ИСТИНА

The glaciological history of Earth spans at least 2.9 billion years as evidenced by diamictites of the Kaapvaal craton in South Africa. Assemblages of mineral and organic species might have been already in place on ice, when subaerial surfaces massively turned into supraglacial during major cooling events like the Huronian glaciation (2.4-2.2 Ga) or the Cryogenian (0.72-0.64 Ga). Microbe-dominated cryoconite granules (Takeuchi et al., 2010) largely studied today are among the best available analogues to approximate early principles of aggregation between minerals and living organisms on ice. Their aggregates could have synergized dark-colored matter affecting energy balance and biogeochemistry of glaciers in ancient times, as it is now. Recent fine-scale measurements revealed a functional heterogeneity of microbial communities inside cryoconite granules including redox stratification linked to the complex internal structure of granular aggregates (Segawa et al., 2020). Here we employ light microscopy, X-ray microtomography, SEM-EDS with MAPS Mineralogy and Raman spectroscopy to explore physical structure and distribution of mineral and organic phases within cryoconite granules from two mountain glaciers: Leviy Aktru (LA), Altay and Garabashi (GB), Caucasus. Despite different size of LA mesogranules (2-6 mm) and GB microgranules (0.1-1 mm) their void space had similar structure and total porosity in the range of 15-16%. The share of connected pores in both types of granules was high and stable (88-89%). LA pore sizes were within 10-220 µm (30-110 µm most frequent) and GB pore sizes – 10-60 µm (14-38 µm most frequent), with larger pores attributed to the granules core in both cases. Granules often had an embryonic grain represented by a single mineral or a rock fragment and a compacted layer at the periphery. LA mesogranules comprised smaller subgranules. Our dataset suggests phyllosilicates (smectite, kaolinite, chlorite, micas) as an important component of granules physical stability along with the previously widely acknowledged role of extracellular polymeric substances (EPS) and filamentous cyanobacteria. The highest concentration of silty and clay particles (up to 31.6% smectites in GB) occurred at the periphery of granules. They were densely packed in the granule wall subparallel to its outer surface. SEM suggests fine multilayer structure of phyllosilicate clusters within granules wall: up to 18 layers in GB microgranules and >50 layers in LA mesogranules. Phyllosilicate clusters were always interlayered with amorphous C-rich cement originating from EPS. According to the multispectral Raman imaging, the clusters of methyl functional groups (–CH3) were confined to the walls of granules suggesting hydrophobic properties on the surface important for stability of cryoconite microecosystem in meltwater. Consistent outer shell was crucial and complex component of the whole granular microecosystem that sustained inner core and created temporally stable environment for transformation of organic matter instantly inside the aggregate. Combination of physical and chemical stabilization mechanisms in cryoconite granules was reminiscent of microaggregates in complex colloidal systems, e.g., soils. Basic principles of biota-to-mineral aggregation exhibited in modern cryoconite granules of microbial origin suggest similar assemblages could have existed on glaciers earlier in Earth history. If true, environment inside granules provided specific soil-like conditions for biogeochemical cycling alternative to the rest of supraglacial area, especially when ice was a dominant and long-living setting on Earth.