Cave Minerals of San Salvador Island, Bahamas Bogdan P. Onac, Jonathan Sumrall, John E. Mylroie & Joe Kearns The University of South Florida Karst Studies Series 1
1 | The University of South Florida Karst Studies Series The University of South Florida Karst Studies Series Series Editors Available Online at the Karst Information Portal 1. Cave Minerals of San Salvador Island, Bahamas E. Spencer Fleury, Todd A. Chavez, H. Len Vacher www.karstportal.org
2 Cave Minerals of San Salvador Island, Bahamas | Cave Minerals of San Salvador Island, Bahamas Bogdan P. Onac University of South Florida/Babes-Bolyai University Jonathan Sumrall University of South Florida John E. Mylroie Mississippi State University Joe Kearns The Pennsylvania State University
3 | The University of South Florida Karst Studies Series Acknowledgements We thank the Gerace Research Centre on San Salvador Island, Bahamas, for financial and logistical support of our research; GRC Executive Directors Kenny Buchan, Vince Voegeli, and Tom Rothfus; and the Bahamian Government for permission to do research. The authors would like to thank their colleagues Jim Carew and Joan Mylroie. Will White was an invaluable colleague for the initial mineral project. We also thank Larry Davis, Doug Gamble, Silvia Onac, and students (Cathy Wade, Matt Reece, Sara Klimek, Tanya Beck, Tiffany Roberts, Glen Hunt, Mark Horwitz) who participated with us in the fieldwork on San Salvador over the years. Karst Logo Design Publication Design Sarah Beth Fratesi Eileen M. Thornton ISBN: 09776744-2-8
4 Cave Minerals of San Salvador Island, Bahamas | Table of Contents Geographic and geologic setting...................................... The karst of San Salvador Island...................................... Investigated caves....................................................... Speleothems and cave minerals....................................... Mineral description..................................................... Carbonates..................................................... Halogenides.................................................... Nitrates ......................................................... Sulfates.......................................................... Phosphates...................................................... References................................................................ 5 9 11 32 34 34 37 40 43 51 66
5 | The University of South Florida Karst Studies Series San Salvador Island, Bahamas: Geographic and Geologic Setting The Bahama Archipelago stretches over some 1000 km (NW-SE) in the Atlantic Ocean, from Little Bahama Bank off the coast of Florida to Great Inagua Island, just off the coast of Cuba ( Fig. 1 ). The northeastern Bahama Islands are isolated landmasses that project above sea level from two large carbonate platforms, Little Bahama Bank and Great Bahama Bank. To the southeast, beginning in the area of San Salvador Island ( Fig. 2a ), the Bahamas comprise small isolated platforms, many of which are capped by islands that make up a significant portion of the available platform area. The Bahamas represent carbonate banks that have, since Mesozoic time, existed as tectonically-stable and isostatically-subsiding platforms. Today, the surficial geology is entirely Quaternary limestone that has been modified by karst processes. The major control of Quaternary Bahamian geology has been glacioeustasy. Fig. 1: Map of the Bahamian Archipelago
6 Cave Minerals of San Salvador Island, Bahamas | The steep-sided platform nature of the Bahama Banks means that the banks have stood above sea level as subaerial platforms for about 85% of the Quaternary. During glacioeustatic highstands, however, the platform tops are partially inundated by marine water and the carbonate factory initiates. During the initial transgression of the sea across the platform, eolianites are produced as wave action and rising sea level constantly keep lagoon and beach sediments mobilized to produce transgressive-phase eolianites. Cockburn Town Member French Bay Member Hanna Bay Member North Point Member Owl's Hole Formatio n Undiff. Grotto Beach Fm. Undiff. Pleistocene Unlithified Holocene Geologic Unit s Fi g. 1: Geo l ogi c M ap of San Sal v ado r Is la nd Fig. 2a: Map of San Salvador Island, Bahamas
7 | The University of South Florida Karst Studies Series During the sea-level highstand, reefs grow up to wave base, lagoons become quiescent, eolian production declines and strand plains prograde into the lagoons. When sea level begins to fall, the declining wave base mobilizes lagoonal sediments, and regressivephase eolianites develop, commonly over-stepping reefs and subtidal deposits. When the platform is again emergent, epikarst development and pedogenic processes influenced by African dustfall dominate the landscape. The Quaternary geologic record of the Bahamas consists of carbonate depositional packages formed during sea-level highstands, separated by terra rossa paleosols formed during the longer sea-level lowstands. The stratigraphy of the Bahamas consists of the Holocene Rice Bay Formation ; a terra rossa paleosol; the trangressive-phase eolianite/subtidal/regressivephase eolianite carbonate package of the Grotto Beach Formation from the last interglacial (MIS 5e), a terra rossa paleosol, and eolianites and terra rossa paleosols from several earlier sea-level fluctuations designated as the Owls Hole Formation ( Fig. 2b ). Full treatments of Bahamian geology can be found in Geological Special Publication 300 (Curran and White, 1995), and in chapters 3 and 4 of Geology and Hydrogeology of Carbonate Islands (Vacher and Quinn, 1997). The book Bahamian Landscapes (Sealey, 2006) offers a broad overview of Bahamian geology targeted to the lay reader. Field guide to the geology and karst geomorphology of San Salvador Island (Mylroie and Carew, 2008) provides abundant, specific information about the geology and karst of San Salvador Island.
8 Cave Minerals of San Salvador Island, Bahamas | HOLOCENE HOLOCENE PLEISTOCENE HANNA BAY MEMBER NORTH POINT MEMBER COCKBURN TOWN MEMBER FRENCH BA Y MEMBER RICE BA Y FORMATION GROTTO BEACH FORMATION OWL'S HOLE FORMATION Fig. 2b: Stratigraphic Units for San Salvador Island, Bahamas
9 | The University of South Florida Karst Studies Series San Salvador Island occupies a small isolated bank within the Bahamian archipelago. All stratigraphic and karst features present must be explicable in terms of the islands small size and the young age of the limestone (<500,000 years). The karst features present are quite different from those found in older, diagenetically mature limestones of continental interiors. The rocks of San Salvador are considered eogenetic ; they have never been buried or undergone significant diagenesis. Subaerial caves fall into three main categories (Mylroie & Mylroie, 2007): Pit caves are vadose shafts that collect water from the epikarst and conduct it towards the water table. Ten samples were collected from such caves throughout the island, namely: Hog Cay, Double Shaft, Triple Shaft, and Owls Hole. Banana holes form from dissolution caused by vadose-phreatic mixing of water at the top of the freshwater lens, supported in part by organic accumulation and decay. Most of these cavity entrances are collapse dolines and have a diameter/depth ratio >1. Because most banana hole entrances are large, speleothems are usually absent, except for some crusts coating the walls. No samples have yet been collected from these open-to-the-sky chambers. The largest voids are which develop in the distal margin of the freshwater lens, under the flank of the enclosing landmass. In this environment, vadose-phreatic mixing at the top of the lens is superimposed on the marine-fresh water mixing zone at the base of the lens. Organic materials collect at both the top The Karst of San Salvador Island
10 Cave Minerals of San Salvador Island, Bahamas | and bottom of the lens where decay produces CO2 and potentially anoxic H2S-derived acidity; the lens margin again superimposes these environments. Finally, as the lens cross section decreases at the lens margin, flow velocities increase, bringing reactants in, and rapidly taking products out. These factors create an exceptional dissolutional setting. Although most flank margin caves are small, showing single or ramiform passages, others are far more complex, with typical spongework or branchwork patterns (Palmer, 2007). Most of the investigated samples were collected in such flank margin caves from the northern (Lighthouse, Reckley Hill Pond, Midget Horror, Garden Cave, Crescent Top, Pipe, and Major) and southern (Dance Hall, Alter, Dripping Rock, Chinese Fire Drill, and Blowhole) part of the island. On San Salvador one can also observe blue holes (flooded karst features of polygenetic origin). These subsurface voids are developed in carbonate banks and islands; are open to the earths surface; contain tidally-influenced waters of fresh, marine, or mixed chemistry; extend below sea level for a majority of their depth; and may provide access to submerged cave passages (Mylroie et al., 1995a). No samples for our mineralogical studies were collected from underwater passages.
