P A P E R Observations of the Middle Island Sinkhole in Lake Huron – A Unique Hydrogeologic and Glacial Creation of 400 Million Years A U T H ORS ABSTRACT Steven A. Ruberg In the northern Great Lakes region, limestone sediments deposited some 400 NOAA-Great Lakes Environmental million ybp during the Devonian era have experienced erosion, creating karst features Research Laboratory such as caves and sinkholes. The groundwater chemical constituents of the shal- Scott T. Kendall low seas that produced these rock formations now contribute to the formation of Bopaiah A. Biddanda a unique physical (sharp density gradients), chemical (dissolved oxygen-depleted, Grand Valley State University sulfate-rich) and biological (microbe-dominated) environment in a submerged sink- hole near Middle Island in freshwater Lake Huron. A variety of methods including Tyrone Black aerial photography, physico-chemical mapping, time series measurements, remotely Michigan Office of Geological Survey operated vehicle (ROV) survey, diver observations and bathymetric mapping were Stephen C. Nold employed to obtain a preliminary understanding of sinkhole features and to observe University of Wisconsin – Stout physical interactions of the system’s groundwater with Lake Huron. High conductivity ground water of relatively constant temperature hugs the sinkhole floor creating a Wayne R. Lusardi distinct sub-ecosystem within this Great Lakes ecosystem. Extensive photosynthetic Russ Green purple cyanobacterial benthic mats that characterize the benthos of this shallow Thunder Bay National Marine sinkhole were strictly limited to the zone of ground water influence. Sanctuary Tane Casserley Introduction tions of chloride, and 100-fold higher NOAA NMS Maritime Heritage ubmerged karst features (sink- concentrations of sulfate (Ruberg et al., Program Sholes) were discovered in Lake Huron 2005). A variety of non-photosynthetic Elliott Smith (Michigan) during a 2001 side scan benthic microbial mats were observed Noble Odyssey Foundation sonar survey expedition conducted in this deepwater aphotic sinkhole by the Thunder Bay National Marine system. Water samples collected from T. Garrison Sanders Sanctuary (TBNMS) and the Institute the sinkhole plume contained bacterial Grand Valley State University for Exploration (Coleman, 2002). Our concentrations (~9x109 cells l-1) an or- Gregory A. Lang initial understanding of the composi- der of magnitude higher than ambient 9 NOAA-Great Lakes Environmental tion of Lake Huron sinkhole systems lake concentrations (~1x10 cells l-1), Research Laboratory is based on a preliminary 2003 ROV and showed evidence for the occur- exploration of the Isolated Sinkhole lo- rence of significant chemosynthesis in Stephen A. Constant cated 10 miles offshore in the TBNMS this lightless deep water environment University of Michigan at a depth of 93 m (Figure 1), reveal- (Biddanda et al., 2006). These rates of ing a visible cloudy layer attributed to chemosynthesis occurring in the Lake groundwater. When compared to am- Huron Isolated sinkhole were com- bient lake water with a temperature of parable to those measured in thermal 3.9 °C and specific conductivity of 140 vents in Yellowstone Lake (Cuhel et µS/cm, groundwater in these systems al., 2002). was observed to be several degrees high- Understanding the nature of the er in temperature, with 10-fold higher groundwater emerging in Lake Huron’s conductivity, 10-fold higher concentra- sinkholes requires an introduction 12 Marine Technology Society Journal FIGURE 1 Map of the North American Laurentian Great Lakes Basin showing regions of Silurian-Devonian aquifers potentially having karst formations (left, modified from Grannemann et al., 2000), and regions of above ground karst formations in Alpena County, MI and submerged sinkholes in the Thunder Bay National Marine Sanctuary (TBNMS), Lake Huron (right, after Coleman, 2002; Biddanda et al., 2006). to the major geologic forces shaping collapse in the recent one million year “exposed” at the edges of the Michi- sediments and lake morphometry in geologic history of Michigan’s northern gan Basin by glacial scouring. Water the Michigan Basin. Most noticeable lower peninsula. easily entered the partially developed are collapse features of the past 10,000 The large-scale loss of evaporites paleokarst (fossil karst) systems and years since the glaciers retreated from and collapse of the Detroit River Group enhanced the karst as dissolution of the the area. One of these, the Middle Is- and formations above it were caused evaporites continued. As the evaporite land sinkhole/resurgence, is the surface by water flow. Surface and ground volumes were diminished, the weight expression of a collapse pipe extending waters flowed into hydraulically open of