Alison Olcott Marshall

The potential of Raman spectroscopy for the analysis of diagenetically transformed carotenoids

Recently, carotenoids have received much attention as target compounds for astrobiological prospecting principally because they are a group of molecules that display unique diagnostic Raman spectra that can be assigned to organic material of unequivocal biological origin. However, no work has been performed on assessing the potential of Raman spectroscopic detection of carotenoids from fossilized microbes. Here, we report the first Raman spectra acquired from ‘perhydro’ derivatives of β-carotene and lycopene formed by hydrogenation of the polyene chain during diagenesis, resulting in much less specific fossil hydrocarbons such as β-carotane and lycopane, respectively. We propose here that diagenetically altered carotenoids formed by hydrogenation reactions during the fossilization processes also provide unique diagnostic spectra that can be interpreted as a biological signature.

Multiple Generations of Carbon in the Apex Chert and Implications for Preservation of Microfossils

While the Apex chert is one of the most well-studied Archean deposits on Earth, its formation history is still not fully understood. Here, we present Raman spectroscopic data collected on the carbonaceous material (CM) present within the matrix of the Apex chert. These data, collected within a paragenetic framework, reveal two different phases of CM deposited within separate phases of quartz matrix. These multiple generations of CM illustrate the difficulty of searching for signs of life in these rocks and, by extension, in other Archean sequences.

Raman Hyperspectral Imaging of Microfossils: Potential Pitfalls

Initially, Raman spectroscopy was a specialized technique used by vibrational spectroscopists; however, due to rapid advancements in instrumentation and imaging techniques over the last few decades, Raman spectrometers are widely available at many institutions, allowing Raman spectroscopy to become a widespread analytical tool in mineralogy and other geological sciences. Hyperspectral imaging, in particular, has become popular due to the fact that Raman spectroscopy can quickly delineate crystallographic and compositional differences in 2-D and 3-D at the micron scale. Although this rapid growth of applications to the Earth sciences has provided great insight across the geological sciences, the ease of application as the instruments become increasingly automated combined with nonspecialists using this techique has resulted in the propagation of errors and misunderstandings throughout the field. For example, the literature now includes misassigned vibration modes, inappropriate spectral processing techniques, confocal depth of laser penetration incorrectly estimated into opaque crystalline solids, and a misconstrued understanding of the anisotropic nature of sp² carbons.

Hematite and carbonaceous materials in geological samples: a cautionary tale.

Over the last few decades Raman spectroscopy has been increasingly applied as an analytical tool in geoscience research. Raman spectroscopy is a powerful tool for geologists as it is non-destructive, requires little to no sample preparation, and can be undertaken in situ on various irreplaceable geological samples. Also, this technique is useful in the identification of minerals and geo-organic material. However, despite this ease of application, there are some facets of Raman spectroscopy data that can lead to erroneous interpretations. For instance, there is much confusion in the geological literature distinguishing the difference between the hematite vibrational mode at ca. 1320 cm(-1) and the disordered sp(2) carbonaceous material D band at 1340 cm(-1). Furthermore, geologists will often collect 2 spectra, one in the mineral finger print region (200-800 cm(-1)) and then a spectrum in the carbon first-order region (1000-1800 cm(-1)), rather than performing a full-region scan. This allows the misidentification of the hematite mode at 1320 cm(-1) as the D band from disordered carbonaceous material. Here we show that it is best practice for geologists to collect spectra between 200 and 1800 cm(-1) to better distinguish between hematite and disordered carbonaceous material, materials that often co-occur in geological samples.

RAMAN SPECTROSCOPIC INVESTIGATIONS OF BURGESS SHALE–TYPE PRESERVATION: A NEW WAY FORWARD

Abstract Study of the Burgess Shale-type deposits of the Cambrian has greatly enhanced scientific understanding of early animal evolution, but as the mechanisms by which these deposits formed are still unclear, here we outline and present data from the application of a new analytical approach, Raman spectroscopy, that can be used to characterize fossils from these deposits. Although these deposits present exceptional views into a diverse, nonbiomineralized to lightly biomineralized biota, this taphonomic regime mostly disappears from the fossil record after the Cambrian, with a few notable exceptions. Numerous detailed taphonomic and chemical studies have provided significant insight into modes of fossil preservation in these deposits, although there is still significant debate regarding the preservational and diagenetic mechanisms that may be involved. Compared to previous electron microscopy-based elemental mapping approaches, which have identified the elemental components of similar fossils, Raman spectroscopy allows a determination of the chemical phases and specific mineralogy at the molecular level, as well as the thermal maturity, of these fossils. This approach therefore provides new types of data, such as hematite phase, that may prove helpful for elucidating some of the mechanisms responsible for the exceptional types of preservation found in these deposits, and potentially helps to resolve the existing taphonomic debates.

