Production and off-bank transport of carbonate sediment, Black Rock, southwest Little Bahama Bank

Abstract

Surficial sediments between the intertidal zone and 600 m were studied with the objectives of characterizing their source, dispersal pathways and sites of accumulation. Bioerosion is a major source of sediment in the intertidal and shallow subtidal environments. In-situ enclosure experiments showed that the urchin Echinometra lucunter produced 667 mg sediment cm −2 yr −1, and an associated diverse rock infauna produced an additional 183 mg cm −2 yr −1. Microborers caused erosion of 25 mg cm −2 yr −1, and feeding activity by the chiton Acanthopleura granulata caused additional erosion of 10 mg cm −2 yr −1. Sequential weighing of tethered limestone plates showed an abrasion rate of 5.5 mg cm −2 yr −1, due chiefly to impacts on the plate edges. Surficial sediments between the intertidal zone and the top of the bank-margin wall at 30 m are gravel-sand mixtures containing 0–2% mud. Molluscs and Halimeda plates are the main constituents of gravel fractions. Sand fractions are composed of bioerosion-produced carbonate rock fragments (1–70%), skeletal material from Halimeda (3–63%), forams (1–33%), molluscs (6–15%) and coral (0–6%). Gravel and sand are transported as bedload in chutes across the narrow, rocky shelf. Chutes end at 45 m, and sediment traps indicate an average flux of 5.4 kg chute −1 yr −1, equivalent to 945 kg km −1 yr −1, moving to deep water. A dense population of Halimeda living on the wall, between 30–70 m, contributes gravel and sand to sediment on wall ledges. At the base of the steep wall (65°), a ramp slopes into the Northwest Providence Channel. Between 120–300 m, the ramp (30° slope) is a zone by-passed by coarse and fine sediment. At 300 m, the slope decreases to an average of 13°, and an apron-shaped body of mud is accumulating. The apron extends parallel to the bank margin and wall, and reaches depths > 600 m. Traps showed that sediment flux to the ramp is 0.12 kg m −2 yr −1, which is 2% of the particle flux overtopping the bank margin. Surficial sediment of the ramp is mostly mud (> 70%) which is primarily of shallow-water origin. Some form of bottom current transporting gravel and sand (mostly Halimeda and abraded coral heads) has cut gullies into the ramp. This bottom flow is probably episodic, and turbulent enough to suspend and supply Halimeda sand to sediment traps on the ramp at 463 m.