When active venting has ceased, reduced minerals in hydrothermal mounds and sediments continue to provide an inorganic energy source for chemolithotrophic microbes. This research focuses on the nature of microbially-mediated metal transformations in hydrothermal sediments during sulphide alteration and their impact on the ultimate fate of hydrothermal sulphides on the seafloor. The core studied is more seawater altered than other cores studied at TAG and provides an insight into the bacterial and archaeal communities as well as the geochemical processes taking place in highly altered metalliferous sediments. This study combines geochemical approaches with microbiological and organic biomarker measurements within the suboxic transition zone of sulphidic sediments to characterise the reactions and microbial communities present. This integrated approach demonstrates that (a) there is biogeochemical zonation within the sediment sequence with distinct microbial communities present at the sulphide-oxic seawater transition, (b) the microbes identified are associated with Fe and S redox cycling, (c) Marinobacter sp. are dominant at the sulphide interface. There is a significant shift in the microbiological community across the redox transition zone in these sulphidic sediments. The microbial assemblage of the suboxic transition zone is dominated by Bacillus sp. which are microaerophilic whereas the sulphide layer assemblage is dominated by Marinobacter sp. which are Fe oxidisers. Based on biomarker assemblages and genetic analyses, archaeal (and to a certain degree bacterial) communities are comparable to other hydrothermal settings despite the low biomass present. Processes inferred to be important in this sediment include the S, Fe and N cycle, all potentially coupled to the release and uptake of a range of transition metals. Significant recycling of redox active species occurs in the suboxic transition zones present in the sediment core. Uranium concentrations are low compared with other less altered sulphidic hydrothermal sediments, and U is associated with the upper regions of the suboxic transition zone and is associated with enrichment of a suite of other transition metals (e.g. Cu, Mo, As and V). Massive electrodes were constructed from CuFeS2 (from Ireland and TAG) and FeS2 (from TAG) and studied in oxygenated artificial seawater under circumneutral conditions using electrochemical methods. The results can be explained by existing hypotheses about pyrite and chalcopyrite oxidation and their oxidation products. The oxidation status of Cu in covellite could be identified as Cu+1. Impurities found (as expected because natural samples were used) have no effect on the electrochemical behaviour of the electrodes. The impact of Marinobacter sp. on sulphide alteration was studied in detail using an electrochemical approach. The results demonstrate that Marinobacter aquaeolei enhances the rates of oxidation. Marinobacter species seem to be of special importance for weathering reactions on the seafloor and in hydrothermal settings like the TAG area.

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.

Tritium is a by-product of civil nuclear reactors, military nuclear applications, fusion programmes and radiopharmaceutical production. It commonly occurs, though not exclusively, as tritiated water (HTO) or organically-bound tritium (OBT) in the environment but may exist as other forms in nuclear-related construction and fabrication materials. During the lifetime of nuclear sites (especially those involving heavy water) tritium becomes variably incorporated into the fabric of the buildings. When nuclear decommissioning works and environmental assessments are undertaken it is necessary to accurately evaluate tritium activities in a wide range of materials prior to any waste sentencing. Of the various materials comprising UK radioactive wastes, concrete and metal account for approximately 20% of the total weight of low level waste (LLW) and 12% and 35% of the total weight of intermediate level waste (ILW). Proper sampling and storage of samples are significant factors in achieving accurate tritium activities. The degree of loss of 3H and cross-contamination can be significantly reduced by storing samples in an air/water tight container in a freezer (-18°C). The potential for tritium contamination is dependent on the 3H form. Most 3H loss originates from tritiated water which is easily exchanged with atmospheric hydrogen in the form of water vapour at room temperature. However, the loss of more strongly bound 3H, produced in-situ in materials by neutron activation, is not significant even at room temperature. Such tritium is tightly retained in materials and does not readily exchange with water or diffuse. In nuclear reactor environments tritium may be produced via several neutron-induced reactions, 2H(n,g)3H, 6Li(n,a)3H, 10B(n,2a)3H and ternary fission (fission yield <0.01%). It may also exist as tritiated water (HTO) that is able to migrate readily and can adsorb onto various construction materials such as structural concrete. In such locations it exists as a weakly-bound form that can be lost at ambient temperatures. Bioshield concretes present a special case and systematic analysis of a sequence of sub-samples taken from a bioshield core (from UKAEA Winfrith) has identified a strongly-bound form of 3H in addition to the weakly bound form. The strongly bound 3H in concrete is held more strongly in mineral lattices and requires a temperature of >850°C to achieve quantitative recovery. This more strongly retained tritium originates from neutron capture of trace lithium (6Li and potentially 10B) distributed throughout minerals in the concrete. The highest proportion of strongly bound 3H was observed in the core sections closest to the core. Weakly bound tritium is associated with water loss from hydrated mineral components. Tritium is retained in metals by absorption by free water, hydrated surface oxidation layer, H ingress into bulk metal and also as lattice-bound tritium produced via in-situ neutron activation. Away from the possible influence of neutrons, the main 3H contamination to metals arises from absorption and diffusion via atmospheric exposure to the HTO. Here contamination is mainly confined to the metal surface layer. The tritium penetration rate into metal surfaces is controlled by the metal type and its surface condition. Where metals are exposed to a significant neutron flux and contain 6Li, 7Li and 10B then in situ 3H production will occur which may propagate beyond the surface layer. In such cases tritium may exist in two forms namely a weakly bound HTO form and a non-HTO strongly bound form. The HTO form is readily lost at moderate temperatures (~120°C) whereas the non-HTO requires up to 850°C for complete extraction.