11 | The University of South Florida Karst Studies Series Fig. 3: Reckley Hill Pond Water Cave 01 2m John Mylroi e Peter Vogel John Schweyen Digitized by Jonathan Sumral l Sample Locatio n Owl's Hole Formatio n Below, we briefly introduce all caves visited and sampled for this mineralogical study. The caves represent a sampling from across the island. While some pit caves are represented, flank margin caves predominate as they provide the best cave conditions for preservation minerals unique to the cave environment. Reckley Hill Pond Water Cave This is the only cave on San Salvador to which a successful dye trace has been done, from the nearby Reckley Hill Pond drain. The cave entrance is a 2 m high, globular phreatic chamber that has just been intersected by retreat of the hillside. Crawling from this chamber leads one back into the hill to a low (1 m high), wide (~10 m) chamber that has been modified by collapse and trends down to water ( Fig. 3 ). The water flows across the chamber, changing direction with the tidal cycle, and then connecting to Reckley Hill Pond. The water enters and leaves the chamber by low, impenetrable horizontal fractures. Investigated Caves
12 Cave Minerals of San Salvador Island, Bahamas | Reckley Hill Pond Maze Cave This cave, accessible via a small breakdown pile along the Reckley Hill Pond hillside, is tiny but complex, consisting of a series of interconnecting phreatic fissures and tubes within a small area, totaling less than 20 m of passage ( Fig. 4 ). Fig. 4: Reckley Hill Pond Maze Cave 0 1m J Mylroi e P. Vogel J Schweyen Sample Locatio n Owl's Hole Formatio n
13 | The University of South Florida Karst Studies Series Fig. 5: Midget Horror Cave Midget Horror Cave This cave entrance was dug open in the early 1990s from an initial 15 cm diameter hole in a low cliff. The cave contains only about 15 m of passage, but the constricted entrance has limited atmospheric exchange with the cave itself. The cave is one elongated chamber 1 to 1.5 m high, 3 to 4 m wide, and about 15 m long ( Fig. 5 ). meters 0 2 mag J.E. Mylroi e M. Ohms Digitized by J. Sumral l Sample Locatio n Owl's Hole Formatio n
14 Cave Minerals of San Salvador Island, Bahamas | Garden Cave Garden Cave is an example of a flank margin cave that has begun to collapse and segment as surficial erosion processes on the low hill containing the cave breach into the voids below. From the north to the south, the cave changes from a mostly intact chamber, though a region of collapses and skylights, to a region of total collapse with small cave remnants still surviving. The northern chamber is the most complete portion of the cave, a roomy, 1 to 2 m high chamber about 6 m wide and 12 m long ( Fig. 6 ). 0 4m mag J.E. Mylroi e J.G. Harri s Digitized by J. Sumral l Sample Locatio n Owl's Hole Formatio n Fig. 6: Garden Cave
15 | The University of South Florida Karst Studies Series Crescent Top Cave Crescent Top Cave was also dug open in the early 1990s, from a low (15 cm high) slit on the hillside to the human-sized dimensions of today. The cave, while not large, is significant as the main chamber has a shaft down to water, which fluctuates with the tidal cycle in nearby Crescent Pond. The small entrance and open water body in the chamber make the cave very humid, in stark contrast to nearby dry caves such as Garden Cave or Midget Horror Hole. To the right (west) of the entrance passage, a series of low crawls trend west and then south for a few meters. From the south end of the main chamber, a narrow passage leads east 3 m to a circular room ( Fig. 7 ). Fig. 7: Crescent Top Cave meters 0 mag 2 Sample Locatio n Owl's Hole Formatio n J.E. Mylroi e J.R. Mylroi e Digitized by J. Sumral l
16 Cave Minerals of San Salvador Island, Bahamas | Fig. 8: Pipe Cave Pipe Cave Pipe Cave was discovered the same day as Crescent Top Cave. It is a very small cave that trends from a 3 m deep hillside drop-in entrance, through small chambers, to a low crawl that again appears on the hillside 15 m east ( Fig. 8 ). That crawl entrance is impassible except for the smallest of people. mag 0 3 meters M. Ohms D. Chaky R. Lips Digitized by J. Sumrall Sample Location Owl's Hole Formation
17 | The University of South Florida Karst Studies Series Lighthouse Cave Lighthouse Cave is the largest cave on San Salvador ( Fig. 9 ). In addition to its size, it is unusual on the island and across the Bahamas for its extensive series of water passages within the high and low tide range. Because of the size of the cave and its geological and mineralogical significance, it is given a detailed description here (condensed from Mylroie and Carew, 2008). The large room at the base of the drop can be entered by climbing down the Main Entrance using the available ladder, or via free climbing using a hole adjacent to the main pit. From this vantage point, a number of significant observations can be made. Directly ahead, to the north, is Aeolian Chamber, the main room of the cave. The rock floor in the Main Entrance area is modified by vadose flow, and the ceiling in this region has numerous small phreatic pockets called bell holes. Down to the left (west), water is visible. The cave walls are white, but near the water they are stained dark brown. The top of the brown stain is the high tide mark. If the water level is near the high tide mark, some of the cave will not be easily accessible. Tidal range in the cave is nearly 1 meter, sometimes more during spring tides. The water is slightly hypersaline (38 ppt), and can be seen to enter and leave the cave by a choked passage at the southwest end of the cave. This spot is only a few tens of meters from an inland lake. The water flow of that lake, as with the inland lakes in general, is tidal. The implication of this is that the cave does not connect in any other direct (conduit) manner to the internal plumbing of the island, which is consistent with a flank margin mode of cave origin. Also consistent with the flank margin model is the lack of a natural entrance to the cave during its origin and growth. The three entrances are all later-forming vadose pit caves superimposed upon a series of sealed phreatic chambers.
18 Cave Minerals of San Salvador Island, Bahamas | entrance Aeolian Chamber Water Loop mag0 10 meters Mike Lace Athena Owen Jon Sumrall Will Waterstrat Digitized by J. Sumrall Sample Location Owl's Hole Formation entrance Fig. 9: Lighthouse Cave
19 | The University of South Florida Karst Studies Series Proceed right and upslope, around the east side of Aeolian Clamber. Climb to the top of the slope, where the terrain levels out. Ahead, continuing to the north, is the rest of Aeolian Clamber. The broad, unsupported span of the ceiling in such young, porous rock is impressive. The floor of the chamber has numerous holes in it that lead to a warren of smaller passages under the room. Note that the room does not contain much breakdown. The absence of breakdown is not unusual in Bahamian caves, although breakdown is common in related areas like Bermuda (Mylroie et al., 1995b). Most of the large loose blocks can be shown to have disarticulated in place as a result of dissolution. The wall rock of this region shows the eolian sedimentary structures, especially the steep, large-scale foreset beds. Downslope to the south-southeast is a crevice in the wall that leads to Hydrology Hall, a three-dimensional maze that leads to the south (back under the trail on the surface). It ends abruptly in a blank wall. A room to the side of Hydrology Hall leads into Bat Series, a roosting spot for the bat colony that lives in the cave. Visitation to this area is discouraged to preserve the biological and geological aspects of this part of Lighthouse Cave. Ten meters north of the crevice leading to Hydrology Hall is a large opening into Bug Passage. This passage leads straight east and down to water and a termination, but by swinging to the north a loop through a guano crawl can be made back to the north end of Aeolian Chamber. The roof of Aeolian Chamber contains many vertical holes that are 25 to 50 cm in diameter, and up to a meter or more in height. They are characterized by very straight walls with a dome on top. Because of their shape, which looks like the inside of a bell with the clapper removed, they have been called bell holes. A review of bell holes is available in Dogwiler (1998). The reason to single out these features for discussion is the wide variety of theories that have been proposed to explain them. Some feel they are vadose
20 Cave Minerals of San Salvador Island, Bahamas | in origin, a result of condensation corrosion (Tarhule-Lips and Ford, 1998), where water condenses on the cave roof, and uses atmospheric CO2 to drive dissolution. The water then falls to the floor, where it re-evaporates, minus its dissolved solute load, to condense on the cave roof again and repeat the process. The other vadose theory states that bats and their metabolic activities form the bell holes (Miller, 1990; Lundberg and McFarlane, in press). The rationale behind this argument is that in chambers where bats cannot reachfor example, dry chambers that are only accessible via water-filled passages, or chambers with entrances blocked by collapsebell holes are not seen. The implication is that without bats, there will be no bell holes. The other theories involve phreatic conditions, such as the establishment of vertical convection cells where water sinks after becoming denser by dissolving the ceiling and gaining a solute load. Unsaturated water would then move to continue the dissolution process (Dogwiler, 1998). At north end of Aeolian Chamber, the chamber wall bends to the northwest, and the room ends in a crevice that also heads northwest. Before heading off into the low crawls that empty into the Water Loop, look back across the top of Aeolian Chamber toward the Main Entrance, which is visible in the distance. This is the largest room in the largest cave on San Salvador, but even bigger chambers can be seen on other Bahamian islands (e.g. Salt Pond Cave, Long Island; Mylroie et al., 1991). The group now will work its way to the northwest down some sloping low crawlways, called The Slide, ending up in the water. The key to successfully reaching this spot is to stay high in the crawls, avoiding the lower-elevation alternatives.
21 | The University of South Florida Karst Studies Series This wet chamber marks the start of the Water Loop, a series of passages that will eventually lead back to the Main Entrance. Straight ahead to the northwest is an example of a tubular passage that abruptly ends. According to the flank margin model, the end of the tube indicates the position of the mixing of diffuse freshwater flow (coming into this dissolutional chamber from the ridge interior) with seawater intruding from the coast. When sea level fell at the end of MIS 5e, the cave passage was abandoned and dissolution stopped. From here, take the opening ahead and to the left, into a high, domed chamber. The way on is sharply to the left (south), through a low arch at water level (notice that the high tide mark reaches the ceiling of this arch), and requires getting partially wet. On the other side of the arch, the ceiling rises and there is a central tunnel heading south with a variety of maze passages on both sides. The bedrock floor is full of potholes, and the unwary can step in up to their neck or deeper. Persistent searching of the right (west) wall will reveal leads into low, mazy areas with no continuing passage except to the south, which will link up with the main passage. Persistent searching along the left (east) wall will eventually lead to discovery of a passage up into Aeolian Chamber. From this junction with Aeolian Chamber, routes lead left (north) back to the route into the Water Loop at The Slide, right (east) back to the Main Entrance, or straight ahead into a warren of small passages beneath Aeolian Chamber. By continuing straight through the Water Loop, some interesting domes can be seen overhead. These lead up and may connect with each other, but no substantial upper level is present. Some domes look entirely phreatic in nature, while others have some vadose modification. Eventually, after a short stretch of deep water, another low arch with its ceiling below the high tide mark is reached. On the other side, the Main Entrance can be seen up and to the left (southeast).
