Earth and Planetary Sciences ETDs

Carol A. Hill

Publication Date

12-6-1978

Abstract

ABSTRACT

Southeastern saltpeter caves have temperatures between 10-15° C and humidities between 90-99%. Saltpeter sediments are alkaline (pH= 6-9), dry and porous (moisture = 5-10 wt. %) and non-organic (low levels of compost, guano and animal remains); sediments have nitrate concentrations ranging between 0.01-4.0 wt. %, low total nitrogen (0.08-0.13 wt. %) and relatively low phosphorous (0.1-1.4 wt. %). 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 sulfate speleothems (up to 1 wt. %). Surface and sinkhole limestones directly exposed to rainfall are leached of nitrate (1-2 ppm); sinkhole limestones in recessed, partly exposed positions have intermediate nitrate values (10-100 ppm); and protected cave limestones are enriched in nitrate (thousands of ppm in 30 an 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 floor sediments are higher (6 x 105 /g of sediment) and of a different species (agilis) than surface soil populations (1 x 103/g; Winogradsky).

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

The nitrate minerals niter (KN03), soda-niter (NaNO3) and darapskit1e (Na3(N03) (S04),Hz0) were identified in non-saltpeter, southwest1ern caves. Niter (crystal size = 1.5 nm) occurs as a transparent, colorless to light brown, bitter tasting wall c11.1St in a lava tube cave near Socorro, New Mexico. Soda-niter occurs as a crystalline wall c11.1St, as two small stalactites and one small stalagmite with the Socorro niter, and as cottony efflorescences along recessed bedding planes in the fissure caves of the Wupatki, Arizona area. The Socorro soda-niter crystals are rhombohedral, 2-3 nm long, transparent and colorless; the Wupatki soda-nitercrystals are prismatic to acicular, 2 nm long, colorless and transparent Sarapskite, Solis Flower Cave, Big Bend National Park, Texas occurs as cave "flowers", crust, ''hair", flowstone and stalactites. The darapskite is colorless with 2V(-) = 20-30°, « = 1.30±.005, a= 1.480±.003, y = 1.489±.003, a= 10.558(3), b = 6.870(2), c = 5.186(1) and a= 101.46.1'(5). Nitrate minerals cannot theoretically crystallize in humid southeastern saltpeter caves. At 12° C, the humidity at which crystallization occurs is 54% for nitrocalcite (Ca(NO3)2-4H20) and nitromagnesite (Mg(NO3)2•6H20), 74% for ammonia-niter (NH/O3), 79% for soda-niter and 92% for niter.

Degree Name

Earth and Planetary Sciences

Level of Degree

Masters

Department Name

Department of Earth and Planetary Sciences

First Committee Member (Chair)

Rodney Charles Ewing

Second Committee Member

Douglas Gridley Brookins

Third Committee Member

Stephen G. Wells

Document Type

Thesis

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