¹⁴C content of ten meteorites measured by tandem accelerator mass spectrometry     

R.M. Brown  H.R. Andrews  G.C. Ball  N. Burn  Y. Imahori  J.C.D. Milton  E.L. Fireman 《Earth and Planetary Science Letters》1984,67(1):1-8

¹⁴C has been measured in three North American and seven Antarctic meteorites with the Chalk River MP tandem accelerator. In most cases cosmogenic¹⁴C, which is tightly bound, was separated from absorbed atmospheric radiocarbon by stepwise heating extractions. Terrestrial ages obtained by comparing cosmogenic¹⁴C in the meteorite to that in Bruderheim are (7.2 ± 0.6) × 10³ years for Yamato 7304, (11.6 ± 0.4) × 10³ years for Estacado, and range from (32.7 ± 0.5) × 10³ to (41.0 ± 0.8) × 10³ years for six meteorites recovered at Allan Hills and its vicinity. The present upper limit to age determination by the accelerator method varies from 50 × 10³ to 70 × 10³ years depending upon mass and carbon content of the sample. The natural limit caused by cosmic ray production of¹⁴C in silicate rocks at 2000 m elevation is estimated to be (55 ± 5) × 10³ years. “Weathering ages” were estimated for the Antarctic meteorites from the specific activity of loosely-bound CO₂ considered to be absorbed from the terrestrial atmosphere on weathering. The accelerator measurements are in accordance with previous low-level counting measurements but have higher precision and sensitivity.  相似文献   

Estimation of the energy balance of the Dischma valley     

Dr. Barbara Hennemuth  Dipl. Met. U. Köhler 《Theoretical and Applied Climatology》1984,34(1-2):97-119

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In-situ neutron flux,Cl production and groundwater evolution in crystalline rocks at Stripa, Sweden     

J.N. Andrews  J.-Ch. Fontes  J.-L. Michelot  D. Elmore 《Earth and Planetary Science Letters》1986,77(1)

The meteoric input of³⁶Cl due to cosmogenic or nuclear-weapon-produced³⁶Cl cannot contribute significantly to the³⁶Cl present in the saline groundwaters (up to 700 mg l⁻¹ Cl⁻) from the Stripa granite. The extent of in-situ production of³⁶Cl has been estimated on the basis of the neutron fluxes within the granite and its surrounding leptite. The³⁶Cl present in the groundwaters is attributed to either admixture of labelled Cl⁻ from the leptite with Cl⁻ from the granite or to the total derivation of groundwater chlorinity within the leptite followed by radiochemical ingrowth of³⁶Cl during subsequent groundwater residence within the granite. The chloride derived from the leptite may be either matrix chloride or chloride from an external source which has had a long residence time within the leptite. The implications of³⁶Cl in-situ production for the estimation of groundwater residence times and for the geochemical evolution of groundwater chlorinity are discussed.  相似文献   

Man and climate in the Maya lowlands     

Barbara W. Leyden   《Quaternary Research》1987,28(3)

A 15-m sedimentary core from Lake Salpeten provides the first complete Holocene sequence for the lowlying Peten District, Guatemala. Today, Lake Salpeten is a brackish, calcium sulfate lake near saturation surrounded by tropical semievergreen forest. The basal pollen record depicts sparse juniper scrub surrounding a lake basin that held ephermal pools and halophytic marshes. The lake rapidly deepened to > 27 m in the early Holocene and may have been meromictic, because nearly 2 m of gypsum “mush” was deposited. Mesic forests were quickly established and persisted until the Maya entered the district 3000 yr ago and caused extensive deforestation. Any climatic information contained in the pollen record of the Maya period is thus masked, but a regional pollen sequence linked to the archaeological record is substantiated because environmental disturbance was pervasive. Local intensification of occupation and population growth are seen as an increased deposition of pollen of agricultural weeds and colluviation into the lake, while the Classic Maya collapse is marked by a temporary decline in Compositae pollen. Effects of perturbations induced by the Maya persist in the pollen and limnetic record 400 yr after the Spanish conquest.  相似文献   

