DOI: 10.5061/dryad.6djh9w13m
title: LIBS and raman spectral data in the qaidam analog

description: Biosignature detection is one of the most important goals in Mars
 missions. Since the Curiosity mission, the laser-induced breakdown
 spectrometer (LIBS) becomes an essential payload due to its convenience
 and versatility in profiling elemental chemistry. To test whether LIBS
 alone could filter potential biosignatures, a clastic quartz stone
 collected from a Mars analog setting, the western Qaidam Basin, was
 selected for LIBS analysis. Raman spectroscopy was used as an indicator of
 organic signals to support the presence of potential hypolithic
 communities and the dearth of epilithic biomass on the rock. A total of
 344 LIBS spectra were determined and statistically analyzed using
 principal component analysis (PCA). Our results indicate that, with a
 sufficient sample size, PCA analysis can partially differentiate biotic
 and abiotic signals based on LIBS measures. This finding is significant
 since it indicates that multivariate analysis of LIBS data can be useful
 for biosignatures filtering on Mars exploration.
description: Located on “the roof of the world” Tibetan Plateau, the western
 Qaidam Basin is a cold, dry, and irradiative environment that shapes
 itself with landforms (e.g., dunes, yardangs, playas, wind streaks,
 polygonal terrains, and gullies) commonly found on Mars. A clastic quartz
 stone was sampled from a Cenozoic gravel deposit (38°35′44″ N, 90°59′6″ E,
 3245.17 m altitude) from the hyperarid Dalangtan Playa, western Qaidam
 Basin, on 29 July 2021. The Cenozoic gravel deposit was likely derived
 from the weathering of Mesozoic (Pre-Jurassic and Jurassic) rocks, and
 quartz stones were common in the deposit. Visible light greenish color
 could be observed at the bottom of the quartz stone.
 Multiple spots of four vertical lines (11 spots for line 1 Qz-l1,
 6 spots for line 2 Qz-l2, 9 spots for line 3 Qz-l3, and 8 spots for line 4
 Qz-l4) of the Qaidam quartz stone were selected to stereoscopically
 investigate the spatial distribution of Raman spectra-based mineralogical
 or organic/biotic signals. An alpha 300R confocal Raman imaging system
 (WITec, Germany) incorporated with a 50x objective lens of numerical
 aperture = 0.55 and an excitation laser source of 532 nm was used for
 Raman spectral measurements. The laser wavelength was corrected using the
 Raman peak of a Si wafer. All spectra were acquired in a spectral range of
 0-4000 cm<sup>−1</sup> with a spectral resolution of 4.8
 cm<sup>-1</sup>. To retain the resolutions of both minerals
 and organic matter as much as possible, laser power was kept at 3.1 mW for
 an integration time of 3 s with the number of accumulations of
 30. To understand the elemental compositions and
 spectral features of chosen samples, the SciAps Z-300 Handheld LIBS
 Analyzer (SciAps Inc, Woburn, MA, USA) was employed for LIBS analysis
 (excitation source: 5-6 mJ·pulse<sup>-1</sup>, 50 Hz
 repetition rate, 1064 nm laser source, argon purge). Z-300 LIBS Analyzer
 measured the signal intensity every 0.0333 nm from 200 to 900 nm. LIBS
 Analyzer was equilibrated with an internal standard prior to determining
 the peak patterns of respective target samples. LIBS was employed to
 construct a pseudo-three-dimensional geochemical profile of spots on the
 four vertical lines identical to Raman spectroscopic measurements. The
 LIBS spectrum of each spot was generated by the LIBS Analyzer in
 quadruplicate. In addition, 140 spots from 9 oval outlines of the top,
 side, and bottom faces of the Qaidam quartz were measured using LIBS: 8
 spots of inner circle, 16 spots of middle circle, and 15 spots of outer
 circle on the top face; 16 spots of upper circle, 23 spots of middle
 circle, and 23 spots of lower circle on the side face; and 8 spots of
 inner circle, 12 spots of middle circle, and 19 spots of outer circle on
 the bottom face. Moreover, 68 random spots (20 from the top, 28 from the
 side, and 20 from the bottom) of the Qaidam quartz were chosen for
 singlicate LIBS measurements and the downstream statistical
 analysis.