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

J. F. Bijkerk  J. ten Veen  G. Postma  D. Mikeš  W. van Strien  J. de Vries 《Basin Research》2014,26(3):351-368

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.  相似文献   

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

G. Postma  F. J. Hilgen  W. J. Zachariasse 《地学学报》1993,5(5):438-444

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.  相似文献   

Release of arsenic associated with the reduction and transformation of iron oxides   总被引：9，自引：0，他引：9  

Hanne D. Pedersen  Dieke Postma  Rasmus Jakobsen 《Geochimica et cosmochimica acta》2006,70(16):4116-4129

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.  相似文献   

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

M. W. I. M. van Heijst  G. Postma  X. D. Meijer  J. N. Snow†  J. B. Anderson† 《Basin Research》2001,13(3):243-268

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.  相似文献   

Sorption and desorption of arsenate and arsenite on calcite     

Helle U. Sø  Dieke Postma  Flemming Larsen 《Geochimica et cosmochimica acta》2008,72(24):5871-5884

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.  相似文献   

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

George Postma  Thomas B. Roep  Gerard H.J. Ruegg 《Sedimentary Geology》1983,34(1):59-82

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.  相似文献   

Zusammenhänge in der Strukturgenese des Rheinischen Schiefergehirges     

Dr. Onno Oncken 《International Journal of Earth Sciences》1984,73(2):619-649

Zusammenfassung Ausgehend vom Maß der Inkohlung organischer Substanz in den Gesteinen des nördlichen Schiefergebirges einschließlich der Eifel können nach der Bestimmung der Beziehungen Aussagen zur tektonischen Evolution und zur Paläogeothermie des betrachteten Gebietes getroffen werden. Es bestätigen sich frühere Ansichten, daß die Großsättel aus Bereichen verminderter Absenkungsgeschwindigkeit in der Geosynklinale mit gleichzeitig relativ höheren geothermischen Gradienten hervorgehen. Obgleich Absenkung und Wärmefluß derart miteinander verbunden scheinen, ist das in den Inkohlungsdaten abgebildete Wärmefeld wahrscheinlich der Effekt einer Überlagerung eines präkinematischen, durch Absenkungsdifferenzen mitbestimmten Wärmefeldes mit einer während der Heraushebung in der Geosynklinalfüllung nordwärts wandernden Abkühlfront. Alle im Kartenbild deutlichen Großstrukturen des nördlichen Schiefergebirges sind auf ein kongruentes synsedimentäres Absenkungsmuster im Verein mit einer — möglicherweise ebenfalls vom thermischen Regime beeinflußten — differentiellen Heraushebung zurückzuführen. Der eigentlichen Faltung kommt dabei nur die Rolle eines die vorhandenen Strukturen überprägenden Faktors zu.

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The coal rank of organic matter dispersed in sediments of the northern Rhenish Massif including the Eifel permits — having determined the relationship — an evaluation of the tectonic and paleogeothermic evolution of the considered area. Earlier views on this subject are confirmed: Anticlinoria develop from geosynclinal zones of relatively lesser subsidence with, at the same time, elevated geothermal gradients opposite to the synclinoria. Although subsidence and heat flow appear to be related in this way, the observed temperature field — frozen in the vitrinite reflectance data — probably is due to the effect of the superposition of a prekinematic, differentiated temperature field, influenced by differential subsidence, with a cooling front moving northwards in the géosynclinal filling during uplift. All greater structures visible in the geologic map of the Rhenish Massif result from a congruent synsedimentary pattern of subsidence coupled with an equally differentiated pattern of postkinematic uplift, the latter also being influenced by the geothermal regime. The proper process of folding does not cause more than an overprinting of the preexisting structures.

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Résumé La dégradation thermique de la matière organique dispersée dans les sédiments du Schiefergebirge septentrional, y compris l'Eifel, permet, après détermination des relations, d'évaluer l'évolution tectonique et paléogéographique de la région considérée. Les vues antérieures sur ce sujet sont vérifiées: les anticlinaux de premier ordre résultent de zones géosynclinales à subsidence relativement réduite avec, en même temps, des gradients géothermiques élevés relativement aux synclinaux. Quoique subsidence et flux thermique paraissent liés de cette façon, le champ de température observé-attesté par les données de réflectance du vitrin-est probablement l'effet de la superposition d'un champ de température précinématique, codéterminé par des différences de subsidence, et d'un front de refroidissement se déplaçant vers le Nord pendant le soulèvement. Toutes les grandes structures que la cartographie a mis clairement en évidence dans le Schiefergebirge septentrional, sont à rapporter à un modèle de subsidence synsédimentaire associé à un soulèvement postcinématique différentiel, ce dernier peut-être aussi influencé par un régime géothermique. Au plissement proprement dit revient seulement le rôle d'un facteur se superposant aux structures déjà présentes.

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40Ar/39Ar geochronology using a quadrupole mass spectrometer     

Björn Schneider  Klaudia Kuiper  Onno Postma  Jan Wijbrans 《Quaternary Geochronology》2009,4(6):508-516

Magnetic sector mass spectrometers dominate the field of ⁴⁰Ar/³⁹Ar geochronology. Recent advances in quadrupole mass spectrometer technology, especially improvements in resolution, have increased the performance of these instruments to the extent that they can be used for isotopic determinations. We describe a triple filter quadrupole mass spectrometer (Hiden HAL 3F Series Pulse Ion Counting Triple Filter QMS) linked to an automated furnace extraction and cleaning system dedicated to ⁴⁰Ar/³⁹Ar incremental heating experiments.The instrument produces peaks with broad flat tops and a width of 0.9 amu at 10 cps height and 0.84 amu at 500,000 cps height on a 1 million cps high peak (peak width at 0.01‰ and 50% peak height respectively). This allows measurement of ratios of the main Ar peaks in the 1‰ range. Measurements of 1.6 × 10^?12 mole of air reference gas over two years yields ⁴⁰Ar/³⁶Ar = 257.9 ± 1.3 (1σ, n = 34). The ability of the instrument to produce ⁴⁰Ar/³⁹Ar ages from rocks/minerals of a wide age range, reaching into the late Quaternary, are demonstrated by a series of tests and comparison with geochronological data from other studies and an in-house MAP 215-50 magnetic sector mass spectrometer. We demonstrate that high-end quadrupole systems can be used for routine ⁴⁰Ar/³⁹Ar dating purposes.  相似文献   

Timestep Buffering to Preserve the Well-Mixed Condition in Lagrangian Stochastic Simulations     

John V. Postma 《Boundary-Layer Meteorology》2015,156(1):15-36

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Comprehensive analytic formulae for stellar evolution as a function of mass and metallicity   总被引：2，自引：0，他引：2  

Jarrod R. Hurley  Onno R. Pols  Christopher A. Tout 《Monthly notices of the Royal Astronomical Society》2000,315(3):543-569

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