Shelfstone in Atlantis, Lechuguilla Cave (Team B....
Transcript of Shelfstone in Atlantis, Lechuguilla Cave (Team B....
Shelfstone in Atlantis, Lechuguilla Cave (Team B. 1991)
Becky Lauren Quinlan
Sulfuric Acid Speleogenesis
Geomorphology Seminar
May 16, 2005
History of cave formation thought
Theorists have written about their speculations of the processes of cave formation since at
least 1503 (Shaw, 2000). Theories formulated before the 20th century were divided between the
catastrophic camp and those that felt that steady processes rule speleogenesis (Shaw, 2000).
Biblical events, earth movement and cosmological theories that showed that caves were formed
with Earth’s creation were the focuses of the catastrophic group (Shaw, 2000). Others were
certain that caves could only be formed over long periods through steady processes like erosion
and dissolution (Shaw, 2000). In the early 20th century, the classic debate between vadose and
phreatic processes was subjected to the scientific method (Lowe, 2000). In his 1930 paper, The
origin of limestone caverns, William Morris Davis wrote that caves formed during very long
stretches of time by fresh water below the water table (Lowe, 2000). Davis acknowledged that
physical exploration of caves needed to be undergone, and he failed to establish models of cave
development to drive investigations into his assertions on drainage and deep phreatic dissolution
(Lowe, 2000). Others, like Swinnerton in 1929, endorsed vadose and shallow phreatic
speleogenesis (Lowe, 2000). Most of the theories proposed and developed before the 1950’s
have been further developed and accepted by geomorphologists and geologists (Lowe, 2000). In
the latter half of the 20th century, in addition to new theories of cave development like the
sulfuric acid theory, scientists began to better understand the interactions between caves and their
surroundings (White, 2000a). An emphasis was placed on the geologic setting of caves and the
chemical processes of karst dissolution (White, 2000a). Geomorphologists agreed that most
caves were formed in limestone, which was dissolved by carbonic acid and carried away, and
many researchers focused on the mechanism by which fresh, undersaturated water could reach
deep carbonate aquifers (White, 2000a). The speed of cave formation is dependant on the rate
of limestone dissolution and the rate at which fresh water can replace that already saturated with
limestone (White, 200a). The latter control is stratigraphically and hydrologically dependant
(White, 2000a). Researchers have attempted to quantify the dissolution rate of limestone, but
have been unsuccessful in agreeing on a number using field observations or lab experiments; the
rate seems to hover between about 25 and 100 micrometers per year of denudation (White,
2000b).
History of the sulfuric acid theory
Though most caves, by far, are formed through carbonic acid speleogenesis, some caves,
like Carlsbad Caverns in the Guadalupe Mountains of southeastern New Mexico, possessed
features inconsistent with the model. While working on his dissertation in the early 1970’s,
Egemeier (1973, p 9, 27) acted under the assumption that the caves in Big Horn Basin, Wyoming
formed like most other caves, aided by the heat provided by deep artesian flow (p 8-9). He
observed the odor of hydrogen sulfide (H2S) in Lower Kane Cave, an active cave in the region,
as well as the formation
of gypsum crusts and
mounds within the
caves. He also
observed the quick
dissolution of gypsum
mounds by water both
within and outside
caves, where gypsum Figure 1. Massive gypsum block in the Big Room, Carlsbad Caverns (Egemeier, 1973).
blocks had been used to dam small streams. He noted the unusual pattern of caves widening
upstream along flowing water. Also confusing to Egemeier was the fact that, despite his
assumption of caves being formed by flowing water features, he observed in the active caves no
or little incision of the flowing water into the cave floors, potholes, or meanders influencing the
shape of cave rooms (p 32). Egemeier observed phreatic features in most of the Big Horn Basin
caves, such as spongework, pockets in the walls, ceilings, and floor, bedding and joint
anastomoses and continuous rock spans (p 34-35). Egemeier proposed without evidence or
citation that he believed the gypsum was formed during speleogenesis (p 36), giving the reader a
hint of the theory to come. In his conclusions, Egemeier accumulated the evidence of H2S in the
air and gypsum crusts on the walls to propose a sulfuric acid induced enlarging of the active
caves (p 39), and boldly stated his belief that the caves of the Big Horn Basin are formed by the
replacement of limestone and dolomite by gypsum through the action of sulfuric acid (p 40).
