Dimethylsulfide production in Sargasso Sea eddies

Lagrangian time series of dimethylsulfide (DMS) concentrations from a cyclonic and an anticyclonic eddy in the Sargasso Sea were used in conjunction with measured DMS loss rates and a model of vertical mixing to estimate gross DMS production in the upper 60 m during summer 2004. Loss terms included biological consumption, photolysis, and ventilation to the atmosphere. The time- and depth (0–60 m)-averaged gross DMS production was estimated to be 0.73±0.09 nM d⁻¹ in the cyclonic eddy and 0.90±0.15 nM d⁻¹ in the anticyclonic eddy, with respective DMS replacement times of 5±1 and 6±1 d. The higher estimated rate of gross production and lower measured loss rate constants in the anticyclonic eddy were equally responsible for this eddy's 50% higher DMS inventory (0–60 m). When normalized to chlorophyll and total dimethylsulfoniopropionate (DMSP), estimated gross production in the anticyclonic eddy was about twice that in the cyclonic eddy, consistent with the greater fraction of phytoplankton that were DMSP producers in the anticyclonic eddy. Higher rates of gross production were estimated below the mixed layer, contributing to the subsurface DMS maximum found in both eddies. In both eddies, gas exchange, microbial consumption, and photolysis were roughly equal DMS loss terms in the surface mixed layer (0.2–0.4 nM d⁻¹). Vertical mixing was a substantial source of DMS to the surface mixed layer in both eddies (0.2–0.3 nM d⁻¹) owing to the relatively high DMS concentrations below the mixed layer. Estimated net biological DMS production rates (gross production minus microbial consumption) in the mixed layer were substantially lower (by almost a factor of 3) than those estimated in a previous study of the Sargasso Sea, which may explain the relatively low mixed-layer DMS concentrations found here during July 2004 (3 nM) compared to previous summers (4–6 nM).     

A predictive numerical model for potential mapping of the gas hydrate stability zone in the Gulf of Cadiz

This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment.The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope.     

Altering intertidal sediment topography: effects on biodiversity and ecosystem functioning

Sedimentary marine systems are often highly productive and perform important nutrient regeneration functions as they efficiently decompose organic material. In recent years the role of habitat effects and of species composition in ecosystem functioning has become of interest. Estuarine environments are frequently subject to considerable anthropogenic pressures whilst supporting a variety of habitats ranging from well sorted soft muds through biogenically stable sediments to highly mobile coarse sands. There is therefore considerable spatial complexity in habitat type and faunal composition. This study set out to observe the effects of altering the topographical habitat features of an estuarine mudflat on a range of porewater nutrient concentrations (NH₄⁺, NO₃⁻, NO₂⁻, PO₄³⁻ and SiO) collected from four depths (3, 5, 9, and 12 cm) and on faunal composition. Two treatments (Shelled Nets and Net Controls) were used to alter the topography from simple mud to a mussel shell crumble and were compared to un-manipulated Control areas. Sediment granulometry and organic matter content analyses alongside biological traits analysis of the fauna were also conducted.
Differences were observed in porewater nutrient concentrations between the Control and both netted treatments at 5 cm depth only; the species diversity and abundance were also different in the netted treatments compared to the Controls, although no difference between the two manipulated treatments were observed. The changes in faunal composition were attributed solely to the altered topography and the observed nutrient changes were attributed to the faunal alteration rather than the topographical manipulation.     

