Graduation date: 1998

Storm-induced open-ocean convective mixing is one of the primary processes controlling the supply of nitrate to the sunlit layer of the oligotrophic North Atlantic Subtropical Gyre (NASG). Yet, the magnitude and timing of nitrate fluxes during winter convection is poorly understood due to an absence of targeted process studies. In the northwest NASG, multiple quasi-Lagrangian studies were conducted during the boreal winters of 2004 and 2005 in an effort to sample strong winter convection. During each of the time-series studies, inventories of vertically fluxed nitrate were quantified approximately every twelve hours using the distribution of helium isotopes ([delta]�He) and nitrate in the water column. This method is known as the Helium Flux Gauge Technique (HFGT). Large variability in surface forcing and density structure of the upper ocean was observed between the two years; however, only winter 2005 experienced convective mixing to depths greater than 150 m. In winter 2004, mild atmospheric conditions coincided with a positive phase in the winter North Atlantic Oscillation (NAO), consistent with the dominant regime experienced during the previous decade. On average 36 � 9 mmol m[-2] of fluxed nitrate was inferred by excess �He in the mixed layer of the ocean during the winter 2004 study period. This inventory of physically transported nitrate is attributed to the sampling of waters laterally advected from nearby eddy features. The sampling of multiple water masses is likely due to the inability of the drogue to persistently follow water masses efficiently. Although physical evidence indicates spatial variability within the time-series data, the length scales of convective mixing appear to be greater than those associated with spatial aliasing as a result of drogue performance. This observation provides us with increased confidence that the objectives for the present study are not compromised by spatial variability in the data. In contrast, winter 2005 experienced a negative NAO, strong physical forcing and convective mixing to depths > 250 m. Two convectively modified water masses, most likely resulting from a single storm event, were sampled at different stages of development. These two water masses exhibit large variability in the magnitude of nitrate entrained in the convective layer from the thermocline. An average inventory of 247 � 56 mmol NO₃[-]m[-2] was entrained in the rapidly expanding convective layer of the first water mass in the first few days following the storm approach. In contrast, ongoing entrainment of nitrate was absent from the second water mass, sampled two weeks later when the depth of the surface mixed layer was consistently ~ 300 m. These results indicate that surrounding fluid is entrained into the convective layer when it is actively expanding in the vertical. On the other hand, significant fluid entrainment does not occur at the base of the plume once sinking waters have reached a level of neutral buoyancy. The persistence of elevated nitrate stocks (~ 100 mmol m[-2]) in the convective layer two to three weeks after the inferred injection event, suggests sub-optimal nitrate uptake by resident phytoplankton. Phytoplankton growth was most likely resource limited by light or a micronutrient such as iron. Despite the implied biolimitation, changes in chlorophyll-a, a proxy for phytoplankton biomass, indicate net production within the convective layer. On average, the convective layer was observed to support an inventory of 62 � 6mg chlorophyll-a m[-2], increasing at an average rate of 3.4mg m[-2] d[-1]. This inventory indicates a slow build-up of phytoplankton biomass to near bloom levels, ahead of the main spring bloom that typically follows formation of the seasonal thermocline near Bermuda. Net production in the convective layer was likely due to transient periods of increased (weak) surface stability that were observed to support high phytoplankton biomass, following the cessation of thermocline fluid entrainment. When nitrate and excess �He in samples collected from the thermocline were regressed for the purpose of quantifying nitrate fluxes, the results showed that between 1.6 - 2.0 [mu]mol kg[-1] of dissolved nitrate was present during formation of the water mass. This suggests the source of this excess (above Redfield ratios) nitrate in the thermocline of the NASG is not local, and has ramifications for local nitrogen fixation budgets determined using geochemical approaches. Thesis supervisors: William J. Jenkins, Senior Scientist, WHOI (United States of America); Philip W. Boyd, Senior Scientist, NIWA (New Zealand); Michael W. Lomas, Senior Scientist, BIOS (Bermuda)

