共查询到20条相似文献，搜索用时 562 毫秒
1.
M Stuiver 《Science (New York, N.Y.)》1968,162(857):994-997
Freshwater lake marl and mollusks from Indiana show variations in ratios of oxygen-18 to oxygen-16 of a few per mil during the Postglacial. For the last 9000 years these variations are about 1 per mil for properly selected lake sediments. Thus the oxygen isotope composition of atmospheric precipitation appears relatively stable over this interval, indicating stationary atmospheric circulation patterns over the Great Lakes region. A reduction in oxygen-18 content around 10,000 years ago corresponds with the climate change indicated by pollen profiles. Large systematic differences were found between carbon-13 and oxygen-18 content of marl and mollusks. 相似文献
2.
Gas vacuolation in Oscillatoria rubescens decreased with increased nitrogen limitation and increased with transitions from nitrogen to inorganic carbon limitation. Gas vacuoles consist of protein vesicles that can accumulate in carbon- limited but not in unenriched nitrogen-limited cells. Nitrogen limitation is a factor in the formation of deep population maxima; carbon limitation can promote surface blooms. 相似文献
3.
Druffel EM 《Science (New York, N.Y.)》1982,218(4567):13-19
Radiocarbon analyses and stable isotope measurements are presented foro recent cores of banded corals from the Florida Straits. These values provide a record of variations in the ratio of carbon-14 to carbon-12 in the dissolved inorganic carbon in the surface waters of the Gulf Stream from A.D. 1642 to 1800. An increase in the carbon-14/carbon-12 ratio of 7 per mil for coral growth during the early 1700's was most likely induced by an increase in the carbon-14/carbon-12 ratio of 20 per mil in the atmospheric carbon dioxide that occurred at about 1700. The ratios of oxygen 18 to oxygen-16 in these coral bands show a small decrease of a water temperature ( approximately 1 degrees C) during the latter part of the Little Ice Age (1700 to 1725). These results support the hypothesis that the increase in atmospheric carbon-14 at about 1700, and possibly the temperature change as well, was caused by a decrease in solar activity (Maunder sunspot minimum). 相似文献
4.
Lowell TV Heusser CJ Andersen BG Moreno PI Hauser A Heusser LE Schlüchter C Marchant DR Denton GH 《Science (New York, N.Y.)》1995,269(5230):1541-1549
A radiocarbon chronology shows that piedmont glacier lobes in the Chilean Andes achieved maxima during the last glaciation at 13,900 to 14,890, 21,000, 23,060, 26,940, 29,600, and >/=33,500 carbon-14 years before present ((14)C yr B.P.) in a cold and wet Subantarctic Parkland environment. The last glaciation ended with massive collapse of ice lobes close to 14,000(14)C yr B.P., accompanied by an influx of North Patagonian Rain Forest species. In the Southern Alps of New Zealand, additional glacial maxima are registered at 17,720(14)C yr B.P., and at the beginning of the Younger Dryas at 11,050 (14)C yr B. P. These glacial maxima in mid-latitude mountains rimming the South Pacific were coeval with ice-rafting pulses in the North Atlantic Ocean. Furthermore, the last termination began suddenly and simultaneously in both polar hemispheres before the resumption of the modern mode of deep-water production in the Nordic Seas. Such interhemispheric coupling implies a global atmospheric signal rather than regional climatic changes caused by North Atlantic thermohaline switches or Laurentide ice surges. 相似文献
5.
A method based on measurements of dissolved molecular nitrogen, molecular oxygen, and argon can distingish biological from physical contributions to oxygen supersaturation in the ocean. The derived values of biological O(2) production can be used as a check on estimates of total organic productivity measured by instantaneous rates of carbon-14 assimilation. Application to the shallow summer O(2) maxima in the North Pacific gyres shows that about 72% of the O(2) supersaturation maximum at 28 degrees N and about 86% of the maximum at 40 degrees N are due to net photosynthetic production. 相似文献
6.
