Identification of the Holocene-Pleistocene boundary in the Bering Sea by diatoms

The modern diatom flora from surface sediment of the Navarin Basin region of the Bering Sea is dominated by Denticulopsis seminae and Nitzschia oceanica. D. seminae , a north boreal species, is most abundant in the deeper waters of the slope-basin regions, whereas N. oceanica , an Arctic species associated with ice cover, dominates the shallow shelf waters. Downcore studies show that these species alternate in dominance within the late Quaternary sediments. This alternation suggests responses to climatic fluctuations. The interval at which D. seminae is replaced downcore by N. oceanica in dominating the assemblage is distinct and can be correlated in cores throughout the study area as well as adjacent regions. This interval represents the transition from glacial to interglacial conditions and the Holocene-Pleistocene boundary.     

Geothermometry, Geobarometry and T-X(Fe-Mg) Relations in Metapelites, Snow Peak, Northern Idaho

The most recent of two metamorphic events (M2) in the Snow Peakarea caused progressive changes in mineral parageneses in peliticrocks ranging from chlorite-biotite to kyanite grade. Systematicpartitioning of elements between coexisting phases indicatesa close approach to equilibrium during M2. Temperature estimatesfor M2 range from 440 ?C in the chlorite-biotite zone to 565?C in the kyanite zone. Coexistence of kyanite, garnet, ilmenite,and quartz places an upper pressure limit of approximately 60kb, and an upper temperature limit at the kyanite-sillimaniteboundary. Equilibrium of garnet, kyanite, plagioclase, and quartzindicates that total pressure of equilibration of kyanite-bearingassemblages was approximately 6 kb. Pressure estimates basedon equilibrium of garnet, muscovite, biotite, and plagioclaseindicate a pressure gradient between garnet and lower staurolitezone samples, which equilibrated at approximately 3? 5 kb, andupper staurolite to kyanite zone samples, which equilibratedat 5? 5 kb. Equilibrium of paragonite component of muscovitewith plagioclase, kyanite and quartz, distribution of speciesin C-O-H fluids in equilibrium with graphite, and the presenceof zoisite in adjacent calc-silicate rocks indicate that themetamorphic fluid in kyanite-bearing assemblages contained 65-90mole per cent H₂O. However, the experimentally calibrated equilibriumof staurolite, quartz, garnet, and kyanite can be reconciledwith estimated temperature only if X_H2O in the fluid was verylow ( 33 mole per cent). T-X(Fe-Mg) relations among chlorite, biotite, garnet, staurolite,kyanite, muscovite and quartz are calculated at 6 kb on thebasis of 3 independent Fe-Mg exchange equilibria: garnet-biotite,chlorite-biotite (empirical, this study), garnet-staurolite(empirical, this study), and three independent net transferequilibria. Alternative sets of data for Mg-chlorite and Fe-stauroliteare evaluated by comparing observed and calculated changes inmineral paragenesis and mineral composition with grade. Chloritedata from Helgeson et al., 1978 give T-X(Fe-Mg) relations consistentwith trends observed in these rocks, whereas data derived frombreakdown of clinochlore and clinochlore + dolomite do not.Calculation of T-X(Fe-Mg) relations consistent with observationsrequires lower values of and than those consistent with experiments on the breakdown of staurolite+quartz.     

DETERMINATION OF TRACE ELEMENTS IN 11 CHINESE GEOLOGICAL REFERENCE MATERIALS BY ICP-MS

More than 40 trace elements in 11 Chinese geological reference materials (stream sediments) GSD-1 to GSD-7 and GSD-9 to GSD-12 were determined by inductively coupled plasma-mass spectrometry (ICP-MS). Open acid digestions were used to prepare samples for the determination of most elements. However, for the measurement of Y, Zr and Hf, it was found necessary to use an alkali fusion in order to make fully quantitative measurements. The results obtained in this work are generally in good agreement with the recommended values from the sub-ppm level to > several hundred ppm and illustrate the versatility of the technique, particularly for some less well characterised elements.     