22 Cave Minerals of San Salvador Island, Bahamas | Between the exit from the Water Loop and the Main Entrance are a series of stalagmites, one of which was collected in 1980 and dated by U/Th methods. The results of that dating are presented in Mylroie and Carew (1988). The stalagmite in question had two growth episodes, one centered at 49,000 years ago and one centered at 37,000 years ago. Between these two growth episodes the stalagmite contains an overgrowth of the marine serpulid worm Filograna sp. Following the trend of the cave wall on the right (west), a low undercut in the wall leads west to a submerged passage that John Schweyen (in January 1987) connected a third entrance (DONT try to free dive this passage!). Continuing ahead in the water leads to a domed chamber as the main entrance passes from view over to the left. Ahead and to the left (southeast) the passage leads up to an alcove with a small opening through a thin partition and into a small set of passages called the Rollar [sic] Coaster. Immediately on the right is a window into the domed chamber, but continuing ahead leads into a nasty, bat-filled area and into a short lower level. No further way on has been found here. By continuing straight ahead from the domed chamber and up an incline, you can see daylight in the roof. From this spot, a short climb leads to the second entrance, Cactus Entrance. At the base of the climb, in the low and rubble-choked crawls, is the water entry/exit point for the cave. If the tide is either rising or falling, a significant flow of water can be observed here. It has not been possible to negotiate this passage for any distance. This water flow seems to be an artifact of current sea level and the lateral breaching of the cave to the saline lake.
23 | The University of South Florida Karst Studies Series Hog Cay Cave (John Winters Cave) Hog Cay Cave is a bit of an enigma. It appears to be a complex pit cave. It descends from near the top of the hill as a series of chambers and passages almost to sea level in the terminal room ( Fig. 10 ). The cave has apparently experienced some infilling events of surface soil and weathered rock, as lithified deposits of that fill are found in portions of the cave. An alternate explanation is that the fill deposits are much older infilled dissolution cavities intersected by the cave. The rooms and passages wind downward over each other almost 13 m, exhibiting both phreatic and vadose features, to a terminal chamber. Elsewhere on Hog Cay are other pit caves that have similar features, but on a smaller scale. Fig. 10: Hog Cay Cave 0 10 m(Cross Section) Sample Location Owl's Hole Formation L. Florea P.J. Moore K. Toepka
24 Cave Minerals of San Salvador Island, Bahamas | Fig. 11: Majors Cave Majors Cave Majors Cave is the second largest cave on San Salvador Island. It has a series of entrances along the west side of the hill containing the cave, the most northerly one is the best to use. The entrance opens up into the main chamber of the cave, which makes up the vast majority of the caves total size ( Fig. 11 ). The passages range from 3 to 1 m in height. The chamber is 20 m long, north to south, and 11 m wide, east to west. Along the east wall, slots in the floor lead down to water at numerous places. High on the northeast corner of the room, a low passage leads to a chamber. Extremely tight and difficult climbing and crawling lead upward into a pit cave with complex passages from which an exit can be made onto the hilltop. Such a connection between a complex pit cave and a flank margin cave is rare in the Bahamas. In the southeast corner of the cave are a series of small rooms that lead down to deep water, and a large entrance at the base of the hill. Main Entrance Pit Surface Pit Entrance Bat Nook 0 3m Sample Location Owl's Hole Formation T.J. Dogweiler A.S. Engel J.E. Myroie M.L. Porter Digitized by J. Sumrall
25 | The University of South Florida Karst Studies Series Fig. 12: Dance Hall Cave Dance Hall Cave Dance Hall Cave is located on the side of a hill, along the west edge of Stoudts Lake. The entrance is 9 m wide and opens directly into the main chamber of the cave, which is 12 m long east to west, and 6 m wide north to south ( Fig. 12 ). Two passages lead off the west side of the roomone for a short distance to a second entrance, the other for almost 20 m to the northwest as a 1 to 1.5 m circular tube. At its far end, the roof is occupied by a large bat colony. Two short side passages lead north and northeast from the main chamber. 0 3 m J. Mylroie P. Vogel Digitized by J. Sumrall Sample Location Owl's Hole Formation
26 Cave Minerals of San Salvador Island, Bahamas | Altar Cave Altar Cave is entered from a 1m high, 13 m wide entrance at the base of a hill. The passage trends due east for 17 m, then rises abruptly into the main chamber, which is 20 m long east to west, and 12 m wide north to south ( Fig. 13 ). A large flowstone mound at the east endthe altar for which the cave is nameddominates the chamber. A short side passage trends south and then east for a few meters in the south wall of the main chamber. The main chamber is up to 6 m high. The entrance passage contains large speleothems that disappear into the sand forming the cave floor, indicating partial infill of the original passage. Dripping Rock Cave Dripping Rock Cave is a wide, high chamber that has been about one-half removed by surficial erosion. The entrance is 24 m wide and 4 m high. The chamber narrows and lowers, and past some Fig. 13: Altar Cave 0 10 Meters L. Florea Digitized by J. Sumrall Sample Location Cockburn Town Member
27 | The University of South Florida Karst Studies Series Fig. 14: Dripping Rock Cave large, desiccated speleothems that disappear into the sandy floor, the cave continues 8 m as a low, wide crawlway before ending ( Fig. 14 ). The main chamber has a well dug 3 m down into the sand fill. The well, now dry and abandoned, dates from the plantation period of the early 1800s. 20 ft mag Sample Location Cockburn Town Member M. Cullison B. Gayle F. Moore D. Spain J.E. Mylroie J.R. Mylroie Digitized by J. Sumrall 0 Owls Hole Shaft Owls Hole is a pit cave, one of the largest in diameter and deepest on San Salvador ( Fig. 15 ). It has an overhung roof, with a large tree growing all the way up to and out of the entrance. Owls commonly roost on ledges in the pit. The pit is 10 m deep, but narrows a bit about 3 m above the floor. This narrowing corresponds to where the pit passes through a terra rossa paleosol, the only pit cave on San Salvador known to do so. This transition is between the Grotto Beach Formation and the underlying Owls Hole Formation
28 Cave Minerals of San Salvador Island, Bahamas | 10 mN S (Cross Section) J.E. Mylroie and others Sample Location Owl's Hole Formation Fig. 15: Owls Hole Shaft eolianites. Above the paleosol, small passages lead a few meters, but end abruptly. Below the paleosol, the pit widens, and the lower eolianite displays complex trace fossils of plants. Double Shaft Double Shaft consists of two small passages (~3-4 m in length each) that open on both sides of the entrance pit situated in the middle part of the cavity. Double Shaft is located in the northern part of the Triple Shaft ( Fig. 16 ).
29 | The University of South Florida Karst Studies Series Fig. 16: Triple Shaft Cave mag 0 5m Double Shaft J.E. Mylroie and Others Digitized by J. Sumrall Sample Location French Bay Member Triple Shaft Triple Shaft is a complex pit cave ( Fig. 16 ). It gathers water through a series of small tubes and passages just below the surface (in the epikarst), and these funnel water to the main pit complex. The main pit provides access to passages to the east and west. To the east, a climb down of about 7 m leads to a series of domes, some of which open to the surface. To the west, a climb down an enlarged foreset bed in the eolianite leads to a chamber. To the east, a dome room with numerous palm frond impressions in the eolianite walls can be entered. To the west, a small hole high up the wall leads up and out another entrance.
