In the northern Great Lakes region, limestone sediments deposited some 400 million ybp during the Devonian era have experienced erosion, creating karst features such as caves and sinkholes. The groundwater chemical constituents of the shallow seas that produced these rock formations
now contribute to the formation of a unique physical (sharp density gradients), chemical (dissolved oxygen-depleted, sulfate-rich) and biological (microbe-dominated) environment in a submerged sinkhole near Middle Island in freshwater Lake Huron. A variety of methods including aerial photography,
physico-chemical mapping, time series measurements, remotely operated vehicle (ROV) survey, diver observations and bathymetric mapping were employed to obtain a preliminary understanding of sinkhole features and to observe physical interactions of the system's groundwater with Lake Huron.
High conductivity ground water of relatively constant temperature hugs the sinkhole floor creating a distinct sub-ecosystem within this Great Lakes ecosystem. Extensive photosynthetic purple cyanobacterial benthic mats that characterize the benthos of this shallow sinkhole were strictly limited
to the zone of ground water influence.
Michigan has three main zones of evaporite karst: collapse breccia in Late Silurian deposits of the Mackinac Straits region; breccia. collapse sinks, andmega-block collapse in Middle
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