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Sinkholes

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Karst Geology

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Expedition Purpose

Why Are Scientists Exploring Submerged Sinkhole Ecosystems in Thunder Bay National Marine Sanctuary?

A key purpose of NOAA's Ocean Exploration Initiative is to investigate the more than 95 percent of Earth's underwater world that until now has remained virtually unknown and unseen. Such exploration may reveal clues to the origin of life on Earth, cures for human diseases, answers to how to achieve sustainable use of resources, links to our maritime history, and information to protect endangered species. Thunder Bay Sinkholes 2008 focuses on unique, recently discovered ecosystems in Earth's largest group of freshwater lakes: the Laurentian Great Lakes of the United States and Canada.

Between 350 and 430 million years ago, during the Paleozoic era, shallow seas covered what is now the border between Canada and the United States between Minnesota and New York. Over thousands of years, sand, minerals, and sediments accumulated on the seafloor, and were gradually compressed to form sandstone, limestone and shale (for an overview of the geologic timeline, visit http://www.sdnhm.org/exhibits/mystery/fg_timeline.html exit icon).

About 1.8 million years ago, the Great Ice Age of the Pleistocene epoch began and continued until about 10,000 years ago. During this time, four major periods of glaciation occurred, separated by three interglacial periods. As the final glacial period (the Wisconsin period, beginning about 70,000 years ago) came to a close, retreating glaciers along the U.S.-Canadian border revealed five huge lakes that we now know as the Laurentian Great Lakes (although we often simply call these “the Great Lakes,” there are actually about 250 other lakes around the world that have a surface area exceeding 500 square kilometers, which is the minimum requirement for a lake to be called “great;” so we use the name “Laurentian” to specify the U.S.-Canadian great lakes).

In June, 2001, the Ocean Explorer Thunder Bay ECHO Expedition used side scan sonar to locate shipwrecks in the deep waters of the Thunder Bay National Marine Sanctuary and Underwater Preserve in Lake Huron. But in addition to many shipwrecks, the explorers discovered something else: dozens of underwater sinkholes in the limestone bedrock, some of which were several hundred meters across and 20 meters deep. The following year, an expedition to survey the sinkholes found that some of them were releasing fluids that produced a visible cloudy layer above the lake bottom. Subsequent studies revealed that chemical characteristics of the fluids were distinctly different from the surrounding lake water, and led scientists to hypothesize that the source of the fluids was the Silurian-Devonian aquifer beneath the sediments of Lake Huron.

Aquifers are rocks and sediments that contain large amounts of water. In the Laurentian Great Lakes region, aquifers are found in deposits of sand and gravel left by glaciers, as well as in porous bedrocks (limestone and sandstone) that were formed much earlier in geologic time. Five major aquifers are recognized in the Great Lakes Basin: one near the land or lake floor surface (the surficial aquifer) and the others in deeper bedrock named for the geologic time periods when they were formed (the Cambrian-Ordovician, Silurian-Devonian, Mississippian, and Pennsylvanian aquifers). The bedrock that forms the Silurian-Devonian aquifer is primarily limestone and mineral formations from evaporating seawater (remember the Great Lakes region has been periodically covered by shallow seas), with smaller amounts of shale and sandstone. Both fresh and saline water are found in the Silurian-Devonian aquifer.

Limestone rocks are soluble in acid. Atmospheric carbon dioxide often dissolves in rainwater to form a weak acid (carbonic acid). Rainwater flowing over land surfaces may also pick up organic acids produced by decaying leaves and other once-living material. The resulting weak acid can slowly dissolve limestone rocks. Land areas that contain large amounts of limestone may be shaped by this dissolution process. The result is known as “karst topography,” and may include caves, springs, and sinkholes (the name “karst” comes from the region in Slovenia where this type of landform was first described). Karst landscapes are found in areas of the Laurentian Great Lakes where limestone forms a major portion of the bedrock.

Sinkholes on land are known recharge areas for the Silurian-Devonian aquifer (areas where water flows into the aquifer). But there has not been a systematic search for submerged sinkholes, and very little is known about the chemistry, geology, and biology of submerged sinkholes that serve as vents for groundwater.

Initial explorations of some Lake Huron sinkholes have found conspicuous green, purple, white, and brownish mats covering the lake floor near the sinkholes, as well as a dark, cloudy plume suspended over areas where fluid was emerging. The overall appearance of these mats (see http://gvsu.edu/wri/envbio/biddanda/sinkhole.htm exit icon) is very similar to mats found near cold seep and hydrothermal vent habitats in the deep ocean (for virtual tours of hydrothermal vent and cold seep communities visit http://www.bio.psu.edu/hotvents exit icon and http://www.bio.psu.edu/cold_seeps exit icon, respectively). Preliminary studies have found that mats near sinkhole vents are composed of green algae where water is shallow (≤ 1.0 m), while mats in deeper (~ 18 m) waters are formed by filamentous purple cyanobacteria. Mats near the deepest (93 m) sinkholes are white or brown, but their composition is presently unknown. The appearance of mats near the deepest sinkholes is very similar to mats observed in the vicinity of cold seeps and hydrothermal vents in the deep ocean, which are often formed by chemosynthetic bacteria and are a food source for a variety of other organisms.