30 Cave Minerals of San Salvador Island, Bahamas | Chinese Fire Drill Cave Chinese Fire Drill Cave opens on to a low cliff at the seacoast ( Fig. 17 ). A collapse entrance is also present on the bench above the cliff. Segmented fragments of the cave exist in the cliff wall immediately to the west. The cave is best entered from the collapse entrance. To the north, two high (1 to 1.5 m), narrow (0.5 to 0.25 m) passages that appear similar to vadose canyons trend north and become too narrow after 3 or 4 m. To the southwest, the passage drops down, and forms a series of small globular phreatic rooms and passages that travel a few meters and open on the cliff face. It has been hypothesized that the cave displays the vadose canyon to phreatic tube transition, and hence, the ancient water table. Fig. 17: Chinese Fire Drill Cave M. Cullison B. Gayle F. Moore D. Spain J.E. Mylroie J.R. Mylroie M. Sumner Digitized by J. Sumrall 10 ft mag Sample Location Cockburn Town Member
31 | The University of South Florida Karst Studies Series Fig. 18: Blowhole Cave Blowhole Cave Blowhole Cave is a chamber 1 to 2 m wide, trending north about 5 m ( Fig. 18 ). It opens onto the bench above the same cliff containing Chinese Fire Drill Cave, with a second entrance low on that cliff. The bench entrance has a slope that is made up of paleosol that drapes down into the cave, indicating that the cave formed prior to the paleosol. This feature is the most significant aspect of the cave. From the main chamber, a low crawl can be followed east for a few meters. The cave can be almost entirely full of sand, or almost entirely empty of sand, depending on wave energies. 10 ft mag Sample Location Cockburn Town Member M. Cullison B. Gayle F. Moore D. Spain J.E. Mylroie J.R. Mylroie Digitized by J. Sumrall
32 Cave Minerals of San Salvador Island, Bahamas | Before taking off on our journey into the mineral kingdom of San Salvador caves, lets briefly cover the basics of speleothems (i.e., formation and composition). Speleothems are defined as secondary mineral deposits that come in a variety of morphologies (stalactites, stalagmites, flowstones, etc.; Hill & Forti, 1997). Remember that the term speleothem refers to the origin of a particular mineral (or mixture of minerals) in the cave, not its composition. Therefore, it would be incorrect to refer to calcite as being a speleothem; however, the term has the proper meaning when articulated as calcite stalactite or Deposition of a specific speleothem in the cave environment is largely controlled by the location within the cave (roof, wall, overhanging, etc) and the way the solutions enter the cave (e.g., dripping water will create stalactites and stalagmites, seeping water will generate crusts, helictites, and blisters; whereas, flowing water is responsible for draperies, rimstone dams, etc.). A number of physico-chemical processes are ultimately responsible for precipitating a variety of minerals in caves. Among the typical reactions involved, the following are very common: (a) dissolution/ precipitation; (b) acid/base reactions; and (c) microbiologicallymediated redox reactions (White, 1997; Onac, 2005). The most common speleothems are made of calcite (CaCO 3 ) because the vast majority of caves are formed in limestone, which is composed of almost pure calcium carbonate. In such an environment (typical for carbonate island such as San Salvador), meteoric water passes through the soil and epikarst zone and combines with CO 2 produced by soil respiration, generating carbonic acid, which dissolves bedrock. Upon entering the cave atmosphere, the percolating water becomes supersaturated with respect to calcium carbonate (due to degassing of CO 2 into the cave atmosphere) and thereby speleothem growth initiates. Speleothems and Cave Minerals
33 | The University of South Florida Karst Studies Series Both bedrock composition and especially water chemistry are the chief parameters that control the diversity of minerals precipitated within the cave environment. If only limestone and rainwater interact (i.e., homogeneous petrography and no allogenic water enter the cave system), one would expect to find speleothems that are made up of calcite and/or aragonite. The situation changes, however, if caves are developed in dolomitic rocks. The presence of magnesium in these rocks will be responsible for deposition of Mgrich carbonate minerals (e.g., dolomite, hydromagnesite, huntite, etc). Mineral diversity is further enhanced by the interaction between bat guano with either limestone bedrock or clay minerals. Occasionally, the presence of ore deposits or hydrothermal activity in the vicinity of a karst cave provides a unique set of conditions that allow the deposition of a suite of exotic minerals (Onac, 2005). Given the lack of lithological variety on San Salvador Island, we expected to find a narrow range of cave minerals (Onac et al., 2001). Our investigations, however, revealed the presence of 20 minerals. This surprising mineral diversity has an explanationthe presence in and around the caves of some particular ingredients such as bat guano, seawater, and sea spray. Another reason for successfully identifying this large number of minerals is that we focused our studies not only on typical speleothems, but mostly on a variety of weathered crusts and earthy aggregates collected from within fresh or desiccated guano piles. The mineralogical data in this booklet rely on 77 samples recovered from 17 flank margin caves and vadose pits. The identified minerals belong to the following chemical classes: carbonates, sulfates, phosphates, nitrates, and halogenides (Table 1). All specimens collected were examined by X-ray diffraction (XRD) using two different diffractometers: (1) Scintag Pad V (The Pennsylvania State
34 Cave Minerals of San Salvador Island, Bahamas | University) and a Rigaku Miniflex 2 (University of South Florida). At both locations, silicon (NBS 640b) was used as the internal standard. In addition to XRD, scanning electron microscope (SEM with energy-dispersive X-ray spectrometer attached (EDS) and Fourier-transformed infrared (FTIR) investigations helped us to better characterize the minerals presented below. Mineral description Carbonates This chemical group is only represented by its chief minerals, calcite and aragonite Nothing surprising, considering that calcite is the thermodynamically stable form of calcium carbonate one would expect to find under typical cave conditions (i.e., microclimate, carbon dioxide partial pressure, etc). The dissolution/precipitation reactions that take place in the CO 2 -H 2 O-CaCO 3 system are summarized below; the first step involves generation of carbonic acid (equation 1, see discussion on previous page). The second reaction consumes CO 2 and dissolves bedrock, while the third reaction precipitates calcium carbonate in form of various speleothems while liberating CO 2 in the cave atmosphere. CO 2 (g) + H 2 O (aq) H 2 CO 3 (carbonic acid) (1) CaCO 3 (limestone) + H 2 CO 3 (aq) dissolution Ca 2 + (aq) + 2HCO 3 (aq) (bicarbonate) (2) Ca 2+ (aq) + 2HCO 3 (aq) precipitation CaCO 3 (speleothem) + CO 2 (g) + H 2 O (aq) (3) Calcite CaCO 3 (trigonal) Calcite comprises over 85% of the speleothemic material collected and investigated from San Salvador caves. Calcite and gypsum are the only minerals that were positively identified in every single visited cave. The following types and subtypes of calcite speleothems are
35 | The University of South Florida Karst Studies Series presented in caves on the island: stalactites, soda straws, stalagmites, columns, domes, fringed draperies, flowstones, rimstone dams, and crusts ( Fig. 19 ). Identification of calcite relies on routine XRD investigations ( Fig. 20 ). Some of the crusts collected in Dance Hall Cave, Double Shaft, and Triple Shaft were X-rayed using very slow scanning of the spectrum and the resulted patterns indicate high-magnesium calcite (Mg-calcite) was the main phase in those samples. Fig. 19: Carbonate speleothems in Lighthouse Cave
36 Cave Minerals of San Salvador Island, Bahamas | The dominant mechanism for calcite precipitation in San Salvador caves is the one outlined above. In a limited number of caves, such as those with large entrances (e.g., Dripping Rock Cave) and/or those close to the sea where wind action may impact cave ventilation (e.g., Chinese Fire Drill, Blowhole), evaporation can cause supersaturation and precipitation. This process, however, is of minor importance. Aragonite CaCO 3 (rhombohedral) Aragonite was only identified in millimeter-size crusts from five cavities, all with exception of Lighthouse Cave, located in the southern part of the island. Aragonite is one of the metastable polymporphs (e.g., graphite and diamond are also polymorphs, i.e., have similar composition but its structure is different) of calcium Fig. 20: X-ray pattern of calcite
37 | The University of South Florida Karst Studies Series carbonate. Its presence in San Salvador caves, as well as anywhere else, raises some questions with respect to how it formed. There is a vast literature focusing on the calcite-aragonite problem (Hill & Forti, 1997 and references therein) from which a number of theories regarding precipitation of aragonite in the cave environment emerge. As for the precipitation of aragonite in San Salvador caves, two possible pathways are considered: either from high-magnesium rich solutions, or where carbon dioxide concentration in the cave atmosphere is high. The first process likely operates in Double Shaft, Triple Shaft, and Chinese Fire Drill, where Mg-calcite was already documented. Altar Cave has reduced air circulation and a fairly large bat colony in the main room where the aragonite crust was sampled. In such caves with limited airflow and biologic activity, carbon dioxide can build up and lower the supersaturation of seeping water to a level that can trigger aragonite crystallization. Another likely explanation for higher concentrations of CO 2 would be that Altar Cave did not open until the Holocene sea-level rise caused hillside wave erosion that breached the entrance (Florea et al., 2004). HALOGENIDES Although over 100 mineral species are listed in this class, only 11 of them were documented from salt caves, lava tubes, or at cave locations where halite beds exists in the overburden. Here we report the first halogenide from a cave in San Salvador. Halite NaCl (cubic) Halite was identified by XRD analysis ( Fig. 21 ) at three locations on the island. In Reckley Hill Pond Water Cave (see Fig. 3 ), a flank margin cave that opens in the eolianite escarpment (Owls Hole Formation) just southeast of the Gerace Research Center, halite has been found closely associated with gypsum in milky crusts (not thicker than 1.5 mm); it forms colorless to white sub-millimeter-
38 Cave Minerals of San Salvador Island, Bahamas | size hopper-like cubic crystals that always occur on the backside of the gypsum crusts. These crusts cover the cave floor just above the highest water level mark around some pools with brackish water of whose level is controlled by tidal fluctuations. Supratidal zones around these pools were fed by capillary concentration (solution moving upwards through the porous eolianites at times of high tides), and it is this action that led to the precipitation of halite underneath gypsum crusts. A strange looking sample (soft, sticky, and highly hydrated crust) was collected from the blind passage that opens in the right hand side of the Main Chamber in Altar Cave. The XRD spectrum of this sample shows few low intensity peaks that clearly document the presence of halite in the mineral phase. The moonmilk-like, yellowish-white material is mostly composed of calcite. How halite got into this deposit remains an open question. Fig. 21: X-ray pattern of halite
39 | The University of South Florida Karst Studies Series Fig. 22: Halite deposits in Blowhole The third occurrence of halite is in the Blowhole (see Fig. 18 ). The cave opens in the southwestern corner of the island in the cliffs overlooking Sandy Point. Blowhole has an entrance up on the cliff face and another one at the sea level well within the wave action zone. The entire cavity is illuminated by daylight. In Blowhole, white-greenish to deep green moonmilk-like deposits (patches of less than 10 cm in diameter) cover the ceiling or fill small dissolution pockets ( Fig. 22 ). Microscopic observation pointed out that subhedral skeletal crystals (stained green by algae) cover a thin granular gypsum crust. In contrast to the previous caves with halite, in Blowhole the mineral is unquestionable precipitating directly from the aerosols and from wave-splashed seawater inside the cave.
