Mineralogy 2009 ICS Proceedings 15th International ...

Mineralogy

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2009 ICS Proceedings

Speleosol: A Subterranean Soil Michael N. Spilde1, Ara Kooser1, Penelope J. Boston2,3, Diana E. Northup4 1 Department of Earth and Planetary Sciences, MSC03-2050, University of New Mexico, Albuquerque, New Mexico, USA 2 New Mexico Institute of Mining and Technology, Socorro, New Mexico, USA 3 National Cave and Karst Research Institute, Carlsbad, New Mexico, USA 4 Department of Biology, MSC03-2020, University of New Mexico, Albuquerque, New Mexico, USA Mud and soil deposits occur in many caves. Frequently, they are transported into caves by flooding or infilling. However, in several caves around the world, soil-like material has been found that appears to be an autochthonous product of pedogenic or soil-forming processes that take place in these caves. The podegenic alteration of bedrock results in mass loss, mass transfer, and mineral transformation via chemical and microbial weathering. The material has been variously known as “corrosion residue” and “cave ferromanganese deposits,” but because of the soil-like character and the processes forming this material, we propose to establish the term speleosol to refer to soil-like material formed in the cave. The term should not include allochthonous sediments washed into the cave or autochthonous sediments such as sand and silt remaining from primary speleogenesis.

1. Introduction

Secondary mineral deposits in caves may include a wide array of unusual minerals, many of which may be mediated by microbial communities. Caves often provide an ideal environment for chemolithoautotrophic growth of microbial communities, and evidence of their existence is often observed in the minerals that are produced. Examples include ferromanganese deposits (FMD) that are discussed here, moonmilk, and pool fingers. FMD are interesting because they involve both alteration of the cave walls and deposition of secondary minerals. These deposits are accumulations of low density, soil-like material that line the walls, floors and ceilings of some caves (Fig. 1). Thus far, FMD have been observed in caves in New Mexico, Arizona, and South Dakota (USA), and in Turkmenistan (Maltsev, 1997). However, the most prodigious deposits occur in Spider and Lechuguilla Caves in Carlsbad Caverns National Park, New Mexico. The deposits are highly enriched in secondary aluminum hydroxide minerals, iron

Figure 1: Sampling chocolate-brown FMD on a wall in Lechuguilla Cave. Photo by Val Hildreth-Werker.

oxyhydroxide and manganese oxide minerals, along with clays, quartz, carbonates, phosphates and sulfates. Cave FMD are similar in many ways to laterite soils (oxisols). In these leached soils, 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 and other elements such as Al, Fe and Mn are enriched in sesquioxides. Insoluble trace elements (Ti, Zr, Nb) are enriched (Buol and Eswaran, 2000). Both chemical and microbial processes influence the formation of surface soils. Similarly, chemical weathering occurs in caves from the condensation of weak carbonic acid on cave walls that may contribute to the breakdown of bedrock carbonate. Likewise, microbial breakdown of bedrock plays a significant role (Boston et al., 2001; Northup et al., 2003, Spilde et al., 2005), as microorganisms present on the cave wall release organic acids. Both the chemical and microbial dissolution of carbonate contributes to a “weathering” process, making Fe and Mn available to microbial communities for energy untilization. Thus, the process of formation of FMD is one of pedogenesis or a soil-forming process. Because of the soillike nature of this material, we will refer to it as “speleosol.” In this paper we will determine the degree of weathering and collapse that results from dissolution of bedrock at the caveair interface and leaching from the underlying bedrock, and the subsequent amount of enrichment in the residual surface layer.

