How sinkholes are raising levels of Great Lakes, feeding rare life forms
Eroding beaches and flooded shorelines along the Great Lakes this summer might be a product of more than the rains, rivers and streams usually contributing to rising lake levels.
Scientists also are studying the role sinkholes play in the process.
National Oceanic and Atmospheric Administration scientists have been studying Lake Huron sinkholes east of Alpena for more than 15 years, particularly their function as an outlet for water that travels underground and out into the sinkhole.
In some cases, the journey from land to the sinkhole is about a mile. In other, newly discovered sinkholes, the water travels 15 miles.
The sinkholes are considered to have a small effect in boosting lake levels, but researchers said they are still trying to verify how much of an effect. They described the nearshore flow as the equivalent of a stream and the offshore sinkhole flow as more of a “seep.”
“If we know what’s happening in a few of them, we can make some estimates about how much groundwater is going into the lakes,” said Steve Ruberg, the NOAA scientist leading the sinkhole study in Lake Huron.
“There are still areas right here in the Great Lakes that we haven’t really explored."
The water flowing into the sinkhole nourishes microbial life along the bottom that is rarely seen and not so easily accessed in other parts of the world, making scientists giddy with possibilities.
“These sinkholes are exceptional in terms of the type of life they hold,” said Greg Dick, an associate professor of earth and environmental sciences at the University of Michigan.
“We consider them to be an extreme environment. Usually, we would have to go to Yellowstone Park or the deep sea to see something similar.”
Great Lakes sinkholes
For years, researchers have focused on three near-shore sinkholes east of Alpena in Lake Huron — particularly one called the Middle Island Sinkhole. They were found during a research cruise in the Thunder Bay National Marine Sanctuary in the early 2000s.
Ruberg and a team of researchers suspected then that others existed and had even investigated one 10 miles east of the nearshore system. But it wasn’t until 2016, using acoustic mapping technology, that the team confirmed 14 other sinkholes in a 1.5- by 3-mile area 15 miles offshore.
The sinkholes are the width of a football field or larger and range from roughly 60 feet deep closer to shore to 350 feet deep among those located farther offshore, Ruberg said.
It’s likely there are other sinkholes throughout Lake Huron and other Great Lakes due to the limestone bedrock that allows for the formation of the structures. There have been reports of as much in Lakes Erie and Michigan, Ruberg said.
The structures are less likely in the Lake Superior basin, which has different geology.
“They’ve actually been there for some time,” Ruberg said. “But anywhere you have a limestone system, there could be new ones that develop.”
The shallow limestone sea bed that Michigan sits atop is porous and, over the years, lake water has dissolved ancient "carbonate critters" in the limestone to form cave structures and fissures in the bedrock, Ruberg said. When the cave structures came too close to the lake bed, they opened into sinkholes.
The remaining underground structures and porous limestone serve as a sort of pipeline through which groundwater is pushed from on land into the sinkhole. That flow can range from a slow, almost imperceptible seep to a regular stream or a reservoir that fills and then spills over into the sinkhole.
“They don’t have a big effect, but to complete the picture we want to keep on measuring flow in as many of these as we can,” Ruberg said. “It will help us complete that picture of what the inputs are.”
Over the years, as technology has developed, the crew has been able to use remotely operated vehicles to place sensors throughout the sinkholes to monitor the flow. Even the sensors themselves have become more advanced.
“We’re going to see higher flow when we have a lot of rainfall,” Ruberg said.
While the sinkholes play an important role in understanding lake levels, it’s the unique water pouring into them that has captured the attention of scientists around the world.
By the time the water reaches the sinkhole, the salty limestone has largely stripped it of oxygen and pushed through a liquid of a largely sulfuric makeup.
The heavy liquid fosters unique microbial mats on the sinkhole bottom that are varying shades of purple closer to shore and white in deeper waters where sunlight is unable to reach. No matter where it lets out in the lakes, the sulfuric water averages a temperature of 49.1 degrees Fahrenheit, said Bopaiah Biddanda, professor of water resources at Grand Valley State University.
“It is cold water that is coming, and they constantly bathe in that,” he said.
The microbial growth of cyanobacteria closer to shore is produced through a photosynthesis process similar to what might have happened eons ago when the Earth’s sulfur-rich environment slowly evolved to the oxygenated environment that allowed plant and animal life to flourish, said Biddanda, who is an aquatic microbial ecologist.
“We started looking at them as windows to early Earth,” he said. Through genome sequencing, scientists have determined that the mats' closest cousins are located in the permanently ice-covered lakes of Antarctica.
The deeper white mats are too deep for light to reach and rely on chemosynthesis instead, similar to the thermal vents in the deep sea but more conveniently located, said Dick, the UM researcher collaborating with Biddanda and Ruberg.
“We don’t really think of the Great Lakes as extreme environments in that sense,” he said.
The mats begin growing in the spring, peak in late summer and die down in the winter, Biddanda said. They also tend to move as one, ballooning up, down and sideways in search of light or chemicals.
Those mats serve as protectors for the carbon-rich sediments they cover. As the carbon sediments break down, they create methane and hydrogen sulfide that balloon the mat further, creating fingers in the matting that spread out, break off and float to the surface.
The current-day relevance of the sinkholes is just as fascinating, though still a far way off from realization, Biddanda said.
If the mats are survivors of the distant past, they must have fought and won a number of biological battles that may have extrapolations for antiviral agents or chemical compounds of human value.
In addition, their parallels to deep-sea survival may even hold secrets to life that humans might encounter on other planets.
“That is like one of the most extreme life situations on Earth, the thermal vents in the deep sea,” Biddanda said. “We have a very similar, possibly the same thing happening here.
“How does life sustain itself in these situations? Those are fundamentally very important questions.”