Great Lakes Sinkholes

March 09, 2009

Photographer: Russ Green, Thunder Bay National Marine Sanctuary
Summary Author: Stu Witmer

The Great Lakes of the US and Canada are fresh water; every school kid knows that. What you may not know is that in deep sinkholes at the bottom of Lake Huron the water is far from fresh and in it lurk strange life forms similar to those found at the bottom of Antarctic lakes or deep sea hydrothermal vents.

A large portion of the 400 million-year-old bedrock beneath Lake Huron is karst limestone, a relatively soft rock easily eroded by more acidic rainwater. The exposure of the bedrock about 10,000 years ago began the erosion process by creating holes in the rock beneath the land surface. Over time, the ceilings above these holes collapsed and the lake levels rose to cover them. The porous nature of the karst goes both ways and, once covered with water; the sinkholes began seeping their ancient groundwater into the once fresh water of the lake. The water in these holes does not circulate to the rest of the lake. Some of the sinkholes are deep enough to be devoid of sunlight thus creating unique environments of warm, dense, salty water with little or no oxygen and much higher concentrations of phosphorus, sulfate and chloride. This is far from the realm of perch, bass or even Muskies. Rather, it is home to brilliant purple mats of cyanobacteria (see photo above) and pallid, floating pony-tails of other microbial life.

Bopaiah A. Biddanda of Grand Valley State University, in Muskegon, Michigan, is one of the leaders of a scientific team studying the sinkhole ecosystems. The team is probing the origins of ancient minerals flowing in from beneath the lake. They also plan to chart transitions from light, oxygen-rich, fresh water near the lake's surface to dark, anoxic, salty soup down inside the sinkholes. Their work is described in a recent issue of Eos, the newspaper of the Earth and Space Sciences, published by the American Geophysical Union (AGU).