4:00 PM - 4:15 PM
[HCG22-08] Preliminary experiments using oxygen optode for visualization of oxygen-concentration-distribution under sand ripples
Keywords:Permeable Sediments , Ripple, Material Cycling, Porewater Flow
(Hemi-) Pelagic sediments are clayey (cohesive sediments) and the mechanism of mass transport within the sediments (inter-particle spaces) is predominantly diffusion, whereas sandy sediments, which are widely distributed along the coast (70% of land shelf and sandy beaches), are permeable with relatively high porosity and, therefore,advection. is considered to be the dominant mode of mass transport in the sediments. Permeable sediments in shallow waters are susceptible to water waves, thus, form sand ripples (i.e., wave ripples). A simple question is raised: what difference does the development of wave ripples make to the distribution of dissolved oxygen in the bottom sediment compared to a flat bottom? Detailed relationships between such microtopography and the material cycle is not well understood. Oxygen optodes, sensor films, visualize the distribution of dissolved oxygen concentration, from the intensity and lifetime of phosphorescence emitted by metal complex compounds. In previous studies, high concentrations of dissolved oxygen were observed in the sediments near the troughs of the ripple and low concentrations near the crests. However, the factors that cause the contrast of the distribution of oxygen concentrations have not yet been revealed. In this study, we conducted a preliminary experiment to explore the factors controlling oxygen dynamics in wave ripples composed of permeable sediments.
Here we deal with only cases without organic matter in the sediment, that is, under conditions where oxygen consumption in the sediment does not occur, to determine the area of oxygen supply when only porewater flow is concerned. A 1.8 m long, 15 cm wide flume was used and a manual wave generator was to form wave ripples. One optode image was taken every 5 min for 25 min and image-analysis was performed to detect the ripple topography and aerobic regions and to measure the width of the depth in the sediment. Two and three runs were carried out at two water depths of 3 cm and 6 cm, respectively. In addition, two experiments without oscillatory flow (in still water) at a depth of 3 cm were conducted.
The results showed that in the presence of oscillatory flow, the variation of the aerobic region fluctuated up and down, whereas in the absence of oscillatory flow, the region widened downwards with elapsed time. These results visualized that formation and migration of microtopographic (-sand ripples) had a significant effect on the distribution of the oxygenic region. Under conditions of this study, the ripple shape was different between 3 cm and 6 cm water depths, but there was no clear causal relationship between the aerobic region depth and ripple index. Although there was a trend towards greater depth of aerobic area below the crest compared to below the trough when viewed within the same ripple, a two-sample t-test on the entire population of data did not support this trend., may be due to the greater variance between ripples. The result of this study is different from the trend of higher oxygen concentrations at the trough and lower at the crest, as suggested in previous studies. In conclusion, the contrast in oxygen concentrations in the trough and crest in the previous study can be attributed to oxygen consumption (by organic matter) after oxygen has been delivered to the substrate, rather than variation of efficiency of inflow from over-substrate water to the substrate between trough and crust. In other words, porewater flow is considered to contribute material cycling but not be a direct factor for spatial variation of oxygen concentrations. The result is only preliminary, and future work is needed for deeper understanding of oxygen dynamics in shallow sediments.
Here we deal with only cases without organic matter in the sediment, that is, under conditions where oxygen consumption in the sediment does not occur, to determine the area of oxygen supply when only porewater flow is concerned. A 1.8 m long, 15 cm wide flume was used and a manual wave generator was to form wave ripples. One optode image was taken every 5 min for 25 min and image-analysis was performed to detect the ripple topography and aerobic regions and to measure the width of the depth in the sediment. Two and three runs were carried out at two water depths of 3 cm and 6 cm, respectively. In addition, two experiments without oscillatory flow (in still water) at a depth of 3 cm were conducted.
The results showed that in the presence of oscillatory flow, the variation of the aerobic region fluctuated up and down, whereas in the absence of oscillatory flow, the region widened downwards with elapsed time. These results visualized that formation and migration of microtopographic (-sand ripples) had a significant effect on the distribution of the oxygenic region. Under conditions of this study, the ripple shape was different between 3 cm and 6 cm water depths, but there was no clear causal relationship between the aerobic region depth and ripple index. Although there was a trend towards greater depth of aerobic area below the crest compared to below the trough when viewed within the same ripple, a two-sample t-test on the entire population of data did not support this trend., may be due to the greater variance between ripples. The result of this study is different from the trend of higher oxygen concentrations at the trough and lower at the crest, as suggested in previous studies. In conclusion, the contrast in oxygen concentrations in the trough and crest in the previous study can be attributed to oxygen consumption (by organic matter) after oxygen has been delivered to the substrate, rather than variation of efficiency of inflow from over-substrate water to the substrate between trough and crust. In other words, porewater flow is considered to contribute material cycling but not be a direct factor for spatial variation of oxygen concentrations. The result is only preliminary, and future work is needed for deeper understanding of oxygen dynamics in shallow sediments.