Physical and geochemical controls on oxygen dynamics at continental margins and shelf seas

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

Loop representation of charged particles interacting with Maxwell and Chern-Simons fields

The loop representation formulation of non-relativistic particles coupled with abelian gauge fields is studied. Both Maxwell and Chern-Simons interactions are separately considered. It is found that the loop-space formulations of these models share significant similarities, although in the Chern-Simons case there exists an unitary transformation that allows to remove the degrees of freedom associated with the paths. The existence of this transformation, which allows to make contact with the anyonic interpretation of the model, is subjected to the fact that the charge of the particles be quantized. On the other hand, in the Maxwell case, we find that charge quantization is necessary in order to the geometric representation be consistent.Comment: 6 pages, improved versio

On the Effective Metric of a Planck Star

Spacetime metrics describing `non-singular' black holes are commonly studied in the literature as effective modification to the Schwarzschild solution that mimic quantum gravity effects removing the central singularity. Here we point out that to be physically plausible, such metrics should also incorporate the 1-loop quantum corrections to the Newton potential and a non-trivial time delay between an observer at infinity and an observer in the regular center. We present a modification of the well-known Hayward metric that features these two properties. We discuss bounds on the maximal time delay imposed by conditions on the curvature, and the consequences for the weak energy condition, in general violated by the large transversal pressures introduced by the time delay.Comment: 10 pages, many figures; v2 added reference

Thermocline mixing and vertical oxygen fluxes in the stratified central North Sea

In recent decades, the central North Sea has been experiencing a general trend of decreasing dissolved oxygen (O2) levels during summer. To understand potential causes driving lower O2, we investigated a 3-day period of summertime turbulence and O2 dynamics in the thermocline and bottom boundary layer (BBL). The study focuses on coupling biogeochemical with physical transport processes to identify key drivers of the O2 and organic carbon turnover within the BBL. Combining our flux observations with an analytical process-oriented approach, we resolve drivers that ultimately contribute to determining the BBL O2 levels. We report substantial turbulent O2 fluxes from the thermocline into the otherwise isolated bottom water attributed to the presence of a baroclinic near-inertial wave. This contribution to the local bottom water O2 and carbon budgets has been largely overlooked and is shown to play a role in promoting high carbon turnover in the bottom water while simultaneously maintaining high O2 concentrations. This process may become suppressed with warming climate and stronger stratification, conditions which could promote migrating algal species that potentially shift the O2 production zone higher up within the thermocline

Discovery of a natural CO2 seep in the German North Sea: implications for shallow dissolved gas and seep detection

A natural carbon dioxide (CO2) seep was discovered during an expedition to the southern German North Sea (October 2008). Elevated CO2 levels of ∼10–20 times above background were detected in seawater above a natural salt dome ∼30 km north of the East-Frisian Island Juist. A single elevated value 53 times higher than background was measured, indicating a possible CO2 point source from the seafloor. Measured pH values of around 6.8 support modeled pH values for the observed high CO2 concentration. These results are presented in the context of CO2 seepage detection, in light of proposed subsurface CO2 sequestering and growing concern of ocean acidification. We explore the boundary conditions of CO2 bubble and plume seepage and potential flux paths to the atmosphere. Shallow bubble release experiments conducted in a lake combined with discrete-bubble modeling suggest that shallow CO2 outgassing will be difficult to detect as bubbles dissolve very rapidly (within meters). Bubble-plume modeling further shows that a CO2 plume will lose buoyancy quickly because of rapid bubble dissolution while the newly CO2-enriched water tends to sink toward the seabed. Results suggest that released CO2 will tend to stay near the bottom in shallow systems (<200 m) and will vent to the atmosphere only during deep water convection (water column turnover). While isotope signatures point to a biogenic source, the exact origin is inconclusive because of dilution. This site could serve as a natural laboratory to further study the effects of carbon sequestration below the seafloor

Simple, robust eddy correlation amplifier for aquatic dissolved oxygen and hydrogen sulfide flux measurements