Egemeier also suggested a phenomenal rate of enlargement 3.7 m3 of limestone removed per
year, allowing Lower Kane Cave, with a volume of 10,000 m3, to be excavated in as little as
3000 years, a process that would take much longer with carbonic acid dissolution (p 53). He
even proposed that the H2S could be derived from a number of sources, simultaneously: the
thermal springs, sulfate reducing bacteria, and the hydrocarbon reservoirs found in the same
limestone formation of the caves (p 64). He extended his replacement theory to Carlsbad
Caverns and refuted Good’s 1957 proposal that the gypsum found in the big room precipitated
from cooling gypsum-rich water, giving good evidence such as the absence of silt and sand from
the deposits, the sheer volume of gypsum deposits, and the lack of limestone blocks embedded in
the gypsum (p 69-70). Egemeier’s amazing dissertation incorporated many of the elements
present today in the sulfuric acid theory of speleogenesis, but he was not the only researcher
beginning to realize that some caves had features inconsistent with carbonic acid dissolution and
to turn to sulfur for an explanation.
In 1971, Dwight Deal, Harvey
DuChene, Carol Hill and Dave Jagnow,
four graduate students, made a visit to
Carlsbad Caverns. They began to
contemplate the unusual cave features, like
the softness of the walls, odd speleotherms
that looked like a line of popcorn (Figure
2) or a wall of corn flakes, and the huge
blocks of gypsum (Figure 3) on the floor
of some rooms (DuChene and Hill, 2000). David Jagnow chose to investigate the speleogenetic
origin of the Guadalupe Mountains caves for his masters thesis. Without knowledge of
Egemeier’s work in the Kane Caves of Wyoming, he also proposed a sulfuric acid origin for the
caves (Jagnow et al, 2000; Jagnow, 1977). Jagnow pointed out that because of Carlsbad
Caverns’ immense size and beauty, many geologists and geomorphologists have contemplated
the origin of the cave system, and had debated the origin’s vadose or phreatic nature (p 10-14).
Despite the great attention paid to the large gypsum blocks on the big room floor, no previous
Figure 2. Gypsum "popcorn" in Lechuguilla Cave (Thompson, 1991).
Figure 3. Large gypsum blocks on the floor of Prickly Ice Cube Room in Lechuguilla Cave (Team B, 1991).
study except Egemeier’s thesis of 1973 proposed an alternative speleogenetic process to carbonic
acid dissolution (Jagnow et al, 2000). Jagnow drew on the work of White in 1965 and
Morehouse and Pohl in 1968, who proposed sulfuric acid dissolution speleogenesis for caves in
Iowa and Kentucky (Jagnow, 1977, p 107). Jagnow believed that sulfuric acid dissolution was
helped along by the carbonic acid process, which he considered possibly more important in the
speleogenesis of caves in the Guadalupe Mountains (p 109). He speculated that the source of
sulfide for the sulfuric acid reactions was the extensive deposits of pyrite in the area (p 109)
possibly converted by iron bacteria (p 120), and cited the large gypsum beds and gypsum
“popcorn” on the cave walls as evidence of the caves’ sulfuric acid origin (p 119). In an
example using the Left Hand Tunnel of Carlsbad Caverns, he proposed a chronology of cave
formation: “1) solution of Left Hand Tunnel 2) deposition of clay and silt banks 3) truncation
and incrustation by gypsum crust 4) final deposition of stalactites and massive popcorn” (p119).
The theory’s time had come and others were beginning to explore the claims made by
Jagnow (1977) and Egemeier (1973). Donald Davis, in 1973 and 1979, cited and refined
Egemeier’s and Jagnow’s proposals (Jagnow et al, 2000). He stated in his 1979 paper that
sulfuric acid was more important in speleogenesis than Jagnow has surmised, and the pyrite
source was not enough to produce the volumes of sulfuric acid needed to carve the caves
(Jagnow et al, 2000). Davis also published the first review of the sulfuric acid theory and
brought together the proposals of Jagnow, Egemeier, and Queen, Palmer and Palmer, who
proposed the brine mixing mechanism for the formation of gypsum blocks, to create a working
theory for speleogenesis (Jagnow et al, 2000). The theory incorporated brine mixing of fresh,
meteoric water with phreatic sulfide brine to create sulfuric acid at the air/water interface
(Jagnow et al, 2000). Carol Hill, in 1979, provided the first real evidence for the origin of sulfur
from hydrocarbons instead of the Castile Formation as proposed by Jagnow (1977) (Jagnow et
al, 2000). She had sampled the gypsum of the Big Room of Carlsbad Caverns, and found the
sulfur to be isotopically light (δ34S = -13.90/00), where the Castile Formation sulfur averaged
much heavier (δ34S = +10.30/00) (Jagnow et al, 2000).
