Design and application of a semi-automated filtration system to study the distribution of particulate organic carbon in the water column of a coastal upwelling system

The processes controlling the production, transport and ultimate fate of particulate organic carbon (POC) in marine systems play a major role in the global carbon cycle. Direct measurements of POC concentrations in oceanic waters have been used extensively to investigate these processes, but because of the time-intensive effort associated with manual collection and filtration of water samples, this approach often results in limited temporal or spatial coverage. To increase the frequency of POC measurements, we designed and built a semi-automated filtration system (SAFS) that can be linked to a ship's underway surface water flow system and/or a towed profiling vehicle with pumping capabilities. Our results show that, compared to manual filtration, SAFS provides the ability to easily perform filtrations at high enough frequency to measure POC concentrations at spatial and temporal resolutions that are closer to those possible with in situ sensors. When combined with optical measurements, SAFS allows for the characterization of several distinct pools of particles within different geographical and depth regions of the water column off the Oregon Coast. These initial applications yielded data that provide considerable insight into the sources and character of particles in the water column and prove that this approach can be used to gain valuable information on the biogeochemistry of this and other similar ocean margins.     

Géochimie de l'or exogène

Résumé Les auteurs ont étudié la stabilité des suspensions colloïdales d'or, leur transport, leur floculation, ainsi que la synthèse des pépites à partir des gels d'or, dans des conditions géologiques de surface. — Les mesures des paramètres tels que le pH, le Eh, et la concentration des ions or, démontrent que la possibilité de transport de l'or dans la nature sous forme ionique a été surestimée. Sous forme ionique ou même métallique l'or peut donner des suspensions colloïdales stables. Il peut être fluculé facilement et par compaction donner naissance à des pépites d'or similaires aux pépites naturelles. — Par diffusion des solutions auriques dans du gel de silice nous avons obtenu la formation directe des films d'or et reproduit des structures très fréquentes dans de nombreux gisements d'or.

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The stability of colloidal suspensions of gold, their transport and flocculation have been investigated; in addition, the experimental formation of gold nuggets under natural conditions was studied. — The measurements of pH, Eh and the concentration of ionic gold, proved that the possibility of transportation of ionic gold in nature was overestimated. Ionic or even metallic gold may give stable colloidal suspensions. It may be easily flocculated and give by compaction gold nuts similar to natural gold nuts. — By diffusion of gold solutions in silica gel we obtained direct gold films and also some textures frequently present in gold deposits.

     

Sur la formation biogéochimique de stalactites de galène

The genetic problem of stalactites with internal parts composed of sulfides is studied in the case of galena. Considerations of the transport of lead at low temperature and on the cycle of sulfur allow to build an experimental model, taking in count the part played by the sulfate reducing bacteria. The results are interpreted by the use of chemical thermodynamics.     

Spatial and temporal distribution of groundwater recharge in northern Nigeria

Moisture samples obtained from unsaturated-zone profiles in sands from northern Nigeria were used to obtain recharge estimates using the chloride (Cl) mass-balance method and to produce records of past recharge and climatic events. Recharge rates range from 14–49 mm/year, on the basis of unsaturated-zone Cl values and rainfall chemistry measured over eight years at three local stations. The unsaturated-zone results also provide a record of the changing recharge and climatic events of the past 80 years; this record compares quite well with modelling results using precipitation data from Maiduguri, especially for the late 20th-century period of drought. The best fit for the model is made, however, by using a lower mean rainfall Cl (0.65 mg/l) than that obtained from the mean of the field results (1.77 mg/l Cl). This result implies that the measured rainfall Cl probably overestimates the depositional flux of Cl, although the lower value is comparable to the minimum of the measured rainfall Cl values (0.6 mg/l Cl). Recharge estimates made using these lower Cl values range from 16–30 mm/year. The spatial variability was then determined using results from 360 regional shallow wells over 18,000 km². Using the revised rainfall estimate, the Cl balance indicates a value of 43 mm for the regional recharge, suggesting that either additional preferential flow is taking place over and above that from the vadose one, or that the regional recharge represents inputs from earlier wetter periods. These recharge estimates compare favourably with those from hydraulic modelling in the same area and suggest that the recharge rates are much higher than values previously published for this area. High nitrate (NO₃) concentrations (NO₃-N>Cl) preserved under aerobic conditions in the vadose zone reflect secondary enrichment from N-fixing vegetation, as occurs elsewhere in the Sahel. Electronic Publication