Evidence of Microbial Activity From PaleoArchean Australia

Carbonaceous microstructures from sedimentary laminated chert within the 3.46 Ga Apex Basalt, Chinaman Creek locality, Pilbara, Western Australia

Authors:

Hickman-Lewis et al

Abstract:

Hydrothermal black chert veins intruding the 3.46 Ga Apex Basalt contain some of Earth’s oldest microfossil-like objects, whose biogenicity has been questioned. Whilst these black chert veins have been studied in great detail, relatively little is known about the stratiform, seafloor, sedimentary cherts that are conformably interbedded with volcanic rocks of the Apex Basalt.

Herein, we document and assess the biogenicity of carbonaceous microstructures present in the lowermost of the stratiform chert units (informally known as the ‘Apex chert’), at the Chinaman Creek locality in the Marble Bar greenstone belt, Pilbara Craton, Western Australia. Carbonaceous material mostly occurs within clotted grey-black cherts and microgranular ‘grainstone-like’ cherts within the stratiform unit, the latter being the major focus of this study. In the clotted cherts, carbon occurs as lobate, fluffy grains, rare compressed flakes, and as a grain boundary phase around spherulitic silica. There is no morphological evidence to support the biogenicity of these microstructures. In contrast, the microgranular chert contains fluffy and flaky carbonaceous grains, plus laminated grains comprising multiple non-isopachous wrinkled carbonaceous laminae, with noted thickening towards some ridge crests, as determined by confocal laser scanning microscopy. Roll-up structures provide evidence of an initial plasticity, interpreted to have formed via the tearing-up and current-induced plastic deformation of microbial mat fragments. Geochemical mapping, using laser Raman micro-spectroscopy and NanoSIMS, respectively demonstrates the antiquity of the carbon, and reveals a close correlation between carbon, nitrogen and sometimes sulphur, concentrated within dark brown to black laminae. Adjacent to microgranular zones are zones of more persistent carbonaceous, undulose, filament-like laminae that entrain relict sediment grains. These microstructures are directly comparable to a sub-type of microbially induced sedimentary structure (MISS), widely reported from younger siliciclastic sediments colonised by microbial biofilms.

The morphology and chemical composition of both the non-isopachous laminated grains and the filament-like laminae are consistent with a biological interpretation, suggesting microscopic MISS were present in the microgranular stratiform ‘Apex chert’. However, the fact that neither macroscopic MISS nor bona fide microfossils have yet been reported from this unit, coupled with the proximity of these structures to active hydrothermal vents, potentially discharging hot carbon-rich fluids, urges caution in such an interpretation. The Chinaman Creek ‘Apex chert’ investigated here is one of at least five sedimentary, laminated cherts within the Apex Basalt. These horizons are promising targets in the search for biological activity within a dominantly volcanic Archaean environment.

Evidence of an Impact From the PaleoArchean in Australia

A new ∼3.46 Ga asteroid impact ejecta unit at Marble Bar, Pilbara Craton, Western Australia: a petrological, microprobe and laser ablation ICPMS study

Authors:

Glikson et al

Abstract:

The Archean record contains seventeen asteroid impact ejecta units interpreted in terms of terrestrial vestiges of an extended late heavy bombardment (LHB) (, and ). Correlated impact ejecta units include the 3.47 Ga in the Barberton Greenstone Belt, Kaapvaal Craton, South Africa, and Pilbara region of Western Australia, with multiple ejecta units in the 3.25–3.22 Ga and 2.63–2.48 Ga intervals. This paper reports the discovery and investigation of a new impact ejecta unit within the Marble Bar Chert Member (MBCM) of the felsic volcanic Duffer Formation, east Pilbara Craton, Western Australia. The age of the MBCM is constrained by a 3459±2 Ma U-Pb zircon date from the uppermost volcanic unit of the Duffer Formation and by a 3449±3 Ma U-Pb zircon date from the overlying felsic volcanic Panorama Formation, stratigraphically above the intervening un-dated Apex Basalt. The ejecta unit, observed in a drill core (ABDP 1) ∼4 km south-southwest of Marble Bar, consists of multiple lenses and bands of almost totally silicified impact spherules 1–2 mm in diameter. All internal primary textures of the spherules have been destroyed. Nonetheless, Fe-rich spherule rims, largely composed of secondary siderite, are well preserved. Chemical analyses of the rims reveal iron-magnesium carbonate displaying high Fe, Mg, Ni, Co and Zn. Whole-rock and in-situ analyses (X-ray fluorescence, Inductively Coupled Plasma Mass Spectrometry (ICPMS), electron-microprobe (EMP) and EMP-calibrated laser ICPMS) reveal that the rims contain high Ni abundances and high Ni/Cr ratios (less than 50). The spherules are separated by an arenite matrix and spherule lenses also occur within bedded chert. The spherules are particularly visible over some ∼14 m of true stratigraphic thickness in which chert breccia is interpreted to represent a tsunami-generated diamictite affected by hydrothermal fragmentation and veining. Despite the almost total silicification of the MBCM whole-rock nickel sulphide (NIS) results indicate high Ir (2 ppb) and a low Pd/Ir ratio (2.0), consistent with geochemical features of impact ejecta units. Dense concentrations of spherules at the 57-58 m level and the 77 m level of the core, separated by banded chert, raise the possibility of two distinct impact events. Stratigraphic and isotopic age data distinguish between the 3459–3449 Ma age of the MBCM ejecta unit and ∼3470.1 ± 1.9 Ma impact ejecta units in the Antarctic Creek Member, Mount Ada Basalt, about 40 km to the west of Marble Bar. In combination with a 3472 ± 2.3 Ma impact unit in the Barberton greenstone belt, these impact ejecta units record large Paleoarchean asteroid impacts, significant for understanding early earth bombardment rates and early crustal evolution.

