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An Investigation of Meromixis in Cave Pools, Lechuguilla Cave, New Mexico

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Abstract:
Levy D B 2008 An Investigation of Meromixis in Cave Pools. Lechuguilla Cave. New Mexico International Journal of Speleology, 37 (2), 113-118 Bologna (Italy) ISSN 0392-6672.

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 < 1 mg L'), biogenic meromixis resulting in 0:(g)-depleted subsurface waters is not expected Four pools at various depths below ground surface (0 rn) 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 02(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 (> 0 9 m), and are probably the result of localized and transient atmospheric C02(g) concentrations At LOBG, an EC increase of 93 pS cnr' 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

Keywords: chemocline, geochemistry, meromixis, mixing, stratification. United States, Carlsbad Caverns National Park

INTRODUCTION
Abundant cave pools exist throughout the world whose water column may be predisposed to a condition of permanent chemical stratification (meromixis), depending on the specific effects of pool morphometry and environmental conditions. Numerous studies have been conducted regarding density stratification within the sinkholes of coastal aquifers, such as in the renowned cenotes of the Yucatan Peninsula, Mexico, although the limnological characteristics of inland, limestone-hosted freshwater cave pools have received less consideration. Meromixis in fresh water cave pools is of particular interest because the physical and chemical characteristics of stratification could provide insight into the geochemical origin and evolution of pool waters.

The physical and chemical processes leading to chemical stratification in cave pools are potentially analogous to those of permanently-stratified (meromictic) inland lakes. Over 100 meromictic lakes have been identified in North America (Anderson et al.f 1985}, and the estimated world-wide frequency of meromictic lakes is 1:1000 (Lindholm, 1995). Meromictic lakes have been used as tools in the study of paleolimnology and trace element redox cycling (Viollier et al., 1995; Taillefert et al.f 2000; Hakala, 2005). Some meromictic water bodies also host specific bacterial populations with potential for future bio tech no logical applications (Overmann et al., 1996; De la Rosa-Garcia et al., 2007).

In the classical scheme for describing meromictic water bodies, the uppermost circulating layer is the mixolimnion and the deep isolated layer is the monimolimnion. The two layers are stabilized by high total dissolved solids (TDS) concentrations in the monimolimnion, which is separated from the mixolimnion by a zone called the chemocline. Thermal stratification is also typically evident, and the monimolimnion is often depleted in dissolved 02(g) (DO) due to subsurface decomposition of organic C. Meromixis in a water body may be caused by: (1) input of saline or fresh water from an external source (ectogenic meromixis), (2) subsurface flow of saline water (crenogenic meromixis), or (3) decomposition of organic C leading to elevated salinity in deeper waters [biogenic merombds) (Hutchinson, 1937).

Meromixis associated with cave and karst systems is most notable in coastal aquifers where density gradients develop between deep saline groundwater and more shallow, dilute groundwater (e.g., Mejia-Ortiz et al., 2007; Schmitter-Soto et al., 2002; Stoessell & Coke, 2006). Cenote Verde, in Quintana Roo, Mexico, is a good example of a meromictic karst pond. Cenote Verde is the deepest known karst pond in the area (49 m) and has seven distinguishable layers in the water column. Several of the layers contain dense bacterial populations and elevated H2S(g) concentrations (> 25 mg I/1) resulting from anaerobic decay of organic C coupled with the slow exchange of groundwater (Wilson & Morris, 1994).

The most notable occurrences of density stratification in inland caves occur in karst systems composed of soluble rocks. The high solubility of gypsum (CaSOa»2H;0) can lead to a significant increase in the density of the dissolving water, causing chemical stratification which may occur in single cave pools, or on an aquifer scale (Kempe, 1972; Klimchouk, 1997). Continuous or periodic inflow of fresh water, such as vadose percolation or upward groundwater recharge, favors the persistence of density stratification (Cordingley, 1991). Klimchouk (1997) provides an excellent discussion of the bevels (flat ceilings) and facets (from horizontal notching) which form when aggressive water recharges a gypsum stratum from an underlying formation under low artesian flow. However, few studies have been conducted which pertain to meromixis in isolated, fresh-water cave pools hosted in limestone bedrock.

Lechuguilla Cave (Lechuguilla), located in New Mexico (USA), contains abundant isolated pools of various size located throughout over 195 km of passage. Lechuguilla is situated in carbonate rocks of the Permian Capitan Reef Complex and was formed over millions of years through an unusual process of H2S04 speleogenesis (Palmer & Palmer, 2000). Results from approximately 200 water samples collected between 1986 and 1999 show that Lechuguilla pools are dominated by Ca, Mg, HC03, and S04 (Fig. 1), due to interaction of groundwater with carbonate rocks and secondary gypsum deposits (Turin & Plummer, 2000|. Lechuguilla pool water has a pH between 7.5 and 8.5, PC02(g) values greater than atmospheric (10149kPa(, and is usually oversaturated with respect to calcite (CaC03) and dolomite [CaMg(C03)2]. Vadose- zone water entering the cave as flowstone seepage and ceiling drips provides recharge to the pools, while water loss from pools can occur through overflow, evaporation, and leakage. Consequently, the pools display a range in chemical composition due to variations in bedrock mineralogy, localized evaporation rates, and residence time (Forbes, 2000; Turin & Plummer, 2000). Typical Lechuguilla pools are of the Ca-Mg-HC03 and Ca-Mg-S04 type, although two unique pools, one representing gypsum saturation (Dilithium Pool) and another signifying extreme evapoconcentration to a Mg-SO^ brine (Briny Pool) have been discovered (Fig. 1).

Lechuguilla Cave therefore provides a suitable setting to investigate meromixis in isolated fresh water pools hosted in limestone caves. If present, chemical stratification in the pools is hypothesized to be ectogenic, resulting from the convergence of waters with contrasting salinity. Because organic C concentrations are typically low (< 1 mg L'1), biogenic meromixis resulting in an 02(g)-depleted monimolimnion is not expected. The discovery of common-ion effect speleothems (subaqueous helictites) in more than 30 locations demonstrates that some Lechuguilla pools have been recharged with Ca-S04 water originating from dissolution of secondary gypsum (Davis et al., 1990). The potential for extreme evapoconcentration is also evident at the Briny Pool, where both Na and CI exceed 4,000 mg L'1 and TDS concentrations are greater than 45,000 mg L'1 (Turin & Plummer, 2000). Identification and characterization of meromixis in Lechuguilla pools could provide insight into the geochemical origin and evolution of particular pools.

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