An investigation of meromixis in cave pools, Lechuguilla Cave, New Mexico

Chemical characteristics of permanent stratification in cave pools (meromixis) may provide insight into the geochemical origin and evolution of cave pool waters. The objective of this study was to test the hypothesis that some pools in Lechuguilla Cave may be subject to ectogenic meromixis, where permanent chemical stratification is induced by input of relatively saline or fresh water from an external source. However, because organic C concentrations in Lechuguilla waters are low (typically \u3c 1 mg L-1), biogenic meromixis resulting in O2(g)-depleted subsurface waters is not expected. Four pools at various depths below ground surface (0 m) were studied: (1) Lake Chandalar (-221 m), (2) Lake of the Blue Giants (LOBG) (- 277 m), (3) Lake Margaret (- 319 m), and (4) Lake of the White Roses (LOWR) (- 439 m). Water column profiles of temperature, pH, dissolved O2(g), and electrical conductivity (EC) were collected down to a maximum depth of 13.1 m using a multi-parameter sonde. Opposite pH gradients were observed at Lake Chandalar and LOBG, where pH at the surface (0.3 m) varied by ±0.20 units compared to the subsurface (\u3e 0.9 m), and are probably the result of localized and transient atmospheric CO2(g) concentrations. At LOBG, an EC increase of 93 μS cm-1 at the 0.9-m depth suggests meromictic conditions which are ectogenic, possibly due to surface inflow of fresh water as drips or seepage into a preexisting layer of higher salinity