Storm-induced open-ocean convective mixing is one of the primary processes controlling the supply of nitrate to the sunlit layer of the oligotrophic North Atlantic Subtropical Gyre (NASG). Yet, the magnitude and timing of nitrate fluxes during winter convection is poorly understood due to an absence of targeted process studies. In the northwest NASG, multiple quasi-Lagrangian studies were conducted during the boreal winters of 2004 and 2005 in an effort to sample strong winter convection. During each of the time-series studies, inventories of vertically fluxed nitrate were quantified approximately every twelve hours using the distribution of helium isotopes ([delta]�He) and nitrate in the water column. This method is known as the Helium Flux Gauge Technique (HFGT). Large variability in surface forcing and density structure of the upper ocean was observed between the two years; however, only winter 2005 experienced convective mixing to depths greater than 150 m. In winter 2004, mild atmospheric conditions coincided with a positive phase in the winter North Atlantic Oscillation (NAO), consistent with the dominant regime experienced during the previous decade. On average 36 � 9 mmol m[-2] of fluxed nitrate was inferred by excess �He in the mixed layer of the ocean during the winter 2004 study period. This inventory of physically transported nitrate is attributed to the sampling of waters laterally advected from nearby eddy features. The sampling of multiple water masses is likely due to the inability of the drogue to persistently follow water masses efficiently. Although physical evidence indicates spatial variability within the time-series data, the length scales of convective mixing appear to be greater than those associated with spatial aliasing as a result of drogue performance. This observation provides us with increased confidence that the objectives for the present study are not compromised by spatial variability in the data. In contrast, winter 2005 experienced a negative NAO, strong physical forcing and convective mixing to depths > 250 m. Two convectively modified water masses, most likely resulting from a single storm event, were sampled at different stages of development. These two water masses exhibit large variability in the magnitude of nitrate entrained in the convective layer from the thermocline. An average inventory of 247 � 56 mmol NO₃[-]m[-2] was entrained in the rapidly expanding convective layer of the first water mass in the first few days following the storm approach. In contrast, ongoing entrainment of nitrate was absent from the second water mass, sampled two weeks later when the depth of the surface mixed layer was consistently ~ 300 m. These results indicate that surrounding fluid is entrained into the convective layer when it is actively expanding in the vertical. On the other hand, significant fluid entrainment does not occur at the base of the plume once sinking waters have reached a level of neutral buoyancy. The persistence of elevated nitrate stocks (~ 100 mmol m[-2]) in the convective layer two to three weeks after the inferred injection event, suggests sub-optimal nitrate uptake by resident phytoplankton. Phytoplankton growth was most likely resource limited by light or a micronutrient such as iron. Despite the implied biolimitation, changes in chlorophyll-a, a proxy for phytoplankton biomass, indicate net production within the convective layer. On average, the convective layer was observed to support an inventory of 62 � 6mg chlorophyll-a m[-2], increasing at an average rate of 3.4mg m[-2] d[-1]. This inventory indicates a slow build-up of phytoplankton biomass to near bloom levels, ahead of the main spring bloom that typically follows formation of the seasonal thermocline near Bermuda. Net production in the convective layer was likely due to transient periods of increased (weak) surface stability that were observed to support high phytoplankton biomass, following the cessation of thermocline fluid entrainment. When nitrate and excess �He in samples collected from the thermocline were regressed for the purpose of quantifying nitrate fluxes, the results showed that between 1.6 - 2.0 [mu]mol kg[-1] of dissolved nitrate was present during formation of the water mass. This suggests the source of this excess (above Redfield ratios) nitrate in the thermocline of the NASG is not local, and has ramifications for local nitrogen fixation budgets determined using geochemical approaches. Thesis supervisors: William J. Jenkins, Senior Scientist, WHOI (United States of America); Philip W. Boyd, Senior Scientist, NIWA (New Zealand); Michael W. Lomas, Senior Scientist, BIOS (Bermuda)

Thesis (M.Phil.) - University of St Andrews, May 2007.

Thèse (D.R.Min.) -- Université du Québec à Chicoutimi, 2002. / La p. de t. porte en outre: Thèse présentée à l'Université du Québec à Chicoutimi pour l'obtention du doctorat en ressources minérales offert à l'Université du Québec à Montréal en vertu d'un protocole d'entente avec l'Université du Québec à Chicoutimi. CaQCU Bibliogr.: f. [121]-128. Document électronique également accessible en format PDF. CaQCU

Thesis (Ph. D.)--Duke University, 2007.

