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  • AGU (American Geophysical Union)  (5)
  • Cambridge University Press  (3)
  • American Institute of Physics (AIP)  (1)
  • 2000-2004
  • 1990-1994  (3)
  • 1985-1989  (6)
  • 1975-1979
  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Review of Scientific Instruments 63 (1992), S. 2485-2487 
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: Accelerator mass spectrometry (AMS) requires ion sources delivering intense negative ion beams of high stability. In addition, a multitarget system and a vacuum lock for sample loading are essential for efficient operation. We present a new design of a cesium (Cs) negative ion sputter source with the following features: the vacuum chamber of the ion source and the target cassette are at ground potential. Batches of targets can be loaded and unloaded without interrupting the operation of the source. All control systems for sample loading and automatic changing are also at ground potential. This ion source concept has already been successfully applied to AMS at our laboratory. The new design incorporates a spherical Cs surface ionizer to increase the negative ion currents. Difficulties in realizing this concept are the high electric fields inside the vacuum chamber in an environment with high Cs vapor pressure. Also critical are the proper and reliable operation of all mechanical components under these conditions and the large temperature gradients.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 1993-01-01
    Description: A 34.5 m borehole, which was drilled near the Dead Sea coast (altitude -394 m) in the southern part of the fan delta of Wadi Zeelim, reveals the geological history of that area from the latest Pleistocene to present. The depositional time frame is based on six 14C dates and two U-Th dates. An erosional (or nondepositional) period is implied by the hiatus between 21,100 yr B.P. (U-Th age, depth 33 m) and 11,315 yr B.P. (14C age, depth 32 m). A subsequent arid phase is recorded by a 6.5-m-thick layer of halite; based on 14C dates this phase relates to the abrupt Younger Dryas cold period reported in temperate to polar regions. The fragility of the environment in this region is indicated by the fact that the region experienced such a severe, short aridification phase (less than 1000 yr), evidence of which is found widely in the desert fringes of the Middle East and North Africa. The aragonite found in most of the Holocene section indicates that the well site was covered by the lake for most of the Holocene. Exceptions are the intervals at 0-3 and 10-14 m depths which represent low stands of the lake.
    Print ISSN: 0033-5894
    Electronic ISSN: 1096-0287
    Topics: Geography , Geosciences
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  • 3
    Publication Date: 1988-01-01
    Description: Measurements of the cosmogenic isotope 10Be (T½ = 1.5 Ma) on Greenland ice cores produced interesting results. Variations in the 10Be concentrations can be interpreted in terms of changes in the production rate and in atmospheric circulation and deposition. During the Holocene, good agreement between short-term variations in 10Be and 14C indicates that the production rate of both isotopes was changing, probably due to solar modulation. During the last ice age, periods with significantly higher 10Be concentrations are observed. The good anti-correlation between 10Be and δ18O suggests that these intervals correspond to periods of low precipitation rates. Work on Antarctic ice cores is in progress, but only relatively few 10Be data have been published yet. 10 Be results from Antarctic ice cores are presented and compared with data from Greenland.
    Print ISSN: 0260-3055
    Electronic ISSN: 1727-5644
    Topics: Geography , Geosciences
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  • 4
    Publication Date: 1988-01-01
    Description: Measurements of the cosmogenic isotope 10Be (T½ = 1.5 Ma) on Greenland ice cores produced interesting results. Variations in the 10Be concentrations can be interpreted in terms of changes in the production rate and in atmospheric circulation and deposition. During the Holocene, good agreement between short-term variations in 10Be and 14C indicates that the production rate of both isotopes was changing, probably due to solar modulation.During the last ice age, periods with significantly higher 10Be concentrations are observed. The good anti-correlation between 10Be and δ18O suggests that these intervals correspond to periods of low precipitation rates.Work on Antarctic ice cores is in progress, but only relatively few 10Be data have been published yet. 10 Be results from Antarctic ice cores are presented and compared with data from Greenland.
