Ferromanganese deposits in the caves of the Guadalupe Mountains

—Cave ferromanganese deposits are an unusual type of mineral deposit present in some caves of the Guadalupe Mountains, NM. These deposits consist of several horizons: a layer of soft, altered “punk rock” underneath a highly colored layer, composed predominantly of Fe, Mn, and Al oxides and hydroxides. The deposits contain a diverse microbial community, and DNA analyses indicate that some identified organisms are closely related to known manganese and iron oxidizers. Originally thought to be derived from chemical corrosion of the cave bedrock, the enrichment of Fe and Mn in these deposits cannot be explained solely by the dissolution of carbonate; Fe and Mn are likely transported from the punk rock zone and enriched in the oxide layer. The accumulation of oxides in one horizon, the breakdown of bedrock in another horizon, and the presence of a microbial community suggests that the deposits are similar to soils and may undergo similar processes. FIGURE 1. Collecting a sample of chocolate-brown ferromanganese from the ceiling of Snowing Passage in Lechuguilla Cave. Photo by Val Hildreth-Werker. See Plate 13B for a color image of a FMD. 162 SPILDE, NORTHUP, AND BOSTON able MnO, whereas the FMD are strongly enriched in Feand Mn oxides (Spilde et al., 2005). Cunningham et al. (1994) observed evidence of microbial products in these deposits using scanning electron microscopy (SEM) and postulated that there may be a biotic process involved in the creation of the cave FMD (Cunningham et al., 1995). Northup at al. (2000, 2003), Boston et al. (2001), and Spilde et al. (2005) developed a model in which FMDs are the result of microbial activity. In this model, shown in Figure 2, microbes release organic acids, which break down the carbonate bedrock in the punk rock layer releasing Fe(II) and Mn(II) present in trace amounts in the carbonate minerals. Ironand manganese-oxidizing microbes utilize the reduced Fe and Mn, oxidizing the elements as an energy source. The microbes may transport the released Fe and Mn from the punk rock zone with chelating ligands or through networks of exopolysaccharides. The oxidized respiration products build up in the oxide layer as Fe(III) and Mn(IV) oxides. CHEMISTRY AND MINERALOGY OF CAVE FMD The cave FMDs are chemically and mineralogically distinct from the underlying bedrock, which consists of dolomite or calcite. In general, backreef bedrock is predominantly dolostone and the reef bedrock is predominantly calcite; detailed stratigraphy of the Guadalupe Mountains can be found in Hill (1996). Spider Cave is located entirely within the backreef, and Lechuguilla Cave spans both backreef and reef rocks. Table 1 lists the minerals that have been identified in the bedrock and FMD and their approximate abundances. Although calcite or dolomite are present in the FMD, their abundances are diminished, and new minerals, such as Al-hydroxides and Fe/Mn oxy-hydroxides have appeared. Lithiophorite [(Al,Li)MnO(OH)2], nordstrandite and gibbsite [Al(OH)3], goethite, kaolinite, and illite have been identified by XRD and SEM/EDX analysis. TEM examination revealed that much of the abundant Feand Mn-oxides are poorly crystalline, consisting of nanometerand micrometer-sized domains of coherent crystal lattice. Todorokite [(Mn2+,Ca, Na, Mg,K)Mn 4+ 3O7•H2O] and birnessite [(Ca,Na)0.5(Mn ,Mn)2O4•1.5H2O] have been identified in the FMD by synchrotron micro-XRD (Xray diffraction) (Boston et al., 2004). Table 2 provides analyses of representative samples of the bedrock, punk rock and several colors of FMD to demonstrate the change in chemistry across the constituent layers. The major carbonate components, Ca and Mg, are depleted relative to original bedrock composition; the absolute concentration of most other elements is increased by the loss of carbonate. However, the relative proportion of the residual elements when compared to an insoluble element such as Ti varies significantly. Silica from clay and detrital quartz and feldspar in the bedrock is depleted relative to TiO2 in the dark FMD whereas Fe and Mn are generally much higher than expected. Iron and Mn in the oxide layer are hundreds to thousands of times enriched relative to the underlying bedrock; Fe2O3 in the