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Developing a Trace Element Biosignature for Early Earth and Mars

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Developing a Trace Element Biosignature for Early Earth and Mars 2nd International Mars Sample Return 2018 (LPI Contrib. No. 2071) 6018.pdf DEVELOPING A TRACE ELEMENT BIOSIGNATURE FOR EARLY EARTH AND MARS. A. Gangidine1, A. D. Czaja1 and J. Havig2. 1Department of Geology, University of Cincinnati ([email protected], [email protected]) 2Department of Earth Sciences, University of Minnesota ([email protected]). Introduction: Impending missions focusing on the to be a valuable tool for the search for extraterrestrial search for life outside of our planet require the devel- life in precious samples, particularly from those col- opment of robust and conclusive biosignatures. Due to lected by the upcoming Mars 2020 mission. metamorphism and diagenesis, determining the bio- genicity of ancient fossils on Earth is difficult and often contentious (1, 2). Some of the oldest evidence for life on Earth comes from hydrothermal silica deposits (3, 4), which may also exist on the surface of Mars (5, 6, 7, 8) in at least one of the candidate landing sites for the Mars 2020 mission. We report here our initial re- sults and plan to further develop a novel biosignature for ancient terrestrial and extraterrestrial life based on trace elements sequestered by life and preserved in the rock record. Preliminary data from modern organisms preserved in terrestrial hydrothermal silica-depositing environments indicate that enrichments of certain trace elements are spatially associated with biological mate- rial relative to the surrounding mineral matrix. Samples were collected from Steep Cone in Yellowstone Na- tional Park, a 9-m-tall active, alkaline, hot spring that has layers of silica precipitate exposed along the side of the cone by a stream cut. The silica-rich water flow- ing from the hot spring preserves the native microbial Figure 1: A sample of BIOSIMS analyses of silici- life in sinter. Samples taken from various levels of the fied filaments from Steep Cone. A) Plain light photo- strata allow for the microbes to be observed in multiple micrograph of a sample taken from the top of Steep stages of silicification/diagenesis. Samples were ana- Cone. The dotted yellow boxes in A & B show the are- lyzed via biological secondary ion mass spectrometry as analyzed via SIMS in subsequent images to the right (BIOSIMS) for primary and trace elements (Figure 1). of A & B, and the dotted red outlines surround the area Using BIOSIMS, we can determine the concentration of the filaments bisecting the surface (and thus ana- and spatial location of trace elements in each sample lyzed by the BIOSIMS) of each thin section. Subse- on a micron scale across biological structures and into quent images show SIMS data for each noted element. the surrounding silica matrix. Most of the measured B) Plain light photomicrograph of a sample taken from trace elements (Fe, As, Al, Cr, and Mn) are found to be the base of Steep cone. Subsequent images show SIMS co-localized with preserved organics, while Ga is data for each noted element. Color bar on right shows found to be co-localized with siliceous coatings that relative values from low (blue) to high (red). The scale surrounded the microbes during life and were subse- bar in panel A = 20 µm and applies to all images. quently preserved. By developing this novel biosigna- References: [1] Brasier M. D. et al. (2002) Nature ture and combining it with multiple techniques for es- 416.6876, 76-81. [2] Schopf J. W. et al. (2002). Na- tablishing biogenicity, we can find more robust evi- ture, 416, 73–76. [3] Walter M. R. et al. (1993) Icarus dence of life. Based on the observation that trace ele- 101.1, 129-143. [4] Djokic T. et al. (2017) Nature ments are sequestered in silica associated with mi- Communications 8. [5] Squyres S. W. et al. (2008)" crobes and not in the surrounding silica matrix, it is Science 320.5879, 1063-1067. [6] Ruff S. W. et al. possible that these elemental distributions can be used (2011) Journal of Geophysical Research: Planets as a biosignature that exists independently from organ- 116.E7. [7] Ruff S. W. and Farmer J. D. (2016) Nature ic and morphological preservation. This technique is communications 7, 13554. [8] Thomas R. J. et al. performed using thin sections and each analysis re- (2017) Geophysical Research Letters, 44(13), 6579- quires the ablation of a volume of material on the order 6588. of 10-5 mm3 and is thus minimally invasive and mini- mally destructive. Therefore, this technique may prove .
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