Reply to Schoell: Implications of a temperature trend in methane evolved from Cumberland during Mars evolved gas analyses experiments

In response to our reporting the carbon isotopic compositions of CH₄ released during pyrolysis of solid Mars sediments (1), Schoell (2) states that our full dataset does not follow a trend with temperature observed for the pyrolysis of Earth kerogen-rich sediments. We appreciate Schoell’s highlighting that the changing isotopic composition is an important constraint useful for understanding the origin of CH₄ released during pyrolysis. Our published dataset (1) includes analyses of five aliquots from the Cumberland drill hole, one of the locations where highly ¹³C-depleted carbon was observed by the tunable laser spectrometer (TLS). A different temperature cut was sent to TLS for each aliquot, but, collectively, CH₄ released from Cumberland during pyrolysis follows a trend with temperature similar to that observed in the West Siberian hydrocarbon source rocks (3) mentioned in Schoell’s (2) critique, once the terrestrial data are adjusted to reflect a more ¹³C-depleted source carbon (Fig. 1). The other samples shown in Fig. 1, single aliquots from multiple other drill holes, are interpreted to have similar carbon isotopic values for their CH₄ sources. These samples, when taken together, also show a trend with temperature similar to that observed during pyrolysis of kerogen-rich rocks on Earth.

Fig. 1.

TLS CH₄ δ¹³C results (±1 SE) from evolved gas analyses of samples of the Cumberland drill hole material (filled circles) as a function of the temperature cut used for TLS (1). Horizontal lines show the complete range of each temperature cut. The gray dashed curve represents the experimental results reported in ref. 3 but adjusted to approximate the isotopic values for methane released from the Cumberland samples. To generate such strong depletions under this scenario, the original δ¹³C value for the Cumberland carbon source would need to be approximately −90‰ (gray cross); the gray square shows, for reference, the δ¹³C of the West Siberian hydrocarbon source rocks studied in ref. 3. The open circles are other Gale crater CH₄ TLS isotope results (GB2, HF, RH, HU, and EB) that, while each from separate experiments, were interpreted to have evolved CH₄ δ¹³C values in family with the Cumberland set, due to the strong ¹³C depletions observed.

Notably, unlike West Siberia’s hydrocarbon source rocks, the Gale crater samples represent multiple geologic assemblages (Yellowknife Bay clastic sediments, mudstone-rich Murray lacustrine sediments, Vera Rubin ridge geochemically altered sediments, sandstone-rich Carolyn Shoemaker sediments, and the Stimson sandstone deposited after a disconformity). The observed geologies (including geochemical alteration, veining, and an erosional disconformity) imply that our full suite of samples represents significant geological time, so it is not surprising that Curiosity’s 25-km traverse encountered a variety of organics with differing isotopic compositions. Additionally, the Gale crater samples release mineral-bound carbon at a variety of temperatures (4), indicating that the full dataset reported includes pyrolysis CH₄ released from a variety of different carbon sources, as previously discussed (1).

Schoell (2) ends his critique with a claim that our paper “infer[s] methane as a proven biosignature on Mars.” We do not. Our report exhaustively discusses mechanisms (abiotic and biotic) by which isotopic fractionations can occur [including serpentinization (5, 6)] and how these processes could apply to Gale crater sediments (1). The discussion narrows the options to a few working hypotheses, each requiring further exploration before being confirmed or refuted. Naturally, one hypothesis invokes large carbon isotopic fractionations mediated by methanogens under certain conditions (7), which result in ¹³C-depleted subsurface CH₄ on Earth (8). The other working hypotheses utilize large isotopic fractionations observed between CO and dust in interstellar giant molecular clouds or suspected for UV-mediated CO₂ reduction in the Martian atmosphere (1). By presenting distinct working hypotheses, we intentionally aimed to remain cautious about biosignature implications of the most ¹³C-depleted values and provide the community with optimistic guidance as to how progress can be made in understanding the origin of the specific carbon we reported.