Kenichiro Sugitani

Geochemical characteristics of Archean cherts and other sedimentary rocks in the Pilbara Block, Western Australia: evidence for Archean seawater enriched in hydrothermally-derived iron and silica

Abstract Geochemical and mineralogical characteristics of Archean sedimentary rocks such as cherts, banded iron-formations (BIFs), shales and sandstones collected from the Pilbara Block, Western Australia were studied. Lithologic features of various types of chert occurring within the Pilbara Supergroup are closely related to their chemical compositions. Red and brown bands of banded cherts generally show a higher Fe 2 O 3 * ( * =total iron as Fe 2 O 3 ) concentration than do other cherts, whereas grey, greenish-grey and pale yellowish-grey cherts are characteristic of relatively higher concentrations of Al and Ti. White cherts are generally composed almost exclusively of SiO 2 . Geochemical characteristics of the cherts suggest that they were precipitated from hydrothermal solutions. Evidence for the hydrothermal origin includes (1) low MnO/Fe 2 O 3 * values, (2) low concentrations of heavy metals, (3) positive Euanomalies, (4) low Co/Zn and Ni/Zn values, and (5) high concentrations of Mn and Fe in carbonates. Positive correlations between Fe and heavy metals such as Ni and Zn imply the intermittent precipitation of Fe-oxides (hydroxides) along with these heavy metals in the ancient ocean. Cherts, except for grey, greenish-grey and pale yellowish-grey cherts, are highly depleted in detrital materials. This may have been caused by rapid precipitation of hydrothermally-derived Fe and Si. The Archean “Pilbara Ocean” is believed to have been enriched in Fe and Si derived from hydrothermal activity.

Anomalously low Al2O3/TiO2 values for Archean cherts from the Pilbara Block, Western Australia—possible evidence for extensive chemical weathering on the early earth

Geochemical, sedimentological and mineralogical features of 3.0 Ga Archean cherts and intercalated BIFs from the Pilbara Block, Western Australia, were studied. Chert layers that are well exposed at Point Samson, 20 km north-northeast of Roebourne consist of various types of chert: white-light gray chert, brownish-yellow chert, gray chert, dark gray chert, massive black chert, and light gray chert. The cherts other than brownish-yellow chert consist mostly of microcrystalline quartz with a minor amount of fine silicate minerals and carbonates. The brownish-yellow chert is characterized by a high concentration of goethite. Most cherts contain thin layers characterized by high concentrations of Ti-bearing minerals such as rutile (anatase), FeTi aggregates, and fine TiO2 grains. Accordingly, Al2O3/TiO2 values in some samples are very low (<1.0), indicating that these samples contain more TiO2 than Al2O3. This geochemical feature is attributed mainly to the supply of detrital materials with high Ti-bearing minerals to the site of deposition, because the Ti-bearing minerals are generally associated with detrital quartz and zircon. Ti-enriched detrital materials are possibly formed by AlTi fractionation during: (1) extensive chemical weathering, or (2) hydrothermal alteration of the source rocks. In the first hypothesis, which we prefer, it is suggested that low pH values (<4) of meteoric waters accelerated the decomposition of rock-forming minerals and prevented precipitation of dissolved Al3+. Such low pH values of meteoric waters could have been attributed to more violent volcanic activities issuing acid materials (e.g., H2S and HCl) and/or a higher concentration of atmospheric CO2 in the Archean than today.

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.

Stratigraphy and sedimentary petrology of an Archean volcanic–sedimentary succession at Mt. Goldsworthy in the Pilbara Block, Western Australia: implications of evaporite (nahcolite) and barite deposition

Abstract A approximately 3.3 Ga sedimentary succession at Mt. Goldsworthy in the northeastern Pilbara Block, Australia contains beds composed of silicified pseudomorphs of nahcolite (NaHCO3) or barite (BaSO4). This sedimentary succession correlates with the Corboy Formation and consists of lower, middle and upper sedimentary units, which conformably overlie mafic to ultramafic volcanic and volcaniclastic rocks. The lower and middle units are predominantly siliciclastic, whereas the upper unit is characterized by ferruginous cherts and banded iron-formations with minor proportions of intercalated mature sandstone. The succession defines an overall upward fining and deepening trend. The detrital materials were derived from older greenstone successions, volcaniclastics and reworked precipitative beds. Silicified pseudomorphs of nahcolite crystals up to 40 cm in length occur in a 20 m thick bed in the upper portion of the lower unit. The formation of this unit was initiated by deposition of immature terrigenous clastic sediments and followed by the precipitation of nahcolite from Na+-HCO3− brines in a closed or semi-closed evaporitic basin. Bladed barite deposits occur mainly in the middle unit, which was deposited in a sub-aerial to shallow marine environment. Barite may have precipitated as a result of mixing of SO42−-rich seawater and Ba2+-rich hydrothermal fluids. The Mt. Goldsworthy sedimentary succession records an Archean shallow to sub-aerial sedimentary environment that probably developed in a continental margin setting.

