The NSS Bulletin - ISSN 1090-6924
Volume 43 Number 4: 106-109 - October 1981
A publication of the National Speleological Society

Origin of Cave Saltpeter
Carol A. Hill

Abstract

Southeastern saltpeter caves have temperatures between 10 and 18°C and humidities between 90 and 99%. Saltpeter sediments are alkaline (pH 6 to 9), dry and porous (moisture 5 to 10% by weight) and non-organic (low levels of compost, guano and animal remains); sediments have nitrate concentrations ranging between 0.01 and 4.0% by weight, low total nitrogen (0.08 to 0.13% by weight) and relatively little phosphorus (0.1 to 1.4% by wight). Lixiviated cave sediments regenerate in nitrate in several years. Areal extent of cave nitrate is uniform; vertical concentration is in the top few meters of sediment. Nitrate is a minor constituent in some sulfate speleothems (up to 1% by weight). Surface and sinkhole limestones directly exposed to rainfall are leached of nitrate (1 to 2 ppm); sinkhole limestones in recessed, partly exposed positions have intermediate nitrate values (10 to 100 ppm); and protected cave limestones are enriched in nitrate (thousands of ppm in 30 cm deep cores). Nitrate concentration increases immediately at the sinkhole-cave boundary and is independent of limestone type or stratigraphy. Nitrobacter bacteria populations on cave bedrock walls, ceilings and follor sediments are higher (6 x 10 5/g of sediment) and of a different species (N. agilis) than surface soil populations ( 1 x 10 3/g; [N. winogradsky]).

Compost, animal remains, rat guano and bat guano do not acccount for the areal extent, vertical extent and regeneration of saltpeter earth nor do they expalain saltpeter depostits that extend to cave ceilings or high (thousands of ppm) nitrate withihn limestone bedrock. Bat guano can enrich saltpeter earth in nitrate, but it is not the only, or even a major nitrate source.

Surface soil nitrate transported into the cave by seeping groundwater is the most likely source of cave saltpeter. The proposed mechanism that drives seeping groundwater towards the cave is evaporation at the cave air-bedrock interface which produces a moisture density gradient wihin the limestone. Reduced nitrogen (NH4+) is transported from the surface through the zone of aeration to cave bedrock and sediments where it is oxidized to nitrate by Nitrosomonas and Nitrobacter. The seeping groundwater model explains such characteristics of saltpeter caves and sediments as regeneration, areal and vertical extent, and nitrate within cave bedrock.

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