Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers.The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn (r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg).These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.     

Beach erosion and acceretion—An example from Kerala,Southwest Coast of India

The thickly populated coastal zone of Kerala, India is facing severe problems due to attack of high waves during the southwest monsoon. Systematic beach profiling at 5-km intervals was carried out along the 560-km stretch of the Kerala coast during the pre-and postmonsoon seasons in 1984. Beach volume changes were calculated at each profile station, and the erosional and accretional trends for the entire coastal tract were demarcated in a map. Total erosion along 55 stations is 1276 m³/m. The general erosional and acceretional trends were also found to coincide with diverging and converging littoral currents deduced from the wave refraction diagrams. Such study at periodic intervals will be highly useful for proper management of the coastal zone.     

SPIRE point source photometry: within the Herschel interactive processing environment (HIPE)

The different algorithms appropriate for point source photometry on data from the SPIRE instrument on-board the Herschel Space Observatory, within the Herschel Interactive Processing Environment (HIPE) are compared. Point source photometry of a large ensemble of standard calibration stars and dark sky observations is carried out using the 4 major methods within HIPE: SUSSEXtractor, DAOphot, the SPIRE Timeline Fitter and simple Aperture Photometry. Colour corrections and effective beam areas as a function of the assumed source spectral index are also included to produce a large number of photometric measurements per individual target, in each of the 3 SPIRE bands (250, 350, 500μm), to examine both the accuracy and repeatability of each of the 4 algorithms. It is concluded that for flux densities down to the level of 30mJy that the SPIRE Timeline Fitter is the method of choice. However, at least in the 250 and 350μm bands, all 4 methods provide photometric repeatability better than a few percent down to at approximately 100mJy. The DAOphot method appears in many cases to have a systematic offset of ～8 % in all SPIRE bands which may be indicative of a sub-optimal aperture correction. In general, aperture photometry is the least reliable method, i.e. largest scatter between observations, especially in the longest wavelength band. At the faintest fluxes, <30mJy, SUSSEXtractor or DAOphot provide a better alternative to the Timeline Fitter.     

Petrography and chemistry of tungsten-rich oxycalciobetafite in hydrothermal veins of the Adamello contact aureole,northern Italy

Mineralogy and Petrology - Tungsten-rich oxycalciobetafite occurs in complex Ti-rich hydrothermal veins emplaced within dolomite marble in the contact aureole of the Adamello batholith, northern...     

Interaction between iron and Pseudomonas aeruginosa biofilms attached to Sepharose surfaces

When Pseudomonas aeruginosa PAO1 biofilms (attached to Sepharose surfaces) were subjected to dissolved Fe³⁺, most Fe was removed from solution within 25 h by surface complexation with negatively charged functional groups on the bacterial cell wall via a nucleation and mineralization process. Chemical formation of Fe-(hydr)oxides was partially responsible for dissolved Fe removal, which stemmed from a pH increase, facilitated by microbial activity. PAO1 used Fe³⁺ as an electron acceptor producing Fe²⁺ under localized anaerobic conditions over the first 50 h. The high ratio of Fe²⁺ to total Fe in solution produced a high proportion of Fe(II) in Fe precipitates; however, as the formation of Fe-(hydr)oxides started after 50 h, the Fe²⁺ content in solution began to diminish. Biofilms can so influence the local chemical conditions and metal speciation that the bulk solution phase is also affected, thereby mediating a wide-range (bio)geochemical cycling of iron. Long-term survival of natural biofilms, even under strict oligotrophic conditions, could have a broad lasting effect on the bulk geochemical environment.     