overlying formations collapsed the down through the Rogers City and tectonic faults to deeply buried layers of group and caused structural adjust- Dundee limestone formations into the the group. Surface and ground waters ment faulting in the formations above, top of the Detroit River Group karst also penetrated the group where it is allowing more water to penetrate. For- system, approximately 200 feet below FIGURE 2 (Figure 2). Some inland sinkholes have collapsed through up to 800 feet of Schematic diagram of water sources and flow along subsurface pathways to discharge into Lake additional overlying formations not Huron. Inland recharge areas exert water pressure on aquifers, forcing water through the karst system to Lake Huron. present at Middle Island (Black, 1983). Gypsum, anhydrite, and salts in the Detroit River Group formation were deposited in the Michigan Basin in the early Middle Devonian Period, ap- proximately 400 million ybp (Gardner, 1974). Toward the end of this deposi- tion there was a brief uplift and disso- lution of some of the evaporites. Basin subsidence and new sediments buried and “fossilized” the karst features that had developed. This originally shallow and minor karst development would set the stage for large-scale dissolution and Winter 2008/2009 Volume 42, Number 4 13 mations were weaker and were more sinkhole (45° 11.911’ N, 83° 19.662’ perature averaging approximately 5 °C. likely to develop sinkholes where faults W; Figure 1) is located approximately Visibility underwater varied depending crossed each other. 2 miles offshore at a depth of 23 m on the season, ranging from only 6 Erosion had exposed the edges of in NW Lake Huron. In the present meters at times during the mid-to-late formations before the last glacial period. study, a variety of methods including summer, to 20 meters or better in the Glaciation scoured the exposed surface physico-chemical mapping, time-series spring and early summer. Diver tasks of collapsed formations, sometimes fill- measurements, diver observations and included mapping of the sinkhole ing in and plugging already developed bathymetric mapping were employed “alcove” (a deep hole that fills with sinkholes. As the glacier retreated, to obtain a preliminary understanding groundwater and spills over a sill into it left behind a cover of diamictite of sinkhole features and to observe the wider sinkhole area) and “arena” (glacial till) from zero to 700 feet that some of the physical aspects of the sink- (the wider bowl-shaped sinkhole area mantled bedrock and sinkholes, it left hole system’s groundwater interaction that opens out to Lake Huron) areas, water available to surface streams and with Lake Huron. deploying light scientific equipment, groundwater, and formations stressed sample collection, and collection of still by the removal of the glacial weight and and video imagery (Figures 3, 4 and 5). crustal rebound. Some of the stress was Survey Procedures Plan and profile views (Figure 5) of the relieved by collapse of windows into (Methods) sinkhole alcove were produced by diver the karst system. If these windows were The project utilized divers from the observation and cursory measurements above the static water level (controlled NOAA TBNMS, NOAA Maritime utilizing hand-held tape measures, a by the Great Lakes levels of the time), Heritage Program, Noble Odyssey dive computer recording depth of wa- water would enter the system at that Foundation (NOF), State of Michigan, ter, and a compass to orient magnetic point (Figure 2). If the window was and East Carolina University. Their north. Observations were recorded below the static level, water was able reports and drawings provided observa- on Mylar attached to a submersible to exit the system under hydrostatic tions and measurements unobtainable plastic slate. pressure into the Great Lakes system, from imagery, surface mounted acous- Bathymetric survey operations, bringing with it the dissolved minerals tics or moored instrumentation. Dives time-series mooring deployments, and and nutrients accumulated during its were conducted from the TBNMS physical and chemical mapping were transit through the formations. The 41-foot R/V Huron Explorer, 28-foot conducted from the NOF’s 28-foot latter formations resulted in the sub- R/V Sweetwater, and the NOF R/V R/V Sounder, NOAA’s R/V Huron merged sinkhole systems that are the Pride of Michigan. Trained chiefly in Explorer and smaller vessels. The NOF subject of our present study. underwater archaeology, the dive team bathymetric mapping system con- Sinkholes have been the source of adapted readily to the range of skills sisted of an Innerspace 455 single-beam scientific curiosity for many years in required for this multi-disciplinary echosounder with a 200 kHz 3° stain- the