Sea-level pressure variability around Antarctica since A.D. 1750 inferred from subantarctic tree-ring records

 A tree-ring chronology network recently developed from the subantarctic forests provides an opportunity to study long-term climatic variability at higher latitudes in the Southern Hemisphere. Fifty long (1911–1985), homogeneous records of monthly mean sea-level pressure (MSLP) from the southern latitudes (15–65^°S) were intercorrelated on a seasonal basis to establish the most consistent, long-term Trans-Polar teleconnections during this century. Variations in summer MSLP between the South America-Antarctic Peninsula and the New Zealand sectors of the Southern Ocean are significantly correlated in a negative sense (r=−0.53, P<0.001). Climatically sensitive chronologies from Tierra del Fuego (54–55^°) and New Zealand (39–47^°) were used to develop verifiable reconstructions of summer (November to February) MSLP for both sectors of the Southern Ocean. These reconstructions, which explain between 37 and 43% of the instrumentally recorded pressure variance, indicate that inverse trends in MSLP from diametrically opposite sides of Antarctica have prevailed during the past two centuries. However, the strength of this relationship varies over time. Differences in normalized MSLP between the New Zealand and the South America-Antarctic Peninsula sectors were used to develop a Summer Trans-Polar Index (STPI), which represents an index of sea-level pressure wavenumber one in the Southern Hemisphere higher latitudes. Tree-ring based reconstructions of STPI show significant differences in large-scale atmospheric circulation between the nineteenth and the twentieth centuries. Predominantly-negative STPI values during the nineteenth century are consistent with more cyclonic activity and lower summer temperatures in the New Zealand sector during the 1800s. In contrast, cyclonic activity appears to have been stronger in the mid-twentieth than previously for the South American sector of the Southern Ocean. Recent variations in MSLP in both regions are seen as part of the long-term dynamics of the atmosphere connecting opposite sides of Antarctica. A detailed analysis of the MSLP and STPI reconstructions in the time and frequency domains indicates that much of the interannual variability is principally confined to frequency bands with a period around 3.3–3.6 y. Cross spectral analysis between the STPI reconstruction and the Southern Oscillation Index suggests that teleconnections between the tropical ocean and extra-tropical MSLP variations may be influencing climate fluctuations at southern latitudes. Received: 18 December 1996/Accepted: 10 January 1997     

Interference effect of two closely-spaced shallow strip foundations on geogrid-reinforced sand

Summary Geotextiles and geogrids are now being used extensively in many civil engineering construction works. This study presents some laboratory model test results for the ultimate bearing capacity of an isolated, and two closely-spaced, strip foundations resting on unreinforced sand, and sand reinforced with layers of geogrid. Based on the model test results, the variation of the group efficiency with the centre-to-centre spacing of the foundation has been determined.     

The application of electromagnetic techniques to groundwater recharge investigations

The use of electrical methods for estimating spatial patterns of groundwater recharge was evaluated at a field site in southeastern Australia. Here, recharge increased from less than 0.2 mm year⁻¹ under native Eucalyptus vegetation, to between 1 and 14 mm year⁻¹ under dryland agriculture. This increase in recharge results in progressive leaching of salts in the soil profile. Differences in recharge can be estimated from differences in depth of leaching. The estimated recharge rates are correlated with soil texture, with higher recharge rates generally occurring through sandier soils. The relationships of recharge to salt content and soil texture both contribute to lower apparent electrical conductivities for higher recharge rates.

The effect of recharge rate on measured apparent electrical conductivities was modelled for various geophysical devices (including frequency-domain (FEM) and time-domain (TEM) electromagnetic instruments and direct current resistivity). The soil-texture effect was shown to have a greater effect than the solute leaching effect in determining the correlation between recharge and apparent electrical conductivity. Analysis of sensitivity to geological noise showed that variations in soil type below 2 m could disguise any correlation.

Correlations between recharge rate, measured at core sites from chloride tracer techniques, and apparent electrical conductivity, measured with FEM electromagnetic devices, supported the conclusions of the model. For DC resistivity and TEM methods, correlations between recharge and apparent electrical conductivity were not significant, although for resistivity this may be due partly to the small number of measurements made. The FEM device most sensitive to variations in recharge had an operating frequency of 9.8 kHz. At lower frequencies the sensitivity is reduced, as the instruments are sensing too deeply. The poor correlations for TEM, as compared with FEM, are due probably to the relatively deeper penetration of the TEM instrument used in the study, rather than any inherent differences between the techniques.

Because the major reason for the correlation between recharge and apparent electrical conductivity is soil texture, in this area the geophysical devices are mostly mapping soil type.     

