Foraminiferal communities of a mid-Holocene reef: Isla Colón, Caribbean Panama

Abstract

The distribution of mid-Holocene, tropical benthic foraminiferal assemblages, including species' proportions, diversity, dominance and wall type, were analyzed for their correspondence to marine habitats and invertebrate facies. Benthic foraminifera are useful for paleoenvironmental reconstructions because the modern ecology of many species found as Quaternary fossils is known. Samples were collected from trenches in ~30,000 m2 of an excavated Acropora cervicornis-dominated, mid-Holocene reef with an age of ~6 kyr on Isla Colón (Colon island), bordering Almirante Bay in Bocas del Toro, Caribbean Panama. Bulk sediment samples were collected from a maximum depth of ~7 m below modern mean sea level and classified in the field into five invertebrate biofacies: 1) A. cervicornis-dominated reef, 2) molluscan mud, 3) Porites-Agaricia reef, 4) mixed coral, and 5) Lobatus-dominated seagrass. Sediment carbon and grain size analyses, a cluster analysis performed on the relative abundance of species per sample, and the Fisher's alpha diversity index were used to compare sample similarity and environmental variables to determine habitat relationships. Most samples contained high total inorganic carbon and poorly sorted, medium-coarse sediments. Principle component analysis of sediment grain size and carbon values did not show a clear correspondence among samples, habitat type or location of trenches. Foraminiferal assemblages in A. cervicornis and other reefal samples (categories 1, 3 and 4, above) had the greatest diversity and did not distinguish between the three reefal types, suggesting similar, normal marine conditions and/or mixing of coral fragments. Molluscan mud samples with high total organic carbon content were least diverse with dominant Ammonia and Elphidium taxa, though foraminifera in other molluscan mud samples showed a transition to proximal reef corals. Seagrass samples were differentiated from molluscan mud samples and had similar diversities and species assemblages to the Porites-Agaricia samples, and taxa known to be temporary grazers and living temporarily attached to seagrass blades are present in relatively greater amounts in these samples.Based on the distribution of foraminiferal species across this mid-Holocene coral reef, we conclude that it was a patch reef that included high-organic and seagrass facies similar to those of modern Almirante Bay. Results from this study can be compared to modern foraminiferal studies to investigate whether the modern habitats of BDT are significantly different from the pristine reefs of the mid-Holocene.