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| Titel |
Carbonaceous matter and putative microfossils of the mid-Archean Kromberg type-section re-visited, Barberton Greenstone Belt, South Africa |
| VerfasserIn |
Nicola McLoughlin, Eugene Grosch |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250093051
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| Publikation (Nr.) |
 EGU/EGU2014-7421.pdf |
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| Zusammenfassung |
| Silicified seafloor sediments of the Kromberg Formation from the Onverwacht Group of the
Barberton greenstone belt (BGB), South Africa, have been argued to contain some of the
world’s oldest preserved carbonaceous microfossils. Previous studies of these cherts have
reported filamentous, spheroidal and ellipsoidal microfossils in thin-section (Walsh 1992);
and bacteriomorph like structures in HF-etched samples (Westall et al. 2001). These
microtextural studies however, lack supporting in-situ geochemical data, and are hampered to
some degree by re-mobilisation of the carbonaceous matter (Van Zuilen et al. 2007). In
light of these concerns, and ongoing debates surrounding carbonaceous remains in
other Archean cherts (e.g., W Australia), further in-situ data from the Kromberg is
required to positively identify carbonaceous matter of biogenic origin. New data will
also help to address outstanding questions regarding the relative contribution of
benthic versus planktonic microorganisms, and the putative microbial metabolisms
involved.
This study focuses on surface samples and drill core from the Barberton Scientific
Drilling Programme, (BSDP, Grosch et al. 2009) from the southeastern limb of the
Onverwacht anticline of the BGB. We sampled the Footbridge chert and a second chert
horizon in drill core KD1 of the BSDP in the upper Kromberg Fm; and surface outcrops of
two black cherts from the lower Kromberg Fm. Sedimentological logging reveals horizons
rich in volcaniclastics with interbedded finely laminated grey-black chert, also intrusive black
cherts, and sulphide rich horizons. The TOC of the sampled cherts is 1.24 to 5.40 wt%.
Preliminary bulk carbon isotope values range from δ13C -21.1 to -35.3o values
that are consistent with organic matter produced by anoxygenic photosynthesis.
Microfabrics preserved in the Kromberg cherts include, primary wispy-laminated
carbonaceous films suggesting compaction of early carbonaceous laminae. Also large
composite carbonaceous grains >30 μm across recording wave-motion on the seafloor.
Secondary fabrics include hydrothermal veins containing remobilized carbon and
sometimes sulphides, also void-filling silica spherulites coated in carbonaceous
matter.
A novel fabric discovered in the lower Kromberg chert is silicified carbonaceous
fragments with plastic deformation that are morphologically comparable to microtextures
reported from the 3.416 Ga Buck Reef Chert (Tice and Lowe 2004) interpreted to be
deformed microbial mat fragments. These fabrics are currently being studied by raman
spectroscopy to assess the effects of taphonomic processes and metamorphic alteration on
this potential biosignature. In-situ sulphur isotope measurements by SIMS on sulphides
associated with primary carbonaceous fabrics of the Footbridge chert found a narrow range in
δ34SCDTvalues of -6.00 to + 1.50 oand positive Δ33S values up to +2.50 o suggesting
the involvement of atmospheric sulphur aerosols, but do not strongly support either microbial
sulphate reduction or disproportionation (Grosch and McLoughlin 2013). The geochemical
evidence for microbial processes must therefore be further tested in the Kromberg cherts to
build upon the bulk carbon isotope data that is consistent with, but alone not diagnostic of,
microbial processes.
In summary, this new survey of carbonaceous cherts from the Kromberg type
section has identified well-preserved candidate microbial microfabrics that will be the
target of ongoing high resolution in-situ geochemical and ultrastructure analysis. |
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