Experimental study and TEM characterization of dusty olivines in chondrites: Evidence for formation by in situ reduction     

Hugues Leroux  Guy Libourel  Laurence Lemelle  Fran&#x;ois Guyot 《Meteoritics & planetary science》2003,38(1):81-94

Abstract— An analytical transmission electron microscopy (ATEM) study was undertaken in order to better understand the formation conditions of dusty olivines (i.e., olivines containing abundant tiny inclusions of Fe‐Ni metal) in primitive meteorites. Dusty olivines from type I chondrules in the Bishunpur chondrite (LL3.1) and from synthetic samples obtained by reduction of San Carlos olivines were examined. In both natural and experimental samples, micron size metal blebs observed in the dusty region often show preferential alignments along crystallographic directions of the olivine grains, have low Ni contents (typically <2 wt%), and are frequently surrounded by a silica‐rich glass layer. These features suggest that dusty olivines are formed by a sub‐solidus reduction of initially fayalitic olivines according to the following reaction: Some volatilization of SiO_gas may account for the apparent excess of metal relative to silica‐rich glass observed in both experimental and natural samples. Comparison with experimentally produced dusty olivines suggests that time scales of the order of minutes usually inferred for chondrule formation are also adequate for the formation of dusty olivines. These observations are in agreement with the hypothesis that at least part of the metal phase in chondrites originated from reduction during chondrule formation.  相似文献   

The effect of rates of sedimentation and tidal submersion regimes on the growth of salt marsh plants   总被引：1，自引：0，他引：1  

Laurence A. Boorman  J. Hazelden  M. Boorman 《Continental Shelf Research》2001,21(18-19)

A system of tidal mesocosms has been constructed in order to determine interactions between rates of accretion and duration of inundation. Plants are grown in three tanks in which different tidal regimes are simulated by a system of electronically controlled pumps. Sediment is added to give four different rates of accretion and the mesocosms are run under controlled greenhouse conditions. To date experiments have been conducted with seedlings of Salicornia europaea and Aster tripolium. The longest duration of immersion reduces the growth rate of root and shoot of Salicornia seedlings but Aster was unaffected by 5 h submersion daily. Both species, however, responded positively to all rates of sediment addition up to the equivalent of 60 mm per year. The implication of these findings are discussed in relation to salt marsh development.  相似文献   

The value of teaching about geomorphology in non-traditional settings     

R. Laurence Davis   《Geomorphology》2002,47(2-4)

Academics usually teach about geomorphology in the classroom, where the audience is enthusiastic, but generally small. Less traditional settings offer opportunities to reach a wider audience, one that is equally enthusiastic, given its love of geomorphic features in the National Parks, but one which has little knowledge of the science behind what they are seeing. I have “taught” geomorphology in four non-traditional settings: at a summer camp, a state wildlife refuge, on community field trips, and at meetings for clubs and government boards. This paper discusses my experiences and offers suggestions to others who may wish to follow this less-traveled educational path.As Head of Nature Programs at Camp Pemigewassett in New Hampshire, I have worked, over the last 33 years, with thousands of campers ranging from 8 to 15 years old. Our setting, in a glaciated valley on a small lake, exhibits a wide range of geomorphic features and offers many opportunities for direct learning through field investigations. I have found that even 8-year olds can do real science, if we avoid the jargon. Once “taught” they carry their knowledge about landforms and processes with them and eagerly share it with their friends and family on outings and trips, thus reaching an even wider public.Parks, wildlife refuges, nature preserves, and other similar areas generally have nature trails, often with educational information about the environment. Generally, interpretive signs are prepared by biologists and the content ignores the site's physical features, as well as the connections between ecological communities and the underlying geology and geomorphology. My students and I have addressed this situation at two places in Connecticut, one a state wildlife management area, also used for training teachers to teach Environmental Education, and the other, a town recreation area. We catalogued the geomorphic features, looked at relationships of the community level ecology to those features, and prepared interpretive signs that added this perspective to the trails. The public response has been extremely favorable.Geomorphology can also be taught by leading field trips for community organizations. I have done this twice, once for the Manchester (NH) Historical Society and once for a small watershed association. The attendance and interest surprised me. We finally had to limit the Manchester trip to one full busload (45) and the watershed trip, which was part of a “trails day,” drew over 90 people.Finally, I have found that organizations such as Sierra Club chapters and town conservation boards are frequently looking for speakers for their periodic meetings. Why not a geomorphologist? After all, much of what conservationists do is related to what geomorphologists do. I have given several of these presentations and the receptions have always been enthusiastic.While the work involved in preparing to teach in one of these non-traditional settings is frequently substantial, the rewards are equally large. It is a way to reach masses of people who know little about the science of geomorphology and to demonstrate its importance to them. Taking our message directly to the public in these settings is an effective way to put geomorphology in the public eye.  相似文献   

