The geological evolution of southern McMurdo Sound — new evidence from a high-resolution aeromagnetic survey

Magnetic anomaly data are presented from a new helicopter-borne high-resolution aeromagnetic survey in southern McMurdo Sound. Anomaly data have been acquired at a common 305 m elevation above the McMurdo and southern McMurdo ice shelves and draped over the volcanic islands that pin them. The resulting anomaly patterns provide a significant advance in the understanding of the rift related geology beneath the floating ice shelves. More extensive Erebus Volcanic Province (McMurdo Volcanic Group) rocks are indicated along with a significant blanket of glaci-volcaniclastic sediment on the seafloor between the volcanic islands in southern McMurdo Sound. These glaci-volcaniclastic sediments are inferred to originate from former grounding of the southern McMurdo Ice Shelf as a marine ice sheet. A strong N–S fabric is also observed in the anomaly data suggesting that the rift structure observed in the Victoria Land basin persists to the south beneath the McMurdo and southern McMurdo ice shelves. W–N–W transfer faults identified within the Transantarctic Mountain rift flank to the west are not obvious in the aeromagnetic data set, implying that the 'Discovery Accommodation Zone' may be restricted to the region between a southward extension of the range bounding fault that marks the limit of the Victoria Land Basin and the right lateral offset in the Transantarctic Mountain front in southern Victoria Land.     

Does a one point sample adequately characterize the lake environment for paleoenvironmental calibration studies?

A major goal of paleolimnological studies is the quantification of past environmental conditions. This is accomplished by computing transfer functions relating organism assemblages to environmental factors. The environmental data are typically comprised of a point sample of water chemistry and other environmental parameters that are collected at the same time as a surface sediment sample. We explore whether the year of sampling of the environmental variables affects the parameterization of organism-environment relations, in particular chironomids, ostracodes and diatoms. Canonical correspondence analyses revealed that the year of sampling is of secondary importance when relating the organism assemblages to environmental variables, but only with the major explanatory variables. A chironomid-inferred bottom water temperature partial least squares transfer function revealed similar performance statistics between the years. Taxon optima and tolerances were computed for both years using weighted averaging, and the results are comparable. A paired t-test computed on the proxy-inferred bottom water temperature values indicated that the results between the 2 years are not statistically different. The results of this study provide guarded optimism that the methodology of estimating transfer functions as currently applied is not entirely determined by the particular year when the data were collected, although more case studies are needed.     

Progress in isotope paleohydrology using lake sediment cellulose

Recent advances in sample preparation techniques and mass spectrometry have fostered more routine oxygen isotope analysis of aquatic cellulose in lake sediment cores, a proxy for lake water oxygen isotope history. These methodological developments have significantly increased the feasibility of incorporating this approach into high-resolution, multi-site, and multi-proxy studies, which are frequently necessary to answer complex hydrological, hydroecological and hydroclimatic questions requiring a paleoenvironmental perspective. Direct translation of lake sediment aquatic cellulose oxygen isotope composition into lake water oxygen isotope composition offers appreciable opportunity for quantitative paleohydrological reconstructions, as evidenced by studies conducted over the past 15 years that span Holocene and pre-historical timescales.     

Evaluation of ostracod-based palaeoenvironmental reconstruction with instrumental data from the arid Faiyum Depression,Egypt

The utility of ostracod-based palaeoenvironmental reconstruction was evaluated using instrumental data for Lake Qarun, Egypt. The euryhaline ostracod Cyprideis torosa was the only species found in the lake’s recent sediment record. This species is known to tolerate salinity levels and water solute compositions that may prevent colonisation by other species. Oxygen and carbon isotope ratios of ostracod carbonate from lake sediments covary with changes in instrumental values for lake level and salinity for the period 1890–1974. δ¹³C-values correlate negatively with lake water salinity (r ² = 0.87) and δ¹⁸O-values correlate negatively with measured lake level changes (r ² = 0.41). Other ostracod proxy data provide qualitative information on lake level trends. Fossil assemblage data (juvenile/adult and valve/carapace ratios and valve preservation) provide information on wave energy. Ecophenotypic variation of C. torosa valves provided some useful palaeolimnological information. Sr/Ca and Mg/Ca ratios in ostracods were not found to reflect water composition, due to the uncoupling of these ratios with salinity in Lake Qarun. Overall, our results highlight the need to calibrate ostracod proxy data in modern systems prior to their use for palaeoenvironmental reconstruction.     