40 Cave Minerals of San Salvador Island, Bahamas | NITRATES Cave nitrates are highly soluble, hence, when found in caves are indicative for rather dry (low relative humidity) or/and well ventilated passages. The hygroscopic (absorb moisture from air) and deliquescent (dissolve in atmosphere moisture) properties of most nitrates indicate they occur as a kind of ghost minerals in caves because they can appear and disappear as the relative humidity seasonally drops or rises. It is not surprising to find them during the dry season (period of lower relative humidity) and not during the wet season (period of higher relative humidity). There are many theories that have been developed to account for the presence of nitrates in caves, but the issue is still far from resolved. The most likely source of nitrate is either highly-organic surface soil from which nitrate-rich seeping groundwater enters the caves, or bat guano deposits. In either case, there is a growing body of evidence that show the role of nitrifying bacteria in deposition of cave nitrates (Northup et al., 1997; Bottrell, 2003; Northup & Lavoie, 2003). Niter (saltpeter) KNO 3 (orthorhombic) Nitratine (soda-niter) NaNO 3 (trigonal) Both niter and nitratine were found as patchy efflorescences within rather dusty guano near the entrance to the big room in Altar Cave. The cave sediment at this location has about 15 to 20 cm in thickness. Niter forms minute, light brown rhombic prisms ( Fig. 23 ), while nitratine appears as colorless micron-size granular crystals as observed under SEM. Identification of these two nitrates relies on XRD and SEM-EDS (Onac et al., 2001). Of the two nitrates, niter is expected to exist even if the relative humidity in caves is slightly above 90%. In order for nitratine to precipitate, however, the relative humidity must drop below 80%
41 | The University of South Florida Karst Studies Series (Hill, 1981); such conditions were encountered in Altar Cave in December 2005. Decomposition of bat guano is considered the primary source of nitrate ions in this cave; whereas limestone and sea-spray mixing with percolating groundwater have supplied the necessary sodium and potassium for the two minerals. Darapskite Na 3 (SO 4 )(NO 3 )H 2 O (monoclinic) White-grayish, dull nodules were collected from the lower part of a desiccated bat guano lenses in Majors Cave (see Fig. 11 ). At this specific location the guano accumulated in a dissolution pocket formed on a limestone breakdown in the backside of the main chamber ( Fig. 24 ). The relative humidity was not measured at the time the samples were collected, but it is reasonable to assume the cave is rather well ventilated (at least during the dry season), given its multiple entrances. Even so, the cave temperature is ~22C. When sampling the white opaque nodules we first thought the XRD would show a phosphate, however, the lines in the X-ray pattern perfectly match the d-spacing values for darapskite ( Fig. 25 ). This mineral wasnt previously documented on the island (Onac et al., 2001). Fig. 23: SEM image of a niter crystal
42 Cave Minerals of San Salvador Island, Bahamas | The soil in shallow dissolution pits and basins can be quite thick, 50 to 100 cm or more, so a localized nitrate source might be available above the cave. Another possibility would be that darapskite precipitated from the reaction between sodium contained in eolianites from sea spray and the nitrate and sulfate supplied by bat guano. Additional studies are required in order to understand the stability field of darapskite in this particular cave and elsewhere. Fig. 24: Sampling location for darapskite in Majors Cave Fig. 25: X-ray pattern of Darapskite
43 | The University of South Florida Karst Studies Series SULFATES The most abundant class of cave minerals is that of sulfates. Over 65 species are known to occur under various cave settings resembling many of the carbonate speleothem type forms (especially crusts, flowers, crystals, and efflorescences). Overall, sulfates are only the second most important class (after the carbonates) because of the overwhelming presence of calcite and aragonite within caves worldwide (Hill & Forti, 1997, 2003; Onac, 2005). One should not be surprised in having such a large number of sulfates in caves. The SO 4 2ion is not only one of the most common ions in circulating groundwater, but it also posses a rather high reactivity. In addition to these characteristics, on continents a number of evaporate minerals (namely gypsum and anhydrite) form thick beds in the overlying limestone or in close vicinity. Some sulfates in caves are closely associated with the presence of bat guano, an important source for SO 4 2production (also true for San Salvador Island). On a lesser degree, basaltic rocks in which lava tubes form and fumarole activity occurs in the proximity of caves are the other sulfate supplier. The most common mechanisms that lead to sulfates deposition in caves are evaporation and replacement reactions. The latter assumes generation of sulfuric acid by oxidizing sulfide minerals (e.g., pyrite, marcasite) or less often hydrogen sulfide. Once sulfuric acid is produced it will react with the carbonate bedrock to primarily form gypsum (see the general reaction below), but other sulfates may well precipitate. CaCO 3 (limestone) + H + + SO 4 2+ 2H 2 O CaSO 4 H 2 O (gypsum) + HCO 3 Although it doesnt apply to San Salvador Island, sulfates may sometimes precipitate directly from saturated brines both subaerial
44 Cave Minerals of San Salvador Island, Bahamas | and subaqueous environments. The famous giant gypsum crystals from Naica Mine (Mexico) are the best example (Garcia-Ruiz et al., 2007). In island and coastal settings, sulfate contained in sea spray and aerosols may enter the bedrock with vadose water flow and be precipitated in the cave setting. Sulfate may also enter the cave as phreatic seawater flow. Gypsum CaSO 4 H 2 O (monoclinic) A large variety of gypsum speleothems (crystals, crusts, cave flowers, moonmilk, stalagmites, etc) have been described from caves around the world (Hill & Forti, 1997). In caves on San Salvador Island, gypsum is omnipresent. It only occurs, however, as non-spectacular crusts (found in all investigated cavities), powdery material (in the cave soil), coralloids (nodular, coral-like speleothem type of various sizes up to 4 cm in diameter), and blisters (only documented from Lighthouse Cave; Fig. 26 ). As seen with the naked eye, the color of the crusts ranges from transparent to milky white. When stained by guano, soil or airborne impurities, gypsum crusts are yellowish, light to dark brown, gray or even green ( Fig. 27 ). An ordinary binocular inspection of gypsum crusts reveals they are made up of fibrous or tabular, millimeter-size crystals. In Lighthouse Cave we observed small (1-2 mm) subhedral prismatic gypsum crystals protruding out from the gypsum crust. Evaporative processes are involved in the precipitation of most gypsum speleothems on the island. The source of the sulfate bearing solutions delivered in caves located near the shore or in those with large entrances is possibly aerosols (directly through the entrance or mixing with the percolating meteoric water) or pumped in by tidal fluctuation (Lighthouse, Reckley Hill Pond Water, Crescent Top, and Majors).