Estimates of vertical fluxes of nitrate (JN) into the subsurface chlorophyll maximum from the bottom mixed layer were made in a variety of hydrological regimes over the Celtic Sea in 2003 and 2005. Over a topographically flat shelf JN varied with the spring-neap tidal cycle (1.1 (neap) – 2.8 (spring) mmol m-2 day-1), driven by changes in barotropic shear generated vertical diffusivities (Kz) at the base of the thermocline. Further increases in nitrate fluxes were possible through small shear perturbations. JN is further enhanced over topographic features, such as banks on the shelf or the shelf break, by the generation and dissipation of lee waves. The strength of mixing driven by the lee waves also varies with the spring-neap cycle, with higher Kz at the base of the thermocline occurring around spring tide, compared to neap tide, associated with the greater occurrence of short period internal waves. Over banks vertical nitrate fluxes varied between 2.9 (neap) – 15.7 (spring) mmol N m-2 day-1 and over the shelf break estimated vertical nitrate fluxes were 4 (neap) –15 (spring) mmol m-2 day-1. These fluxes are capable of supporting new production of 207 mg C m-2 day-1 over the Celtic Sea shelf, which over the summer stratified period is potentially greater than the new production taking place in the spring bloom. Enhanced production of 1200 mg C m-2 day-1 is supportable over regions of the shelf affected by the generation of lee waves over banks. This equated to a 4% increase in new production within the SCM over the Celtic Sea shelf. 31% of new production in the Celtic Sea was associated with the shelf break, where 660 mg C m-2 day-1 could be supported in the shelf break region.

The aim of this study was to improve our understanding of the marine iron cycle using a newly implemented technique to measure dissolved iron in seawater. The setting up of a flow-injection analyser with chemiluminescence detection (FIACL) for Fe(II) proved to be non-trivial. Extensive work was undertaken to solve problems relating to our limited level of understanding of the CL reaction, and the variable behaviour of the resins prepared to preconcentrate iron. An analyser for Fe(II)+(III) was optimised, and careful assessment of data demonstrated the high quality of the information interpreted in this study, from the Celtic Sea shelf edge (Northeast Atlantic), and from the North Scotia Ridge (Southern Ocean). The distribution of iron at the Celtic Sea shelf edge was examined, and was used to provide a conceptual framework for future studies. Dissolved Fe (< 0.4 µm) concentrations were measured in samples from nine vertical profiles taken across the continental slope (160 – 2950 m water depth). Dissolved iron concentrations varied between 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the iron distributions. The presence of elevated levels of dissolved Fe near the seafloor was consistent with release of Fe from in situ particulate organic matter remineralisation at two upper slope stations, and possibly of pore water release upon resuspension on shelf. Lateral transport of dissolved iron was evident in an intermediate nepheloid layer and its advection along an isopycnal. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. The possibility of iron limitation of phytoplankton at the shelf edge was not ruled out despite obvious depletion of nitrate. This study supports the view that export of dissolved iron laterally to the ocean’s interior from shelf and coastal zones may have important implications for the global budget of oceanic iron. A set of surface samples collected on a survey between the Falkland Islands and South Georgia were analysed for total dissolvable iron. Results suggested a source of benthic iron near South Georgia. A shift in photo-physiology of phytoplankton towards South Georgia was probably influenced by the transition from iron-limited to iron-replete populations. These results therefore strongly support the hypothesis that South Georgia may be a "pulse-point" of iron to high-nutrient low-chlorophyll waters.

