Nitrogen fixation in the western English Channel (NE Atlantic Ocean)

In temperate Atlantic waters (18.8 to 20.1°C), biological nitrogen fixation has beendemonstrated by 2 independent measurements: 15N-N2 incorporation and nifH identification in theDNA and expressed messenger RNA (mRNA). At 2 stations in the western English Channel, bulkwaters were incubated with 15N-N2. At the high levels of particulate nitrogen (?11.5 ?mol N l–1),absolute fixation rates of 18.9 ± 0.01 and 20.0 nmol N l–1d–1 were determined. While a caveat mustaccompany the magnitude of the rates presented due to the limited number of data, the presence andactivity of diazotrophic organisms in these waters is of ecological significance and may affect currentattitudes to nitrogen and carbon budgets. In particular, our estimate of the rate of N fixation(0.35 mmol N m–2 d–1) is comparable to that of denitrification rates in UK shelf seas. Molecular analysisidentified a diversity of expressed nifH genes, and 21 different prokaryotic nifH transcripts wereidentified

Modelling sedimentary biogeochemical processes in a high nitrate, UK estuary (the Gt. Ouse) with emphasis on the nitrogen cycle

The description, calibration and application of a reaction-diffusion model of early diagenesis is presented. Unlike previous models it has been developed for a temperate latitude estuary (upper and lower Gt. Ouse, UK) impacted by high nitrate concentrations (annual mean 700 mM). Five variables, O2, NO-3, NH4+, SO4= and S=, are modelled from the steady state distributions of bulk total organic carbon (TOC). Different representations of the first order rate constant, k, for TOC mineralisation are tested. Use of separate k values for individual mineralisation pathways is the only way to reproduce the data but at the cost of 1) increasing the degrees of freedom in the model and 2) conceptual simplicity. This casts doubt over the universal applicability of diagenetic models in high NO3- environments. Underestimation of the observed ammonium fluxes leads to the inclusion of dissimilatory nitrate reduction to ammonium (DNRA) into a diagenetic model for the first time. Use of an empirical temperature function successfully simulates rates of denitrification and DNRA. It is concluded that temperature is an important control in partitioning nitrate reduction into DNRA and denitrification in the Gt. Ouse sediments. This temperature effects implies that during an extended warm summer in temperature estuaries receiving high nitrate inputs, nitrate reduction may contribute to, rather than counteract a eutrophication event. A literature review showing that DNRA can account for up to 100% of the nitrate reduction in different locations around the world, means that diagenetic models of the nitrogen cycle in coastal areas should include DNRA. A parameter sensitivity analysis (PSA) reveals a highly non linear model response to parameter changes of ±50%. The variability in model response among the sites in the Gt. Ouse highlights the importance of accounting for differences in 1) the relative contributions of oxic, suboxic and anoxic mineralization to total organic carbon mineralization; 2) rates of oxygen consumption and 3) oxygen penetration depths.</p

Modelling sedimentary biogeochemical processes in a high nitrate, UK estuary (the Gt. Ouse) with emphasis on the nitrogen cycle

The description, calibration and application of a reaction-diffusion model of early diagenesis is presented. Unlike previous models it has been developed for a temperate latitude estuary (upper and lower Gt. Ouse, UK) impacted by high nitrate concentrations (annual mean 700 mM). Five variables, O2, NO-3, NH4+, SO4= and S=, are modelled from the steady state distributions of bulk total organic carbon (TOC). Different representations of the first order rate constant, k, for TOC mineralisation are tested. Use of separate k values for individual mineralisation pathways is the only way to reproduce the data but at the cost of 1) increasing the degrees of freedom in the model and 2) conceptual simplicity. This casts doubt over the universal applicability of diagenetic models in high NO3- environments. Underestimation of the observed ammonium fluxes leads to the inclusion of dissimilatory nitrate reduction to ammonium (DNRA) into a diagenetic model for the first time. Use of an empirical temperature function successfully simulates rates of denitrification and DNRA. It is concluded that temperature is an important control in partitioning nitrate reduction into DNRA and denitrification in the Gt. Ouse sediments. This temperature effects implies that during an extended warm summer in temperature estuaries receiving high nitrate inputs, nitrate reduction may contribute to, rather than counteract a eutrophication event. A literature review showing that DNRA can account for up to 100% of the nitrate reduction in different locations around the world, means that diagenetic models of the nitrogen cycle in coastal areas should include DNRA. A parameter sensitivity analysis (PSA) reveals a highly non linear model response to parameter changes of ±50%. The variability in model response among the sites in the Gt. Ouse highlights the importance of accounting for differences in 1) the relative contributions of oxic, suboxic and anoxic mineralization to total organic carbon mineralization; 2) rates of oxygen consumption and 3) oxygen penetration depths.</p

Assessing the Impact of Surface and Upper-Air Observations on the Forecast Skill of the ACCESS Numerical Weather Prediction Model over Australia

