INFLUENCES ON CARBONATE GEOCHEMISTRY AND MINERALOGY IN COASTAL LAGOONS, NORTHERN JAMAICA

Lagoons along the northern coast of Jamaica are in many aspects similar to the “classical” Bahamian carbonate-producing windward lagoons but are unique in their sediment composition with varied amounts of older carbonate and siliciclastic particles. Contemporary sedimentation of the autochthonous modern carbonates and the allochthonous carbonates and siliciclastics derived from the island interior by the rivers, wind, or via artificial beach nourishment, results in syndepositional mixing. This study focuses on the Little Laughlands Bay, a narrow (max. 400m) and shallow lagoon (mostly <1.5 m) that is protected by a narrow (<50 m) barrier reef and has one major perennial and one smaller ephemeral freshwater inflow. Here, the analysis of the stable-isotope composition, major- and minor elements, and CaCO₃ polymorphs is used to assess the mechanisms responsible for their distribution within the lagoon-floor sediments. Bulk sediment samples were collected along 11 shoreline-to-reef transects and analyzed (granulometry, petrography, X-ray diffraction, X-ray fluorescence, and carbon-isotope analysis).

The diversity in grain producers, coupled with varying depth, hydrodynamics, and proximity to the stream mouths results in a highly diversified environment with several distinct habitats. Low-Mg calcite predominates in river washes, and its abundance gradually drops away from the river mouths. High-magnesium calcite and aragonite predominate in reef flat, seagrass meadow, sparse seagrass with sand patches, and sandy skeletal shoal habitats. The stable isotopic composition of bulk sediment samples gradually increases from the beach towards the reef crest where it reaches values comparable to similar modern settings. Sr/Ca ratio also gradually increases, whereas SiO₂ decreases towards the reef crest; the latter is also abundant in the deep channel that traverses the reef. The geochemical and mineralogical composition of the in-situ carbonates is reflective of the influence of the diagenetically modified older limestones from the island interior, as well as siliciclastics from the beach. The results emphasize the importance of marine and freshwater hydrodynamics in governing the habitat distribution and sediment composition in a mixed carbonate-siliciclastic shallow photic zone.