Depleted carbon isotope compositions observed at Gale crater, Mars

Significance

Carbon isotopic analysis is among the most pervasive geochemical approaches because the fractionation of carbon isotopes produces a natural tracer of biological and chemical processes. Rover-based carbon isotopic analyses of sedimentary rocks on Mars have the potential to reveal modes of Martian carbon cycling. We report carbon isotopic values of the methane released during pyrolysis of samples obtained at Gale crater. The values show remarkable variation indicating different origins for the carbon evolved from different samples. Samples from multiple locations within Gale crater evolved methane with highly fractionated carbon isotopes. We suggest three routes by which highly fractionated carbon could be deposited on Mars, with each suggesting that Martian carbon cycling is quite distinct from that of the present Earth.

Abstract

Obtaining carbon isotopic information for organic carbon from Martian sediments has long been a goal of planetary science, as it has the potential to elucidate the origin of such carbon and aspects of Martian carbon cycling. Carbon isotopic values (δ¹³C_VPDB) of the methane released during pyrolysis of 24 powder samples at Gale crater, Mars, show a high degree of variation (−137 ± 8‰ to +22 ± 10‰) when measured by the tunable laser spectrometer portion of the Sample Analysis at Mars instrument suite during evolved gas analysis. Included in these data are 10 measured δ¹³C values less than −70‰ found for six different sampling locations, all potentially associated with a possible paleosurface. There are multiple plausible explanations for the anomalously depleted ¹³C observed in evolved methane, but no single explanation can be accepted without further research. Three possible explanations are the photolysis of biological methane released from the subsurface, photoreduction of atmospheric CO₂, and deposition of cosmic dust during passage through a galactic molecular cloud. All three of these scenarios are unconventional, unlike processes common on Earth.

Results

For evolved gas analysis (EGA) at Gale crater, drilled powder or scooped fines are heated at 35 °C min⁻¹ under a helium flow in quartz cups up to ∼850 °C. A fraction of the evolved gas mixture is directly analyzed with a quadrupole mass spectrometer, and a specific preselected temperature cut of the remaining gas is diverted to the Herriott cell of the TLS. The redox state of the oven, in practice, depends on the relative abundances of oxychlorine species and reduced minerals in each sample. During EGA, organic material can produce a range of products including CO₂, CH₄, CO, OCS, CS₂, and molecular organic fragments. EGA δ¹³C CH₄ values were measured by the TLS instrument of the SAM suite for 24 samples from Gale crater, Mars (Table 1) using methods described in this paper’s supplement. The amount of CH₄ observed by the TLS-SAM instrument for the TLS temperature cut is also indicated in Table 1. This includes five analyses of the Cumberland (CB) sample drilled in the Sheepbed member of the Bradbury group rocks at Yellowknife Bay, as well as 15 samples from the Mount Sharp group, three from the overlying Stimson formation, and one scooped sand sample. Shown in Table 2 are the posterior probabilities of reduced sulfur presence (based on several evolved sulfur gases observed compared to laboratory data) and the δ³⁴S values calculated from SO₂ evolved between ∼500 and 600 °C during EGA and measured by the SAM quadrupole mass spectrometer (QMS).

As seen in Fig. 2, the TLS CH₄ δ¹³C values that are highly ¹³C depleted correspond predominantly with the ³⁴S-depleted δ³⁴S QMS values observed for evolved SO₂, reported by Franz et al. (4) and Wong (5). Four of the most depleted TLS CH₄ δ¹³C values (Table 1) are from samples [CB2, CB3, CB5, and Edinburgh (EB)] that also clearly evolve ³⁴S-depleted SO₂ (Table 2 and Fig. 2). Such δ³⁴S-depleted SO₂ evolved at mid (500 to 600 °C) temperature has been interpreted to indicate Martian sulfides (4). Reduced sulfur has also been inferred based on the evolution of OCS and CS₂ compared to analyses of laboratory sulfur samples (6). Interestingly, 8 out of the 10 samples showing strong TLS δ¹³C depletions (i.e., <−70‰) in evolved CH₄ also evolved sulfur gases (OCS and CS₂) indicative of a reduced sulfur detection based on statistical comparisons to laboratory runs (Table 2). One of the two samples in which the EGA data do not support a reduced sulfur detection is CB6, which had an unusual EGA procedure that prohibited an effective analysis of the sulfur gases. The other is Hutton (HU), which occurs just below the Greenheugh pediment (a gently sloping erosional surface downslope from Gediz Vallis) and the Basal Siccar Point group unconformity. HU shows evidence for geochemical alteration by later fluids flowing near the Basal Siccar Point group unconformity (e.g., ref. 7).

