Sinkhole backgrounder | Great Lakes Echo
Additional information about Lake Huron sinkholes The Lake Huron Sinkholes Overview El Cajon Sinkhole Middle Island Sinkhole Isolated
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Map of submerged sinkholes (including the study sites – Misery Bay containing the El Cajon Bay Blue Hole, Middle Island Sinkhole and Isolated Sinkhole) in the Thunder Bay National Marine Sanctuary (TBNMS), Lake Huron. Image courtesy of Thunder Bay Sinkholes 2008, NOAA, OceanExplorer.noaa.gov
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.
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.
