Influence of sedimentary gas ebullition on interfacial transport in permeable marine sands

The main goal of this study was to assess the role of ebullition on solute flux and oxygen dynamics in the sediment. I conducted seasonal field measurements of gas ebullition and sediment free gas content, and analyzed the composition of these gases. The highest gas ebullition volumes were generally observed in the afternoon and evening hours before midnight, a pattern reflecting the build-up of oxygen in the sediment after sunrise, reaching supersaturation in the late morning hours and bubble formation thereafter. The lowest bubble volumes were measured after midnight and in the early morning. Ebullition rates reached up to a maximum 15.6 ml m−2 h−1 with a peak of 62.3 ml m-2 h−1 between the hours of 12:00–18:00 during the summer season. On a yearly average, about 59 ± 32 percent of the gas that was measured from the bay was released from the sediment. There is a noticeable offset between bubble formation in the sediment to maximum rates of gas release. The latter became most significant and exceeded the sedimentary gas volumes mainly during the afternoon hours. The maximum volume of gas was recorded in August at 492.4 ml m−2 d−1, which integrates gas volume released from the sediment and the free gas still present in the sediment. In this study, most of the measured gas was not collected in the sediment, but rather in the overlying water column. The maximum volume of gas that a given volume of sediment can contain before ebullition occurs, at 1 cm sediment depth, was determined to be around 0.064 ± 0.005 ml in the lab.;I conducted laboratory experiments utilizing sediment columns with pore water that contained inert fluorescein dye. Bubbles released at six different depths in the sediment resulted in a 4 to 21 fold enhancement of sediment-water solute flux relative to diffusion-only transport. The strongest increase of dye flux from the pore water into the overlying water was recorded when bubbles were released closest to the sediment-water interface, where individual gas bubbles were the smallest in volume but the release rate of individual bubbles was the highest. Subsequent analyses of larger sediment cores (57 cm diameter, ∼5 cm in height) revealed a distinct pattern of dye washout caused by the ebullition that extended below the level at which the gas was released at 2.5 cm depth. With a transfer of the laboratory experimental design to the field using 19-cm diameter chambers with rotating discs and chamber water spiked with inert bromide (Br−) tracer, we could show that the average volume of pore water exchange in five different ebullition chambers was 231 ± 96 L m−2 d−1. Compared to the average from four non-ebullition chambers (79 ± 96 L m −2 d−1), the difference in interfacial tracer flux caused by ebullition was almost 3-fold.;Investigations were also undertaken on oxygen gas bubble behavior in both the field and laboratory. Correlation comparisons between the collected gas samples from St. Joseph Bay and the measured environmental factors show that the change in the gas volume is most closely associated with sunlight, albeit with a lag time. While the rate of increase in the oxygen concentration was the greatest at the onset of the day (6:00–12:00), the volume maxima was not typically reached until the afternoon hours. The average concentration of oxygen in the bubbles during the summer at the bay exceeded 50% in both the sediment and bubble trap samples. The flux of oxygen from the sediment through ebullition was up to 5.93 ± 0.63 mmol m−2 d−1 during August. Pure oxygen gas was released at the sediment-water interface, as well as 2 cm in the sediment and collected at 20-cm intervals up to 1 m to see if any immediate changes in composition would occur. The gas stripping was further pursued in the lab by injected known volumes of pure oxygen (5, 10 or 15 ml) into seawater control vials and (5 or 10 ml) into glass containers filled with fresh, wet sediment collected from the bay. The loss of oxygen in the rising bubbles in the water column at St. Joseph Bay was 21% greater when gas was injected at 2 cm in the sediment versus at the sediment surface. In the lab experiments the gas samples in the sediment loss 43 and 33 percent more oxygen following 24 hours, with rates of decrease approximately 4.9 and 5.5 times (5 and 10 ml experiments) higher than the controls. In all experiments, the sediment demonstrated higher rates of oxygen consumption than in the water column, and N2 was the only other gas found in all of our gas samples. (Abstract shortened by UMI.).