Mineral-trapping of greenhouse gases in Arctic glendonites

Fossil ikaite, preserved as the pseudomorph glendonite, occurs in vast amounts in Jurassic and Cretaceous successions in the high Arctic. Thermodynamics predict that ikaite is only stable at near-freezing temperatures and glendonite is thus widely used as a paleo-indicator of cold climate conditions, conflicting with traditional views of a very warm and equable Mesozoic greenhouse. Here, we show based on a multi-proxy investigation of Jurassic and Cretaceous glendonites from Siberia and Svalbard that this one-dimensional view detracts from their exceedingly complex biogeochemistry. NanoSIMS analyses of a Jurassic glendonite from Siberia produced large C-isotope gradients (> 40‰) over micrometer distances hinting at strong kinetic fractionation that is coupled to the formation of various precipitates, including an inclusion-rich primordial phase with C-isotope values as low as -38‰ that records methane oxidation. In line with previous results from Siberia, all investigated glendonites from the Cretaceous of Svalbard contain methane gas (700 - 2500 ppb/g) with enriched δ13C-CH4 signatures (-44 to -50‰ V-PDB), depleted δ2H--CH4 (-285 to -245‰ V-SMOW), and relatively large proportions of C2-C5 gas. Such values are potentially indicative of thermogenic methane gas sourced from structure II gas hydrates. Organic geochemistry of glendonites from Svalbard shows the presence of abundant hopanes, including bisnorhopanes with a CSIA signature of -41‰, suggesting activity of sulfide oxidizing bacteria possibly also linked to the inclusion of oil droplets. Moreover, exceptionally heavy bulk δ34Scas values of +46.2‰ clearly link marine ikaite formation in deep time to sulfate-driven anaerobic methane oxidation. Marine ikaite formation and preservation is thus a highly complex process, driven by temperature and (bio)chemical processes in the sea floor, complicating its use as a simple paleoclimate proxy. Regardless, glendonite episodically trapped large amounts of greenhouse gases and stored those for hundreds of millions of years, making this authigenic mineral a potential recorder of past carbon cycle perturbations.