The impact of the Australian Bureau of Meteorology’s in situ observations (land and sea surface observations, upper air observations by radiosondes, pilot balloons, wind profilers, and aircraft observations) on the short-term forecast skill provided by the ACCESS (Australian Community Climate and Earth-System Simulator) global numerical weather prediction (NWP) system is evaluated using an adjoint-based method. This technique makes use of the adjoint perturbation forecast model utilized within the 4D-Var assimilation system, and is able to calculate the individual impact of each assimilated observation in a cycling NWP system. The results obtained show that synoptic observations account for about 60% of the 24-h forecast error reduction, with the remainder accounted for by aircraft (12.8%), radiosondes (10.5%), wind profilers (3.9%), pilot balloons (2.8%), buoys (1.7%) and ships (1.2%). In contrast, the largest impact per observation is from buoys and aircraft. Overall, all observation types have a positive impact on the 24-h forecast skill. Such results help to support the decision-making process regarding the evolution of the observing network, particularly at the national level. Consequently, this 4D-Var-based approach has great potential as a tool to assist the design and running of an efficient and effective observing network

Authigenic barite records of methane seepage at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanoes (MVs) are dynamic features that episodically expel gas-charged fluids and mud onto the seafloor, transferring various chemical constituents into the overlying water column. The temporal variability in MV activity is, however, poorly understood, so their importance as a source of methane (CH4) and higher hydrocarbons for the oceanic carbon budget, although thought to be significant, cannot be properly constrained. In this study, the history of fluid and gas seepage at the Carlos Ribeiro MV (Gulf of Cadiz) is assessed via geochemical analyses and transport-reaction modelling of pore fluids and barium (Ba) rich layers (Ba fronts) in sediment cores, recovered along a transect from the eye to the periphery of the MV. X-ray fluorescence data reveal that Ba fronts are absent at the eye, while a single front (with up to 1740 ppm Ba) is present at the margin of the summit. Three Ba fronts occur at 45, 85 and 130 cm depth within a mudflow to the southeast of the crater. Spectrometric analyses indicate that barite is the Ba-rich mineral in these layers. Upward advecting pore fluids are enriched in barium but depleted in calcium (Ca2 +) relative to seawater. Modelling of the Ba2 + and Ca2 + pore fluid profiles indicates that the positions of the Ba fronts reflect both the present-day hydrodynamic conditions as well as higher fluxes of methane in the past. Fluid advection appears to have decreased since 340 cal yr before present, but degassing of the mudflow is ongoing and is potentially an important source of CH4

Spatial variation in fluid flow and geochemical fluxes across the sediment–seawater interface at the Carlos Ribeiro mud volcano (Gulf of Cadiz)

Submarine mud volcanism is an important pathway for transfer of deep-sourced fluids enriched in hydrocarbons and other elements into the ocean. Numerous mud volcanoes (MVs) have been discovered along oceanic plate margins, and integrated elemental fluxes are potentially significant for oceanic chemical budgets. Here, we present the first detailed study of the spatial variation in fluid and chemical fluxes at the Carlos Ribeiro MV in the Gulf of Cadiz. To this end, we combine analyses of the chemical composition of pore fluids with a 1-D transport-reaction model to quantify fluid fluxes, and fluxes of boron, lithium and methane, across the sediment–seawater interface. The pore fluids are significantly depleted in chloride, but enriched in lithium, boron and hydrocarbons, relative to seawater. Pore water profiles of sulphate, hydrogen sulphide and total alkalinity indicate that anaerobic oxidation of methane occurs at 34–180 cm depth below seafloor. Clay mineral dehydration, and in particular the transformation of smectite to illite, produces pore fluids that are depleted in chloride and potassium. Profiles of boron, lithium and potassium are closely related, which suggests that lithium and boron are released from the sediments during this transformation. Pore fluids are expelled into the water column by advection; fluid flow velocities are 4 cm yr?1 at the apex of the MV but they rapidly decrease to 0.4 cm yr?1 at the periphery. The associated fluxes of boron, lithium and methane vary between 7–301, 0.5–6 and 0–806 mmol m?2 yr?1, respectively. We demonstrate that fluxes of Li and B due to mud volcanism may be important on a global scale, however, release of methane into the overlying water column is suppressed by microbial methanotrophy

