Springs of Great Britain

Predictably, in a country such as Britain, with its preponderance of consolidated, sedimentary, mainly fissure-flow aquifers, there is a very large number of springs, many of which are, or have been, used for public supply. Migratory springs are a feature of the British (Ur. Cretaceous) Chalk, the most important British aquifer. The Chalk's low specific yield and high capillary moisture retention together give rise to very considerable fluctuations (more than 33 m in some areas) of the unconfined water table. Along the gentle dip slopes of the Chalk (North and South Downs of southern and southeastern England) springs may migrate laterally for several miles, giving rise to seasonal streams locally known as bournes or lavants. However, springs such as at Duncton, West Sussex, at the base of the much steeper scarp slopes of the Chalk, form point sources, the flows from which tend to be relatively steady; such springs commonly supply and are the original reason for the existence of many of the small towns and villages which nestle along the bases of the chalk scarps of Sussex and Kent.Where the Chalk forms coastal cliffs, a number of springs break out at the base of the cliff between high and low tide levels; there are major chalk coastal springs, for instance, at St. Margaret's Bay (Kent) and at Arish Mells, east of Lulworth Cove, Dorset. Such springs are not used for direct supply (their salinity is usually too high) but are indicators of the presence of local reserves of groundwater for possible future development.     

Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia

Carbonate-rich sedimentary rocks of the western Anabar region, northern Siberia, preserve an exceptional record of evolutionary and biogeochemical events near the Proterozoic/Cambrian boundary. Sedimentologically, the boundary succession can be divided into three sequences representing successive episodes of late transgressive to early highstand deposition; four parasequences are recognized in the sequence corresponding lithostratigraphically to the Manykal Formation. Small shelly fossils are abundant and include many taxa that also occur in standard sections of southeastern Siberia. Despite this coincidence of faunal elements, biostratigraphic correlations between the two regions have been controversial because numerous species that first appear at or immediately above the basal Tommotian boundary in southeastern sections have first appearances scattered through more than thirty metres of section in the western Anabar. Carbon- and Sr-isotopic data on petrographically and geochemically screened samples collected at one- to two-metre intervals in a section along the Kotuikan River, favour correlation of the Staraya Reckha Formation and most of the overlying Manykai Formation with sub-Tommotian carbonates in southeastern Siberia. In contrast, isotopic data suggest that the uppermost Manykai Formation and the basal 26 m of the unconformably overlying Medvezhya Formation may have no equivalent in the southeast; they appear to provide a sedimentary and palaeontological record of an evolutionarily significant time interval represented in southeastern Siberia only by the sub-Tommotian unconformity. Correlations with radiometrically dated horizons in the Olenek and Kharaulakh regions of northern Siberia suggest that this interval lasted approximately three to six million years, during which essentially all 'basal Tommotian' small shelly fossils evolved.     

Derivation of A-type Granites from a Dehydrated Charnockitic Lower Crust: Evidence from the Chaelundi Complex, Eastern Australia

Triassic I- and A-type granites of the Chaelundi Complex, NewEngland Fold Belt, eastern Australia, were generated in a subduction-relatedtectonic setting. Although isotopic ages of the suites are indistinguishable,field relations indicate that the A-type is younger. The mostmafic granitoids from each suite have similar silica contents(66–68% SiO₂), slightly LREE enriched patterns withoutEu anomalies, low Rb/Sr and K/Ba ratios, and high K/Rb ratios,suggesting that both represent parental magmas. The A-type isdistinguished mineralogically by abundant orthoclase and sodicplagioclase (total >60%), ferro-hornblende, annite and allanite.In contrast, the I-type has more hornblende and biotite, whichare more magnesian in composition, and less feldspar. The parentalmagmas of both suites have many similar geochemical characteristics,although the A-type has slightly higher alkalis, Zr, Hf, Znand LREE, and lower CaO, MgO, Sr, V, Cr, Ni and Fe³⁺/Fe. Thegeochemical properties characteristic of leucocratic A-typegranites, such as high Ga/Al, Nb, , HREE and F contents, areonly manifest in the more felsic members of the A-type suite.These features were produced by 70% fractional crystallizationof feldspar, hornblende, quartz and biotite. Both granite suites were generated by water-undersaturated partialmelting of a similar source, but the A-type parent magma resultedfrom lower aH₂O conditions during partial melting. Generationand rapid ascent of the earlier 1-type magma during disequilibriumpartial melting produced a relatively anhydrous, but not refractory,charnockitic lower crust. Continued thermal input from mantle-derivedmagmas, during continuing subduction, partially melted the ‘charnockitized’lower crust at temperatures in excess of 900C, to produce A-typemagmas. Charnockitic magmas (C-type) form in a similar way toA-type magmas, although their different composition reflectsvariations in the anhydrous lower-crustal mineral assemblagesthat remain after the previous (1-type) granite-forming event. The New England Fold Belt was a subduction—accretion complexuntil the late Carboniferous, when the deeper parts underwentpartial melting to produce S-type granites. As the I-and A-typegranites intruded penecontemporaneously, a tonalitic sourcemodel for genesis of the Chaelundi A-type is untenable. KEY WORDS: A-type; charnockitization; eastern Australia ^*Corresponding author.     

