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Coastal caves
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Coastal caves John E. Mylroie Mississippi State University, Starkville, MS, United States
Introduction Caves that form in coastal environments will be controlled by factors that separate them from caves that form in traditional inland settings. The first and most obvious factor will be the physical and chemical power of waves, freshwater (lakes) and saltwater (oceans) acting on coastal rocks. Second, and less obvious, is the fact that sea level can change, and with that change, the position of the coastline moves. Therefore, the position of cave development by coastal processes will also move. Sea level can change in a variety of ways, but there are two ways that are of particular importance to cave formation on coasts. First, sea level can change on a global scale, called eustatic sea-level change. The most common way for this change to happen is by changing the amount of ice on the continents. From 10,000 to 2,600,000 years ago, the Pleistocene Epoch, the earth underwent a series of ice advances called glaciations (the “Ice Ages” of the popular press) and a series of ice retreats called interglacials. As ice sheets grew, evaporated seawater falling as snow on land was trapped as ice, and sea-level dropped worldwide. When the ice melted as an interglacial occurred, sea level rose as the melt water flowed back into the ocean basins. Eustatic sea-level change of this type is called glacioeustatic sea-level change. Evidence indicates that the earth went through more than 15 of these glacial cycles, and hence sealevel changes, in the Pleistocene. Second, sea-level position can also shift as isolated events at specific locations because the land is either subsiding or being uplifted. Such sea-level change is called local sea-level change, as only that local area is affected, and is commonly caused by tectonic movements of the earth, or compaction and subsidence of sediments, as in river deltas. Caves found in coastal areas fall into two major categories, karst caves and pseudokarst caves. The latter can form in almost any rock material by a variety of mechanisms. The most common pseudokarst cave in a coastal environment is the sea cave, produced by wave action. Sea caves are usually single chambers or a small collection of chambers and fissures open to the sea. The precise scientific name is littoral cave, meaning a cave formed within the range of tides. Coastal weathering and erosive conditions commonly create coastal cliffs, which in turn allow production of other pseudokarst cave types, such as tafoni (subaerial weathering pockets), talus caves (boulder and block rubble deposits), and fissure caves (cliff failure fractures). While tafoni are only isolated pockets, talus and fissure caves can sometimes be extensive. However, talus and fissure caves are a direct result of coastal erosion, and as a result are in an environment of continued modification and destruction, which results in loss of these cave types through time. Sea caves, being in solid bedrock, persist longer.
Sea caves The most common of the coastal caves are sea caves, and they are found the world over. Sea caves are caves that form by wave erosion in coastal areas that contain exposed bedrock. They can develop in almost any type of bedrock, with wave energy utilizing fractures and other preexisting weaknesses in the rock to quarry out voids by mechanical action. The chemical action of saltwater can also exploit rock weakness. The compression of air caused by water flowing forcefully into cracks and fractures in the rock can break rock, including rock above sea level. In addition to exploiting weaknesses in the rock, waves also interact with themselves and the sea floor to create constructive interference patterns that can focus wave energy at specific sites along a uniform coast. As a result, seas caves can form in rock where no obvious weakness exists. Sea caves can vary from arches and small voids only a few meters across to very large chambers up to 100 m deep and wide (Fig. 1). The sea caves seen on coast lines today have formed rapidly, as sea level has only been at its present elevation for perhaps 3000–5000 years, following melting of the large continental glaciers at the end of the last glaciation. In areas such as Alaska and Norway, where the earth’s crust was depressed by large masses of ice during glaciation, the shoreline is now rising as the crust rebounds to a stable position following melting of the ice. In so doing, sea caves formed many thousands of years ago have now been carried high above modern sea level. Encyclopedia of Caves. https://doi.org/10.1016/B978-0-12-814124-3.00033-9 Copyright © 2019 Elsevier Inc. All rights reserved.
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FIG. 1 Left, looking out of a large sea cave on Eleuthera Island, Bahamas. Right, Sea arches and waves, Barbados.
The earth is currently in an interglacial, or between glaciations, so glacial ice is at a minimum and sea level is high. The last interglacial occurred 124,000–115,000 years ago, and during that time the ice melted back a bit more than present conditions, and sea level was about 6 m higher than it is today. On some rocky coasts, sea caves produced at that time are still visible, 6 m above the ocean, if more recent erosion has not obliterated them. Sea caves have had a long history of interaction with people, especially wherever sailors have used the ocean on rocky coastlines. Sea caves were particularly favored by smugglers to hide stolen goods, and to also hide the small, fast sailing ships that carried such cargo out of reach of the taxman. Pirates allegedly buried treasure in sea caves, but most sea caves are in an active erosional environment, and anything buried would not survive long. Pirates who chose old sea caves above modern sea level would have had better success. Some sea caves formed during a past sea level higher than today contain significant archeological and paleontological remains, as the coastal environment was a rich ecology and food source for hunters and gatherers. Sea caves are ubiquitous on the rocky coasts of the world. Fingal’s Cave in Scotland, the Blue Grotto of Capri in the Mediterranean, Sea Lion Cave on the coast of Oregon, and Arcadia Cave on the coast of Maine are well-known sea caves visited by tourists on a regular basis. Many organisms use sea caves as a refuge, particularly seals, sea lions, and other marine mammals, as well as birds, which roost in the ceiling ledges above the reach of waves. From the viewpoint of cave exploration, sea caves are not of major category, primarily because they are short in length. In areas where other types of caves are rare, such as the west coast of the United States, sea caves offer the best cave exploration option. Occasionally, sea caves can have spacious chambers and over 500 m of passages. Exploration of sea caves can be very dangerous for those not experienced in handling strong waves, tides, and currents.
