Genesis of saltpeter deposits in karst caves in Southwest China

Saltpeter deposits preserved in karst caves are large and widely distributed in southwest China. These deposits historically served as the principal source of saltpeter for traditional gunpowder production in pre-industrial China. Despite their long mining history, the sources and enrichment mechanisms of these cave nitrate deposits are poorly constrained. This study presents a comprehensive isotopic analysis of cave nitrate deposits, combining geological field observations in Southwest China. The active formation of nitrate deposits in the Dashiwei Tiankeng (DT) system (Guangxi) was discovered. The continuous emission and accumulation of nitrate–bearing aerosols occurs through gas emission pores distributed across ground and cave walls in the DT system. Consistent δ15N and δ18O values of nitrate in between Dashiwei Cave and Jinfo Cave reveal a common nitrate formation pathway across temporal and spatial scales. The absence of oxygen isotope mass–independent fractionation definitively excludes atmospheric nitrate sources, confirming that soil ammonium is the important precursor through microbial nitrification. Exceptionally low sulfate δ34S values in Jinfo Cave occur due to large–scale sulfide oxidation in coal–bearing strata. We propose that these cave nitrate systems originate primarily from water vapor above subterranean rivers rather than from conventional bat guano or drip water sources, with transport and enrichment mediated by a novel evaporative-refrigeration mechanism of the karst cave system. When gas flows through narrow cave passages into large chambers, rapid pressure release induces water vapor cooling condensation, and humidity drop results in the supersaturation of soluble salts, leading to the precipitation of salt aerosols.