45 | The University of South Florida Karst Studies Series Fig. 26: Gypsum blisters in Lighthouse Cave Fig. 27: Brown-stained gypsum crusts in Lighthouse Cave
46 Cave Minerals of San Salvador Island, Bahamas | Let us now introduce the basics of sulfur isotope geochemistry for better understanding of how they can help distinguish between different sulfate sources in caves. Sulfur has four stable isotopes: 32 S (abundance = 95.02%), 33 S (0.75%), 34 S (4.21%), and 36 S (0.02%). Stable isotopic compositions are reported as ratios of 34 S/ 32 S ( 34 S) and expressed in per mil () relative to the standard Canyon Diablo Troilite (CDT). Variations in the 34 S values are caused by two kinds of processes: equilibrium fractionation during inorganic reactions and, more important, kinetic fractionation due to the reduction of sulfate to sulfide by anerobic bacteria (Sharp, 2007). Modern marine sulfate has a 34 S value of about +21 and gypsum (or any other sulfate) precipitated there from should have very close values (Seal et al., 2000). The 34 S values of gypsum crusts from Chinese Fire Drill and Lighthouse caves are around +21 +22 CDT, which tell us that seawater is the ultimate sulfate source from which gypsum precipitated in these caves (Bottrell et al., 1993). A completely different situation was reported by Bottrell et al. (1993) after investigating gypsum crusts from Reckley Hill Pond Water Cave. They found all 34 S values of gypsum were below -20 CDT, a typical isotopic signature for sulfur that has been fractionated by sulfate-reducing bacteria; this process is responsible for precipitation of biogenic pyrite within the carbonate bedrock. Oxidation of these pyrite grains caused generation of sulfuric acid, which then reacted with the eolianite bedrock to form thin gypsum crusts. Gypsums identification mainly relies on X-ray diffraction analysis because it always gives (even when a multi-mineral phase sample) a rather nice and clean spectrum ( Fig. 28 ). Celestine SrSO 4 (orthorhombic) Investigating the lower part of Owls Hole pit (below the paleosoil horizon) we noticed patches of gray to light-green wall crust patches
47 | The University of South Florida Karst Studies Series Fig. 28: X-ray pattern of gypsum (less than 5 cm in diameter) covering older and more extensive calcite and/or gypsum crusts. The XRD analyses (3 samples from different locations) indicate without doubt that the crusts consist of almost pure celestine ( Fig. 29 ). Furthermore, rhombic prisms are visible under SEM images ( Fig. 30 ). Blooming of algae, as sunlight penetrates all the way down through its large entrance shaft, may be responsible for the greenish stain on some of the crusts. Celestine is not a common sulfate in caves worldwide, unless particular conditions (thermal activity, etc.) for a given cave are met (Onac, 2005). One of the theories in explaining the origin of celestine in Owls Hole considers Sr originating from sea-spray aerosols. However, this is the only location where celestine was positively identified, while sea-spray aerosols are present all over the island. Therefore another source/mechanism should be invoked,
48 Cave Minerals of San Salvador Island, Bahamas | Fig. 29: X-ray pattern of celestine Fig. 30: SEM image of celestine crystals without ignoring the role of sea-spray. It is known that some marine carbonate sediments are enriched in Sr during diagenesis due to the transformation of Sr-bearing aragonite or biogenic calcite to lowSr calcite. A chemical analysis of the French Bay eolianite bedrock (collected above the paleosoil horizon in Owls Hole) indicates
49 | The University of South Florida Karst Studies Series Table 1: List of minerals identified in caves on San Salvador Island
50 Cave Minerals of San Salvador Island, Bahamas | Sr concentrations up to 379 ppm, thus celestine is apparently deposited from Sr-rich solutions flowing down the shaft. It might be that these solutions enrich in Sr as they pass through the dense, micritic paleosoil causing celestine to precipitate only on Owls Hole Formation bedrock below the lowest paleosoil horizon. We have to keep in mind that Owls Hole is the only known pit cave on San Salvador island where a lower paleosoil is breached. Cesanite Na 3 Ca 2 (SO 4 ) 3 (OH) (hexagonal) Calcite and gypsum crusts are common in Lighthouse Cave. Routine microscope observations on one of these crust samples deposited above the high-tide mark in one of the phreatic domes revealed two distinct layers (calcite precipitated directly on the wall and gypsum on top of it). Between those layers and predominantly inside small cavities located on the backside of corroded gypsum crust, we observed a third mineral phase that was further investigated by XRD and SEM (+EDS). Separating the third phase was time consuming; however, well worth the effort. The XRD spectrum of this phase revealed some peaks identified as cesanite ( Table 1 ), a mineral that was never mentioned in a cave environment, but only from a geothermal field in Italy (Cavarretta et al., 1981). To confirm the XRD identification was correct, we conducted an additional SEM-EDS investigation. The SEM analysis showed the presence of outward-radiating crystals (less than 50 m in size; Fig. 31 ) of which EDS spectrum consists of a series of peaks of the following elements: Na, Ca, S, O, Sr, and Cl. Based on this semi-quantitative analysis and the XRD, the presence of cesanite is confirmed in Lighthouse Cave. A single crystal X-ray investigation and proper quantitative chemical composition (electron microprobe) are planned when quality material becomes available.
51 | The University of South Florida Karst Studies Series Cesanite is isostructural (have the same structure, but not necessarily the same chemical composition and have a considerable tendency for ionic substitution) with apatite-(CaOH), a common phosphate mineral in caves; the three Ca 2+ replaced by three Na + and three PO 4 3replaced by three SO 4 2to maintain the charge balance within the structure. Since there is no information (that the author is aware of) on the stability of cesanite in low temperature environments, the only scenario envisaged for precipitation of this mineral would be an apatite-(CaOH) precursor. This phosphate is widespread within Lighthouse Cave and in conjunction with the presence of seawater that ensures an alkaline environment and supplies Na and SO 4 2one can advocate for the possible precipitation of cesanite in caves. If near-neutral drip or seeping water was involved, another double salt (i.e., eugsterite) would have likely been precipitated (Vergouwen, 1981). PHOSPHATES Phosphates are the second most important class of caves minerals, when considering the number of mineral species (over 50). This statement holds true only if a global perspective is taken into Fig. 31: SEM image of fibrous crystals of cesanite
52 Cave Minerals of San Salvador Island, Bahamas | account. Phosphate minerals in caves have two distinct features: 1) They never form spectacular speleothems that most people are accustom to seeing in caves, mainly occurring as crusts, nodules, lenses, and inconspicuous earthy masses and thus are ignored by most individuals; and 2) they control cave color tonalities (from yellow, ochre, shades of brown, all the way to black). On the other hand, cave scientists find a large variety among the numerous phosphate species because so many are found only if a unique set of conditions are met (Onac et al., 2005; Onac & Effenberger, 2007). Deposition of phosphates in caves is almost always associated with the presence of fresh or desiccated bat/bird guano or large accumulations of fossil skeletal bones. The largest guano deposits are concentrated in caves from low-latitude humid regions, therefore, the highest diversity of phosphate minerals occur there. When the phosphate (PO 4 3) ions are released from guano or bone deposits and combine with the calcium (Ca 2+ ) ions in carbonate rocks, a suite of calcium phosphate minerals will form (e.g., brushite, apatite-(CaOH), whitlockite, etc). If guano accumulates over or near aluminum-rich clay sediments, the Al-rich phosphate assemblage generated will normally include taranakite, crandallite, variscite, etc. The minerals association becomes more exotic if one of the following situations is encountered: ore-related minerals (present in carbonate rocks) interact with phosphate derived from leached guano to form hopeite, tarbuttite, and sampleite (among others) or combustion of guano occurs causing the formation of berlinite. In some caves a little luck is required to collect samples at the right spot and the right time in order to document the presence of ammonia (NH 4 ) rich phosphates. To accomplish such a task, samples must be collected underneath bat hibernation or maternity
53 | The University of South Florida Karst Studies Series locations. Even so, a short time frame cannot be exceeded because the very soluble nitrogen or ammonium is partially or entirely leached away and the NH 4 -rich phosphates will either transform to other mineral phases or simply disappear. Once ammonia is leached away, only the more common calcium phosphate brushite, ardealite, apatite-(CaOH), etc., remain. Given some particular settings existing within the San Salvador Island caves (mainly the interaction between bat guano and seawater and cave climate), phosphate minerals are more numerous than others from any given chemical class. Below, we characterize each identified phosphate mineral giving some insights on its origin and deposition/stability conditions. Mineral species are considered in alphabetical order. APATITE GROUP Apatite-(CaCl) Ca 5 (PO 4 ) 3 Cl (hexagonal) Apatite-(CaF) Ca 5 (PO 4 ) 3 F (hexagonal) Apatite-(CaOH) Ca 5 (PO 4 ) 3 OH (hexagonal) Following the current guidelines adopted and imposed by the Commission of New Minerals, Nomenclatures and Classification (CNMNC) of the International Mineralogical Association, a suffix nomenclature should be used where multiple chemical elements in parentheses indicate different structural positions such as in the apatite group: Ca 5 (PO 4 ) 3 (Cl,F,OH) (Back & Mandarino, 2008; Burke, 2008). Under the new nomenclature, chlorapatite becomes apatite-(CaCl); hydroxylapatite becomes apatite-(CaOH) and so forth. Carbonate-hydroxylapatite (earlier known as dahllite ), is a very common phosphate mineral that no longer has its original name. Of the three species, by far the most common in caves is apatite(CaOH). This holds true for San Salvador, where it was identified by routine XRD analyses in 11 cavities ( Fig. 32 ). At any of these
54 Cave Minerals of San Salvador Island, Bahamas | locations, apatite-(CaOH) develops crusts of various sizes on walls, fallen blocks, or on other speleothems ( Fig. 33 ). It also forms earthy aggregates in the cave soil close to the guano deposits. Apatite(CaOH) forms in all 11 caves as a result of reactions between phosphate-rich solutions (draining out from the guano masses) and carbonate rock. This mineral typically precipitates in slightly alkaline environments. Only Lighthouse Cave hosts all three apatite species. In Altar Cave, apatite-(CaF) has been traced by means of FTIR analysis in the composition of dark brown crusts covering eolianite breakdown blocks. An interesting situation occurs in this cave. Light-colored layers of apatite-(CaCl) alternates with thin (mm) horizons of dark brown apatite-(CaF) in thick crusts precipitated directly under a bell hole inhabited by bats. The crust covers the floor in a section where the cave is periodically flooded during high tides. Our interpretation of this unusual pattern is that apatite-(CaCl) mainly precipitates at times when seawater covers the cave floor (supply of Cl), whereas apatite-(CaF) would preferentially be precipitated during low tides. Archerite (K,NH 4 )H 2 PO 4 (tetragonal) Close to the northwestern entrance in Aeolianite Chamber (Lighthouse Cave), delicate gypsum blisters protrude from the dullwhite gypsum crusts located on the wall. Some of these blisters are stained light brown by urine and guano coming from small size bat colonies (10 to 15 individuals). The color drew our attention and prompted an investigation. As expected, an XRD analysis of the bulk sample confirmed the presence of gypsum, but along with it, the remaining reflections [3.73 (10), 2.913 (6), 1.955 (4), 5.1 (2), 3.013 (1), 1.982 (1)] conform closely to those of archerite, a rare mineral known from only a handful of locations around the world (Hill & Forti, 1997; Forti et al., 2004).