Flank margin caves The coastal environment creates a very unusual type of cave when limestones are present. The interaction of freshwater and seawater produces a unique geochemical situation that allows caves to form by dissolution that are very different from both sea caves, made by mechanical wave action, and most limestone caves located in the interior of continents, which are usually underground stream conduits formed by freshwater dissolution, or deep-seated acids at depth. Freshwater is slightly less dense than seawater, because of the extra salt dissolved in seawater. Average fresh water has a density of 1.0 g/cm3, average seawater has a density of 1.025 g/cm3. The difference in density is only one part in 40, but it is sufficient that when freshwater flows toward the ocean inside an aquifer, it floats on top of the seawater that has invaded the aquifer from the ocean. The boundary between the freshwater and saltwater is called the halocline (“halo” meaning salt, “cline” meaning boundary) if it is a sharp boundary. If the boundary is broad, containing water of brackish salinity, it is called a mixing zone. The freshwater flows toward the ocean because rainfall infiltrates the land behind the coast, piling up in the aquifer until there is sufficient slope to drive the water toward the ocean. Where the freshwater is piled up inland, because of buoyancy, the halocline sinks downward into the seawater, much like a piece of wood floating in water. Because the difference in density is one part in 40, for each centimeter, (or each meter) that the freshwater piles up above sea level, it sinks 40 cm (or 40 m) into the seawater. As the freshwater flows down this small slope toward the ocean, its elevation above sea level decreases, and in buoyant response, the halocline rises up toward sea level, a 40 cm rise for each 1 cm of elevation loss of the water table. At the coast, the freshwater discharges to the sea as a thin sheet. This configuration of fresh water over seawater is called the freshwater lens, because seen in cross section in an island (where the water discharges to coasts on either side), the freshwater body is seen to
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FIG. 2 Cartoon diagram of a freshwater lens in a limestone island, showing flank margin cave development in the lens margin, under the flank of the enclosing landmass. The lens is drawn with vertical exaggeration.
have the shape of a lens, similar to a lens in a magnifying glass. Fig. 2 shows this relationship in a diagrammatic fashion, with vertical exaggeration. Understanding the freshwater lens is critical to successful exploitation of fresh ground water in islands and coastal areas. When the freshwater lens is formed within a limestone aquifer in a coastal region, a very unique type of cave, called a flank margin cave, can develop. Limestones are made up of CaCO3, either as the mineral calcite, or its close polymorph (i.e., alternate shape), aragonite. Seawater is usually saturated with CaCO3, and cannot dissolve limestone very well. Freshwater that has had a long residence time in a limestone aquifer is also commonly saturated with CaCO3 and also cannot dissolve more limestone. However, because the freshwater and seawater became saturated with CaCO3 under different initial conditions, when they mix they are capable of doing more dissolution, a process called mixing corrosion or mixing dissolution. Where the freshwater lens meets the seawater at the halocline, it is possible to dissolve out large voids, or caves that otherwise would not be able to form. The top of the freshwater lens is also a place where waters can mix. In this case, the freshwater at and below the water table (in this case, the top of the lens) is called phreatic water, and the water descending from the ground surface above is called vadose water. It is common for both the phreatic and vadose waters to be saturated with respect to CaCO3, but as with the case of mixing seawater and freshwater, the phreatic and vadose waters saturated at different initial conditions, so that when they mix, the water can dissolve more CaCO3. Therefore, both the top and bottom of the freshwater lens is a favorable environment for the dissolution of CaCO3. The top of the lens (the water table), and the bottom of the lens (the halocline), represent density interfaces. Organic particulate material transported by the vadose water flow from the land surface commonly floats on the top of the water table. Some of this organic material may then become water logged, and work its way to the bottom of the lens where it floats on denser seawater at the halocline. The decay of the organic matter at these interfaces creates CO2, which dissolves in the water to make carbonic acid that promotes CaCO3 dissolution. In both situations, if the amount of organic material becomes too much, its decay will use up the local oxygen supply to create anoxic conditions. If the anoxic conditions persist, anaerobic bacteria will create H2S, which can later encounter water with oxygen in it to create H2SO4 or sulfuric acid, a very powerful acid which can dissolve even more CaCO3. The mixing, and the possible anoxic conditions, that promote dissolution can occur at the top and bottom of the lens throughout the lens area. However, these environments are superimposed on each other at the edge, or margin of the freshwater lens where the top of the lens slopes down to sea level, and the bottom on the lens rises up to sea level. Because the lens is thin at this point, its flow velocity is high because the entire lens discharge is being forced through a thinning wedge. The combination of increased flow and superposition of the favorable geochemical environments for CaCO3 dissolution result in large voids forming very rapidly at the margin of the lens. As noted earlier, this discharge of the lens occurs at the margin of the lens, at the flank of the land, so the caves thus developed are called flank margin caves. In addition, in tropical islands and coasts, the limestone is commonly very young. Unlike the ancient limestones of the mid-continent regions, these young limestones have not been buried, squeezed, or greatly altered. The rocks still have a high degree of primary porosity, and water moves through them easily. Such rock is said to be eogenetic. True conduit flow is difficult to develop, as the rock has a high degree of permeability, meaning that the water has many flow routes to choose from. So how do caves form in this setting? Flank margin caves are not true conduits, like caves formed by sinking streams in continental interiors. They are instead mixing chambers. Conduit caves form by turbulent water flow, but flank margin caves develop in the laminar (nonturbulent) flow
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FIG. 3 Interior of flank margins caves, showing the complex dissolutional morphology. (A) Small pores, vugs, and pockets (ruler in lower right is 10 cm long), Cumulus Cave, Crooked Island, Bahamas. (B) Small pockets bordering a chamber in Cueva del Agua Sardinera, Isla de Mona, Puerto Rico. (C) Chamber with wall pockets and pores, Ten Bay Cave, Eleuthera, Bahamas. (D) Large chamber with pockets, arches and tubes, 1702 Cave, Crooked Island, Bahamas.
