Pervasive speleogenetic modification of cave passages by nitrification of biogenic ammonia

Abstract

It has long been known that guano deposits from animals in caves can cause localised biogenic modification through a combination of acidity and altered environmental conditions, such as increased humidity and CO₂. Geomorphological and geochemical evidence from the caves in the Gunung Mulu National Park, Sarawak, suggest this biogenic overprint may be far more widespread than previously thought due to microbial metabolic activity. Based on our observations, we propose a novel method of secondary cave enlargement by the conversion of highly soluble ammonia gas released by bat and swiftlet guano to NOx on surfaces by microbial ammonia oxidation. Our data suggest this activity produces aggressive nitric acid solutions on moist cave walls, accelerating limestone dissolution. This previously undescribed cave enlargement process has potentially profound geomorphological implications, as the original passage morphologies (which are used to interpret speleogenesis and landscape evolution) are erased and replaced with a distinctive suite of biogenic corrosion features. Such findings significantly alter our understanding of post-speleogenetic modification and secondary enlargement of caves in tropic environments.