Cottonballs, a unique subaqeous moonmilk, and abundant subaerial moonmilk in Cataract Cave, Tongass National Forest, Alaska

Abstract

The Tongass National Forest is known for its world-class karst features and contains the largest concentration of dissolutional caves in Alaska. Within these karst systems exist unusual and possibly unique formations exhibiting possible biological origin or influence. Cataract Cave is an example of such a system. This cave hosts a unique depositional setting in which so-called "cottonballs" line two permanent pools. The cottonballs are a calcitic deposit heavily entwined within a mass of microbial filaments. They are juxtaposed with extensive subaerial calcitic moonmilk wall deposit of a more conventional nature but of an extraordinary thickness and abundance. Both the cottonballs and moonmilk are composed of microcrystalline aggregates (0.20 wt.%) compared to the cottonballs (0.12 wt.%). However, the cottonballs are dominated by monocrystalline needles, whereas the moonmilk is mainly composed of polycrystalline needles. The microbial environments of both displayed similar total microbial cell counts; however, culturable microbial counts varied between the deposits and among the various media. For both, in situ cultures and isolates inoculated in a calcium salt medium produced calcium carbonate mineralization within biofilms. Geochemical variations existed between the deposits. Moonmilk displayed a slightly higher abundance of organic carbon (0.20 wt%) compared to the cottonballs (0.12 wt%). Stable isotopic analysis revealed that the moonmilk (δ13C = -1.6‰) was isotopically heavier compared to the cottonballs (δ13C = -8.1‰) but both are lighter than the host rock (δ13C = +1.1‰). However, the organic carbon δ13C values of both deposits were similar (δ13C = -27.4 and -26.7‰) and isotopically lighter compared to other overlying surface organic carbon sources. Due to the similarities between the deposits, we infer that both the cottonballs and moonmilk are subject to a set of related processes that could collectively be accommodated by the term "moonmilk". Thus, the cottonball pool formation can be characterized as a type of subaqueous moonmilk. The differences observed between the moonmilk and cottonballs may be largely attributable to the changes in the depositional environment, namely in air or water.

Microbiological activities in moonmilk monitored using isothermal microcalorimetry (Cave of Vers chez le Brandt, Neuchatel, Switzerland)

“…This deposit has strong similarities with secondary calcium carbonate accumulations observed in soils that are mainly composed of NFC and nanofibres. Many papers have provided strong supporting evidence for the biogenic origin of NFC in soils (Callot et al, 1985;Phillips and Self, 1987;Cailleau et al, 2009a,b;Curry et al, 2009). Moreover, recent investigations emphasize the potential implication of organic templates as a precursor to mineralized nanofibres (Cailleau et al, 2009b;Bindschedler et al, 2010).…”

High-resolution signatures of oxygenation and microbiological activity in speleothem fluid inclusions

“…We see evidence of this in the presence of efflorescent cave minerals like magnesium sulfate (Hill & Forti, 1997) that come and go with season, and bedrock breakdown products that have an evaporative mechanism as well as microbial and chemical degradative processes (e.g., moonmilk, cf. Curry et al, 2009). Thus, evaporatively driven processes contribute to the movement of materials through the carbonates (and some other minerals) are deposited in caves, they develop small inhomogeneities that result in such inclusions.…”

Biogenicity and Characterization of Moonmilk in the Grotta Nera (Majella National Park, Abruzzi, Central Italy)

“…It is a microcrystalline aggregate, typically found on the ceilings, floors, and walls of carbonate caves and on speleothems. Moonmilk deposits have been reported in numerous caves worldwide, in a variety of different countries and in climates from alpine to tropical (Onac and Ghergari, 1993;Hill and Forti, 1997;Chirienco, 2002;Lacelle et al, 2004;Ford and Williams, 2007;Blyth and Frisia, 2008;Richter et al, 2008;Curry et al, 2009). Frequently, moonmilk is the only speleothem present in cold, high-altitude or high-latitude caves, where massive calcite speleothems such as stalagmites do not form (Onac and Ghergari, 1993;Hill and Forti, 1997;Borsato et al, 2000;Lacelle et al, 2004).…”

“…The morphology of moonmilk is as varied as its composition (Curry et al, 2009). Microscopically, the most diagnostic calcitic moonmilk feature is needle-shaped or fibrous crystal morphology that appears as unstructured aggregates of micrometer-to nanometer-sized crystals with no apparent preferred orientation (Onac and Ghergari, 1993;Gradziñ ski et al, 1997;Hill and Forti, 1997;Borsato et al, 2000;Cañ averas et al, 2006).…”

“…Microscopically, the most diagnostic calcitic moonmilk feature is needle-shaped or fibrous crystal morphology that appears as unstructured aggregates of micrometer-to nanometer-sized crystals with no apparent preferred orientation (Onac and Ghergari, 1993;Gradziñ ski et al, 1997;Hill and Forti, 1997;Borsato et al, 2000;Cañ averas et al, 2006). Moonmilk that is biologically active also contains significant amounts of cells, filaments, and apparent biofilms (Gradziñ ski et al, 1997;Boston et al, 2001;Curry et al, 2009). The origin of moonmilk has long been discussed, and many hypotheses for its genesis have been proposed since its first description was made by Nicolas Langh in 1708 (Bernasconi, 1976).…”