Dataset: Sediment surface elevation change of sediments collected from the Northern Gulf of Mexico following laboratory resuspension at the Dauphin Island Sea Lab in 2020

Sediment cores were collected from 10 meters (m) depth in the northern Gulf of Mexico offshore of Dauphin Island, Alabama, in cohesive muddy sediment on January 26, 2020 (30° 13.333' N, 88° 8.348' W). Cores were collected from a Dauphin Island Sea Lab vessel, the R/V Alabama Discovery. Polycarbonate sediment cores (9.6 centimeter (cm) inner diameter x 60 cm height) were collected using an MC-400 multicorer (Ocean Instruments, Fall City, WA).

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 repeated measurements of sediment-water interface height. 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 a related dataset. Detailed data on erosion measurements, as well as repeated non-destructive measurements of sound speed on cores processed on day 30 are provided in separate datasets.

To quantify short- and long-term sediment compaction continuously and non-destructively, we measured post-disturbance sediment-water interface height above the core base 16 times over 30 days following disturbance. For each core, the pre-disturbance sediment-water interface height was subtracted from the sediment-water interface heights at each timepoint to provide sediment-water interface height above or below the undisturbed height. Sediment-water interface heights higher than the undisturbed height indicated less compact sediment.

We performed acoustic measurements following methods from Dorgan et al. (2020). Within a seawater tank, a 400 kHz three-cycle sinusoidal tone burst was transmitted horizontally through sediment cores to a receiver at 3 depths below the sediment surface (2.5, 5, 10 cm) (see Fig. 1 in Dorgan et al., 2020). To account for sound speed differences due to temporal variability in temperature and salinity, sound speed through sediment was normalized by the sound speed in seawater to obtain sound speed ratio (SSR). Each day, we also performed acoustic measurements on cores filled with seawater and with no core present. Sound speed in seawater and the lag time between the transmitted and received signals (time of flight) through sediment and seawater cores were used to calculate sound speed in sediment (νp):

ν_p=c_w/(1-(c_w * ∆t/d_s )

where cw is sound speed in water, Δt is the difference in time of flight between seawater core (tw) and sediment core (ts), and ds is the inner diameter of the core (Jackson and Richardson, 2007; Dorgan et al., 2020). SSR was then calculated by dividing νp by cw, where a higher SSR indicates more compact sediment.

Instruments:
Acoustics measurements were done following Dorgan et al. 2020, JASA.