Oxygen dynamics and organic carbon remineralization in the sands of high-energy beaches

Coastal regions are hotspots of biological productivity and biogeochemical substance transformations, and play a key role in global element cycles. Within them, beach aquifers beneath sandy coasts, which make up about a third of the world's ice-free shorelines, function as prominent biogeochemical reactors that regulate the transport and fate of carbon, nutrients, and trace elements at the land-ocean interface. At high-energy coasts, tides and waves drive the infiltration of large volumes of seawater into the upper saline plume (USP) of the beach aquifer, introducing fresh organic carbon (OC) into the beach sands and inducing deep porewater advection. This input stimulates the activity of the sedimentary microbial communities, remineralizing OC and recycling nutrients. However the regulation of microbially mediated biogeochemical processes under variable boundary conditions, such as tides, waves, and seasonal inputs, in these systems remains poorly understood and substance transformations are not sufficiently quantified. This doctoral thesis aims to improve our mechanistic understanding of how OC remineralization and redox zonation in high-energy beach aquifers respond to variable physico-chemical boundary conditions, such as the seasonally variable inputs of oxygen (O2) and labile organic matter (OM). To achieve this, an integrated approach combining a year-long field campaign, direct rate measurements and reactive transport modelling captures both large-scale patterns and small-scale heterogeneity of OC mineralization rates and redox dynamics. The first study examined the seasonal and spatial patterns of O2 consumption as a proxy for OC turnover in the upper sediment layers of the USP’s infiltration zone. This investigation employed novel batch incubations for a high sample throughput that spanned an entire year and provided a comprehensive dataset of aerobic respiration rates. The interpretation of the data was informed by the seasonal inputs of OM and O2, filtration efficiency of the sediment, and beach morphodynamics. The results revealed a pronounced seasonal regulation of OC remineralization, with a highly reactive surface layer that closely correlated with labile OC inputs and particulate organic matter (POM) retention. These patterns demonstrate that aerobic respiration in the USP is limited by the availability of labile OC and O2 is advected beyond the investigated upper sediment layers. The study characterized the USP of the beach aquifer as a high-throughput system for OC with rapid remineralization in a reactive retention layer, particularly in summer, but little OM storage. Building on these findings, the second study integrated field data into a reactive transport model to simulate O2 dynamics and OC remineralization across the full depth of the USP under winter and summer conditions. The tide-resolving model accounted for periodic tidal desaturation of the surface layer and indicated that atmospheric O2 inputs during desaturation contributes significantly to total O2 supply. In contrast, O2 from infiltrating seawater largely bypassed the upper reactive layer, establishing an oxycline at depths over ten meters, depending on the season. The aeration mechanism dampened seasonal variability in deeper sediment layers while maintaining high aerobic OC remineralization rates in shallow sediments. The study provided a mechanistic understanding how variable boundary conditions and OM inputs as well as the desaturated layer within the infiltration zone determine the aerobic OC turnover and drive significant changes in redox conditions in the USP of beach aquifers. The third study shifted focus from OC mineralization in the bulk sediment to the microscale variability of aerobic respiration. Using steady-state and transient flow-through reactor experiments with sandy sediments from the upper POM retention layer, it showed that small-scale heterogeneity in reactivity fosters localized anoxic zones within otherwise oxic sediments. This enables the spatial coexistence of aerobic and anaerobic processes and highlights the role microscale biogeochemical heterogeneity in creating microbial niches for anaerobic metabolism under bulk oxic conditions. The fourth study expanded the scope of the thesis to include trace metal cycling in the USP, focusing on the redox-sensitive mobilization of elements such as cobalt. Experimental flow-through reactor incubations complemented field observations and geochemical analyses, contributing to a mechanistic understanding of trace metal behavior in sandy beach sediments. The fifth study presents experimental rate measurements as part of the DynaDeep Observatory, which provides the conceptual and infrastructural framework for this thesis. Together, these studies identify the USP as a high-throughput, yet carbon-limited system, where biogeochemical functioning is strongly influenced by seasonal variability, POC retention and tide-induced desaturation at the upper infiltration boundary. By combining field observations, laboratory experiments, and numerical modelling, this thesis offers an integrative framework for understanding O2 dynamics and OC remineralization in intertidal permeable beach sediments. It emphasizes their role as effective biogeochemical filters at the land–ocean interface and highlights the importance of continued research on these systems.