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Modern Facies Change: The Replacement of Corals by Crustose Coralline Algae and Rhodoids in Southwestern Puerto Rico Reefs

Abstract

Ongoing rapid environmental changes offer a unique opportunity to witness biological and geological changes at a global scale within the time span of less than half a century. This study integrates diverse field and laboratory quantitative data with the specific objective to investigate the controls of climate and ocean chemistry on the recent genesis and distribution of rhodoids and crustose coralline algae in shallow environments. This modern environmental change is present in reefs located in Media Luna, Margarita, Laurel, San Cristobal, and Playa Buye in southwestern Puerto Rico. Rhodoids and crustose coralline algae in these locations are colonizing shallow back reef and reef flat environments and are expanding their habitat from the mesophotic zone (>50 m). Previously, modern extensive rhodolith beds have been restricted to deeper reef communities within 50 meters of water depth, such as the Agelas Reef at Isla de Desecheo, Puerto Rico and the world's largest continuous rhodolith bed ~21,000 km located in eastern Brazil's Abrolhos Bank. Increasing the understanding of modern shallow water systems containing crustose coralline red algae and rhodoids offers the ability to establish quantitative and qualitative criteria to predict the environmental conditions needed to produce important hydrocarbon reservoir rocks, such as those found in the Sirte Basin of Libya and the Salawati Basin of Indonesia. Thus, we hypothesize that four main factors are driving transitional change from coral dominated to red algal dominated systems, including: lowering of pH, warmer shallow water temperature, periodic anomalously high temperature episodes resulting in coral bleaching, and destructive storm events. This project is coupling field and laboratory studies of rhodoids to better understand the processes behind their formation by integrating high-resolution geochemical analysis, biomarker analysis, petrographic and CT scan analysis, and assessment of environmental conditions. Results will offer an increased understanding of carbonate systems containing crustose coralline algae and rhodoids in the geologic past. In particular, this research will provide insight into why rhodoids are dominant in shallow water systems during certain periods of time, such as the Late Carboniferous and Permian.