55 | The University of South Florida Karst Studies Series Fig. 32: X-ray spectrum of Apatite-( CaOH ) Fig. 33: Apatite-( CaOH ) crusts in Altar Cave
56 Cave Minerals of San Salvador Island, Bahamas | Unfortunately for now, given the small amount of sample available and the difficulty to separate a purer phase for a more conclusive XRD run, the discovery of new material has become absolutely necessary. If the presence of archerite will be fully confirmed, its deposition possibility relates to a warm (~23C or more) and highly evaporative microenvironment, under which potassium and phosphorus leaching out from fresh bat guano crystallizes on top of the gypsum crust. Chemical analyses on bat guano indicate concentrations of potassium of up to 5000 ppm (Fenolio et al., 2006). Ardealite Ca 2 (SO 4 )(HPO 4 )H 2 O (monoclinic) Ardealite occurs in Lighthouse Cave as white to yellowish crusts (few millimeters in thickness) lining wall shoulders along and above the high tide mark ( Fig. 34 ). Trace amounts of guano accumulated on these ledges helped us to explain the origin of ardealite. We believe the mineral forms in a two-step process: first, brushite is derived from decaying bat guano. Afterwards, when pH drops under 5.5 Fig. 34: Ardealite crusts
57 | The University of South Florida Karst Studies Series Fig. 35: X-ray pattern of biphosphammite and destabilizes the brushite in the presence of SO42(derived from seawater in the bedrock or directly from guano), favorable conditions for ardealite formation are met. Ardealite is a fairly common phosphate mineral in caves (Hill & Forti, 1997; Onac, 2005), but it is not known why it is missing from all but one cave on San Salvador (in spite of the fact that all chemical ingredients are available). Biphosphammite (NH 4 ,K)H 2 PO 4 (tetragonal) Biphosphammite, the ammonium analogue of archerite, was identified by XRD in Midget Horror Cave, closely associated with gypsum and apatite-(CaOH) ( Fig. 35 ). This is a very low and dry cave in which the floor is covered by a thick (up to 10 cm) loose sediment composed from a mixture of silt-size weathered bedrock and guano dust produced by coprophagic (fecal eating) insects. The material that contains biphosphammite is light buff in color and was collected from the downside part of some gypsum crusts extending from the cave wall down to the floor. Biphosphammite is the decomposition product of bat guano and urine (at the time the cave was inhabited by bats) in a relatively dry environment. All ingredients are supplied by bat guano.
58 Cave Minerals of San Salvador Island, Bahamas | Brushite CaHPO 4 H 2 O (monoclinic) Brushite has been found in only four caves on the island (Table 1). In every location it forms light brown thin wall crusts or is coating breakdown blocks covered by guano ( Fig. 36 ). Interestingly, when forming crusts, brushite is always coated over apatite-(CaOH). In Crescent Top and Lighthouse caves, brushite occurs as earthy material or efflorencences on the cave floor in close proximity to guano piles. The XRD spectra for brushite are identical for all investigated samples. Therefore, we only showed the one XRD from Reckley Hill Pond Water Cave ( Fig. 37 ). It has to be mentioned that brushite and gypsums XRD patterns are very similar (the two minerals are isostructural), but distinguishable. To confirm the presence of brushite, we further investigated our samples using the SEM/EDS technique. SEM microphotographs of brushite ( Fig. 38 ) show micron-size subhedral crystals, tabular on (010). EDS scans over these crystals identified Ca, P, and O but no S, confirming the presence of brushite over gypsum. Fig. 36: Breakdown blocks covered by guano and brushite crusts
59 | The University of South Florida Karst Studies Series Fig. 37: X-ray spectrum of brushite (sample from Reckley Hill Pond water Cave) Fig. 38: SEM microphotograph of brushite crystals The textural relationship between the two phosphate minerals in the investigated crusts suggests that brushite formed through
60 Cave Minerals of San Salvador Island, Bahamas | replacement of apatite-(CaOH). In contrast, brushite speleothems occurring on the floor likely originate from the direct reaction between phosphate solutions (derived from guano) and the dusty carbonate sediment deposited on the cave floor. Collinsite Ca 2 (Mg,Fe 2+ )(PO 4 ) 2 H 2 O (triclinic) Our previous mineralogical investigations reported the presence of collinsite in Altar Cave in the form of aggregates of fragile white crystals. These crystals were deposited at the surface of fossil guano around a dripping point (Onac et al., 2001). A second occurrence of this rare mineral on the island is Reckley Hill Pond Water Cave. Until then, collinsite was identified only from three other caves in South Africa, Romania, and Spain, respectively (Martini & Moen, 1997; Onac et al., 2002, 2005). In Altar Cave, collinsite is associated with apatite-(CaOH) and brushite in a soft, light brown material underlying a thin gypsum crust on the cave floor ( Fig. 39 ). Fig. 39: Floor sediment in Altar Cave
61 | The University of South Florida Karst Studies Series Fig. 40: Hopeite-rich phosphate crust in Lighthouse Cave High magnification binocular microscope investigations revealed that minute, transparent crystals of collinsite are disseminated within this earthy mass. As with all the other phosphates, collinsite derives its phosphorous from bat guano, whereas calcium and magnesium comes from bedrock. The iron for the mineral comes from the paleosol horizons above the cave. In both caves, precipitation of collinsite seems to be promoted by a rather damp environment. Hopeite Zn 3 (PO 4 ) 2 H 2 O (orthorhombic) The identification of hopeite is not only unexpected for San Salvador caves, but also is problematic when tracing the source of Zn. All previous reports indicate that hopeite is an ore-related mineral, and thus its presence in a cave environment indicates that either the rocks overlying the cave or those in its proximity contain significant concentrations of Zn. A dark brown crust (with some grayish staining) was collected in the Sitting Room (Lighthouse Cave, Fig. 40 ). The XRD spectrum
62 Cave Minerals of San Salvador Island, Bahamas | and the EDS microchemical test both indicate that the Majors phase is apatite-(CaF). In addition, some reflections are best fitted to those of hopeite ( Fig. 41 ). EDS scans along some irregular rhombic prisms revealed the presence of P, O, and Zn. The unanswered question: Where is the Zn coming from? Three working hypotheses are considered: (1) Zinc is leached from soil ; zinc is bound to the soil and normally does not dissolve in water. Root-induced changes, however, along with the presence of certain microorganisms can directly increase solubility and mobility of zinc in the rhizosphere (region of soil in the vicinity of plant roots in which the chemistry and microbiology is influenced by their growth, respiration, and nutrient exchange). Hence, Zn may enter the cave via percolating water; (2) Zinc is supplied by bat guano ; it is known that the digestive efficiency of some bats is up to 78%, meaning that they expel unabsorbed nutrients in guano and urine (Stalinski, 1994). Chemical analysis of bat guano show that Zn can exceed 400 ppm (Fenolio et al., 2006) and its concentration may vary according to the diet of different bat species; (3) Zinc is derived Fig. 41: X-ray pattern of hopeite
63 | The University of South Florida Karst Studies Series Fig. 42: X-ray spectrum of monetite (Lighthouse Cave) from human activities such as waste burning and pollution from domestic wastewater. Most likely, the first and the second source seem plausible for the source of Zn. Monetite CaHPO 4 (triclinic) Monetite was discovered along with whitlockite and gypsum in Lighthouse Cave and Dance Hall Cave, respectively ( Fig. 42 ). At both locations, monetite occurs as well-crystallized dark brown crusts, either overlying bedrock or underlying desiccated bat guano. In Lighthouse Cave, the crust composed of monetite is precipitated in the apex of a bell hole, near the southern entrance in the water loop ( Fig. 43 ). Monetite in Dance Hall Cave occurs along the gallery that gives access into the bat colony passage (see Fig. 12 ). The origin of monetite in these two caves is the most common one, i.e., dehydration of brushite (the final product of the reaction between fresh guano and bedrock) under dry, acidic conditions. In Lighthouse Cave, the presence of brushite has already been documented. In Dance Hall Cave, however, we only suspect the monetites precursor was brushite.
64 Cave Minerals of San Salvador Island, Bahamas | Fig. 43: Patchy crusts of monetite in Dance Hall Cave
65 | The University of South Florida Karst Studies Series Fig. 44: SEM image of whitlockite crystals Whitlockite Ca 9 Mg(PO 3 OH)(PO 4 ) 6 (trigonal) Whitlockite was first described on San Salvador from Altar Cave where it forms patches of brown microcrystalline coatings in the lower part of the cave wall (Onac et al., 2001). Micron size rhombohedral crystals of whitlockite are seen under SEM ( Fig. 44 ). During this study, whitlockite has also been found in two more caves: Lighthouse and Reckley Hill Pond Water caves ( Fig. 45 ). In each of these caves, whitlockite is associated with apatite-(CaOH) and gypsum. The XRD spectrum is shown in Fig. 46 This study suggests that the main initial physico-chemical mechanism for the observed occurrence of whitlockite is the dissolution of the CO 3 2and Mg z from other minerals (apatite-(CaOH), high-Mg calcite, etc.) and their re-precipitation from descending solutions lower in calcium concentration, but enriched in iron. Possible sources for Fe are the paleosol or bat guano. Whether or not this mechanism is the result of microbial-triggered reactions or that of chemical or physico-chemical reactions or both, remains to be ascertained.
