Oxycline Variabilities in Intertidal Beach Aquifers Under Seasonally Variable Oxygen Consumption and Physical Forcing Regimes

High-energy beaches mark a highly variable land-ocean transition where matter fluxes are modulated by dynamic subsurface biogeochemical reactions. At the beach face, seawater infiltration into the saline recirculation cell of the intertidal beach aquifer creates a high input of electron acceptors and organic matter. Microorganisms rapidly degrade fresh organic matter in the upper sandy beach layer under advective flow conditions. Filtration of particulate organic matter and constant supply of oxygen (O₂) in the shallow sand body result in much of this turnover taking place under predominantly oxic conditions. In temperate regions, this filter effect combined with seasonal seawater inputs results in a strong seasonality of reaction rates as well a seasonally heterogeneous distribution of rates. Additionally, subsurface transport dynamics of seawater containing biogeochemical reactants highly depends on the physical forcings such as tides, waves and the beach morphology, adding complexity to the system. We assume that the variable O₂-consuming degradation processes in the upper layer in combination with dynamic physical forcing regimes lead to a fluctuating oxycline in the beach aquifer. Therefore, the aim of our study was to investigate the impact of seasonally variable oxygen demand under different physical forcing regimes on redox zonation in the beach subsurface. We used O₂ consumption rates from the beach face at Spiekeroog Beach (Germany), measured down to 1m depth and over a year-long sampling campaign within the project DynaDeep, to develop a numerical reactive transport model at field scale. The results from the field data showed a strong seasonal depth dependency of O₂ consumption rates. Lowest rates were found in winter and increased substantially in summer, with the strongest increase in rates in the upper decimeters.Modelling case studies for a summer and a winter situation were carried out to simulate both, quasi-stationary and dynamic conditions. Model results show that the oxic zone is significantly larger in winter than in summer, aligning with the general O₂ distribution measured in the field. We found that in summer, dynamic tidal conditions lead to greater variations in O₂ concentrations than in winter. In addition, the model shows that during tidal inundation, O₂ can overcome the high consumption rates in the upper decimeters in summer, thereby increasing the oxic zone. Finally, the model will be used to explore the impact of additional physical forcings in order to better constrain the oxycline as a variable redox boundary for subsequent anoxic processes.