Introduction

The Gunflint chert of the northern Lake Superior region is of particular interest to the field of astrobiology because of its extraordinarily well-preserved microfossils and Precambrian age. Microfossils ~1.9 Ga in age from the Gunflint formation provided the first well-supported evidence suggesting the existence of life during the Precambrian. These specimens containing ancient life are critical to understanding both the origins of life on earth and in the great difficulty of detecting extraterrestrial life in extraterrestrial samples. Using a variety of advanced microscopy and microanalysis tools, this study focused on characterizing the microchemistry of the carbon in the Gunflint chert. Further microchemical evaluation should lead to an improved understanding of chemical biosignatures for preserved primitive life forms.

Description of Microfossils

• Microfossils observed in the Gunflint chert include filaments ranging from .5-5 µm in diameter, and spheroidal bodies 1-20 µm in diameter consistent with earlier studies.
• Microorganisms have been classified as extinct prokaryotic cyanobacteria.
  Organisms are found in association with kerogen and other organic residues in an impure chert matrix.

Matrix Characterization

• Chert is composed of fine-grained SiO2 crystals with minor pyrite.
  Quartz grains are randomly oriented in the matrix with no apparent order near microfossils.
• Chemical impurities in quartz are homogenously distributed (CL).
  Organic residues within quartz are non-uniformly distributed.

SEM and X-Ray Analysis

• Low-vacuum scanning electron microscopy of uncoated samples was used to avoid carbon typically applied during specimen preparation.
• Concentrations of elements of interest including nitrogen, iron, and sulfur were below the detection limits of this method.
• X-ray analysis revealed small differences in carbon content of microfossils. Variations were in part due to removal of carbon from microfossils during sample preparation.
  
• Difficulty in detecting carbon was attributed to strong absorption of carbon X-rays by silicon. “Electron Flight Simulator,” a computer program used to mimic electron scattering and X-ray generation, yielded simulations showing no carbon signal emission from below ~0.5 µm for 5-13 kV electron beams.

Laser Raman Imaging Spectroscopy

• Raman spectroscopy is highly sensitive to carbon-bearing molecules. It is also capable of distinguishing graphitic (G) from disordered (D) carbon.
  
  
• The ratio of graphitic (crystalline) carbon to disordered (organic) carbon is considered an indicator of thermal maturity.
  
  
• Average G/D ratio in sample area was 1.9. Fries and Steele (in progress) recently reported an average G/D ratio in the same sample as 1.4.
  
  
• Organic residues in microfossils and in the chert matrix have the same G/D ratio.

Focused Ion Beam

A focused ion beam (FIB) uses an energetic beam of Ga ions focused to the nanometer length scale capable of performing micro-milling of a specimen. Such ion milling was used to extract a thin wafer from the sample.  The wafer is sufficiently thin (100s of nanometers) to transmit electrons through the sample using scanning transmission electron microscopy (STEM).

This method can be used to look for elemental species associated with the carbon on a submicrometer length scale. STEM work is currently in progress.

Summary

• Low-vacuum SEM x-ray method was not useful for identifying carbon because the carbon signal was strongly absorbed by the silica matrix.
• Increased G/D carbon ratio suggests local heating has occurred in the chert. This may be due to small-scale hydrothermal veins.
• Equivalent G/D carbon ratios in the microfossils and in the chert suggest carbon in both types have the same or similar sources.

Acknowledgements

We gratefully acknowledge Marc Fries and Andrew Steele from the Carnegie Institution of Washington, Babak Nikoobakht from the National Institute of Standards and Technology, Tim Rose, Tim Gooding, Scott Whittaker, Peter Heaney (Penn State University), and S.M.C. for their technical help, discussions, and support.

This research was supported by the Bill and Jean Lane Endowment.