oxide layer may be as high as 78 wt% compared to less than 1 wt% in the bedrock and MnO as much as 22 wt% compared to 200 ppm or less in the bedrock. Not only are these elements strongly enriched, the Mn/Fe ratio increases by an order of magnitude from around 0.07 in the bedrock and most of lighter colored FMDs to 0.8 in the dark FMD, suggesting an enrichment of manganese over iron (Spilde et al., 2005). Likewise, the porosity increases and bulk density decreases dramatically from bedrock, through punk rock, to the oxide layer, which is usually much less than 1 g/cm3 after drying. The strong enrichment of Fe and Mn oxides in the FMD and the marked increase of Mn: Fe ratios indicate a mass transfer from the punk rock to the outer oxide layer, while Ca and Mg are removed from both layers and and Si is depleted in the outer layer. MICROBIOLOGY OF CAVE FMD Northup et al. (2000, 2003) demonstrated that there is a diverse microbial community present in the FMD in Lechuguilla and Spider Caves, including a high percentage of mesophilic Archaea in one site. Identified from community DNA were clones whose closest relatives are iron or manganese oxidizers or reducers, although, in general, the similarity values were low. Nearest relatives known to oxidize or reduce iron or manganese include Hyphomicrobium, Pedomicrobium, Leptospirillum, Stenotrophomonas, and Pantoea (Northup et al., 2003). DNA extracted from cultures inoculated with FMD from the Rainbow Room in Lechuguilla Cave more clearly demonstrated the presence of putative manganese oxidizers with closest relatives including species of Bacillus and Alcaligenes, organisms that have been reported to oxidize manganese (e.g., Francis and Tebo, 2002 for Bacillus and Abdrashitova et al., 1990 for Alcaligenes). Epifluorescent microscopy showed extensive microbial communities present in both the oxide layer and punk rock (on the FIGURE 2. Schematic model of the ferromanganese deposit. Microbes (1) generate organic acids (2) that break down carbonate bedrock (3) releasing Fe(II) and Mn(II) from the carbonate mineral structure. The reduced elements are transported in the form of metal chelates (4) or exopolysaccharides to the microbial community where microbial oxidation takes place, with oxides accumulating at the cave-air interface (5). Modified from Northup et al., 2000. 163 FERROMANGANESE DEPOSITS IN THE CAVES OF THE GUADALUPE MOUNTAINS order of 107 cells per cm3). Analysis of total cell numbers showed that highest numbers of cells were seen at interfaces, the pink/ white interface between oxide layer and punk rock, and the black/ brown interface between the oxide layer and the atmosphere of the cave. Studies done to ascertain the portion of metabolically active cells present demonstrated that higher activities are seen in darker FMDs and in the punk rock, where higher numbers of prosthecate (i.e., stalked) bacteria are observed. We believe that this lends support to a biogenic hypothesis for FMDs because two known manganese oxidizers (Pedomicrobium and Hyphomicrobium) are prosthecate bacteria that are similar in morphology to those observed in FMDs and punk rock. Organisms cultured from the cave deposits have slowly produced in the laboratory an array of Feand Mn oxideminerals that are present in the cave FMDs, e.g., birnessite (Boston et al., 2001). SPELEOSOL: A NEW TYPE OF SUBTERRANEAN SOIL Cave FMD are similar in many ways to laterite soils (oxisols). Sedimentary 2:1 clays (e.g., smectite, illite) are converted to 1:1 clays (e.g., kaolinite) (Sposito,1989); soluble elements such as K, Ca, Na, Mg, Si are leached from the system; other elements such as Al, Fe and Mn are enriched in sesquioxides; and insoluble trace elements (Ti, Zr, Nb) are enriched (Buol and Eswaran, 2000). Other trace elements like Ni are sequestered in oxides, especially Mn-oxides that are powerful scavengers of transition metals (Manceau et al., 2002). Both todorokite and birnessite have been identified in these deposits. These minerals are also present in both soils and desert varnish, environments believed to be strongly influenced by microbial communities. Birnessite is the most common Mn-mineral found in soils (Sposito,1989) and todorokite is widely cited Mineral Formula Approximate abundance: omanganese Bedrock