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746.
Sabine Hanisch Martin Wessels Frank Niessen Antje Schwalb 《Journal of Paleolimnology》2009,41(3):393-406
This study used organic matter in oligotrophic Lake Constance (southern Germany) to reconstruct lake environment and to disentangle
the multiple factors, such as climate change and human impacts, which influence sedimentation in large lakes. A sediment core
from Upper Lake Constance, which represents 16,000 years of Late Glacial and Holocene lake history, was analysed for organic
biomarkers, hydrogen index and elements calcium, strontium, and magnesium. Magnetic susceptibility was measured to establish
a high-resolution stratigraphic framework for the core and to obtain further information about changes with respect to relative
allochthonous versus autochthonous sedimentation. Dinosterol—a biomarker for dinoflagellates—and calcium have low concentrations
in Younger Dryas sediments and consistently high concentrations between 10,500 and 7,000 cal. years BP. These variations are
attributed to changes in lake productivity, but are not reflected in the proportion of total organic carbon within the sediment.
During the Younger Dryas and between 6,000 and 2,800 cal. years BP, concentrations and accumulation rates of land-plant-derived
C29-steroids (β-sitosterol, stigmastanol and stigmasterol), in combination with a relatively low HI, indicate periods of enhanced
terrigenous input to the lake. For the Younger Dryas, higher runoff can be attributed to a cold climate, leading to decreased
vegetation cover and increased erosion. After 6,000 cal. years BP, high terrestrial input may be explained by enhanced precipitation.
Biomarker and HI results, in combination with archaeological studies, raise the question as to whether lakeshore settlements
affected sedimentation in Upper Lake Constance between 6,000 and 2,800 cal. years BP. 相似文献
747.
Martin Reinhardt Nielsen 《Polar research》2009,28(2):238-245
This paper evaluates recent changes in narwhal ( Monodon monoceros ) catches in Siorapaluk, the northernmost community in Greenland, in consideration of the effects of changing climate and uncertainty of stock delineation. The catch statistics show a significant increase in narwhal catches by hunters in Siorapaluk after 2002, which does not appear to be a result of increased effort. Hunters attribute the increase to changed sea-ice conditions providing access by boat to Smith Sound as early as June and July. This indicates that climate change is likely to have a considerable impact on narwhal hunting in northern Greenland. Traditional ecological knowledge and scientific surveys suggest that narwhal in Smith Sound constitute an independent stock. The absence of scientific recommendations for this stock has been seen as an opportunity to increase quotas in West Greenland. Scientific management recommendations are urgently needed to allow the authorities to assign sustainable quotas for this stock. The development of collaborative management agreements and locally based monitoring are recommended to ensure local acceptance of regulations, and to allow rapid responses to climate change. 相似文献
748.
The Liard Lobe formed a part of the north‐eastern sector of the Cordilleran Ice Sheet and drained ice from accumulation areas in the Selwyn, Pelly, Cassiar and Skeena mountains. This study reconstructs the ice retreat pattern of the Liard Lobe during the last deglaciation from the glacial landform record that comprises glacial lineations and landforms of the meltwater system such as eskers, meltwater channels, perched deltas and outwash fans. The spatial distribution of these landforms defines the successive configurations of the ice sheet during the deglaciation. The Liard Lobe retreated to the west and south‐west across the Hyland Highland from its local Last Glacial Maximum position in the south‐eastern Mackenzie Mountains where it coalesced with the Laurentide Ice Sheet. Retreat across the Liard Lowland is evidenced by large esker complexes that stretch across the Liard Lowland cutting across the contemporary drainage network. Ice margin positions from the late stage of deglaciation are reconstructed locally at the foot of the Cassiar Mountains and further up‐valley in an eastern‐facing valley of the Cassiar Mountains. The presented landform record indicates that the deglaciation of the Liard Lobe was accomplished mainly by active ice retreat and that ice stagnation played a minor role in the deglaciation of this region. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
749.
The period and Q of the Chandler wobble 总被引:3,自引:0,他引:3
Summary. We have extended our calculation of the theoretical period of the Chandler wobble to account for the non-hydrostatic portion of the Earth's equatorial bulge and the effect of the fluid core upon the lengthening of the period due to the pole tide. We find the theoretical period of a realistic perfectly elastic Earth with an equilibrium pole tide to be 426.7 sidereal days, which is 8.5 day shorter than the observed period of 435.2 day. Using Rayleigh's principle for a rotating Earth, we exploit this discrepancy together with the observed Chandler Q to place constraints on the frequency dependence of mantle anelasticity. If Qμ in the mantle varies with frequency σ as σα between 30 s and 14 months and if Qμ in the lower mantle is of order 225 at 30 s, we find that 0.04 ρα ≤ 0.11; if instead Qμ in the lower mantle is of order 350 near 200 s, we find that 0.11 ≤α≤ 0.19. In all cases these limits arise from exceeding the 68 per cent confidence limits of ± 2.6 day in the observed period. Since slight departures from an equilibrium pole tide affect the Q much more strongly than the period we believe these limits to be robust. 相似文献
750.
Kevin L. Shelton Justin M. Beasley Jay M. Gregg Martin S. Appold Stephen F. Crowley James P. Hendry Ian D. Somerville 《Mineralium Deposita》2011,46(8):859-880
A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones
overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined
historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates
the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic,
cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises
four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage
I, high-temperature (T
h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This
vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite,
quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of
~15–20 wt.% equiv. NaCl. The δ34S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines
in the Lower Palaeozoic metamorphic rocks. The δ18O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted
brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in
the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary
brines. 相似文献