Volume Pulsation of Small Gas Bubbles in the Surface Layer of Coastal Sands Caused by Surface Gravity Waves

In the uppermost millimeters of shallow submerged coastal sediments, photosynthesis by microalgae and cyanobacteria during daylight hours can cause oxygen supersaturation of sediment porewater, leading to bubble formation. In shallow water depths, the seabed is affected by the pressure maximum beneath the wave crest and pressure minimum beneath the wave trough. While the photosynthetically-generated gas bubbles persist within the surface layer highly permeable sand sediments, they may be exposed to pressure pulsations caused by tides and passing surface gravity waves, because pressure is not significantly attenuated in the upper few centimeters of permeable sediments. The question arises, whether the tens of thousands of millimeter-size bubbles that are produced on sunny days in each square meter of nearshore sands respond to these pressure oscillations and if so, what consequences these responses may have. The main goals of the research thus were the demonstration of the bubble pulsation within the sediment and the quantification of the pore water flow, associated interfacial solute flux, and sand grain movement caused by the pulsation. The central working hypotheses tested in this research were: 1) Millimeter-size photosynthetic gas bubbles buried in the surface layer of submerged permeable coastal sands respond to passing surface gravity waves by volume changes leading to bubble volume pulsation. 2) This bubble volume pulsation causes pore water flows and thereby exchange across the sediment-water interface and an increase of the net solute flux from the sediment. 3) The bubble volume pulsation causes sand grain movement and thereby local sediment compaction, alteration of sediment surface topography and vertical transport of substances attached to the sand grains. These three working hypotheses were addressed in Thesis chapters 1, 2 and 3, respectively. In-situ video observations with a buried camera showed that the bubbles (1-3 mm diameter) buried in the surface layer ([less than] 10 cm) of nearshore sand respond to passing waves by volume pulsation and allowed estimation of the oscillating volume change of the bubbles visible in the sediment-cross section. These observations revealed bubble volume oscillation with compressions of 7.4% caused by ~1 meter water waves producing a temporary 10 kPa pressure increase. Laboratory measurements in a custom-built pressure tank confirmed the in-situ observations: Bubbles with 1.24 mm to 2.12 mm diameter (1 mm3 to 5 mm3), embedded in transparent Nafion[TM] sand sediment, at 1 m water depth were compressed by 8.7 ± 1.3 % of their volume when exposed to the same pressure increase of 10 kPa as produced by a 1 meter water wave. With an observed abundance of 50,000 bubbles m-2 in sandy Gulf of Mexico sediments, the pulsation of bubbles with 2 mm diameter produced by the passing of thirty 1 m-waves per minute (8.3% compression) could pump 31.3 L m-2 h-1 (or 750 L m-2 d-1) of water across the sediment-water interface. Once it was established that surface layer bubbles pulsate due to pressure oscillations associated with passing surface gravity waves at a rate consistent with Boyle's Law, the effect of pulsating bubbles on pore water movement and grain movement was investigated. Fluorescein dye tracer experiments conducted at the same wave frequency (0.5 Hz) and wave height (75-100 cm) showed a linear correlation (R2 = 0.99) between interstitial gas bubble volume and interfacial tracer flux. The pulsation of 200 five-microliter bubbles embedded in the top 2 cm of a 10 cm (L) x 10 (W) x 45 cm (D) wet sediment core (bubbles occupied 0.5% of the volume of the 2 cm thick upper sediment layer) increased interfacial flux initially (first 7 min) by a factor of 17 compared to the control experiment, where tracer transport was limited to molecular diffusion and some tracer release caused by the setup of the experiment. The increase of flux caused by the bubbles is produced by the oscillating water flow across the sediment-water interface that pushes pore water out of the sediment that then is mixed into the overlying water through dispersion and turbulence. At an observed in-situ abundance of 50,000 bubbles m-2, bubble pulsation can be estimated to increase solute transport across the highly-active sediment-water interface by a factor of 30 when compared to molecular diffusion. Over time, the intermittent movement of sand grains driven by the pulsating bubbles resulted in a tighter packing of the sand, with a decrease in pore space, and an overall downward migration of grains above and around the buried bubbles. Particle Image Velocimetry showed that the cross-sectional area around the bubble, in which grains were moved by its pulsation, decayed exponentially over time. In the natural environment, decompaction of the sediment surface layer by bioturbation and sediment resuspension by bottom currents counteracts the compaction caused by the bubble pulsation, resulting in a continuous cycle of compaction/de-compaction that keeps a substantial fraction of the grains in the surface layer moving. With observed in-situ abundances of 50,000 bubbles m-2 and an average bubble volume of 5 mm3, approximately 5,000 cm3 m-2 or 50% of the grains in the top 1 cm of the sand bed are moved by the pulsation of buried gas bubbles every hour. The movement and net downward migration of surface layer sand grains towards pulsating bubbles has broad implications for sediment physical characteristics, sediment geochemistry and pore water flow.