Guest Blog: How Speleothems Are Used To Determine Past Climates?

About author: Alex Graham is an undergraduate student at University of Newcastle, Australia. He is interested in Geology as a whole but his major interests include fluvial processes, karst systems and ocean science. During his visit to New Zealand, he has obeserved the glow worms in Waitomo Caves and spelunking in Nikau Caves.

Speleothems, more commonly known as stalactites or stalagmites, consist of calcium carbonate (calcite or aragonite) crystals of various dimensions, ranging from just a few micrometers to several centimetres in length, which generally have their growth axis perpendicular to the growth surface. Speleothems are formed through the deposition of calcium carbonate minerals in karst systems, providing archives of information on past climates, vegetation types and hydrology, particularly groundwater and precipitation. However, they can also provide information on anthropogenic impacts, landscape evolution, volcanism and tectonic evolution in mineral deposits formed in cave systems.

Stalagmite Formation

Rainfall containing carbonic acid weathers the rock unit (generally either limestone or dolomite) and seeps into the cracks, forming caverns and karst systems. The groundwater, percolating through such cracks and caverns, also contains dissolved calcium bicarbonate. The dripping action of these groundwater droplets is the driving force behind the deposition of speleothems in caves.

Core drilling of an active stalagmite in Hang Chuot cave.

Speleothems are mainly studied as paleoclimate indicators, providing clues to past precipitation, temperature and vegetation changes over the past »500,000 years. Radioisotopic dating of speleothems is the primary method used by researchers to find annual variations in temperature. Carbon isotopes (d^13C) reflect C3/C4 plant compositions and plant productivity, where increased plant productivity may indicate greater amounts of rainfall and carbon dioxide absorption. Thus, a larger carbon absorption can be reflective of a greater atmospheric concentration of greenhouse gases. On the other hand, oxygen isotopes (d^8O) provide researchers with past rainfall temperatures and quantified levels of precipitation, both of which are used to determine the nature of past climates.

Stalactite and stalagmite growth rates also indicate the climatic variations in rainfall over time, with this variation directly influencing the growth of ring formations on speleothems. Closed ring formations are indicative of little rainfall or even drought, where-as wider spaced ring formations indicate periods of heavy rainfall or flooding. These ring formations thus enable researchers to potentially predict and model the occurrence of future climatic patterns, based off the atmospheric signals extrapolated from speleothems. Researchers also use Uranium –Thorium radioisotopic dating, to determine the age of speleothems in karst formations. Once the layers have been accurately dated, researchers record the level of variance in groundwater levels over the lifetime of the karst formation. Hydrogeologists specialise in such areas of quantitative research. As a result, speleothems are widely regarded as a crucial geological feature that provide useful information for researchers studying past climates, vegetation types and hydrology.


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Fingal's Cave

What is Fingal's Cave?

Fingal's Cave is a sea cave on the uninhabited island of Staffa, in the Inner Hebrides of Scotland, known for its natural acoustics. The National Trust for Scotland owns the cave as part of a National Nature Reserve. It became known as Fingal's Cave after the eponymous hero of an epic poem by 18th-century Scots poet-historian James Macpherson.

Fingal was immortalised by Mendelssohn in his Hebrides Overture, after he visited the island in 1829. The island’s distinctive six-sided columns of rock are formed from basalt, the same as the Giant's Causeway in Northern Ireland. During spring and early summer, the cliffs and grassy slopes provide nesting sites for various seabirds including guillemots, razor-bills and puffins.

History

At 72 feet tall and 270 feet deep, what makes this sea cave so visually astoundingly is the hexagonal columns of basalt, shaped in neat six-sided pillars, that make up its interior walls.

The cave was a well-known wonder of the ancient Irish and Scottish Celtic people and was an important site in the legends. Known to the Celts as Uamh-Binn or “The Cave of Melody,” one Irish legend in particular explained the existence of the cave as well as that of the similar Giant’s Causeway in Ireland. As both are made of the same neat basalt columns, the legend holds that they were the end pieces of a bridge built by the Irish giant Fionn mac Cumhaill (a.k.a. Finn McCool), so he could make it to Scotland where he was to fight Benandonner, his gigantic rival.

