Rovelli, Lorenzo (2014) Physical and geochemical controls on oxygen dynamics at continental margins and shelf seas. (PhD/ Doctoral thesis), Christian-Albrechts-Universität zu Kiel, Kiel, Germany, 144 pp.

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Abstract

In light of increasing anthropogenic influences on natural waters and climate change, it is important to advance our understanding of the intricate interactions between biological, geochemical and physical processes that control constituent dynamics within aquatic systems. Among those constituents, dissolved oxygen (O2) is a well-established indicator for biological activity and is also involved in most biogeochemical processes in both the water column as well as in the upper region of the sediment. While the different aspects of O2 dynamics and their flux pathways are well investigated with the boundaries of each discipline, interdisciplinary studies are, at present, still relatively scarce. Within this multidisciplinary thesis, O2 dynamics in the water column and at the sediment-water interface (SWI) were investigated using state-of-the-art high-resolution tools on seasonally stratified shelf seas (central North Sea) as well as on cold seep habitats at continental margins (off Chile). The aim of those studies is that of further investigating the role of the hydrodynamics in modulating costituent transport, with emphasis on O2 transport. The central North Sea process study on thermocline mixing and O2 fluxes, which was performed with a turbulence profiler, fast O2 microsensors and moored current measurements, revealed the occurrence of a second-mode, near-inertial internal wave. Zones of enhanced vertical shear of horizontal velocity and concomitant strong stratification were observed at the upper and lower limits of the interior layer where turbulence levels were also found to be a factor of ten higher than in the central interior region. High-resolution O2 measurements further revealed a well-established O2 maximum which occurred at the lower limit of the interior layer, from which a considerable, yet overlooked, O2 flux towards the bottom boundary (BBL) was observed. It was hypothesized that due to this additional O2 source, the carbon turnover between the thermocline and the BBL is much larger than previously regarded. To overcome the shallow-depth rating limitations of fast O2 sensor systems such as that used on the presented North Sea study, a newly designed fast O2 system with deep sea ratings was developed based on pressure-compensated Clark-type microsensors. The system was embedded on a microstructure profiler and successfully tested at the Chilean continental margin. The O2 gradients above the O2 minimum were twice as high as reported by the standard O2 sensor from the ship-operated CTD (Conductivity-Temperature-Depth). The fast O2 system also proved to be accurate and fast enough to detected step-like structures in water-column O2 profiles. Those were similar to the step-like structures observed in temperature and salinity that characterize a double-diffusive system (i.e., finger regime). Controls on benthic O2 and hydrogen sulfide (H2S) fluxes were also investigated on two cold-seeps habitats at the Chilean continental margin using a state-of-the-art in-situ microprofiling transecting unit and corresponding flow measurements. The first habitat was characterized by recurrent bacterial mat coverage and the frequent occurrence of sulfide; conversely, the second habitat was less sulfidic with limited bacterial mat coverage. While H2S fluxes were found to vary little between the habitats, the average diffusive O2 uptake rate (DOU) was a factor of two higher in the more sulfidic habitat. The major contributions to the observed DOUs were seemingly dominated by sulfide oxidation and, to a lesser extent, by the particulate organic matter input from the overlaying water. Both habitats showed the occurrence of periods of transport limitation resulting from a flow-driven diffusive boundary layer (DBL) which modulated DOU. This limitation was more pronounced in the more sulfidic habitat, suggesting that increased geochemical activity might lead to increased physically-driven dampening of O2 uptake. The implications of this transport limitation are therefore not only of importance for seep O2 dynamics but also for organically enriched continental margins and shelfs characterized by enhanced O2 uptake rates. The results of this thesis also showed that the combination of high-resolution constituent measurements and accurate physical characterization is currently the best approach to further advance the knowledge on the O2 dynamics.

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