The 1980’s brought further refinement of the sulfuric acid speleogenesis theory with the
posthumous 1985 publication of Eigemeier’s Theory for the Origin of Carlsbad Caverns and
Carol Hill’s 1987 Geology of Carlsbad Caverns and Other Caves of the Guadalupe Mountains,
New Mexico and Texas in which she drew the connection between cave origin and sulfur
deposits in the Delaware Basin (Jagnow et al, 2000). Jagnow et al (2000) claimed that Hill’s
publication by the New Mexico Bureau of Mines and Mineral Resources showed that the sulfuric
acid theory had entered the mainstream of speleogenetic thought. With the breakthrough in 1986
into the vast expanses of Lechuguilla Cave, pristine evidence was rapidly collected that
supported the sulfuric acid theory and the 1990’s were a time of theoretical verification and
refinement of the theory (Jagnow et al, 2000).
Evidence for the theory
Polyak and Provencio (2000b) assert that, unlike carbonic acid dissolution caves, sulfuric
acid caves leave by-products of the speleogenetic process that can be studied. One of these by-
products, alunite, can be dated, which is just what Polyak and Provencio undertook in their
2000b paper. Samples of alunite were taken from Guadalupe Mountain caves at various
elevations. Testing dated the speleogenesis of caves at higher elevations (Virgin and
Cottonwood) at 11 Ma and those at lower elevations (Lechugilla, Carlsbad, and Endless) at 6, 5,
and 4 Ma. These dates correspond to the previously determined aquifer water level patterns,
affirming the theory of sulfuric acid generation and cave formation at the water table in the
Guadalupe Mountains (Polyak and Provencio, 2000b). Polyak and Provincio (2000a) did not
confine themselves to alunite, but studied all
the known and suspected by-products of
sulfuric acid speleogenesis. They divided the
by-products into primary, those formed directly
from speleogenesis, and secondary, which are
altered primary by-products. Primary by-
products are found in cave locations, which are
protected from fresh water and consist of
gypsum, elemental sulfur, hydrated halloysite,
alunite, natroalunite, jaorsite, hydrobasaluminite,
quartz, todorokite, rancieite and amorphous silica
and aluminum sulfates (Polyak and Povincio,
2000a). Gypsum is the most prevalent of the
primary by-products, and can be seen in the large
Figure 4. Gypsum glacier in the Prickly Ice Cube Room of Lechuguilla Cave (Team B, 1991).
Figure 5. Researcher rappelling through a drip-pit and airflow corrsion shaft in a 10 m thick gypsum glacier, Glacier Bay, Lechuguilla Cave (Team B, 1991).
floor blocks in Carlsbad, Lechuguilla and other caves in the Guadalupe, though gypsum may also
be a secondary by-product. Palmer, Palmer and Davis (1991) and Davis (2000) described the
gypsum mounds in Lechuguilla Cave, which include crevasse features and the calving of blocks,
as resembling ice glaciers (Figure 4) that have been observed up to 10 m in thickness (Figure 5).
Primary gypsum is also found in the form of wall and ceiling rinds (Jagnow, 1977; Polyak and
Provincio, 2000a), and the occurrence of gypsum rinds and blocks were described in Egemeier
(1973) in both active and relict caves. Another primary by-product of sulfuric acid speleogenesis
is elemental sulfur (Figure 6), which was shown by Hill in 1987, along with gypsum from the
Big Room of Carlsbad, to be of hydrocarbon
origin, due to the light isotopic properties of
the sulfur (Polyak and Provencio, 2000a).
Montmorillonite altered by sulfuric acid
during speleogenesis transforms to hydrated
halloysite, blue or white waxy nodules like
those seen in pockets of the Green Clay
Room of Carlsbad Caverns (Polyak and
Provencio, 2000a). In the same pockets as hydrated halloysite, alunite and natroalunite can often
be found (Polyak and Provencio, 2000a). Among the secondary by-products are two varieties of
moonmilk formations, gypsum and aluminite moonmilks (Polyak and Provencio, 2000a).
