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991.
992.
Robert H. NICHOLS Frank A. PODOSEK Brad S. MEYER Cristine L. JENNINGS 《Meteoritics & planetary science》1999,34(6):869-884
Abstract— The presence of several short-lived (now extinct) radionuclides in the early solar system demands that they were synthesized and added to preexisting solar system materials shortly (on a time scale on the order of the relevant radionuclide lifetime) before formation of solar system solids. For diverse reasons, it is often suggested that the solar system distributions of these radionuclides were radically heterogeneous, perhaps because of the late addition. Much attention has been given to the astrophysical circumstances that might govern the synthesis and distribution of these short-lived radionuclides, but comparatively little attention has been devoted to the distribution of cosynthesized isotopes. The focus of this paper is a systematic, quantitative evaluation of the collateral consequences in stable and long-lived isotopes that might be expected if short-lived radionuclides, in particular 26Al or 53Mn, were injected at their canonical levels and inhomogeneously distributed in the early solar system. We mix model massive star yields of Meyer et al. (1995) and Woosley and Weaver (1995) into a reservoir of cosmic composition, as tabulated by Anders and Grevesse (1989). To mitigate the effects of systematic deviations that may be present in these mixtures due to uncertainties in model stellar yields, we follow Timmes and Clayton (1996) and also mix into a “renormalized” proxy solar system composition computed from a galactic chemical evolution model based primarily on the stellar yields of Woosley and Weaver (1995). The results are very unfavorable to the likelihood of heterogeneously distributed 26Al derived from supernova ejecta. If a massive star is invoked to account for 26Al, its ejecta must have been rather uniformly distributed, as inferred from the lack of measured collateral anomalies in several elements, notably Ca, Cr, and Ni. Conversely, if 26Al were indeed radically heterogeneously distributed, some other nucleosynthetic source more efficient at producing 26Al is required. In principle, a similar statement applies to 53Mn, but the situation is more complicated. The inferred anomalies at 53Cr will depend not only on how much 53Mn is added by a heterogeneous component, but also more sensitively on the contributions to the associated stable nuclides, 53Cr, 52Cr, and 50Cr. Consideration of predicted collateral anomalies provides no direct support for heterogeneously distributed supernova-derived 53Mn, but the required quantity of supernova contribution, and thus also the collateral anomalies, are much less for 53Mn than for 26Al. With allowance for model calculation uncertainties, it could be argued that anomalies collateral to heterogeneous 53Mn might be small enough to have evaded detection. 相似文献
993.
994.
Ferromanganese crusts from the Atlantic, Indian and Pacific Oceans record the Nd and Pb isotope compositions of the water
masses from which they form as hydrogenous precipitates. The10Be/9Be-calibrated time series for crusts are compared to estimates based on Co-contents, from which the equatorial Pacific crusts
studied are inferred to have recorded ca. 60 Ma of Pacific deep water history. Time series of ɛNd show that the oceans have
maintained a strong provinciality in Nd isotopic composition, determined by terrigenous inputs, over periods of up to 60 Ma.
Superimposed on the distinct basin-specific signatures are variations in Nd and Pb isotope time series which have been particularly
marked over the last 5 Ma.
It is shown that changes in erosional inputs, particularly associated with Himalayan uplift and the northern hemisphere glaciation
have influenced Indian and Atlantic Ocean deep water isotopic compositions respectively. There is no evidence so far for an
imprint of the final closure of the Panama Isthmus on the Pb and Nd isotopic composition in either Atlantic or Pacific deep
water masses. 相似文献
995.
Variations in sediment input and distribution to the Laptev Sea continental margin during the Holocene and Termination I could be identified based on radiocarbon dated magnetic susceptibility logs and sediment thickness in high-resolution seismic profiles. Magnetic susceptibility of surface samples reveals an increased input of magnetic grains to the Laptev Sea deriving from the Anabar and Khatanga river catchments. Exposed magnetite schists and volcanic rocks of the Anabar shield and Putoran Plateau, respectively, function as major source of magnetic material. The distribution of magnetic susceptibility in association with the thickness of the Holocene sediments indicates bottom-current induced sediment transport guided by major submarine valleys on the Laptev Sea shelf. The sites of filled paleoriver channels identified in the seismic profiles suggest that during the Late Weichselian sea-level lowstand river runoff continued through four of the major valleys on the exposed Laptev Sea shelf. The sediments at the top of the lowstand deposits in front of the Anabar-Khatanga valley, represented in the seismic profiles by prograding deltas, are characterized by outstandingly high magnetic susceptibility values. Radiocarbon datings approximate the deposition of these high magnetic sediments between 10 and 13.4 ka. It is suggested that this increased input of magnetic material is related to the deglaciation of the Anabar shield and the Putoran Plateau and thus support their glaciation during marine isotope stage (MIS) 2. 相似文献
996.
