[H-CG23] Earth surface processes related to deposition, erosion and sediment transport

[HCG23-05] Temporal changes in the distribution of dissolved oxygen concentration in sediments during sand ripple generation: Attempt at visualization in a tank experiment

*Koki Shibuya¹, Noritaka Endo¹ (1.Kanazawa University Graduate School of Natural Science and Technology )

Keywords:Ripple, Advection, Diffusion, Oxygen, Permeable sediments, Shallow water

Sandy sediments are ubiquitous material composing substrates of various locations in not only terrestrial areas but also shallow water areas such as continental shelves. The substrate material is one of the factors that determines the characteristics of each environment. Compared to cohesive clayey sediments, sandy sediments are permeable because of higher porosity, so mass transport by porewater flow, instead of diffusion has been considered to be predominant in sandy substrate, providing an environment that promotes oxygen consumption for microorganisms. Some previous studies suggested that the oxygen flux dynamics in permeable sediments is generally governed by a transportation mechanism driven by advective porewater flow, in which the sediment-water interface constrains the rate of oxygen supply to the bottom sediment. More recently, it has been pointed out that the activation of the porewater-exchange-cycle due to microtopography-formation may enhance oxygen transport and consumption in sandy sediments. In relatively shallow water environments where sandy sediments are common, specific microtopographies called sand-wave-ripples are generally formed by oscillatory currents due to waves. Kessler et al. (2012) created an artificial sand-ripple in the flume and took oxygen optode images below a flow velocity that collapses the ripple. Precht et al. (2004) performed experiments generating oscillatory flow that spontaneously formed sand wave ripples in a tank. In both cases, highly dissolved oxygen areas were observed in the sediments near the troughs of the ripples, and low concentrations were found near the crests. The details of the relationship between microtopography and material circulation, however, have not yet been fully elucidated because of limited number of studies. We conducted a model experiment using oxygen optodes to explore the factors that govern oxygen dynamics when ripples form in a permeable sediment environment and to capture the changes in the oxygen supply region.In our preceding experiment (run duration was 25 min), the temporal development of aerobic zone caused by advective pore water flow was visualized under the condition in which there was no organic matter in the sediment. In that experiment, the aerobic zone was zoned but no spatial differences were observed between the trough and crest of ripples, which suggests that (1) advection is a main mechanism driving of dissolved oxygen for sand sediments, and (2) spatial differences in the aerobic zone will not occur unless elements causing oxygen consumption are present in the sediment. In order to test the hypothesis, we conducted the similar experiments but with a longer observation time of 360 minutes. As the initial condition, the dissolved oxygen in the tank water is completely removed using sodium sulfite. A microtopography was not artificially created in advance, but was generated spontaneously by the flow applied. We plan to conduct experiments under various hydraulic conditions in the future, but we first conducted a preliminary experiment to see how the aerobic region expands by diffusion alone without flow when the bottom topography is flat, for the purpose of comparison with the following experiments. The results showed an active migration of the aerobic zone downward within the first 120 minutes that was more widespread than the advection in short time period-observed in the previous experiment, which means that diffusion is not ignorable in sandy sediments. Both advection and diffusion processes, therefore, should be considered to understand the dynamics of the aerobic zone in terms of the interaction between hydraulic conditions and microtopographies, which influences microbial ecosystems.