Oceanic lithosphere constitutes the upper boundary layer of the Earth’s convecting mantle. Its structure and evolution provide a vital window on the dynamics of the mantle and important clues to how the motions of Earth’s surface plates are coupled to convection in the mantle below. The three-dimensional shear-velocity structure of the upper mantle beneath the Atlantic Ocean is investigated to gain insight into processes that drive formation of oceanic lithosphere. Travel times are measured for approximately 10,000 fundamental-mode Rayleigh waves, in the period range 30-130 seconds, traversing the Atlantic basin. Paths with >30% of their length through continental upper mantle are excluded to maximize sensitivity to the oceanic upper mantle. The lateral distribution of Rayleigh wave phase velocity in the Atlantic upper mantle is explored with two approaches. One, phase velocity is allowed to vary only as a function of seafloor age. Two, a general two-dimensional parameterization is utilized in order to capture perturbations to age-dependent structure. Phase velocity shows a strong dependence on seafloor age, and removing age-dependent velocity from the 2-D maps highlights areas of anomalously low velocity, almost all of which are proximal to locations of hotspot volcanism. Depth-dependent variations in vertically-polarized shear velocity (Vsv) are determined with two sets of 3-D models: a layered model that requires constant VSV in each depth layer, and a splined model that allows VSV to vary continuously with depth. At shallow depths (~75 km) the seismic structure shows the expected dependence on seafloor age. At greater depths (~200 km) high-velocity lithosphere is found only beneath the oldest seafloor; velocity variations beneath younger seafloor may result from temperature or compositional variations within the asthenosphere. The age-dependent phase velocities are used to constrain temperature in the mantle and show that, in contrast to previous results for the Pacific, phase velocities for the Atlantic are not consistent with a half-space cooling model but are best explained by a plate-cooling model with thickness of 75 km and mantle temperature of 1400oC. Comparison with data such as basalt chemistry and seafloor elevation helps to separate thermal and compositional effects on shear velocity.

Part I. (1) An elementary treatment of the motion of charged particles in a magnetic field is presented. The concept of guiding centre motion is introduced, and is used in outlining the theory of particle drifts. (2 ) The motion of charged particles in the geomagnetic field is discussed, and the concept of adiabatic invariance introduced. (3) Mc Iliwains coordinates for mapping the distribution of charged particles trapped in the geometric field are defined and briefly discussed. (4) A survey of present knowledge of the Van Allen radiation zones is made. Particular attention is given to the distribution, characteristics, and variability of the trapped radiation. (5) The Cape Town magnetic anomaly, the Brazil radiation anomaly and the South Atlantic radiation anomaly are discussed. The electrons entering the South Atlantic radiation anomaly are shown to be those monitored over Iowa by the US satellite Injun I. Part 11. (1) It is shown how the geomagnetic field can, at high altitudes and over relatively short distances, be approximated by the field of a monopole. A new method is developed which enables one to plot the energy absorbed from an electron (which moves in a monopole field in the atmosphere) against altitude, given the initial energy and pitch angle of the electron. Some numerical computations using this method are described, and the results discussed. These results are used, in conjunction with US satellite Injun I for the Iowa region, to estimate the energy inout to the atmosphere in the South Atlantic radiation anomaly. The main approximations and simplifying assumptions made in this treatment, are discussed. (2) Geophysical effects generally recognized to be connected with the precipitation of charged particles are discussed. In the course of this discussion the two main theories of the connection between the radiation zones and the auroras are examined. (3) A preliminary discussion, based on the work summarized in point (1) of detectable geophysical events associated with the precipitation of electrons into the South Atlantic radiation anomaly is given. It is concluded that auroral emission, X-ray bursts, and ionospheric ionization in the E region should be more frequent and pronounced in the South Atlantic radiation anomaly than in any other region of comparable invariant latitudes and that the effects of atmospheric heating by precipitated electrons should be detectable over the anomaly. (4) An assessment is made of the value of the method referred to in point (1). Suggestions for its modification and extension are put forward. It is suggested that if more extensive rocket and satellite data on the low energy component of the electron flux become available, this method can be employed in a rigorous theoretical investigation of the South Atlantic radiation anomaly. Summary, p. 98-100.

Although it has long been known that cold dense waters from the Nordic Seas overflow the Wyville Thomson Ridge, the water masses' subsequent pathways and fate have been uncertain. This study conclusively places Wyville Thomson Ridge Overflow Water (WTOW) as an important water mass in the eastern subpolar North Atlantic for the first time. Using a variety of chemical tracer s (chlorofluorocarbons, oxygen, nutrients and aluminium) in conjunction with temperature and salinity, WTOW is traced southwards into the northern and central Rockall Trough as well as into the channels between the western banks. The overflow water has a clear temperature, salinity and chlorofluorocarbon (CFC-11 and CFC-12) signature. Additionally, levels of aluminium are elevated in WTOW suggesting that this element is potentially a useful and novel water mass tracer. The lower oxygen layer complicates the use of dissolved oxygen and nitrate as tracers in the mid water column. However, higher and lower concentrations respectively in the western trough reveal the presence of WTOW in this area. The overflow water does not appear to have a silicate or phosphate signature. Two branches of WTOW exist in the Rockall Trough: a slow-moving indistinct intermediate branch (600-1200 m) f ound in both the east and west of the basin; and a coherent deep branch (> 1200 m) that flows southward along the western banks of the trough. As well as having a large spatial footprint within the Rockall Trough, intermediate a nd deep WTOW are temporally persistent being present 65-75 % of the time between 1975 and 2008. The signature of WTOW at intermediate depths is absent from the Ellett Line record in the mid-1980s and early-1990s. As deep WTOW is still observed during these periods flow over the Wyville Thomson Ridge cannot have ceased. Instead, it is proposed that the strength of the Subpolar Gyre is an important driver in the temporal distribution of intermediate WTOW within the Rockall Trough. When the gyre is strong, such as in the mid-1980s and early-1990s, the mid water column is dominated by waters originating from the west which block the southward flow of intermediate WTOW. In contrast, when the gyre is weak, such as in the late-1990s and 2000s, subpolar waters lie further west enabling intermediate waters within the Rockall Trough to be dominated by the southern orig inating Mediterranean Overflow Water and the northern water mass of WTOW.