Authigenic carbonates from the Darwin Mud Volcano, Gulf of Cadiz: A record of palaeo-seepage of hydrocarbon bearing fluids

of the oceans, but little is known about the long-term variation in fluid fluxes at MVs. The Darwin MV is one of more than 40 MVs located in the Gulf of Cadiz, but it is unique in that its summit is covered by a thick carbonate crust that has the potential to provide a temporal record of seepage activity. In order to test this idea, we have conducted petrographic, chemical and isotopic analyses of the carbonate crust. In addition a 1-D transport-reaction model was applied to pore fluid data to assess fluid flow and carbonate precipitation at present. The carbonate crusts mainly comprise of aragonite, with a chaotic fabric exhibiting different generations of cementation and brecciation. The crusts consist of bioclasts and lithoclasts (peloids, intraclasts and extraclasts) immersed in a micrite matrix and in a variety of cement types (microsparite, botryoidal, isopachous acicular, radial and splayed fibrous). The carbonates are moderately depleted in 13C (?13C = ? 8.1 to ? 27.9‰) as are the pore fluids (?13C = ? 19.1 to ? 28.7‰), which suggests that their carbon originated from the oxidation of methane and higher hydrocarbons, like the gases that seep from the MV today. The carbonate ?18O values are as high as 5.1‰, and it is most likely that the crusts formed from 18O-rich fluids derived from dehydration of clay minerals at depth. Pore fluid modelling results indicate that the Darwin MV is currently in a nearly dormant phase (seepage velocities are &lt; 0.09 cm yr? 1). Thus, the thick carbonate crust must have formed during past episodes of high fluid flow, alternating with phases of mud extrusion and uplift

Impact of resuspension of cohesive sediments at the Oyster Grounds (North Sea) on nutrient exchange across the sediment–water interface

Benthic-pelagic exchange processes are recognised as important nutrient sources in coastal areas, however, the relative impact of diffusion, resuspension and other processes such as bioturbation and bioirrigation are still relatively poorly understood. Experimental ship-based data are presented showing the effects of diffusion and resuspension on cohesive sediments at a temperate shelf location in the North Sea. Measurements of diffusive fluxes in both spring (1.76, 0.51, −0.91, 17.6 μmol/m2/h) and late summer (8.53, −0.03, −1.12, 35.0 μmol/m2/h) for nitrate, nitrite, phosphate and dissolved silicon respectively, provided comparisons for measured resuspension fluxes. Increases in diffusive fluxes of nitrate and dissolved silicon to the water column in late summer coincided with decreases in bottom water oxygen concentrations and increases in temperature. Resuspension experiments using a ship board annular flume and intact box core allowed simultaneous measurement of suspended particulate matter, water velocity and sampling of nutrients in the water column during a step wise increase in bed shear velocity. The resuspension of benthic fluff led to small but significant releases of phosphate and nitrate to the water column with chamber concentration increasing from 0.70–0.76 and 1.84–2.22 μmol/L respectively. Resuspension of the sediment bed increased water column concentrations of dissolved silicon by as much as 125% (7.10–15.9 μmol/L) and nitrate and phosphate concentrations by up to 67% (1.84–3.08 μmol/L) and 66% (0.70–1.15 μmol/L) respectively. Mass balance calculations indicate that processes such as microbial activity or adsorption/desorption other than simple release of pore water nutrients must occur during resuspension to account for the increase. This study shows that resuspension is potentially an important pathway for resupplying the water column with nutrients before and during phytoplankton blooms and should therefore be considered along with diffusive fluxes in future ecosystem models

Modelling macronutrients in shelf sea sediments: fitting model output to experimental data using a genetic algorithm

Purpose:Diagenetic modelling, the mathematical simulation of the breakdown of sedimentary organic matter and subsequent fate of associated nutrients, has progressed to a point where complex, non-steady state environments can be accurately modelled. A genetic algorithm has never been used in conjunction with an early diagenetic model, and so we aim to discover whether this method is viable to determining a set of realistic model parameters, which itself is often a difficult task.Materials and methods:A range of sensitivity analyses were conducted to establish the parameters for which the model was most sensitive before a micro-genetic algorithm (?GA) was used to fit an output from a previously published diagenetic model (OMEXDIA) to observational data, taken at the North Dogger site from a series of cruises in the North Sea. Profiles of carbon, oxygen, nitrate and ammonia were considered. The method allows a set of parameters to be determined in a manner analogous to natural selection. Each iteration of the genetic algorithm within each experiment decreases the variance between the observed profiles and those calculated by OMEXDIA.Results and discussion:Despite some of the observed profiles, particularly for carbon, showing unusual patterns, the genetic algorithm was able to generate a set of parameters which was able to fit the observations. The genetic algorithm can therefore help to determine the values of other parameters used in the model, for which observational values are difficult to measure (e.g. the flux of organic matter to the sediment from the overlying water column and the rates of degradation of organic matter). We also show that the values of the parameters determined by the ?GA technique are able to be used in a potentially temporally predictive manner.Conclusions:The ?GA used is a viable method to fit carbon and nutrient sedimentary profiles observed in complex, dynamic shelf sea systems, despite OMEXDIA originally being designed for a different sedimentary environment. The results therefore show that this novel use of a genetic algorithm is a suitable method for both model calibration and validation and that the technique may help in explaining processes which are poorly understood