Pleistocene decalcification of Late Pliocene Red Crag shelly sands from Walton-on-the-Naze, England

Uppermost sands of the Red Crag at Walton-on-the-Naze (Essex) and elsewhere in East Anglia have been decalcified to iron-stained quartz sands. In contrast, lower sands are only minimally altered and contain aragonitic and calcitic shells. Aragonitic shells are slightly dissolved (chalkified), but calcitic shells are unaffected. Cementation is limited to an addition of iron oxides, now mainly haematite, which also coat carbonate grains. Abundant iron-oxide fines in the upper decalcified sands were liberated from the coatings of shells; shells that have since dissolved. The diagenetic nature of the contact between decalcified upper and unaffected lower sands is evident where it transects cross-bedding. The contact is knife-sharp, even smoothly truncating large shells, and is usually planar and subhorizontal. Shelly sands immediately beneath the boundary contain similar amounts of aragonitic material, as do sands further below. Locally the decalcification boundary has been contorted by cryoturbation, implying that carbonate dissolution was a Pleistocene event. Decalcification probably occurred when the area was affected by permafrost. Lower sands were cemented by ice and protected from dissolution. Upper sands were above the ice table and subject to chemically aggressive waters during summer thaws. Decalcification is believed to have taken place during an episode of climate amelioration when downward retreat of the ice table accompanied replacement of tundra by boreal forest. Highly acidic and podzolic soils developed, beneath which shell-carbonate dissolved. Sharp based decalcified zones in Lowestoft Till and Devensian deposits in other parts of England can also be attributed to dissolution associated with permafrost.     

体视学对于睾丸生物学研究的贡献

男性和女性的生育控制问题是本世纪一个重要的生物学研究领域。女性生育的有效控制方法已存在好多年了，然而，用于男性的有效的方法还没有找到。近年来，人们试图确定荷尔蒙对精子产生的调节作用，期望找到与女性避孕同样有效并可接受的方法。睾丸包括生精管和裂隙腔，生精管中容纳的是滋养幼芽上皮细胞的Sertoli细胞，裂隙腔中容纳的是产生睾丸激素的Leydig细胞。在精子产生过程中，精原细胞通过有丝分裂繁殖，其下一代经过减数分裂产生单倍体圆形精子细胞，接着发生一系列复杂的形态变化成为成熟的长形精子细胞，成熟后的精子就具有受精能力。 维持幼芽上皮细胞的Sertoli细胞在人的一生中不发生大的变化。Sertoli细胞维持幼雅细胞的能力是有限的，但在不同人之间其能力有一个较大的变化范围。因而，Sertoli细胞的数目决定了睾丸产生精子的最大能力。对于精子产生的调节涉及到内分泌、副分泌和自分泌机制。体视学和各种形态测定工具已成为实验条件和自然条件下睾丸生物学研究的一把钥匙，这些研究包括了解Sertoli细胞的繁殖以及调节精原细胞转变为成熟细胞的效率的因素。从而提出精子发育的规律性，由荷尔蒙对精子产生调节作用的模型。     

Falling-stage features of a precambrian braided stream: Criteria for sub-aerial exposure

In a Precambrian braided stream deposit in north Norway a series of small terraces are exposed in a plan view over an area of 1500 sq. metres. These features compare closely with features formed during falling stages on exposed bars in modern rivers. Using the evidence of terracing a detailed history of deposition and erosion during fluctuating discharge has been unravelled. Since these structures provide direct evidence of both sub-aerial exposure and fluctuating water level they are of considerable palaeoenvironmental importance.     

Turbulent magnetic fields in the Sun

Nigel Weiss recounts his Presidential Address 2001, given to the RAS A&G Ordinary Meeting on 9 February 2001.
Recent high-resolution observations, from the ground and from space, have revealed the fine structure of magnetic features at the surface of the Sun. At the same time, advances in computing power have at last made it possible to develop models of turbulent magnetoconvection that can be related to these observations. The key features of flux emergence and annihilation, as observed by the MDI experiment on SOHO, are reproduced in kinematic calculations, while three-dimensional numerical experiments reveal the dynamical processes that are involved. The pattern of convection depends on the strength of the magnetic field: as the mean field decreases, slender rising plumes give way to a regime where magnetic flux is separated from the motion and then to one where locally intense magnetic fields nestle between broad and vigorously convecting plumes. Moreover, turbulent convection is itself able to act as a small-scale dynamo, generating disordered fields near the solar surface.     

Limiting apparent-resistivity values for dipping-bed earth models

Consideration is given to some special features of normalized apparent-resistivity (NAR) curves, resulting from the deployment of Wenner electrode arrays on the surface above dipping earth structures. Limiting values of the potential are derived when a direct-current source is located at points on the surface above a simple two-region dipping-bed earth model and their influence on corresponding NAR curve characteristics is investigated. Particular attention is given to the exploitation of such features to provide a new and direct approach to model parameter estimation, either as an alternative to traditional curve-matching techniques or as a source of supporting information when other earth model characterization methods have been employed. Throughout, the emphasis is on the single-dipping-bed model, but application to more complex structures is discussed, including examples of two dipping beds, dipping dikes and more general tilted unconformities.     

