Cornelis H. Van der Weijden

Chemical processes affecting the mobility of major, minor, and trace elements during weathering of granitic rocks

Abstract The behaviour of 38 major and trace elements as well as changes in the mineralogy have been examined in 10 weathering profiles developing on some Portuguese granitic rocks. Element mobilities are calculated from geochemical data normalized with respect to Ti in the fresh parent rock. Chemical elements are divided into two groups, immobile and mobile, on the basis of their geochemical distribution during weathering. Elements that are immobile during weathering are Zr, Hf, Fe, Al, Th, Nb, Sc and the REE. Very mobile are Ca, Na, P, K, Sr, Ba, Rb, Mg and Si. Mobile elements are derived mainly from leachable minerals such as feldspars, micas and apatites, whereas immobile elements are either concentrated in resistate phases or strongly adsorbed by secondary minerals. The geochemical behaviour of Mn, Cr, V, Fe and Ce is very dependent on redox conditions. Redox transformations of these elements can be used to set limits on the oxidation state of a weathering suite. The REE are mobilized or fractionated during late stages of weathering, but not during moderate stages of weathering. This fractionation is caused by selective leaching of rocks composed of both stable and unstable minerals containing REE.

Pitfalls of normalization of marine geochemical data using a common divisor

Abstract Normalization of trace element contents of sediment through division by the content of an immobile element – usually aluminum – is common practice in marine geochemical studies. This might appear to be a simple way of correcting for dilution by sedimentary phases barren of a particular trace element – for instance carbonates or silica – and for comparison with its content in a standard clay or shale. The purpose of this paper is to revive the awareness of the pitfalls of this practice by giving numerical examples of the often-unexpected results of normalization. Taking a statistical perspective, it is first shown that uncorrelated variables acquire spurious correlations when normalized. But normalization can also increase, decrease, change sign of, or even blur the correlations between unmodified variables. Next, a number of realistic scenarios are worked out to show that the correlations between normalized element contents still suffer from the closure effect. Only in a few simple cases it is possible to extract, from the normalized data, realistic estimates of trace element contents in distinct sedimentary phases, such as organic matter. This is ascertained from data on copper contents in sediment from the Black Sea and Arabian Sea. When the coefficient of variation (i.e. standard deviation divided by the mean) of the aluminum data is relatively low and much lower than for the original values of the trace elements, comparisons of correlations can equally well or even better be made on the basis of the unmodified values. Then, the comparison of trace element data with their values for standard shales is discussed. The inherent problem is that the composition of the commonly used standard shale and, consequently, the reference values of normalized elements are not necessarily representative of the local or regional sediments in the study area. Also, this method falls short of identifying the processes responsible for enrichment or diminution in trace element contents. Finally, consideration is given to some aspects of a proper use of Al normalization

Geochemistry of dissolved trace metals (cadmium, copper, zinc) in the Scheldt estuary, southwestern Netherlands: Impact of seasonal variability

The distribution of dissolved Cd, Cu, and Zn in the Scheldt estuary has been studied during eight axial surveys, carried out between February 1987 and February 1988. The observed metal-salinity profiles depend on the season. During spring and summer, when the river water is anoxic (containing traces of dissolved sulfide), the dissolved metal concentrations in the riverine endmember are extremely low. This observation is ascribed to formation of sparingly soluble metal sulfides in the water column. During winter, when the river water is not totally devoid of oxygen (10–40% saturation), the dissolved Cu and Zn concentrations in the riverine endmember are an order of magnitude higher, but rapid removal is apparent in the very low salinity zone. Flocculation (of organometal complexes) or coagulation (of colloid-associated metals), sediment resuspension and formation of particulate Fe and Mn oxyhydroxides are likely to be involved in the removal process. At higher salinities, maxima of dissolved Zn (at 6–9 × 10−3), Cu (at 9–18 × 10−3), and Cd (at 12–21 × 10−3) are consistently found over the year. These maxima are ascribed to dissolution and desorption of particulate metal forms with increasing salinity. Reoxidation of trace metal sulfides during transport from the anoxic (or suboxic) upper estuary to the fully oxygenated lower estuary is suggested as the first step in the mobilization process. During phytoplankton blooms, desorption of Cd and Zn (but not Cu) is suppressed, which is attributed to the pH increase related to primary production, and to biological uptake. The impact of mobilization processes in the Scheldt estuary is reflected by effective dissolved Cd and Cu concentrations which are much higher than the observed metal concentrations in the river water. Based on these findings, it is expected that restoration of the dissolved oxygen concentration, which is a major goal of the present-day management of the Scheldt estuary, will lead to an increase in the dissolved metal transport to the North Sea.

