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Aquatic Geochemistry (v.11, #1)
Toward an Understanding of Biogenic-silica Dissolution in Seawater – An Initial Rate Approach Applied between 40 and 90 °C by JIM E. GREENWOOD; VICTOR W. TRUESDALE; ANDREW R. RENDELL (pp. 1-20).
The kinetics of phytoplankton frustule dissolution has generally been studied as the appearance of silicic acid in a batch reactor. Unfortunately, this approach, though often illuminating, has not so far been successful because of the difficulty of parameterising the full reaction curve. This current study shows how the initial rate approach to chemical kinetics offers a way around this bottleneck, thereby allowing much chemical kinetics information about frustule dissolution to be collected. The technique is shown to be flexible and suited to short reaction times which facilitate detailed quantitative kinetics investigation, indeed, as would be expected in a solution phase, kinetics study. The technique is exemplified by a dissolution study of uncleaned frustules of Cyclotella crypticaat 40 °C and above. The frustules were found to yield the same dissolution rate after 5 weeks dark storage, at 4 °C. Meanwhile, log dissolution rate was found to vary linearly with pH, with gradient 0.38 ± 0.01 (r 2=0.990). Linearity was upheld even at pHs as high as 14. Finally, a robust Arrhenius plot was established between 40 and 90 °C yielding an activation energy for dissolution of 84 ± 3 kJ mol −1. Follow through with the Eyring equation yielded an activation enthalpy, ΔH ‡, and an activation entropy, ΔS ‡, of 81 and 85 J mol −1 K −1, respectively. The discussion brings salient aspects of existing knowledge about diatom frustule dissolution kinetics into the wider context of silicate mineral dissolution.
Keywords: biogenic silica; dissolution kinetics; initial rate
On the Stability of the AlOSi(OH) 3 2+ Complex in Aqueous Solution by Lorenzo Spadini; Paul W. Schindler; Staffan Sjöberg (pp. 21-31).
The complexation of aluminium(III) and silicon(IV) was studied in a simplified seawater medium (0.6 M Na(Cl)) at 25 °C. The measurements were performed as potentiometric titrations using a hydrogen electrode with OH − ions being generated coulometrically. The total concentrations of Si(IV) and Al(III) respectively [Si tot ] and [Al t ot], and −log[H +] were varied within the limits 0.3 < [Si tot ] < 2.5 mM, 0.5 < [Al tot ] < 2.6 mM, and 2 ≤ -log[H +] ≤ 4.2. Within these ranges of concentration, evidence is given for the formation of an AlSiO(OH) 3 2+ complex with a formation constant log β1,1-1 = −2.75 ± 0.1 defined by the reaction Al 3++Si (OH)4 ↔ AlOSi(OH) 3 2+ +H + An extrapolation of this value to I=0 gives log β1,1-1 = −2.30. The calculated value of log K (Al 3++SiO(OH) 3 − ↔ AlOSi(OH) 3 2+ ) = 6.72 (I=0.6 M) can be compared with corresponding constants for the formation of AlF 2+ and AlOH 2+ , which are equal to 6.16 and 8.20. Obviously, the stability of these Al(III) complexes decreases within the series OH −>SiO(OH) 3 − > F −
Keywords: aluminium; equilibrium analysis; silica; solution chemistry; speciation
Biogeochemistry of Major Redox Elements and Mercury in a Tropical Reservoir Lake (Petit Saut, French Guiana) by T. Peretyazhko; P. Van Cappellen; C. Meile; M. Coquery; M. Musso; P. Regnier; L. Charlet (pp. 33-55).
The hydroelectric reservoir of Petit Saut, French Guiana, was created in 1994–1995 by flooding 350 km2 of tropical forest. When sampled in 1999, the lake exhibited a permanent stratification separating the 3–5 m thick, oxygenated epilimnion from the anoxic hypolimnion. The rate of anaerobic organic carbon mineralization below the oxycline was on the order of 1 μmol C m−2 s−1 and did not show a pronounced difference between wet and dry seasons. Methanogenesis accounted for 76–83% of anaerobic carbon mineralization, with lesser contributions of sulfate reduction and dissimilatory iron reduction. Upward mixing of reduced inorganic solutes explained 90% of the water column O 2 demand during the dry season, while most O 2 consumption during the wet season was coupled to aerobic respiration of organic matter synthesized in the surface waters. Inorganic mercury species represented 10–40% of total dissolved mercury in the epilimnion, but were of relatively minor importance (≤10%) in the anoxic portion of the water column. Net production of soluble organic mercury compounds in the flooded soils and anoxic water column did not vary significantly between wet and dry seasons. Methylmercury accounted for about 15% of total dissolved mercury below the oxycline. Its estimated net production rate, 0.04 mg m−2 yr−1, is of the same order of magnitude as values reported for contaminated lakes and flooded terrestrial ecosystems.