Diversity in the Archean Biosphere: New Insights from NanoSIMS

The origin of organic microstructures in the approximately 3 Ga Farrel Quartzite is controversial due to their relatively poor state of preservation, the Archean age of the cherts in which they occur, and the unusual spindle-like morphology of some of the forms. To provide more insight into the significance of these microstructures, nano-scale secondary ion mass spectrometry (NanoSIMS) maps of carbon, nitrogen, sulfur, silicon, and oxygen were obtained for spheroidal and spindle-shaped constituents of the Farrel Quartzite assemblage. Results suggest that the structures are all bona fide approximately 3 Ga microfossils. The spindles demonstrate an architecture that is remarkable for 3 Ga organisms. They are relatively large, robust, and morphologically complex. The NanoSIMS element maps corroborate their complexity by demonstrating an intricate, internal network of organic material that fills many of the spindles and extends continuously from the body of these structures into their spearlike appendages. Results from this study combine with previous morphological and chemical analyses to argue that the microstructures in the Farrel Quartzite comprise a diverse assemblage of Archean microfossils. This conclusion adds to a growing body of geochemical, stromatolitic, and morphological evidence that indicates the Archean biosphere was varied and well established by at least approximately 3 Ga. Together, the data paint a picture of Archean evolution that is one of early development of morphological and chemical complexity. The evidence for Archean evolutionary innovation may augur well for the possibility that primitive life on other planets could adapt to adverse conditions by ready development of diversity in form and biochemistry.

Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils

The Strelley Pool Formation (SPF) is widely distributed in the East Pilbara Terrane (EPT) of the Pilbara Craton, Western Australia, and represents a Paleoarchean shallow-water to subaerial environment. It was deposited ~3.4 billion years ago and displays well-documented carbonate stromatolites. Diverse putative microfossils (SPF microfossils) were recently reported from several localities in the East Strelley, Panorama, Warralong, and Goldsworthy greenstone belts. Thus, the SPF provides unparalleled opportunities to gain insights into a shallow-water to subaerial ecosystem on the early Earth. Our new micro- to nanoscale ultrastructural and microchemical studies of the SPF microfossils show that large (20-70 μm) lenticular organic-walled flanged microfossils retain their structural integrity, morphology, and chain-like arrangements after acid (HF-HCl) extraction (palynology). Scanning and transmitted electron microscopy of extracted microfossils revealed that the central lenticular body is either alveolar or hollow, and the wall is continuous with the surrounding smooth to reticulated discoidal flange. These features demonstrate the evolution of large micro-organisms able to form an acid-resistant recalcitrant envelope or cell wall with complex morphology and to form colonial chains in the Paleoarchean era. This study provides evidence of the evolution of very early and remarkable biological innovations, well before the presumed late emergence of complex cells.

Carbon isotopic analyses of ca. 3.0 Ga microstructures imply planktonic autotrophs inhabited Earth’s early oceans

The ca. 3 Ga Farrel Quartzite (FQ, Western Australia) contains possible organic microfossils of unusual spindle-like morphology that are surprisingly large and complex, preserved along with spheroids. The unusual nature of the possible fossils, coupled with their antiquity, makes their interpretation as biogenic difficult and debatable. Here, we report 32 in situ carbon isotopic analyses of 15 individual FQ specimens. The spheroids and the spindle-like forms have a weighted mean δ 13 C value of –37‰, an isotopic composition that is quite consistent with a biogenic origin. Both the spheroids and the spindle-like structures are isotopically distinct from the background organic matter in the same thin section (weighted mean δ 13 C value of –33‰), which shows that the preserved microstructures are not pseudofossils formed from physical reprocessing of the bulk sedimentary organic material. When considered along with published morphological and chemical studies, these results indicate that the FQ microstructures are bona fide microfossils, and support the interpretation that the spindles were planktonic. Our results also provide metabolic constraints that imply most of these preserved microorganisms were autotrophic. The existence of similar spindles in the ca. 3.4 Ga Strelley Pool Formation of Australia and the ca. 3.4 Ga Onverwacht Group of South Africa suggests that the spindle-containing microbiota may be one of the oldest, morphologically preserved examples of life. If this is the case, then the FQ structures represent the remains of a cosmopolitan biological experiment that appears to have lasted for several hundred million years, starting in the Paleoarchean.