In addition to the by-products of cave formation, there are distinctive characteristics of
the sulfuric acid caves, themselves. These caves often cut across stratigraphic layers, ignoring
the boundaries between layers that carbonic acid caves seem to honor (Palmer and Palmer,
2000). They typically have huge rooms (Figure 7) and passages, regularly surpassing 15 m in
Figure 6. Elemental sulfur in snowy white gypsum deposits near Ghost Town in Lechuguilla Cave (Thompson, 1991).
height and width (Hill,
2000). They have large
vertical passages and
have many passages
and rooms that end
abruptly with no
fissures for the entrance
or escape of flowing
water (Hill, 2000). The
relationship between
cave entrances and the land surface seem to be
random, and have no relationship with current or past
springs or recharge paths (Hill, 2000). The continued
study of Lechuguilla Cave in the Guadalupe
Mountains has revealed unique features that may
result from sulfuric acid speleogenesis, according to
Davis (2000). Rillenkarren, deeply grooved floors
near acid pools, seem to be the result of rapid
evaporation and condensation of aggressive water.
Rillenkarren (Figure 8) commonly have been
observed on the surface of karst features, but their
subterranean appearance seems to be unique to
Figure 7. Western Borehole in Lechuguilla Cave (Team A, 1991).
Figure 8. Rillenkarren in Acide Lake Basin, East Rift, Lechuguilla Cave (Team B, 1991).
sulfuric acid caves, appearing
in Carlsbad, Kane and
Lechuguilla caves (Davis,
2000). Rimmed vents and
small hydromagnesite
bubbles, are seen in the
rooms of Lechuguilla Cave,
though the mechanisms for
their formation are not well
understood (Davis, 2000).
Unusual corrosion/deposition lines along the walls of some rooms signify long-term layering of
air (Davis, 2000). Associated with these lines are gypsum crusts, like the lines of “popcorn”
observed by the graduate students in 1971, and beautiful aragonite frostwork. Another feature
occurring only in sulfuric acid caves so far are “rusticles” (Figure 9), stalactites and columns
consisting of a core of bacterial colonies and a shell of black oxidized iron produced by the
bacteria (Davis, 2000). Peculiar subaqueous helictites (Figure 10) are formed only where small
trickles of water flow under substantial
amounts of gypsum, then dripping into pools
where the gypsum precipitates and forms
tiny strings. All of the above features
described by Davis (2000) and others
(Palmer, Palmer and Davis, 1991), as well
as the beautiful gypsum chandeliers (Figure
Figure 9. Rusticles in Lechuguilla Cave (Team B, 1991).
Figure 10. Subaqueous helictites in Sugarland, Lechuguilla Cave (Team B, 1991).
11) in Lechuguilla Cave can be considered secondary by-products of sulfuric acid speleogenesis.
Egemeier (1973) noticed the sulfur smell
of Lower Kane Cave and witnessed the
formation of gypsum rinds and gypsum mounds
in an active sulfuric acid cave in Wyoming.
Similarly, Hose and Pisarowicz (1999) have
begun to study another active sulfuric acid cave,
Cueva de Villa Luz in Tabasco, Mexico.
Though the cave had been previously studied for
its biologic features it had never been explored
from a geomorphologic perspective (Hose and
Pisarowicz, 1999). Two cavers heard a rumor of
a sulfur cave in 1987 and decided to take a look
before their flight back to the US. Pisarowicz
had never seen features like the elemental sulfur,
massive amounts of gypsum and what are now known as snottites (Figure 12). Since his first
excursion to the Cueva de Villa Luz in 1987, Pisarowicz, in the company of other researchers,
has returned a number of times to test the acidity of snottites and water drops (pH 1), sample the
levels of H2S in the air which they found to be isotopically light, as Carol Hill did for Guadalupe
caves, and to create a detailed survey of the cave. Explorers reported a sulfur smell and a
slightly thermal nature to the cave. The sulfur smell became more prevalent toward the
unvented, back portion of the cave and, when wading in the milky-white stream that runs
through the center of the cave floor, researchers noted a mild burning sensation on their feet
Figure 11. Gypsum chandeliers, Chandelier Ballroom, Lechuguilla Cave (Thompson, 1991).
(Hose and Pisarowicz, 1999). The air of the cave is toxic: carbon monoxide levels of 45 ppm,
H2S levels at 152 ppm, and O2 at a low 9% have been measured during extreme events in the
cave, and cavers left feeling ill before protective gear was adopted (Hose and Pisarowicz, 1999).