66 Cave Minerals of San Salvador Island, Bahamas | Fig. 45: Whitlockite crusts (brown) at the entrance in Reckley Hill Pond Water Cave Fig. 46: X-ray pattern of whitlockite
67 | The University of South Florida Karst Studies Series References Back, M.E., Mandarino, J.A. 2008: Fleischers Glossary of mineral species 2008 The Mineralogical Record Inc., Tucson, 346 p. Bottrell, S. 2003: Microbial processes in caves In: Encyclopedia of caves and karst science (Gunn, J., ed.). Fitzroy Dearborn: New York, 505-506. Bottrell, S.H., Carew, J.L., Mylroie, J.E. 1993: Inorganic and bacteriogenic origin for sulfate crusts in flank margin cave, San Salvador Island, Bahamas. In Proc. of the 6th Symposium on the Geology of the Bahamas (White, B., ed.), Bahamian Field Station, San Salvador, pp. 17-21. Burke, E. 2008: Tiding up mineral names: an IMA-CNMNC scheme for suffixes, hyphens, and diacritical marks. Mineral. Rec. 39: 131-135. Cavarretta, G., Mottana, A., Tecce, F. 1981: Cesanite, Ca 2 Na 3 [(OH)(SO 4 ) 3 ], a sulphate isotypic with apatite from the Cesano geothermal field (Latium, Italy). Mineral. Mag. 44: 269-273. Curran, H.A., White, B. (eds.) 1995: Terrestrial and Shallow Marine Geology of the Bahamas and Bermuda. Geological Society of America Special Paper 300, 344 p. Dogwiler, T.J. 1998: Analysis of bell hole morphology and distribution: A tool for evaluating formational processes Master thesis, Mississippi State University, 106 p. Fenolio, D.B., Graening, G.O., Collier, B.A., Stout, J.F. 2006: Coprophagy in a caveadapted salamander; the importance of bat guano examined through nutritional and stable isotope analyses. Proc. Royal Society B, 273: 439-443. Florea, L.J., Mylroie, J.E., Price, A. 2004: Sedimentation and porosity enhancement in a breached flank margin cave. Carbonates and Evaporites 19: 82-92. Forti, P., Galli, E., Rossi, A., Pint, J., Pint, S. 2004: Ghar Al Hibashi lava tube: the richest site in Saudi Arabia for cave minerals. Acta carsologica 33(2): 189-205. Garca-Ruiz, J.M., Villasuso, R., Ayora, C., Canals, A., Otlora, F. 2007: Formation of natural gypsum megacrystals in Naica, Mexico. Geology 35(4): 327-330. Hill, C.A. 1981: Mineralogy of cave nitrates. NSS Bull ., 43: 127-132. Hill, C.A., P. Forti 1997: Cave minerals of the world (2 nd ed.). National Speleological Society, Huntsville, Alabama. Martini, J.E.J., Moen, H.F.G. 1998: Blue Lagoon, Afrique du Sud. Une grotte remplissage palokarstique permien et concretions daragonite. Karstologia 32(2): 27-38. Miller, T.E. 1990: Bellholes: Biogenic (bat) erosion features in tropical caves: Geo 2 Occasional Paper No. 4, 17(2-3): 76. Mylroie, J.E., Carew, J.L. 1995: Karst development on carbonate islands. In Unconformities and Porosity in Carbonate Strata (Budd, D.A., Harris, P.M., Saller, A., eds.). American Association of Petroleum Geologists Memoir 63: 55-76. Mylroie, J.E., Carew, J.L. 1988: Solution conduits as indicators of Late Quaternary sea level position. Quaternary Science Reviews 7: 55-64. Mylroie, J.E., Carew, J.L. 2008: Field guide to the geology and karst geomorphologyof San Salvador Island Bahamian Field Station, 88 p. Mylroie, J.R., Mylroie, J.E. 2007: Development of the carbonate island karst model. Journal of Cave and Karst Studies 69(1): 59. Mylroie, J.E., Carew, J.L., Vacher, H.L. 1995a: Karst development in the Bahamas and Bermuda. In Terrestrial and Shallow Marine Geology of the Bahamas and Bermuda (Curran, H.A., White, B., eds.) Geological Society of America Special Paper 300: 251-267. Mylroie, J.E., Carew, J.L., Moore, A.I. 1995b: Blue holes: Definition and genesis: Carbonates and Evaporites 10(2): 225-233. Mylroie, J.E., Carew, J.L., Sealey, N.E., Mylroie, J.R. 1991: Cave development on New Providence Island and Long Island, Bahamas. Cave Science 18: 139-151.
68 Cave Minerals of San Salvador Island, Bahamas | Northup, D.E., Lavoie, K.H. 2003: Microorganisms in caves In: Encyclopedia of caves and karst science (Gunn, J., ed.). Fitzroy Dearborn: New York, 506-509. Northup, D.E., Reysenbach, A.-L., Pace, N.R. 1997: Microorganisms and speleothems In: Cave minerals of the world (2nd ed., Hill, C.A. & Forti, P., eds.). National Speleological Society, Huntsville, Alabama, 261-266. Onac, B.P. 2005: Minerals. In: Encyclopedia of Caves (Culver, D & White, W.B., eds.), Academic Press, New York, p. 371-378. Onac, B.P. Effenberger, H. 2007: Re-examination of berlinite from Cioclovina Cave, Romania. Amer. Mineral ., 92: 1998-2001. Onac, B.P., Mylroie, J.E., White, W.B. 2001: Mineralogy of cave deposits on San Salvador Island, Bahamas. Carbonates and Evaporites 16: 8-16. Onac, B.P., Breban, R., Kearns, J., T ma T. 2002: Unusual minerals related to phosphate deposits in Cioclovina Cave, ureanu Mts. (Romania). Theor. Appl. Karstol ., 15: 27-34. Onac, B. P., Ettinger, K., Kearns, J., Balasz, I. I. 2005: A modern, guano-related occurrence of foggite, CaAl(PO 4 )(OH) 2 H 2 O and churchite-(Y), YPO 4 H 2 O in Cioclovina Cave, Romania. Mineralogy and Petrology 85: 291-302. Palmer, A.N. 2007: Cave geology Cave Books, Dayton, 454 p. Seal, R.R., Alpers, C.N., Rye, R.O. 2000: Stable isotope systematics of sulfate minerals. In Sulfate minerals. Crystallography, geochemistry, and environmental significance. Rev. Mineral. Geochem ., 40: 541-602. Sealey, N.E. 2006: Bahamian landscapes. An introduction to the geology and geography of the Bahamas (3rd ed.). Macmillan Caribbean, 174 p. Stalinski, J. 1994 Digestion, defecation and food passage rate in the insectivorous bat Myotis myotis Acta Theriol ., 39: 1. Sharp, Z. 2007: Principles of stable isotope geochemistry Prentice Hall: New Jersey, 360 p. Tarhule-Lips R.F.A., Ford D.C. 1998: Condensation corrosion in caves on Cayman Brac and Isla de Mona. Journal of Caves and Karst Studies 60: 84-95. Vacher, H.L., Quinn, T.M. (eds.) 1997: Geology and hydrogeology of carbonate islands Elsevier Science Publishers, 948 p. Vergouwen, L. 1981: Eugsterite, a new salt mineral. Am. Mineral. 66: 632-636. White, W.B. 1997: Thermodynamic equilibrium, kinetics, activation barriers, and reaction mechanisms for chemical reactions in karst terrains. Environ. Geol. 30(1-2): 46-58.
69 | The University of South Florida Karst Studies Series
70 Cave Minerals of San Salvador Island, Bahamas | Bogdan P. Onac is Assistant Professor in the Department of Geology, University of South Florida. Over the last 20 years he has studied the mineralogy and crystallography of speleothems from Romania and other locations around the world, describing many minerals new to the cave environment. More recently, he has broadened his interests within karst, working on paleoclimate reconstructions based on cave deposits (speleothems and ice), stable isotopes of sulfates from caves affected by thermal waters, and sea-level changes recorded in speleothems from the littoral caves of Mallorca. Jonathan Sumrall received his bachelors degree from Mississippi State University in Geology. He continued his interest in caves and karst by attending graduate school at the University of South Florida for his masters degree in Geology. His research interests involve stable isotope geochemistry of cave speleothems, cave mineralogy, and island karst. He is currently working on karst and cave projects in Romania, San Salvador (Bahamas), and Mallorca (Spain). John E. Mylroie is Professor of Geology at Mississippi State University. For the last three decades he has been active in cave and karst research, particularly paleoclimate interpretations. He has worked on glaciated karst in New York and Norway, but for the last decade has been focused on island karst systems around the world, including the Bahamas, Bermuda, Puerto Rico, the Mariana Islands, Australia, New Zealand, and Croatia. He created the flank margin model of cave formation in fresh-water lenses, and helped develop the Carbonate Island Karst Model as a unifying explanation of carbonate island karst. Joe B. Kearns has been part of the research staff at the Materials Research Institute at The Pennsylvania State University for over 25 years. He does materials characterization using scanning electron microscope and X-ray diffraction techniques. He also has done research on mineralogy, karst hydrology, and geomorphology. He is an avid caver having caved all over the world. He holds a Bachelor of Science degree from The Pennsylvania State University. About the Authors
The Karst Information Portal (KIP) is a community of scientists, information specialists, and speleologists creating an open system for accessing karst-related information online. The KIP user community includes scientists, decision makers, educators and students, and cavers. Our goal is to leverage this high level of connectivity and interaction to advance karst, cave, and aquifer research far more quickly than has previously been possible. The Karst Information Portal is housed by the University of South Florida Libraries. More at: www.lib.usf.edu K A R ST INFORMATION PORTAL www.karstportal.org 09776744-2-8