From PaleoValleys to Drowned Reefs: Shelf Morphology as a Paleo Sea Level Indicator

Submerged or drowned reefs constitute an important geological record of sea level variations, forming the substrate for the colonization of modern benthic mesophotic communities. Paleovalleys are also an important indicator of sea level changes. Here we present the result of several acoustic surveys that mapped mesophotic reefs and paleovalleys along the Eastern Brazilian Continental Margin. Seabed mapping was carried out using multibeam and side scan sonars, but available bathymetric datasets were also compiled. High resolution seismic sources were used to define the seismic stratigraphy. Ground truthing was obtained using drop camera or scuba diving and grab sampling. Mesophotic reefs were mapped in water depths varying from 30 to 100m and under distinct oceanographic conditions, especially in terms of river load input and shelf width. Reefs showed distinct morphologies, from low relief banks and bioconstructions associated with paleovalleys to shelf edge ridges. Extensive occurrence of low relief banks were mapped along the most important coralline complex province in the South Atlantic, the Abrolhos Shelf. These 30 to 40m deep banks have no more than 3 meters in height and may represent fringing reefs formed during sea level stabilization. Paleovalleys mapped along the eastern margin showed the occurrence of coralgal ledges along the channel margins. Paleovalleys are usually deeper than 45m and are associated with outer shelf rhodolith beds. Shelf edge ridges (80 to 120m deep) were mapped along both margins and are related to red algal encrusting irregular surfaces that have more than 3m in height, forming a rigid substrate for coral growth. Along the Equatorial Margin, off the Amazon mouth, shelf edge patch reefs and rhodolith beds forming encrusting surfaces and shelf edge ridges were mapped in water depths greater than 100m. Thus, the occurrence of mesophotic reefs along the Brazilian Margin is influenced by transgressive morphological features, which could be used as a surrogate for mesophotic reef distribution. The extensive occurrence of rhodolith beds on the outer shelf characterizes most of these reefs.