Constraining the magmatic systems of active volcanoes is important for comprehending the mechanisms that drive magma supply. Particularly important are the geometry and characteristics of magma storage regions in the upper and middle crust, which can in uence the style and time-scales of eruptions. Seismic tomography can provide detailed resolution of the physical characteristics of complex three-dimensional structures in the subsurface, and has been successfully employed at several active volcanoes to constrain the structure of their magmatic systems. The island of Montserrat, Lesser Antilles, has been the subject of an land-sea active-source seismic experiment to constrain its upper crustal structure, with particular focus on the magma reservoir feeding the current eruption. A two-dimensional inversion of seismic travel-times from a subset of the data collected, including wide-angle refractions and reflections, constrains the upper crust to a depth of 5-6 km, along a south-east to north-west section. The two-dimensional model delineates the high-velocity cores of the volcanic edifices that make up the island, the slower surrounding volcaniclastic deposits and pelagic sediments, and the underlying upper crust. Analysis of field recordings and synthetic waveforms calculated with a viscoelastic finite-difference method indicates that the high-velocity cores correspond to a geologically heterogeneous region which causes anomalous scattering of the seismic wave field. A three-dimensional seismic velocity model from first-arrival travel-time tomography delineates the high-velocity cores in greater detail and reveals a low-velocity volume at 4 to 8 km depth beneath the active Soufriere Hills volcano, inferred to correspond to the active upper-crustal magma chamber. The magma chamber is further constrained with the help of numerical models of incremental magma chamber growth, which suggests that a magma chamber of about 18 km3 formed at 5.5 to at least 7.5 km depth by incremental intrusion of sills over a few thousand years and is therefore larger and deeper than previously inferred. The new tomographic results are integrated with previous geological, petrological and geodetic constraints to provide one of the most complete models of the magmatic system of an active volcano from the magma source in the mantle wedge to the volcanic edifice at the surface. The volume estimate and chamber location are critical parameters required for models of eruption dynamics, which in turn are key to forecasting the likelihood and characteristics of future eruptions and the mapping of hazard.

Submarine landslides at open continental slopes are the largest mass movements on Earth and can cause damaging tsunamis. To be able to predict where and when such large landslides may occur in the future requires fundamental understanding of the mechanisms that cause them. Due to the inaccessibility of these features this understanding is based on poorly tested hypotheses. Recent studies have proposed that more landslides occur during periods of sea level rise and lowstand, or during periods of rapid sedimentation. These hypotheses are tested by comparing a comprehensive global data set of ages for large submarine landslides to global mean sea level and local sedimentation rates. The data set does not show statistically significant patterns, trends or clusters in landslide abundance, which suggests that the link between sea level and landslide frequency is too weak to be detected using the available global data base. The analysis also shows no evidence for an immediate influence of rapid sedimentation on slope stability, as failures tend to occur several thousand years after periods of increased sedimentation rates. Large submarine landslides occur on remarkably low slope gradients (<2�), which makes them difficult to explain. A widely used explanation for failure of such low angle slopes is high excess pore pressure due to rapid sedimentation and/or focused pore fluid flow to the toe of the slope. If these hypotheses are universal, and therefore also hold for continental margins with comparatively low rates of sediment deposition (where numerous large landslides are observed), is tested in this thesis. Fully coupled 2D stress-fluid flow finite element models are created that simulate the excess pore pressure and drainage response of a continental slope to the deposition of new sediment. Homogeneous models with a wide range of physical-mechanical properties as well as models with an aquifer are loaded by low rates of sediment deposition. All models turn out stable and resulting excess pore pressures are too low to significantly decrease effective stress anywhere along the slope. Hence, factors other than sediment deposition must be fundamental for initiating slope failure, at least in locations with slow sedimentation rates. The results obtained in this thesis not only indicate that failure mechanisms that have previously been considered important may not be universal. They also emphasise the large uncertainties in our current understanding of the occurrence, timing and frequency of large submarine landslides at open continental slopes.

No description available.