Comment on “new evidence from beneath the western North Atlantic for the depth of glacial erosion in Greenland and North America” by E. P. Laine     

J.T. Andrews 《Quaternary Research》1982,17(1):123-124

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Observations of tidal and residual currents in the Sizewell-Dunwich area,East Anglia,UK     

Barbara J. Lees 《Ocean Dynamics》1983,36(1):1-24

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Effets combinés de la pêche et de l'eutrophisation sur la perche (Perca fluviatilis L.) dans les eaux vaudoises du Léman     

Barbara Lang  Claude Lang 《Aquatic Sciences - Research Across Boundaries》1983,45(2):480-494

Catches of perch have strongly decreased in Lake of Geneva from 1975 to 1981. This decrease was attributed to the impact of overfishing, enhanced by the accelerated growth of perch. Therefore, too many perches were captured before their first reproduction and the stocks of reproducers were strongly reduced. Furthermore, adverse meteorological conditions occurred probably during several periods of reproduction. The accelerated growth of perch was attributed to the combined effects of increasing eutrophication (more food available) and of overfishing (decreased perch density in the shoals).   相似文献   

Lead isotopic compositions of South Sandwich Island volcanic rocks and their bearing on magmagenesis in intra-oceanic island arcs     

Barbara Barreiro 《Geochimica et cosmochimica acta》1983,47(4):817-822

Pb isotope ratios have been measured in 12 volcanic rocks from the South Sandwich Islands. The ranges are ${}^{206}\text{Pb}{}^{204}\text{Pb}\text{= 18.51–18.66;}{}^{207}\text{Pb}{}^{204}\text{Pb}\text{= 15.55–15.64;}{}^{208}\text{Pb}{}^{204}\text{Pb}\text{= 38.42–38.64}$. In ${}^{207}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}\text{-}{}^{206}\text{Pb}{}^{204}\text{Pb}$ correlation diagrams, the South Sandwich data plot distinctly above the fields for ocean ridge basalts, and yield trends showing apparent mixing with a sedimentary end member similar to South Atlantic pelagic sediments as reported by Chow and Patterson (1962) and this study. Armstrong and Cooper (1971) have likewise shown that volcanics from the Lesser Antilles show mixing trends with North Atlantic sediments in Pb isotope correlation diagrams. The North Atlantic sediments have distinctly higher ${}^{206}\text{Pb}{}^{204}\text{Pb}$ and ${}^{208}\text{Pb}{}^{204}\text{Pb}$ ratios compared to the South Atlantic sediments. The parallel relationships between sediments and volcanic island arc rocks of the North and South Atlantic provide strong evidence for a component of Pb from subducted sediments in the lavas of the west Atlantic basin. In contrast to these data, lavas from the Mariana Arc in the western Pacific show little or no component of Pb from pelagic sediments. The reason for the different behaviors in the two settings is speculative.  相似文献   

Honduras: Caribbean Coast     

Harborne AR  Afzal DC  Andrews MJ 《Marine pollution bulletin》2001,42(12):1221-1235

The coast of Honduras, Central America, represents the southern end of the Mesoamerican Barrier Reef System, although its marine resources are less extensive and studied than nearby Belize and Mexico. However, the coastal zone contains mainland reef formations, mangroves, wetlands, seagrass beds and extensive fringing reefs around its offshore islands, and has a key role in the economy of the country. Like most tropical areas, this complex of benthic habitats experiences limited annual variation in climatic and oceanographic conditions but seasonal and occasional conditions, particularly coral bleaching and hurricanes, are important influences. The effects of stochastic factors on the country's coral reefs were clearly demonstrated during 1998 when Honduras experienced a major hurricane and bleaching event. Any natural or anthropogenic impacts on reef health will inevitably affect other countries in Latin America, and vice versa, since the marine resources are linked via currents and the functioning of the system transcends political boundaries. Much further work on, for example, movement of larvae and transfer of pollutants is required to delineate the full extent of these links.

