Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)

Iron oxides may undergo structural transformations when entering an anoxic environment. These transformations were investigated using the isotopic exchange between aqueous Fe(II) and iron oxides in experiments with ⁵⁵Fe-labelled iron oxides. ⁵⁵Fe was incorporated congruently into a ferrihydrite, two lepidocrocites (#1 and #2), synthesised at 10°C and 25°C, respectively, a goethite and a hematite. The iron oxides were then submerged in Fe²⁺ solutions (0-1.0 mM) with a pH of 6.5. In the presence of aqueous Fe²⁺, an immediate and very rapid release of ⁵⁵Fe was observed from ferrihydrite, the two lepidocrocites and goethite, whereas in the absence of Fe²⁺ no release was observed. ⁵⁵Fe was not released from hematite, even at the higher Fe²⁺ concentration. The release rate is mainly controlled by characteristics of the iron oxides, whereas the concentration of Fe²⁺ only has minor influence. Ferrihydrite and 5-nm-sized lepidocrocite crystals attained complete isotopic equilibration with aqueous Fe(II) within days. Within this timeframe ferrihydrite transformed completely into new and more stable phases such as lepidocrocite and goethite. Lepidocrocite #2 and goethite, having larger particles, did not reach isotopic equilibrium within the timeframe of the experiment; however, the continuous slow release of ⁵⁵Fe suggests that isotopic equilibrium will ultimately be attained.Our results imply a recrystallization of solid Fe(III) phases induced by the catalytic action of aqueous Fe(II). Accordingly, iron oxides should properly be considered as dynamic phases that change composition when exposed to variable redox conditions. These results necessitate a reevaluation of current models for the release of trace metals under reducing conditions, the sequestration of heavy metals by iron oxides, and the significance of stable iron isotope signatures.     

Sorption of phosphate onto calcite; results from batch experiments and surface complexation modeling

The adsorption of phosphate onto calcite was studied in a series of batch experiments. To avoid the precipitation of phosphate-containing minerals the experiments were conducted using a short reaction time (3 h) and low concentrations of phosphate (?50 μM). Sorption of phosphate on calcite was studied in 11 different calcite-equilibrated solutions that varied in pH, P_CO2, ionic strength and activity of Ca²⁺, and . Our results show strong sorption of phosphate onto calcite. The kinetics of phosphate sorption onto calcite are fast; adsorption is complete within 2-3 h while desorption is complete in less than 0.5 h. The reversibility of the sorption process indicates that phosphate is not incorporated into the calcite crystal lattice under our experimental conditions. Precipitation of phosphate-containing phases does not seem to take place in systems with ?50 μM total phosphate, in spite of a high degree of super-saturation with respect to hydroxyapatite (SI_HAP ? 7.83). The amount of phosphate adsorbed varied with the solution composition, in particular, adsorption increases as the activity decreases (at constant pH) and as pH increases (at constant activity). The primary effect of ionic strength on phosphate sorption onto calcite is its influence on the activity of the different aqueous phosphate species. The experimental results were modeled satisfactorily using the constant capacitance model with >CaPO₄Ca⁰ and either >CaHPO₄Ca⁺ or > as the adsorbed surface species. Generally the model captures the variation in phosphate adsorption onto calcite as a function of solution composition, though it was necessary to include two types of sorption sites (strong and weak) in the model to reproduce the convex shape of the sorption isotherms.     

Morphodynamics and sedimentary structures of bedforms under supercritical‐flow conditions: New insights from flume experiments

Supercritical‐flow phenomena are fairly common in modern sedimentary environments, yet their recognition and analysis remain difficult in the stratigraphic record. This fact is commonly ascribed to the poor preservation potential of deposits from high‐energy supercritical flows. However, the number of flume data sets on supercritical‐flow dynamics and sedimentary structures is very limited in comparison with available data for subcritical flows, which hampers the recognition and interpretation of such deposits. The results of systematic flume experiments spanning a broad range of supercritical‐flow bedforms (antidunes, chutes‐and‐pools and cyclic steps) developed in mobile sand beds of variable grain sizes are presented. Flow character and related bedform patterns are constrained through time‐series measurements of bed configurations, flow depths, flow velocities and Froude numbers. The results allow the refinement and extension of some widely used bedform stability diagrams in the supercritical‐flow domain, clarifying in particular the morphodynamic relations between antidunes and cyclic steps. The onset of antidunes is controlled by flows exceeding a threshold Froude number. The transition from antidunes to cyclic steps in fine to medium‐grained sand occurs at a threshold mobility parameter. Sedimentary structures associated with supercritical bedforms developed under variable aggradation rates are revealed by means of combining flume results and synthetic stratigraphy. The sedimentary structures are compared with examples from field and other flume studies. Aggradation rate is seen to exert an important control on the geometry of supercritical‐flow structures and should be considered when identifying supercritical bedforms in the sedimentary record.     