of a porous, eogenetic limestone aquifer. The freshwater enters the developing flank margin cave as diffuse flow and exits, after mixing with seawater, as diffuse flow. Flank margin caves form without human-sized entrances. As a result of their nonconduit origin, flank margin caves do not have long tunnels or a dendritic pattern found in most conduit caves. The flank margin caves are a series of oval rooms from pore size up to large chambers, which tend to be extensive in the horizontal direction, but limited in the vertical direction, a result of developing in the thinning margin of the freshwater lens (Fig. 3). The chambers can connect in a somewhat random manner, creating caves that are unpredictable in their pattern. Maze-like areas are common, indicating regions where chamber development did not go to completion. As the caves were growing, the mixing zone advanced into them, such that the back wall of the cave (the wall farthest from the ocean) is the youngest. Complex cross-connections between chambers can develop, and the caves can be quite complex despite their simple mode of development. The typical flank margin cave consists of one or more large chambers located just inside the edge of the island. The cave may trend parallel to the coast for some distance, but rarely penetrates very far inland, as its development site was restricted to the margin of the lens (Fig. 4). The caves form entrances when the erosion of the hillside which contains them breaches into the underlying cave. Initially, this entrance may be a small opening, but through time it can enlarge as more of the outer wall of the cave erodes away. Throughout the Bahamas and other carbonate island areas, flank margin caves in all states of erosional destruction, from almost intact to almost entirely removed, can be seen. On islands like Bermuda, where weathering and erosion of the limestone hills is more rapid than in the Bahamas, most flank margin caves have been entirely removed. Because they develop in the freshwater lens, flank margin caves are sensitive to sea-level change. If sea-level falls, the caves will be drained as the freshwater lens follows sea-level downwards, and cave development will cease. If sea level goes up, the lens will be pushed higher, and the caves will become flooded with pure sea water from below, and dissolution and cave enlargement will stop. The Bahama Islands are tectonically stable, meaning they are not rising or falling because of tectonic forces. The dry flank margin caves that explorers enter there today resulted from the freshwater lens being 6 m higher than today 124,000–115,000 years ago, during the last interglacial. That sea-level highstand lasted only about 9000 years, but caves with individual chambers up to 14,000 m3 in volume developed in that time frame, indicating how rapidly this mixing dissolution process can occur (Fig 3). On
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FIG. 4 Map of Sistema del Faro on Isla de Mona, Puerto Rico, a very large flank margin cave. Note how the cave wraps around the edge of the island, following the margin of a past freshwater lens. Cartography by Marc Ohms.
stable limestone coasts around the world are many flank margin caves that developed during the last interglacial sea-level highstand. Isla de Mona, halfway between Puerto Rico and the Dominican Republic, is a small island that has been uplifted by tectonics. It has huge flank margin caves that formed almost 2 million years ago (Fig. 4). These caves are very big because they developed in a freshwater lens before the Pleistocene glaciations began and so had a longer time to dissolve before sea-level fluctuation caused the freshwater lens to change position. When the initial glaciation began early in the Pleistocene, these caves were drained and speleothems such as stalactites and stalagmites developed. When a subsequent interglacial sea-level highstand occurred, sea level rose and the caves were reinvaded by the freshwater lens, partially dissolving the speleothems produced during the dry phase. The caves were then uplifted by tectonics well beyond any further glacioeustatic sea-level changes, and have been preserved for exploration today. In this case, local sea-level change affected only this one island. Some of the cave chambers are over 400,000 m3 in volume, much wider than they are high, but with many complex connections with adjacent chambers. The chambers have ancient speleothems much modified by attack from an invading freshwater lens, and more modern speleothems that have grown since the last uplift event, which are in pristine condition. The longest flank margin cave in the world, Sistema del Faro, is located on Isla de Mona, and has over 19 km of survey on its map (Fig 4). Flank margin caves commonly have numerous entrances, and in tropical settings are warm and friendly. The pleasant conditions and many entrances make them easy to explore. When movies and TV dramas show pirates in caves, it seems they always have enough light, and they move easily through large open passages. Flank margin caves actually do look a bit like this show-biz characterization (Fig 5).
Blue holes Blue holes are names for large, deep, and water-filled pits that form on islands and lagoons in tropical waters. They are named blue holes because their great depth gives them a very dark blue color (Fig. 6). They commonly connect to cave systems at depth. The
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FIG. 5 Hamilton’s Cave, Long Island, Bahamas. Spacious, well lit chambers like this one are common in flank margin caves.