Additionally, explorers reported dead and dying bats hanging from the walls and ceilings of the
cave, though bats seem to be in residence year-round (Hose and Pisarowicz, 1999). The snottites
observed most thickly in the rear of the cave are sulfur-loving microbes that provide the
foundation of a curious food web: midges feed on the microbes, bats and fish feed on the midges,
and the local indigenous people have a traditional fishing ceremony during which they once
obtained their winter food from the sulfur-laden
stream (Hose and Pisarowicz, 1999). These
observations, and those of Lower Kane Cave,
have contributed to the current understanding of
sulfuric acid speleogenesis.
Theory as it stands
Palmer and Palmer (2000) have
explained well the current theory of
speleogenesis by a sulfuric acid mechanism in
their paper Hydrochemical interpretation of cave
patterns in the Guadalupe Mountains, New
Mexico. Palmer and Palmer (2000) reported
that the caverns seem to have formed with
complete disregard to the rock layers in which they reside. The caverns’ openings are also very
Figure 12. Snottites in Cueva de Villa Luz (PBS, 2005)
large and level in places, even though the stratigraphy lies at an angle. Palmer and Palmer
(2000) thought that indicated some control other than the lithography of the area. The current
theory is that the level rooms were formed at prehistoric water table levels where H2S in solution
and in tiny gas bubbles ascended through fissures in the bedrock until it met highly oxygenated
water and formed sulfuric acid (H2SO4). The H2SO4, with a little help from high CO2
concentrations and perhaps some help from sulfur-oxidizing bacteria to maintain low pH, then
began to enlarge the caverns at a very rapid rate, as much as 23 m3 per year (p 98). Palmer and
Palmer (2000) examined how the two essential compounds (H2S and O2 rich water) might have
come to meet. Some of the caverns were apparently formed at the water line where O2 mixing
would be expected, but some of the caverns seem to have formed deep below the water line,
which suggests that an active flow was essential in the cavern formation. In addition to the
water-line mixing that took place to create the great, level caverns, Palmer and Palmer (2000)
asserted that phreatic water must have been aggressive at depths up to 200m. They believed that
meteoric water flowing though joints may have been subjected to tectonically-induced
hydrostatic pressures that pushed the water table higher episodically.
Palmer and Palmer (2000) integrated previous studies to support their explanation for the
mixing necessary for sulfuric acid speleogenesis. They turned to Carol Hill (1996) for a detailed
description of the stratigraphy and geology of the Guadalupe Mountains, Polyak and Provencio
(2000b) to provide the dating of speleogenesis of particular caves, and Egemeier (1973) and
others for observations of H2S associated with similar cave features. Also, Palmer and Palmer
(2000) described in detail the replacement of limestone and dolomite by gypsum and the
formation of the great gypsum blocks as in-situ replacement instead of falling speleotherm
accumulation speculated upon by Egemeier (1973). In addition to clarifying the chemical and
physical mechanisms for sulfuric acid speleogenesis, the Palmers acceded that not all is
understood and further research is necessary to better understand the process. They felt that
more complete mapping of the Guadalupe caves might aid correlation of water table-formed
rooms. It is now understood that the Guadalupe caves formed from a combination of phreatic
and vadose processes; water from above combined with H2S-rich water from below caused the
rapid dissolution and replacement of limestone and dolomite. Vadose flow removed much of the
resulting gypsum and created many of the speleotherms seen in the caves today. Palmer and
Palmer (2000) asserted that further distinctions between vadose and phreatic speleogenetic
mechanisms are still needed.
Conclusion
Now that the sulfuric acid speleogenesis theory has gained acceptance and the evidence
for cave development by the sulfuric acid mechanism is becoming well defined, more cave
systems are joining the speleogenetic ranks of the Guadalupe Mountains caves and those of the
Big Horn Mountains in Wyoming. Other caves known or suspected to have been formed
through sulfuric acid speleogenesis are Fiume-Vento Cave in Italy, La Cueva de Villa Luz,
Mexico (active), Las Brujas Cave in Argentina, Kugitangtou caves of Turkmenistan, Redwall
caves, Grand Canyon, Arizona, Movile Cave, Romania(active), and Mbobo Mkulu Cave, South
Africa (Jagnow et al, 2000). Researchers continue to explore sulfuric acid caves, mapping,
testing ad photographing their new discoveries and aiding the consolidation and development of
the theory.
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