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Degrading windward patch reefs and processes influencing composition, mineralogy and stable- isotope record of peri-reefal sediment, San Salvador Island, Bahamas

Patch reefs and their surrounding skeletal- peloidal-intraclast grainstone facies are very abundant on modern (sub-)tropical carbonate platforms. Compared to the barrier and fringing reef facies, the processes influencing composition, mineralogy and stable-isotope record of peri-reefal sediment have received much less attention, despite a potentially significant volume of such sediment in many Phanerozoic carbonate platforms. To better understand the evolution and sedimentary record of patch reef settings, this study investigates the patch reef biota (scuba diving surveys) and composition (petrography), mineralogy (X-ray diffraction) and stable isotope record (mass spectrometry) of peri- reefal sediments at East Beach on the northeastern, windward margin of San Salvador Island, the Bahamas. The macroalgae-dominated ecology is assessed, and a decadal decline in stony coral coverage with a shift to more opportunistic, stress-tolerant species is documented. The most common stony corals on these shallow-water (3-5 m) patch reefs are Diploria strigosa, Porites astreoides, and P. porites, and Halimeda lacrimosa dominates among green algae. Peri-reefal sediment is pure carbonate sand composed of roughly equal volumes of aragonite and high-Mg calcite (HMC), with mean MgCO3 content in HMC of 15.2%. The bulk-sediment isotope record shows very limited variation (δ13C between +2.3‰ and +3.2‰, and δ18O between –0.8‰ and –0.6‰) and this is consistent with similar modern settings. A lack of very fine particles and an abundance of intraclasts suggests that the environment is strongly influenced by bio- and physical erosion. Fast-growing algae have likely reached an abundance beyond the grazing capacity, which has drastically reduced the space for coral settlement and recruitment, and thus has compromised the patch reef resilience in this part of the Bahamian Archipelago.

coral reef degradation ; patch reefs ; carbonate sediment ; stable isotopes ; San Salvador Island ; Bahamas

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