Long-Term Temperature Trends and Tree Growth in the Taymir Region of Northern Siberia 总被引:2,自引:0,他引:2
Gordon C. Jacoby Nikolai V. Lovelius Oleg I. Shumilov Oleg M. Raspopov Juri M. Karbainov David C. Frank 《Quaternary Research》2000,53(3):312
The northernmost conifers in the world are located well above the Arctic Circle in the Taymir region of northern Siberia and have been recording the thermal environment for centuries to millennia. The trees respond to temperatures beyond the narrow season of actual cambial cell division by means of root growth, photosynthesis, lignification of cell walls, and other biochemical processes. Data from annual tree-ring widths are used to reconstruct May–September mean temperatures for the past four centuries. These warm-season temperatures correlate with annual temperatures and indicate unusual warming in the 20th century. However, there is a loss of thermal response in ring widths since about 1970. Previously the warmer temperatures induced wider rings. Most major warming and cooling trends are in agreement with other high-latitude temperature reconstructions based on tree-ring analyses with some regional differences in timing of cooling in the late 18th century and of warming in the late 19th century. 相似文献
997.
998.
Clayton J. Williams Rudolf Jaffé William T. Anderson Frank J. Jochem 《Estuarine, Coastal and Shelf Science》2009
A stable carbon isotope approach was taken to identify potential organic matter sources incorporated into biomass by the heterotrophic bacterial community of Florida Bay, a subtropical estuary with a recent history of seagrass loss and phytoplankton blooms. To gain a more complete understanding of bacterial carbon cycling in seagrass estuaries, this study focused on the importance of seagrass-derived organic matter to pelagic, seagrass epiphytic, and sediment surface bacteria. Particulate organic matter (POM), seagrass epiphytic, seagrass (Thalassia testudinum) leaf, and sediment surface samples were collected from four Florida Bay locations with historically different organic matter inputs, macrophyte densities, and primary productivities. Bulk (observed and those reported previously) and compound-specific bacterial fatty acid δ13C values were used to determine important carbon sources to the estuary and benthic and pelagic heterotrophic bacteria. The δ13C values of T. testudinum green leaves with epiphytes removed ranged from −9.9 to −6.9‰. Thalassia testudinum δ13C values were significant more enriched in 13C than POM, epiphytic, and sediment samples, which ranged from −16.4 to −13.5, −16.2 to −9.6, and −16.7 to −11.0‰, respectively. Bacterial fatty acid δ13C values (measured for br14:0, 15:0, i15:0, a15:0, br17:0, and 17:0) ranged from −25.5 to −8.2‰. Assuming a −3‰ carbon source fractionation from fatty acid to whole bacteria, pelagic, epiphytic, and sediment bacterial δ13C values were generally more depleted in 13C than T. testudinum δ13C values, more enriched in 13C than reported δ13C values for mangroves, and similar to reported δ13C values for algae. IsoSource mixing model results indicated that organic matter derived from T. testudinum was incorporated by both benthic and pelagic bacterial communities, where 13–67% of bacterial δ13C values could arise from consumption of seagrass-derived organic matter. The IsoSource model, however, failed to discriminate clearly the fraction of algal (0–86%) and mangrove (0–42%) organic matter incorporated by bacterial communities. These results indicate that pelagic, epiphytic, and sediment surface bacteria consumed organic matter from a variety of sources. Bacterial communities incorporated consistently seagrass-derived organic matter, the dominant macrophyte in Florida Bay, but seagrass δ13C values alone could not account fully for bacterial δ13C values. 相似文献
999.
Alistair McIlgorm Susan Hanna Gunnar Knapp Pascal Le Floc’H Frank Millerd Minling Pan 《Marine Policy》2010
We examine the implications of climate change for fishery governance using seven international fishery case studies in low, mid and high latitudes, including eastern Australia, the western Pacific Ocean, Alaska, west coast United States, Hawaii, west coast Canada and France. Climate change adds uncertainty about fish stock productivity, migratory patterns, trophic interactions and vulnerability of fish populations to fishing pressure. 相似文献
1000.