FIG. 6 Blue holes, so named for their deep blue color. Left, an inland blue hole, Andros Island, Bahamas. Right, an ocean hole, Dean’s Blue Hole, the world’s second deepest at 200+ m, Long Island, Bahamas.
name blue hole was first published in 1725, and later appeared on British Admiralty charts from the Bahama Islands in the 1840s. Blue holes became popularized in the early 1970s when cave divers began to make the first serious investigations of their depths. Since that time, blue holes have been the subject of a number of major scientific investigations, including the discovery in 1979 of a new class of Crustaceans. Blue holes are defined as: “subsurface voids that are developed in carbonate banks and islands; are open to the earth’s 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., 1995, p. 225). Blue holes can be additionally characterized as being found in two settings: ocean holes open directly into the present marine environment and contain marine water, usually with tidal flow; inland blue holes are isolated by present topography from marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally influenced water of a variety of chemistries from fresh to marine (Fig. 6). While blue holes are best known from the Bahama Islands, they are found in a wide variety of tropical coasts and islands. Their origin is tied to the coast, island and lagoon environments where they are found, which means that their development has been influenced by glacioeustatic sea-level changes of the Pleistocene. Blue holes commonly contain many stalactites and stalagmites that are now under water, indicating that the blue holes were drained by glacioeustatic sea-level lowstands, allowing the speleothems to form from drip water, then were flooded by return of sea level as the ice sheets melted on the continents at the end of the last glaciation. Some of these speleothems are more than 350,000 years old, indicating that the blue holes containing
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them are very old, and have undergone repetitive sea-level lowstands and highstands. Thousands of blue holes are believed to exist out on the open Bahama Banks, but infilled now by modern sediments (Larson and Mylroie, 2014). These infilled blue holes are now thought to be the origin of whitings, clouds of carbonate sediment that appear on the banks. Tidal forces make these infilled blue holes “burp” CaCO3 saturated water, which warms and degasses in the shallow bank water, precipitating aragonite. There are three main hypotheses of how blue holes form: (1) drowning of surface karst features such as pits and sinkholes by sea-level rise; (2) bank margin fracturing; and (3) collapse of deep-seated phreatic dissolution voids. As islands become larger, their perimeter increases linearly, but the area goes up by the square. As a result, diffuse flow becomes inefficient as more meteoric water is being forced through a relatively small perimeter. As a result, conduit flow develops. The islands of the Bahamas today, and 120,000 years ago, are too small to support conduits, so only flank margin caves have formed. But when sea level was lower, the entire Bahama Banks were exposed to make a huge meteoric catchment, and large conduits formed. It is these conduits that have collapsed in places to make blue holes. Blue holes come in a variety of morphologies, and may represent features of polygenetic (i.e., many origins) development, in that case a combination of the above hypotheses may be correct. Exploration of blue holes generally involves cave diving to great depths. Such exploration is at the leading edge of technology and stamina, requiring the use of mixed gases, long decompression stops, total darkness, the danger of silt-out (stirring up silt so that the way out cannot be seen), and tight passages. Cave diving in blue holes is extremely dangerous, and many cave divers and scientists have lost their lives trying to penetrate into the unknown. Unlike many other types of exploration, however, there is no substitute for direct human exploration. The blue holes and their associated caves, and the contents of those caves, cannot be viewed, measured, or sampled without someone going there.
Conclusion A complete review of coastal caves can be found in the book Coastal Karst Landforms (Lace and Mylroie, 2013). Coastal caves are important to science as they contain information about present and past sea-level conditions. Their utilization as a habitat makes them important for many organisms over the breadth of the Animal Kingdom. The speleothems contained within blue holes and flank margin caves contain within their layers evidence about the changes in earth climate over hundreds of thousands of years. Flank margin caves reveal past conditions of the freshwater lens, an essential component of island habitation. While flank margin caves are generally easy to explore, sea caves can be dangerous for the unwary traveler, and blue holes exceptionally dangerous even for the well trained.
Bibliography Bunnell, D., 1988. Sea Caves of Santa Cruz Island. McNally and Loftin, Santa Barbara, CA. Lace, M.J., Mylroie, J.E. (Eds.), 2013. Coastal Karst Landforms. Coastal Research Library. In: vol. 5. Springer, Dordrecht. https://doi.org/10.1007/978-94-0075016-6_1. Larson, E.B., Mylroie, J.E., 2014. A review of whiting formation in the Bahamas and new models. Carbon. Evaporites 29, 337–347. https://doi.org/10.1007/ s13146-014-0212-7. Mylroie, J.E., Mylroie, J.R., 2007. Development of the carbonate island karst model. J. Cave Karst Stud. 69, 59–75. Mylroie, J.E., Carew, J.L., Moore, A.I., 1995. Blue holes: definition and genesis. Carbon. Evaporites 10, 225–233. Palmer, R., 1997. Deep into Blue Holes. Media Publishing, Nassau, Bahamas. Vacher, H.L., 1988. Dupuit-Ghyben-Herzberg analysis of strip-island lenses. Geol. Soc. Am. Bull. 100, 580–591. Waterstrat, W.J., Mylroie, J.E., Owen, A.M., Mylroie, J.R., 2010. Coastal caves in Bahamian eolian calcarenites: differentiating between sea caves and flank margin caves using quantitative morphology. J. Cave Karst Stud. 72, 61–74.
