Identifying elements in rocks from the Dry Valleys desert (Antarctica) by ion beam proton induced X-ray emission

Abstract

In some zones of Antarctica’s cold and dry desert, the extinction of cryptoendolithic microorganisms leaves behind inorganic traces of microbial life. The extinction of these microorganisms is considered to be the best terrestrial analogue of the disappearance of possible life on early Mars. In the present study, sandstone rock samples from several sites of the Dry Valleys known to harbour endolithic microorganisms, as well as samples with microbial fossils, were analysed by proton induced X-ray emission (PIXE). Our findings suggest significant differences in major element concentrations among the different zones within the same sample. In some sample fractions, these differences could be considered as traces of the microbial origin. However, other samples reveal contamination produced by allochthonous minerals of abiogenetic origin.

Introduction

The endolithic microbial ecosystems of the Dry Valleys of Antarctica are a good terrestrial model of the last stages of life on early Mars [1]. In this cold desert, microbial life is mostly confined to a narrow layer beneath the rock surface [2]. In several zones of Dry Valleys microbial life expires. The extinction of microorganisms could leave behind indirect signs. The first life trace observed was a whitish iron-leaching layer attributed to the mobilization of iron hydroxides due to activity of cryptoendolithic lichens [3], [4]. Later, it was demonstrated that some minerals in Antarctic rocks are biogenetically transformed to generate inorganic biomarkers [5]. Recently through in situ scanning electron microscopy in backscattered electrons detection mode (SEM-BSE), microbial fossils left behind by endolithic microorganisms in Antarctica were demonstrated for the first time [6]. However, research into detecting inorganic traces of life, past and present, in these rocks requires much further work using unconventional microanalytical techniques. Indeed, according to [7], chemical analysis might be an essential tactic for exploring fossil evidence also on Mars. All robotic missions to Mars have been equipped with microanalytical devices based on detection of X-rays in order to determine chemical composition of the minerals.

The aim of our study was to give response on the question: could the present and/or past microbial life in Antarctic sandstones left chemical traces detectable with microanalytical systems? In the present study, rock samples from several sites of the Dry Valleys known to harbour endolithic microorganisms, as well as samples in which life was not present, were analysed by proton induced X-ray emission (PIXE) in order to detail characterization of rock fragments. For this purpose we have performed a macro PIXE measurement for the determination of the elemental composition in different fractions of the rocks from Dry Valley.