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

The Tongass National Forest is known for its world-class karst features and contains the largest concentration of dissolutional cavesin 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 arejuxtaposed with extensive subaerial calcitic moonmilk wall deposit of a more conventional nature but of an extraordinary thicknessand 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 polycrystallineneedles. The microbial environments of both displayed similar total microbial cell counts; however, culturable microbial counts variedbetween the deposits and among the various media. For both, in situ cultures and isolates inoculated in a calcium salt mediumproduced calcium carbonate mineralization within biofilms. Geochemical variations existed between the deposits. Moonmilk displayeda slightly higher abundance of organic carbon (0.20 wt%) compared to the cottonballs (0.12 wt%). Stable isotopic analysis revealedthat the moonmilk (δ13C = -1.6‰) was isotopically heavier compared to the cottonballs (δ13C = -8.1‰) but both are lighter than thehost rock (δ13C = +1.1‰). However, the organic carbon δ13C values of both deposits were similar (δ13C = -27.4 and –26.7‰) andisotopically 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 processesthat could collectively be accommodated by the term “moonmilk”. Thus, the cottonball pool formation can be characterized as atype of subaqueous moonmilk. The differences observed between the moonmilk and cottonballs may be largely attributable to thechanges in the depositional environment, namely in air or water.