The legend, which connects the two structures, is in effect geologically correct. Both the Giant’s Causeway and Fingal’s Cave were indeed created by the same ancient lava flow, which may have, at one time formed a “bridge” between the two sites. Of course, this happened some 60 million years ago, long before people would have been around to see it. Nonetheless, the deductive reasoning of the ancient peoples formed the connection and base of the legend that the two places must be related.

The cave was rediscovered when naturalist Sir Joseph Banks visited it in 1772. At the time of Banks’ discovery, Fingal, an Ancient Epic Poem in Six Books was a very popular poetic series, supposedly translated from an ancient Gaelic epic by Irish poet James Macpherson. The book was an influence on Goethe, Napoleon, and Banks, who promptly named the Scottish cave, which already had the name Uamh-Binn, after the Irish legend, calling it “Fingal’s Cave”.

And though Banks is responsible for both rediscovering and renaming the cave, it would be a romantic German composer who truly vaulted the cave to world fame.

So moved was famed composer Felix Mendelssohn by the splendour of the cave that he sent the opening phrase of an overture on a postcard to his sister with the note: “In order to make you understand how extraordinarily the Hebrides affected me, I send you the following, which came into my head there.” The Hebrides Overture, also known as Fingal’s Cave, premiered on May 14, 1832, in London. (The original name may have been based on the amazing noises the cave sometimes produces.)

In a one-two Romantic punch, artist J. M. W. Turner painted “Staffa, Fingal’s Cave” in the same year and together these launched the cave from a little-known wonder into a must-see Romantic-Victorian tourist site. William Wordsworth, John Keats, Lord Tennyson, and Queen Victoria all visited the cave as did consummate traveller and lover of wonders, Jules Verne.

After this, the cave never left the public imagination. Pink Floyd named one of their early, unreleased songs after the cave, and Matthew Barney used the cave in his Cremaster cycle.

One can visit the cave via cruise (though boats cannot enter the cave, they make regular passes by it so boat trips to Staffa cannot be made) or can travel to the small island of Staffa and hike into the cave by stepping from column to column.

Caves and Karst

Caves and Karst 

The Development of Caves 

In 1799, as legend has it, a hunter by the name of Houchins was tracking a bear through the woods of Kentucky when the bear suddenly disappeared on a hillslope. Baffled, Houchins plunged through the brambles trying to sight his prey. Suddenly he felt a draft of surprisingly cool air flowing down the slope from uphill. Now curious, Houchins climbed up the hill and found a dark portal into the hillslope beneath a ledge of rocks. Bear tracks were all around was the creature inside? He returned later with a lantern and cautiously stepped into the passageway. After walking a short distance, he found himself in a large, underground room. Houchins had discovered Mammoth Cave, an immense network of natural tunnels and subterranean chambers a walk through the entire network would extend for 630 km.

Most large cave networks develop in limestone bedrock because limestone dissolves relatively easily in corrosive groundwater. Generally, the corrosive component in groundwater is dilute carbonic acid (H2CO3), which forms when water absorbs carbon dioxide (CO2) from materials, such as soil, that it has passed through. When carbonic acid comes in contact with calcite (CaCO3) in limestone, it reacts to produce HCO31- and Ca2 ions, which then dissolve. 

In recent years, geologists have discovered that about 5% of limestone caves around the world form due to reactions with sulfuric-acid-bearing water Carlsbad Caverns in New Mexico serves as an example. Such caves form where limestone overlies strata containing oil, because microbes can convert the sulfur in the oil to hydrogen sulfide gas, which rises and reacts with oxygen to produce sulfuric acid, which in turn eats into limestone and reacts to produce gypsum and CO2 gas. 

Geologists debate about the depth at which limestone cave networks form. Some limestone dissolves above the water table, but it appears that most cave formation takes place in limestone that lies just below the water table, for in this interval the acidity of the groundwater remains high, the mixture of groundwater and newly added rainwater is not yet saturated with dissolved ions, and groundwater flow is fastest. The association between cave formation and the water table helps explain why openings in a cave network align along the same horizontal plane.