Field-Based Raman Spectroscopic Analyses of an Ordovician Stromatolite

Raman spectrometers are being miniaturized for future life-detection missions on Mars. Field-portable Raman spectrometers, which have similar spectral parameters to the instruments being developed for Mars rovers, have been used to examine extant biosignatures, but they have not yet been used to examine ancient biosignatures. Here, a portable Raman spectrometer was used to analyze an Ordovician stromatolite at the outcrop, revealing both its mineralogy and the presence of sp² carbonaceous material. As stromatolites are often used as proof of the presence of life in Archean rocks and are searched for on Mars, the ability to analyze them in the field with no sample preparation has important ramifications for future Mars missions. However, these results also reveal that a 785 nm excitation source, rather than the 532 nm excitation source planned for future missions, might be a better choice in the search for fossil biosignatures.

Microchemical differentiation of conodont and scolecodont microfossils

ABSTRACT Conodonts and scolecodonts are common Paleozoic microfossils that are often used to determine the geothermometry of Paleozoic sequences. The two fossils, which can be difficult morphologically to distinguish one from the other, undergo a different thermal alteration pathway. In this study, Fourier transform infrared (FTIR) spectroscopy of scolecodont and conodont microfossils from the Woodford Shale of southern Oklahoma, United States, confirmed they have different chemical compositions. Infrared (IR) spectra acquired from conodonts show a predominance of an inorganic carbonated hydroxylapatite (CO3OHAp) with a minor organic composition of aliphatic hydrocarbon, containing carbonyl substituent functional groups. In contrast, IR spectra acquired from scolecodonts show no inorganic mineralogy but instead confirm that these microfossils are composed of organic material consisting of an aliphatic and aromatic hydrocarbon network with ether linkages and carbonyl substituent functional groups. These data reveal that scolecodont elements can easily be distinguished from conodont elements with FTIR microspectroscopy due to their different chemical compositions. This study provides future fossil workers with a viable method to independently identify enigmatic toothlike microfossils that cannot confidently be assigned to either scolecodont or conodont groups by morphology alone.

Raman spectroscopy as a screening tool for ancient life detection on Mars

The search for sp2-bonded carbonaceous material is one of the major life detection strategies of the astrobiological exploration programmes of National Aeronautics and Space Administration and European Space Agency (ESA). The ESA ExoMars rover scheduled for launch in 2018 will include a Raman spectrometer with the goal of detecting sp2-bonded carbonaceous material as potential evidence of ancient life. However, sp2-bonded carbonaceous material will yield the same Raman spectra of well-developed G and D bands whether they are synthesized biologically or non-biologically. Therefore, the origin and source of sp2-bonded carbonaceous material cannot be elucidated by Raman spectroscopy alone. Here, we report the combined approach of Raman spectroscopy and gas chromatography–mass spectrometry biomarker analysis to Precambrian sedimentary rocks, which taken together, provides a promising new methodology for readily detecting and rapidly screening samples for immature organic material amenable to successful biomarker analysis.

Raman spectroscopic documentation of Oligocene bladder stone.

Discovery of a fossil (30–35 million-year-old) urolith from Early Oligocene deposits in northeastern Colorado provides the earliest evidence for the antiquity of bladder stones. These are spherical objects with a layered phosphatic structure and a hollow center. Each layer is composed of parallel crystals oriented perpendicular to the surface. Macroscopic and microscopic examination and X-ray diffraction analysis, along with comparison with 1,000 contemporary uroliths, were used as evidence in the confirmation of this diagnosis. Raman microspectroscopy verified the presence of organic material between layers, confirming its biologic origin.

Imaging of Vanadium in Microfossils: A New Potential Biosignature

The inability to unambiguously distinguish the biogenicity of microfossil-like structures in the ancient rock record is a fundamental predicament facing Archean paleobiologists and astrobiologists. Therefore, novel methods for discriminating biological from nonbiological chemistries of microfossil-like structures are of the utmost importance in the search for evidence of early life on Earth. This, too, is important for the search for life on Mars by in situ analyses via rovers or sample return missions for future analysis here on Earth. Here, we report the application of synchrotron X-ray fluorescence imaging of vanadium, within thermally altered organic-walled microfossils of bona fide biological origin. From our data, we demonstrate that vanadium is present within microfossils of undisputable biological origin. It is well known in the organic geochemistry literature that elements such as vanadium are enriched and contained within crude oils, asphalts, and black shales that have been formed by diagenesis of biological organic material. It has been demonstrated that the origin of vanadium is due to the diagenetic alteration of precursor chlorophyll and heme porphyrin pigment compounds from living organisms. We propose that, taken together, microfossil-like morphology, carbonaceous composition, and the presence of vanadium could be used in tandem as a biosignature to ascertain the biogenicity of putative microfossil-like structures. Key Words: Microfossils-Synchrotron micro-X-ray fluorescence-Vanadium-Tetrapyrrole-Biosignature. Astrobiology 17, 1069-1076.