2. Chemistry and Mineralogy of Speleosol Like most soils, speleosols consist of chemically and

15th International Congress of Speleology

Mineralogy 339 mineralogically distinct layers. In Lechuguilla and Spider Caves, three layers make up the speleosol: (1) a colorful outer layer highly enriched in Fe- and Mn-oxides that ranges in thickness from less than a cm to a number of cms; (2) a leached layer of soft, altered bedrock that may extend many cms into the cave wall; and (3) the underlying, unaltered bedrock that may be either dolostone or limestone (Fig. 2). The outer layer consists mainly of oxide minerals and clays in a spectrum of color from light pink, to blood red, brick red, yellow, orange, ocher, brown, chocolate brown, gunmetal grey, or jet black. Lithiophorite [(Al,Li)Mn4+O(OH)2], nordstrandite and gibbsite [both Al(OH)3], goethite, kaolinite, and illite have been identified in the oxide layer by X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray microanalysis (Spilde et al., 2005).

Transmission electron microscopy has revealed that much of the abundant Fe- and Mn-oxides are poorly crystalline, consisting of nanometer- and micrometer-scale domains of coherent lattice. Todorokite [(Mn2+,Ca,Na,Mg,K)Mn34+ O7•H2O] and birnessite [(Ca,Na)0.5(Mn4+,Mn3+)2O4•1 .5H2O] have been identified by synchrotron micro-XRD (Boston et al., 2004). Iron and manganese 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 MnO2 as much as 57 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 in the lighter colored oxide layers to 0.8 in the dark oxide layers, suggesting an enrichment of manganese over iron in the dark material (Spilde et al., 2005).

Figure 2: Schematic diagram of cave speleosol. Top: illustration of an idealized cross section through the cave wall, from bedrock (left) through punk rock and oxide layer and into the cave passage on the right side, with an approximate scale shown. Gray arrows represent microbial leaching of the punk rock and black arrows represent chemical weathering by condensation corrosion. Middle: diagrammatic representation of the original volume of bedrock, which after leaching and chemical weathering, consists of remaining bedrock, a leached zone (punk rock), an enriched zone (oxide layer), and the volume lost to weathering and leaching. Bottom: Hypothetical geochemical profile through the speleosol showing enrichment of immobile element due to carbonate dissolution at surface and in leached zone and enrichment of mobile elements in the enriched zone.



Under the oxide layer is a soft, leached layer of bedrock of variable thickness called “punk rock” by Hill (1987), which occurs in shades of pink, yellow or white. This layer consists of partially dissolved carbonate bedrock, and in the SEM, severely etched calcite or dolomite crystals are abundant (Spilde et al., 2005). Where the punk rock occurs in dolomite, intermixed calcite is often selectively removed and the dolomite recrystallized into euhedral crystals. Minor clays (illite and kaolinite) and trace gibbsite and quartz are the only notable minerals in this layer besides calcite and/or dolomite. The carbonate bedrock, below the punk rock layer, contains major calcite or dolomite, depending on where the cave passage is located in the Capitan Reef complex. Accessory

Mineralogy minerals include small amounts of clays (dickite, illite, kaolinite, or smectite), some of which are allogenic; other detrital minerals include quartz, feldspar, apatite, monazite, hematite, and rutile.

3. Origin of Cave Speleosol

The oxide-rich layer was originally called “corrosion residue” and was believed to be the insoluble residue from either attack of corrosive air on the carbonate bedrock (Queen, 1994) or insoluble material remaining from sulfuric acid speleogenesis (Davis, 2000). In the process of condensation corrosion, warm moist air rises in RayleighBernard convection cells and water is condensed on the ceiling and upper cave walls, presumably because these areas are slightly cooler due to the geothermal gradient (Sarbu and Lascu, 1997). The condensed water absorbs CO2 from the air to form carbonic acid that corrodes the carbonate bedrock. Thus, the speleosol in this model represents the insoluble residue left after the dissolution of the bedrock by the weak acid. However, the Fe and Mn in the oxide layer are many times more enriched than can be explained by acidic corrosion of carbonate, either as result of condensation corrosion or as a residue of speleogenesis. Simple dissolution of Guadalupe carbonate bedrock by acids ultimately leaves a silica-rich residue with slightly enriched Fe2O3 and barely detectable MnO (Spilde et al., 2005). Thus, processes more complex than simple dissolution are necessary to explain this enrichment. Boston et al. (2001) and Northup et al. (2003) demonstrated that speleosols host an active microbial community that includes iron and manganese oxidizers, acid-producing, and nitrogen-cycle bacteria. Spilde et al. (2005) showed that the deposits were highly enriched in Mn- and Fe-oxides and presented a model in which Fe and Mn enrichment is the result of microbial activity. In this model, microbes release organic acids that break down the carbonate bedrock in the punk rock layer, releasing Fe(II) and Mn(II) present in trace amounts in the carbonate minerals. Iron- and 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 as networks of exopolysacchorides. The oxidized respiration products build up in the oxide layer as Fe- and Mn(hydr)oxides. This microbial model does not preclude the additional influence of the previous corrosion condensation model and both processes may be active simultaneously.