Solar forcing of drought frequency in the Maya lowlands 总被引：1，自引：0，他引：1
We analyzed lake-sediment cores from the Yucatan Peninsula, Mexico, to reconstruct the climate history of the region over the past 2600 years. Time series analysis of sediment proxies, which are sensitive to the changing ratio of evaporation to precipitation (oxygen isotopes and gypsum precipitation), reveal a recurrent pattern of drought with a dominant periodicity of 208 years. This cycle is similar to the documented 206-year period in records of cosmogenic nuclide production (carbon-14 and beryllium-10) that is thought to reflect variations in solar activity. We conclude that a significant component of century-scale variability in Yucatan droughts is explained by solar forcing. Furthermore, some of the maxima in the 208-year drought cycle correspond with discontinuities in Maya cultural evolution, suggesting that the Maya were affected by these bicentennial oscillations in precipitation. 相似文献
7.
Carbon and oxygen isotope analysis through a 30-year (1944 to 1974) growth of Montastrea annularis from Hen and Chickens Reef (Florida Keys) shows a strong yearly variation in the abundances of both carbon-13 and oxygen-18 and a broad inverse relationship between the two isotopes. Normal annual dense bands are formed during the summer and are characterized by heavy carbon and light oxygen. "Stress bands" are formed during particularly severe winters and are characterized by heavy carbon and heavy oxygen. The isotopic effect of Zooxanthellae metabolism dominates the temperature effect on the oxygen-18/oxygen-16 ratio. The isotopic results on the deep-sea solitary coral Bathypsammia tintinnabulum, where Zooxanthellae are nonexistent, indicates that the abundance of the heavy isotopes carbon-13 and oxygen-18 is inversely related to the growth rate, with both carbon and oxygen approaching equilibrium values with increasing skeletal age. 相似文献
8.
History of atmospheric lead deposition since 12,370 (14)C yr BP from a peat bog, jura mountains, switzerland 总被引：3，自引：0，他引：3
W Shotyk D Weiss PG Appleby AK Cheburkin RFM Gloor JD Kramers S Reese Van Der Knaap WO 《Science (New York, N.Y.)》1998,281(5383):1635-1640
A continuous record of atmospheric lead since 12,370 carbon-14 years before the present (14C yr BP) is preserved in a Swiss peat bog. Enhanced fluxes caused by climate changes reached their maxima 10, 590 (14)C yr BP (Younger Dryas) and 8230 (14)C yr BP. Soil erosion caused by forest clearing and agricultural tillage increased lead deposition after 5320 (14)C yr BP. Increasing lead/scandium and decreasing lead-206/lead-207 beginning 3000 (14)C yr BP indicate the beginning of lead pollution from mining and smelting, and anthropogenic sources have dominated lead emissions ever since. The greatest lead flux (15.7 milligrams per square meter per year in A.D. 1979) was 1570 times the natural, background value (0.01 milligram per square meter per year from 8030 to 5320 (14)C yr BP). 相似文献
9.
Carbon isotopic evidence for different feeding patterns in two hyrax species occupying the same habitat 总被引：1，自引：0，他引：1
The carbon-13/carbon-12 ratios of the carbonate and collagen fractions of bone of the sympatric hyrax species Procavia johnstoni and Heterohyrax brucei indicate that the former obtains most of its diet by grazing while the latter is primarily a browser. The carbon-13/carbon-12 ratios of these fractions in fossil bone will record information about diet if they have not been altered during diagenesis. 相似文献
10.
M Lacroix F Mosora M Pontus P Lefebvre A Luyckz G Lopez-Habib 《Science (New York, N.Y.)》1973,181(98):445-446
The ratio of carbon-13 to carbon-12 is much higher in most commerical preparations of glucose used for oral glucose tolerance tests than it is in carbon dioxide in expired air. This recent discovery provided a novel and potentially significant means of studying glucose metabolism. The changes in the ratio of carbon-13 to carbon-12 in carbon dioxide expired after oral glucose administration were determined by mass spectrometry. In six healthy male volunteers, the administration of glucose resulted in a marked, reproducible rise in the isotopic ratio in expired carbon dioxide; the ratio reached its maximum at 4 hours and then declined progressively. 相似文献
11.
Hormone-induced stabilization of soluble RNA in pea-stem tissue 总被引：2，自引：0，他引：2
When indoleacetic acid labeled with carbon-14 in the carboxyl group is fed to excised green pea-stem segments, growth is initiated, and there is a parallel progressive labeling of the RNA extracted by cold phenol. The bulk of the label is found in the 4S fraction. This fraction is more resistant to degradation by ribonuclease than a similar fractian obtained from tissue not treated with indoleacetic acid. 相似文献
12.