The predicted cycling of the whole ocean through hydrothermal plumes is comparable to the mixing time of the oceans (few thousand years). Hence, understanding hydrothermal plume processes is crucial if their impact on the global geochemical cycles of elements is to be assessed. One of the most important processes that has been demonstrated to modify the gross chemical flux from venting to the oceans is the oxidative precipitation of dissolved Fe (II). It has been hypothesised that this might vary significantly from one ocean basin to another along the path of thermohaline circulation. To test that hypothesis, hydrothermal plume samples were collected from the first confirmed hydrothermal vent fields in the Indian Ocean, at Kairei and Edmond, close to the Rodriguez Triple Junction, during the RRS Charles Darwin cruise CD128 in 2001. The samples were analysed to determine the concentrations of dissolved iron and manganese and particulate Fe, Mn, Al, Ca, Mg, Cu, Zn, P, V, As, Y and the rare earth elements. For a subset of the samples, the concentrations of Fe, Mn, Cu and P in different size fractions of the particulate phase were also measured. Dissolved Fe and Mn concentrations are high in the Kairei and Edmond hydrothermal plumes compared to Atlantic and Pacific hydrothermal plumes previously studied. Particulate Fe concentrations are also high while particulate Mn concentrations remain low throughout the plume. Of the total (i.e. particulate plus dissolved) Fe which emerges from the vents, approximately 20-30% is lost from the plume via the removal of Fesulfide phases formed early in the buoyant plume. Further loss of Fe due to the oxidation and formation of particulate Fe-oxide phases results in a total Fe loss of 50-70%. For the very young non-buoyant plume samples, there is very little in situ particulate Fe present. The behaviour of the chalcophile elements (Cu, Zn, Cd and Pb), elements which exist as oxyanions in seawater (P, V and As), as well as the rare earth elements and Y are consistent with previous studies of elemental behaviours in hydrothermal plumes in the Atlantic and Pacific Oceans. The observed behaviours of these elements with respect to particulate Fe suggests that the differing Fe (II) oxidation rates between ocean basins do not impact the processes taking place within hydrothermal plumes. In addition, fractionation of Fe, Mn, Cu and P in the Edmond hydrothermal plume between the dissolved, colloidal and fine and coarse particulate phases shows consistency with previous conclusions based on only one of these phases. The P/Fe and V/Fe ratios of the hydrothermal particulate samples are intermediate to those of particulate samples from the Atlantic and Pacific Oceans suggesting that as previously hypothesised, these ratios are dependent on dissolved ambient phosphate concentrations. Hence, there remains the potential to use these ratios from sediment cores as paleo-proxies for dissolved phosphate concentrations.

