Gregor P. Eberli, PhD , University of Miami
A quantitative analysis was conducted to assess, for the first time, the resolution necessary for mapping cold-water coral habitats. Comparing AUV and ship-borne multi-beam datasets collected from the same site at 3 m resolution (AUV data), as well as 20 and 50 m resolution (ship-borne data) shows that only the 3 m resolution AUV-generated map accurately depicts the complete mound size spectrum, as well as fine-scale details of mound morphology. 844 mounds are detected based on the 3 m resolution map, whereas only 10 mounds are detected using the 50 m resolution map. Thus most bathymetric surveys from coral mound fields using ship-borne multi-beam systems probably greatly underestimate the abundance of individual mounds and mound complexity.
Individual mounds are the dominant features in the three sites surveyed at the Great Bahama Bank (GBB) slope. An average of 14 mounds per km2 is detected at these sites. This estimate is significantly higher than reports from the nearby Florida-Hatteras slope, where approximately 0.3 mounds per km2 are identified. Morphometric analyses show that mounds vary significantly in height (1-83 m), area (81-600,000 m2), shape (mound aspect ratio 0.1-1), and orientation (mound long axis 0-180°) within and among the three sites. Mounds at the GBB slope are not lithified but instead consist of a coral framework filled with unconsolidated sediments. Together, the morphometric and sedimentologic analyses from the GBB sites contradict the paradigm that mound fields in the Straits of Florida are uniform, with most mounds being cemented, teardrop-shaped, and aligned northward.
Some cold-water coral habitats in the Straits of Florida are so distinctive that they cannot be called mounds. These features include kilometre long, low-relief ridges of 20 m height that cover ~18 km2 of the base of the Miami Terrace (Site 5) and low-relief set of ridges that are arranged in a chevron shape that cover ~70% (8.5 of 12.2 km2) of the mapped area in the centre of the Straits. Discrete cold-water coral mounds are not observed in Sites 4 or 5, however.
The seismic data show that 20 m-high ridges at the Miami Terrace are regularly spaced, asymmetrical in profile, and oriented perpendicular to the current. They have a waveform morphology that resembles the sediment dunes. But the seismic facies, depositional profiles, and sedimentary regimes characterizing the coral ridges clearly exclude these ridges as sediment dunes that are overgrown by corals. These coral ridges are entirely biogenic in origin and that they accrete vertically, independent of their underlying topography. The entire ridge succession is formed through the baffling of suspended particles into asymmetrical coral frameworks.
A similar mechanism for the development of coral habitats occurs at the center of the Straits, where corals form knobs that coalesce into several sets of ridges. These ridges have teardrop- and chevron-like patterns that are aligned in various directions with respect to the bottom current regime. Although the underlying topography is poorly resolved for this site, observed spatial and backscatter distributions of coral facies indicate that the ridges at the center of the Straits are also bio-constructional in nature.
Abiotic factors, such as hydrodynamics, sedimentation and antecedent topography, vary significantly among and within the five mapped sites. For example, in a < 50 km E-W transect across the Straits, current regime changes from tidal motions at the slope of GBB (Site 1-3) to a northward current in the center of Straits (Site 4), to a countercurrent at the Miami Terrace (Site 5). Sedimentation also varies across the Straits with the highest deposition rates occurring at sites on the GBB slope. At the GBB, slope architecture is shaped by gravity mass flow deposits and slump scars. High sedimentation rates documented at Sites 1-3 partially explain the unlithified mound surfaces observed in this area. The fine sediments that is shed off of the platform to the GBB slope likely contribute to the mound construction as it is trapped within the coral framework and at the same time prevent early cementation. Away from the GBB mound fields, fine sediment deposits give way to winnowed sands in Site 4 and to a sediment-starved condition in the eastern part of Site 5. At Site 5, coral ridges sit atop a mid-Miocene unconformity, which outcrops to the surface in some areas. Like coral features at the GBB slope, the coral ridges away from the GBB are not cemented, indicating that lithoherms off Little Bahama Bank are the exception and not the rule in the Straits of Florida. Taken together, differences in environmental conditions as well as variability in slope architectures control the spatial variability in coral habitats among the slope of GBB, the center of the Straits, and the Miami Terrace.