13C-depleted methane pyrolyzed from contaminated Gale Crater sediment cores, Mars

This work re-interprets published SAM-EGA (Sample Analysis at Mars-Evolved Gas Analysis) geochemical data for twenty-four sediment cores sampled by the Curiosity rover. The samples were pyrolyzed at 35 degrees C/min in the range - 100-850 degrees C. The amount of methane generated from carbonaceous matter in the cores and its stable carbon isotope ratio (813C-CH4) in selected cuts within the full temperature range were determined by tunable laser spectrometry (TLS cut). Chemometric analysis of five independent variables for eighteen of the cores identifies four genetic families in which three endmembers explain most data variance. One endmember is contamination by silylating agent (MTBSTFA) introduced unintentionally from wet-chemistry cups during flight and/or after landing of the rover. Five subsamples from the Cumberland (CB) core show a linear relationship between 813C-CH4 and mean pyrolysis temperature (R2 = 0.77) within TLS cuts in the range 99-786 degrees C, which conforms with temperature-dependent kinetic theory. Based on dates of pyrolysis for the five CB samples, BSW (bisylylated water, a marker of MTBSTFA contamination) peaked on sol 281 and progressively decreased to sol 382. Linear regression of 813C-CH4 versus BSW concentration (R2 = 0.98) yields 813C-CH4 - -70%o for carbonaceous matter in uncontaminated CB core. Higher BSW yields systematically more negative 813C-CH4 to the most negative value of -133%o for CB1, which equates to an apparent fractionation of -98%o from bulk MTBSTFA (813C = -35%o) to CB1 methane. Previous workers suggested that 813C-CH4 from MTBSTFA could not be more than -5%o depleted compared to bulk MTBSTFA. However, their SAM-like laboratory pyrolysis experiments for analogs of MTBSTFA yield 813C-CH4 values that correspond only to gas trapped within the analyzed pyrolysis temperature cut (455-755 degrees C). Lower temperature TLS cuts for CB1 and CB2 (220-349 degrees C and 99-349 degrees C) trapped more 13C-depleted methane as reflected in their anomalous 813C-CH4 of -133 and - 115%o, respectively. In addition, pyrolysis of MTBFTSA byproducts, residual solvent, and internal standards with Martian minerals, perchlorates, and sorption or desorption on clays may contribute to more negative 813C-CH4 than expected from laboratory experiments that lack these components. It is not possible to determine bulk 813C or 813C-CH4 for the remaining two carbonaceous endmembers because of the effects of three factors: (1) Chemometric results by alternating least squares regression (ALS) define the relative contributions of endmembers based on five independent variables, not just 813C-CH4. (2) Samples from distinct locations and stratigraphic ages are subject to variations in 813C of deposited carbonaceous matter. (3) 813C-CH4 values for samples from different TLS cuts differ due to temperature-dependent kinetic fractionation.