Thermogenic Methane Production in Antarctic Subglacial Hydrocarbon Seeps

Gavin Piccione , Jared Nirenberg, Joseph Novak, Abigale Hawthorn, Paul Northrup, Terrence Blackburn, Daniel Enrique Ibarra 

Methane forms beneath ice sheets through microbial methanogenesis and thermogenic breakdown of organic matter, creating a potentially large greenhouse gas reservoir prone to release during glacial retreat. Subglacial thermogenic methanogenesis can increase gas buildup and create oases for life, but this process has not yet been observed in Antarctica, contributing to uncertainty in the spatial distribution and magnitude of methane reserves beneath the ice sheet. Here, we present evidence of Antarctic subglacial thermogenic methane production in shallow basal sediments, preserved in carbonate nodules collected at Elephant Moraine and the Pensacola Mountains, East Antarctica. Oxygen and uranium-series isotope analyses indicate that these carbonates precipitated from glacial meltwater during deglacial periods in the late Pleistocene. Carbonate δ13C values as low as -32.75 ‰ identify thermogenic methane as a primary carbon source, while clumped isotope measurements indicate formation temperatures of 12 - 20˚C, consistent with a geothermal origin. Lipid biomarker analyses further show that organic matter preserved in the nodules is highly thermally matured. These findings indicate that deep-sourced thermogenic methane migrated as hydrocarbon seeps to shallow pore spaces within basal sediments, demonstrating that geothermally active areas can be hotspots for methane accumulation below the Antarctic Ice Sheet. High manganese concentrations and the presence of pyrite oxidation products in the carbonate nodules suggest that this methane was oxidized via sulfate or manganese reduction by chemosynthetic microbes. Subglacial hydrocarbon seeps may thus represent habitable environments in isolated subglacial environments on Earth and icy planetary bodies.