Discussion

Some of the ¹³C depletions reported here are anomalously large, especially with respect to the carbon isotopic composition of the Martian atmosphere, whose δ¹³C value reported earlier by TLS-SAM is ∼+46‰ (8). This atmospheric value reflects the integrated largescale loss of volatiles from the Martian atmosphere and thus, the δ¹³C composition of the atmosphere may have been less ¹³C enriched when Gale crater sediments were deposited. Because of the magnitude of the ¹³C depletions observed in CH₄ evolved during EGA runs, the authors have considered potential rover-induced origins for the observations without uncovering any explanation. In fact, the TLS spectra obtained on Mars from evolved CH₄ (SI Appendix, Fig. S1) are exceptionally clean and provide multiple ¹²C and ¹³C lines with which to calculate δ¹³C values, making it unlikely that the ¹³C depletions observed are due to an interfering organic molecule. From the repeat CB analyses, it appears that the isotope depletion observed in CH₄ is most pronounced at lower temperatures. This observation suggests that precursors to the evolved CH₄ are relatively volatile organic molecules. However, strong depletions were still observed in several samples using high-temperature cuts (>450 °C) in which the temperature cut includes a tail of a CH₄ release centered at a lower temperature (e.g., SI Appendix, Fig. S2). The CH₄ isotopic variation in CB samples, though, are not completely explained by differences in the temperature cut. Additionally, at Yellowknife Bay, the TLS analyses of CB using the 2.78-μm laser produced highly ¹³C-depleted CO₂ δ¹³C values (SI Appendix, Table S1). While these TLS analyses of CB produced CO₂ δ¹³C values that were, in some cases, comparable to CH₄ δ¹³C (SI Appendix, Table S1), such depleted CO₂ δ¹³C values have not been observed in later samples of the mission. In contrast, TLS CH₄ δ¹³C values using the 3.27 μm laser show strong depletions in multiple different drill samples from vastly different parts of the mission. Because the observation of anomalous ¹³C values in evolved CH₄ are repeatable with different samples spread out in space and time, we have focused on those values for this study.

We have considered the SAM instrument background of N-tert-butyldimethylsilyl-N-methyl-trifluoroacetamide (MTBSTFA (δ¹³C = −35‰; refs. 9 and 10) as a reasonable source of evolved CH₄ to consider. Early in the mission, this background was estimated to contribute up to 900 nmol of CO₂ during pyrolysis (11). In addition to oxidizing to CO₂, MTBSTFA is known to react with water resulting in 1,3-bis(1,1-dimethylethyl)-1,1,3,3-tetramethyldisiloxane [or bisilylated water (BSW)], which can be monitored by the SAM QMS. Based on the levels of BSW detected, the level of MTBSTFA background has been variable between runs depending on how long it has been since a wet chemistry experiment and, for a few cases in which it was relevant, how long a sample was stored before analysis. For 16 of the samples, the mean of the total BSW observed during the course of the EGA runs was 1.0 nmol with an SD of 1.5 nmol (Table 2). HU had over 22 times this average, and Duluth (DU) had over 30 times more than this average. In the case of DU, the extreme amount of BSW observed was also concurrent with an anomalous amount of total EGA methane (774 nmol) and a TLS CH₄ isotopic value (δ¹³C = −45‰) similar to MTBSTFA (δ¹³C = −35‰), which demonstrated that with elevated levels of MTBSTFA in the SAM background, a portion can end up as evolved CH₄. Further, an intramolecular isotopic analysis of methyl-trifluoroacetamide from the hydrolysis of MTBSTFA showed that the types of carbon (methyl-carbon and carbonyl C) most likely to contribute CH₄ from the MTBSTFA background were the most ¹³C enriched (SI Appendix, Fig. S3), making it unlikely that the observed anomalies stemmed from a site-specific carbon depletion obscured in the bulk δ¹³C value for MTBSTFA.

Major endmember carbon reservoirs (SI Appendix, Table S4) presently on Mars are the atmospheric CO₂ (δ¹³C = +46 ± 4‰; ref. 8) and the igneous carbon (δ¹³C = −20 ± 4‰; ref. 12). TLS δ¹³C values between −17 ± 2‰ and −57 ± 2‰ were found for 10 samples including DU. The isotopic composition of the CH₄ evolved during pyrolysis of these samples may reflect the MTBSTFA SAM background, Martian igneous carbon, and/or meteoritic infall along with any isotopic fractionations the occur during oven reactions. Laboratory experiments using solid materials were conducted to explore the magnitude of carbon isotopic fractionation possible during pyrolysis under conditions similar to SAM (SI Appendix, Table S2). We found that cleavage and reduction of methyl groups to CH₄, as would happen for most CH₄ derived from MTBSTFA products, produced little ¹³C depletion (0.4 to 4.6‰; SI Appendix, Fig. S4). CH₄ evolved from recalcitrant sources (graphite and diamond) showed ¹³C-depletion of 8 to 21‰, CH₄ from oxalate/oxamide showed moderate ¹³C depletion (25.0 to 25.3‰), and bicarbonate reduction showed the largest ¹³C depletions (28.5 to 49‰). Both our laboratory pyrolysis experiments (SI Appendix, Table S2) and modeling of possible isotopic pseudoequilibration during pyrolysis (SI Appendix, Figs. S5–S8 and Table S3) considering several different scenarios suggested that oven processes would typically produce fractionations less than 50‰ and, therefore, cannot account for the large ¹³C depletions observed in multiple samples at Gale crater. Applying the most extreme oven fractionation imagined to these carbon reservoirs results in CH₄ with δ¹³C values of about −5 to −70‰. CB (CB1, CB2, CB3, CB5, CB6), Gobabeb (GB2), Highfield (HF), Rock Hall (RH), Hutton (HU), and EB showed TLS CH₄ values more ¹³C depleted than this range, indicating that oven reactions are not likely to be causing their anomalous ¹³C-depleted values observed in evolved CH₄.

It may be notable that the highly depleted ¹³C values for evolved CH₄ have so far been found in five distinct locations at Gale crater, Mars (Table 2 and Fig. 1 A–F). The highly ¹³C-depleted signal was first seen in the mudstones of Yellowknife Bay on the crater floor (Fig. 1F) in a location where high thermal inertia values were measured from orbit and have been potentially attributed to secondary alteration at the end of the Peace Vallis fan (13). N