Sinkhole backgrounder

Additional information about Lake Huron sinkholes

The Lake Huron Sinkholes Overview

El Cajon Sinkhole

Middle Island Sinkhole

Isolated Sinkhole

Glossary of terms and concepts

The Lake Huron Sinkholes

This map shows the locations of three sinkholes scientists are studying in northern Lake Huron.  The gray area is Michigan and the color gradient represents lake depth.

Sinkholes and caves are karst formations created when mildly acidic rain and groundwater dissolve calcium carbonate in the limestone, carving tunnels and holes into the rock.

The sinkholes in Lake Huron were most likely formed thousands of years ago, before the formation of the Great Lakes but after glaciers retreated.  When the Pleistocene glaciers retreated 10,000 years ago, they scraped the landscape clean of any older karst formations.  As a result, karst formations in the Great Lakes region formed between 10,000 and 8,000 years ago when low lake levels exposed the limestone bedrock.

The sinkholes in Lake Huron range in diameter from a tabletop to a football field. They lie both near the shoreline and in the deeper waters. Groundwater entering the sinkholes maintains a constant temperature, about 45-48 degrees Fahrenheit, despite the lake water temperatures ranging from 39 to 77 degrees.

The groundwater passes through 400-million-year-old bedrock before venting into Lake Huron. Hundreds of millions of years ago, the bedrock was a seafloor. It still has deposits from the ancient sea’s sulfur and salt. These deposits are known as evaporites – salts left behind when the sea evaporated.  The groundwater traveling through the bedrock delivers the evaporites into Lake Huron, entering the lake with 83 times as much sulfur as the surrounding lake water. The groundwater is also nearly 100 times saltier than the lake water, although it has far less salt than the ocean. While trapped underground, the water exhausts its oxygen supply. By the time it arrives at the lake the water resembles conditions last seen on the Earth billions of years ago.

The groundwater’s depleted oxygen and high sulfur and salt content are hostile to fish but host a number of exotic microorganisms. The microbes use a variety of survival strategies in the sinkholes, and scientists are finding that light plays a key role in dictating those strategies.

El Cajon Sinkhole

The El Cajon sinkhole in Misery Bay is the shallowest of the three sinkholes. At a little more than three feet deep, light penetrates the water, allowing photosynthetic green algae and cyanobacteria to thrive.

The cyanobacteria live alongside sulfate-reducing bacteria, which get energy by converting sulfate in the groundwater to hydrogen sulfide. The cyanobacteria use this sulfide for photosynthesis, much as plants and green algae use water to help convert carbon dioxide into energy.

Middle Island Sinkhole

The Middle Island sinkhole is 75 feet below Lake Huron’s surface. Only 5 percent of sunlight reaches the sinkhole, so it is primarily occupied by the purple cyanobacteria mats and chemosynthetic bacteria.

The groundwater at the Middle Island sinkhole does not vent from the bottom. It originates in a small side area and then cascades into the rest of the sinkhole in what researchers call an underwater waterfall. The dense groundwater pours over the lip of the sinkhole, but due to the resulting turbulence, it mixes with the lake water, creating a gradient of salt and oxygen. The water becomes more oxygenated and less salty the farther it flows from the source. This gradient influences the types of communities living in the sinkhole.

Isolated Sinkhole

The Isolated sinkhole is one of the deep, dark sinkholes in Lake Huron that were discovered in 2001. The sinkhole is more than 300 feet below the lake’s surface and 10 miles from shore. While in a deeper part of the lake, the sinkhole itself is only 10 feet deep.

Light does not reach the isolated sinkhole, so the purple cyanobacteria are absent. Instead, mats of white chemosynthetic bacteria coat the sinkhole floor. Scientists studying the Isolated Sinkhole recently discovered that the groundwater feeding the sinkhole is even saltier than the water found in sinkholes nearer to shore. They think the groundwater may be passing through even older bedrock, most likely from the Silurian period, 440 million years ago, though this has yet to be confirmed.

GLOSSARY

Acid rain: Rainwater becomes mildly acidic when it mixes with carbon dioxide in the atmosphere to form carbonic acid. While pure water has a pH of 7, rainwater’s pH is generally around 5.6 as a result of this mixing. Carbonic acid is a weak acid used in soda beverages to produce the bubbling effect. The acid rain responsible for damaging forests and statues has an even lower pH (typically below 4.5) and forms when the water mixes with sulfur dioxide to form sulfuric acid or nitrogen dioxide to form nitric acid. The more acidic acid rain can form naturally after a volcano eruption, or, more commonly, as a result of man-made pollution. Return

Calcium carbonate: Calcium carbonate (CaCO3) is a common mineral that is found in many rocks and is a major component in the shells of marine and aquatic animals such as mussels and clams.  In the presence of mildly acidic rain or groundwater, the calcium carbonate in limestone becomes calcium bicarbonate, which is soluble in water.  As a result, when rain and groundwater comes in contact with limestone, which is largely composed of calcium carbonate, the water dissolves the rock.  The actual formation of caves and tunnels takes many thousands or even millions of years.   Return

Chemosynthesis: Chemosynthetic organisms use chemical energy, rather than light energy, to turn carbon dioxide into food. Some of the major players in chemosynthesis are similar to those found in photosynthesis. Like plants and algae, bacteria performing chemosynthesis rely on combining carbon dioxide and water to produce sugars that can be turned into energy. The difference is that chemosynthetic bacteria do this without the aid of sunlight. The bacteria add an additional molecule to the equation, typically methane or hydrogen sulfide, which provides the energy to convert the carbon dioxide into a sugar molecule. Chemosynthetic bacteria thrive in absolute darkness and are the foundation for foodwebs in deep sea thermal vents. Return

