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User:Cshirc1/Archean Life in the Barberton Greenstone Belt

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The Barberton Greenstone Belt of eastern South Africa contains some of the most widely accepted fossil evidence for Archean life. These cell-sized prokaryote fossils are seen in the Barberton fossil record in rocks as old as 3.5 billion years [1]. The Barberton Greenstone Belt is an excellent place to study the Archean Earth due to exposed sedimentary and metasedimentary rocks.

Studying the earliest forms of life on Earth can provide valuable information to help understand how life can evolve on other planets. It has long been hypothesized that life may exist on Mars due to the similarity of environmental and tectonic conditions during the Archean time [2]. By knowing the environments early life evolved in on Earth, and the rock types they are preserved in, scientists can have a better understanding of where to look for life on Mars.

The greenstone belt is located in the red highlighted area of eastern South Africa.

Global beginnings of life

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Fossil life of 3.5 billion years of age is also found in the Pilbara Craton of western Australia[3]. This evidence, along with Barberton fossils, show that cellular life must have existed by this point in the evolution of Earth. There is work that potentially demonstrates life at 3.8 billion years ago, in what is now western Greenland [4][5], but it is highly debated. It is important to note that cellular life existed 3.5 billion years ago and thus it evolved prior to this time. Because the Earth is 4.5 billion years old[6], there is a window of about one billion years for cellular life to evolve from an abiogenic earth.

Archean tectonic history of the Barberton Greenstone Belt

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The Barberton Greenstone Belt is located on the Kaapvaal craton, which covers much of the southeastern part of Africa, and was formed by the emplacement of granitoid batholiths[7]. The Kaapvaal Craton was once part of a supercontinent termed Vaalbara that also included the Pilbara Craton of western Australia [7]. Though the exact timing is still debated, it is likely that Vaalbara existed from approximately 3.6 to 2.2 billion years ago [8], and then split into two different continents.

Evidence for life

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Preserved life in Archean rocks has been altered over its 3.5 billion year history, and thus can be difficult to distinguish. The cell wall structure can be preserved, but the original composition changes over time and becomes mineralized. There are six established criteria to determine the plausibility of a given microstructure being a microfossil[9][10]

True Microfossils should...

(1) be of relatively abundant occurrence.

(2) be of carbonaceous composition, or, if mineralic, be biologically precipitated (for example, some bacteria form pyrite due to metabolic processes[11]

(3) exhibit biological morphology (see following section)

(4) occur in a geologically plausible context (for example, there are no microfossils in igneous rock, because life cannot grow in molten lava).

(5) fit within a well-established evolutionary context (for example, complex microfossils are highly unlikely to exist at 3.5 million years, as they have yet to evolve from their more simple cellular ancestors).

(6) be dissimilar from non-biogenic carbonaceous matter (see Isotope Analysis section)

Cell morphology
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Cells are preserved in the rock record because their cell walls are made of proteins which convert to the organic material kerogen as the cell breaks down after death. Kerogen is insoluble in mineral acids, bases, and organic solvents[12]. Over time, it is mineralized into graphite or graphite-like carbon, or degrades into oil and gas hydrocarbons[13].

Three main types of Archaean cell morphologies

There are three main types of cell morphologies. Though there is no established range of sizes for each type, spheroid microfossils can be as small as about 8μm, filamentous microfossils have diameters typically less than 5μm and have a length that can range from 10s of μm to 100μm, and spindle-like microfossils can be as long as 50 μm [1][14] In the Barberton Greenstone Belt, all types of morphologies are found, typically in black or banded cherts[1].

Isotope analysis
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Stable isotope fractionation is a useful way of characterizing organic carbon and inorganic carbon. These numbers are reported as δ13C values, where C is for the chemical element carbon. Isotope Analysis of inorganic carbon typically yields δ13C values heavier than -10 per mil, with numbers usually falling between -5 and 5 per mil. Organic carbon, however, has δ13C values that range from -20 per mil for photoautotrophic bacteria [15] to -60 per mil for microbial communities that recycle methane[16]. The large range in values for organic carbon has to do with the cellular metabolism. For instance, an organism that uses photosynthesis (a phototroph) will have a different isotope δ13C value than an organism that relies on chemical substances for energy (an autotroph). Many isotope analyses have been done on possible organic material in the Barberton Greenstone Belt. Recently, however, isotope data interpretation has been questioned, due to the fact that other, non-biologic processes could cause such negative fractionation.

Fossil record

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The oldest microfossils from the Barberton Greenstone belt are found in the Onverwacht Group, specifically, in both the Kromberg and Hooggenoeg Formations[1]. Both of these formations are predominantly igneous rock; the sedimentary rock has been metamorphosed. However, it is still possible to find microfossils in chert, a type of evaporite that forms in sedimentary environments. From the evidence in these rocks, it is likely that early life existed in the form of microbial mats and stromatolites. Evidence for this hypothesis is preserved in both chert and lithified stromatolites [1]

Stromatolites represent large colonies of microorganisms, and are found both in the fossil record and in modern environments. A typical stromatolite consists of alternating layers of sediment and microbes. The microbes are photosynthetic; thus stromatolites represent shallow water environments in fossil record due to their necessity to exist in the photic zone of water bodies. Stromatolites typically consist of filamentous microfossils[17]. The oldest stromatolites have been dated to approximately 3.5 billion years old[18]. Stromatolites in Barberton have been dated to about 3.3 billion years.

