Researchers are exploring sinkholes in Lake Huron off the coast of Alpena, Mich. Photo: NOAA
By Sarah Coefield, coefield@msu.edu
Great Lakes Echo
July 14, 2009
Lake Huron’s depths hide a colorful, ancient world that holds keys to the planet’s history and clues for new cancer treatments and antibiotics.
The locals in Alpena have long known about sinkholes just offshore from their northeast Michigan community. But it will take researchers several years to unravel the local diving spots’ mysteries.
The story of the Lake Huron sinkholes and their exotic ecosystems begins on a ship. While surveying shipwrecks in the Thunder Bay National Marine Sanctuary in 2001, Steve Ruberg and his colleagues were surprised to detect underwater basins 300 feet below the surface. To their trained eyes, the basins looked like sinkholes.
The discovery warranted further investigation.
Steve Ruberg
“Looking at the data and understanding what was going on, we actually came back and revisited the sites in 2003,” Ruberg said. Ruberg is an engineer with the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory and a project leader for the sinkhole research.
When the scientists returned they discovered salty groundwater venting into underwater sinkholes.
Sinkholes are not unusual. On land they can form wherever limestone is prevalent. Groundwater and rainwater dissolve the rock, carving underground caves and tunnels. If too much limestone dissolves, the surface collapses, forming a depression that can fill with water. The sinkholes in Lake Huron likely formed thousands of years ago, before the lake covered the limestone bedrock.
Though sinkholes are common, scientists have found that the ecosystems flourishing in them under Lake Huron are far from the ordinary.
Bopi Biddanda was immediately intrigued: “I said, what if it’s a really different composition (of water) coming out. They’d be seeing very different types of life. Let’s go and look there.”
Biddanda, a research scientist at the Grand Valley State University Annis Water Resources Institute in Muskegon, Mich., co-leads the sinkhole research with Ruberg.
Beginning in 2003, divers and remotely operated vehicles collected samples and images from the sinkholes. They revealed conditions never before documented in the Great Lakes. The mats of purple and white bacteria thriving in them are reminiscent of life found in permanently ice-covered Antarctic lakes and deep sea vents, Biddanda said. At that point “we knew we were onto something really cool,” he said. “Really unique life and possibly life processes that … could add to the exploration of life on the planet itself.”
Before bubbling into the sinkholes in Lake Huron, the groundwater passes through 400-million-year-old bedrock rich with ancient marine salts and sulfur. By the time it reaches the lake, the water has little oxygen left. The combination of low oxygen and the presence of salt and sulfur is reminiscent of prehistoric Earth.
The water “is filling food webs that almost resemble what might have prevailed in the ancient shallow seas, probably a couple of billion years ago,” Biddanda said.
This summer researchers are aging the water to see if it is as ancient as the bedrock it passes through.
The dense salt-and-sulfur-laden groundwater hugs the lake floor. Its constant temperature provides a nursery for exotic bacteria, but the low oxygen keeps fish at bay.
The brilliant purple mats in the shallow, sunlit sinkholes are composed of cyanobacteria – ancient, complex bacteria that rely on photosynthesis for energy, like plants, but use sulfur compounds in place of water. In the deep, dark sinkholes farther off shore, mats of bright white bacteria coat the sinkhole bottom. These white bacterial mats rely on chemosynthesis – using sulfur for energy in the absence of light or oxygen.
“You know, we never expected that in our own backyard, in the Great Lakes,” Biddanda said. “It was totally, totally cool.”
The unique bacteria and geology lend themselves to several research projects, including new disease treatments and a search for clues to ancient life and for similar ecosystems in the Great Lakes.
Tomorrow: Lake Huron sinkholes provide clues to ancient life.