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Aquatic Geochemistry (v.17, #1)

Sources and Distribution of Particulate Organic Matter of a Tropical Estuarine-Lagoon System from NE Brazil as Indicated by Lipid Biomarkers by Talitha L. F. Costa; Michelle P. Araújo; Bastiaan A. Knoppers; Renato S. Carreira (pp. 1-19).

In the present work, we evaluated the origin of organic matter in the tropical estuarine-lagoon system of Mundaú–Manguaba, NE Brazil, by considering the bulk (organic carbon and chlorophyll-a) and lipidic (n-alcohols and sterols) composition of suspended particles. Water samples were collected in August 2006 from 24 stations covering the salinity gradient from the rivers down to the sea outlet. Chlorophyll-a (Chl-a) varied from 22.7 to 134.1 μg L⁻¹ in the lagoons, indicating eutrophic to hypertrophic conditions at the time of sampling. The high correlation between Chl-a and phytol together with the molar C:N ratio indicated the presence of fresh and recently produced autochthonous particulate organic matter throughout the system, except for the river samples. The elevated concentrations of short-chain n-alcohols and phytosterols, mainly 24-methylcholesta-5,24(28)-dien-3β-ol, also corroborated the predominance of autochthonous organic matter in the lagoons but were generated by distinct sources: cianobacteria in the freshwater Manguaba lagoon and diatoms in the brackish Mundaú lagoon compartments. Input of terrestrial organic matter was only detected in the rivers themselves or at the upper river–lagoon interfaces. Coprostanol indicated contamination by sewage in Mundaú lagoon and in some rivers, but at lower levels when compared to other Brazilian coastal lagoons and estuaries.

Keywords: Lipid biomarkers; Organic matter; Suspended particles; Tropical lagoon system; NE Brazil

Generic Issues of Batch Dissolution Exemplified by Gypsum Rock by Victor W. Truesdale (pp. 21-50).

Recent work has emphasized that the empirical rate equation for batch dissolution of a solid consists of a forward term involving the surface area minus a back reaction term involving surface area and concentration of dissolved solid. Integrated forms exist for use at extremes of high under-saturation and of very heavy solid loadings which lead to saturation. A middle condition allows for significant decrease in solid supply and simultaneous arrival at saturation. This study tests the three approaches simultaneously to the batch dissolution of gypsum, thereby demonstrating a consistent applicability of the afore-mentioned rate equation. Previously, some mineral dissolutions have displayed so-called nonlinear kinetics and hence have not appeared to conform to this rate equation. This paper provides a template for future investigation of those situations; dissolution experiments are not easy to perform, and instances of the so-called nonlinear kinetics may represent experimental artefact. The relationship between this empirical approach and that of Transition State Theory used in mineral dissolution is discussed, and a new, linear proof for the applicability of the ‘middle ground’ equations is demonstrated. Stirring experiments highlight the difference between the conditions in fluidized bed and laminar flow reactors. Gypsum dissolution is found to be transport limited at all but very vigorous laboratory stirring conditions, although the relationship between the rate of shrinkage of gypsum particles and stirring seems to be relatively simple. A stirring factor is applied to the rate equation overall to allow for differences in reactor design, and it is suggested that this should also be applicable to laminar flow reactors. The link between batch and chemo-stat dissolutions is stressed, together with a need to contour dissolution data on a new graph of particle size versus stirring rate.

Keywords: Batch dissolution; Dissolution kinetics; Shrinking object; Shrinking sphere; Gypsum dissolution; Carbonate dissolution; Silica dissolution; Geo-engineering; Transition State Theory; Biogenic silica; Gypsum; Stirring rate; Common ion

Geochemistry and Behavior of Trace Elements During the Complete Evaporation of the Merouane Chott Ephemeral Lake: Southeast Algeria by Messaoud Hacini; Eric H. Oelkers (pp. 51-70).