4. Methods: Mass Balance Analysis

A quantitative mass balance approach makes it possible to

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use chemical elements in rocks and soils as geochemical tracers of specific hydrochemical processes during weathering or supergene enrichment (Brimhall et al., 1985). Such an approach must take into account chemical, physical, volumetric, and mechanical properties of the weathered product to compare with the unaltered prolith or bedrock. This technique quantitatively calculates mass loss and changes in volume (strain) due to dissolution of mobile elements. Residual enrichment of immobile elements (such as Ti, Al, Zr or Nb; indexed here as i) occurs by the removal of more soluble or locally mobile species (indexed as j; Eqn.1). In the absence of lateral flux, the mass of an immobile element (i) contained in the original protolith volume (p) before weathering is retained in the rock after weathering (w) by chemical or microbial processes, and is given as the product of volume, density, and concentration (Brimhall and Dietrich, 1987). The 3-dimensional volume may be simplified to consider only the thickness of the individual zones in the profile, since the flow path of water is essentially into or out of the cave wall surface. Thus, B can be used to represent the columnar length of the representative volume of protolith and weathered equivalent, substituting Bp and Bw for the respective V terms. Since the original length of bedrock is unknown, we need to introduce a term for one-dimensional strain εi,w , which we can define as the change in length divided by the original length, εi,w = (Biw – Bip)/Bip. In this manner, the strain can be stated in terms of the measurable quantity Biw = Bip(εiw + 1). Thus we can define a nondimensional equation describing residual enrichment, in which strain is the only unknown (Brimhall and Dietrich, 1987).

(1)

The first term in the equation is the enrichment factor, calculated on the concentration of Ti as an immobile element. The second term yields the accumulated strain and the third term is the density ratio between protolith and weathered product, which in our case is the carbonate bedrock and FMD, respectively. Bulk chemical analysis was performed on 21 colored FMD oxides, 13 punk rock, and 10 bedrock samples (including 4 surface bedrock samples) from Lechuguilla and Spider Caves. Of these, half represented dolostone protolith and half limestone. Cave samples were collected under permit from Carlsbad Caverns National Park, and surface bedrock was collected from road cuts on public land outside of


