Various organic compounds, including graphitic carbon, can be formed abiotically in hydrothermal systems, such that evidence for early life must necessarily combine geological, morphological and geochemical data to be compelling. Carbonaceous materials (CM) have been isolated from three rock packages of mid to early Archaean age from the Pilbara Craton of Western Australia. This CM has been subjected to a multidisciplinary approach utilising a variety of analytical and observational techniques, in an attempt to establish the occurrence, associations, mineral affinities, historical environments of growth, and the metamorphic/thermal history experienced by what may be some of the earliest, relatively pristine record of 3500 million year old life on this planet. CM isolated from drillcore obtained from the first of these localities, the 3.24 Ga Sulphur Springs volcanic hosted massive sulphide (VHMS) deposit, occurs as isotopically light (δ13C values of −34.0 ‰ to −26.8 δ13C) finely striated, lenticular to banded material emplaced parallel to original sedimentary bedding planes within the fine-grained silicified epiclastic hanging wall sediments. Petrological and transmission electron microscopy (TEM) observations have revealed well-preserved bundles of filamentous and tubular structured microbial remains closely resembling both modern-day and more ancient microbial forms documented from sea floor hydrothermal environments. Total organic carbon (TOC) has a range of <1.0 to 2.3 %, while the thermal maturity (%Ro) of the filamentous bundles points to maximum temperatures since deposition of around 90–100 °C, a factor that has enabled the preservation of their morphology. These results are suggestive of a well-developed Archaean sediment-hosted microbial community, situated within a basinal environment associated with an active centre of seafloor hydrothermal activity. The majority of the CM isolated from drillcore samples of the second locality, the 3.46 Ga Salgash Subgroup, a lower member of the Apex Basalt, also appears as in situ, bedding parallel bands intercalated with foliated altered argillaceous sandstone beds. TOC of the samples ranges from 1.25 to 11.48 %, while carbon content varies from 2.05 to 32.17 %. δ13C results are relatively heavy, varying from -30.4 to -22.5 ‰. Thermal maturity indicators of 10-13 %Ro suggest the CM having been subjected to temperatures greater than normally obtained from processes associated with burial. Electron paramagnetic resonance (EPR) results showed this CM in a highly ordered graphitic state. Optically, the graphite lacks the typical pronounced anisotropy characterising graphites in metamorphic terranes. Graphitisation therefore, is likely the result of rapid heating at very high temperature. HRTEM of this material revealed an extremely high level of molecular ordering contemporaneous with the presence of the C60 fullerene molecules within carbon nanotubes. These forms are a key to the distinction between biologically and abiotically synthesized CM, both by their small size and their resistance to thermal degradation. The occurrence of these carbon forms in terrestrial deposits is rare, and usually associated with wildfires, lighting strike or meteoritic impact. In the case of the Salgash CM, the formation of these molecules and the isotropic graphitised state of the CM is interpreted as a result of emplacement under pressure of very high temperature (komatiitic/ peridotite) lavas. The thermal overprint of the CM by such a high temperature process resulted in the volatilisation of the organic material, destruction of any primary biological morphology and the subsequent reorganisation of the residual CM, resulting in increased molecular ordering. In the third part of the study, CM isolated from drillcore samples of the ca. 3.5 Ga Dresser Formation bedded black chert-barite units, occurs in both dispersed and layered forms, interlayered with fine-grained silica. The intimate association of the CM and silica strongly resembles silicified microbial colonies from active hydrothermal systems, which have been previously proposed as analogues of Archaean hydrothermal sites. Isotopically light δ13C values from -38.2 to -32.1 ‰, and the association of C, H, and N, are highly indicative of a biological origin for the material. Palaeotemperatures calculated from δ18O isotope analysis of quartz chips indicate a depositional temperature for the hydrothermal veins ranging from ~120 °C to ~200 °C. 207Pb-206Pb isotope analyses conducted on pyrites extracted from the interbedded barite units reveal a dual MORB and Erosion mix source for the Pb, which gives an average 207Pb/206Pb age of 3531±42 Ma for the deposit. Ro measurements reveal four distinct CM populations, defined as ACM, A1CM, BCM, and CCM, which represent temperatures ranging from 170 °C to potentially >400 °C. TEM and HRTEM observations of the lower temperature CM population show morphological entities strongly suggestive of microbial remains, including possible cell wall remnants. Higher Ro rank CM commonly fills or coats mineral grains and lacks distinguishable structures, which is consistent with an increased thermal degradation /hydrothermal overprint. The geological setting and mineralogy of the Dresser Formation endorse its formational history as a silica-barite dominated seafloor hydrothermal deposit, most likely analogous to modern “white smokers”. The occurrence of the predominant CM (type ACM) in more or less continuous bands and laminae within the sedimentary rocks suggest an in situ, syndepositional source for the majority of this material, whereas the dispersed nature of type BCM particles indicates a recycled nature. The occurrence of type CCM within fluid inclusions gives an insight into the primary morphology of the non-degraded original microbial cells that may have existed at that time. These observations, combined with the carbon isotopic heterogeneity and fractionations are suggestive of chemosynthetic microbes occupying a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM. Finally, in an effort to further understand the CM structures observed in the rocks of the Dresser Formation in the context of present day microbial colonies in similar environments, a comparative morphological study was conducted using a potential modern analogue derived from an active seafloor hydrothermal environment. Such methodology utilises the standard classification used in biological species identification, which is initially based on visual identification of specific features, whether by the naked eye, light microscopy or electron microscopy. The extant hyperthermophilic microbe Methanocaldococcus jannaschii was cultured under conditions similar to the Archaean seafloor, simulating an increased thermal maturity by artificially induced autoclaving at 100 °C (1 atm) and 132 °C (2 atm). A striking resemblance to the early Archaean forms observed in the Dresser CM was evident in both wall structure and thermal degradation mode of the cultured microbe. Cell disintegration of the cultures occurred at 100 °C marking the limits of life, whereas complete disintegration, deformation and shrinkage of the cells occurred at 132 °C. These comparative observations present as a feasible way of understanding the structural features in CM identified in Archaean sedimentary packages.
| Identifer | oai:union.ndltd.org:ADTP/279229 |
| Creators | Lawrence Duck |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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