Paleomagnetic Study of a Reversal of the Earth's Magnetic Field     

Dunn JR  Fuller M  Ito H  Schmidt VA 《Science (New York, N.Y.)》1971,172(3985):840-845

A detailed record of a field reversal has been obtained from the natural remanent magnetization of the Tatoosh intrusion in Mount Rainier National Park, Washington. The reversal took place at 14.7 +/- 1 million years and is interpreted to be from reverse to normal. A decrease in the intensity of the field of about an order of magnitude occurs immediately before the reversal, while its orientation remains substantially unchanged. The onset of the reversal is marked by abrupt swinging of the virtual geomagnetic pole along an arc of a great circle. During the reversal the pole traces a path across the Pacific. In the last stage of the process recorded in the sections, the succession of virtual geomagnetic poles is very similar to those generated by secular variation in the recent past. Although the cooling rate of the intrusion is not sufficiently well known to permit a useful calculation of the duration of the reversal process, an estimate based on the length of the supposed secular variation cycles gives 1 to 4 x 103 years for the reversal of field direction and approximately 1 x 104 years for the time scale of the intensity changes.  相似文献   

Ice on venus: can it exist?     

Owen T  Businger JA  Holton JR  Libby WF 《Science (New York, N.Y.)》1968,161(3844):915-916

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The Fusarium graminearum genome reveals a link between localized polymorphism and pathogen specialization     

Cuomo CA  Güldener U  Xu JR  Trail F  Turgeon BG  Di Pietro A  Walton JD  Ma LJ  Baker SE  Rep M  Adam G  Antoniw J  Baldwin T  Calvo S  Chang YL  Decaprio D  Gale LR  Gnerre S  Goswami RS  Hammond-Kosack K  Harris LJ  Hilburn K  Kennell JC  Kroken S  Magnuson JK  Mannhaupt G  Mauceli E  Mewes HW  Mitterbauer R  Muehlbauer G  Münsterkötter M  Nelson D  O'donnell K  Ouellet T  Qi W  Quesneville H  Roncero MI  Seong KY  Tetko IV  Urban M  Waalwijk C  Ward TJ  Yao J  Birren BW  Kistler HC 《Science (New York, N.Y.)》2007,317(5843):1400-1402

We sequenced and annotated the genome of the filamentous fungus Fusarium graminearum, a major pathogen of cultivated cereals. Very few repetitive sequences were detected, and the process of repeat-induced point mutation, in which duplicated sequences are subject to extensive mutation, may partially account for the reduced repeat content and apparent low number of paralogous (ancestrally duplicated) genes. A second strain of F. graminearum contained more than 10,000 single-nucleotide polymorphisms, which were frequently located near telomeres and within other discrete chromosomal segments. Many highly polymorphic regions contained sets of genes implicated in plant-fungus interactions and were unusually divergent, with higher rates of recombination. These regions of genome innovation may result from selection due to interactions of F. graminearum with its plant hosts.  相似文献   

In situ evidence for an ancient aqueous environment at Meridiani Planum, Mars     

Squyres SW  Grotzinger JP  Arvidson RE  Bell JF  Calvin W  Christensen PR  Clark BC  Crisp JA  Farrand WH  Herkenhoff KE  Johnson JR  Klingelhöfer G  Knoll AH  McLennan SM  McSween HY  Morris RV  Rice JW  Rieder R  Soderblom LA 《Science (New York, N.Y.)》2004,306(5702):1709-1714

Sedimentary rocks at Eagle crater in Meridiani Planum are composed of fine-grained siliciclastic materials derived from weathering of basaltic rocks, sulfate minerals (including magnesium sulfate and jarosite) that constitute several tens of percent of the rock by weight, and hematite. Cross-stratification observed in rock outcrops indicates eolian and aqueous transport. Diagenetic features include hematite-rich concretions and crystal-mold vugs. We interpret the rocks to be a mixture of chemical and siliciclastic sediments with a complex diagenetic history. The environmental conditions that they record include episodic inundation by shallow surface water, evaporation, and desiccation. The geologic record at Meridiani Planum suggests that conditions were suitable for biological activity for a period of time in martian history.  相似文献   

Response     

Finnerty JR  Block BA 《Science (New York, N.Y.)》1994,265(5176):1250-1251

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Factors in ethanol tolerance     

Wenger JR  Woods SC 《Science (New York, N.Y.)》1984,224(4648):524

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AMA Research Institute To Close     

Kramer JR 《Science (New York, N.Y.)》1969,166(3913):1604

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Are the data worth owning?     

Macdonald JR 《Science (New York, N.Y.)》1972,176(4042):1377

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Ammonia flux into the atmosphere from a grazed pasture     

Denmead OT  Simpson JR  Freney JR 《Science (New York, N.Y.)》1974,185(4151):609-610

A micrometeorological technique has been used to measure the flux of ammonia and related gaseous nitrogen compounds into the atmosphere from a pasture grazed by sheep. During 3 weeks in late summer, the average daily flux density of nitrogen in these forms was 0.26 kilogram per hectare. This is a substantial part of the nitrogen turnover in grazed pastures.  相似文献   

不同分散温度、时间对双波长法测定大米中淀粉质量分数的影响研究          下载免费PDF全文

张雪梅  张玲  高飞虎  曾志红  李雪  梁叶星  张欢欢  杨世雄 《西南大学学报(自然科学版)》2020,42(4):49-55

为了测定大米中直、支链淀粉及总淀粉质量分数,采用双波长法研究了不同测定条件(温度、时间)对大米中直链、支链淀粉及总淀粉质量分数的影响.结果表明,在70～100℃温度条件下,样品75℃、处理15 min测得的支链淀粉质量分数最高, 80℃时样品的支链淀粉相对质量分数最低;在80℃、处理15 min时,样品直链淀粉质量分数最高;处理温度和时间对大米中总淀粉质量分数的影响与支链淀粉相似.由此可知,不同的测定条件(温度、时间)对双波长法测定大米中各淀粉质量分数的影响不同.  相似文献