Land, lakes linked underground by sinkholes

Robert Ballard, discoverer of the sunken Titanic, and a team of researchers set out in 2001 to map shipwrecks offshore of Michigan in Lake Huron’s Thunder Bay National Marine Sanctuary. A geologist by training, Ballard took notice when imagery from the team’s side-scan sonar revealed dozens of large depressions in the lakebed.

Upon further inspection, the depressions proved to be sinkholes. Researchers were surprised to find some of them teeming with brilliant purple bacteria and other peculiar, seemingly primitive organisms (see March Bay View Compass). Similar life forms previously had been found only in extreme places like the deep sea and Antarctica.

So how is it that humble Lake Huron came to harbor these unusual oases? 

Dissolved Gypsum Left Cavities

The likely answer begins about 400 million years ago, when a shallow sea covered much of what is now the Great Lakes region, said Tyrone Black, a geologist with the Michigan Office of Geological Survey. Over millions of years, sediment accumulated on the seabed and slowly hardened into rock like dolomite and limestone. In some areas-including around the northern tip of present-day Michigan’s Lower Peninsula-pockets of gypsum, a rock formed from mineral deposits left by the sea, also developed.

Soon after, the sea receded, leaving the rocks exposed. Precipitation trickled through them, dissolving portions of the most soluble rock-gypsum-and creating minor cavities. Other seas then came and went, depositing sediment that eventually formed new rock over the old. Finally, the sea disappeared from the area altogether.

But through tectonic faults in the newer rock-mostly limestone and dolomite-freshwater again reached the 400-million-year-old gypsum, entering its existing cavities and dissolving it further. In some places the overlying rock then collapsed down upon these cavities, creating sinkholes at the surface. These new fractures channeled even more water into the ground, thereby continuing and accelerating the cycle of degradation and sinkhole formation, Black said.

Sinkholes Today

Today, hundreds of sinkholes pepper the Lower Peninsula landscape, and others dot the bottom of Lake Huron, as Ballard discovered. The on-land sinkholes are still swallowing water and feeding the area’s groundwater system, said Black, who has studied the features for 33 years.

The lake sinkholes, however, actually release water from the groundwater system, up into the lake. That’s because the surface of the groundwater system is higher than the surface of the lake; the water is therefore pushed lakeward, just as water in a water tower (higher) is pushed toward your faucet (lower). One nearshore sinkhole is estimated to release about 120 gallons of groundwater per second into the lake, Black said.

The outflowing groundwater probably comes from the on-land sinkholes as well as ancient underground stores of saltwater and glacial freshwater, Black said. During its travels through perhaps miles of rock, the water picks up minerals and loses dissolved oxygen. By the time it exits through the lake sinkholes, it is high in sulfur and low in oxygen-just what the odd organisms living there need to survive.

Links to Other Systems

Black said that given the area’s geology, other as-of-yet-undiscovered sinkholes probably exist in Lake Huron, and possibly Lake Michigan. Each represents an opportunity to learn about not only the sinkholes themselves, but also the groundwater, surface water, and geological systems to which they are inextricably linked.

Black said the outflowing groundwater’s chemical characteristics give scientists clues about “where the water’s been and what it’s reacting with [deep underground].” That, in turn, reveals information about the area’s geology that is otherwise difficult to obtain.

The sinkholes can also help scientists better understand the contribution of groundwater to lake levels, said Val Klump, a biogeochemist and director of the UWM Great Lakes WATER Institute. “We don’t know a lot about groundwater inputs to the lakes,” he said. “It’s sort of an unknown term that is usually calculated, not observed.”

Klump and a team of researchers are using radiochemical techniques, sensors, and other equipment to study sinkhole outflow and the rate at which it mixes with lake water, and to identify the factors that influence outflow.

At one sinkhole, the team found that groundwater is usually present in just a thin layer, but that occasionally the layer thickens considerably, said Steve Ruberg, project leader and researcher at the Great Lakes Environmental Research Laboratory in Michigan.

The change could simply reflect a weakening in local lake currents that normally wash the groundwater away, Ruberg said. Or, it could be the result of a more distant influence: increased inputs to the inland groundwater system. “If you see lots of precipitation or thaw on land, do you see more [outflow from] these [sinkhole] systems? We don’t know yet,” he said, but he expects future research will help answer that question.

Overall, said Klump, the sinkholes provide a good illustration of how land and water are connected. “These sinkholes are a reminder that what we do on land affects our lakes, even if the flow from one to the other is not obvious.”

Jennifer Yauck is a science writer at the Great Lakes WATER Institute. GLWI (glwi.uwm.edu) is the largest academic freshwater research facility on the Great Lakes.

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