Styles and rates of mesophotic reef accretion on a Caribbean insular slope

Structural framework is fundamental to the concept of a coral-algal reef. However, our current understanding of reef framework is based largely upon studies of relatively shallow systems that formed at water depths of less than 30 m. There is comparatively limited information on deeper, mesophotic reef frameworks. Short drill cores collected at depths of \textasciitilde 50 m on the steep insular slope of southwest Puerto Rico allow for documentation of the composition and accretion rates of mesophotic reef framework. Framework is dominated by platy agariciid corals (Agaricia spp.) and autochthonous micrite (automicrite) with calcareous algae, corallines and peyssonnelids, forming a secondary component. Conspicuously, well-lithified automicrite constitutes a major structural and volumetric component of the framework. It is composed of magnesian calcite and displays irregular clotted, peloidal and fenestral fabrics, consistent with in situ formation. Radiocarbon ages of corals and automicrite indicate that they have formed recently (< 4 ka) and coevally in their current setting over a period of relative environmental stability. The steep slope gradient and associated dynamic oceanographic conditions may allow for extensive automicrite formation. Mesophotic reef accretion is occurring on the steep slope, though at slow rates of \textasciitilde 0.3 m ka−1.