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Yeala Shaked 《Geochimica et cosmochimica acta》2008,72(6):1540-1554
Redox transformations of iron in the surface waters of the Gulf of Aqaba, Red Sea, were studied on recurrent cruises from September 2006 to May 2007. Fe(II) concentrations and oxidation kinetics were measured in situ using luminol chemiluminescence. High Fe(II) concentrations of 200-400 pM were recorded in the autumn, followed by low concentrations of 20-130 pM in the winter-spring. A distinct diurnal pattern in Fe(II) concentrations was observed in the autumn with maximum values coinciding with maximum solar irradiance. In situ and in vitro Fe(II) oxidation rates showed temporal and spatial variability that was accounted for by changes in water temperature and pH. Dissolved oxygen was found to be the dominant oxidant in all but one cruise. In situ photoreduction rates (deduced from oxidation rates) were linearly correlated with solar irradiance during the autumn, suggesting that the reducible iron pool was not exhausted even at the strongest irradiances and that it was kept constant throughout the season. Phytoplankton had no discernible influence on Fe(II) production, consumption, or oxidation kinetics. Given the fast oxidation and photoreduction rates of up to 180 pM min−1, the turn-over rates of iron were estimated at 10-30 per day. Such a dynamic Fe redox cycle probably influences the chemical reactivity and bioavailability of iron and may enhance the solubility of the abundant aerosol dust. 相似文献
2.
Iron biogeochemical cycling and distribution between particulate, reactive (colloidal + dissolved, oxine-labile, Fe(II)) fractions were studied in the seasonally stratified, mesotrophic Lake Kinneret. This article presents various aspects of the Fe budget in the lake and relates them to the chemical reactivity of various physicochemical forms of Fe.The budget of Fe in Lake Kinneret is dominated by fluvial Fe load, rather than by internal recycling of Fe from the sediment, as shown by the fact that 75 to 94% of the variance in Fe concentrations in the lake can be explained by the fluctuation in the water discharge of the Jordan River. Iron associated with phytoplankton accounts for 9-16% of the bulk particulate Fe in the lake. However, within patches of the dominate algae, the dinoflagellate Peridinium gatunense, algal Fe accounts for more than 70% of the lake’s particulate Fe. The algal Fe is predominantly intracellular, and the Fe: chlorophyll a ratios were within the range of published values for dinoflagellates and cyanobacteria. Iron associated with particles larger than 0.025 μm (20-300 nM) accounts for 80-95% of total Fe in the epilimnion of Lake Kinneret throughout the year. In contrast, this fraction of Fe is dominant in the hypolimnion only during the period of lake mixing. Iron concentration of different size fractions (<0.025 μm, <0.2 μm and >0.025 μm) in the surface water covaried throughout the research period. These covariations suggest dynamic transformations of Fe between different size fractions, either due to partial dissolution/precipitation or desorption/adsorption. Oxine-labile Fe concentrations, the Fe fraction considered chemically labile and available for phytoplankton, ranged from 15 to 75 nM. In wintertime, the oxine-labile Fe accounts for only 10-20% of the total Fe, while in other seasons most of the Fe is oxine-labile. Oxine-labile Fe concentrations always exceed the 0.025 μm-filtered Fe, implying that some of the larger particles contain oxine labile-Fe, and therefore are reactive. The fraction of reactive particles in Lake Kinneret (10-80%) is high relative to that of the marine environments, and can most likely be attributed to its fluvial source, which contains a significant proportion of reactive Fe oxide and hydrous oxide particles.The annual variability in the epilimnetic concentrations of other trace elements and nutrients, such as Al, Mn, Cd, Zn, Pb, and P were studied and grouped according to their resemblance with river water discharge, redox processes, or biological uptake and scavenging. Distribution patterns of Al, Pb and Cd resembled that of Fe, suggesting that similar processes control the concentrations of these metals, namely significant contribution from the watershed, high affinity to particulate matter and little control of biota on their fate in the lake. Other elements such as Zn and P are more affected by biological uptake, while Mn is more affected by redox cycling within the lake. 相似文献
3.
