SKELETAL CARBONATE SEDIMENT DISTRIBUTION PATTERNS IN ROÁTAN ISLAND LAGOONS, HONDURAS

Skeletal sands composed of fragments of calcareous red and green algae, corals, mollusks, and foraminifera, predominate in moderate- and high-energy shallow-water lagoons of the Caribbean. This study focuses on the modern carbonate-sediment distribution patterns in shallow lagoons along the northern coast of Roátan Island, Honduras, which is an excellent location to investigate the relationship between the carbonate sand composition and the marine environment. A total of 47 sediment samples, coupled with data on depth, habitat, and biota were collected from five different lagoons. Detailed beach-to-reef crest transects were studied in Sandy Bay and West End Beach; transects were 140m to 350m long, with depth measurements taken every 10m and sediment samples collected every 50m. A set of sieves was used to analyze the grain size distribution from each sample (>2mm, 1-2mm, 0.5-1mm, 0.125-0.5mm, >0.125mm). Petrographic analysis of 39 collected sediment samples was carried out on thin-section grain-mounts by counting 300 data points per slide. Powdered samples were also analyzed for their mineralogical composition and stable isotope values.

In Sandy Bay, major components include corals (27%), igneous/ metamorphic lithoclasts (19.6%), red algae (14.1%) and Halimeda (12.9%). Non-carbonate grains sourced from the island bedrock dominate the beach but become negligible 20-30 m into the lagoon. At the higher-energy West End Beach there is no siliciclastic poisoning, and major grains are corals (31.5%), red algae (25.5%), Halimeda (9.6%) and mollusks (8%). In both lagoons, grains are coarser and well-sorted along the beach and on the reef flat; the deepest parts of lagoons have finer grains that are poorly sorted. Carbon isotope values (δ13C) range from +0.7 to +4.1 ‰VPDB) and oxygen isotopes (δ18O) from −2.3 to −1.0 ‰VPDB. The isotope values exhibit strong positive correlation (R = +0.80). The results provide new insights into the relationship between carbonate sand composition, and energy levels (depth).