The Merouane Chott, located in arid southeastern Algeria, experiences annual cycles of filling from September through February followed by its complete evaporation from February through June. The concentration of 15 trace elements (Li, B, Ti; V, Cr, Mn, Co, Cu, Ni, Zn, As, Sr, Ba, Pb, Bi, and U) were measured in chott water samples collected from January through June 2003 during the complete evaporation of the lake. The corresponding concentrations of these trace elements in the major external inputs to this closed basin chott were also obtained. The trace metals show two distinct behaviors. Li, B, Cr, Co, and U tend to be conserved in the chott waters throughout its evaporation. Much of Cr, Co, and U originated from external sources. It is likely, therefore, that the concentration of these elements will increase in the chott waters in future years. In contrast, Ti, Sr, Ba, Zn, Ni, and Pb precipitate continuously during chott evaporation. Of these elements, most of the Sr, Ba, and Zn originated from outside the chott, and thus it is likely these elements will become increasingly concentrated in the chott bottom salts with time. V, As, and Cu exhibit intermediate behaviors. These contrasting behaviors are confirmed by analysis of chott bottom solids.

Keywords: Chott Merouane; Saline lakes; Trace elements; Water–rock interaction

Salt Waters of the Northern Apennine Foredeep Basin (Italy): Origin and Evolution by Tiziano Boschetti; Lorenzo Toscani; Orfan Shouakar-Stash; Paola Iacumin; Giampiero Venturelli; Claudio Mucchino; Shaun K. Frape (pp. 71-108).

The salt waters from the Emilia-Romagna sector of the Northern Apennine Foredeep have been investigated using major and trace element and stable isotope (δ²H, δ¹⁸O, δ³⁷Cl, δ⁸¹Br and ⁸⁷Sr/⁸⁶Sr ratio). Ca, Mg, Na, K, Sr, Li, B, I, Br and SO₄ vs. Cl diagrams suggest the subaerial evaporation of seawater beyond gypsum and before halite precipitation as primary process to explain the brine’s salinity, whereas saline to brackish waters were formed by mixing of evaporated seawater and water of meteoric origin. A diagenetic end-member may be a third component for mud volcanoes and some brackish waters. Salinization by dissolution of (Triassic) evaporites has been detected only in samples from the Tuscan side of the Apennines and/or interacting with the Tuscan Nappe. In comparison with the seawater evaporation path, Ca–Sr enrichment and Na–K–Mg depletion of the foredeep waters reveal the presence of secondary processes such as dolomitization–chloritization, zeolitization–albitization and illitization. Sulfate concentration, formerly buffered by gypsum-anhydrite deposition, is heavily lowered by bacterial and locally by thermochemical reduction during burial diagenesis. From an isotopic point of view, data of the water molecule confirm mixing between seawater and meteoric end-members. Local ¹⁸O-shift up to +11‰ at Salsomaggiore is related to water–rock interaction at high temperature (≈150°C) as confirmed by chemical (Mg, Li, Ca distribution) and isotopic (SO₄–H₂O) geothermometers. ³⁷Cl/³⁵Cl and ⁸¹Br/⁷⁹Br ratios corroborate the marine origin of the brines and evidence the diffusion of halogens from the deepest and most saline aquifers toward the surface. The ⁸⁷Sr/⁸⁶Sr ratio suggests a Miocene origin of Sr and rule out the hypothesis of a Triassic provenance of the dissolved components for the analyzed waters issuing from the Emilia-Romagna sector of the foredeep. Waters issuing from the Tuscan side of the Apennines and from the Marche sector of the foredeep show higher ⁸⁷Sr/⁸⁶Sr ratios because of the interaction with siliciclastic rocks.

Keywords: Salt waters; Chemical and isotope composition; Seawater evaporation; Northern Apennine Foredeep

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Aquatic Geochemistry (v.17, #1)

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Aquatic Geochemistry (v.17, #1)