Mars Sediment Analog? Dark Biomineralized Mn-oxide/hydroxide Cemented Sandstone of Low-t Spring Origin

SANDSTONE OF LOW-T SPRING ORIGIN. J. L. Berkley, Department of Geosciences, SUNY Fredonia, Fredonia, NY 14063 and Kaitlyn Dykstra, Department of Biological Sciences, Carnegie Mellon University, Pittsburgh, PA 15213. Introduction: Detailed petrological analysis of a dark black to dark brown conglomeratic sandstone in southwestern New York State shows that its cementing agent is composed mainly of mixed Mn oxidehydroxide minerals (MOH), some with significant Ba (~10 wt%). Lamellar textures of these mineral precipitates strongly suggest bacterial or other microbiotic deposition. This study includes an attempt to characterize possible microbial species involved in precipitating this cementing agent. Although our sandstone occurs in an inactive, fossil spring setting, active springs and aqueous seeps occur nearby that may portray a model environment for the creation of our samples. The deposition environment of this sandstone may mimic early Martian environments where subsurface bacteria-laden water deposited Mn minerals as thinlylaminiated biofilm structures along clastic grain boundaries. The resulting indurated sedimentary rocks would likely take on a uniformly dark color. Analytical: Samples of the black sandstone were analyzed using standard light-optic petrographic techniques, scanning electron microscopy with quantitative EDS spectrography (SEM incl. BSE; SUNY Buffalo), x-ray fluorescence (XRF) semi-quantitative analysis (SUNY Fredonia), and by quantitative electron microprobe (SEMQ; RPI, Troy, NY). Genomic DNA was isolated from 4 samples of the active biofilm using a Qiagen DNeasy kit. Sequencing of the PCR amplified 16S rRNA gene was performed on a LiCor 4300 DNA Analyzer at SUNY Fredonia and was classified using the Ribosomal Database Project [http://rdp.cme.msu.edu]. Textures: Framework grain size ranges from siltsize through small pebbles (~1-3 cm dia.), but most grains fall in the “sand” range, averaging 1.5 mm dia. Grains are generally angular and consist of mostly quartz, feldspar (Kand plagioclase), mafic silicates, and opaque oxides along with shale fragments from the local Devonian bedrock (Gowanda Fmn.). Manual point counts suggest a porosity of about 15%. MOH cementing minerals attached to framework grains occur as compositionally homogeneous spacefilling patches, and as finely laminated colloform or botryoidal structures (fig. 1). The latter texture is dominant and ubiquitous throughout the rocks, and is consistent with an origin by bacterial agents, e.g., [1,2,3]. Biofilm production progressed by adding new MOH layers upon early layers at the interface with permeating hydrous fluids. The speckled layers in fig. 1 commonly produce lower-than-normal electron microprobe totals along with chlorine peaks, possibly indicating the presence of trapped organic material some of which may be remnant bacteria remains [2,3]. Mineral Compositions: SEMQ spot analyses of botryoidal precipitates show at least two different dominant compositions, one Ba-rich and the other Bapoor. Ba-rich areas average about 50 wt.% Mn and 10 wt% Ba, while Ba-poor areas average 51 wt% Mn and 0.5 wt% Ba. Calculated oxygen in both cases is slightly over 16% with minor (<<1%) abundances of Na, Al, Si, Cl, K, Ti, and Ca. Totals for analyses are consistently low, ~ 78% for high-Ba points and 70% for low-Ba points. These low totals suggest the presence of hydroxide or other amorphous or volatile components. Mn oxides and hydroxides commonly occur as intimate intergrowths in "bog" ores and other terrestrial Mn deposits. Point analyses of Mn-rich cement suggest a fine mixture of pyrolusite (MnO2) plus holandite (Ba(0-2)(Mn,Fe,Al)8(O,OH)16 or romanechite (Ba,H2O)2