Infaunal burrow ventilation and pore-water transport in muddy sediments

The ventilation of burrows by tube-dwelling benthos is understood to be important in determining rates of exchange of solutes between the sediment and overlying water. However, few models have attempted to link the burrow ventilation behavior of tube-dwelling organisms with their geochemical consequences. The classic cylinder model of bioirrigation in muddy sediments (Aller, R.C., 1980. Quantifying solute distributions in the bioturbated zone of marine sediments by defining an average microenvironment. Geochimica et Cosmochimica Acta 44, 1955–1965) links pore-water processes and burrow sizes and distributions in the sediment by assuming that burrows are fully flushed. The equivalence between the cylinder model and the more commonly used one-dimensional non-local exchange model depends upon this assumption. However, this assumption has seldom been tested in the field. We have extended the cylinder model of bioirrigation to include burrow ventilation activities of organisms. Burrow ventilation is modeled as a simple non-local exchange of burrow water with overlying water. Model simulations indicate that burrow ventilation has a large effect on vertical profiles and fluxes of solute tracers. We collected data on burrow geometry in the field by CT-scanning freshly collected sediment cores. At the same study sites, we measured activity profiles of ²²²Rn, a naturally occurring radionuclide tracer of pore-water transport. With model geometry independently constrained, we tested the model by comparing our model-predicted profiles with measured profiles. Our results demonstrate that burrows in the field are not fully flushed. Our estimated burrow ventilation rates compare favorably with previous laboratory measurements. The inclusion of realistic burrow ventilation in this pore-water transport model strongly affects modeled solute profiles and fluxes. We demonstrate how model parameters can be determined from field samples and present a model that more realistically simulates pore-water transport processes in muddy sediments.     

MedFlux: Investigations of particle flux in the Twilight Zone

The MedFlux project was devised to determine and model relationships between organic matter and mineral ballasts of sinking particulate matter in the ocean. Specifically we investigated the ballast ratio hypothesis, tested various commonly used sampling and modeling techniques, and developed new technologies that would allow better characterization of particle biogeochemistry. Here we describe the rationale for the project, the biogeochemical provenance of the DYFAMED site, the international support structure, and highlights from the papers published here. Additional MedFlux papers can be accessed at the MedFlux web site (http://msrc.sunysb.edu/MedFlux/).     

EINSTEIN and MICHELSON The Context of Discovery and the Context of Justification

The philosopher of science is not much intersted in the thought processes which lead to scientific discoveries; he looks for a logical analysis of the completed theory, including the relationships establishing its validity. That is, he is not interested in the context of discovery, but in the context of justification … it appears amazing to what extent the logical analysis of relativity coincides with the original interpretation by its author, as far as it can be constructed from the scanty remarks in EINSTEIN 's publications. In contradistinction to some developments in quantum theory, the logical schema of the theory of relativity corresponds surprisingly with the program which controlled its discovery.     

The WOCE-era 3-D Pacific Ocean circulation and heat budget

To address questions concerning the intensity and spatial structure of the three-dimensional circulation within the Pacific Ocean and the associated advective and diffusive property flux divergences, data from approximately 3000 high-quality hydrographic stations collected on 40 zonal and meridional cruises have been merged into a physically consistent model. The majority of the stations were occupied as part of the World Ocean Circulation Experiment (WOCE), which took place in the 1990s. These data are supplemented by a few pre-WOCE surveys of similar quality, and time-averaged direct-velocity and historical hydrographic measurements about the equator.An inverse box model formalism is employed to estimate the absolute along-isopycnal velocity field, the magnitude and spatial distribution of the associated diapycnal flow and the corresponding diapycnal advective and diffusive property flux divergences. The resulting large-scale WOCE Pacific circulation can be described as two shallow overturning cells at mid- to low latitudes, one in each hemisphere, and a single deep cell which brings abyssal waters from the Southern Ocean into the Pacific where they upwell across isopycnals and are returned south as deep waters. Upwelling is seen to occur throughout most of the basin with generally larger dianeutral transport and greater mixing occurring at depth. The derived pattern of ocean heat transport divergence is compared to published results based on air–sea flux estimates. The synthesis suggests a strongly east/west oriented pattern of air–sea heat flux with heat loss to the atmosphere throughout most of the western basins, and a gain of heat throughout the tropics extending poleward through the eastern basins. The calculated meridional heat transport agrees well with previous hydrographic estimates. Consistent with many of the climatologies at a variety of latitudes as well, our meridional heat transport estimates tend toward lower values in both hemispheres.