Samples for the analysis of dissolved Mn, Co, Fe, Pb, Cu, Ni, Cd, Zn, nitrate plus nitrite, silicon and phosphate were collected at 7 stations in the northern and 6 stations in the southern Aegean Sea, eastern Mediterranean, in both March and September 1997. This represents the first extensive dissolved trace metal data set for this region. Additional samples were collected near hydrothermal vents in Milos, and across the Thermaikos Gulf. Trace metal distributions and concentrations were interpreted with respect to the particularly complex hydrography and circulation patterns of the Aegean Sea. The results show elevated surface concentrations of Mn, Co, Fe, Pb, Cu, Ni and Zn which can be attributed to both atmospheric deposition in the southern Aegean and a combination of atmospheric deposition, the outflow of the Black Sea Water and potential coastal inputs within the northern Aegean. Spatial and temporal metal variability in surface waters was observed, with higher concentrations for all metals in the northern Aegean due to additional surface sources. In September 1997 the overall higher concentrations of Mn, Co, Pb, Cu, Ni and Zn are attributed to the shallowing of the thermocline located at approximately 100 m in March to around 50 m in September, although the possibility of enhanced aolian deposition is not excluded. Elevated signatures of dissolved Mn, Co, Fe and Pb are associated with Black Sea Water and, to a lesser extent with Levantine Intermediate Water in the northern Aegean for March 1997. In the more complex hydrography of the south Aegean the elevated dissolved nitrate plus nitrite and silicon are associated with an intrusion of Transitional Mediterranean Water. In addition, dissolved trace metal results from Thermaikos Gulf and Milos do not suggest a significant input of metals from these sources into the Aegean Sea during the period of sampling. Atmospheric metal fluxes for the central and north-western Mediterranean Sea were used to calculate the residence times in the Aegean surface waters. These fluxes were compared with metal fluxes from sediment traps and particulate metal data in the northern and southern Aegean during the period of sampling. Linear regression analysis was applied to Black Sea influenced surface water waters in the northern Aegean in order to estimate the concentrations of Mn, Co, Fe, Cu and Ni in the Straits of Dardanelles. For the metals Mn, Fe and Pb the calculated short residence times in southern Aegean surface waters of 0.3 to 6.0 years reflect their high particle reactivity and scavenging onto particles and transport to depth. Co exhibits a similar cycling pathway to Mn but with longer surface residence times of 4 to 40 years. The vertical distributions of Ni, Cu, Cd and Zn do not resemble those of nutrients. Ni, Cu, and Zn exhibit surface elevation in the northern Aegean, and to a lesser extent in the southern Aegean, whereas Cd depth profiles were found to be homogenous. These profiles are the result of surface metal sources combined with the oligotrophic nature of the Aegean Sea. The present work suggests that the Aegean Sea, as well as the Mediterranean is not in a steady state with respect to Ni, Cu, Zn and Cd. Concentrations in the southern Aegean are overall comparable to recent high quality observations in the open Mediterranean. However, there appears to be an increase in dissolved Cu and Ni from the western to the eastern basins of the Mediterranean Sea due to continuous inputs to surface waters.

This thesis explores the nature and ubiquity of iron (Fe) inputs from sediments to the oceans. In the last 10 years continental shelf sediments have become widely recognised as important vectors for dissolved Fe inputs to the oceans, where bacterial dissimilatory Fe-reduction (DIR) promotes the flux of Fe to the water column during the oxidation of sedimentary organic matter. Deep-sea and volcanogenic sediments however, are important reservoirs of Fe, which have not yet been investigated as sources of Fe to seawater. Furthermore knowledge of the nature of Fe phases involved in sediment, porefluid and seawater cycling is limited. The nature of Fe cycling was investigated in deep-sea volcaniclastic surface-sediments (0-20 cmbsf). Pore-fluid and sediment samples were collected from tephra-rich sites near the active volcanic island of Montserrat, Caribbean Sea, and mixed biosiliceous sites around the dormant Crozet Island archipelago, Southern Ocean. Analyses reveal both regions maintain high pore-fluid Fe concentrations close to the sediment surface (up to 20 μM 0-5 cmbsf), despite relatively low organic carbon supply and contrasting oxygen utilization pathways. The oxidation of young tephra is thought to maintain the steep oxygen gradient measured in Montserrat sediments, and is considered to be an important component of Fe, and in particular manganese (Mn), cycling with local bottom water. Unlike Montserrat dissolved Fe and Mn in Crozet pore-fluids are dominated by colloidal phases (0.02-0.2 μm), and in both oxic and sub-oxic sediment layers. Thus mixed biosiliceous-volcaniclastic sediments are shown to host important colloidal-Fe generating reactions, which it is argued, promote the exchange of Fe with the overlying bottom waters. Re-cycling processes close to the seafloor are likely to determine the impact of this flux on seawater Fe budgets. Low-cost ex-situ incubation experiments were used to measure a benthic Fe flux on sediments from the river-dominated Californian margin (6.3 ± 5.9 μmol Fe m-2 yr-1) consistent with previous studies. Fe and Mn fluxes from Montserrat tephra deposits were also assessed; Differences in oxidation kinetics are shown to prevent the accumulation of Fe, yet permit the accumulation of Mn (~27 μmol m-2 yr-1) in Montserrat bottom waters. Studies indicate temporospatial variations to bioirrigation and sediment re-suspension are important aspects of sedimentary Fe inputs that are poorly represented by conventional sampling methods. In an effort to trace the biogeochemical processing of pore-fluid Fe in Crozet sediments, its isotopic composition was determined, representing the first measurements of their kind in deep-sea pore-fluids. Unique relative to previous studies of pore-fluid Fe isotopes, the near-crustal δ56Fe compositions, demonstrate that DIR does not impart the same light Fe-isotopic signature that characterises previous sub-oxic pore-fluids. Comparison of reactive Fe contents between Crozet and pacific margin sediments indicates pore-fluid Fe isotopes reflect the extent to which Fe is recycled by redox processes. This discovery brings to light the potential for Fe isotopes to trace the input of Fe from shelf sediments, where redox re-cycling of Fe is extensive. The mean oceanic Fe isotope composition (δ56Fe) is predicted to be -0.1 to -3.2‰ depending on the balance of uncertainty in input terms. The predicted surface water Fe isotope composition in the Crozet region (-2.0 to - 2.2‰) is shown to reflect the light composition of shelf-derived Fe for a Fe inventory already constrained for this region.

