Kinetic fractionation of Fe isotopes during transport through a porous quartz-sand column     

Alan Matthews  Simon Emmanuel  Haim Gvirtzman 《Geochimica et cosmochimica acta》2008,72(24):5908-5919

Sorption and desorption processes are an important part of biological and geochemical metallic isotope cycles. Here, we address the dynamic aspects of metallic isotopic fractionation in a theoretical and experimental study of Fe sorption and desorption during the transport of aqueous Fe(III) through a quartz-sand matrix. Transport equations describing the behavior of sorbing isotopic species in a water saturated homogeneous porous medium are presented; isotopic fractionation of the system (Δ_{sorbedmetal-soln}) being defined in terms of two parameters: (i) an equilibrium fractionation factor, α_e; and (ii) a kinetic sorption factor, α₁. These equations are applied in a numerical model that simulates the sorption-desorption of Fe isotopes during injection of a Fe(III) solution pulse into a quartz matrix at pH 0-2 and explores the effects of the kinetic and equilibrium parameters on the Fe-isotope evolution of porewater. The kinetic transport theory is applied to a series of experiments in which pulses of Na and Fe(III) chloride solutions were injected into a porous sand grain column. Fractionation factors of α_e = 1.0003 ± 0.0001 and α₁ = 0.9997 ± 0.0004 yielded the best fit between the transport model and the Fe concentration and δ⁵⁶Fe data. The equilibrium fractionation (Δ⁵⁶Fe_{sorbedFe-soln}) of 0.3‰ is comparable with values deduced for adsorption of metallic cations on iron and manganese oxide surfaces and suggests that sandstone aquifers will fractionate metallic isotopes during sorption-desorption reactions. The ability of the equilibrium fractionation factor to describe a natural system, however, depends on the proximity to equilibrium, which is determined by the relative time scales of mass transfer and chemical reaction; low fluid transport rates should produce a system that is less dependent on kinetic effects. The results of this study are applicable to Fe-isotope fractionation in clastic sediments formed in highly acidic conditions; such conditions may have existed on Mars where acidic oxidizing ground and surface waters may have been responsible for clastic sedimentation and metallic element transport.  相似文献   

Biologically mediated dissolution of calcium carbonate above the chemical lysocline?     

《Deep Sea Research Part I: Oceanographic Research Papers》1999,46(10):1653-1669

We find a diverse but increasing amount of evidence to suggest considerable dissolution of calcium carbonate, perhaps as much as 60–80%, in the upper 500–1000 m of the ocean, well above the chemical lysocline. The same biological processes that promote the rapid settling of carbonate particles (i.e. ingestion, digestion and egestion by zooplankton) as well as biologically mediated processes within flocculates and aggregates may be responsible – directly or indirectly – for much of this dissolution. The significance of shallow-water dissolution of carbonate particles necessitates further research into both its validity and, if true, specific causal mechanisms.  相似文献   

Fluvial incision and coarse gravel redistribution across the modern Dead Sea shelf as a result of base-level fall     

Haggai Eyal  Elad Dente  Itai Haviv  Yehouda Enzel  Thomas Dunne  Nadav G. Lensky 《地球表面变化过程与地形》2019,44(11):2170-2185

Global eustatic lowstands can expose vast areas of continental shelves, and occasionally the shelf edge and the continental slope. The degree of fluvial connectivity to receding shores influences the redistribution of sediments across these emerging landscapes. Shelf and slope emergence in the Dead Sea since the middle of the 20th century, offers a rare opportunity to examine evolution of stream connectivity in response to continuous base-level decline. We characterize the connectivity evolution of two streams, using high-resolution time series of aerial imagery and elevation models, field mapping, and grain-size analyses. Our rich spatiotemporal dataset of evolving channel geomorphology, sediment transport conditions, and sediment redistribution, allows calculating potential coarse sediment mobility in response to base level decline. Following shelf emergence, alluvial fans first prograded onto the low-gradient shelf under unfavourable conditions for transporting coarse sediment to the regressing shoreline. Then, with shelf and slope emergence, the two adjacent streams evolved differently. The smaller, more arid watershed still maintains its highstand delta progradation on the shelf and is practically disconnected from the receding lake. The larger catchment, heading in wetter environments and having a narrower shelf, has incised the shelf and renewed and gradually intensified the sediment transport from the highstand to the lowstand delta. Sediment mobilization to lowstand shorelines is controlled by the evolution of the channel profile and by the average speed of gravel transport (10s-100s m yr^-1). These findings from the Dead Sea are relevant to fluvial processes operating on continental shelves during glacial maxima. Streams would have commonly stored high proportions of their coarse sediment on the continental shelves rather than efficiently connecting with the lowstand level. Additionally, differences in sediment routing patterns should exist among nearby streams, primarily due to continental margin geometry and watershed hydrology. © 2019 John Wiley & Sons, Ltd.  相似文献   

How common is the Milky Way–satellite system alignment?     