Coastal caves John E. Mylroie Mississippi State University, Starkville, MS, United States
Introduction Caves that form in coastal environments will be controlled by factors that separate them from caves that form in traditional inland settings. The first and most obvious factor will be the physical and chemical power of waves, freshwater (lakes) and saltwater (oceans) acting on coastal rocks. Second, and less obvious, is the fact that sea level can change, and with that change, the position of the coastline moves. Therefore, the position of cave development by coastal processes will also move. Sea level can change in a variety of ways, but there are two ways that are of particular importance to cave formation on coasts. First, sea level can change on a global scale, called eustatic sea-level change. The most common way for this change to happen is by changing the amount of ice on the continents. From 10,000 to 2,600,000 years ago, the Pleistocene Epoch, the earth underwent a series of ice advances called glaciations (the “Ice Ages” of the popular press) and a series of ice retreats called interglacials. As ice sheets grew, evaporated seawater falling as snow on land was trapped as ice, and sea-level dropped worldwide. When the ice melted as an interglacial occurred, sea level rose as the melt water flowed back into the ocean basins. Eustatic sea-level change of this type is called glacioeustatic sea-level change. Evidence indicates that the earth went through more than 15 of these glacial cycles, and hence sealevel changes, in the Pleistocene. Second, sea-level position can also shift as isolated events at specific locations because the land is either subsiding or being uplifted. Such sea-level change is called local sea-level change, as only that local area is affected, and is commonly caused by tectonic movements of the earth, or compaction and subsidence of sediments, as in river deltas. Caves found in coastal areas fall into two major categories, karst caves and pseudokarst caves. The latter can form in almost any rock material by a variety of mechanisms. The most common pseudokarst cave in a coastal environment is the sea cave, produced by wave action. Sea caves are usually single chambers or a small collection of chambers and fissures open to the sea. The precise scientific name is littoral cave, meaning a cave formed within the range of tides. Coastal weathering and erosive conditions commonly create coastal cliffs, which in turn allow production of other pseudokarst cave types, such as tafoni (subaerial weathering pockets), talus caves (boulder and block rubble deposits), and fissure caves (cliff failure fractures). While tafoni are only isolated pockets, talus and fissure caves can sometimes be extensive. However, talus and fissure caves are a direct result of coastal erosion, and as a result are in an environment of continued modification and destruction, which results in loss of these cave types through time. Sea caves, being in solid bedrock, persist longer.
Sea caves The most common of the coastal caves are sea caves, and they are found the world over. Sea caves are caves that form by wave erosion in coastal areas that contain exposed bedrock. They can develop in almost any type of bedrock, with wave energy utilizing fractures and other preexisting weaknesses in the rock to quarry out voids by mechanical action. The chemical action of saltwater can also exploit rock weakness. The compression of air caused by water flowing forcefully into cracks and fractures in the rock can break rock, including rock above sea level. In addition to exploiting weaknesses in the rock, waves also interact with themselves and the sea floor to create constructive interference patterns that can focus wave energy at specific sites along a uniform coast. As a result, seas caves can form in rock where no obvious weakness exists. Sea caves can vary from arches and small voids only a few meters across to very large chambers up to 100 m deep and wide (Fig. 1). The sea caves seen on coast lines today have formed rapidly, as sea level has only been at its present elevation for perhaps 3000–5000 years, following melting of the large continental glaciers at the end of the last glaciation. In areas such as Alaska and Norway, where the earth’s crust was depressed by large masses of ice during glaciation, the shoreline is now rising as the crust rebounds to a stable position following melting of the ice. In so doing, sea caves formed many thousands of years ago have now been carried high above modern sea level. Encyclopedia of Caves. https://doi.org/10.1016/B978-0-12-814124-3.00033-9 Copyright © 2019 Elsevier Inc. All rights reserved.
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FIG. 1 Left, looking out of a large sea cave on Eleuthera Island, Bahamas. Right, Sea arches and waves, Barbados.
The earth is currently in an interglacial, or between glaciations, so glacial ice is at a minimum and sea level is high. The last interglacial occurred 124,000–115,000 years ago, and during that time the ice melted back a bit more than present conditions, and sea level was about 6 m higher than it is today. On some rocky coasts, sea caves produced at that time are still visible, 6 m above the ocean, if more recent erosion has not obliterated them. Sea caves have had a long history of interaction with people, especially wherever sailors have used the ocean on rocky coastlines. Sea caves were particularly favored by smugglers to hide stolen goods, and to also hide the small, fast sailing ships that carried such cargo out of reach of the taxman. Pirates allegedly buried treasure in sea caves, but most sea caves are in an active erosional environment, and anything buried would not survive long. Pirates who chose old sea caves above modern sea level would have had better success. Some sea caves formed during a past sea level higher than today contain significant archeological and paleontological remains, as the coastal environment was a rich ecology and food source for hunters and gatherers. Sea caves are ubiquitous on the rocky coasts of the world. Fingal’s Cave in Scotland, the Blue Grotto of Capri in the Mediterranean, Sea Lion Cave on the coast of Oregon, and Arcadia Cave on the coast of Maine are well-known sea caves visited by tourists on a regular basis. Many organisms use sea caves as a refuge, particularly seals, sea lions, and other marine mammals, as well as birds, which roost in the ceiling ledges above the reach of waves. From the viewpoint of cave exploration, sea caves are not of major category, primarily because they are short in length. In areas where other types of caves are rare, such as the west coast of the United States, sea caves offer the best cave exploration option. Occasionally, sea caves can have spacious chambers and over 500 m of passages. Exploration of sea caves can be very dangerous for those not experienced in handling strong waves, tides, and currents.
