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The Great Blue Hole of Belize
In 1997, geologist Robert F. Dill and divers from the Cambrian Foundation, an organization of mixed-gas divers (also called tech divers), visited the Great Blue Hole to collect stalactites for isotopic dating and sea-level studies. Two divers descended 407 ft and obtained two short push cores. The 2-ft long cores revealed outstanding sedimentary laminations. There is no oxygen near the bottom, and hydrogen sulfide prevents bottom dwellers from burrowing and disturbing the sediment. Preliminary analyses of the short cores showed fluctuating pollen, spores, mercury, and arsenic levels. Arsenic ranges between 15 and 21 ppm. That's right! ppm as in parts per million. Other events recorded in the short cores included hurricane or large storm layers. The storm layers are light-colored, beautifully laminated, and are distinct from the darker organic and clay-rich sediments representing slow deposition. Could these sediments provide a long-term record of climate, mercury, arsenic, and African dust flux? We think so!
On May 8th, Gene Shinn, Chuck Holmes, Chris Reich, and Don Hickey entered the deep blue hole aboard Winning Ticket, a 55-ft boat out of Miami operated by Captain Barry Denton with Randy Migdalski as First Mate. Winning Ticket traveled to Belize with the coring equipment where the USGS crew boarded for the final run out to the blue hole. Along the way, we picked up student/volunteer Elizabeth Williams who served as observer for Audubon during the sampling. Our purpose was to obtain long cores and hopefully a several-thousand-year record of dust flux. Coring was done with a Rossfelder electro-vibracoring system using 20-ft-long core tubes. We lowered the 300 lbs. of equipment to the bottom on a 500-ft length of rope from the bow of the boat. Cores and equipment were retrieved with the boat's electric anchor windlass and divers removed the cores from the apparatus at the surface. By the third and last day, we had obtained two 18-ft cores. In all, seven cores were recovered. Sound easy? Fieldwork is seldom easy. On this trip, the electric vibracoring equipment failed. But as luck would have it, First Mate Randy's other job is running an electric motor rewinding shop. In a few hours he rewired components and we were back in business. Thanks to Randy, we got the 18 footers. Why this blue hole? The Belize Great Blue Hole is located far from industrial sources of aerosols but is well within the area affected by silica- and clay-bearing African soil dust. African dust is deposited in the Caribbean mainly between June and October. The hole thus serves as a giant sediment trap where the overall sedimentation rate is slow and layering is preserved due to anoxic conditions. A high rate of atmospheric deposition relative to normal shallow-marine sedimentation should make it easier to identify and separate African soil dust from local carbonate mud. If all goes as planned, these cores will provide the centerpiece of a study aimed at determining the geologic history of dust deposition over the past few thousand years. Stay Tuned!
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in this issue:
Global Assessment of Geologically-Sourced Methane
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U. S. Department of the Interior | U.S. Geological Survey
cover story:
