Browsing by Author "Zheng, H"

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    Neolithic agriculture, freshwater resources and rapid environmental changes on the lower Yangtze, China
    (Elsevier, 2011-01) Qin, J; Taylor, D; Atahan, P; Zhang, X; Wu, G; Dodson, JR; Zheng, H; Itzstein-Davey, F
    Analyses of sedimentary evidence in the form of spores, pollen, freshwater algae, dinoflagellate cysts, phytoliths and charcoal from AMS 14C-dated, Holocene-aged sequences provide an excellent opportunity to examine the responses of Neolithic agriculturalists in the lower Yangtze to changing environments. Evidence from two sites close to the southern margin of the Yangtze delta and separated by what is now Hangzhou Bay attests the critical importance to early attempts at food production of access to freshwater resources. More readily, if episodically, available freshwater resources during the early to mid-Holocene on the Hangjiahu plain may have encouraged an early reliance on rice-based agriculture, which in turn facilitated the accumulation of agricultural surpluses and cultural diversification. Cultural change was relatively attenuated and human population pressures possibly lower on the Ningshao plain, seemingly because of much more profound environmental impacts of variations in local hydrological conditions, and because predominantly saline conditions, associated with rising relative sea level, hampered the early development of irrigated agriculture. The evidence, although largely dating to the early and middle parts of the Holocene, provides a timely warning of the complexity of vulnerability to climate change-induced processes of agriculture, and indeed human activities more generally, on megadeltas in Asia. Analyses of sedimentary evidence in the form of spores, pollen, freshwater algae, dinoflagellate cysts, phytoliths and charcoal from AMS 14C-dated, Holocene-aged sequences provide an excellent opportunity to examine the responses of Neolithic agriculturalists in the lower Yangtze to changing environments. Evidence from two sites close to the southern margin of the Yangtze delta and separated by what is now Hangzhou Bay attests the critical importance to early attempts at food production of access to freshwater resources. More readily, if episodically, available freshwater resources during the early to mid-Holocene on the Hangjiahu plain may have encouraged an early reliance on rice-based agriculture, which in turn facilitated the accumulation of agricultural surpluses and cultural diversification. Cultural change was relatively attenuated and human population pressures possibly lower on the Ningshao plain, seemingly because of much more profound environmental impacts of variations in local hydrological conditions, and because predominantly saline conditions, associated with rising relative sea level, hampered the early development of irrigated agriculture. The evidence, although largely dating to the early and middle parts of the Holocene, provides a timely warning of the complexity of vulnerability to climate change-induced processes of agriculture, and indeed human activities more generally, on megadeltas in Asia. © 2011, Elsevier Ltd.
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    Tetragonal YBaFe4O7.0: A stoichiometric polymorph of the "114" ferrite family
    (Elsevier, 2012-07-01) Duffort, V; Caignaert, V; Pralong, V; Barrier, N; Raveau, B; Avdeev, M; Zheng, H; Mitchell, JF
    The exploration of the phase diagram of the ferrite YBaFe(4)O(7+delta) versus the oxygen content delta and temperature shows the complex crystal chemistry of this system. Besides the cubic form (F (4) over bar 3m), which is observed up to 600 degrees C and for 0 < delta <= 0.65, a stoichiometric tetragonal form (delta= 0) is isolated below 300 degrees C that is stable only in the absence of oxidizing atmosphere. The resolution of the structure of this new YBaFe(4)O(7.0) form, from combined neutron and synchrotron data, in the space group I<(4)over bar>, shows significant displacements of the atoms with respect to the cubic form, especially concerning the oxygen atoms surrounding the barium cations. The decrease of several Ba-O distances around the under-bonded barium cations is explained by the existence of hybridized Ba((2-delta)+) -O(2-) -Fe((2+delta)+) bonds, in agreement with Mossbauer spectroscopy. The role of coulombic repulsions in the [Fe(4)O] and [Fe(4)] tetrahedra of the [Fe(4)](infinity) sublattice on the structural transition is also discussed. © 2012, Elsevier Ltd.