In Fe-limited or Fe-stressed waters Fe(II) cycling is an important feature of Fe biogeochemistry because processes favouring Fe(II) production or stability always expand the pool of Fe available to aquatic microorganisms. However almost nothing is known about Fe(II) organic speciation in natural waters and this limits our understanding of how this metal ion may interact with biological Fe uptake systems. Therefore I investigate how organic material affects Fe(II) biogeochemistry in rainwater, riverwater and in suspensions of aged glacial particulates. We compare Fe(II) concentrations (12-3600 nM) and, by adapting a ferrozine based reverse titration technique, ligand binding constants (logKFe(II) <5.5-11) in 5 temperate and sub-tropical river-estuary systems with varying dissolved organic carbon (DOC 200-1300 μM), labile dissolved Fe (Fe <0.2μm available to ferrozine after reduction with ascorbic acid 100 nM-20 μM) and pH (5.5-8.5). In riverwater we identify a natural class of humic Fe(II) ligands with weak Fe(II) binding constants (logKFe(II) <8) and concentrations in excess of Fe(II). Stronger ligands (logKFe(II) 11) were found only in the Itchen (Hampshire, UK) and may be associated with anthropogenic waste inputs. Similarly, the previously identified stabilisation of Fe(II) within North Carolina rainwater may be associated with anthropogenic emissions. Fe(II) concentrations in Wilmington (North Carolina, USA) rainwater have decreased from a high of 52 nM (and a 1:1 ratio of Fe(II):Fe(III)) in summer 2000 to a record low of 3 nM (and a 0.23:1 ratio of Fe(II):Fe(III)) in summer 2013 concurrently with improving air quality. The weak (logKFe(II) <8), terrestrially derived ligands we report in 4 temperate/sub-tropical river systems are likely to be found in most surface freshwater systems, but their impact in higher salinity coastal seawater will be minimal. Stronger Fe(II) ligands (logKFe(II) 11) associated with effluent emissions however may exert a stabilising effect on Fe(II) concentrations in the natural environment. This may result in localised increases in bioavailable Fe concentrations and have implications for aquatic ecosystems in industrialised areas.

Much of the primary production in northern latitudes is associated with the relatively short spring phytoplankton bloom. Quantifying the bloom is essential to understanding export production and energy transfer to higher trophic levels. This study focuses on the physical forcing controlling the spring bloom in the Irminger Basin (IB), situated between Greenland and Iceland. In situ data are available from four cruises to the region carried out under the UK Marine Productivity programme. This data set is extended with six years of SeaWiFS satellite chlorophyll-a concentration (chl-a) data, together with the corresponding model net heat flux (NCEP reanalysis) and satellite measured wind speed (QuikSCAT), sea surface temperature (SST; AVHRR) and photosynthetically available radiation (PAR; SeaWiFS). The remotely sensed data are complemented by a 1-D vertical mixing model and temperature and salinity profiles from Argo drifting profilers. The seasonality in temperature-nutrient (TN) relationships is investigated and the TN relationships are improved by including chlorophyll in the regressions. Basin-wide, daily estimates of nitrate, phosphate and silicate are made from satellite SST and chl-a. The study focuses on three biogeographical zones determined by cluster and Empirical Orthogonal Function analysis of SeaWiFS chl-a data. The three areas have distinct chl-a signatures and cover the East Greenland shelf, the Reykjanes Ridge and the Central Basin. An ANOVA analysis reveals that significant interannual variability is occurring in chlorophyll-a. An objective method for determining the start day of the spring bloom is described. Interannual variability in the timing of the initiation of the bloom and its magnitude and duration is discussed. The influence of the prevailing meteorology on chl-a in different seasons are investigated using generalized linear modelling. Whilst net heat flux and PAR are the dominating factors in spring, wind speed and SST become increasingly influential during summer and autumn. A method for estimating time series of Sverdrup’s critical depth from remotely sensed PAR and attenuation coefficient data is outlined. It is found that the spring bloom never begins before the mixed layer depth becomes shallower than the critical depth, and there is a delay of ~10 days. Specific criteria for the start of the bloom in terms of net heat flux and PAR are determined. The effect of nutrient depletion on the decline of the bloom is discussed. The East Greenland coastal zone is used as an example of the lasting impact that anomalous meteorological conditions can have on the following spring’s bloom. In 2002 the East Greenland region experienced anomalously low chl-a concentrations. Strong easterly winds, associated with the tip-jet phenomena, occurred throughout winter and spring and net heat flux was anomalously low in 2002. The spring bloom in the Irminger Basin can be affected by large scale climatic events, such as shifts in the North Atlantic Oscillation. Finally, the timing of nutrient depletion and its impact on community succession is considered. The possibility of iron limitation in the basin is discussed. A lower bound estimate of export production is made based on the timing of silica availability, and hence diatom dominance, of the spring bloom. The contributions to export production by diatoms and non-diatoms are estimated.

