Noam Lotem

The organic carbon content of ancient rocks provides a fundamental record of the biosphere on early Earth. For over 50 y, the high organic content of Archean (>2.5 Ga) mudrocks has puzzled geologists and evolutionary biologists, because high biological primary productivity was unexpected for the nascent biosphere before the rise of O ₂ . Here, we took a different approach to resolve this apparent paradox, by studying the accumulation rates of Archean organic-rich mudrocks. We evaluated the sedimentation rates of three sections of the Mount McRae Shale and Jeerinah Formation (2.68 to 2.48 Ga, Pilbara Craton, Australia) with new and recently published U–Pb zircon ages from intraformational ash beds. For comparison, we compiled Phanerozoic (<500 Ma) data from comparable depositional settings and developed an idealized model that considers the sedimentation rates for predicting rock organic content. We found that organic-rich Archean mudrocks were deposited under exceptionally low sedimentation rates (~1 m/Ma), in sharp contrast to organic-rich rocks from the Phanerozoic Eon (10 to 100 m/Ma). Constrained by observations, model results indicated that the Archean data reflect low primary productivity (~100-fold lower than during the Phanerozoic) and enhanced preservation under anoxic conditions, with the principal control on organic carbon content provided by dilution with inorganic sediment. Thus, the high organic carbon content which is typically attributed to high productivity instead reflects slow accumulation, high preservation, and minimal inorganic dilution—reconciling the geological evidence with a slow carbon cycle cadence during Archean time.

About 40% of the annual methane emissions originate from natural, non-anthropogenic sources. These include mainly freshwater sediments, in which significant increase in methane emissions has been observed throughout the past decades with the ongoing global temperature rise. Thermokarst lakes, formed by abrupt thawing of permafrost, play a significant role in this observed increase in methane emissions. However, methane production rates and natural consumption controls there are not well constrained, as well as their response to global warming. We explore the rates and mechanisms of methane production and anaerobic oxidation (AOM) processes several interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice-rich permafrost thaw. This is mainly through geochemical and microbial profiles combined with slurry incubation experiments with labeled isotopes, potential electron acceptors and several inhibitors in different temperatures. Our manipulated experiments shed insight on the controls of methanogenesis onset and the mechanisms of both methanogenesis and AOM. Direct rate measurements using two isotope methods and modeling provide robust rate estimations for methanogenesis and AOM. They indicate that the role of AOM in these lakes is less significant than previous estimations, and that AOM will probably not attenuate the methanogenesis increase in a warmer climate.

The ongoing global temperature rise enhances permafrost thaw in the Arctic, allowing Pleistocene‐aged frozen soil organic matter to become available for microbial degradation and production of greenhouse gases, particularly methane. Here, we examined the extent and mechanism of anaerobic oxidation of methane (AOM) in the sediments of four interior Alaska thermokarst lakes, which formed and continue to expand as a result of ice‐rich permafrost thaw. In cores of surface (~ 1 m) lake sediments we quantified methane production (methanogenesis) and AOM rates using anaerobic incubation experiments in low (4°C) and high (16°C) temperatures. Methanogenesis rates were measured by the accumulation of methane over ~ 90 d, whereas AOM rates were measured by adding labeled‐¹³CH₄ and measuring the produced dissolved inorganic ¹³C. Our results demonstrate that while methanogenesis was vigorous in these anoxic sediments, AOM was lower by two orders of magnitude. In almost all sediment depths and temperatures, AOM rates remained less than 2% of the methanogenesis rates. Experimental evidence indicates that the AOM is strongly related to methanogens, as the addition of a methanogens' inhibitor prevented AOM. Variety of electron acceptor additions did not stimulate AOM, and methanotrophs were scarcely detected. These observations suggest that the AOM signals in the incubation experiments might be a result of enzymatic reversibility (“back‐flux”) during CH₄ production, rather than thermodynamically favorable AOM. Regardless of the mechanism, the quantitative results show that near surface (< 1‐m) thermokarst sediments in interior Alaska have little to no buffer mechanisms capable of attenuating methane production in a warming climate.

Methane (CH ₄ ) release to the atmosphere from thawing permafrost contributes significantly to global CH ₄ emissions. However, constraining the effects of thaw that control the production and emission of CH ₄ is needed to anticipate future Arctic emissions. Here are presented robust rate measurements of CH ₄ production and cycling in a region of rapidly degrading permafrost. Big Trail Lake, located in central Alaska, is a young, actively expanding thermokarst lake. The lake was investigated by taking two 1 m cores of sediment from different regions. Two independent methods of measuring microbial CH ₄ production, long term (CH ₄ accumulation) and short term ( ¹⁴ C tracer), produced similar average rates of 11 ± 3.5 and 9 ± 3.6 nmol cm ⁻³ d ⁻¹ , respectively. The rates had small variations between the different lithological units, indicating homogeneous CH ₄ production despite heterogeneous lithology in the surface ~1 m of sediment. To estimate the total CH ₄ production, the CH ₄ production rates were multiplied through the 10–15 m deep talik (thaw bulb). This estimate suggests that CH ₄ production is higher than emission by a maximum factor of ~2, which is less than previous estimates. Stable and radioactive carbon isotope measurements showed that 50% of dissolved CH ₄ in the first meter was produced further below. Interestingly, labeled ¹⁴ C incubations with 2‐ ¹⁴ C acetate and ¹⁴ C CO ₂ indicate that variations in the pathway used by microbes to produce CH ₄ depends on the age and type of organic matter in the sediment, but did not appear to influence the rates at which CH ₄ was produced. This study demonstrates that at least half of the CH ₄ produced by microbial breakdown of organic matter in actively expanding thermokarst is emitted to the atmosphere, and that the majority of this CH ₄ is produced in the deep sediment.