A. Matmon Y. Shaked N. Porat Y. Enzel R. Finkel N. Lifton E. Boaretto A. Agnon 《Earth and Planetary Science Letters》2005,240(3-4):803-817
In this study, we explored the spatial and temporal relations between boulders and their original in-situ locations on sandstone bedrock cliffs. This was accomplished by combining field observations with dating methods using cosmogenic isotopes (10Be and 14C) and optically stimulated luminescence (OSL). Our conclusions bear both on the landscape evolution and cliff retreat process in the hyperarid region of Timna and on the methodology of estimating exposure ages using cosmogenic isotopes.
We recognize three discrete rock fall events, at 31 ka, 15 ka, and 4 ka. In this hyperarid region, the most plausible triggering mechanism for rock fall events is strong ground acceleration caused by earthquakes generated by the nearby Dead Sea fault (DSF). Our record, however, under represents the regional earthquake record implying that ongoing development of detachment cracks prior to the triggering event might be slower than the earthquake cycle.
Cliff retreat rates calculated using the timing of rock fall events and estimated thickness of rock removed in each event range between 0.14 m ky− 1 and 2 m ky− 1. When only full cycles are considered, we derive a more realistic range of 0.4 m ky− 1 to 0.7 m ky− 1. These rates are an order of magnitude faster than the calculated rate of surface lowering in the area. We conclude that sandstone cliffs at Timna retreat through episodic rock fall events that preserve the sharp, imposing, landscape characteristic to this region and that ongoing weathering of the cliff faces is minor.
A 10%–20% difference in the 10Be concentrations in samples from matching boulder and cliff faces that have identical exposure histories and are located only a few meters apart indicates that cosmogenic nuclide production rates are sensitive to shielding and vary spatially over short distances. However, uncertainties associated with age calculations yielded boulder and matching cliff face ages that are similar within 1 σ . The use of external constraints in the form of field relations and OSL dating helped to establish each pair's age. The agreement between calculated 14C and 10Be ages indicates that the accumulation of 10Be at depth by the capture of slow deep-penetrating muons was properly accounted for in the study. 相似文献
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Our aim is to theorize the shifting relationship between cities and the biosphere in ways that can incorporate vanguard scientific, technical and social innovations. We specify that the city (a) generates third natures - specific new environments - such as heat islands, that today are destructive of the biosphere, and (b) that the city has systemic properties that correspond to those of the biosphere, but today are mostly flattened out of action through the ruptures that dominate today's articulation between cities and biosphere. That is to say, our specific project agrees with the problematizing of the category “nature,” which pertains to our presence in the biosphere. But we do not take Harvey's more absolute statement that the city itself is nature nor do we confine our analysis only to Latourian natures-cultures. Our analysis is less centered in the work of correcting a false binary, as is the case with both Latour and Harvey, notwithstanding their different objects of study. We focus on the complex in-between space that is the site of both the transactions between city and biosphere, as well as the site of the ruptures that characterize these transactions. 相似文献
5.
Jordanova V.K. Thorne R.M. Farrugia C.J. Dotan Y. Fennell J.F. Thomsen M.F. Reeves G.D. McComas D.J. 《Solar physics》2001,204(1-2):361-375
We study the development of the terrestrial ring current during the time interval of 13–18 July, 2000, which consisted of
two small to moderate geomagnetic storms followed by a great storm with indices Dst=−300 nT and Kp=9. This period of intense geomagnetic activity was caused by three interplanetary coronal mass ejecta (ICME) each driving
interplanetary shocks, the last shock being very strong and reaching Earth at ∼ 14 UT on 15 July. We note that (a) the sheath
region behind the third shock was characterized by B
z fluctuations of ∼35 nT peak-to-peak amplitude, and (b) the ICME contained a negative to positive B
z variation extending for about 1 day, with a ∼ 6-hour long negative phase and a minimum B
z of about −55 nT. Both of these interplanetary sources caused considerable geomagnetic activity (Kp=8 to 9) despite their disparity as interplanetary triggers. We used our global ring current-atmosphere interaction model
with initial and boundary conditions inferred from measurements from the hot plasma instruments on the Polar spacecraft and the geosynchronous Los Alamos satellites, and simulated the time evolution of H+, O+, and He+ ring current ion distributions. We found that the O+ content of the ring current increased after each shock and reached maximum values of ∼ 60% near minimum Dst of the great storm. We calculated the growth rate of electromagnetic ion cyclotron waves considering for the first time wave
excitation at frequencies below O+ gyrofrequency. We found that the wave gain of O+ band waves is greater and is located at larger L shells than that of the He+ band waves during this storm interval. Isotropic pitch angle distributions indicating strong plasma wave scattering were
observed by the imaging proton sensor (IPS) on Polar at the locations of maximum predicted wave gain, in good agreement with model simulations. 相似文献
6.