Geochemical framework for water quality studies in sub-Saharan Africa

The scope for using hydrogeochemical techniques in water quality studies in Africa is reviewed as a background to a set of thematic papers. Water quality problems are emerging as a key issue in Africa either:

• 1. 

  i) in view of the pressures of man-made pollution on finite resources; or

• 2. 

  ii) the existence of regions with naturally induced geological problems, for example fluoride endemic areas.

Such natural problems are the focus of this paper and the following topics were emphasised during a workshop in Sodere, Ethiopia: the need to determine natural baselines as a means of recognizing anthropogenic pollution; the need for high quality samples and field data, but relatively basic analytical data; the use of chloride to assist in recharge estimation and water-balance studies; an understanding of depth stratification of water quality as part of the design of well/borehole drilling programmes; the use of hydrogeochemistry in geothermal studies; the recognition of health and/or acceptability problems, especially for F, Fe, Mn, As, I and Al. Water quality standards for African countries need to be considered in the context of local geochemical environments and some of the WHO limits, especially for major ions, may be unattainable owing to naturally high total mineralization.     

Possible consequences of absence of “jupiters” in planetary systems

The formation of the gas giant planets Jupiter and Saturn probably required the growth of massive 15 Earth-mass cores on a time scale shorter than the 10⁷ time scale for removal of nebular gas. Relatively minor variations in nebular parameters could preclude the growth of full-size gas giants even in systems in which the terrestrial planet region is similar to our own. Systems containing failed Jupiters, resembling Uranus and Neptune in their failure to capture much nebular gas, would be expected to contain more densely populated cometary source regions. They will also eject a smaller number of comets into interstellar space. If systems of this kind were the norm, observation of hyperbolic comets would be unexpected. Monte Carlo calculations of the orbital evolution of region of such systems (the Kuiper belt) indicate that throughout Earth history the cometary impact flux in their terrestrial planet regions would be 1000 times greater than in our Solar System. It may be speculated that this could frustrate the evolution of organisms that observe and seek to understand their planetary system. For this reason our observation of these planets in our Solar System may tell us nothing about the probability of similar gas giants occurring in other planetary systems. This situation can be corrected by observation of an unbiased sample of planetary systems.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

The ‘ultimate objective’ of the framework convention on climate change requires a new approach in climate change research

In the Framework Convention on Climate Change an ultimate objective is formulated that calls for stabilization of the concentrations of greenhouse gases in the atmosphere at a level that would allow ecosystems to adapt naturally, safeguard food supply and enable sustainable development to proceed in a sustainable manner. This paper addresses the possible contribution of science to translate this rather vague and ambiguous objective into more practicable terms. We propose a regionalized, risk-based six-step approach that couples an analysis of ecosystem vulnerability to the results of simulations of climate change. An ultimate objective level could be determined in terms of stabilized concentrations of greenhouse gases in the atmosphere. The level and timing of this stabilization would be determined by a political appreciation of associated risks for managed and unmanaged ecosystems. These risks would be assessed by region in an internationally coordinated scientific effort, followed by a global synthesis.     

Nitrogen sulfide in quiescent dark clouds

We report the first detection of interstellar nitrogen sulfide (NS) in cold dark clouds. Several components of the 2 pi 1/2, J = 3/2 --> 1/2 and J = 5/2 --> 3/2 transitions were observed in TMC-1 and L134N. The inferred column density for TMC-1 is NNS approximately 8 x 10(12)cm-2 toward the NH3 peak in that cloud, and in L134N is NNS approximately 3 x 10(12)cm-2 toward the position of peak NH3 emission. These values correspond to fractional abundances relative to molecular hydrogen of fNS approximately 8 x 10(-10) for TMC-1, and fNS approximately 6 x 10(-10) for L134N. The NS emission is extended along the TMC-1 ridge and is also extended in L134N. The measured abundances are significantly higher than those predicted by some recent gas phase ion-molecule models.     

Solar and stellar chromospheric contrast

Positions of active regions estimated from observations of the whole solar disk in Caii K _iv during the period 1977–1989 at the Coimbra Astronomical Observatory are compared with the time-dependent latitudinal distribution of background solar magnetic fields and with the latitudinal shifts of boundaries of their polarities. We confirm that the sunspot groups are located near the zonal boundaries between the opposite polarities of the solar background magnetic field during different phases of the two recent consecutive cycles of activity. We demonstrate a probable connection between the increased number of groups and the commencement of poleward migration of zonal boundaries in both hemispheres. But the influence of the dominant convective rolls seems to he still unclear. A new problem of interrelation between the zonal and sector boundaries has also appeared.     

WKB estimates for the onset of ideal MHD instabilities in solar coronal loops

A WKB approach, based on the method of Connor, Hastie, and Taylor (1979), is used to obtain simple estimates of the critical conditions for the onset of ideal MHD instabilities in line-tied solar coronal loops. The method is illustrated for the constant twist, Gold-Hoyle (1960) field, and the critical conditions are compared with previous and new numerical results. For the force-free case, the WKB estimate for the critical loop length reduces to . For the sufficiently non-force-free case the critical length can be expressed in the forml ₀ +l ₁/m. The results confirm the findings of De Bruyne and Hood (1992) that for force-free fields them = 1 mode is the first mode to become unstable but for the sufficiently strong non-force-free case this reverses with them → ∞ mode being excited first.