Trace elements in marine biogenic carbonates may be used as proxies for past ocean chemistry provided that there is an established relationship between the trace element proxy and a parameter of interest, this relationship is preserved within biogenic carbonate, and the trace element can be determined sufficiently accurately. Successful application of any trace element proxy requires both development of the analytical methodology to ensure accurate data with the necessary sensitivity, and an understanding of the relationship between proxy and seawater chemistry. Herein I develop methods for the determination of Mg/Ca, Sr/Ca and Cd/Ca in planktonic foraminiferal calcite, using inductively coupled plasma optical emission spectrophotometry and isotope dilution thermal ionisation mass spectrometry, and propose a potential reference material for Mg/Ca in foraminiferal calcite. The developed techniques are applied to an investigation of the Mg/Ca temperature proxy over Chatham Rise in the Southwest Pacific Ocean and a calibration study of the partition coefficient, DCd, for cadmium incorporation into planktonic foraminifera. Comparisons of planktonic foraminiferal Mg/Ca, shell weight and oxygen isotope records from sites north and south of the Subtropical Front on Chatham Rise, demonstrate the effects of hydrography, foraminiferal habitat and dissolution as controls on Mg/Ca. Determinations of Cd/Ca in seven species of planktonic foraminifera confirm that the dominant controls on Cd/Ca are foraminiferal habitat and hydrography, with only a minor influence of post depositional dissolution. The major uncertainty in determination of DCd from core top samples comes from uncertainty in estimation of the depth distribution and seasons of calcification of planktonic foraminifera.

The aim of this thesis was to improve our understanding of the natural iron fertilisation processes that can alleviate the High Nutrient Low Chlorophyll conditions normally associated with the Southern Ocean. The annual phytoplankton bloom occurring north of the Crozet Plateau (46°26’S - 52°18’E) provided a good opportunity to study these phenomena during CROZEX, a multidisciplinary study performed in austral summers 2004/2005 and 2005/2006. Particular emphasis has been placed on the sources of Fe to the upper water column, and on the different processes that drive the distribution of iron, such as mixing with deeper waters, advection of Fe rich waters from the islands, or particles and atmospheric inputs. A flow-injection analyser with DPD catalytic spectrophotometric detection (FIA-DPD) was first set up for the determination of total dissolved Fe (DFe, ≤ 0.2 µm) and careful assessment of data demonstrated the high quality of the information obtained in this study. Analytical measurements of DFe were performed in twenty vertical profiles from the North to the South of the islands that show evidence of a range of processes influencing the iron distributions. Most significantly, an enrichment of DFe (>1 nM) has been identified at close proximity to the islands, which suggests that the plateau and the associated sediments are a source of iron. Waters further north also appear to be affected by this input of both coastal and shelf origin, although dissolved iron concentrations decrease as a function of distance to the north of the plateau with a gradient of 0.07 nM.km-1 as a result of dispersion and mixing. This gradient was then combined with short-lived Radium isotopes profiles, allowing the determination of a horizontal advective flux of Fe. Estimates of atmosphere and vertical fluxes of Fe to surface waters were also calculated. It was then possible to estimate a pre-bloom concentration of ~ 0.44 nM, which is sufficient to drive the inferred level of the new production in the bloom area. Labile iron has been distinguished from the refractory fraction of iron in the suspended particulate matter exported from the mixed layer by developing a two-stage leaching technique. The resulting solutions were analyzed using ICP-MS. By using associated 234Th fluxes, vertical fluxes of particulate iron in both phases were determined as well as their residence times. Data clearly show that carbon export was linked to the iron residence time and enhanced in the northern region where iron inputs occur. This thesis therefore provides a comprehensive data set of DFe and particulate iron in waters surrounding the Crozet Islands. It also presents the first attempt to establish an iron budget that can be developed into a model and provides a good reference point for subsequent studies of natural iron fertilization processes that occur around these islands.