DOCUMENTING THE TIMING AND DEPOSITION OF CORAL RUBBLE RIDGES ON CATALINA ISLAND, DOMINICAN REPUBLIC

In regions of the world where intense tropical storms and hurricanes are common, evidence of their passage is often preserved in sediments and rubble ridges. These deposits can provide crucial information about the magnitude and timing of historic and prehistoric storms, which aids in preparation for future storms. Catalina Island is located in the Caribbean hurricane belt 2 km off the southeastern coast of the Dominican Republic. On the windward side of the island, discontinuous coral rubble ridges line the beach-rock coast. Ridges are predominantly composed of Acropora palmata fragments ranging in size from gravel to boulder (B-axis size 2 - 75 cm). Finer grained sediments are restricted to sheltered areas along the coast. Terrestrial black mold continuously colonizes the surface of the coral clasts, and individual clasts range from white to almost black in color. In this study, we investigate the physical forces necessary to transport the coral blocks, and attempt to differentiate depositional events based on the degree of colonization by the black mold. Here, we develop a new technique for analyzing mold colonization based on the average grayscale hue of each coral fragment. Preliminary analyses of one photo transect across a rubble ridge show that grayscale hues vary from 25-205 on a 255-shade scale. K-means cluster analysis shows that 95% of the variance in hue values can be explained by five clusters, which suggests that individual storms can be distinguished. Analysis of additional transects will test this hypothesis. Two-dimensional cluster analysis shows that while there are small fragments of every grayscale hue, larger coral fragments fall in predictable grayscale clusters. This indicates that storms capable of transporting large coral fragments occur infrequently enough to be distinguished from the continuum of grayscale hues produced by smaller storms. Standard hydrodynamic equations show that to entrain, transport and deposit these large coral clasts, waves of 4.5-5 meters (storm surge + tide + wave setup) are necessary. The last historic hurricane capable of producing waves this size was category 3 Hurricane Georges in 1998, which produced storm surges of approximately 3 meters in Puerto Rico. Storms of similar or larger size affected Catalina Island in 1979 and 1930.