Anthropogenic impacts, largely driven by the increasing population and proportion of people living in coastal areas, are numerous and include key factors such as agricultural run-off, over-fishing, urban and industrial pollution (particularly sewage) and infrastructure development. Many of these threats act synergistically and, for example, poor watershed management via shifting cultivation, increases sedimentation and pesticide run-off onto coral reefs, which increases stress to corals already affected by decreasing water quality and coral bleaching. Threats from agriculture and fishing are particularly significant because of the size of both industries. The desire to generate urgently required revenue within Honduras has also led to increased tourism which provides an over-arching stress to marine resources since most tourists spend time in the coastal zone. Hence the last decade has seen a dramatic increase in coastal development, a greater requirement for sewage treatment and more demand for freshwater, particularly in the Bay Islands.

Although coastal zone management is relatively recent in Honduras, it is gaining momentum from both large-scale initiatives, such as the Ministry of Tourism's ‘Bay Islands Environmental Management Project', and national and international NGO projects. For example, a series of marine protected areas and legislative regulations have been established, but management capacity, enforcement and monitoring are limited by funding, expertise and training. Existing and future initiatives, supported by increased political will and environmental awareness of stakeholders, are vital for the long-term economic development of the country.  相似文献   

Ice core evidence for secular variability and 200-year dipolar oscillations in atmospheric circulation over East Antarctica during the Holocene   总被引：3，自引：0，他引：3  

Barbara Delmonte  Jean Robert Petit  Gerhard Krinner  Valter Maggi  Jean Jouzel  Roberto Udisti 《Climate Dynamics》2005,24(6):641-654

Two Holocene ice core records from East Antarctica (Vostok and EPICA-Dome C) were analysed for dust concentration and size distribution at a temporal resolution of 1 sample per ~50 years. A series of volcanic markers randomly distributed over the common part of the ice cores (from 9.8 to 3.5 kyear BP) ensures accurate relative dating (±33 years). Dust-size records from the two sites display oscillations structured in cycles with sub-millennial and secular scale frequencies that are apparently asynchronous. The power spectra of the composite sum (Σ) of the two dust-size records display spectral energy mostly for 150- to 500-year periodicities. On the other hand, the 200-year band is common to both records and the 200 year components of the two sites are out-of-phase (100-year lead or lag) over ~5.5 kyear, a phenomenon also reflected by a significant (>99% conf. lev.) band in the power spectra of the composite difference (Δ) of the two size records. During long-range transport, mineral dust originating from the Southern Hemisphere continents is graded to a variable extent depending on the altitude and duration of atmospheric transport. Relatively coarse dust is associated with air mass penetration from the middle–lower troposphere and conversely relatively fine dust with upper troposphere air masses or the influence of subsidence over the Antarctic plateau, a hypothesis already proposed for the changes that occurred during the Last Glacial Maximum to Holocene transition (Delmonte et al. 2004b). Moreover, we assume that the overall fluctuation of air mass advection over Antarctica depends on the meridional pressure gradient with respect to low latitudes, i.e. the Antarctic Oscillation (AAO). We therefore suggest a regional variability in atmospheric circulation over East Antarctica. The 150–500 year power spectrum of the composite (Σ) parameter represents the long term variability of the AAO, imprinted by secular internal oscillations probably related to the southern ocean-climatic system. On the other hand, the Δ dust composite parameter suggests a persistent atmospheric dipole over East Antarctica delivering coarser (finer) dust particles alternatively to Vostok and Dome C regions with a bi-centennial periodicity. Indeed, a seesaw phenomenon in dust size distribution was already observed at three East Antarctic sites during the last deglaciation (Delmonte et al. 2004b) and was interpreted as a progressive reduction of the eccentricity of the polar vortex with respect to the geographic south pole. Interestingly, the Δ parameter shows a pronounced 200-year oscillation mode, throwing new light on the unresolved question of a possible relationship between climate and solar activity.  相似文献