On the central and western side of the island of South Andros in the Bahamas, one can find a series of cavern systems formed in the limestone rock platforms that comprise most of the islands. These submerged cave systems are commonly referred to as Blue or Black Holes. Blue Holes are horizontal caves, often with lateral passages that connect to the sea; Black Holes are vertical caves with no known lateral passages. Blue Holes appear blue due to the reflection of blue sky from the water surface, and the white carbonate sand deposits on the cave floor and walls. Conversely, Black Holes appear black in color, due to a 1 m thick microbial layer located at a depth of about 18 m.
On any given day, you might run into Dr. Stephanie Schwabe, Department of Earth and Environmental Sciences at the University of Kentucky, exploring these unique cavern systems. Dr. Schwabe also serves as Director of the Rob Palmer Blue Holes Foundation, a non-profit created to further the scientific examination of these caves for exploration, research, and education. In her role as Director, Schwabe leads groups of students on teaching expeditions to Andros Island, in addition to her own scientific studies on the formation and hydrology of the Blue and Black Holes.
How Holes are Formed
One focus of Schwabe’s research in the Bahamas has been to try to determine how the Black Holes are formed. Black Holes are unique, in that they develop from the top down, and generally have no direct connection to the sea. They also exhibit a unique vertical morphology; the upper third of the water column (18-19 m) consists of oxygen rich, low salinity, brackish water. A denser, anoxic saline layer below is separated by a 1 m thick microbial layer. Little exchange takes place between the two layers. A dominant feature of the microbial layer is the presence of anaerobic phototropic bacteria, including purple sulfur bacteria. Dr. Schwabe discovered a species of purple sulfur bacteria, Allocromatium palmerii (named after Rob Palmer, for whom the Foundation is named). Temperatures are much warmer (to 41 °C in some holes) below the microbial layer. Dr. Schwabe postulated that the phototropic purple bacteria have relatively low efficiency in converting light energy at their photosynthetic reaction centers, thus dissipating the unused light energy as heat. The highly reactive chemical environment of this microbial layer rapidly dissolves the limestone, resulting in lateral widening, rather than vertical deepening, of the holes giving them their typical round, top-down morphology.
To measure light and chemical profiles in the Black Hole water columns, Dr. Schwabe used a Datasonde Hydrolab (Hach Hydromet, Loveland, CO) fitted with a LI-COR LI-193 Underwater Spherical Quantum Sensor. Blooms of purple sulfur bacteria usually develop at shallow depths (5-10 m) where light intensities are high. In the South Andros Black Holes, however, the water columns have a higher transparency to visible light wavelengths, allowing the light to penetrate to greater depths where it can be utilized by the bacteria. Dr. Schwabe used the LI-193 to make profile measurements to determine the amount of light penetrating the water column, and to see if light was making it through the microbial layer. Dr. Schwabe discovered that although the water below the microbial layer appeared devoid of light, a small amount of light was actually penetrating the layer. The LI-193 measures photon flux from all directions in the 400-700 nm range, which is ideal for this study, as the maximal absorption range of the purple sulfur bacteria is between 480 and 550 nm. This measurement is referred to as Photosynthetic Photon Flux Density (PPFD), and is important for studying phytoplankton, which utilize radiation from all directions for photosynthesis.
In the near future, Dr. Schwabe hopes to lower the Hydrolab from a helicopter into holes that are currently inaccessible due to difficult terrain, or because they are too small to dive. This will allow Dr. Schwabe to make many depth profiles to build a database of the Blue and Black holes located throughout the islands. These studies will ultimately help establish the geological timescale over which these cave systems form, and provide additional information about underground water resources, pollution control, and hydrological relationships with limestone formations.
More information about the Rob Palmer Blue Holes Foundation can be found at www.blueholes.org.
Thanks to Dr. Schwabe for photos and personal correspondence.