If mats near the deepest sinkholes are also composed of chemosynthetic bacteria, the variety of mats may indicate a shift from photosynthetic species in shallower waters to chemosynthetic organisms in habitats where light does not penetrate. Under this hypothesis, primary production in the shallowest mats takes place through photosynthesis in green algae. In deeper waters with reduced light, primary production also occurs through photosynthesis in cyanobacteria, but does not produce oxygen. In the deepest areas where there is virtually no light, chemosynthetic bacteria are responsible for primary production (for more information, see “More About Chemosynthetic Communities and Photosynthesis That Doesn't Produce Oxygen,” below).

Water samples collected within the sinkhole plume had high concentrations of sulfate, phosphorus, and particulate organic matter, as well as ten times more bacteria compared to nearby lake water. These observations suggest that submerged sinkholes may be biogeochemical “hot spots” inhabited by unusual and possibly unknown life forms. At the same time, water flow through submerged sinkholes depends upon recharge from land. This means that sinkhole ecosystems are likely to be very sensitive to changes in rainfall patterns that may accompany climate change, as well as human alterations of these landscapes surrounding recharge areas. These factors make understanding sensitive sinkhole ecosystems an urgent necessity.

Expedition Questions
Key questions of the Thunder Bay Sinkholes 2008 Expedition include:

Exploration Technology
A combination of instruments and techniques will be used to answer key questions, including:

Remotely Operated Vehicles

Remotely operated vehicles (ROVs) are unoccupied robots linked to an operator by a group of cables. Underwater ROVs are usually controlled by an operator aboard a surface ship. Most are equipped with one or more video cameras and lights, and may also carry other equipment such as a manipulator or cutting arm, water samplers, and measuring instruments to expand the vehicle's capabilities. The Thunder Bay Sinkholes 2008 Expedition will use an ROV called M-ROVER to carry a CTD (see below) and other sampling instruments. M-ROVER carries video and still cameras, an articulated arm for sampling and other tasks, sonar imaging equipment, and can carry up to 100 lb of additional equipment. The ROV is capable of speeds up to 3-knots on the surface and 3/4 knot underwater, and is rated for a maximum depth of 450 meters. For more information about M-ROVER, visit http://www.engin.umich.edu/dept/name/facilities/oel/mrover.html exit icon. For more information about other ROVs, visit http://oceanexplorer.noaa.gov/technology/subs/rov/rov.html.

CTD - A CTD collects data on seawater conductivity, temperature, and depth. These data can be used to determine salinity of the seawater which is a key indicator of different water masses. For more information about CTDs, visit http://www.oceanexplorer.noaa.gov/technology/tools/sonde_ctd/sondectd.html.

Radioactive Isotope Techniques - Isotopes are forms of an element that have different numbers of neutrons. For example, carbon-13 (13C) contains one more neutron than carbon-12 (12C). Both forms occur naturally, but carbon-12 is more common.

Ecologists use a tool called “stable isotope analysis” to study food webs. When an animal eats food that contains both carbon isotopes, carbon-12 is selectively metabolized, so the ratio of carbon-12 to carbon-13 in the tissues of the animal is higher than the ratio of these isotopes in the food they consumed. In other words, carbon-13 is “enriched” in the animal's tissues. If this animal is eaten by another consumer, the enrichment process will be repeated. So the ratio of carbon-13 to carbon-12 increases with each increase in trophic level (i.e., each step up the food chain). For additional discussion of stable isotope analysis, see “Who Is Eating Whom?” (http://oceanexplorer.noaa.gov/explorations/07mexico/logs/june15/june15.html).

Isotopes of hydrogen, oxygen, and radon will be used to study groundwater flow. The ratios of 2H/H and 18O/16O in rainfall are known to vary according to elevation, latitude, and distance from the ocean. These differences are preserved when rainwater infiltrates into an aquifer, providing a sort of “fingerprint” that can be used to identify the recharge location for water in a specific portion of an aquifer. 222Radon is a radioactive gas that is produced naturally in groundwater from the radioactive decay of 226radium in uranium-bearing rocks and sediment. Several studies have documented that radon can be used to identify locations of significant groundwater input. Preliminary data from some Lake Huron sinkholes indicate that 226Ra concentrations are 10-20 times higher in the sinkholes than in surrounding lake water. In addition, because radiochemical tracers decay with known half-lives (that range from a few days to hundreds of years), they also can be used to estimate the time scales involved in various processes related to water flow and mixing within an aquifer.  