A 225 kyr record of dust supply, paleoproductivity and the oxygen minimum zone from the Murray Ridge (northern Arabian Sea)

Abstract Analysis of a (15 m long) sediment core recovered from the Murray Ridge (northern Arabian Sea) reveals a 225,000 year record of climate-induced variations in surface water productivity, mid-water ventilation, and dust input in the northern Arabian Sea. Productivity and dust input records closely resemble those from the Owen Ridge and Oman Margin. Also the phase lag of ∼10 kyr between maximum productivity and precession minimum is in phase with the summer monsoon upwelling record from the Owen Ridge and the Oman Margin. This implies that the productivity variations in the northern Arabian Sea during the last 225 kyr are primarily controlled by changes in the Indian Ocean summer monsoon. The winter monsoon obviously played a minor role in the productivity history of the northern Arabian Sea. Another result from this study is that the Mg Al record from the Murray Ridge nicely documents the post-glacial flooding of the Persian Gulf, which is the main glacial source for dolomite dust in this region. Finally, it is concluded that the intensity of the OMZ co-varies with surface water productivity, being weak during periods of reduced summer monsoon, especially during glacial periods.

Manganese solubility control in marine pore waters

Abstract A theoretical study of the system CaCO3-MnCO3-H2O provides evidence that solid solution is limited to three mole fraction ranges corresponding to Mn-calcite, kutnohorite and calcic-rhodochrosite. All these authigenic phases may control the concentration of manganese in suboxic marine pore waters. As a consequence the solubility of manganese cannot be adequately described by a single thermodynamic equilibrium constant.

Arsenic, antimony and vanadium in the North Atlantic Ocean

Abstract New measurements of arsenic, antimony and vanadium in samples from vertical profiles at nine stations between 50° and 70°N in the Atlantic Ocean are presented. Antimonate concentrations exhibit little variation with depth and location, the average concentration being 1.16 nmol 1-1. The vertical distribution of arsenate is similar to that reported for the South Atlantic and Pacific Ocean, with somewhat lower surface concentrations (14 to 18 nmol 1-1) and uniform deep water concentrations (mean, 19 nmol 1-1). Uniform vertical profiles for vanadium with a mean concentration of 32.6 nmol 1-1 are in agreement with data for the Central Atlantic, but differ from profiles in the Pacific which exhibit surface water depletion. The presented data for As and V have extended the limited basis for comparison, so that inter-ocean fractionation processes can now be recognized. The concentrations of both As and V show regular increases in the ocean deep water as it passes from the North Atlantic (As 19.0; V 32.6 nmol 1-1) via the South Atlantic (As 21.1 nmol 1-1) and Indian Ocean (As 22.5; V 35.6 nmol 1-1) to the Pacific Ocean (As 24.0; V 36.4 nmol 1-1). The magnitude of biological recycling for V and As, as deduced from interoceanic fractionation, is consistent with estimates based on particle or algal compositions.

Enhanced preservation of organic matter in sediments deposited within the oxygen minimum zone in the northeastern Arabian Sea

Abstract The presence of a strongly developed oxygen minimum zone (OMZ; [O 2 ] 2+ and Fe 2+ show that reduction of manganese and iron oxides plays a minor role in sediments lying within the OMZ and a larger role in sediments lying below the OMZ.

Sorption of Benzo[a]pyrene and Phenanthrene on Suspended Harbor Sediment as a Function of Suspended Sediment Concentration and Salinity: A Laboratory Study Using the Cosolvent Partition Coefficient.