Submitted by Renata Lopes (renatasil82@gmail.com) on 2015-12-08T14:30:37Z No. of bitstreams: 1 julianarodriguesmoron.pdf: 5395147 bytes, checksum: 0f2d0052bd8730bc31eb9bf590e8a739 (MD5) / Approved for entry into archive by Adriana Oliveira (adriana.oliveira@ufjf.edu.br) on 2015-12-09T13:42:27Z (GMT) No. of bitstreams: 1 julianarodriguesmoron.pdf: 5395147 bytes, checksum: 0f2d0052bd8730bc31eb9bf590e8a739 (MD5) / Made available in DSpace on 2015-12-09T13:42:27Z (GMT). No. of bitstreams: 1 julianarodriguesmoron.pdf: 5395147 bytes, checksum: 0f2d0052bd8730bc31eb9bf590e8a739 (MD5) Previous issue date: 2015-03-30 / CAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Apesar de investigações bioacústicas de golfinhos-rotadores já terem sido abordadas e descritas na literatura, a região do Oceano Atlântico Sul Ocidental só abrangeu a população do arquipélago de Fernando de Noronha. Portanto, a fim de contribuir para um melhor conhecimento desta espécie cosmopolita, este estudo teve como objetivo descrever os sons através de seus parâmetros acústicos de uma gravação oportunista realizada ao longo da quebra da plataforma continental Brasileira. Um grupo de aproximadamente 400 golfinhos Stenella longirostris foi registrado através de uma matriz de arrasto composta por um hidrofone realizando gravação mono contínua a 96kHz / 24bits. Após uma seleção de 12 minutos a partir do registro, 1.100 assobios foram extraídos e classificados em sete tipos de contornos, juntamente com outras dez variáveis extraídas. Os assobios dos golfinhos-rotadores foram constituídos em sua maioria por contorno ascendente (41,2%). O grupo apresentou ainda assobios com maior índice de modulação de frequência do que já havia sido descrito na literatura, apresentando uma frequência máxima de 26,4 kHz. Apesar da curta duração desses assobios, variando de 0,03 s a 2,56 s (média = 0,67 s), esses valores também foram mais altos do que a maioria dos estudos havia relatado. Apesar de diferenças estatisticamente significativas, a comparação a partir da média dos valores dos parâmetros extraídos apontou que dois estudos anteriores realizados no Oceano Pacífico Norte são mais semelhantes em relação a este estudo. Dessa forma, as possibilidades a partir das comparações discutidas no presente trabalho não só auxiliam a identificar a variabilidade nas emissões de forma a melhorar as classificações de detecção acústica em pesquisas e mitigação de impacto; como também contribuem para a compreensão de populações distintas e / ou espécies distribuídas em diferentes regiões oceânicas. / Spinner dolphins’ bioacoustics have already been described in literature, meanwhile the Western South Atlantic Ocean was only approached inside Fernando de Noronha Archipelago. Therefore, in order to contribute to a better knowledge of this cosmopolitan species, this study aimed to describe the sounds through their acoustic parameters detected during an opportunistic recording carried out at the Brazilian continental shelf break. A group of approximate 400 Stenella longirostris dolphins was recorded through a one-element hydrophone array performing continuous mono recording at 96 kHz/24 bits. After a 12 minutes selection of the record, 1,100 whistles were extracted and classified into seven contours shapes, along with other ten variables extracted. The spinner dolphins whistles was mostly comprised by upsweep (41.2%) contour. The group produced higher pitch frequency modulated whistles than what has been previously described, presenting a maximum frequency of 26.4 kHz. Despite the short whistle durations, ranging from 0.03 s to 2.56 s (mean = 0.67 s), those values were also higher than most of previous works had reported. Despite statistically significant differences, the comparison from the average of the extracted parameters pointed out that two previous studies in the North Pacific Ocean are more similar in relation to this study. Thereby, the possibilities from the comparisons discussed in this paper not only assist in identifying the variability in the emissions in order to improve acoustic detection classifications in research and impact mitigation; but also contribute to the understanding of distinct populations and/or species distributed in different ocean basins.