Astrobiology Workshop, Macquarie University July 12-13 2001

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Bacterial remains in Archaean and Palaeoproterozoic sedimentary rocks

M. Glickson and S.D. Golding (Department of Earth Sciences, University of Queensland)

Bacterial remains including a 3.5 Ga barite smoker deposit in Western Australia, 2.5 Ga bacteria in Mt McRae shale of the Hamersley Group, and the Paleoproterozoic Karelian Shungite deposit in Russia are some examples of micro-organisms in extreme environments [1, 2]. Bacteria resembling modern hypothermophile methanogenic microbes were observed in samples from Hamersley Group sediments. The bacteria display a range of shapes, and often appear to be encased in membrane-like capsules. Some bacteria are distinguished by appendages, and others are connected by narrow bridge-like material reminiscent of dividing organisms. The size of microorganisms varies from nanno scale (20-200 nm) in the 3.5 Ga Western Australian smoker deposit and the Shungite deposit to 1.5-5 microns length in the Hamersley Group sediments. Microanalysis showed the electron dense parts of the latter to have consistently iron sulphide incorporated in the organic material. This is also characteristic of the WA barite deposit microbial remains. Incorporation of metals in bacterial cell walls, and partial replacement of organic carbon would have enhanced preservation.

Whereas the relatively low reflectance (low thermal stress organic matter) Mt McRae shale samples yielded well preserved and structured micro-organisms, higher reflectance organic matter (>3.0 % Ro) consisted of morphologically poorly defined masses/bundles of filamentous material with no recognisable detailed structural features, grading into fine isotropic to weakly anisotropic mesophase. The mesophase when observed in TEM showed only moderately developed molecular ordering, and a probable Type I kerogen source material. Mineral assemblages associated with this type of organic matter were predominantly silica, with only minor contribution from iron sulphide, and other metals. On the other hand, the mineral assemblage associated with the methanogenic-like bacteria consisted of predominantly metallic elemental iron particles of fairly consistent size, around 5 microns, and euhedral iron sulphide of varying sizes, from very small, less than 2 microns to over 20 microns. These observations indicate that highly anoxic conditions alternated with somewhat more oxidising conditions during sedimentation.

Carbon isotope compositions of Mt McRae shale organic concentrates show increasingly negative values with maturation, as expressed in Ro. Relatively low maturation samples, in the range of 1.8 - 2.2 % Ro range in carbon isotope composition from 29 to 33 per mil. Organic matter above 2.2 % Ro is more 13C depleted, with carbon isotope compositions ranging from 35 to 42 per mil; the most negative values are associated with anomalously high reflectance values. The increasingly isotopically lighter organic matter hand in hand with increase in Ro (representing oxidation) may be the result of incorporation of CO2 derived from oxidised methane that diffused upwards through the sediments. An alternative explanation in light of latest research in the area [3, 4] is anaerobic methane oxidation by a consortia of archaea and sulphate reducing bacteria. The latter process may also explain the ever-present pyrite both in sediments as well as replacing the organic carbon in the bacterial cell walls.

References:

[1] Glikson M. et al. (2000) in Organic Matter and Mineralisation, M. Mastalerz and M. Glikson, eds., Kluwer Academic Publishers, Great Britain, pp.66-101.

[2] Mastalerz M. et al. (2000) in Organic Matter and Mineralisation, M. Mastalerz and M. Glikson, eds., Kluwer Academic Publishers, Great Britain, pp.102-119.

[3] Hinrichs K.-U. et al. (1999) Nature, 398, 802-805.

[4] Boetius A. (2000) Nature, 407, 623-626.