BLUE HOLES

Executive summary by darmansjah

1,000 blue caves can be found along the Bahamian shoreline.

THE BAHAMAS Inland caves flooded by the sea, called blue holes, are unlike any other environment on Earth. Reduced tidal flow results in a sharp stratification of water. A thin lens of fresh water-supplied by rain-tops a denser layer of salt water. The fresh water isolates the salt water from atmospheric oxygen.

Of the more than a thousand blue holes believed to be in the Bahamas, few than 20 percent have been probed. But the few explorers who have ventured there have brought back data that may deepen our understanding of geology, water chemistry, biology, and even astrobiology. By studying bacteria that thrive in these anoxic waters, scientists can postulate about distant oxygen-free planets and moons.

Millennia in the Making

Divers swim up into the Crystal Palace section of Dan’s Cave, a blue hole in Abaco, the Bahamas. The delicate mineral formations in this and other blue holes are thousands of years old.

Indigo Invitation

Dean’s on Long Island, Bahamas, is the deepest known blue hole, dropping 663 feet (202 m). the holes’ azure waters lure divers, such as those exploring North Passage of Stargate, on Andros Island.

Explained by Science

BLUE holes of the Bahamas SAWMILL SINK a blue hole is a flooded cavern with an eye to the sky, a sinkhole with a twist its opening, created by a cave-in, leads to a deep void and side passages, filled with seawater. Conditions in this inland blue hole on Abaco island make it ideal for reconstructing the ancient natural history of the Bahamas and can even mirror life on the planet billions of years ago. The cave-in that opened Sawmill Sink as early as 120,000 years ago filled it with a cone of limestone devris.

Water Chemistry AN Inland blue hole’s water is  very still and highly stratified. A lens of fresh water, from rainfall, floats on the denser salt water and isolates it from oxygen in the atmosphere. Brightly colored bacteria thrive where the layers meet. They need light but can’t tolerate oxygen. Other bacteria here produce hydrogen sulfide, which the colored bacteria consume.

Sawmills’s two side passages, each about 2,000 feet (610 m) long, descend as deep as 180 feet (55 m). stalagmites and stalactites grow only when sea level is too low to flood the caves. Some formations merge into massive columns.

Climate Clues  Sawed lengthwise to reveal its core, a blue hole stalagmite, 14,5 inches (36,8 cm) tall, holds 36,000 years of climate history. Growing drop by drop an rainwater leaches calcite form limestone, a stalagmite becomes a climate time line. Colors may reflect the rate of formation. Chemical analysis show high levels of iron at five intervals, evidence of dust blown from the Sahara. Their dates match episodes of rapid climate change (from drier to wetter in the Bahamas) previously detected in ice cores and ocean floor sediments.