The sorption of benzo[a]pyrene (BaP) and phenanthrene (PHE) on sediment collected in the harbor of Rotterdam was studied in batch experiments using 14 C-labeled compounds. In order to simulate various estuarine conditions, we investigated the effects of various sediment concentrations and salinities on sorption of these compounds. We determined the apparent partitioning as well as the cosolvent partitioning between the liquid and solid phases. The apparent partition coefficient (K p App ) is influenced by the presence of colloids and dissolved organic matter. The influence of these components is minimized by the use of the cosolvent method in which the partition coefficient (K p * ) is determined by extrapolation of the partition coefficients obtained in a variety of solutions with water and methanol. The effect of sediment concentration on sorption was investigated at five different sediment concentrations ranging from 0.064 to 10.8 g/L. In contrast to K p * , K p App was found to be sensitive to the sediment concentration. Log K p * (with K p * in L/kg), determined for different sediment concentrations, was 5.2-6.3 for BaP and 2.8-3.6 for PHE,whereas log K p App was 4.1-5.6 for BaP and 2.9-4.0 for PHE. The salt concentration was varied with dilutions of Milli-Q water and seawater ; the salinity ranged from 0 to 35. The influence of salinity on K p * was small ; the increase in log K p * from freshwater to seawater is approximately 0.2 for BaP and 0.1 for PHE.

Profiles of the redox-sensitive trace elements As, Sb, V, Mo and U in the Tyro and Bannock Basins (eastern Mediterranean)

Abstract Concentration profiles of dissolved As, Sb, V, Mo and U across the interface between hypersaline anoxic brines and overlying Mediterranean seawater are presented for the Tyro and Bannock Basins. In the oxic Mediterranean seawater, the concentrations of these elements show little variation with depth and location. The average normalized concentrations are: 18.2 nM for ΣAs, 1.6 nM for ΣSb, 31.1 nM for V, 140 nM for Mo, and 14.5 nM for U. The concentrations of the trivalent species of As and Sb increase from 3.2 ± 1.4 and 0.22 ± 0.06 nM respectively in the overlying Mediterranean seawater to ∼ 150 and 5.3 nM respectively in the deepest parts of the brines. Similarly, the concentrations of the pentavalent species of As and Sb show an increase from 16.9 ± 2.4 and 1.56 ± 0.10 nM respectively to ∼ 125 and 2.6 nM respectively across the interface between Mediterranean seawater and hypersaline anoxic brines. However, at greater depths concentrations of pentavalent species decrease again, but remain high in view of the prevailing reducing conditions. The concentrations of V show an increase of 3–8 nM at the interface, as a consequence of the decomposition of organic matter. Below the interface, concentrations of V are low (∼ 7 nM). In the hypersaline anoxic brines the concentrations of Mo and U decrease strongly to values below their detection limits:

Hydrochemistry, weathering and weathering rates on Madeira island

Madeira island consists of Miocene to Pleistocene lavas and pyroclasts. Major rock types are alkali-basalts, basanites and hawaiites; principal soil types are leptosols, andosols and cambisols. Our main objective was to link the chemistry of ground waters to weathering reactions and rates. We collected 40 shallow groundwater samples, remote from human activities. With a few exceptions, the ranges of electrical conductivities were 29 – 176 mS/cm and of pH 5.8– 8.5. The calculated PCO2 was generally higher than the atmospheric value. The contribution of sea salt to the water chemistry was 30 ^ 9%. Corrected for sea salt, the cation concentrations (in meq/l) decrease in the order Ca 2þ < Mg 2þ . Na þ .. .K þ . The concentrations of SO42 and NO3 are very low. We calculated that the total annual chemical denudation rate in the studied area amounts to 37 ^ 12 g/m 2 , consuming 0.86 ^ 0.38 mol CO2/m 2 . To achieve our main objective, a set of mole balance equations— ðAX ¼ BÞ—was used, where A is a composite matrix of coefficients, including ratios between stoichiometric coefficients as determined by the weathering reactions and coefficients accounting for unconstrained contributions, B is the vector with a water composition, and X is the set of mole fractions of dissolved primary minerals plus the residual concentrations of the unconstrained contributions. Olivine (Ol), pyroxene (Py) and plagioclase (Pl) were considered to be the major primary minerals, and smectite, vermiculite, halloysite, allophane, gibbsite and hematite the secondary minerals in the weathering reactions. Using iterative procedures, whereby mixtures of secondary products as well as the composition of plagioclase are allowed to change, we selected one best-fit set of weathering reactions for each spring by checking all possible solutions of the mole balances against predefined boundary conditions. At odds with Goldich (1938) sequence, our model results indicate—for most best –fit sets—a weathering rate sequence of Pl . Ol . Py, but such reverse order is not unique. On average, the annual weathering rates (in mol/(ha y vol% mineral)) are 44 ^ 19 (Pl), 29 ^ 14 (Ol) and 22 ^ 13 (Py). q 2003 Elsevier B.V. All rights reserved.