Biosignature analysis: Analysis and interpretation of new petrographic and micro-Raman data on a specimen relevant to the origin of life, geobiology or exobiology

Description: Analysis and interpretation of new petrographic and micro-Raman data on a specimen relevant to the origin of life, geobiology or exobiology

Learning outcomes:

• –  Introduce rock and mineral specimens related to the origin of life, biosignatures and exobiology

• –  Create a complete sketch of a key hand sample or mineral assemblage

• –  Describe scientific observations completely and concisely

• –  Interpret observations of the abiotic and biological carbon cycle in the context of the literature

• –  Write a report of the new information

  Practical skills to gain:

  – How to microscopy image files should be named
  – Analyse micro-Raman data of organic matter in the mineral assemblage of a target of interest – Basic description of rocks and minerals
  – Writing a brief scientific report with new data

  Plan:

• –  Choose a geobiological or cosmochemical rock specimen and I will provide you with a new unpublished dataset for you to analyze.

• –  Do some background search in the peer-reviewed literature on the chosen rock and organic matter occurrences that have been reported in similar rocks.

• –  Write a short paper-style report of the new findings along with interpretations in the context of exobiology.

  Introduction:

  Your task now is to collect background information on your chosen rock specimen and contribute a new analysis of unpublished microscopy and micro-Raman data from a target with organic matter and relate it to the origin of life, biosignatures, or exobiology.

Description of a rock specimen

• –  In a hand specimen, field outcrop, or polished slab or section, describe colours of phases, notable features, patterns, and grain size and shape.

• –  List/enumeration of minerals and their observed diagnostic properties in the polarising microscope (taking note of relief, colour, cleavage, pleochroism, birefringence, and reflective colour).

• –  Details from micro-Raman observations (evidence for organic matter including key peak positions and qualitative description of peaks shape (width is narrow, medium, or broad? and intensity is weak, medium or strong?), peak positions of associated minerals, comparison with other spectra (literature or rruff.info), and patterns from hyperspectral images).

• –  Other relevant details from the rock type (provenance, age constraints, peak metamorphic grade) – For this, you need to peruse the scientific literature.

  For example, in a text form, here are some brief examples:

  In outcrop, slab, or whole thin section

  The rock specimen of stromatolitic phosphorite is composed of a uniform white-cream coloured carbonate groundmass with millimetre-wide veinlets of recrystallised quartz and carbonate. In outcrops, two minerals can be distinguished based on their variable weathering, whereby apatite is harder and topographically high, and carbonate is softer and topographically low. Non-branching columnar stromatolites are dominantly composed of apatite and vary in diameter between 2 and 6 cm. Some outcrops have occasional decimetre size cavities lined with green coloured botryoidal quartz (hardness is 7). In whole thin section, the stromatolite column is dark gray in colour, whereas the carbonate is distinctively white in colour. Convex upward laminations of carbonate are interlayered in the apatite-dominated stromatolite columns.

  In the polarising microscope

  In the thin section, the rock has a micro-crystalline grain size and both calcite and dolomite can be distinguished from their cleavage, best seen in some larger grains. Apatite crystals have dimensions less than 1 mm, and these crystals are microcrystalline and most often (>90% of all apatite) between 4 and 20 μm in size. Calcite and dolomite co-occur in stromatolite columns, but dolomite is dominant in the intercolumnar space. In plane polarised light, the apatite is easily distinguished from carbonate, because the apatite colour is darkened by disseminations of graphitic carbons and the lighter white-cream colour phase is carbonate. In cross-polarised light, calcite and dolomite both display up to 8th order white-pink-green colours, while apatite is dark gray from the combination of low first order interference colours and ubiquitous graphitic stains. There are occurrences of large (up to 100 micron) single crystals of euhedral carbonate that form subhedral rhombohedra as well as opaque euhedral cubes that have a yellow reflection, probably pyrite. All crystals appear to be randomly oriented within the rock specimen. At the millimetre scale, there are bands of darker gray, pepper-like disseminations in the carbonate matrix, which are likely nanoscopic particles of graphitic carbons.

  In the micro-Raman

An area selected for its highly variable concentrations of graphitic carbons was analysed from micro-Raman spectroscopy. Spectra show a systematic co-occurrence of graphitic carbon in apatite, located inside the stromatolite columns. The spectra of the dominant graphitic carbon shows a strong and medium D1 peak at 1345-1349 cm-1 and a strong and narrow G peak at 1585- 1600 cm-1. A weak and narrow peak at 2710 cm-1 also occurs in this graphitic carbon. The spectrum resembles those of graphitic carbons heated at 370 to 440oC (based on Beyssac et al., 2002 (J. Met. Petrol.)). In comparison, some spectra of another minor type of graphitic carbon have strong and broad D1 peaks 1355-1360 cm-1 and strong and medium G peaks at 1595-1602 cm-1, which yields a peak metamorphic temperature of 315 +/- 50oC (based on Kouketsu et al., 2014 (Island Arc); Lafhid et al., 2010 (Terra Nova)). Spectra of carbonate show small variations in the main carbonate peak position between 1088 and 1097 cm-1, independently confirming the co- occurrence of calcite and dolomite. The Raman spectrum of apatite shows a dominant strong and narrow peak at 965 cm-1 (P-O vibrations) as well as a medium and broad peak at 3300 cm-1 (O-H vibrations). Lastly, the presence of medium and narrow peaks forming a doublet at 343 and 375 cm-1, independently confirms the presence of pyrite in these stromatolites.

Contents of the project report – one report per student

– Introduction (10 points)
o Identify a scientific problem about the rock type you chose
o Pose a specific scientific question about the sample you chose
o Propose a hypothesis that is addressed by the data provided (normally, you would

pose this hypothesis before testing it with an instrument)

– Materials and Methods (10 points)
o Obtain and describe briefly some background geological information on the locality

and age of your chosen sample
o Describe briefly (<100 words) the methods inspired by descriptions in the literature.

§ Polarising optical microscopy § Micro-Raman spectroscopy

– Results (30 points)
o Describe in words and present figures of new data of the specimen collected at low

spatial resolution
o Describe the observations of the selected target in the specimen at high spatial

resolution
o Create adequate figures with multiple panels, annotations, scales, and informative

captions.

– Discussion (30 points)
o Develop arguments that your new data is representative of the original

sedimentary environment, or not.
o Describe and compare to occurrences in the literature how might it have been

related to the presence or activity of microorganisms or abiotic carbon cycling? o How does your (a)biosignature(s) compare to the peer-reviewed literature?

o Was your hypothesis adequately (or fully, or partly) tested? – Why? How do your new data advance knowledge about the origin of life and exobiology?

– Conclusion (10 points)
o One or two sentences about the context of your new observations on your chosen

rock type
o Briefly and specifically summarize your new observations
o Summarize the main interpretations and implications
o Propose a future direction that could be used to further address the scientific

question in the future
– References, presentation, organization and grammar (10 points)

Geobiological sample to investigate (subject to availability):

• carbonaceous chondrite

  
  

• Marking:

  – You will be evaluated on the basis of the completeness of your descriptions, the quality of English writing, the substance of your comparisons and arguments, the organization and presentation of your report, the presentation of your data (figures according to what you have been asked before in this module), analysis and discussion of your data, and for the main elements described in the sections above.

  – The figure(s) will be evaluated for relevance, correctness, and quality.
  – It is also important that you focus primarily on the scientific aspects of your topic and not on the opinions of others, the socio-economic context, political implications, etc. Stay focused on the origin of life, biosignatures, and exobiology