Cyanobacterial weathering in warming periglacial sediments: Implications for nutrient cycling and potential biosignatures

The cryosphere hosts a widespread microbial community, yet microbial influences on silicate weathering have been historically neglected in cold‐arid deserts. Here we investigate bioweathering by a cold‐tolerant cyanobacteria (Leptolyngbya glacialis) via laboratory experiments using glaciofluvial drift sediments at 12°C, analogous to predicted future permafrost surface temperatures. Our results show threefold enhanced Si weathering rates in pre‐weathered, mixed‐lithology Antarctic biotic reactors compared to abiotic controls, indicating the significant influence of microbial life on weathering. Although biotic and abiotic weathering rates are similar in Icelandic sediments, neo‐formed clay and Fe‐(oxy)hydroxide minerals observed in association with biofilms in biotic reactors are common on Icelandic mafic minerals, similar to features observed in unprocessed Antarctic drifts. This suggests that microbes enhance weathering in systems where they must scavenge for nutrients that are not easily liberated via abiotic pathways; potential biosignatures may form in nutrient‐rich systems as well. In both sediment types we also observed up to fourfold higher bicarbonate concentrations in biotic reactors relative to abiotic reactors, indicating that, as warming occurs, psychrotolerant biota will enhance bicarbonate flux to the oceans, thus stimulating carbonate deposition and providing a negative feedback to increasing atmospheric CO2.