Flank margin caves The coastal environment creates a very unusual type of cave when limestones are present. The interaction of freshwater and seawater produces a unique geochemical situation that allows caves to form by dissolution that are very different from both sea caves, made by mechanical wave action, and most limestone caves located in the interior of continents, which are usually underground stream conduits formed by freshwater dissolution, or deep-seated acids at depth. Freshwater is slightly less dense than seawater, because of the extra salt dissolved in seawater. Average fresh water has a density of 1.0 g/cm3, average seawater has a density of 1.025 g/cm3. The difference in density is only one part in 40, but it is sufficient that when freshwater flows toward the ocean inside an aquifer, it floats on top of the seawater that has invaded the aquifer from the ocean. The boundary between the freshwater and saltwater is called the halocline (“halo” meaning salt, “cline” meaning boundary) if it is a sharp boundary. If the boundary is broad, containing water of brackish salinity, it is called a mixing zone. The freshwater flows toward the ocean because rainfall infiltrates the land behind the coast, piling up in the aquifer until there is sufficient slope to drive the water toward the ocean. Where the freshwater is piled up inland, because of buoyancy, the halocline sinks downward into the seawater, much like a piece of wood floating in water. Because the difference in density is one part in 40, for each centimeter, (or each meter) that the freshwater piles up above sea level, it sinks 40 cm (or 40 m) into the seawater. As the freshwater flows down this small slope toward the ocean, its elevation above sea level decreases, and in buoyant response, the halocline rises up toward sea level, a 40 cm rise for each 1 cm of elevation loss of the water table. At the coast, the freshwater discharges to the sea as a thin sheet. This configuration of fresh water over seawater is called the freshwater lens, because seen in cross section in an island (where the water discharges to coasts on either side), the freshwater body is seen to
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FIG. 2 Cartoon diagram of a freshwater lens in a limestone island, showing flank margin cave development in the lens margin, under the flank of the enclosing landmass. The lens is drawn with vertical exaggeration.
have the shape of a lens, similar to a lens in a magnifying glass. Fig. 2 shows this relationship in a diagrammatic fashion, with vertical exaggeration. Understanding the freshwater lens is critical to successful exploitation of fresh ground water in islands and coastal areas. When the freshwater lens is formed within a limestone aquifer in a coastal region, a very unique type of cave, called a flank margin cave, can develop. Limestones are made up of CaCO3, either as the mineral calcite, or its close polymorph (i.e., alternate shape), aragonite. Seawater is usually saturated with CaCO3, and cannot dissolve limestone very well. Freshwater that has had a long residence time in a limestone aquifer is also commonly saturated with CaCO3 and also cannot dissolve more limestone. However, because the freshwater and seawater became saturated with CaCO3 under different initial conditions, when they mix they are capable of doing more dissolution, a process called mixing corrosion or mixing dissolution. Where the freshwater lens meets the seawater at the halocline, it is possible to dissolve out large voids, or caves that otherwise would not be able to form. The top of the freshwater lens is also a place where waters can mix. In this case, the freshwater at and below the water table (in this case, the top of the lens) is called phreatic water, and the water descending from the ground surface above is called vadose water. It is common for both the phreatic and vadose waters to be saturated with respect to CaCO3, but as with the case of mixing seawater and freshwater, the phreatic and vadose waters saturated at different initial conditions, so that when they mix, the water can dissolve more CaCO3. Therefore, both the top and bottom of the freshwater lens is a favorable environment for the dissolution of CaCO3. The top of the lens (the water table), and the bottom of the lens (the halocline), represent density interfaces. Organic particulate material transported by the vadose water flow from the land surface commonly floats on the top of the water table. Some of this organic material may then become water logged, and work its way to the bottom of the lens where it floats on denser seawater at the halocline. The decay of the organic matter at these interfaces creates CO2, which dissolves in the water to make carbonic acid that promotes CaCO3 dissolution. In both situations, if the amount of organic material becomes too much, its decay will use up the local oxygen supply to create anoxic conditions. If the anoxic conditions persist, anaerobic bacteria will create H2S, which can later encounter water with oxygen in it to create H2SO4 or sulfuric acid, a very powerful acid which can dissolve even more CaCO3. The mixing, and the possible anoxic conditions, that promote dissolution can occur at the top and bottom of the lens throughout the lens area. However, these environments are superimposed on each other at the edge, or margin of the freshwater lens where the top of the lens slopes down to sea level, and the bottom on the lens rises up to sea level. Because the lens is thin at this point, its flow velocity is high because the entire lens discharge is being forced through a thinning wedge. The combination of increased flow and superposition of the favorable geochemical environments for CaCO3 dissolution result in large voids forming very rapidly at the margin of the lens. As noted earlier, this discharge of the lens occurs at the margin of the lens, at the flank of the land, so the caves thus developed are called flank margin caves. In addition, in tropical islands and coasts, the limestone is commonly very young. Unlike the ancient limestones of the mid-continent regions, these young limestones have not been buried, squeezed, or greatly altered. The rocks still have a high degree of primary porosity, and water moves through them easily. Such rock is said to be eogenetic. True conduit flow is difficult to develop, as the rock has a high degree of permeability, meaning that the water has many flow routes to choose from. So how do caves form in this setting? Flank margin caves are not true conduits, like caves formed by sinking streams in continental interiors. They are instead mixing chambers. Conduit caves form by turbulent water flow, but flank margin caves develop in the laminar (nonturbulent) flow
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FIG. 3 Interior of flank margins caves, showing the complex dissolutional morphology. (A) Small pores, vugs, and pockets (ruler in lower right is 10 cm long), Cumulus Cave, Crooked Island, Bahamas. (B) Small pockets bordering a chamber in Cueva del Agua Sardinera, Isla de Mona, Puerto Rico. (C) Chamber with wall pockets and pores, Ten Bay Cave, Eleuthera, Bahamas. (D) Large chamber with pockets, arches and tubes, 1702 Cave, Crooked Island, Bahamas.