The aquatic application of the eddy correlation (EC) technique is growing more popular and is gradually becoming a standard method for resolving benthic O2 fluxes. By including the effects of the local hydrodynamics, the EC technique provides greater insight into the nature of benthic O2 exchange than traditional methods (i.e., benthic chambers and lander microprofilers). The growing popularity of the EC technique has led to a greater demand for easily accessible and robust EC instrumentation. Currently, the EC instrumentation is limited to two commercially available systems that are still in the development stage. Here, we present a robust, open source EC picoamplifier that is simple in design and can be easily adapted to both new and existing acoustic Doppler velocimeters (ADV). The picoamplifier has a response time of < 0.1 ms and features galvanic isolation that ensures very low noise contamination of the signal. It can be adjusted to accommodate varying ranges of microelectrode sensitivity as well as other types of amperometric microelectrodes. We show that the extracted flux values are not sensitive to reduced microelectrode operational ranges (i.e., lower resolution) and that no signal loss results from using either a 16- or 14-bit analog-to-digital converter. Finally, we demonstrate the capabilities of the picoamplifier with field studies measuring both dissolved O2 and H2S EC fluxes. The picoamplifier presented here consistently acquires high-quality EC data and provides a simple solution for those who wish to obtain EC instrumentation. The schematic of the amplifier’s circuitry is given in the Web Appendix

Fe, Co, and Ni Ions Promote the Catalytic Activity of Amorphous Molybdenum Sulfide Films for Hydrogen Evolution

Molybdenum sulfide materials had been shown as promising non-precious catalysts for hydrogen evolution. This paper describes the study of the promotional effect of certain transition metal ions on the activity of amorphous MoS3 films. Ternary metal sulfide films, M-MoS3 (M = Mn, Fe, Co, Ni, Cu, Zn), have been prepared by cyclic voltammetry of aqueous solutions containing MCl2 and (NH4)2[MoS4]. Whereas the Mn-, Cu-, and Zn-MoS3 films show similar or only slightly higher catalytic activity as the MoS3 film, the Fe-, Co-, and Ni-MoS3 films are significantly more active. The promotional effects of Fe, Co, and Ni ions exist under both acidic and neutral conditions, but the effects are more pronounced under neutral conditions. Up to a 12-fold increase in exchange current density and a 10-fold increase in the current density at an overpotential of 150 mV are observed at pH = 7. It is shown that Fe, Co, and Ni ions promote the growth of the MoS3 films, resulting a high surface area and a higher catalyst loading. These changes are the main contributor to the enhanced activity at pH = 0. However, at pH = 7, Fe, Co, and Ni ions appear to also increase the intrisinc activity of the MoS3 film

Maxwell Chern Simons Theory in a Geometric Representation

We quantize the Maxwell Chern Simons theory in a geometric representation that generalizes the Abelian Loop Representation of Maxwell theory. We find that in the physical sector, the model can be seen as the theory of a massles scalar field with a topological interaction that enforces the wave functional to be multivalued. This feature allows to relate the Maxwell Chern Simons theory with the quantum mechanics of particles interacting through a Chern Simons fieldComment: 12 pages, LaTe

Benthic O-2 uptake by coral gardens at the Condor seamount (Azores)

Using the non-invasive aquatic eddy covariance technique, we provide the first oxygen (O-2) uptake rates from within coral gardens at the Condor seamount (Azores). To explore some of the key drivers of the benthic O-2 demand, we obtained benthic images, quantified local hydrodynamics, and estimated phototrophic biomass and deposition dynamics with a long-term moored sediment trap. The coral gardens were dominated by the octocorals Viminella flagellum and Dentomuricea aff. meteor. Daily rates of O-2 uptake within 3 targeted coral garden sites (203 to 206 m depth) ranged from 10.0 t 0.88 to 18.8 +/- 2.0 mmol m(-2) d(-1) (mean +/- SE) and were up to 10 times higher than 2 local sandy reference sites within the seamount summit area. The overall mean O-2 uptake rate for the garden (13.4 mmol m(-2) d(-1)) was twice the global mean for sedimentary habitats at comparable depths. Combined with parallel ex situ incubations, the results suggest that the octocorals might contribute just -similar to 5% of the observed O-2 uptake rates. Deposition of particulate organic matter (POM) assessed by the sediment trap accounted for less than 10% of the O-2 demand of the coral garden, implying a substantial POM supply circumventing the deployed traps. Our results expand the database for carbon turnover rates in cold-water coral habitats by including the first estimates from these largely understudied coral gardens.Peer reviewe