In August, scientists with the National Oceanic and Atmospheric Administration’s Great Lakes Environmental Research Laboratory discovered signs of sinkholes on the floor of Lake Michigan, in the national marine sanctuary off Wisconsin's lakeshore, near Sheboygan. They found around 40 large underwater features, most of which are about as long as an Olympic swimming pool. They’re cold, dark, and weirdly circular.
The features are similar to sinkholes previously discovered in Lake Huron. In that case, groundwater eroded the layer of limestone that sits on the bottom of the lake, creating caverns. When the roof of a cavern collapsed, a sinkhole was born.
But Steve Ruberg, a physical scientist with the lab, says the researchers need more information before they can confirm that what’s in Lake Michigan is just like those in Lake Huron.
“I'm not ready to say that this is a sinkhole that's similar to Lake Huron,” he said. “These are much bigger, and they're just very strangely circular. They're very interesting.”
They were, however, discovered in a manner very much like what happened in Lake Huron — accidentally. Acoustic surveys in the Thunder Bay National Marine Sanctuary, intended to uncover historic shipwrecks, revealed the bottom features almost 20 years ago. A similar, recent hunt for shipwrecks in Wisconsin’s Shipwreck Coast National Marine Sanctuary uncovered signs of similar depressions in the limestone layer on the lake’s bottom.
That led Ruberg and other researchers to return to the site this summer to take a closer look.
Life in a Great Lakes sinkhole
The subterranean holes, with their extreme environments, offer scientists a look at the Great Lakes’ distant past. In Lake Huron, they are home to a rich tapestry of microbial life.
“Presumably, life was like this everywhere in the early biosphere,” said Bopi Biddanda, a microbial ecologist at Grand Valley State University in Michigan who has studied the microbes in Lake Huron’s sinkholes.
Some two to three billion years ago, low-oxygen, high-sulfur conditions — much like those in the sinkholes — in shallow, light-filled holes fueled the growth of photosynthesizing cyanobacteria, which first oxygenated the planet. Studying similar microbial mats of these bacteria today provides insight to how microbes of the past helped produce the oxygen that life needs to thrive.
In Lake Huron’s deeper holes, where light wasn’t available, bacteria made use of sulfur to produce their food. These microbial colonies grew into white mats, much like those that live around deep sea vents today.
“So, right in our backyard, we have a window to the past, as well as to the deep sea,” Biddanda said.
Studying the microbes that live in such extreme environments can also offer insights to modern problems, from biochemicals that could be of pharmaceutical value to carbon sequestration.
“It’s very interesting to me that these things — hard to find and very little in stature — might have had an impact on our being here today, in evolutionary terms,” Biddanda said. “And that threatened as they might be, they might even survive us in the long run.”
Dive into the discovery
Steve Ruberg joins Lake Effect to share more about the potential for sinkholes in Lake Michigan.
This interview has been edited for length and clarity.
What are we even talking about here? What is a sinkhole?
We've been looking at sinkholes on Lake Huron, and the sinkholes in that situation are eroded limestone features. When you look at the geology of the Great Lakes — Lake Michigan and Lake Huron, especially — they sit on a limestone geologic layer that was laid down four to five hundred million years ago. And then on top of that, you have glacial activity.
In Lake Huron, those are karst features. Limestone geology that is eroded, and a cavern forms. That cavern collapses and forms a sinkhole. It appears that's what's happening on Lake Michigan. But we haven't been able to verify that to a high degree of certainty. We feel like they probably are, but there are other options for what they could be as well.
The interesting thing about the Great Lakes in general is that a very small portion of the Great Lakes have been explored. We haven't mapped a lot of the bottom, and we haven't gone and looked at the things that we see when we map the bottom. What we want to do, in this case, is not just to go look — we're trying to understand the amount of groundwater that might be coming into these systems. And along with groundwater, we want to know what the chemical makeup is.
At first, there was this hint that they might be there through the acoustic surveys. What did that look like?
We took a remotely operated vehicle with some sensors on it that would allow us to detect groundwater. We’re measuring something called specific conductivity. We’re measuring temperature.
When we get into the zone where there's groundwater, in Lake Huron, we're seeing higher than normal water temperatures — that's an indication of groundwater. We're seeing much higher than normal levels of specific conductivity, which indicates that water is going through this limestone system, picking up a lot of salt and sulfur and other minerals. So that’s an indication that we’re seeing groundwater coming into these systems too.
What we wanted to do is quantify the amount of groundwater that's residing in a sinkhole. Unfortunately, [when] we looked in Wisconsin and on Lake Michigan, we did not detect a groundwater signature.
So, what does that mean? Jury's still out?
Jury's still out, that's exactly what it means. We did see a lot of interesting life down there. We saw deep water sculpin and, of course, unfortunately, a lot of quagga mussels and creatures like that.
I'm not ready to say that this is a sinkhole that's similar to Lake Huron. These are much bigger, and they're just very strangely circular. They're very interesting. It's possible that if we get out deeper, just [due to] the natural erosion of the limestone, the groundwater will eventually stop coming into them. It's possible that the deeper ones may still have groundwater.
In the long run, we want to know how much water is coming in from these features. That adds to our model of lake water levels. We know that the groundwater input is very, very small, but still, for completion, we'd like to try to get an idea of how much groundwater is coming in. The other thing is that the groundwater has a lot of salt and sulfur. Knowing how much salt and sulfur chloride is getting into the system is important to get a baseline for what the inputs of those sorts of things into the Great Lakes are.
What kinds of things might we learn from these features? What could it tell us about Lake Michigan, or the geological history of the Great Lakes?
That's what we're hoping. If there are these limestone karst features, you're going back four to five hundred million years ago. The shallow sea that we sit on now is providing this, this signal of the chloride and sulfate. But in more recent times, we’ve had glacial activity that formed the Great Lakes, and that could have laid down some of the clay that we're seeing too. So it's just telling us the history of these systems and how they've changed over time.