Zircon (U-Th)/He thermochronometry: He diffusion and comparisons with Ar/Ar dating

(U-Th)/He chronometry of zircon has a wide range of potential applications including thermochronometry, provided the temperature sensitivity (e.g., closure temperature) of the system be accurately constrained. We have examined the characteristics of He loss from zircon in a series of step-heating diffusion experiments, and compared zircon (U-Th)/He ages with other thermochronometric constraints from plutonic rocks. Diffusion experiments on zircons with varying ages and U-Th contents yield Arrhenius relationships which, after about 5% He release, indicate E_a = 163-173 kJ/mol (39-41 kcal/mol), and D₀ = 0.09-1.5 cm²/s, with an average E_a of 169 ± 3.8 kJ/mol (40.4 ± 0.9 kcal/mol) and average D₀ of 0.46^+0.87_−0.30 cm²/s. The experiments also suggest a correspondence between diffusion domain size and grain size. For effective grain radius of 60 μm and cooling rate of 10°C/myr, the diffusion data yield closure temperatures, T_c, of 171-196°C, with an average of 183°C. The early stages of step heating experiments show complications in the form of decreasing apparent diffusivity with successive heating steps, but these are essentially absent in later stages, after about 5-10% He release. These effects are independent of radiation dosage and are also unlikely to be due to intracrystalline He zonation. Regardless of the physical origin, this non-Arrhenius behavior is similar to predictions based on degassing of multiple diffusion domains, with only a small proportion (<2-4%) of gas residing in domains with a lower diffusivity than the bulk zircon crystal. Thus the features of zircon responsible for these non-Arrhenius trends in the early stages of diffusion experiments would have a negligible effect on the bulk thermal sensitivity and closure temperature of a zircon crystal.We have also measured single-grain zircon (U-Th)/He ages and obtained ⁴⁰Ar/³⁹Ar ages for several minerals, including K-feldspar, for a suite of slowly cooled samples with other thermochronologic constraints. Zircon He ages from most samples have 1 σ reproducibilities of about 1-5%, and agree well with K-feldspar ⁴⁰Ar/³⁹Ar multidomain cooling models for sample-specific closure temperatures (170-189°C). One sample has a relatively poor reproducibility of ∼24%, however, and a mean that falls to older ages than predicted by the K-feldspar model. Microimaging shows that trace element zonation of a variety of styles is most pronounced in this sample, which probably leads to poor reproducibility via inaccurate α-ejection corrections. We present preliminary results of a new method for characterizing U-Th zonation in dated grains by laser-ablation, which significantly improves zircon He age accuracy.In summary, the zircon (U-Th)/He thermochronometer has a closure temperature of 170-190°C for typical plutonic cooling rates and crystal sizes, it is not significantly affected by radiation damage except in relatively rare cases of high radiation dosage with long-term low-temperature histories, and most ages agree well with constraints provided by K-spar ⁴⁰Ar/³⁹Ar cooling models. In some cases, intracrystalline U-Th zonation can result in inaccurate ages, but depth-profiling characterization of zonation in dated grains can significantly improve accuracy and precision of single-grain ages.     

Spatial distribution and temporal trends of persistent organochlorine pollutants in sediments from Lake Maryut, Alexandria, Egypt

Organochlorine compounds (OCs) in surface and core sediments collected from Lake Maryut, Egypt, were examined to elucidate their distribution, ecological risk and historical trend. To our knowledge, this is the first study on residue levels of OCs in sediments from Lake Maryut. Concentrations of PCBs and DDTs were higher than other OCs, ranging from 3.06 to 388 and from 0.07 to 106 ng/g dry wt., respectively. The highest concentrations of OCs were found at stations near the discharge point of sewage and close to industrial areas. The distribution of DDT and its metabolites suggest no recent inputs into the lake environment. Contamination levels of sedimentary PCBs and DDTs, can be categorized moderate to high compared to other urbanized regions worldwide. Temporal trends in OCs levels were influenced by input pathways at two sites. Evaluation of ecotoxicological risks suggests that adverse biological effects are expected mainly in the main basin area.     

A strange symbiosis: Seismic ray theory and Saturday touch football — Donald V. Helmberger as a graduate advisor

At the end of December of 1978 I was sitting in the office of Caltech Professor Don Helmberger,he was peering out his window towards the San Gabriel Mountains,s...