No description available.

A significant proportion of the world’s organic carbon is trapped in submarine methane hydrates. When ocean bottom waters warm, these hydrates may be destabilised, releasing gaseous methane into the surrounding sediments and potentially into the overlying water column and atmosphere. Increased atmospheric methane contributes to further warming as methane is a potent greenhouse gas. Release of methane from hydrate may have accompanied some paleoclimate warming events, but observations of hydrate destabilization due to current global warming remain unconfirmed. The discovery of more than 250 seafloor methane bubble plumes close to the limit of the gas hydrate stability zone offshore Western Svalbard has recently been linked to increases in bottom water temperature in this region over the past 30 years. To assess the source and fate of this methane, this thesis presents a geochemical study of hydrate, sediments, seawater, and gas in the vicinity of the seafloor methane seepage. Analyses of the gas molecular and isotopic compositions reveals that hydrate-bound gas, free gas in shallow sediments, and gas bubbles entering the water column at seafloor seep sites all have the same source. The gas is thermogenic gas produced offshore that has migrated laterally to the continental slope and shelf region. Transport-reaction modelling of pore water chemistry shows that active anaerobic oxidation of methane in sediments is an effective barrier to release of methane into ocean bottom waters. However, small fractures and faults allow ~90% of the methane that enters near-surface sediments to bubble into the water column at localized seafloor seeps. Analyses of the methane distribution in the water column indicate that the methane in the bubbles rapidly dissolves in seawater, and is transported northwards at depth in the West Svalbard Current. As a result, there is limited vertical exchange of methane between deep and surface waters. Surface waters are nevertheless supersaturated due to isopycnal mixing with methane-rich waters from the shallow shelf onto the upper slope. Measurements of methane mixing ratios in air indicate that the sea to air methane flux offshore Western Svalbard does not make a significant contribution to the local atmospheric methane budget. Sedimentary records of the δ13C-CH4 signature of benthic foraminifera provide evidence for intermittent methane seepage at the current limit of hydrate stability (~400 m water depth) over the last ~20,000 years. Although this is likely due to changes in hydrate stability as a result of changes in bottom water temperature, we find no evidence for this in the current data set.

