the Strelley Pool Formation, this study presents evidence for the existence of morphologically complex and large microfossils at 3400 Ma in the Pilbara Craton, which can be correlated to the contemporaneous, possible microfossils reported from South Africa. Although there is still much to be learned, they should provide us with new insights into the early evolution of life and shallow water ecosystems.

Onset of Plate Tectonics

Analysis of diamonds from the subcontinental mantle reveals that plate tectonics started 3 billion years ago. The further back we look into the geological past, the more obscured the view, masked by an increasingly fragmentary geological record. This has resulted in a controversy on the rates and mechanisms of early continental crust formation and whether plate tectonics—the dominant crust-forming process over at least the past 2.5 billion years—operated the same way, or even at all, during early Earth history (1, 2). On page 434 of this issue, Shirey and Richardson (3) shed light on this issue by looking at it from a new angle—from the bottom up. They investigated the composition of a suite of minerals occurring as inclusions in diamonds dredged up by young kimberlite volcanoes. These diamonds derive from great depths (125 to 175 km) within the ancient lithospheric mantle keels that underpin the stable continental crustal regions known as cratons. Because the minerals can be precisely dated, they can provide a snapshot of the subcontinental lithospheric mantle (SCLM) composition at the time when the crust was being formed.