Microfossils found in chert extend the Barberton microfossil record back to 3.5 billion years. All three types of microfossil morphologies are found in cherts. Chert can have a variety of colors, but microfossils are typically found in black cherts, as the dark color can indicate organic material[1].

Future applications

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Scientists have established the approximate age that life first appears in the fossil record, but that doesn't necessarily equal the time that life evolved on Earth. Though fossils have not been found in older rocks, evidence for life can be found in other ways, such as extended carbon isotope data and Raman Spectroscopy. There is also ongoing work within the scientific community to solve the problem of how cellular life evolved in a hostile early earth.

References

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  1. ^ a b c d e f Walsh, M. (1991). "Microfossils and possible microfossils from the early Archean Onverwacht Group, Barberton mountain land, South Africa". Precambrian Research. 54 (2–4): 271–293. doi:10.1016/0301-9268(92)90074-X. PMID 11540926.
  2. ^ Westall, F. (2000). "Extracellular polymeric substances as biomarkers in terrestrial and extraterrestrial materials". Geophisical Research. 105 (10): 24511–24527. doi:10.1029/2000JE001250. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ Schopf, J.W. (2006). "Fossil evidence of Archaean life". Philos Trans R Soc. 361 (1470): 869–885. doi:10.1098/rstb.2006.1834. PMC 1578735. PMID 16754604.
  4. ^ Mojzsis, S.J. (2007). "Evidence for life on earth 3,800 million years ago". Nature. 384 (6604): 55–58. doi:10.1038/384055a0. PMID 8900275. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  5. ^ McKeegan, K. D. (2007). "Raman and ion microscopic imagery of graphitic inclusions in apatite from older than 3830 Ma Akilia supracrustal rocks, west Greenland". Geology. 35 (7): 591–594. doi:10.1130/G23465A.1. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Patterson, C. (1998). "Age of meteorites and the Earth". Geochimica et Cosmochimica Acta. 10 (4): 230–237. doi:10.1016/0016-7037(56)90036-9.
  7. ^ a b Cheney, E.S. (1996). "Sequence stratigraphy and plate tectonic signifigance of the Transvaal succession of southern Africa and it's equivalent in Western Australia". Precambrian Research. 79 (1–2): 3–24. doi:10.1016/0301-9268(95)00085-2.
  8. ^ Zegers, T.E. (1998). "Vaalbara, Earth's oldest assembled continent? A combined structural, geochronological, and palaomagnetic test". Terra Nova. 10 (5): 250–259. doi:10.1046/j.1365-3121.1998.00199.x. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  9. ^ Schopf, J.W. (1983). Archean microfossils: new evidence of ancient microbes. In: Schopf, J.W., Earth's Earliest Biosphere. Princeton University Press, New Jersey. pp. 214–239. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ Buick, R. (1984). "Carbonaceous filaments from North Pole, Western Australia: are they fossil bacteria in Archaean stromatolites?". Precambrian Research. 24 (2): 157–172. doi:10.1016/0301-9268(84)90056-1.
  11. ^ Ohmoto, H. (1993). "3.4-Billion-year-old biogenic pyrites from Barberton, South Africa: sulfur isotope evidence". Science. 262 (5133): 555–557. doi:10.1126/science.11539502. PMID 11539502. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ Philp, R.P. (1976). "Possible origin for insoluble organic (kerogen) debris in sediments from insoluble cell-wall materials of algae and bacteria". Nature. 262 (5564): 134–136. doi:10.1038/262134a0. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  13. ^ Tegelaar, E.W. (1989). "A reappraisal of kerogen formation". Geochimica et Cosmochimica Acta. 53 (11): 3103–3106. doi:10.1016/0016-7037(89)90191-9. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  14. ^ Oehler, D.Z. (2006). "Chemical Mapping of Proterozoic Organic Matter at Submicron Spatial Resolution". Astrobiology. 6 (6): 838–850. doi:10.1089/ast.2006.6.838. PMID 17155884. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  15. ^ Schidlowski, M. (1983). Earth's Early Biosphere: 149-186 (Princeton University Press). {{cite journal}}: Missing or empty |title= (help); Text "(ed. J.W. Schopf)" ignored (help)
  16. ^ Schidlowski, M (1988). "A 3,800-million-year isotope record of life from carbon in sedimentary rocks". Nature. 333 (6171): 313–318. doi:10.1038/333313a0.
  17. ^ Byerly, G.R. (1986). "Stromatolites from the 3,300 to 3,500-myr Swaziland Supergroup, Barberton Mountain Land, South Africa". Nature. 319 (6053): 489–491. doi:10.1038/319489a0. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  18. ^ Allwood, A. (2010). "Trace elements record depositional history of an Early Archaean stromatolitic carbonate platform". Chemical Geology. 270 (1–4): 148–163. doi:10.1016/j.chemgeo.2009.11.013. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
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