The precarious status of flat oyster Ostrea edulis stocks in Europe is widely acknowledged. To build a scientific basis for oyster restoration, an elevated experimental reef stocked with O. edulis was established within Poole Bay (Dorset, UK). Oysters were planted out on twenty four oyster reef modules (80cm above sea bed) and compared with oysters held on the sea bed close to each reef module to test the hypothesis that a reef habitat enhances physiological performance of O. edulis and improves local biodiversity. The environmental data indicated that there was no significant difference in temperature or salinity between the elevated reefs and sea bed. Whilst total suspended solids were significantly higher at the sea bed than at 80 cm above the sea bed at every sampling interval. The filtration rates of oysters varied with elevation (reef/sea bed) and months. Filtration rates of reef oysters were significantly higher than sea bed oysters. Respiration rates varied among months but were not significantly affected by elevation. Elevation and month also affected the total number of haemocytes and the granulocyte population; reef oysters had significantly higher numbers of haemocytes than sea bed oysters. Althought geographical variation in filtration rate was also observed, this study suggested that the improved physiology of reef oysters in summer was still notable when comparing between different populations. An increase in filtration rate for reef oysters may, however, increase the likelihood of uptake of the protozoan parasite, Bonamia sp. However, haemocyte and haemolymph protein data suggested that there was no differential impact on physiology between reef and sea bed oysters as a result of Bonamia sp. infection. Fifty-four epifaunal species including oyster spat were found on oyster valves in reef boxes whilst only 23 species with no oyster spat were found on oyster valves in oyster cages laid on sea bed. The epifaunal community on oyster valves on reefs was significantly different from those sea bed valves but also had some species in common. Sixty-five mobile species were observed in reef boxes, whilst 47 species were recorded in oyster cages. These differences in faunal communities observed on the elevated reefs and sea bed implied that the presence of elevated reef habitats created by O. edulis valves can enhance or accelerate local diversity in Poole Bay. As current stocks of European flat oysters (O. edulis) in Europe have declined in both abundance and distribution, the results of this pilot study suggest that the culture of oysters on elevated reef structure represents at least a partial solution to improve O. edulis physiology for restoration in Europe.

The Bermuda Atlantic Time-series Study (BATS) site represents a unique hydrographical oceanic environment to study the factors controlling bacterial growth dynamics. The region was sampled at monthly intervals from 1991 to 1996. A typical annual cycle was defined by a deep winter mixing, followed by an increasing stratification of the mixed layer through summer and fall. There were striking seasonal patterns in phytoplankton productivity with a strong maximum immediately following the deep winter mixing and the intrusion of inorganic nutrients. Bacterial growth rates showed a similar pattern but had a secondary peak in late summer/fall of the same magnitude as the spring bloom. Bacterial abundance showed only slightly elevated concentrations in spring. A number of time course storage experiments showed that bacterial abundance decreased by 24-50 % within 7-29 days in samples preserved with 2.5 % glutaraldehyde. By adding a protease inhibitor prior to the addition of glutaraldehyde, the loss of bacterial cells was reduced to 17-18 % over a 30 day period. These findings lead to the recommendation that samples for bacterial abundance should be processed immediately for epifluorescence enumeration. An average of 47 % of all bacterial cells passed the pore size of a Whatman GF/F filter and these viable cells should be included in biomass estimates. An annual average of 26 % of estimated C settled below the spigot of a Niskin water sampler. Consequently, C measurements made on GF/F filters must account for the particles settling below the spigot of a water sampler as well as the number of bacteria lost during the process of filtration. This study is the first to present single cell elemental C, N and P measurements from natural bacteria in the Sargasso Sea. A wide range in elemental content was found between single cells and this could be expressed as a function of the cell size. By applying an average cell volume, an annual average of 10 fg C, 1.9 fg N and 0.28 fg P was calculated per bacterial cell. The average percentage integrated stocks of C in the upper 250 m of the water column, was 20 (phytoplankton), 18 (microheterotrophs) and 62 (other non-living detrital matter). Bacterial biomass was higher than phytoplankton outside the spring bloom period, but non-living carbon showed an overall dominance through out the year. Phytoplankton generation time was relatively constant over the season. Bacterial generation time was ten times longer and showed a greater seasonal variation, but largely followed the changes in primary production. Assuming that 50-70 % of the bacterial cells were non-living, the mean bacterial generation time was estimated to be 7 times (0-60 m) and 1.4 times (80-140 m) longer than phytoplankton generation time. During the spring bloom event, an average of 85 % of the bacterial growth rate was removed by grazing and viral lysis. This was the only noted decoupling between growth and removal of bacteria at BATS. During the remainder of the year bacterial growth was balanced by the loss rate, due to grazing and viral lysis. A linear relationship was found between net DOC accumulation and primary production in natural surface waters at BATS. Phytoplankton net DOC excretion constituted 42 % of the primary production rate, while Trichodesmium colonies only excreted 12 % (puffs) and 23 % (tufts). By using a conservative estimate of the bacterial growth efficiency (14 %) and the net DOC accumulation rate from this study, gross DOC excretion was equivalent to the rate of primary particulate C production. Results from this study suggests that bacteria at the BATS site are using the majority of the DOC generated by primary production. Prior to the spring bloom and the associated increase in DOC excretion, bacteria appeared to be C limited in the surface waters at BATS. Following the peak in primary production and coinciding with the depletion of inorganic nutrients in the euphotic zone, the bacterial cells became less C starved, but never reached a true N or P limitation. Regenerated nutrients from grazing and viral lysis of bacteria and new production by diazotrophic Trichodesmium colonies and trichomes, may support the bacteria with N and P in the euphotic zone in summer and fall. The substrate dependent growth and increase in biomass of bacteria exhibited at BATS in spring, is indicative of a bottom-up controlled system, whereas the bacteria appeared to be topdown controlled by grazing and viral lysis for the remainder of the year.