Sources of nitrogen and phosphorus to Northern San Francisco Bay     

Stephen W. Hager  Laurence E. Schemel 《Estuaries and Coasts》1992,15(1):40-52

We studied nutrient sources to the Sacramento River and Suisun Bay (northern San Francisco Bay) and the influence which these sources have on the distributions of dissolved inorganic nitrogen (DIN) and dissolved reactive phosphorus (DRP) in the river and bay. We found that agricultural return flow drains and a municipal wastewater treatment plant were the largest sources of nutrients to the river during low river flow. The Sutter and Colusa agricultural drains contributed about 70% of the transport of DIN and DRP by the river above Sacramento (about 20% of the total transport by the river) between August 8 and September 26, 1985. Further downstream, the Sacramento Regional Wastewater Treatment Plant discharged DIN and DRP at rates that were roughly 70% of total DIN and DRP transport by the river at that time. Concentrations at Rio Vista on the tidal river below the Sacramento plant and at the head of the estuary were related to the reciprocals of the river flows, indicating the importance of dilution of the Sacramento waste by river flows. During very dry years, elevated DIN and DRP concentrations were observed in Suisun Bay. We used a steady-state, one-dimensional, single-compartment box model of the bay, incorporating terms for advection, exchange, and waste input, to calculate a residual rate for all processes not included in the model. We found that the residual for DIN was related to concentrations of chlorophylla (Chla). The residual for DRP was also related to Chla at high concentrations of Chla, but showed significant losses of DRP at low Chla concentrations. These losses were typically equivalent to about 80% of the wastewater input rate.  相似文献   

Potassium-argon dating of volcanic rocks     

J. Laurence Kulp 《Bulletin of Volcanology》1963,26(1):247-258

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The geochemistry of strontium   总被引：2，自引：0，他引：2  

Karl K. Turekian  J. Laurence Kulp 《Geochimica et cosmochimica acta》1956,10(5-6):245-296

The geochemistry of strontium has been investigated, using an emission spectrographic technique checked by isotope dilution analyses described elsewhere ( et al., in press). Hence the determinations are claimed to have good accuracy as well as precision.

Approximately 700 analyses have been made on silicate rocks and over 300 on carbonate materials (a detailed analysis of the latter appearing in other papers). An attempt was made at a representative sampling of the crust of the earth.

The difference in behaviour of strontium relative to calcium in granitic rocks as compared to basaltic rocks is striking. For granitic rocks there is an increase of strontium content with an increase in calcium not only for samples from a single batholith but also when extended to a universal sampling. On the other hand, though some differentiated basaltic bodies show a covariance of strontium and calcium, it is always with the strontium content increasing as the calcium content decreases. On a worldwide basis, however, this relationship is destroyed because marked regional variations subdue any differentiation effects that might be present locally.

The crustal abundance of strontium in basaltic rocks may be estimated without difficulty by taking a simple average, but a similar procedure for granitic rocks is ruled out because of the dependence of strontium content on rock-type. The table below summarizes the abundances of strontium in the various phases of the earth's crust.  相似文献   

Solar and cosmic X-rays above 7.7 keV     

Hugh S. Hudson  Laurence E. Peterson  Daniel A. Schwartz 《Solar physics》1969,6(2):205-215

Since its launch on March 8, 1967, the OSO-III has continuously observed solar and cosmic X-rays over the 7.7–210 keV range. The sun emits many impulsive X-ray bursts having fluxes several orders of magnitude above the background level of 8 × 10^–9 ergs(cm²-sec)^–1 at 7.7 keV and characteristic times on the order of 5 min. Ninety-five such events having fluxes >3 × 10^–5 ergs(cm²-sec)^–1 were detected in the period from March 8 to June 15, 1967. The cosmic X-ray source Lupus XR-1 has been observed to have a power law spectral form and no significant time variations over a 40-day period. Upper limits have been obtained on the hard X-ray flux of the peculiar galaxy M 87.  相似文献   

Phospholipid ester-linked fatty acids composition of size-fractionated particles at the top ocean surface   总被引：1，自引：0，他引：1  

Anissa Brinis  Laurence Mjanelle  Andr Momzikoff  Genevive Gondry  Joëlle Fillaux  Vanessa Point  Alain Saliot 《Organic Geochemistry》2004,35(11-12):1275

Surface microlayer and subsurface water samples were collected at an oligotrophic Mediterranean site during a diel cycle. The composition of phospholipid ester-linked fatty acids (PLFA) was determined in size-fractionated particles (0.2–0.7 μm; 0.7–200 μm) in order to characterize the major contributors of organic matter to different size fractions. GF/F-retained particles (0.7–200 μm) from the surface microlayer were consistently enriched in PLFA relative to the underlying water. Molecular markers revealed a substantial difference in biological assemblages in both particle sizes. The larger particles were dominated by dinoflagellates, cyanobacteria, microzooplankton and attached bacteria, whereas particles filtered through GF/F and collected on 0.2 μm porosity Durapore filters (0.2–0.7 μm) were mostly bacteria and heterotrophic flagellates. Bacterial PLFA associated with 0.7–200 μm particles were more abundant than those in the 0.2–0.7 μm particles. Specific markers in the branched series appeared more representative of bacteria of smaller particle size.  相似文献   

Effects of size and origin of natural dissolved organic matter compounds on the redox cycling of iron in sunlit surface waters     

Laurence Meunier  Hansulrich Laubscher  Stephan J. Hug  Barbara Sulzberger 《Aquatic Sciences - Research Across Boundaries》2005,67(3):292-307

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Economic restructuring, housing policy and Maori housing in Northland, New Zealand   总被引：1，自引：0，他引：1  

Laurence Murphy  Dorothy Urlich Cloher 《Geoforum》1995,26(4):325-336

Since 1984 the New Zealand economy has experienced a significant phase of economic restructuring which has had a differential impact on various sectors, regions and social groups within the economy. This paper examines the extent to which economic change has affected the marginalized status of Maori households in the predominantly rural region of Northland. The social implications of economic restructuring are assessed by focusing on the housing circumstances of this indigenous people. Using a newly developed Maori housing database, we argue that relatively progressive housing policies, developed in the 1980s, have failed to address the housing problems of Maori and that the recent reliance on ‘market’ mechanisms to meet housing needs is likely to exacerbate problems of housing access and cost.  相似文献