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the National Park. Major and minor elements were analyzed by means of X-ray fluorescence (XRF), with a detection limit of 0.01 wt% in most cases; trace elements, including Ti, were analyzed by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), providing a detection limit of < 1 ppm for Ti. Bulk density of bedrock and particle density of the oxide layer and punk rock was determined by a standard soil science technique (ROWELL, 1994). The FMD bulk density was calculated from particle density and the weight lost in drying the samples prior to bulk chemical analysis. This assumes that the water in the sample Figure 3: Enrichment factors for the insoluble element Ti are plotted against density maintains porosity and ratio for oxide layer and punk rock samples. Samples from both limestone (squares) and that the volume of pore dolostone protoliths (circles) are shown; the unaltered protolith plots at 1 (star) for both space is equal to the limestone and dolostone. The data for Al (open symbols) is also plotted. Various degrees of volume of water lost, a reasonable assumption in an strain () are plotted as lines originating from the origin. Positive strain represents dilation and negative strain, collapse. Inset: Expanded origin corner showing clustering of punk rock environment of saturated samples (circled). humidity. Since the dark oxide layer samples experience as much as 70% loss of weight have undergone as much as 75% collapse from removal in drying, this gives an upper limit on density. of soluble carbonates. This is a fairly small range of values compared to the limestone protolith, where most samples 5. Results have experienced between 99 and 99.9% collapse. The fact The data for Ti and Al are plotted in Figure 3 showing that the Al is more enriched than the Ti in many of the density ratio vs. enrichment factor. The high level of limestone samples indicates that Al has been mobilized and enrichment for both Ti and Al indicate that a large amount enriched in the oxide layer and cannot be regarded as an of carbonate has been removed by dissolution. A simple immobile element. residual weathering path would follow the ε = 0 line. Higher degrees of residual enrichment will plot along the strain line Porosity is defined as the ratio of void volume to total representing the amount of collapse. For example, a number volume and is calculated in soil from the ratio of the bulk of the samples from the dolostone protolith plot along the sample density (r w) to the grain density (rg ) by n = 1 - r w ε = -0.9 line, indicating that 90% of the soluble material / rg (Brimhall and Dietrich, 1987). By plotting has been removed. Thus, 10 mm of initial bedrock would porosity as 1 /(1 - n), as porosity n increases, 1 /(1 - n) collapse into to a 1 mm oxide layer. The punk rock samples increases as well, and the enrichment factor varies linearly cluster between a density ratio of 1 and 2 (Fig. 3 inset) and with density ratio. Figure 4 illustrates porosity data mostly plot between ε = 0 and ε = -0.75, indicating that from the limestone protolith samples. In this figure, two some samples have experienced little collapse while others distinct trends are observed: one with a very high degree


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enrichment of mobile elements such as Fe and Mn. If a soluble element is removed from the system, the degree of enrichment of Ti can be used to establish the degree of enrichment or depletion of other elements. Elements within a sample should have an enrichment factor similar to Ti, in the absence of mobilization or deposition of that element. If enrichment factors are appreciably different, then we can state that those with lower enrichment factors have been mobilized and those that are higher have been mobilized from Figure 4: Punk rock and oxide layer samples from limestone protolith are plotted in terms of elsewhere and precipitated. porosity (n). For those where paired samples were collected, tie-lines connect the oxide layer Aluminum, for example, is samples with the punk rock directly underneath. The constant mass line is noted (Σ = 0). significantly more enriched than Ti in some samples of porosity representing the darker-colored oxide samples from limestone protolith (Fig. 3), indicating mobilization and the other trend of high enrichment with only a small from the punk rock and deposition in the oxide layer. Other change in porosity, representing the lighter-colored oxide major element enrichment factors are shown in Figure 5. layer samples (pink, yellow, and reds). Similar trends are Calcium and magnesium are significantly depleted in the found in the dolostone protolith samples. The black, manganese-rich oxide layers tend to be anomalous compared to the other dark brown samples in that they exhibit low porosity change and low Ti enrichment. This suggests that the range of colors, from light (pink,reds, etc) to the darkest samples is not a continuum and different processes are acting to produce the dark samples and high manganese samples. With the degree of Ti enrichment due to carbonate dissolution now established, we have a way to determine the

Figure 5: Enrichment factors for major mobile elements in all samples. Squares represent limestone; triangles and squares, dolostone; punk rock in open symbols (A) Ca and Mg. Values above an enrichment factor of 1 are enriched and below 1, depleted or leached. (B) Si enrichment factors in limestone and dolostone. Inset: expanded origin corner showing punk rock (open triangles) and oxides samples (black triangles) from both protoliths.


Mineralogy 343 oxide layer samples whereas most of the punk rock samples exhibit enrichment factors between 1 and 2, indicating that minor dissolution of carbonate has occurred in the punk rock but significant dissolution has taken place in the oxide layer. Likewise, Si is much less enriched than Al (