A Modern Analog Model for Ancient Shallow-Water Tropical Carbonate Systems Developed Under Adverse Photic Zone Conditions: Puerto Rico

Current carbonate models applied to the rock record are heavily influenced by studies of modern tropical photozoan reef systems (e.g., the Bahamas) that develop in clear, warm water photic-zone conditions. However, tropical shallow-water carbonate systems also develop in areas with adverse photic-zone conditions (typically due to excess nutrients, turbidity, cooler water) and are increasingly being recognized in the rock record. These systems are commonly characterized by abundant heterozoans and some photozoan components that can tolerate adverse conditions. We are utilizing an extensive dataset from previous studies of modern shallow-marine (<35 m deep) environments surrounding Puerto Rico. The data highlight the variable distributions of photozoan and heterozoan biotic and non-biotic components as a result of natural and human-induced variations in nutrients, turbidity, water chemistry, energy, currents, and water temperature. For example, high-resolution remote sensing data show the influence of land runoff, fluvial sediment, and nutrients on distribution of nearshore coral reef environments. Well-developed Acropora palmata reefs and high coral cover occur away from rivers. In contrast, areas near the mouths of rivers, especially on the north, southwest and west coasts, are characterized by low coral cover, and increased density of macroalgae (e.g., Ramicrusta) and sponges (e.g., Cliona). The corals are dominated by Porites astreoides and Siderastrea, which are known to tolerate turbid conditions. This ongoing study will document in detail the conditions that affect distribution of heterozoan and photozoan facies components around Puerto Rico to provide a modern analog model that can be applied to similar systems in the rock record.