Laboratory Dissolution Experiments Research Articles

Impact of post‐mixing chemical reactions on the major ion chemistry of bulk meltwaters draining the haut glacier d'arolla, valais, Switzerland

AbstractUntil now, alpine glacial meltwaters have been assumed to consist of two components, dilute quickflow and concentrated delayed flow, the mixing of which has been regarded as chemically conservative for the major dissolved ions and electrical conductivity. Dye tracing results suggest that this two‐component model adequately represents the sub‐glacial hydrology of the Haut Glacier d'Arolla, Switzerland. However, laboratory dissolution experiments in which various concentrations of glacial rock flour are placed in dilute solutions show that this rock flour is highly reactive and suggest that bulk meltwaters may acquire significant amounts of solute through rapid chemical reactions with suspended sediment which occur after mixing of the two components. This view is supported by detailed analysis of variations in the hydrochemistry of meltwaters draining from the Haut Glacier d'Arolla over three diurnal cycles during the 1989 melt season. Variations in the composition of bulk meltwaters are controlled by two main factors: dilution of the delayed flow component by quickflow, and the extent of post‐mixing reactions. The latter depends on the suspended sediment concentration in bulk meltwaters and on the duration of contact between these waters and suspended sediment. Seasonal changes in the magnitude of these factors result in changes in the character and causes of diurnal variations in meltwater chemistry. In June, these variations reflect discharge‐related variations in residence time within a distributed subglacial drainage system; in July, when a channelized drainage system exists beneath the lower glacier, they primarily reflect the dilution of delayed flow by quickflow; in August, when suspended sediment concentrations are particularly high, they reflect varying degrees of solute acquisition by post‐mixing reactions with suspended sediment that take place in arterial channels at the glacier bed.

Mineral dissolution rates in plot-scale field and laboratory experiments

Mineral dissolution rates were measured on identical mineral material in field and laboratory experiments. Field dissolution rates were measured in 6 small (2 m 2) plots on a spodosol in eastern Maine, U.S.A. The plots were irrigated with HCl at pH's 2, 2.5 and 3; soil solutions were collected by tension lysimeters at 25-cm depth. The composition of the soil solutions, together with the grain-size distribution and mineralogy of the soil, were used to calculate mineral dissolution rates. Laboratory dissolution experiments were performed on the 75–150-μm size fraction of soil from the site in flow-through reactors at pH-values corresponding to the pH of the bulk soil solution. The use of small plots and “untreated” minerals from the same plots eliminates many of the uncertainties encountered in previous field-laboratory comparisons. Dissolution rates observed in the field, normalized on the basis of geometrical mineral surface area, were smaller than laboratory rates by a factor of ∼200–400. The discrepancy can be explained by a number of factors. The major reason is probably imperfect contact between soil minerals and percolating solution. Macropore flow or limited exchange between water in micropores and the bulk soil solution could reduce the mineral surface area participating in the dissolution process. Retention of water in micropores could result in elevated pH-values and solute concentrations (particularly Al) near the mineral surface, resulting in lower dissolution rates. Differences in mineral surface characteristics between field and laboratory experiments can be largely excluded as a reason for the observed discrepancy since identical mineral material was used in both sets of experiments. However, part of the discrepancy might be explained by the dissolution rate not scaling with geometrical surface area.

Dissolution and Preservation of Antarctic Diatoms and the Effect on Sediment Thanatocoenoses

AbstractComparison of Southern Ocean diatom populations from (i) surface ocean production, (ii) underlying Antarctic sediments, and (iii) laboratory dissolution experiments demonstrates that dissolution can account for the temporal and spatial variations in sedimentary diatom assemblages observed in Southern Ocean sediments. Increasing dissolution causes relative depletions in N. kerguelensis (K), enrichments in T. lentiginosa (L), and slight enrichments in E. antarctica (A). This reflects the relative susceptibility to dissolution of the three species that dominate Antarctic sediments. We have devised a preservation index for the Southern Ocean based on the ratio K/(K + L) to estimate relative extents of dissolution and applied it to natural assemblages. Holocene Southern Ocean sediments display increasing opal preservation toward higher latitudes, but south of the Antarctic Polar Front preservation decreases in the order: well preserved = SE Indian > S. Atlantic ∼ SW Indian > SE Pacific = poorly preserved. Dissolution also accounts for the pattern of diatom assemblages in the last glacial maximum (LGM) sediments of the Indian and Pacific sectors, but in the Atlantic, increased E. antarctica abundances at LGM must have resulted from an increase in surface ocean production of this species. Holocene and LGM diatoms in Atlantic and Pacific sector sediments are equally well preserved, but in the Indian sectors, Holocene sediments are better preserved than those of LGM age. Paleoceanographic and paleoclimatic transfer functions derived from factor analyses of variations in the sedimentary abundances of these three diatoms have ignored the effects of differential dissolution on thanatocoenosis and thus should be interpreted with caution.

Kinetic study of the dissolution of calcined trona ore in aqueous solutions

In the chemical reaction control regime employed in the laboratory dissolution experiments, calcined trona ore dissolves in aqueous solutions according to a first order, irreversible reaction rate. The study showed that the reaction is first order with respect to solution Na2CO3 concentration. The activation energy was determined to be 20.233 kJ/g · mol with a rate constant equal to 1235 mm/s (2917 in. per min). The dissolving particle may be described by shrinking particle behavior.

On the origin of Ethmodiscus ooze

Ethmodiscus ooze is essentially a monospecific diatomaceous deposit found in the equatorial parts of the oceans. Laboratory dissolution experiments on fossil diatom assemblages from the deep-sea floor show that Ethmodiscus rex is very resistant to dissolution. This species is therefore concentrated in the diatom assemblages subjected to experimentally controlled dissolution in seawater. Examination of deep-sea sediments, labelled in the literature as E. rex oozes, indicates that two different types of sediments are involved. In the first type the entire siliceous assemblage is made up by Ethmodiscus fragments. The second type is numerically dominated by Ethmodiscus fragments but includes an amount of etched, dissolution-resistant species. It is suggested that the first type of Ethmodiscus ooze is formed by a combination of differential dissolution and winnowing, whereas the second is created by differential dissolution.

The Early Nonstructural Chemical Diagenesis of Foraminiferal Calcite

ABSTRACT Magnesium/calcium, Sr/Ca, and Na/Ca ratios were determined for planktonic forminiferal tests taken from core top samples at water depths of 500 to 4,700 m along the East Pacific Rise and central Pacific. Six species with varying susceptibilities to calcium carbonate dissolution were analyzed by neutron activation and atomic absorption techniques. These natural dissolution experiments were supplemented by laboratory dissolution and exchange experiments. The magnesium content of planktonic foraminifera was found to be highly susceptible to diagensis; for tests of a given species, Mg/Ca ratios decreased with increasing water depth. Sodium/calcium ratios decreased slightly with depth, and Sr/Ca ratios did not change as a result of the partial dissolution of test material with increasing w ter depth.