Noam I. Libeskind  Carlos S. Frenk  Shaun Cole  Adrian Jenkins  John C. Helly 《Monthly notices of the Royal Astronomical Society》2009,399(2):550-558

The highly flattened distribution of satellite galaxies in the Milky Way (MW) presents a number of puzzles. First, its polar alignment stands out from the planar alignments commonly found in other galaxies. Secondly, recent proper-motion measurements reveal that the orbital angular momentum of at least three, and possibly as many as eight, of the MW's satellites points (within  30°  ) along the axis of their flattened configuration, suggesting some form of coherent motion. In this paper, we use a high-resolution cosmological simulation to investigate whether this pattern conflicts with the expectations of the cold dark matter model of structure formation. We find that this seemingly unlikely setup occurs often: approximately 35 per cent of the time, we find systems in which the angular momentum of three individual satellites points along, or close to, the short axis of the satellite distribution. In addition, in 30 per cent of the systems we find that the net angular momentum of the six best-aligned satellites lies within  35°  of the short axis of the satellite distribution, as observed for the MW.  相似文献   

Holocene climate variability and cultural evolution in the Near East from the Dead Sea sedimentary record   总被引：4，自引：2，他引：2  

Claudia Migowski  Mordechai Stein  Sushma Prasad  Jrg F.W. Negendank  Amotz Agnon 《Quaternary Research》2006,66(3):421-431

A comprehensive record of lake level changes in the Dead Sea has been reconstructed using multiple, well dated sediment cores recovered from the Dead Sea shore. Interpreting the lake level changes as monitors of precipitation in the Dead Sea drainage area and the regional eastern Mediterranean palaeoclimate, we document the presence of two major wet phases ( 10–8.6 and  5.6–3.5 cal kyr BP) and multiple abrupt arid events during the Holocene. The arid events in the Holocene Dead Sea appear to coincide with major breaks in the Near East cultural evolution (at  8.6, 8.2, 4.2, 3.5 cal kyr BP). Wetter periods are marked by the enlargement of smaller settlements and growth of farming communities in desert regions, suggesting a parallelism between climate and Near East cultural development.  相似文献   

Dissolved oxygen in the Dead Sea— seasonal changes during the holomictic stage     

Michal Shatkay  David A. Anati  Joel R. Gat 《International Journal of Salt Lake Research》1993,2(2):93-110

Measurements of dissolved oxygen, density and temperature were con ducted in the Dead Sea between August 1987 and December 1989. Oxygen concentrations were found to vary around an average value of 0.8 mlSTP (standard temperature and pressure) per kg. Dissolved oxygen variations were found to reflect both physical processes (a deepening of the epilimnion, a turnover etc.) and chemical processes (oxidation reduction). Transport coefficients for the oxygen exchange across the air/sea interface were found to be generally low compared to expected values based on laboratory experiments; the presence of surface films which inhibit the gas exchange rates under calm weather conditions is suspected. During stormy weather, the exchange rates were considerably higher, suggesting that the entrainment of bubbles was involved in the gas exchange process. Contribution No. 95, Department of Environmental Sciences and Energy Research. The Weizmann Institute of Science  相似文献   

Investigation of small-scale droplet concentration inhomogeneities in a turbulent flow     

B. Grits  M. Pinsky  A. Khain 《Meteorology and Atmospheric Physics》2006,92(3-4):191-204

Summary This paper is focused on the numerical investigation of spatial characteristics of droplet concentration fluctuations, originating in a turbulent flow. The turbulent flow is simulated using a model of a statistically stationary, homogeneous and isotropic turbulent flow. We found that preferential droplet concentration with the maximum of spatial spectrum at about 1.5 cm is formed regardless of the droplet size (up to the droplet radius of 20 μm). The amplitude of fluctuations depends on the droplet radius and constitutes a significant part of mean concentration. Analogous simulations with non-inertial droplets (passive scalar) do not reveal such concentration fluctuations. Our main conclusion is, therefore, the following: preferential droplet concentration at the cm-scale forms in a turbulent flow due to droplets inertia.  相似文献   

Greigite detected as dominating remanence carrier in Late Pleistocene sediments, Lisan formation, from Lake Kinneret (Sea of Galilee), Israel     

H. Ron  N. R. Nowaczyk  U. Frank  M. J. Schwab  R. Naumann  B. Striewski  A. Agnon 《Geophysical Journal International》2007,170(1):117-131

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Sr and Nd isotopes as tracers in pedogenic studies: Evidence for Saharan dust contribution to the soils of Muravera (Sardinia,Italy)     

《Chemie der Erde / Geochemistry》2015,75(3):301-315

Sr and Nd isotopes were applied to 5 soil profiles from the Muravera area, in south-eastern Sardinia.All the soils, which have developed during the Quaternary on the Lower Paleozoic metamorphic basement except for one on Eocene carbonates, are located far from major sources of pollution. Therefore, they are suitable for testing pedogenic processes and geochemical evolution to benefit for environmental studies.The Sr isotopic ratios range largely (δ⁸⁷Sr = 1.7–65.9‰), even in each soil profile. In particular, the observed increase of δ⁸⁷Sr with depth in the most of the metamorphic rock-based soils can be accounted for by the downward decrease of Sr contributions from organic matter and Saharan dust, both displaying lower isotopic ratios than the soil bedrocks. The carbonate rock-based soil exhibits δ⁸⁷Sr higher (1.7–18.1‰) than the bedrock, indicating a significant contribution of radiogenic Sr from the siliciclastic fraction of the soil, and probably from dust input. The Nd isotopic ratios are slightly variable through the profiles (ɛ_Nd from −7.8 to −14.5), confirming little mobility of Nd and Sm during the pedogenesis. Among the minerals present in the soils, phosphates, albite, and calcite are those important in providing low radiogenic Sr and Nd to organic matter of the soils.Lastly, this isotopic study has in particular allowed for evaluating the potential proportion of contribution of Saharan dust to south-eastern Sardinia, thus corroborating the findings of other studies related to soils from the central-western Mediterranean.  相似文献   

Deformation within an exposed salt wall: Recumbent folding and extrusion of evaporites in the Dead Sea Basin     

《Journal of Structural Geology》2015

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