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

Abstract

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 +\u20092.3‰ and +\u20093.2‰, and δ18O between −\u20090.8‰ and −\u20090.6‰) and this is consistent with similar modern settings. A lack of very fine particles and an abundance of intraclasts suggest 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.