FIELD EXPERIMENT SHOWS MANGANESE OXIDE AEROSOL DEPOSITION NEAR CAVE STREAM

Black manganese oxide coatings are frequently seen as secondary mineral deposits in association with cave streams in the eastern United States. Although less common then much-studied speleothems such as carbonate stalagmites, these materials have the potential to contain significant paleoenvironmental information. But, there is limited knowledge of their mode of deposition. In order to develop an understanding of the processes, a field study was made in an underground perennial master stream (Mystic River) in Scott Hollow Cave, Monroe County, West Virginia. Methods included observations along a 0.3 km reach, collection of existing coating samples, installation of ceramic coupons both below water and above water (elevation range 2.5 m) to identify/quantify ongoing deposition, logging of stream stage and temperature (0.5-hour interval), optical microscopy and SEM/EDAX. Leucoberbelin blue dye was used to confirm the presence of oxidized Mn in various settings. During a 134-day monitoring period (9/16/2018 to 5/5/2018) stream stage occasionally rose up to 1.35 m, and water temperature varied between 10.3 and 12.7 degrees C. Mn-oxide coatings are found on bedrock walls, on relict alluvium, on carbonate speleothems, and on the floor of the channel. Thick, soft coatings close to stream level suggest ongoing deposition sub-aqueously, but are not definitive as to mode of emplacement. Presence of spotty coatings on active speleothems that are seemingly beyond the current flood stage of the stream support the concept that a mechanism other than direct precipitation from stream water is occurring. The morphology of such coatings also suggests that there is microbial involvement. Accumulation of Mn-oxides on installed clean ceramic plates that are 1.5 m above stream level over a period of 8 months confirms that Mn-oxides can be, and are being, deposited via aerosol (mist) mechanisms. This previously undocumented mechanism may be important in any cave where there is the potential for aerosols generated via turbulence or thermal differences.