Lithoherms on the Florida–Hatteras slope

Abstract

Sections of the Florida–Hatteras Slope were investigated during dives of the Johnson Sea-Link I submersible and US Navy's submarine NR-1. Numerous calcareous coral-capped lithoherms, often exceeding 40 m in elevation, were encountered in the subphotic depths below 440 m. One huge lithoherm complex was explored that forms a long, slope-parallel ridge perched on the seaward edge of mid-slope terrace. This lithoherm belt is ∼400 m wide, more than 4.4 km long and ∼150 m high on its down slope flank. Direct observations and side-scan imaging indicate that the long axes of individual ridge crest mounds trend perpendicular to the regional current flow. Away from the ridge individual lithoherms trend more parallel with regional current flow and occur without obvious bedrock control. All of these structures have very steeply sloping flanks (frequently 30–60°) where cemented crusts are exposed by erosion. Thickets of living ahermatypic coral coat the crest and up-stream flanks of these topographic features. Living deep-water coral have modern 14C ages (700±80 year bp) and trap older (3250±100 year bp) sediment rich in fine-grained aragonite. Elsewhere, older deep-water coral rubble (20,230±230 year bp) traps younger sediment (13,760±140 year bp). The trapped sediments contain >50% aragonite fines (<62.5 μm). The friable crusts exposed on these flanks are generally similar in composition to the sediments that accumulate on the crests of these structures, they are older (14C ages of 17,770±330 and 32,710±570 year bp) and contain less aragonite (∼20%) and more high-magnesium calcite. Apparently, lithoherms grow upwards by trapping fine sediments within ahermatypic coral thickets on their crests and upstream flanks. Cemented crusts may be a consequence of earlier paleoceanographic conditions or a result of gradual and ongoing cementation and diagenesis. The large size, platform edge position, and linear trend of this deep-water lithoherm complex are strikingly similar to a shallow marginal reef system.