http://hdl.handle.net/1721.1/7803 2013-06-18T19:12:31Z http://hdl.handle.net/1721.1/79346 Nitrogen cycling in oxygen deficient zones : insights from [delta]¹⁵N and [delta]¹⁸O of nitrite and nitrate Buchwald, Carolyn The stable isotopes, [delta]¹⁵N and [delta]¹⁸O, of nitrite and nitrate can be powerful tools used to interpret nitrogen cycling in the ocean. They are particularly useful in regions of the ocean where there are multiple sources and sinks of nitrogenous nutrients, which concentration profiles alone cannot distinguish. Examples of such regions are "oxygen deficient zones" (ODZ). They are of particular interest because they are also important hot spots of fixed N loss and production of N₂O, a potent greenhouse gas. In order to interpret these isotope profiles, the isotope systematics of each process involved must be known so that we can distinguish the isotopic signature of each process. One of the important processes to consider here is nitrification, the process by which ammonium is oxidized nitrite and then to nitrate. This thesis describes numerous experiments using both cultures of nitrifying organisms as well as natural seawater samples to determine the oxygen isotope systematics of nitrification. These experimental incubations show that the accumulation of nitrite has a large effect on the resulting [delta]¹⁸ONO3. In experiments where nitrite does not accumulate, [delta]¹⁸ONO3 produced from nitrification is between -1 to l%o. These values will be applicable for the majority of the ocean, but the nitrite isotopic exchange will be important in the regions of the ocean where nitrite accumulates, such as the base of the euphotic zone and oxygen deficient zones. [delta]¹⁸ONO2 was developed as a unique tracer in this thesis because it undergoes abiotic equilibration with water [delta]¹⁸O at a predictable rate based on pH, temperature and salinity. This rate, its dependencies, and how the [delta]¹⁸ONO2 values can be used as not only biological source indicators but also indicators of age are described. This method was applied to samples from the primary nitrite maximum in the Arabian Sea, revealing that the dominant source and sinks of nitrite are ammonia oxidation and nitrite oxidation with an average age of 37 days. Finally, using the isotope systematics of nitrification as well as the properties of nitrite oxygen isotope exchange described in this thesis, the final chapter interprets multiisotope nitrate and nitrite profiles in the Costa Rica Upwelling Dome using a simple ID model. The nitrite isotopes showed that there were multiple sources of nitrite in the primary nitrite maximum including (1) decoupling of ammonia oxidation and nitrite oxidation, (2) nitrate reduction during assimilation and leakage of nitrite by phytoplankton. In the oxygen deficient zone and secondary nitrite maximum, there were equal contributions of nitrite removal from nitrite oxidation and nitrite reduction. This recycling of nitrite to nitrate through oxidation indicates that the percentage of reduced nitrate fully consumed to N2 gas is actually smaller than previous estimates. Overall, this thesis describes new nitrogen and oxygen isotopic tracers and uses them to elucidate the complicated nitrogen biogeochemistry in oxygen deficient zones. Thesis (Ph. D.)--Joint Program in Oceanography/Applied Ocean Science and Engineering (Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences; and the Woods Hole Oceanographic Institution), 2013.; In title on title page, "[delta]" appears as lower case Greek letters. Cataloged from PDF version of thesis.; Includes bibliographical references. 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79329 Equilibration of an atmosphere by geostrophic turbulence Jansen, Malte F. (Malte Friedrich) A major question for climate studies is to quantify the role of turbulent eddy fluxes in maintaining the observed atmospheric mean state. It has been argued that eddy fluxes keep the mid-latitude atmosphere in a state that is marginally critical to the deepest mode of baroclinic instability, which provides a powerful constraint on the response of the atmosphere to changes in external forcing. A similar criterion does, however not hold in the Southern Ocean, a region whose dynamics are otherwise very similar to the mid-latitude atmosphere. This thesis resolves this apparent contradiction, using a combination of theoretical considerations and eddy-resolving numerical simulations. It is shown that the adjustment of the extra-tropical troposphere to states of marginal criticality does not follow from a fundamental constraint, but is rather the result of the particular parameters characterizing Earth's troposphere. Both marginally critical and strongly supercritical zonal mean flows can be obtained in planetary atmospheres if external parameters are varied. We argue that changes in the equilibrated mean state over a wide range of simulations can better be understood in terms of a balance between the diabatic forcing and the eddy driven overturning circulation. Using a diffusive closure for the eddy flux of potential vorticity, we can relate the eddy-driven overturning transport to properties of the mean flow, and derive scaling relations for both the baroclinicity and vertical stratification of the equilibrated state. Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 183-187). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79300 Influence of topographic stress on rock fracture : a two-dimensional numerical model for arbitrary surface topography and comparisons with borehole observations Slim, Mirna I Theoretical calculations indicate that topographic stresses in some landscapes may be large enough to fracture rocks, which in turn could influence slope stability, erosion rates, and bedrock hydrologic properties. These predictions typically have involved idealized topographic profiles, with few direct comparisons of predicted topographic stresses and observed fractures at specific field sites. I use a numerical model to calculate the stresses induced by measured topographic profiles and specified far-field tectonic stress. I compare the calculated stress field and potential shear fracture orientations with fracture abundance and fracture orientations observed in shallow boreholes. The model uses a boundary element method to calculate the stress distribution beneath an arbitrary topographic profile. When applied to topographic profiles extracted from a laser altimetry map of the Susquehanna/Shale Hills Critical Zone Observatory in central Pennsylvania, the model predicts considerable differences in depth profiles of stresses beneath ridgelines and valley floors. Using a representative value for the friction angle of shale, we calculate the minimum cohesion required to prevent shear failure, Cmin, as a proxy for the potential for fracturing or reactivation of existing fractures. We compare depth profiles of Cmin with structural analyses of image logs from four boreholes located on the valley floor, and find that fracture abundance declines sharply with depth in the uppermost 10 m of the boreholes, consistent with the modeled profile of Cm.. In contrast, Cmin increases with depth below ridgetops, suggesting that future analyses of ridgetop wells should observe a different trend in fracture abundance if topographic effects are indeed important. The numerical model used assumes the subsurface to be homogeneous and isotropic. The model-predicted fracture orientations do not reflect the scatter in fracture orientations seen in the wells. Thus, the present results support the hypothesis that topography can influence subsurface rock fracture patterns, suggest the imitation and reactivation of fractures of different generations, and provide a basis for further observational tests. Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 73-75). 2013-01-01T00:00:00Z http://hdl.handle.net/1721.1/79299 Surface deformation analysis over a hydrocarbon reservoir using InSAR with ALOS-PALSAR data Şahin, Sedar Cihan InSAR has been developed to estimate the temporal change on the surface of Earth by combining multiple SAR images acquired over the same area at different times. In the last two decades, in addition to conventional InSAR, numerous multiple acquisition InSAR techniques have been introduced, including permanent scatterer (PS) (Ferretti et al., 2001) and small baseline subset (SBAS) (Berardino et al., 2002). Stanford method for persistent scatterers (StaMPS) (Hooper, 2006) is another multiple acquisition method that has been developed for estimating ground deformation and differs from the permanent scatterer technique through the method used for pixel selection. In this project, we used the SBAS method to detect the surface deformation over a hydrocarbon reservoir in Adiyaman Providence, Turkey. The SBAS technique is performed on combinations of SAR images that are characterized by small orbital distances with large time intervals. By applying singular value decomposition (SVD), the temporal sampling rate is increased and those subsets are connected. We applied the SBAS method to five ALOS-PALSAR fine-beam dual (FBD) mode images, and removed the topographic phase by using a 3 arc-sec SRTM digital elevation model (DEM). The atmospheric artifacts are determined and filtered out based on available spatial and temporal information on processed data. Our analysis has revealed that due to the effective mitigation measures taken by the oil company, the maximum observed LOS displacement velocity in the oil field is 5 mm/yr with a likely uncertainty of a similar magnitude in the period of 2007-2010. The high uncertainty estimate is due to the other spatially correlated signals of similar and larger magnitude seen in regions outside of the oil field. Thesis (S.M. in Geophysics)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 2013.; Cataloged from PDF version of thesis.; Includes bibliographical references (p. 92-97). 2013-01-01T00:00:00Z