of a porous, eogenetic limestone aquifer. The freshwater enters the developing flank margin cave as diffuse flow and exits, after mixing with seawater, as diffuse flow. Flank margin caves form without human-sized entrances. As a result of their nonconduit origin, flank margin caves do not have long tunnels or a dendritic pattern found in most conduit caves. The flank margin caves are a series of oval rooms from pore size up to large chambers, which tend to be extensive in the horizontal direction, but limited in the vertical direction, a result of developing in the thinning margin of the freshwater lens (Fig. 3). The chambers can connect in a somewhat random manner, creating caves that are unpredictable in their pattern. Maze-like areas are common, indicating regions where chamber development did not go to completion. As the caves were growing, the mixing zone advanced into them, such that the back wall of the cave (the wall farthest from the ocean) is the youngest. Complex cross-connections between chambers can develop, and the caves can be quite complex despite their simple mode of development. The typical flank margin cave consists of one or more large chambers located just inside the edge of the island. The cave may trend parallel to the coast for some distance, but rarely penetrates very far inland, as its development site was restricted to the margin of the lens (Fig. 4). The caves form entrances when the erosion of the hillside which contains them breaches into the underlying cave. Initially, this entrance may be a small opening, but through time it can enlarge as more of the outer wall of the cave erodes away. Throughout the Bahamas and other carbonate island areas, flank margin caves in all states of erosional destruction, from almost intact to almost entirely removed, can be seen. On islands like Bermuda, where weathering and erosion of the limestone hills is more rapid than in the Bahamas, most flank margin caves have been entirely removed. Because they develop in the freshwater lens, flank margin caves are sensitive to sea-level change. If sea-level falls, the caves will be drained as the freshwater lens follows sea-level downwards, and cave development will cease. If sea level goes up, the lens will be pushed higher, and the caves will become flooded with pure sea water from below, and dissolution and cave enlargement will stop. The Bahama Islands are tectonically stable, meaning they are not rising or falling because of tectonic forces. The dry flank margin caves that explorers enter there today resulted from the freshwater lens being 6 m higher than today 124,000–115,000 years ago, during the last interglacial. That sea-level highstand lasted only about 9000 years, but caves with individual chambers up to 14,000 m3 in volume developed in that time frame, indicating how rapidly this mixing dissolution process can occur (Fig 3). On
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FIG. 4 Map of Sistema del Faro on Isla de Mona, Puerto Rico, a very large flank margin cave. Note how the cave wraps around the edge of the island, following the margin of a past freshwater lens. Cartography by Marc Ohms.
stable limestone coasts around the world are many flank margin caves that developed during the last interglacial sea-level highstand. Isla de Mona, halfway between Puerto Rico and the Dominican Republic, is a small island that has been uplifted by tectonics. It has huge flank margin caves that formed almost 2 million years ago (Fig. 4). These caves are very big because they developed in a freshwater lens before the Pleistocene glaciations began and so had a longer time to dissolve before sea-level fluctuation caused the freshwater lens to change position. When the initial glaciation began early in the Pleistocene, these caves were drained and speleothems such as stalactites and stalagmites developed. When a subsequent interglacial sea-level highstand occurred, sea level rose and the caves were reinvaded by the freshwater lens, partially dissolving the speleothems produced during the dry phase. The caves were then uplifted by tectonics well beyond any further glacioeustatic sea-level changes, and have been preserved for exploration today. In this case, local sea-level change affected only this one island. Some of the cave chambers are over 400,000 m3 in volume, much wider than they are high, but with many complex connections with adjacent chambers. The chambers have ancient speleothems much modified by attack from an invading freshwater lens, and more modern speleothems that have grown since the last uplift event, which are in pristine condition. The longest flank margin cave in the world, Sistema del Faro, is located on Isla de Mona, and has over 19 km of survey on its map (Fig 4). Flank margin caves commonly have numerous entrances, and in tropical settings are warm and friendly. The pleasant conditions and many entrances make them easy to explore. When movies and TV dramas show pirates in caves, it seems they always have enough light, and they move easily through large open passages. Flank margin caves actually do look a bit like this show-biz characterization (Fig 5).
Blue holes Blue holes are names for large, deep, and water-filled pits that form on islands and lagoons in tropical waters. They are named blue holes because their great depth gives them a very dark blue color (Fig. 6). They commonly connect to cave systems at depth. The
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FIG. 5 Hamilton’s Cave, Long Island, Bahamas. Spacious, well lit chambers like this one are common in flank margin caves.
