Dataset: Sediment resuspension erosion

We resuspended the surface 5 cm of natural muddy sediment cores in the lab and compared temporal changes in sediment compaction to changes in surface and subsurface cohesion over 30 days post resuspension. Sediment-water interface (SWI) height and acoustic sound speed through sediment, which depends on bulk density, provided continuous and nondestructive metrics of compaction, and sediment porosity and grain size were measured destructively to characterize sediment physical structure. We determined surface cohesion by measuring both eroded mass and turbidity resulting from increasing shear stress. Subsurface cohesion was determined from the force required for sediments to fail in tension. We compared surface and subsurface exopolymeric substance (EPS) concentrations to surface and subsurface cohesion measurements. We differentiated between water-soluble (colloidal) and sediment-bound EPS as we expected bound EPS to contribute more to sediment-organic matrix development and thus cohesion because they are directly bound to sediment grains rather than dissolved in porewater.

These data include the erosion measurements from this experiment. A summary of data collected on cores processed over time points 0 days (no resuspension), then 1, 2, 3, 7, 14, and 30 days post resuspension is given in dataset 1. Repeated non-destructive measurements of sediment-water interface height and sound speed on cores processed on day 30 are provided in separate datasets.

To examine changes in sediment surface cohesion, we measured total eroded mass and turbidity using a custom-built Gust erosion chamber (Fig. 2 B, Gust & Muller 1997, Thomsen & Gust 2000, U-GEMS Manual, Green Eyes, LLC, 2015). Cores were capped with the erosion chamber cap in which a rotating disc generated increasing levels of shear stress (0.1, 0.2, 0.3, 0.45, 0.6 Pa). Each stress level was maintained for 20 min before increasing to the next level. At each stress level, water and eroded material were removed by a pump at the center of the disc 10 cm above the sediment surface. This effluent passed through a C-Star transmissometer (Sea-Bird Scientific, Bellevue, WA, USA) recording light attenuation coefficient (m-1) at 650 nm to determine turbidity over time and was then captured for later filtration. An initial 0.01 Pa interval was used as a flushing step and not filtered, but seawater used to replace the effluent was filtered to determine background suspended sediment concentration.

Total eroded mass at each stress level was obtained by filtering the effluent through 47mm Whatman GF/F filters (1.5 μm pore size). Filters were dried at 65° C for 24 h, then weighed. We calculated suspended sediment concentration, Cs (kg m-3), for each core at each stress level from the dry mass (kg) of filtered sediment divided by the volume (m-3) of water filtered. Cs was converted to eroded mass per area (E; kg m-2):

(3)    E=(C_s V_c)/A_c

where Vc is chamber volume (7.24 x10-4 m3), and Ac is sediment surface area within the core (7.24 x 10-3 m2). To generate specific shear stresses, we set cap rotation and pumping rate using calibration equations from the University of Maryland Center of Environmental Science Gust Erosion Microcosm System (U-GEMS) Manual (Green Eyes, LLC, 2015):

(4)    u_15^*=0.0318n^0.763

(5)    Q=-28.31u_15^(*2)+170.2u_15^*-23.85

where u*15 is shear velocity at 15 °C (cm s-1), n is rotations per minute, and Q is pumping rate (mL min-1). Shear velocity at 15 °C was converted to shear velocity at the average water temperature measured during the erosion tests (20 °C) as:

(6)    u_15^*=u_20^* [1+0.006(20-15)]

where u*20 is shear velocity at 20 °C (cm s-1) (U-GEMS Manual, Green Eyes, LLC, 2015). Shear stress (τb; Pa) was calculated from shear velocity (u*20; m s-1) and seawater density (ρw; kg m-3) as:

(7)    τ_b=〖ρ_w u_20^*〗^2

(U-GEMS Manual, Green Eyes, LLC, 2015).

We determined turbidity, as suspended sediment concentration (kg m-3), from light attenuation coefficient, measured continuously throughout each stress level. We calibrated the transmissometer with muddy seawater of 4 suspended sediment concentrations (0.0038, 0.018, 0.030, and 0.050 kg m-3) made from sediment from the coring site. We then filtered each muddy water sample following the steps above to determine suspended sediment concentration and determined the relationship of light attenuation and suspended sediment concentration:

(8)    C_s=0.17c+0.0015

where Cs is suspended sediment concentration (kg m-3) and c is light attenuation coefficient (m-1).

These data were collected using a custom-built Gust chamber. For more detail, see Clemo et al., submitted, Limnology and Oceanography.