Unconsolidated, highly porous, permeable siliceous sediments found in shallow-water pools on the middle and distal parts of Geysir's discharge apron are formed of cryogenic opal-A (COA), opal-A spheres, opal-A cement, silicified microbes, and detrital lithic grains. Silicified leaves and silica-coated grass stems and twigs are locally present with these pool sediments, and microbial mats commonly cover the sediment surface. Partially consolidated layers (< 0.5 cm thick) form when the constituent grains are bound together by filamentous microbes, exopolysaccharides, and/or opal-A cement. Most components of the sediment can be related to processes that are controlled largely by seasonal variations. Low temperatures in winter promote freezing of the medial to distal discharge waters and lead to formation of COA in ice. When the ice melts, the released COA grains become part of the pool sediments. Microbial mats trap and bind sediment and provide templates for silica precipitation during spring and summer, but become (semi-)dormant during the constant darkness that Iceland experiences during winter. When the waters are sufficiently supersaturated, opal-A precipitates as spheres in the water column and cement in pool deposits. In some pools, seasonal changes are evident from the alternation of “soft” and “hard” (partly cemented) laminae. Elsewhere, however, the distinction between the different time-dependent components is lost because they become superimposed on each other or as aperiodic changes in flow patterns affect different pools.

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