Three-Dimensional Morphological and Textural Complexity of Archean Putative Microfossils from the Northeastern Pilbara Craton: Indications of Biogenicity of Large (>15 μm) Spheroidal and Spindle-Like Structures

We recently reported a diverse assemblage of carbonaceous structures (thread-like, film-like, spheroidal, and spindle-like) from chert in the ca. 3.0 Ga Farrel Quartzite of the Gorge Creek Group in the Pilbara Craton, Western Australia. Results from a rigorous examination of occurrence, composition, morphological complexity, size distributions, and taphonomy provided presumptive evidence for biogenicity. In this study, we present new data of morphological and textural complexity of large (>15 microm) spheroidal and spindle-like structures, using an in-focus, 3-D image reconstruction system, which further raises the scale of credibility that these structures are microfossils. While many of the large spheroids are single-walled, and the wall is irregularly folded, a few specimens are partially blistered, double walled, or have a dimpled wall. The wall-surface texture varies from smooth and homogeneous (hyaline) to patchy, granular or reticulate. Such variation is best explained as resulting from taphonomic processes. Additionally, an inner solitary body, present in some large spheroids, is hollow and partially broken, which indicates a primary origin for this substructure. Spindle-like structures have two types of flange-like appendage; one is attached at the equatorial plane of the body, whereas the other appears to be attached peripherally. In both cases, the appendage tends to have a flat geometry, a tapering thickness, and constancy in shape, proportions, and dimensions. Spindle-wall surfaces are variously textured and heterogeneous. These morphological and textural complexities and heterogeneity refute potential abiogenic formation models for these structures, such as crystals coated with organic matter, fenestrae, and the diagenetic redistribution of carbonaceous matter. When coupled with other data from Raman spectroscopy, NanoSIMS analysis, and palynology, the evidence that these large carbonaceous structures are biogenic appears compelling, though it is still equivocal as to whether they are cells or outer envelopes of colonies of smaller cells.

Compositional Variations in Manganese Micronodules: A Possible Indicator of Sedimentary Environments

Manganese micronodules from various environments were analyzed with the electron microprobe. Chemical characteristics of marine manganese nodules were examined in relation with their depositional environments and compared with those of manganese macronodules. The compositional variation has a connection with accumulation rates and chemical features of the associated sediments. Mg and Ti tend to be enriched in micronodules relative to the host sediments; these elements may produce authigenic mineral phases in micronodules. These observations show that manganese micronodules are chemically formed under approximately the same condition as that for macronodules, but coexisting pairs of micronodules and macronodules collected from the same sediments suggest that oxic diagenesis plays a more important role in the formation of micronodules than that of macronodules. The chemical features of manganese micronodules collected from several marine environments such as a marginal sea, hemipelagic regions, and the central Pacific show that their compositions have a close relation to their depositional environments. Micronodules of hydrothermal origin show chemical characteristics different from those of hydrogenous and diagenetic nodules. We propose several criteria for the discrimination of micronodules of various types. Examination of DSDP cores indicates that micronodules disperse abundantly in marine sediments and their chemical composition remains unchanged through a long span of geological time. Furthermore, preliminary assessment of sedimentary rocks on land shows the preservation of ancient micronodules. Thus, manganese micronodules are a useful indicator of the depositional environment of sediments, and ancient micronodules from sedimentary rocks may offer a possible key to the origin of sedimentary rocks and to the history of geological terranes.

Redox change in sedimentary environments of Triassic bedded cherts, central Japan: possible reflection of sea-level change

Middle Triassic radiolarian bedded cherts in the Mino Belt, central Japan, include a sequence showing an abrupt facies change from the lower to the upper, where grey-black bedded cherts enriched in carbonaceous matter and framboidal pyrite are overlain by brick-red hematitic bed- ded cherts. Brownish-yellow chert enriched in goethite and purple-red chert occur at the boundary between the grey-black bedded cherts and the brick-red bedded cherts. This facies change is in accor- dance with stratigraphic variations of geochemical characteristics; the lower section grey-black bedded cherts, compared with the upper section brick-red bedded cherts, are enriched in Ctot and Stot, and are characterized by lower MnO/TiO 2 , higher FeO/Fe 2 O 3 * (total iron as Fe 2 O 3 ) and more variable Fe2O3*/TiO2 values. Some of the lower section samples, in addition, are characterized by an enrich- ment in some transition metals (Ni, Cu, and Zn). The covariation of mineralogical and geochemical characteristics indicates that sedimentary environments and diagenetic processes were different between the lower and the upper section bedded cherts. During the deposition of the lower section bedded cherts, the sedimentary environment was anoxic and bacterial sulphate reduction occurred during the early diagenetic stage. In contrast, the upper section bedded cherts were subjected to less reducing diagenetic processes; active sulphate reduction did not occur. The change of sedimentary environment and diagenetic process at the site of deposition is likely to be attributed to the fluctuated concentration of dissolved oxygen in the water mass of a semi-closed marginal ocean basin, which was potentially caused by sea-level change that occurred during Middle Triassic time.