The Character of Cave Networks 

Development of karst, dripstone and flowstone.

As we have noted, caves in limestone usually occur as part of a network. Cave networks include rooms, or chambers, which are large, open spaces sometimes with cathedral-like ceilings, and tunnel-shaped or slot-shaped passages. Some chambers may host underground lakes, and some passageways may serve as conduits for underground streams. The shape of the cave network reflects variations in permeability and in the composition of the rock from which the caves formed. Larger open spaces developed where the limestone was most soluble and where groundwater flow was fastest. Thus, in a sequence of strata, caves develop preferentially in the more soluble limestone beds. Passages in cave networks typically follow pre-existing joints, for the joints provide secondary porosity along which groundwater can flow faster (figure above a). Because joints commonly occur in orthogonal systems (consisting of two sets of joints oriented at right angles to each other), passages may form a grid. 

Precipitation and the Formation  of Speleothems 

When the water table drops below the level of a cave, the cave becomes an open space filled with air. In places where downward percolating groundwater containing dissolved calcite emerges from the rock above the cave and drips from the ceiling, the surface of the cave gradually changes. As soon as this water re-enters the air, it evaporates a little and releases some of its dissolved carbon dioxide. As a result, calcium carbonate (limestone) precipitates out of the water and produces a type of travertine. The various intricately shaped formations that grow in caves by the accumulation of dripstone are called speleothems. 

Cave explorers (spelunkers) and geologists have developed a detailed nomenclature for different kinds of speleothems (figure above b). Where water drips from the ceiling of the cave, the precipitated limestone builds dripstone. Initially, calcite precipitates around the outside of the drip, forming a delicate, hollow tube called a soda straw. But eventually, the soda straw fills up, and water migrates down the margin of the cone to form a more massive, solid icicle-like cone called a stalactite. Where the drips hit the floor, the resulting precipitate builds an upward-pointing cone called a stalagmite. 

If the process of dripstone formation in a cave continues long enough, stalagmites merge with overlying stalactites to create travertine columns. In some cases, groundwater flows along the surface of a wall and precipitates to produce drape-like sheets of travertine called flowstone (figure above c). The travertine of caves tends to be translucent and, when lit from behind, glows with an eerie amber light.

The Formation of Karst Landscapes 

Features of Karst landscapes.

Limestone bedrock underlies most of the Kras Plateau in  Slovenia, along the east coast of the Adriatic Sea. The name kras, which means rocky ground, is apt because this region includes abundant rock exposures (figure above a). Geologists refer to regions such as the Kras Plateau, where surface landforms develop when limestone bedrock dissolves both at the surface and in underlying cave networks, as karst landscapes or karst terrains from the Germanized version of kras. 

Karst landscapes typically display a number of distinct landforms. Perhaps the most widespread are sinkholes, circular depressions that form either when the ground collapses into an underground cave below (as we discussed early in this chapter) or when surface bedrock dissolves in acidic water on the floor of a bog or pond. Not all of the caves or passageways beneath a karst landscape have collapsed, and this situation leads to unusual drainage patterns. Specifically, where surface streams intersect cracks (joints) or holes that link to caverns or passageways below, the water cascades downward into the subsurface and disappears (figure above b). Such disappearing streams may flow through passageways underground and re-emerge from a cave entrance downstream. In cases where the ground collapses over a long, joint-controlled passage, sinkholes may be elongate and canyon-like. Remnants of cave roofs remain as natural bridges. Ridges or walls between adjacent sinkholes tend to be steep-sided. Over time, the walls erode, leaving only jagged, isolated spires a karst landscape dominated by such spires is called tower karst. The surreal collection of pinnacles constituting the tower karst landscape in the Guilin region of China inspired generations of artists who portray them on scroll paintings (figure below).

Tower karst forms a spectacular landscape in southern China.

Karst landscapes form in a series of stages (figure below a–c).

The progressive formation of caves and a karst landscape.