The Murchison and Allende chondrites contain up to 5 parts per million carbon that is enriched in carbon-13 by up to + 1100 per mil (the ratio of carbon-12 to carbon-13 is approximately 42, compared to 88 to 93 for terrestrial carbon). This "heavy" carbon is associated with neon-22 and with anomalous krypton and xenon showing the signature of the s-process (neutron capture on a slow time scale). It apparently represents interstellar grains ejected from late-type stars. A second anomalous xenon component ("CCFXe") is associated with a distinctive, light carbon (depleted in carbon-13 by 38 per mil), which, however, falls within the terrestrial range and hence may be of either local or exotic origin. 相似文献
13.
The carbon-14-labeled carbon dioxide that is released by respiration after glucose labeled with carbon-14 is applied to fungal mycelium can be reabsorbed in highly significant amounts by distant mycelium and agar media in the same petri dishes. Atmospheric transfer of carbon-14 must be considered when using labeled organic compounds to study translocation in fungi. 相似文献
14.
Calcareous particles present in Pacific waters at depths of 50 to 3500 meters were collected by filtering seawater through spongin matrix. The specific activity of carbon-14 could be measured in two of these collections from depths of 2300 and 3500 meters. The ratios of carbon-14 to carbon-12 correspond to values observed in surface waters in recent years as a result of the addition of manmade carbon-14, thus indicating that the calcareous particles resulted from recent biological productivity. The results are related to the mean settling rates and the sizes and dissolution rates of biogenic calcareous particles in transit through a seawater column. 相似文献
15.
碳-14考古断代原理 总被引：1，自引：0，他引：1
马莹 《吉林农业科技学院学报》2004,(3)
放射性元素碳 - 1 4的半衰期为 5 730年。自然界中活着的生物体中碳 - 1 4含量应为定值 ,生物死亡之后 ,碳 - 1 4含量按一定规律衰减。考古学家根据死亡后生物体内碳 - 1 4含量相对于其原始量的变化来断定它所属年代。 相似文献
16.
富硒极大螺旋藻整细胞、藻胆体及藻蓝蛋白的光谱特性 总被引：8，自引：0，他引：8
对培养富硒极大螺旋藻的最佳硒处理浓度作了研究。结果表明,40mg/L的硒可促进该藻的生长。与对照组相比，富硒极大螺旋藻的整细胞、藻胆体及藻蓝蛋白的光谱特性无显著差异，这表明硒没有影响到极大螺旋藻光能的吸收与传递。 相似文献
17.
对培养富硒极大螺旋藻的最佳硒处理浓度作了研究。结果表明,40mg/L的硒可促进该藻的生长。与对照组相比，富硒极大螺旋藻的整细胞、藻胆体及藻蓝蛋白的光谱特性无显著差异，这表明硒没有影响到极大螺旋藻光能的吸收与传递。 相似文献
18.
The values of length of synodic month, obtained from tidally controlled periodical growth patterns in mollusks and stromatolites for several geologic periods, indicate that the deceleration rate of the earth's rotation has not been constant. Two breaks in slope, in the Pennsylvanian and Cretaceous, may be related to changes in distribution of continents, oceans, and adjacent shallow seas. 相似文献
19.
The variability of the carbon-13/carbon-12 ratio in honeys was evaluated preliminary to use of the ratio to detect the addition of high-fructose corn syrup to honey. Eighty-four honey samples representing 34 states and including 37 floral types from 17 plant families were analyzed. The mean value of the per mil increment in carbon-13 (delta13C) for all samples is -25.2 per mil, and the coefficient of variation is 3.7 percent. This is the smallest variation yet encountered for a honey constituent or physical property. The range and magnitude of the values suggest that the floral sources are C(3) plants. 相似文献
20.
Taylor RE Payen LA Gerow B Donahue DJ Zabel TH Jull AJ Damon PE 《Science (New York, N.Y.)》1983,220(4603):1271-1273
A morphologically modern human skeleton from Sunnyvale, California, previously dated by aspartic acid racemization to be approximately 70,000 years old and by uranium series isotopic ratios to be 8300 and 9000 years old, appears to be younger when dated by the carbon-14 method. Four carbon-14 determinations made by both decay and direct counting on three organic fractions of postcranial bone support a middle Holocene age assignment for the skeleton, probably in the range of 3500 to 5000 carbon-14 years before the present. This dating evidence is consistent with the geologic, archeological, and anthropometric relationships of the burial as well as previously determined carbon-14 determinations on associated materials. 相似文献