Evidence of Cyanobacterial Inhabitation on Archean Rock Surfaces

Cyanobacterial Inhabitation on Archean Rock Surfaces in the Pilbara Craton, Western Australia

Authors:

Yosuke et al

Abstract:

High abundances of 7- and 6-monomethylalkanes as well as C17 n-alkane, indicative of cyanobacteria, have been discovered near the surfaces of Archean carbonate rocks of the Fortescue Group in the Pilbara region, Western Australia. The presence of cyanobacterial biomarkers is mostly limited to the surface layer (less than 1 cm thickness) of the rocks, indicating that the cyanobacteria are an endolithic species. Biomarkers are found in bitumen I (solvent-extracted rock) and also in bitumen II (solvent-extracted decarbonated rock). The abundance of biomarkers is generally the same between both bitumen fractions in the surface layer, which suggests that the cyanobacteria penetrated into the carbonate minerals. Trace amounts of the biomarkers have also diffused into a deeper part of the rocks, but this influence is only seen in bitumen I. This implies that hydrocarbons moved toward the inside of the rock through pores and fissures in the rock fabric. In contrast, hydrocarbons in bitumen II, which mainly come from within the carbonate minerals, are isolated from the hydrocarbon migration from the outside of the rock and may be ancient indigenous organic matter. To the best of our knowledge, this is the first report of the past or modern inhabitation of cyanobacteria on Archean rocks in the Pilbara region for which hydrocarbon biomarker analyses was used.

Trace Hydrocarbons in Archean Rocks are NOT Reliable Biomarkers

Reappraisal of hydrocarbon biomarkers in Archean rocks

Authors:

French et al

Abstract:

Hopanes and steranes found in Archean rocks have been presented as key evidence supporting the early rise of oxygenic photosynthesis and eukaryotes, but the syngeneity of these hydrocarbon biomarkers is controversial. To resolve this debate, we performed a multilaboratory study of new cores from the Pilbara Craton, Australia, that were drilled and sampled using unprecedented hydrocarbon-clean protocols. Hopanes and steranes in rock extracts and hydropyrolysates from these new cores were typically at or below our femtogram detection limit, but when they were detectable, they had total hopane (less than 37.9 pg per gram of rock) and total sterane (less than 32.9 pg per gram of rock) concentrations comparable to those measured in blanks and negative control samples. In contrast, hopanes and steranes measured in the exteriors of conventionally drilled and curated rocks of stratigraphic equivalence reach concentrations of 389.5 pg per gram of rock and 1,039 pg per gram of rock, respectively. Polycyclic aromatic hydrocarbons and diamondoids, which exceed blank concentrations, exhibit individual concentrations up to 80 ng per gram of rock in rock extracts and up to 1,000 ng per gram of rock in hydropyrolysates from the ultraclean cores. These results demonstrate that previously studied Archean samples host mixtures of biomarker contaminants and indigenous overmature hydrocarbons. Therefore, existing lipid biomarker evidence cannot be invoked to support the emergence of oxygenic photosynthesis and eukaryotes by ∼2.7 billion years ago. Although suitable Proterozoic rocks exist, no currently known Archean strata lie within the appropriate thermal maturity window for syngenetic hydrocarbon biomarker preservation, so future exploration for Archean biomarkers should screen for rocks with milder thermal histories.

Pilbara Craton Controversy: Were There or Were There NOT Hadean Continents?

Hf isotopes in detrital and inherited zircons of the Pilbara Craton provide no evidence for Hadean continents

Authors:

Kemp et al

Abstract:

Predictions of large volumes of stabilized continental crust by the early Archaean stand in stark contrast to the actual amount of pre-3.5 Ga rocks presently exposed on Earth's surface. The Pilbara Craton of Western Australia, one of the best preserved Paleoarchean crustal blocks on Earth, is believed to have developed on a cryptic, possibly ≥3.8 Ga continental basement. If substantiated, this could support the notion of a widespread and enduring Hadean (ca. 4.5–4.0 Ga) felsic–intermediate crust. To test this, and to elucidate the earliest evolution of the Pilbara Craton, we report Hf isotope data from previously dated detrital zircon grains, and inherited zircon crystals hosted by granitic gneisses, the crystallization ages (3.80–3.55 Ga) of which substantially exceed those of the oldest exposed igneous rocks of the craton (∼3.52 Ga). The Hf isotope compositions of the ancient zircons analyzed in this study are consistent with most of the earliest components of the Pilbara Craton being extracted from near chondritic mantle between ∼3.7 and 3.6 Ga, with little or no input from significantly older crust. These new data suggest either that the Pilbara Craton developed remote from the isotopic influence of the putative Eoarchean to Hadean continental masses, or that the stabilized volumes of the earliest continents have been overestimated. The latter scenario would be consistent with the extreme scarcity of greater than 3.9 Ga rocks and minerals, and the dominantly chondritic Hf isotope composition of the oldest continental rocks in Earth's most ancient Archaean cratons.