Discovery of Naturally Etched Fission Tracks and Alpha-Recoil Tracks in Submarine Glasses: Reevaluation of a Putative Biosignature for Earth and Mars

Jason E French,David F Blake

doi:10.1155/2016/2410573

Abstract

Over the last two decades, conspicuously “biogenic-looking” corrosion microtextures have been found to occur globally within volcanic glass of thein situoceanic crust, ophiolites, and greenstone belts dating back to ~3.5 Ga. These so-called “tubular” and “granular” microtextures are widely interpreted to representbona fidemicrobial trace fossils; however, possible nonbiological origins for these complex alteration microtextures have yet to be explored. Here, we reevaluate the origin of these enigmatic microtextures from a strictly nonbiological standpoint, using a case study on submarine glasses from the western North Atlantic Ocean (DSDP 418A). By combining petrographic and SEM observations of corrosion microtextures at the glass-palagonite interface, considerations of the tectonic setting, measurement of U and Th concentrations of fresh basaltic glass by ICP-MS, and theoretical modelling of the present-day distribution of radiation damage in basaltic glass caused by radioactive decay of U and Th, we reinterpret these enigmatic microtextures as the end product of the preferential corrosion/dissolution of radiation damage (alpha-recoil tracks and fission tracks) in the glass by seawater, possibly combined with pressure solution etch-tunnelling. Our findings have important implications for geomicrobiology, astrobiological exploration of Mars, and understanding of the long-term breakdown of nuclear waste glass.

Highlights

Understanding and successfully identifying examples of preserved microbial life from extreme environments on planet Earth are pertinent to the astrobiological exploration of Mars, and this was highlighted during recent debates over Martian meteorite ALH84001 (e.g., [1,2,3,4,5,6])
From (1), we calculate the presentday alpha-recoil track areal density in Deep Sea Drilling Project (DSDP)-418A basaltic glass to be very high at 148,000,000 alpha-recoil tracks/cm2, which indicates that these glasses are absolutely riddled with alpha-recoil track damage (Figures 10(h), 10(i), 15(c), and
Our discovery in the present study that microscopic etch-tunnels and granular palagonite microtextures in DSDP-418A basaltic glass originate primarily by preferential corrosion of radiation damage has immediate implications for geomicrobiology, microbial ecology, and studies aimed at identifying microscopic morphological biomarkers in volcanic/impact glasses on planet Earth

Summary

Introduction

Understanding and successfully identifying examples of preserved microbial life from extreme environments on planet Earth are pertinent to the astrobiological exploration of Mars, and this was highlighted during recent debates over Martian meteorite ALH84001 (e.g., [1,2,3,4,5,6]). Among these possible microbial biosignatures in rocks, probably the most contentious of all are the recognition of nano- to microscopic morphological biomarkers, especially because there is typically a great deal of subjectivity involved in deciding which of these tiny shapes and forms appear to look like microbial remains/traces based only on visual interpretations and comparisons to known terrestrial biotic microstructures, and, many such micro-/ nanofeatures have straightforward and readily deduced nonbiological explanations It is quite common at this scale of observation (e.g., under petrographic microscope or in high resolution scanning electron microscope (SEM) images) that there may be multiple explanations for such tiny physical structures in rock samples, including both biogenic and nonbiogenic (e.g., mineralogical) explanations, and three well-known examples of this include (1) abiotically produced nanoscopic mineral grains (i.e., calcite) in carbonate rocks that exhibit spherical, rod, and ovoid shapes resembling bacterial remains [20]; (2) filamentous and segmented carbonaceous microstructures in the ∼3.5 Ga Apex cherts of Western Australia resembling bacterial and cyanobacterial remains [21] that have been reinterpreted as abiogenic amorphous graphite [22]; and (3) concentrically zoned carbonate globules in Martian meteorite ALH84001 originally interpreted as bacterially induced carbonate precipitates [3] that were later reinterpreted as abiotic, high temperature, hydrothermally deposited minerals associated with volcanic activity—based on the discovery of similar carbonate globules in Spitsbergen, Norway [1, 6]. As highlighted above and in Section 3.4.2, the typical size of fully etched “pristine” fission tracks in volcanic glass (and tektites) is ∼8-9 μm long (Figure 3(b) [126, 128]; Figure 1 in Sandhu et al [198]; Figure 8(b) in Westgate and Naeser [199]; Figures 1 and 2 in Storzer and Wagner [218])

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Results

Conclusion