Glaciolacustrine varved sediment as an alpine hydroclimatic proxy

The physical significance of a negative correlation between a varve record from Mud Lake, British Columbia, and temperature is discussed in the context of a process-network. The process-network is defined as the system of temporally and spatially connected processes involved in the transfer of a signal from climate to varved glaciolacustrine sediment. The six systems defining the network include climate, glacier, fluvial, geomorphic, terrestrial biologic and lacustrine systems to which each process belongs. A literature review outlines significant variation in the strength and character of correlations between components of the process-network and highlights that more comprehensive interpretations of varves as a hydroclimatic proxy require an improved understanding of the process-network. Documenting each process in the network is integral to informing a more complete model of this system, identifying processes that constitute signal transfer and assessing hydroclimatic proxies based on linear correlation. Such documentation is of growing importance as varved lacustrine sediments are increasingly used as a hydroclimatic proxy. The complex nature of the process-network requires greater emphasis on interdisciplinary cooperation and alternative methods to the linear statistical model.     

关于"冰臼"问题与韩同林先生商榷

跨世纪的"冰臼"与"壶穴"之争,尘埃未定,缘于尚无定论."发现冰臼"却常见于报道.本文根据国内研究冰川的成果就韩同林先生<发现冰臼>书中"冰臼"的定义、表现、成因、分布、"冰臼"遗迹的保存机理、"冰说"理论以及模拟"冰臼"的实验设计的合理性等问题提出质疑.从而阐明了:天坑、天井、冰洞、冰湖不是"冰臼";闽、粤无"冰臼";"冰说"理论不成立的观点.     

Geometry and kinematics of dunes during steady and unsteady flows in the Calamus River, Nebraska, USA

The geometry and kinematics of river dunes were studied in a reach of the Calamus River, Nebraska. During day-long surveys, dune height, length, steepness, migration rate, creation and destruction were measured concurrently with bedload transport rate, flow depth, flow velocity and bed shear stress. Within a survey, individual dune heights, lengths and migration rates were highly variable, associated with their three-dimensional geometry and changes in their shape through time. Notwithstanding this variability, there were discernible changes in mean dune height, length and migration rate in response to changing discharge over several days. Changes in mean dune height and length lagged only slightly behind changes in discharge. Therefore, during periods of both steady and unsteady flow, mean dune lengths were quite close to equilibrium values predicted by theoretical models. Mean dune steepnesses were also similar to predicted equilibrium values, except during high, falling flows when the steepness was above that predicted. Variations in mean dune height and length with discharge are similar to those predicted by theory under conditions of low mean dune excursion and discharge variation with a short high water period and long low water period. However, the calculated rates of change of height of individual dunes vary considerably from those measured. Rates of dune creation and destruction were unrelated to discharge variations, contrary to previous results. Instead, creations and destructions were apparently the result of local variations in bed shear stress and sediment transport rate. Observed changes in dune height during unsteady flows agree with theory fairly well at low bed shear stresses, but not at higher bed shear stresses when suspended sediment transport is significant.     

Attenuation of electrons flux penetrating through the lower ionosphere

The angular variation of elastic and inelastic scattering cross-sections has been calculated and used to study the energy deposition by precipitating electrons with the help of Monte Carlo Method. Monoenergetic, power law electron spectrum with isotropic and monodinational incidence starting at an altitude of 300 km have been used to obtain the angular and energy distributions at certain height intervals. In these calculations constant magnetic field has been used.