Cyanobacteria: Cyanobacteria evolved more than 3 billion years ago.  Two billion years ago, they changed the world.  There are many different kinds of cyanobacteria and they can live in terrestrial, marine and freshwater environments.  They also come in different colors, including blue and purple.  Some cyanobacteria live alone, while others form colonies or long filaments.  The mats in the Lake Huron sinkholes are composed of filamentous cyanobacteria.   While cyanobacteria are diverse in location and appearance, they are all photosynthetic.  Cyanobacteria were some of the first organisms on Earth to use photosynthesis for energy.  Photosynthesis produces oxygen as a waste product and it is likely that photosynthesizing cyanobacteria were responsible for pumping oxygen into the Earth’s atmosphere.  All that oxygen caused a huge die-off of bacteria 1.8 billion years ago, but set the stage for life as we know it. Return

Evaporites: When seas evaporate they leave behind the salts and minerals that were in their waters. These deposits are known as evaporites. Four hundred million years ago, the Great Lakes Basin was underwater. When the sea evaporated it left behind considerable salt, gypsum and anhydrite deposits. In some areas, the salt evaporites are nearly 500 feet thick. The salts and minerals are now underground and can be picked up by groundwater passing through the bedrock. Return

Green Algae: While green algae, like cyanobacteria, conduct photosynthesis and live either as single cells or in colonies, they are not bacteria.  The green algae are closely related to plants and evolved shortly after cyanobacteria filled the Earth’s atmosphere with oxygen.      Return

Karst formations: Karst topography is characterized by limestone formations such as sinkholes and caves.  It is also frequently populated with disappearing streams, underground streams and blind valleys.  Karst formations are created over many thousands or millions of years by water dissolving the rock’s calcium carbonate.  Sometimes the calcium carbonate picked up by the water is later deposited elsewhere, leading, among other things, to the formation of stalactites and stalagmites in caves.  Well known karst landscapes in the United States include Mammoth Cave in Kentucky and the Driftless area, which covers parts of Minnesota, Wisconsin, Illinois and Iowa.
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Photosynthesis: Plants, algae and cyanobacteria use photosynthesis to convert carbon dioxide and water into organic compounds, such as sugars, that they can use for energy. The process uses the sun’s energy to split electrons from the water and start a cascade of reactions that ultimately produces organic compounds and releases oxygen. Photosynthesis that releases oxygen is called oxygenic photosynthesis. Some organisms, such as the cyanobacteria in the Lake Huron sinkholes, can perform photosynthesis using alternative electron donors, such as hydrogen sulfide. The bacteria release sulfur instead of oxygen as a waste product. If oxygen is not released, the photosynthesis is called anoxygenic photosynthesis. While the cyanobacteria in the sinkholes perform anoxygenic photosynthesis, many other types of cyanobacteria release oxygen. In fact, cyanobacteria performing oxygenic photosynthesis were most likely responsible for pumping oxygen into the Earth’s atmosphere billions of years ago.
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Pleistocene: The Pleistocene epoch lasted from 1.8 million to 10,000 years ago, and is characterized by an ice age, large mammals, and the arrival of modern humans. During the late Pleistocene, in the Wisconsinan glacial age, the Great Lakes region was covered in massive glaciers. The ice sheet, known as the Laurentide ice sheet, covered the northern part of North America, stretching from Alberta in Canada to Maine in the United States. The ice sheet covered what would become the Great Lakes Basin 20,000 years ago. It retreated and advanced at least 10 times before beginning its final retreat about 14,000 years ago. Return

Silurian period: The Silurian period lasted from 443 million to about 416 million years ago. During the Silurian, the Earth was warmer than modern times, and shallow seas were prevalent. Coral reefs and bony fish appeared during the Silurian, as did vascular plants. The Great Lakes Basin was covered by a sea that deposited salt, gypsum and dolomite evapirates. Return

Sulfate: Sulfate reducing bacteria use sulfate in place of oxygen during respiration. Respiration is the process of converting food into energy. When animals and many microorganisms respirate they take in oxygen (O2) and release carbon dioxide (CO2) as a waste product. Not all organisms live in areas with available oxygen, however, and must use alternatives from their environment. The sulfate reducing bacteria take in sulfate (SO4) and release hydrogen sulfide (H2S). Return

Hydrogen sulfide: Hydrogen sulfide is responsible for the rotten egg smell frequently associated with sulfur.   The purple cyanobacteria in the Lake Huron sinkholes use hydrogen sulfide instead of water when they perform photosynthesis.  While photosynthesis usually requires water, hydrogen sulfide’s chemical formula (H2S) is similar to water (H2O), and can be used by organisms performing photosynthesis in environments with little oxygen.  Hydrogen sulfide is released by sulfate reducing bacteria during chemosynthesis.
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Sources

Bopi Biddanda, project co-leader Interview

Steve Ruberg, project co-leader Interview

Biddanda, B.A., Nold, S.C., Rubero, S.A., Kendall, S.T., Sanders, T.G., Gray, J.J. 2009. Great Lakes Sinkholes: A Microbiogeochemical Frontier. Eos 90 (8): 61-68

Biddanda, B.A., Coleman, D.F., Johengen, T.H., Ruberg, S.A., Meadows, G.A., Van Sumeren, H.W., Rediske, R.R., Kendall, S.T. 2006. Exploration of a Submerged Sinkhole Ecosystem in Lake Huron. Ecosystems 9: 828-942

NOAA Great Lakes Environmental Research Laboratory. 2007. Exploration of Submerged Sinkholes in Lake Huron

NOAA Ocean Explorer Thunder Bay Sinkholes 2008