Evaluating Sediment Dynamics and Biotic Influences in the Shallow Lagoons of San Andrés Island, Colombia

Session: 37th Annual Undergraduate Research Exhibition Sponsored by Sigma Gamma Epsilon (Posters)

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Abstract:

The igneous basement of San Andrés Island, Colombia, is unconformably overlain by Miocene and Pleistocene shallow-marine limestones, which in turn are rimmed by modern shallow-marine carbonates. Despite its geological and ecological significance, limited data exist on the island’s marine habitats, including those associated with the shallow lagoon and fringing reef system along its eastern margin. This study focuses on Mateo Beach, a ~1.5 km long and up to ~200 m wide, very shallow-water (<1.3 m) lagoon in the southwest part of the island. The goal of the study was to identify and map lagoon-floor habitats and evaluate the major contributors to carbonate sand. Bulk sediment samples (n=17) were collected along six shoreline-to-reef transects and analyzed for grain size and type, carbonate polymorphs, and stable isotopes (C and O). Four distinct lagoon-floor habitats were identified, including rocky sea floor, seagrass beds with skeletal sands and coral rubble, coral rubble with mixed algal cover, and coral rubble with brown algal cover. All habitats are dominated by high-Mg calcite and aragonite, with minor amounts of low-Mg calcite, the latter mostly due to the presence of intraclasts derived from older limestones. The coral rubble with mixed algal cover contains the coarsest sediment (gravelly coarse sand), while the coral rubble with brown algal cover represents the highest-energy habitat near the reef crest, with poorly to moderately sorted sandy gravel accumulating as interreefal sediment.  The best-sorted grains are represented by moderately to well-sorted extraclasts, the abundance of which declines away from the shore. The major carbonate sand producers include corals, mollusks, and Halimeda, followed by echinoids and benthic foraminifera, whereas red algae are less abundant. The study underscores the dynamic relationship among benthic habitat types, biological productivity, and sediment characteristics in low-energy tropical lagoons, and provides a valuable reference for interpreting similar facies in the geologic record.