    Print ISSN: 0260-3055
    Electronic ISSN: 1727-5644
    Topics: Geography , Geosciences
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  • 5
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    AGU (American Geophysical Union)
    In:  Paleoceanography, 3 (4). pp. 509-515.
    Publication Date: 2016-06-15
    Description: A radiocarbon-calibrated box model for today's ocean suggests that a lag of about 1750 years should exist between the arrival of the midpoint of the deglaciation 18O signal in the deep Atlantic Ocean and its arrival in the deep Pacific Ocean. In order to assess the actual lag, we have carried out accelerator radiocarbon measurements on two cores from the Atlantic Ocean and one core from the Pacific Ocean. Although the results are not definitive, there is a suggestion that the actual time lag was about 1000 years.
    Type: Article , PeerReviewed
    Format: text
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  • 6
    Publication Date: 2016-06-16
    Description: It has long been recognized that the transition from the last glacial to the present interglacial was punctuated by a brief and intense return to cold conditions. This extraordinary event, referred to by European palynologists as the Younger Dryas, was centered in the northern Atlantic basin. Evidence is accumulating that it may have been initiated and terminated by changes in the mode of operation of the northern Atlantic Ocean. Further, it appears that these mode changes may have been triggered by diversions of glacial meltwater between the Mississippi River and the St. Lawrence River drainage systems. We report here Accelerator Mass Spectrometry (AMS) radiocarbon results on two strategically located deep-sea cores. One provides a chronology for surface water temperatures in the northern Atlantic and the other for the meltwater discharge from the Mississippi River. Our objective in obtaining these results was to strengthen our ability to correlate the air temperature history for the northern Atlantic basin with the meltwater history for the Laurentian ice sheet.
    Type: Article , PeerReviewed
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  • 7
    Publication Date: 2016-06-15
    Description: Radiocarbon ages for benthic and planktonic foraminifera from the late glacial sections of two Atlantic and two Pacific cores are reported. The differences for benthic-planktonic pairs suggest that the radiocarbon age for deep Atlantic water was somewhat larger than today's (i.e., 600±250, as opposed to 400 years) and that the radiocarbon age for deep Pacific water was also slightly larger than today's (2100±400, as opposed to 1600, years). Our results suggest that during glacial time, the deep Pacific was, as it is today, significantly depleted in radiocarbon relative to the deep Atlantic. As many questions remain unanswered regarding the reliability of this approach, these conclusions must be considered to be preliminary.
    Type: Article , PeerReviewed
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  • 8
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    AGU (American Geophysical Union)
    In:  Paleoceanography, 6 (5). pp. 593-608.
    Publication Date: 2016-06-16
    Description: Radiocarbon ages on CaCO3 from deep-sea cores offer constraints on the nature of the CaCO3 dissolution process. The idea is that the toll taken by dissolution on grains within the core top bioturbation zone should be in proportion to their time of residence in this zone. If so, dissolution would shift the mass distribution in favor of younger grains, thereby reducing the mean radiocarbon age for the grain ensemble. We have searched in vain for evidence supporting the existence of such an age reduction. Instead, we find that for water depths of more than 4 km in the tropical Pacific the radiocarbon age increases with the extent of dissolution. We can find no satisfactory steady state explanation and are forced to conclude that this increase must be the result of chemical erosion. The idea is that during the Holocene the rate of dissolution of CaCO3 has exceeded the rain rate of CaCO3. In this circumstance, bioturbation exhumes CaCO3 from the underlying glacial sediment and mixes it with CaCO3 raining from the sea surface.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2016-06-15
    Description: As a test of the reliability of paleocean ventilation rates reconstructed from radiocarbon age differences between planktonic and benthic foraminifera, measurements have been made on coexisting species of planktonic foraminifera. While ideally no differences should exist, we do find them. In this paper we discuss the possible causes for these differences and attempt to evaluate their impact on the interpretation of benthic-planktonic age differences.
    Type: Article , PeerReviewed
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