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Solute fluxes in Antarctic meltwaters indicate microbial processes influence chemical weathering. Antarctic cyanobacterial mats dominated by Leptolyngbya glacialis enhance weathering rates at 12 °C. Yet, their effects on nutrient fluxes in colder, nutrient-limited conditions, similar to the McMurdo Dry Valleys environments, are unknown. Here, we investigate biotic and abiotic weathering rates of glaciofluvial sediments at 4 °C and compare results to previous experiments at 12 °C. We also examine the effects of nutrient and salt concentrations on weathering fluxes by comparing the effects of different media concentrations (0.1X and 0.001X: 10 and 1000 times diluted) at both temperature conditions. Our results show limited evidence of biologically mediated silica release at 4 °C, yet microbe-mineral interactions still affect nutrient fluxes, particularly for Ca, Mg, Mn, P, and N. However, a higher initial salt concentration (0.1X media) increased the concentration of solutes released under abiotic conditions. These results indicate that aqueous solutes, temperature and microbial processes are all important factors controlling weathering rates and nutrient fluxes in cold settings. [ABSTRACT FROM AUTHOR]

Siliciclastic muds (clay-and silt-sized sediment) concentrate physical and chemical weathering products. However, both rock composition and climate can affect the mineralogy and geochemistry of these sediments. We quantitatively assessed the influence of provenance and climate on muds collected from end-member climates to test, which, if any, of these potential weathering signatures are indicative of climate in finegrained, fluvial sediments. Granulometry, mineralogy, and geochemistry of the studied muds indicated that provenance and mineral sorting hinder interpretation of (paleo)climate signals. These issues also affect chemical index of alteration (CIA) values, as well as mafic-felsic-weathering (MFW), Al2O3-(CaO* + Na2O)-K2O (A-CN-K), and Al2O3-(CaO* + Na2O + K2O)-(FeOT + MgO) (ACNK-FM) ternary plots, decreasing their utility as paleoclimate proxies. CaO content is heavily weighted within the calculations, resulting in even felsic-sourced sediment commonly plotting as mafic owing to the relative enrichment in CaO from preferential sorting of Ca-rich minerals into the mudsized fraction during transport. These results cast doubt on the indiscriminate use of CIA values and ternary plots for interpreting chemical weathering and paleoclimate within muds, particularly from glacial systems. Most notably, the positive correlations between CIA and climatic parameters (mean annual temperature and mean annual precipitation) diminished when sediments that had formed in nonglacial settings were filtered out from the data sets. This implies that CIA may only be applicable when used in nonglacial systems in which the composition of the primary source material is well constrained-such as soil/paleosol profiles. Within this end-member climate data set, CIA was only useful in discriminating hothumid climates. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]