Quaternary coastal lithofacies,sequence development and stratigraphy in a passive margin setting,North Carolina and Virginia,USA

Lithofacies analysis is fundamental to unravelling the succession of depositional environments associated with sea‐level fluctuations. These successions and their timing are often poorly understood. This report defines lithofacies encountered within the north‐eastern North Carolina and south‐eastern Virginia Quaternary section, interprets their depositional environments, presents a model for coastal depositional sequence development in a passive margin setting and uses this understanding to develop the stratigraphy and Quaternary evolutionary history of the region. Data were obtained from numerous drill cores and outcrops. Chronology was based on age estimates acquired using optically stimulated luminescence, amino acid racemization, Uranium series and radiocarbon dating techniques. Geomorphic patterns were identified and interpreted using light detection and ranging imagery. Since lithofacies occurrence, distribution and stratigraphic patterns are different on interfluves than in palaeo‐valleys, this study focused on interfluves to obtain a record of highstand sea‐level cycles with minimal alteration by fluvial processes during subsequent lowstands. Nine primary lithofacies and four diagenetic facies were identified in outcrops and cores. The uppermost depositional sequence on interfluves exhibits an upward succession from shelly marine lithofacies to tidal estuarine lithofacies and is bounded below by a marine ravinement surface and above by the modern land surface. Older depositional sequences in the subsurface are typically bounded above and below by marine ravinement surfaces. Portions of seven depositional sequences were recognized and interpreted to represent deposition from late middle Pleistocene to present. Erosional processes associated with each successive depositional sequence removed portions of older depositional sequences. The stratigraphic record of the most recent sea‐level highstands (Marine Isotope Stage 5a and Marine Isotope Stage 3) is best preserved. Glacio‐isostatic adjustment has influenced depositional patterns so that deposits associated with late Quaternary sea‐level highstands (Marine Isotope Stages 5c, 5a and 3), which did not reach as high as present sea‐level according to equatorial eustatic records, are uplifted and emergent within the study area.     

MIGRATION TO ZERO OFFSET (DMO) FOR A CONSTANT VELOCITY GRADIENT: AN ANALYTICAL FORMULATION1

Migration to zero offset (MZO) is a prestack partial migration process that transforms finite-offset seismic data into a close approximation to zero-offset data, regardless of the reflector dips that are present in the data. MZO is an important step in the standard processing sequence of seismic data, but is usually restricted to constant velocity media. Thus, most MZO algorithms are unable to correct for the reflection point dispersal caused by ray bending in inhomogeneous media. We present an analytical formulation of the MZO operator for the simple possible variation of velocity within the earth, i.e. a constant gradient in the vertical direction. The derivation of the MZO operator is carried out in two steps. We first derive the equation of the constant traveltime surface for linear V(z) velocity functions and show that the isochron can be represented by a fourth-degree polynomial in x, y and z. This surface reduces to the well-known ellipsoid in the constant-velocity case, and to the spherical wavefront obtained by Slotnick in the coincident source-receiver case. We then derive the kinematic and dynamic zero-offset corrections in parametric form by using the equation of the isochron. The weighting factors are obtained in the high-frequency limit by means of a simple geometric spreading correction. Our analytical results show that the MZO operator is a multivalued, saddle-shaped operator with marked dip moveout effects in the cross-line direction. However, the amplitude analysis and the distribution of dips along the MZO impulse response show that the most important contributions of the MZO operator are concentrated in a narrow zone along the in-line direction. In practice, MZO processing requires approximately the same trace spacing in the in-line and cross-line directions to avoid spatial aliasing effects.     

Power intermittency, redundancy and tidal phasing around the United Kingdom

The tidal resource is large in north-western European waters and a number of axial flow, vertical rotor and oscillating hydroplane schemes are approaching full-scale design and construction. However, the intermittency of the output from such devices that only generate electricity during the flood and ebb suggests that back-up generating capacity would be required with the associated inbuilt redundancy. Basic, shallow water tidal theory is reviewed and it is shown that carefully sized tidal stream installations in the Clyde, Tees, Humber, Severn, Menai Straits and the Mersey can utilise the phase locked occurrence of tidal flow and provide a more or less regular and continuous supply of electrical power to the National Grid. Estimates are given for the required capacity of the installations and a scheme is proposed by which an installed capacity of little more than, for example, 200 MW can provide a continuous supply of about 45 MW from these sites, thus achieving a significant and uninterrupted contribution to the Government's 2010 and 2020 targets.     

On and Off the North China Craton: Where is the Archaean Keel?

Geophysical data indicate that the lithosphere beneath the NorthChina Craton (NCC) is     

Use of the Model T Coulter Counter* in size analysis of fine to coarse sand

An electrical sensing-zone particle size analyser has been calibrated for use with sands. A 10: 90 saline/glycerol electrolyte is used with a concentration of 0-1-0.5 g/1 of suspended sand. Calibration error is ± 1.6% (± 0.02 φ). Comparison of counter and sieving results shows close agreement. Advantages of the machine are rapidity—a prepared sample can be analysed in 60 sec—and the small size of the sample required.