More About Chemosynthetic Communities and Photosynthesis That Doesn't Produce Oxygen

The “big picture” of chemosynthesis and photosynthesis is that they are both processes that organisms use to obtain energy needed for life functions (reproducing, locomotion, synthesizing tissues, etc.). Energy in living organisms is stored and transported in the form of adenosine triphosphate (ATP) molecules. The energy used to produce ATP comes from reactions that transfer electrons from an electron donor molecule to an electron acceptor molecule. When these reactions take place, the molecule that loses an electron is said to be “oxidized” and the molecule that gains an electron is said to be “reduced.” One basic way to distinguish chemosynthesis from photosynthesis is the source of these electrons.

In photosynthesis, light energy absorbed by pigments (e.g., chlorophyll) is transferred to electrons in the pigment molecule, and these electrons are transferred to other molecules in a series of oxidation-reduction reactions. What happens to the chlorophyll molecule that loses its electron? In some cases, the electron is eventually recycled to the chlorophyll molecule; a process called “cyclic photophosphorylation.” In an alternative process called “noncyclic photophosphorylation” the electron is replaced by splitting another molecule through a process called “photolysis” (which means “light splitting”).  The general equation for photosynthetic photolysis is

H2X + 2 photons > 2e- + 2H+ + X

”X” may be one of several elements. In the most familiar form of photosynthesis, “X” is oxygen, and the photosynthetic photolysis of water produces oxygen gas. In some purple bacteria, however, hydrogen sulfide is oxidized and particles of sulfur are produced. Note that while photosynthesis is often explained as noncyclic photophosphorylation and photolysis of water, some photosynthetic organisms use other pathways.

In chemosynthesis, electrons are also transferred between molecules to provide the energy needed for ATP production. The key difference is that light does not play a part in these reactions. A variety of electron donors are found in chemosynthetic systems; hydrogen sulfide is common in chemosynthetic organisms associated with hydrothermal vents, while methane is often the electron donor in cold seep communities.

In both photosynthetic and chemosynthetic communities, a significant amount of the energy captured as ATP is used to synthesize organic molecules (note that highly simplified descriptions of photosynthesis imply that light energy is used to combine carbon dioxide and water to form glucose in a single reaction; but the reality is that many reaction sequences are involved). If an organism synthesizes organic molecules from inorganic compounds (such as carbon dioxide), then the organism is called “autotrophic.” If an organism can only use organic compounds as “building blocks,” then the organism is called “heterotrophic.”

Unlike biological communities in shallow-water ocean habitats, some deep-water ecosystems do not depend upon sunlight as their primary source of energy. Instead, these communities derive their energy from chemicals through a process called chemosynthesis (in contrast to photosynthesis in which sunlight is the basic energy source). Some chemosynthetic communities have been found near underwater volcanic hot springs called hydrothermal vents, which usually occur along ridges separating the Earth's tectonic plates. Other such communities are found in the vicinity of “cold seeps” where gases (such as methane and hydrogen sulfide) and oil seep out of sediments. The first deepwater chemosynthetic communities were discovered in 1977 in the vicinity of hydrothermal vents near the Galapagos Islands. Cold seep communities were found by accident in the Gulf of Mexico in 1984. Both communities are home to many species of organisms that have not been found anywhere else on Earth.

Hydrothermal vent communities are often inhabited by large tubeworms known as vestimentiferans, sometimes growing in clusters of millions of individuals. At present, vestimentiferans are generally considered to be part of the phylum Annelida, but they are sometimes grouped as a separate phylum (Pogonophora). These unusual animals do not have a mouth, stomach, or gut. Instead, they have a large organ called a trophosome, that contains chemosynthetic bacteria. Vestimentiferans have tentacles that extend into the water. The tentacles are bright red due to the presence of hemoglobin which can absorb hydrogen sulfide and oxygen which are transported to the bacteria in the trophosome. The bacteria produce organic molecules that provide nutrition to the tubeworm. A similar symbiotic relationship is found in clams and mussels that have chemosynthetic bacteria living in their gills. Bacteria are also found living independently from other organisms in large bacterial mats. A variety of other organisms are also found in cold seep communities, and probably use tubeworms, mussels, and bacterial mats as sources of food. These include snails, eels, starfish, crabs, lobsters, isopods, sea cucumbers, and fishes. Specific relationships among these organisms have not been well-studied.

Typical features of cold seep communities that have been studied so far include mounds of frozen crystals of methane and water called methane hydrate ice, that are home to polychaete worms. Brine pools, containing water four times saltier than normal seawater, have also been found. Researchers often find dead fish floating in the brine pool, apparently killed by the high salinity.