• The establishment of a water table in limestone: The story of a karst landscape begins after the formation of a thick interval of limestone in which the water table lies underground. 
• The formation of a cave network: Once the water table has been established, dissolution begins and a cave network develops. 
• A drop in the water table: If the water table later becomes lower, either because of a decrease in rainfall or because nearby rivers downcut and drain the region, newly formed caves dry out. Downward-percolating water emerges from the roofs of the caves; dripstone and flowstone precipitate. 
• Roof collapse: If rocks fall off the roof of a cave for a long time, the roof eventually collapses. Such collapse creates sinkholes and troughs, leaving behind hills, ridges, and natural bridges.

Life in Caves 

Despite their lack of light, caves are not sterile, lifeless environments. Caves that are open to the air provide a refuge for bats as well as for various insects and spiders. Similarly, fish and crustaceans enter caves where streams flow in or out. Species living in caves have evolved some unusual characteristics. For example, cave fish lose their pigment and in some cases their eyes. Recently, explorers discovered caves in Mexico in which warm, mineral-rich groundwater currently flows. Colonies  of bacteria metabolize sulphur-containing minerals in this water and create thick mats of living ooze in the complete darkness of the cave. Long gobs of this bacteria slowly drip from the ceiling. Because of the mucus-like texture of these drips, they have come to be known as “snotites”.
Figures credited to Stephen Marshak.

Amazing caves of the world

The Naica mine caves in Mexico bears the largest crystals ever seen. The Crystal Cave, where these crystals are located, is closed to the public because of its depth, heat and other issues.

The Son Doong cave in Vietnam is the largest currently known cave in the world. It is filled with countless wonders including isolated ecosystems, weather systems and geological formations.

Ice caves like these form in the glaciers surrounding the Mutnovsky Volcano in Russia. Some of them are formed by vents that release volcanic heat and gases called fumaroles.

This cave is located in Iceland’s Vatnajokull Glacier, the largest glacier in Europe. Caves like these form due to melting glacial icewater, but they can be dangerous because glaciers are constantly breaking and changing.

The Batu Caves in Malaysia have been used by English and Chinese settlers as well as the indigenous Temuan people. The bat guano in the cave was mined for agricultural purposes, but now the cave is filled with statues and is open to visitors.

This ice cave is part of the Mendenhall Glacier near Juneau, Alaska. The spectacular cave was carved out of the glacier by melting icewater. Due to the ever-changing conditions at glaciers, it is unclear whether this cave will still be around for long.

The Algarve region in Portugal, where this cave is located, is prone to various seaside formations because of the rock face’s relative solubility in water. This specific cave near Lagos is accessible only by water.

The Waitomo glow worm caves in New Zealand are home to a unique insect, the glow worm. This insect hangs glistening silken strands from the ceiling of the cave and glows to attract unsuspecting prey.

The Nam Lang river runs through the Tham Lod cave in northern Thailand. The cave is filled with beautiful stalactites and stalagmites and is home to hundreds of thousands of Pacific swifts that have adapted to spend parts of their lives in caves.

Very little is known about this cave in Kyaut Se, Myanmar, other than that the inside has been fitted as a Buddhist temple.

The Marble Caves in Patagonia are known for the spectacular reflections that the turquoise water casts on the white marble ceiling of the cave. They are also called the Marble Cathedral because of their beautiful and arching forms.

Antelope Canyon in Arizona was carved out by thousands of years of persistent wind and flash floods, which is why its smooth walls look so smooth and fluid. During the desert’s monsoon season, dangerous flash floods can occur without warning from rains that have fallen miles away. These flash floods have taken the lives of quite a few unwary tourists.

The Phraya Nakhon Cave in Thailand was historically a popular visiting place for local kings because of the illumination provided by the collapsed roofs. The pavilion in the center was built for the visit of King Chulalongkorn in 1890.

This is the Fantastic Cave pit, part of Ellison’s Cave in Georgia, U.S.A. It is a popular attraction for pit cavers, those who enjoy rappelling down vertical subterranean drops.

The Reed Flute Cave in Guangxi, China has been visited by tourists for at least 1200 years. The cave is home to a spectacular array of stalagmites and stalactites. It is named for the reeds that grow at its mouth, which can be made into flutes.