The onset of separation in neutral,turbulent flow over hills

The onset of separation in turbulent, neutrally stratified, boundary-layer flow over hills is considered. Since the flows are fully turbulent, the occurrence of intermittent separation, in the sense of any reversal of near surface flow, will depend strongly on the detailed structure and behaviour of the turbulent eddies. Very little is known about such intermittent separation and the phenomenon cannot be studied with numerical models employing standard turbulence closures; eddy-resolving models are required. Therefore, here, as elsewhere in the literature, the arguably less physically significant process of mean flow separation is studied. Numerical simulations of flow over idealised two- and three-dimensional hills are examined in detail to determine the lowest slope, _crit, for which the mean flow separates.Previous work has identified this critical slope as that required to produce a zero surface stress somewhere over the hill. This criterion, when a mixing-length turbulence closure is applied, reduces to requiring the near-surface vertical velocity shear to vanish at some point on the hill's surface. By applying results from a recent linear analysis for the flow perturbations to this condition, a new expression for _crit is obtained. The expression is approximate but its relative simplicity makes it practically applicable without the need for use of a computer or for detailed mapping of the hill. The approach suggested differs from previous ones in that it applies linear results to a non-linear expression for the surface stress. In the past, a linear expression for the surface stress has been used. The proposed expression for _crit leads to critical angles that are about twice previous predictions. It is shown that the present expression gives good agreement with the numerical results presented here, as well as with other numerical and experimental results. It is also consistent with atmospheric observations.     

First measurements from a deep-tow transient electromagnetic sounding system

An electromagnetic sounding system has been developed to map the shallow electrical conductivity structure of the deep sea floor. The instrument consists of a magnetic source and several colinear magnetic receivers forming an array which is towed along the seafloor. The source generates a time varying magnetic field; the shape of the resulting magnetic field waveform at the receivers depends on the electrical conductivity below the seafloor between the receivers and the source. The instrument can be towed systematically over a study area under acoustic transponder or GPS navigation to construct a map of the electrical conductivity. Towing speeds of greater than 1 m s^–1 (2 knots) can be achieved without adversely effecting data quality. The instrument is sufficiently robust to survive continual contact with thinly sedimented, abrasive basalt. We present the first results from a deployment in August, 1990 near the Cleft Segment of the Juan de Fuca Ridge along an 8 km track to the west of the spreading center. Unforeseen problems with the instrument restricted the utility of the measurements for constructing detailed vertical conductivity profiles, but the measurements were adequate to determine an average conductivity in the upper 25 m, at more than 70 stations. The conductivity was found to vary from 0.1 to 0.4 S/m along the track.     

Little evidence for faunal zonation or communities in deep sea demersal fish faunas

Demersal fish data from 9 deep-sea surveys in the North Atlantic were examined to assess (1) the coherence and continuity of zones around the ocean basin, and (2) the persistence of structured communities over more than just a local scale. The 63 cruises took 96,779 specimens and 325 species in 692 benthic trawl hauls between 204 and 5345m depth. Measures of similarity and overlap applied to this faunal data did not produce patterns other than those reflecting the occurrence of two widespread and abundant species - Synaphobranchus kaupi at slope depths and Nematonurus armatus on the rise. Communities defined within a survey area by commonly-used analytical procedures cannot be identified from place to place over broader areas, and the concept along with its implication should be abandoned.     

The use of ultra-sound in the preparation of carbonate and clay sediments for chironomid analysis

Initial investigations of Holocene carbonate sediment from Hawes Water, Northwest England, yielded lower numbers of chironomid head capsules than anticipated. Standard techniques used to prepare sediment for chironomid analysis were ineffective in breaking up the coarse crystalline sediment structure sufficiently. This led to large amounts of sediment being retained and increased sample processing times. The low yield of head capsules also meant that more sediment was needed to produce adequate numbers of head capsules for analysis. The use of ultra-sound as part of the sediment processing was investigated. This technique reduced the amount of sediment left for sorting and yielded significantly more head capsules which were of equivalent structural condition and cleaner than those produced by conventional methods. The technique was extended to clay samples where similar results were obtained although shorter treatment times are recommended. The proportion of Tanytarsini and Tanypodinae heads increased significantly in carbonate and clay samples, respectively; both sediment types showed a significant decline in the proportion of Chironomini. The results indicate that ultra-sonic preparation of samples will yield a more accurate representation of chironomid assemblages in sediments leading to greater sensitivity and reliability in analysing past environmental conditions.     

An Active Sun Throughout the Maunder Minimum

Measurements of ¹⁰Be concentration in the Dye 3 ice core show that magnetic cycles persisted throughout the Maunder Minimum, although the Sun's overall activity was drastically reduced and sunspots virtually disappeared. Thus the dates of maxima and minima can now be reliably estimated. Similar behaviour is shown by a nonlinear dynamo model, which predicts that, after a grand minimum, the Sun's toroidal field may switch from being antisymmetric to being symmetric about the equator. The presence of cyclic activity during the Maunder Minimum limits estimates of the solar contribution to climatic change.