Methane hydrate is a type of clathrate, a chemical substance in which the molecules of one material (water, in this case) form an open lattice that encloses molecules of another material (methane) without actually forming chemical bonds between the two materials.  Methane is produced in many environments by a group of Archaea known as the methanogenic Archaeobacteria. These Archaeobacteria obtain energy by anaerobic metabolism through which they break down the organic material contained in once-living plants and animals. When this process takes place in deep ocean sediments, methane molecules are surrounded by water molecules, and conditions of low temperature and high pressure allow stable ice-like methane hydrates to form. These deposits are significant for several reasons:

Deepwater chemosynthetic communities are fundamentally different from other biological systems, and there are many unanswered questions about the individual species and interactions between species found in these communities. These species include some of the most primitive living organisms (Archaea) that some scientists believe may have been the first life forms on Earth. Many species are new to science, and may prove to be important sources of unique drugs for the treatment of human diseases. Because their potential importance is not yet known, it is critical to protect these systems from adverse impacts caused by human activities.

More About SCUBA Diving Technology

Observations and sampling of shallow (less than 30 m deep) sinkholes will be done by divers breathing ordinary air using conventional SCUBA equipment. As anyone knows who has ever tried, it is impossible to breathe underwater through a snorkel, pipe, or hose that is more than a few feet long. This is because the surrounding water exerts pressure on a diver's body that is equal to one atmosphere (about 14 pounds per square inch) for every 33 feet of depth. To breathe through a long snorkel underwater, the muscles we use to fill our lungs have to overcome this water pressure - and they are not designed to do that.

Conventional SCUBA technique overcomes this problem by using a device called a demand regulator (invented in 1943 by Jacques Cousteau and Emile Gagnan). A demand regulator supplies air at a pressure that matches the pressure in the water (ambient pressure), so there is no additional pressure for breathing muscles to overcome. A diver using this type of regulator breathes gas at higher pressures than at the surface. Instead of a normal pressure of one atmosphere, a diver at 33 feet is breathing air pressurized to two atmospheres; at 66 feet, the pressure is three atmospheres, and so on. But there are several problems with this system.

One problem is that if a diver takes a lungful of air at a depth of 66 feet and then ascends to 33 feet, the external pressure drops from 3 atmospheres to 2 atmospheres. Since the volume of a gas is inversely proportional to the pressure of the gas, as the pressure drops the volume will increase. So the air in the diver's lungs would expand, and if the diver was holding his breath the expanding air could rupture his lungs. When this happens, air bubbles can enter the bloodstream causing a condition known as “air embolism.” If the bubbles block important blood vessels, the result can be paralysis or death.

Another problem is that as the pressure of a gas increases, the solubility of that gas in a liquid increases as well (Henry's law). So if a diver breathes air from a demand regulator at 66 feet for a while, her blood will contain twice the amount of dissolved gases from the air than it did at the surface. Again, the problem comes when the pressure is reduced. Anyone who has ever opened a can of soda knows what happens when you suddenly release the pressure on a liquid containing dissolved gas: bubbles form in the liquid. If the diver rapidly ascends from 66 feet, the dissolved gases in her blood may form bubbles, creating a problem that is somewhat similar to an air embolism in that the critical blood vessel may become blocked. This condition is called “decompression sickness” or “the bends,” and was first seen in miners working in pressurized coal mines (it was also a problem for workers constructing the Brooklyn Bridge, who spent hours working underwater in pressurized iron boxes called caissons, so yet another name for the condition is “caissons disease”). Since air is about 70% nitrogen, more nitrogen is dissolved in the blood than other gases and the bubbles of decompression sickness are bubbles of nitrogen gas. Oxygen isn't believed to be involved, since much of the oxygen dissolved in a diver's blood is quickly bound by hemoglobin, and normal metabolism reduces blood oxygen concentration.

Divers avoid decompression sickness by closely monitoring their dive time and depth, since they both affect the amount of gas that dissolves in the blood. Decompression tables and dive computers show how long a diver may stay at a particular depth without having a high risk of decompression sickness. If they stay longer than this time, then they have to return to the surface in stages, stopping for a specific amount of time at shallower depths (“decompression stops”) to allow the nitrogen to diffuse out of their blood without forming bubbles. For more information on technical diving, visit the Cayman Islands Twilight Zone Expedition Web page (http://oceanexplorer.noaa.gov/explorations/07twilightzone/background/techdive/techdive.html)

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For More Information

Contact Paula Keener-Chavis, national education coordinator for the NOAA Office of Ocean Exploration, for more information.

Other lesson plans developed for this Web site are available in the Education Section.

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