BELLHOLES AND BELLBASINS: BIOGENIC (BAT) FEATURES OF CARIBBEAN CAVES

Mature bellholes are vertical, symmetrical, bullet-shaped cavities in ceilings of tropical caves, in association with bats. Commonly 30 cm in diameter, they may exceed a meter in height. Bellbasins are shallow depressions in rock floors, located vertically beneath a bellhole. They occur in such numbers in the Caribbean- tens of thousands in Puerto Rico- that they are an important process in post-formational cave enlargement. Their genetic relation to bats could help interpret past bat populations (e.g. past colony locations) or climate.

Both are vadose features. In Puerto Rico, hundreds of bellholes occur in Cueva Murcielago in entrance areas colonized by thousands of bats; phreatic, water-filled passages immediately beyond have been cave-dived and contain neither bellholes nor bellbasins. Entrance passages of Cueva Perro have numerous bats and bellholes, neither found past a 5 cm airspace in the streamway, beyond which are sizable air-filled passages, but no exit. In Cueva Represa, bellholes have developed in stalactites, with paired bellholes in stalagmites below. No correlation was found with joints or bedding. Bellholes also occur in ceilings above post-formational collapse blocks.

Densities are high: Cueva Represa has 395 bellholes in one chamber, with at least 1000 estimated for the first 250 meters of streamway. Dark brown streaks on rock sampled in 7 bellholes were greatly enriched in phosphorus; XRD analysis also revealed hydroxylapatite in several, a mineral often resulting from the reaction of limestone with bat guano. Bellbasins in Cueva Represa almost always contain bat guano: XRD of their altered limestone rinds showed both hydroxylapatite and apatite.

Development is hypothesized as chemical alteration of carbonate bedrock by bat feces and urine, and physical contact with claws and wings. The uniformity of size as they develop from initial ceiling saucers to mature bellholes is constrained by social habits of several species of bats common to them.