Various factors may increase the supply of organic matter content (eutrophication status) of estuarine and coastal systems. The most common cause of eutrophication is considered to be nutrient enrichment. In this study data has been collected in monthly surveys from January 1998 to April 1999 in the Southampton Water system, and the major features of nutrients (nitrogen, phosphorus and silicon) and suspended particulate matter (SPM) thus obtained. Additionally phosphorus in porewaters, and the solid phase in sediments were measured. The forms of phosphorus in the solid phase of sediment (deposited and suspended) were also measured using a modified sequential phosphorus extraction protocol (SPEPs) method. The ranges of the dissolved inorganic phosphorus (DIP) and other nutrients are similar to the data from the SONUS investigations. The overall distribution of DIP are strongly influenced by the sewage inputs to the system, with a peak in concentration in the River Itchen from the sewage works followed by apparently conservative behaviour. The concentration of dissolved organic phosphorus (DOP) were often low (rarely exceeded 5 |iM) and irregularly distributed in the system. The concentrations of particulate phosphorus in the water column and in the particles decreased seaward. The P:Fe ratio in the bicarbonate-dithionite released phase from particles implied that the phosphorus "saturated" particles near the sewage outfall are likely buried before they reach the high salinity waters and the particles in the high salinity waters are mainly from sediment resuspension, the detritus of phytoplankton or from offshore. The N/P ratios obtained implies that the system has potential phosphorus limitation of phytoplankton growth. The concentrations of the forms of inorganic phosphorus in the sediment generally decreased seaward. The Ca-bound phosphorus appears to be the dominant form of inorganic phosphorus (about 55%-85% of total inorganic phosphorus) in most of the sediments in the system. Benthic flux measurements revealed that the sediments in the system generally act as a sink of dissolved inorganic phosphorus and a source of dissolved organic phosphorus. This is in contrast to the fluxes of DIP predicted from the pore-water concentration gradients and implied the presence of a ferric iron-rich oxidising cap on the sediment surface. The results of a Kd model applied on the freshwater-sea boundary revealed that within the range of the riverine DIP concentration only a small amount of removal or addition of DIP is likely to occur. The Kd model is useful in predicting the influence of resuspended particles on DIP in the water column after resuspension "events" caused by storm and dredging activities. The total phosphorus input to the system is estimated as 144 ± 23.7 tonnes per year. The riverine input is the major source of phosphorus (about 69% of the total inputs). The direct sewage input (30% of the total phosphorus inputs) is also an important source of phosphorus in the system, especially in summer. The atmospheric input (1% of the total phosphorus inputs) is small. A considerable exchange flux with adjacent coastal waters (about 40% of total phosphorus inputs) reveals that the system is a phosphorus source to the English Channel and adjacent areas. The particle burial is an important process and retains about 60% of the phosphorus entering the system. However, significant removal of phosphorus from the water column was not obviously observed, suggesting that there are other sources of phosphorus in the system or the estimate of internal loss is overestimated. Further study is needed to reaffirm the behaviour of all forms of phosphorus in the system, and assess their yearly variation. An investigation of the forms of particulate phosphorus and phosphorus in surfacial sediments on better temporal and spatial resolution is also needed together with more geological information on the origin and sizes of particles.