Distribution, growth rates, and diagenesis of coralline algal structures on the Flower Garden Banks, northwestern Gulf of Mexico

Abstract

The East and West Flower Garden Banks are located in the northwestern Gulf of Mexico, approximately 190 km SSE of Galveston, Texas. These bathymetric highs are surface expressions of salt domes, and are two of the many offshore banks that have been extensively examined by Texas A&M University oceanographers during the past ten years. Although the crests of the East and West Flower Gardens are capped by living coral reefs with 18 species of hermatypic corals (20-50 m depths), crustose coralline algae are the primary framework builders and sediment contributors below 50 m. A depth zonation spanning 20-90 meters has been established on the banks and slopes using eleven genera of corallines (Lithothamnium, Mesophyllum, Melobesia, Archaeolithothamnium, Lithophyllum, Tenarea, Hydrolithon, Porolithon, Paragoniolithon, Lithoporella, and Fosliella), one squamariacean (Peyssonnelia spp.), and an encrusting foraminifer (Gypsina plana). On the upper coral reefs, the most common coralline algae genera are Hydrolithon, Lithoporella, Lithophyllum, Paragoniolithon, and Porolithon. On the flanks of the East Flower Gardens, coralline algae have stabilized ridges of coral debris at 30-35 m depths. These Madracis-algal ridges also support a lush growth of leafy brown algae owing to the absence of intensive grazing, which is typical of the main coral reefs. Between depths of 50-75 m, vast fields of coralline algal nodules, ranging in size from 1-20 cm, cover 60-90% of the bottom. There is a general increase in nodule size with depth. The algal nodule zone can be roughly divided into upper and lower units on the basis of coralline taxonomic composition and the internal structural morphology of the nodules. Nodules decrease in abundance below 75-80 m, and laterally extensive coralline algal pavements and rigid, 1-2 m high algal reefs become the dominant structures. Lithothamnium, Tenarea, and Peyssonnelia are the primary framework builders between 75-90 m. Although constructional void space in the algal reefs is high, most of these cavities have been occluded by micritic high-Mg calcite, the most common cement in algal reefs and nodules. Aragonite cement is also common, especially as spherulites and botryoidal aggregates in coral-line conceptacle voids. The internal morphology and taxonomic composition distinguishes these deep water algal reefs from shallow water algal ridges and cup reefs described by previous authors. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI