Astrobiology Science Conference 2017 (LPI Contrib. No. 1965)
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BIOLOGICAL AFFINITIES OF ARCHEAN SPINDLE-LIKE MICROFOSSILS A. C. Robinette, and G. J.
Retallack, Dept. of Earth Sciences, University of Oregon Eugene, OR 974031272 ([email protected] and
[email protected]).
Introduction: Archaean spindle-like microfossils
of the 3.0 Ga Farrell Quartzite [1] and 3.42 Ga Strelley
Pool Chert [2] of Western Australia and the 3.43 Ga
Kromberg Formation of South Africa [3] have attracted
very different biological interpretations.
Acritarch hypothesis: The spindle-like structures
may have been the resting cysts of prokaryotic photosynthetic marine planktonic cells [4]. Expectations for
that interpretation include radially symmetrical shape,
limited size range (5-30 μm), limited range of carbon
isotopic composition (δ13C -25±5‰ for cyanobacteria;
δ13C -34±5‰ for sulfur bacteria), few appendages or
attached filaments, and no internal bodies.
Figure 1. Spindle-like microfossil in a petrographic
thin section from the 3.0 Ga Farrel Quartzite Western
Australia. University of Oregon specimen number
R4348.
Actinobacteria hypothesis: Another idea is that
the spindle-like structures were sporangia of Actinobacteria, now best known as prokaryotic soil decomposers [5]. Expectations for that hypothesis include
asymmetric and elongate shape, large size (5-100 μm),
highly varied carbon isotopic composition (δ13C -13 to
-50 ‰), attached filaments and multiple internal bodies.
Our observations: The spindle-like structures in
thin section show an elliptical shape with flange-like
projections. Our 3D imaging of the structures confirms
that most are asymmetrical and more like a calzone
than a discus. Published isotopic compositions of spin-
dlelike microfossils [4,6] are quite varied (δ13C range
of -30 to -40.5 ‰). We have also noted filaments attached to one end, and occasionally coming from walls.
They are much larger (20-60 µm in length) than other
spheroidal microfossils in the same thin section. Most
spindles are empty of inclusions, but some have as
many as three near-spheroidal internal bodies.
Conclusions: Our observations support interpretation of the spindle-like microofssils as actinobacterial
sporangia of permineralized paleosols [5].
References: [1] Sugitani et al. (2009) Astrobiology
9, 603-615. [2] Sugitani et al. (2013) Precambrian
Research 226, 59-74. [3] Walsh, M. (1992) Precambrian Research 54, 271-293. [4] House et al. (2013)
Geology 42, 651-654. [5] Retallack et al. (2016)
Gondwana Research 40, 1-20. [6] Lepot et al. (2013)
Geochim. Cosmochim. Acta 112, 66-86.