FIG. 6 Blue holes, so named for their deep blue color. Left, an inland blue hole, Andros Island, Bahamas. Right, an ocean hole, Dean’s Blue Hole, the world’s second deepest at 200+ m, Long Island, Bahamas.
name blue hole was first published in 1725, and later appeared on British Admiralty charts from the Bahama Islands in the 1840s. Blue holes became popularized in the early 1970s when cave divers began to make the first serious investigations of their depths. Since that time, blue holes have been the subject of a number of major scientific investigations, including the discovery in 1979 of a new class of Crustaceans. Blue holes are defined as: “subsurface voids that are developed in carbonate banks and islands; are open to the earth’s 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., 1995, p. 225). Blue holes can be additionally characterized as being found in two settings: ocean holes open directly into the present marine environment and contain marine water, usually with tidal flow; inland blue holes are isolated by present topography from marine conditions, and open directly onto the land surface or into an isolated pond or lake, and contain tidally influenced water of a variety of chemistries from fresh to marine (Fig. 6). While blue holes are best known from the Bahama Islands, they are found in a wide variety of tropical coasts and islands. Their origin is tied to the coast, island and lagoon environments where they are found, which means that their development has been influenced by glacioeustatic sea-level changes of the Pleistocene. Blue holes commonly contain many stalactites and stalagmites that are now under water, indicating that the blue holes were drained by glacioeustatic sea-level lowstands, allowing the speleothems to form from drip water, then were flooded by return of sea level as the ice sheets melted on the continents at the end of the last glaciation. Some of these speleothems are more than 350,000 years old, indicating that the blue holes containing
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them are very old, and have undergone repetitive sea-level lowstands and highstands. Thousands of blue holes are believed to exist out on the open Bahama Banks, but infilled now by modern sediments (Larson and Mylroie, 2014). These infilled blue holes are now thought to be the origin of whitings, clouds of carbonate sediment that appear on the banks. Tidal forces make these infilled blue holes “burp” CaCO3 saturated water, which warms and degasses in the shallow bank water, precipitating aragonite. There are three main hypotheses of how blue holes form: (1) drowning of surface karst features such as pits and sinkholes by sea-level rise; (2) bank margin fracturing; and (3) collapse of deep-seated phreatic dissolution voids. As islands become larger, their perimeter increases linearly, but the area goes up by the square. As a result, diffuse flow becomes inefficient as more meteoric water is being forced through a relatively small perimeter. As a result, conduit flow develops. The islands of the Bahamas today, and 120,000 years ago, are too small to support conduits, so only flank margin caves have formed. But when sea level was lower, the entire Bahama Banks were exposed to make a huge meteoric catchment, and large conduits formed. It is these conduits that have collapsed in places to make blue holes. Blue holes come in a variety of morphologies, and may represent features of polygenetic (i.e., many origins) development, in that case a combination of the above hypotheses may be correct. Exploration of blue holes generally involves cave diving to great depths. Such exploration is at the leading edge of technology and stamina, requiring the use of mixed gases, long decompression stops, total darkness, the danger of silt-out (stirring up silt so that the way out cannot be seen), and tight passages. Cave diving in blue holes is extremely dangerous, and many cave divers and scientists have lost their lives trying to penetrate into the unknown. Unlike many other types of exploration, however, there is no substitute for direct human exploration. The blue holes and their associated caves, and the contents of those caves, cannot be viewed, measured, or sampled without someone going there.
Conclusion A complete review of coastal caves can be found in the book Coastal Karst Landforms (Lace and Mylroie, 2013). Coastal caves are important to science as they contain information about present and past sea-level conditions. Their utilization as a habitat makes them important for many organisms over the breadth of the Animal Kingdom. The speleothems contained within blue holes and flank margin caves contain within their layers evidence about the changes in earth climate over hundreds of thousands of years. Flank margin caves reveal past conditions of the freshwater lens, an essential component of island habitation. While flank margin caves are generally easy to explore, sea caves can be dangerous for the unwary traveler, and blue holes exceptionally dangerous even for the well trained.
Bibliography Bunnell, D., 1988. Sea Caves of Santa Cruz Island. McNally and Loftin, Santa Barbara, CA. Lace, M.J., Mylroie, J.E. (Eds.), 2013. Coastal Karst Landforms. Coastal Research Library. In: vol. 5. Springer, Dordrecht. https://doi.org/10.1007/978-94-0075016-6_1. Larson, E.B., Mylroie, J.E., 2014. A review of whiting formation in the Bahamas and new models. Carbon. Evaporites 29, 337–347. https://doi.org/10.1007/ s13146-014-0212-7. Mylroie, J.E., Mylroie, J.R., 2007. Development of the carbonate island karst model. J. Cave Karst Stud. 69, 59–75. Mylroie, J.E., Carew, J.L., Moore, A.I., 1995. Blue holes: definition and genesis. Carbon. Evaporites 10, 225–233. Palmer, R., 1997. Deep into Blue Holes. Media Publishing, Nassau, Bahamas. Vacher, H.L., 1988. Dupuit-Ghyben-Herzberg analysis of strip-island lenses. Geol. Soc. Am. Bull. 100, 580–591. Waterstrat, W.J., Mylroie, J.E., Owen, A.M., Mylroie, J.R., 2010. Coastal caves in Bahamian eolian calcarenites: differentiating between sea caves and flank margin caves using quantitative morphology. J. Cave Karst Stud. 72, 61–74.