Down‐faulting at the north‐west margins of the Gulf of Aqaba is inferred to have triggered a catastrophic sedimentary event at 2.3 ka that killed the Elat fringing coral reef. Whereas segments of the Holocene reef were perfectly fossilized and preserved beneath a veneer of siliciclastic sediments, other segments were abraded, settled by nomads, and later re‐submerged under 4 m of water. Repeated damage triggered by down‐throwing earthquakes degenerate the fringing reefs of the north‐west end of the gulf. Conversely, on the north‐eastern and southern parts of the gulf, where earthquakes uplift the margins, modern reefs are thriving, attached to uplifted fossil reef terraces. Therefore, coastal subsidence moderates the development of fringing coral reefs during the late Holocene sea‐level stand still. 相似文献
7.
We describe a design for a compact cooled spectrograph which incorporates two cameras and two collimators to achieve high throughput from 0.8 to 2.5m. The spectrograph utilizes multiple NICMOS 3 arrays, specially mounted to minimize their separation in the spectral dimension. This accommodates the broad wavelength range while providing extensive coverage in the spatial dimension. 相似文献
8.
Dotan Shaked Gelband Yael Edelman‐Furstenberg Mordechai Stein Abraham Starinsky 《Sedimentology》2019,66(7):2950-2975
The study focuses on the formation of lacustrine dolomite in late Miocene lakes, located at the East Mediterranean margins (Northern Israel). These lakes deposited the sediments of the Bira (Tortonian) and Gesher (Messinian) formations that comprise sequences of dolostone and limestone. Dolostones are bedded, consist of small‐sized (<7 μm), Ca‐rich (52 to 56 mol %) crystals with relatively low ordering degrees, and present evidence for replacement of CaCO3 components. Limestones are comprised of a wackestone to mudstone matrix, freshwater macrofossils and intraclasts (mainly in the Bira Formation). Sodium concentrations and isotope compositions differ between limestones and dolostones: Na = ~100 to 150 ppm; ~1000 to 2000 ppm; δ18O = ?3·8 to ?1·6‰; ?2·0 to +4·3‰; δ13C = ?9·0 to ?3·4‰; ?7·8 to 0‰ (VPDB), respectively. These results indicate a climate‐related sedimentation during the Tortonian and early Messinian. Wet conditions and positive freshwater inflow into the carbonate lake led to calcite precipitation due to intense phytoplankton blooms (limestone formation). Dry conditions and enhanced evaporation led to precipitation of evaporitic CaCO3 in a terminal lake, which caused an increased Mg/Ca ratio in the residual waters and penecontemporaneous dolomitization (dolostone formation). The alternating lithofacies pattern reveals eleven short‐term wet–dry climate‐cycles during the Tortonian and early Messinian. A shift in the environmental conditions under which dolomite formed is indicated by a temporal decrease in δ18O of dolostones and Na content of dolomite crystals. These variations point to decreasing evaporation degrees and/or an increased mixing with meteoric waters towards the late Messinian. A temporal decrease in δ13C of dolostones and limestones and appearance of microbial structures in close association with dolomite suggest that microbial activity had an important role in allowing dolomite formation during the Messinian. Microbial mediation was apparently the main process that enabled local growth of dolomite under wet conditions during the latest Messinian. 相似文献
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