In-Situ Erodibility of Siliciclastic and Calcareous Sediments

Two field experiments, one in 10-m water depth west of Trinity Shoal, off the Louisiana coast and one in 17-m water depth in Southwest Channel, Dry Tortugas, Florida, were conducted in 2014 and 2015, respectively. A diver-operated mini-flume was deployed to measure the erosion of the respective sand/mud siliciclastic and calcareous sediments. In addition to measuring suspended sediments over a 0.16-m² area with an optical backscatter sensor (OBS), the non-recirculating, linear mini-flume measured the fluid velocity exiting the flume using an acoustic Doppler velocimeter (ADV) and a profiling ADV in the case of the calcareous sediments. Time series of the observed turbulent kinetic energy were correlated with time series from the OBS. Fluid velocities from 15 – 56 cm/s (stress levels from 0.02 – 0.32 Pa) were used on the siliciclastic sediments and the sampled effluent was an organically rich mélange of polychaete fecal pellets, marine snow, and very fine sand. Effluent from different stress levels was characterized by examining 60-ml water samples with a light microscope and a Cilas laser particle size analyzer. The calcareous sediment was coarser than the siliciclastic sediment, containing less mud and coarser sand fractions. Despite higher stress levels of 0.10 – 0.56 Pa, less calcareous sediment was eroded and the sampled effluent was primarily weathered fragments of corals, calcareous alga (Halimeda) skeletons, and mollusk shells. Episodic turbulence was detected in mini-flume runs at both study sites, but mobilization of surface sediment was more highly correlated with turbulence at the siliciclastic site. The calcareous site at the Dry Tortugas was subject to stronger tidal currents than the siliciclastic site near Trinity Shoal, which may advect most of the easily eroded organic material into deeper water. Sediment at the siliciclastic site was likely influenced by advection of organic material derived from the Atchafalaya River.