The role of climate variation in delta architecture: lessons from analogue modelling

Sequence‐stratigraphic models for fourth to sixth order, glacio‐eustatic sequences based only on relative sea‐level variations result in simplified and potentially false interpretations. Glacio‐eustatic sea‐level variations form only one aspect of cyclic climate variation; other aspects, such as variations in fluvial water discharge, vegetation cover, weathering and sediment supply can lead to variable sediment yield, thus adding complexity to sequence‐stratigraphic patterns normally attributed to sea‐level variations. Analogue flume models show a significant impact of water discharge on the timing and character of sequence boundaries, and on changes in the relative importance of systems tracts, as expressed in sediment volumes. Four deltas, generated under the influence of an identical sea‐level curve, and affected by different water‐discharge cycles were generated in the Eurotank facility: (1) constant discharge; (2) high‐frequency discharge variations (HFD); (3) discharge leading sea level by a quarter phase; (4) discharge lagging sea level by a quarter phase. HFD shift the parasequence stacking pattern consistently but do not alter large‐scale delta architecture. Water‐discharge changes that lead sea‐level changes result in high sediment yield during sea‐level rise and in the poor development of maximum flooding surfaces. Delta‐front erosion during sea‐level fall is expressed by multiple, small channels related to upstream avulsions, and does not result in an incised valley that efficiently routs sediment to the shelf edge. When water‐discharge changes lag sea‐level changes, sediment yield is high during falling sea level and results in rapid progradation during forced regression. Erosion from incised valleys is strong on the proximal delta top and dissipates towards the delta front. The combination of high discharge and sea‐level fall provides the most efficient mode of valley incision and sediment transport to the shelf edge. During sea‐level rise, low water discharge results in sediment starvation and well‐developed maximum flooding surfaces. Water‐discharge variations thus alter sequence‐stratigraphic patterns and provide an alternative explanation to the amplitude of sea‐level fall for generating either type 1 or 2 erosional unconformities.     

Precession-punctuated growth of a late Miocene submarine-fan lobe on Gavdos (Greece)

Sedimentary cycles in an upper Miocene succession of hemipelagic sediments (marls) and laminites (sapropels) were deposited in an outerarc basin and are related to the astronomical cycles of precession and eccentricity. Individual marl-laminite couplets correspond with the cycle of precession which has a periodicity of about 22 kyr. The lower part of the succession contains a turbidite interval comprising a number of distinct turbidite sequences. The turbidite sequences occur within or substitute entirely the laminite beds, so that turbidite deposition is similarly precession punctuated. The turbidite facies is characteristic for small, prograding fan lobes fed by small-volume turbidites. The abundant plant remains, the local palaeogeographic setting and the association with laminites (related to wet climate) suggest a river-fed submarine fan-lobe, where the timing of sediment transport is largely controlled by river floods during periods of high precipitation and continental run-off. The onset and ending of the turbidite interval is most likely linked with either autocyclic processes or by tectonic steepening of the hinterland relief. Sea-level changes seem least important for the triggering of turbidites, which is in contrast with current beliefs.     

Release of arsenic associated with the reduction and transformation of iron oxides

The behaviour of trace amounts of arsenate coprecipitated with ferrihydrite, lepidocrocite and goethite was studied during reductive dissolution and phase transformation of the iron oxides using [⁵⁵Fe]- and [⁷³As]-labelled iron oxides. The As/Fe molar ratio ranged from 0 to 0.005 for ferrihydrite and lepidocrocite and from 0 to 0.001 for goethite. For ferrihydrite and lepidocrocite, all the arsenate remained associated with the surface, whereas for goethite only 30% of the arsenate was desorbable. The rate of reductive dissolution in 10 mM ascorbic acid was unaffected by the presence of arsenate for any of the iron oxides and the arsenate was not reduced to arsenite by ascorbic acid. During reductive dissolution of the iron oxides, arsenate was released incongruently with Fe²⁺ for all the iron oxides. For ferrihydrite and goethite, the arsenate remained adsorbed to the surface and was not released until the surface area became too small to adsorb all the arsenate. In contrast, arsenate preferentially desorbs from the surface of lepidocrocite. During Fe²⁺ catalysed transformation of ferrihydrite and lepidocrocite, arsenate became bound more strongly to the product phases. X-ray diffractograms showed that ferrihydrite was transformed into lepidocrocite, goethite and magnetite whereas lepidocrocite either remained untransformed or was transformed into magnetite. The rate of recrystallization of ferrihydrite was not affected by the presence of arsenate. The results presented here imply that during reductive dissolution of iron oxides in natural sediments there will be no simple correlation between the release of arsenate and Fe²⁺. Recrystallization of the more reactive iron oxides into more crystalline phases, induced by the appearance of Fe²⁺ in anoxic aquifers, may be an important trapping mechanism for arsenic.     

