CARBONATE STORM DEPOSITS AND VARVE-LIKE SEDIMENTS WITHIN THE GREAT BLUE HOLE ON LIGHTHOUSE REEF ATOLL, BELIZE, CENTRAL AMERICA

Vibracores taken within the oceanic blue hole located on Lighthouse Reef atoll, Belize, provide a continuous record of carbonate sedimentation. The blue hole is rimmed by coral reef with the exception of two shallow (<6m) inlets and is approximately 300m in diameter with vertical walls reaching depths greater than 120m. At depths between 30-40m, an overhang with stalagtites and a series of caves represents a time when this area was subaerially exposed (~18 ka).

The cores were described and sampled to obtain carbon and oxygen isotopes, radiocarbon ages and trace element geochemistry data. The sediments are organized into distinct packages of finely laminated alternating light and dark colored muds and coarser storm deposits of varying thicknesses. Water in the blue hole lacks circulation, becomes anoxic at ~95m, and allows deposition of finely laminated mud that resembles deposition in a lacustrine environment. It is possible that the sediments in the blue hole are varved, punctuated only by events such as storms or minor debris flows. Trace element analyses record different signatures for the laminated mud and storm deposits. Some elements switch correlation patterns in the upper portion of the longest core denoting a change in the chemical nature of younger versus older sediments. The shift from negative to positive correlation coefficients may be the result of changes in atmospheric controls, source material, water chemistry, diagenetic alteration or anthropogenic influence.

Radiocarbon ages and differences in sedimentary packages in cores taken from the perimeter toward the center of the hole, suggest either a slightly dipping floor or mounds of deposition close to the walls, with the highest topography closer to the inlets of the blue hole. As the perimeter of the blue hole receives coarser sediment more frequently, the center of the blue hole is characterized primarily by laminated mud with finer grained storm detritus layers. With geochronological control, these sediments may provide insight to storm frequency and intensity in the western Caribbean for the past 2700 years.