Quantum yield for the photochemical production of dissolved inorganic carbon in seawater

The direct photooxidation of coloured dissolved organic matter (CDOM) to dissolved inorganic carbon (DIC) may provide a significant sink for organic carbon in the ocean. To calculate the rate of this reaction on a global scale, it is essential to know its quantum yield, or photochemical efficiency. We have determined quantum yield spectra, φ(λ), (moles DIC/mole photons absorbed) for 14 samples of seawater from environments ranging from a turbid, eutrophic bay to the Gulf Stream. The spectra vary among locations, but can be represented quite well by three pooled spectra for zones defined by location and salinity: inshore φ(λ)=e^{−(6.66+0.0285(λ−290))}; coastal φ(λ)=e^{−(6.36+0.0140(λ−290))}; and open ocean φ(λ)=e^{−(5.53+0.00914(λ−290))}. Production efficiency increases offshore, which suggests that the most highly absorbing and quickly faded terrestrial chromophores are not those directly responsible for DIC photoproduction.     

High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m² in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.     

Seasonal variation of Mn adsorption on to calcareous surfaces in the English Channel, and its implication on the manganese distribution coefficient

Dissolved and particulate manganese in seawater samples derived from the English Channel has been analyzed using graphite furnace atomic absorption (GFAAS), and inductively coupled plasma atomic emission (ICP—AES) spectroscopies. Because of the high contents of carbonate minerals found in the suspended matter samples, the English Channel constitutes an ideal field area for the study of the Mn²⁺/Ca²⁺/CaCO₃ system. Owing to the chemical speciation of particulate manganese and the combined use of the X-ray diffraction and electron spin resonance spectroscopy we have shown the importance of the carbonate phase in the stabilization of manganese (II). This has been confirmed by p-pH measurements in the field. All these studies have also indicated that: (1) manganese is associated with calcite in the form of a solid solution, Mn_xCa_1−xCO₃; and (2) significant increases in the concentrations of particulate manganese, especially in offshore waters, occur in summer. This seasonal phenomenon has been attributed to the proliferation of coccolithophorids, which are known to be covered with calcified skeletons at high specific surface areas. To appraise the implication of the coccolithophorid-blooms phenomenon on the Mn²⁺/Ca²⁺/CaCO₃ system, we have used the manganese distribution coefficient, D_i, between the liquid phase and CaCO₃ particles. Overall we have shown that: (1) D_i in summer (i.e. when coccoliths considered as very fine-grained calcite are abundant) is much higher than that obtained in winter; and (2) in the vicinity of the French coast, D_i does not vary significantly even in summer. This is because of the high content of chalk-derived particles found in the near-shore waters.     

Uptake and transformation of C-labelled tributyltin chloride by the dog-whelk, nucella lapillus: Importance of absorption from the diet

The importance of the diet as a source of tributyltin (TBT) in Nucella lapillus was studied using [¹⁴C]tributyltin chloride. In N. lapillus provided with prelabelled mussels, Mytilus edulis, in labelled water (mean 20·5 ng/litre TBT) the rate of accumulation of total ¹⁴C was 2–3 times that in unfed animals. Owing to its degradation in the tissues of both fed and unfed animals, concentrations of [¹⁴C]TBT tended to reach a plateau after only 28 days. However, total concentrations of ¹⁴C were still increasing after 49 days. Under experimental conditions (15°C, ample food, no disturbance) the diet accounted for about 50% of the body burden of TBT in N. lapillus after 49 days exposure: concentration factors (dry tissue/water) for [¹⁴C]TBT in both male and female N. lapillus were similar at about 60 000 in fed and 30 000 in unfed animals. It is concluded that the diet may contribute less than half of the body burden of TBT found in natural populations subjected to life-long exposure.