Quantitative analogue flume-model study of river–shelf systems: principles and verification exemplified by the Late Quaternary Colorado river–delta evolution

ABSTRACT Physical modelling of clastic sedimentary systems over geological time spans has to resort to analogue modelling since full scaling cannot be achieved within the spatial and temporal restrictions that are imposed by a laboratory set‐up. Such analogue models are suitable for systematic investigation of a sedimentary system's sensitivity to allocyclic changes by isolating governing parameters. Until now, analogue models of landscape evolution were mainly qualitative in nature. In this paper, we present a quantitative approach. The quantitative experimental results are verified and discussed by comparison with high‐resolution data from the Colorado river–shelf system of the Texas shelf that we used as a prototype. The model's dimensions are proportionally scaled to the prototype, except for a vertical exaggeration. Time is scaled using a Basin Response factor to maintain a similar ratio between the period of change and the system's equilibrium time for model and prototype. A Basin Fill factor was used to compare the ratio between the time‐averaged sedimentation rate and the rate of change in accommodation space of model and prototype. The flume‐model results are in the form of sediment budgets that are related to shelf cannibalism and fluvial supply, which are compared with the ancestral Colorado river–delta evolution of the last 40 kyr. Model and prototype have similarities in delta evolution in response to one cycle of sea‐level change. With sea‐level change as the isolated variable, the flume model generates a significant supply pulse caused by headward erosion of the shelf in response to the sea‐level fall. This pulse adds to the yield of the hinterland. The supply induced by sea‐level change persists during the early rise, although its rate declines. A similar trend is observed on the east Texas shelf. We argue that shelfal and fluvial degradation cycles induced by sea‐level changes can significantly influence the timing and amount of sediment supply to basins and must therefore be taken into consideration.     

Sorption and desorption of arsenate and arsenite on calcite

The adsorption and desorption of arsenate (As(V)) and arsenite (As(III)) on calcite was investigated in a series of batch experiments in calcite-equilibrated solutions. The solutions covered a broad range of pH, alkalinity, calcium concentration and ionic strength. The initial arsenic concentrations were kept low (<33 μM) to avoid surface precipitation. The results show that little or no arsenite sorbs on calcite within 24 h at an initial As concentration of 0.67 μM. In contrast, arsenate sorbs readily and quickly on calcite. Likewise, desorption of arsenate from calcite is fast and complete within hours, indicating that arsenate is not readily incorporated into the calcite crystal lattice. The degree of arsenate sorption depends on the solution chemistry. Sorption increases with decreasing alkalinity, indicating a competition for sorption sites between arsenate and (bi)carbonate. pH also affects the sorption behavior, likely in response to changes in arsenate speciation or protonation/deprotonation of the adsorbing arsenate ion. Finally, sorption is influenced by the ionic strength, possibly due to electrostatic effects. The sorption of arsenate on calcite was modeled successfully using a surface complexation model comprising strong and weak sites. In the model, the adsorbing arsenate species were and . The model was able to correctly predict the adsorption of arsenate in the wide range of calcite-equilibrated solutions used in the batch experiments and to describe the non-linear shape of the sorption isotherms. Extrapolation of the experimental results to calcite bearing aquifers suggests a large variability in the mobility of arsenic. Under reduced conditions, arsenite, which does not sorb on calcite, will dominate and, hence, As will be highly mobile. In contrast, when conditions are oxidizing, arsenate is the predominant species and, because arsenate adsorbs strongly on calcite, As mobility will be significantly retarded. The estimated retardation factors for arsenate in carbonate aquifers range from 25 to 200.     

Sandy-gravelly mass-flow deposits in an ice-marginal lake (Saalian,Leuvenumsche Beek Valley,Veluwe, the Netherlands), with emphasis on plug-flow deposits

In three pits in the Leuvenumsche Beek Valley (near Ermelo and Elspeet), massive and diffusely banded clean gravelly sands, found in association with sandy sediments showing some similarities to the Bouma sequence, were interpreted as originating from sediment gravity flows. The flows, which came to rest on a ca. 6° paleoslope, probably originated from slumping of parts of the ice-pushed ridges at either side of an ice-marginal lake.Major characteristic features in the three sand pits studied are U-shaped channel forms with a maximum depth and width of 10 × 25 rmm. The fills consist of sand and gravels, locally containing giant sand-clasts. The sediments just below the base of these channel-forms are commonly distorted and folded. Especially the massive more sandy fills are surrounded by a finer-grained diffusely bedded and inversely graded zone.These sedimentary features suggest that “rigid” sediment plugs sheared downslope, generating a finer-grained shear zone around the plug scouring a slide scar till its present semi-cylindrical form and preserving the steep sides (exceeding the angle of repose) of the channel-form by “freezing”.The U-shaped channel forms (plugged troughs) are locally overlain by shallower gently sided trough structures, with mainly diffusely banded infillings. Very shallow and often smaller troughs are found again on top of these massive and diffusely banded infillings. The sediments in these troughs consist of repetitions of two types of microsequences: (1) thickly bedded sequences of normally graded massive sands, near-horizontal stratifications, plane beds and cross-bedded sets; and (2) thinly bedded sequences of relatively thick units T_A and thin units T_C and T_D of the Bouma sequence.The repetitions of the microsequences are interpreted as flow pulsations. Upwards thinning of the sequence, together with a finer grain size may point to waning flow conditions. Their association with plugged troughs is suggestive of retrogressive flow-slides.