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141.
The establishment of a chronology of landscape-forming events in lowland and mid-altitude Tasmania, essential for assessing the relative importance of climatic and human influences on erosion, and for assessing present erosion risk, has been limited by the small number of ages obtained and limitations of dating methods. In this paper we critically assess previous Tasmanian studies, list published radiocarbon ages considered to be dependable, present new radiocarbon and thermoluminescence (TL) ages for 25 sites around Tasmania, and consider the evidence for the hypotheses that erosion processes at low and mid altitudes have been: (1) purely climatically controlled; and (2) influenced both by climatic and anthropogenic (increased fire frequency) effects. A total of 94 dependable finite ages (calibrated for radiocarbon and ‘as measured’ for TL and optically stimulated luminescence (OSL) determinations) are listed for deposits comprising dunes, colluvium, alluvium and loess-like aeolian deposits. Two fall in the >100 ka period, 15 fall in the period 65–35 ka, and 77 fall in the period 35–0.3 ka. There was a sustained increase in erosion recorded in the period 35–15 ka, as reflected by a greater number of dated aeolian deposits during this period.We considered three possible biases that may have affected the age distribution obtained: the limitations of radiocarbon dating, sampling bias, and preservation bias. Sampling bias may have favoured more recent dune strata, but radiocarbon dating and preservation biases are unlikely to have significantly distorted the age distribution obtained.Long but intermittent aeolian deposition is recorded at two sites (Southwood B; c. 59–28 ka and Dunlin Dune; c. 29–14 ka) but there is no evidence of regional loess deposits such as found in New Zealand. The timing of increased erosion in Tasmania between 35 and 30 ka approximately coincides with the intermittent ten-fold increase of dust accumulation between 33 and 30 ka in the Antarctic Dome C ice core. The absence of widespread erosion before 35 ka, the abrupt increase of erosion around this time, the frequent association of erosion products with charcoal, the arrival of people in Tasmania at c. 40 cal ka, and the known use of fires by Aborigines to maintain areas of non-climax vegetation suggest that ecosystem disturbance by anthropogenic fires, in a drier climate than that presently prevailing, may have contributed to erosion in lowland and mid-altitude Tasmania after 35 ka. Thus the Tasmanian erosion record provides circumstantial support for the proposition that human dispersal in southeast Australia was accompanied by significant ecological change. 相似文献
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The angular distribution of solar flare associated hard X-rays ( 10 keV) is calculated on the assumption that they originate as bremsstrahlung emission of energetic electrons with a power law spectrum. For the cross section the relativistic Sauter formula was used. Supposing the electrons to move in a fixed direction, the X-radiation is considerably anisotropic, especially at high photon energies. Taking into account a magnetic field, the anisotropy decreases with increasing pitch angles of the electrons. The anisotropic angular distribution of solar X-radiation seems to be connected with the centre-to-limb variation of hard X-ray bursts and with the correlation of shortwave fadeouts and geomagnetic crochets to H flares. 相似文献
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The progressive intensification of northern hemisphere glaciation as seen from the North Pacific 总被引:4,自引:0,他引:4
M. A. Maslin G. H. Haug M. Sarnthein R. Tiedemann 《International Journal of Earth Sciences》1996,85(3):452-465
Ocean Drilling Project (ODP) site 882 (50°22′N, 167°36′E) provides the first high-resolution GRAPE density, magnetic susceptibility,
carbonate, opal and foraminifera (planktonic and benthic) stable isotopes records between 3.2 and 2.4 Ma in the Northwest
Pacific. We observed a dramatic increase in ice rafting debris at site 882 at 2.75 Ma, which is coeval with that found in
the Norwegian Sea, suggesting that the Eurasian Arctic and Northeast Asia were significantly glaciated from 2.75 Ma onwards.
Prior to 2.75 Ma planktonic foraminifera δ18O records indicate a warming or freshening trend of 4°C or 2‰ over 80 ka. If this is interpreted as a warm pre-glacial Pliocene
North Pacific, it may have provided the additional moisture required to initially build up the northern hemisphere continental
ice sheet. The dramatic drop in sea surface temperatures (SST>7.5°C) at 2.75 Ma ended this suggested period of enhanced SST
and thus the proposed moisture pump. Moreover, at 2.79 and 2.73 Ma opal mass accumulation rates (MAR) decrease in two steps
by five fold and is accompanied by a more gradual long-term decrease in CaCO3 MARs. Evidence from the Southern Ocean (ODP site 704) indicates that just prior to 2.6 Ma there is a massive increase in
opal MARs, the opposite to what is found in the North Pacific. This indicates that the intensification of northern hemisphere
glaciation was accompanied by a major reorganisation of global oceanic chemical budget, possibly caused by changes in deep
ocean circulation. The initiation of northern hemisphere glaciation occurred in the late Miocene with a significant build
up of ice on southern Greenland. However, the progressive intensification did not occur until 3.5–3 Ma when the Greenland
ice sheet expanded to include northern Greenland. Following this stage we suggest that the Eurasian Arctic and Northeast Asia
glaciated at 2.75 Ma, approximately 100 ka before the glaciation of Alaska (2.65 Ma) and 200 ka before the glaciation of the
North East American continent (2.54 Ma). 相似文献
147.
Gerhard Kramm Hans Müller David Fowler Klaus D. Höfken Franz X. Meixner Eberhard Schaller 《Journal of Atmospheric Chemistry》1991,13(3):265-288
A modified profile method for determining the vertical deposition (or/and exhalation) fluxes of NO, NO2, ozone, and HNO3 in the atmospheric surface layer is presented. This method is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these trace gases. The analysis (aerodynamic profile method) includes a detailed determination of the micrometeorological quantities (such as the friction velocity, the fluxes of sensible and latent heat, the roughness length and the zero plane displacement), and of the height-invariant fluxes of the composed chemically conservative trace gases with group concentrations c
1=[NO]+[NO2]+[HNO3], c
2=[NO2]+[O3]+3/2·[HNO3], and c
3=[NO]–[O3]–1/2·[HNO3]. The fluxes of the individual species are finally determined by the numerical solution of a system of coupled nonlinear ordinary differential equations for the concentrations of ozone and HNO3 (decoding method). The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The model requires only the vertical profile data of wind velocity, temperature and humidity and concentrations of NO, NO2, ozone, and HNO3.The method has been applied to vertical profile data obtained at Jülich (September 1984) and collected in the BIATEX joint field experiment LOVENOX (Halvergate, U.K., September 1989). 相似文献
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