Keywords: biogeochemistry; hydroelectric reservoir; mercury; Petit Saut; redox stratification
Geochemistry of Major Elements in a Pristine Boreal River System; Hydrological Compartments and Flow Paths by J. Ingri; A. Widerlund; M. Land (pp. 57-88).
Once or twice weekly, water sampling was undertaken for a two and a half year period in the Kalix River, northern Sweden. Soil water, groundwater, water in tributaries and mire water were also sampled at several occasions. Samples were filtered and analysed for major dissolved elements and TOC. Although only 5 of the bedrock in the Kalix River drainage basin is situated in the Caledonian mountains (mostly schist, with some outcrops of dolomite and limestone), the chemical composition of the river, at the river mouth, is clearly influenced by water from the mountain areas. High dissolved Ca/Mg ratios in June and July indicate a large influence of water from the mountain areas during summer. The dissolved Si/Mg ratio increases when water from the woodland (bedrock consisting of Precambrian granitoids) predominates during snowmelt in May, but the ratio is low during summer when water from the mountains is increased. However, the low Si concentrations in the mountain areas are probably not primarily the result of the different rocks but more a reflection of the less intense weathering of silicate minerals in the mountains. High Si/Mg ratios are closely related to high TOC. All the major dissolved elements, except TOC, are diluted by snowmelt in May. However, the dilution varies for different elements. Based on the interpretations of major element ratios the melt water discharge in May reflects two major compartments in the woodland; peatland areas and the upper section of the soil. During summer and autumn storm events in the woodland most of the storm water originated from peatland. High K/Mg ratios in the river in May are related to water discharge from the upper section of the till. Low S/Mg ratios in the river indicate an influence of mire water from the woodland both during melt water discharge in May and during increased water discharge in autumn. The Ca/Mg ratios in tributaries in the woodland are consistently lower during melt water discharge compared with values in August. The lower Ca/Mg ratio in May probably reflects water that has been in contact with the B-horizon in the till during spring flood. Data show that the TOC discharged during spring flood originates from two major compartments in the landscape, the upper soil profile and peatland. Storm discharge of TOC during the rest of the year originates mostly from peatland.
Keywords: major elements; Kalix River; Northern Sweden; normalisation; hydrogeochemistry; flow paths
Macrophytes Increase Spatial Patchiness of Fluvial Sedimentary Records and Effect Temporal Particulate Nutrient Storage by MARCUS SCHULZ; BJÖRN GÜCKER (pp. 89-107).
Previous studies on fluvial substrates often lack high spatial resolutions for sedimentary characteristics in relation to biotic structures, such as the aquatic vegetation. A sedimentological survey was therefore performed in a section of the Lower River Spree, in order to analyse the riverbed substrates for small-scale changes in grain size and nutrient parameters, indicative of the impact of submerged macrophytes on sediment composition and particulate nutrient retention. We cored the riverine substrate using a systematic grid of 3 m width and 10 m length, and mapped the aquatic vegetation. Sedimentary records were characterised according to facies, grain size and carbon, nitrogen and phosphorus analyses. Facies description and statistics for the sedimentary parameters provided evidence for a significant riverbed patchiness, which was related to the occurrence of macrophytes. In vegetated areas, stable fine sands were covered by an organic layer, a facies significantly different from that of non-vegetated areas, which comprised coarse sandy to gravely sediments. Distinct facies alternated in short distances across the river, partly due to the effects of macrophytes on the flow regime. Mean particulate nutrient concentrations were two orders of magnitude higher in vegetated (TOC 5.16, TN: 0.50 and TP: 0.24 of dry matter (DM)) than in non-vegetated sediments (TOC: 0.27, TN: 0.03 and TP: 0.06 DM). Therefore, the organic layer contained nearly the entire nutrient pool of the surface sediments, and thus significantly contributed to a temporary retention of particulate nutrients (at maximum 80 g nitrogen m−2 month−1 and 38 g phosphorus m−2 month−1) during vegetation period. Submerged macrophytes were found to effect a considerable spatial heterogeneity in riverbeds, and to cause high seasonality in particulate nutrient retention, a result amending previous geological facies analyses.
Keywords: fluvial sediments; grain size properties; particulate nutrients; aquatic macrophytes; nutrient retention