Diorites and associated rocks in the Anglem Complex at The Neck, northeastern Stewart Island, New Zealand: an example of magma mingling

Exposures of the Anglem Complex (Cretaceous) at The Neck, northeastern Stewart Island consist dominantly of diorite, microdiorite, tonalite and granite. Dyke-rocks include microgranite, microdiorite, andesite and microgranodiorite.

A Diorite Suite composed of hornblende gabbro, diorite and quartz-bearing diorite forms a complex association of basic to intermediate lithologies with highly variable grainsize. The Heterogeneous Suite formed in part by mingling of diorite and tonalite intrudes the Diorite Suite. Enclaves within this suite show highly variable margins, from sharp sometimes crenulated and apparently chilled contacts, through to diffuse contacts. Prior to, and during intrusion of this heterogeneous magma, significant textural modifications of dioritic enclaves occurred, and some important controls on this modification process are discussed. Aspects considered include contrasts between the host melt and enclaves in temperature, viscosity, chemistry, volatile content, deformation and recrystallisation. A liquid immiscibility model is discounted in favour of the magma mingling model. Subsequent intrusion of granite was followed by a porphyritic microgranodiorite. Microdiorite and microgranite dykes intrude all of the above rocks with the exception of the microgranodiorite. Coexisting microdiorite, microgranite, and composite microdiorite-microgranite dykes in the one dyke swarm also provide compelling evidence for magma mingling.     

Preliminary transient heat flow model of the Rio Grande rift in southern New Mexico

A positive gravity anomaly, anomalous seismic structure, and abnormally high reduced heat flow occur in a possible extension of the Rio Grande rift in southern New Mexico. The reduced heat flux is explained by the transient effects of basaltic intrusions with dimensions based on interpretations from gravity and seismic data. Ages of volcanic activity in the rift area are used to determine times of thermal events. The effect of latent heat and sharp boundaries of the reduced heat flow anomaly also restrict modeling. The thermal model consists of three intrusions at the base of the crust at 35 m.y., 10 m.y. and 5 m.y. The initial temperatures of these intrusions would differ from those in the surrounding crust and upper mantle by about +800°C, +800°C, and +600°C, respectively. The 35-m.y. intrusion is consistent with Oligocene volcanism and tectonic activity in southwestern New Mexico. This mass also contributes to the Bouguer gravity high in the rift area, as do the intrusions at 10 and 5 m.y. The intrusions at 10 and 5 m.y. explain the excess reduced flux, with the 5-m.y. pulse accounting for most of the curvature in the sharp boundaries of the anomaly. The idealized model is consistent with late Tertiary andesites, the periods of little extrusive activity, and Pliocene-Quaternary basalts in the rift area.     

The mechanisms of the formation and growth of water bubbles and associated dislocation loops in synthetic quartz

The development of water bubbles in synthetic quartz has been monitored by measurements of (i) the intensity of the light scattered and (ii) the increase in volume of the crystal, both as a function of temperature and time. These macroscopic measurements have been complemented by observations of the resulting microstructures, using transmission electron microscopy (TEM). A mechanism is proposed on the assumption that hydrogen is incorporated in the quartz structure by means of (4 H)_Si defects. On heating, these defects diffuse and clusters develop. A cluster of n(4 H)_Si produces a water bubble of (n?1)H₂O, without any change of volume of the crystal. At any temperature T there is a critical bubble diameter above which the “steam” pressure P exceeds the pressure p for a spherical bubble in mechanical equilibrium. If P becomes greater than p, then the bubble increases in volume until P=p, the increase in volume being achieved by the pipe diffusion of Si and O away from the bubble site into a linked edge dislocation loop. This process produces the observed increase in volume of the crystal. The two diffusion processes take place virtually simultaneously and continue until all the (4 H)_Si defects have been trapped in the bubbles. Values of the diffusion constant and the activation energy for the diffusion of the (4 H)_Si defects are deduced. The relevance of these observations to the hydrolytic weakening of quartz is briefly discussed.     

Field measurement of small ozone fluxes to snow,wet bare soil,and lake water

Eddy-correlation measurements over snow, wet bare soil, and lake water indicate very small vertical ozone fluxes. Adjustments to the small vertical fluxes are needed to take into account the effect of mean Stefan flow associated with evaporation at the surface and the effects of correlation between density variations and vertical wind fluctuations. For snow, the residual resistance calculated for the surface is about 34 s cm^-1, indicating that the maximum deposition velocity is abut 0.03 cm s^-1. For cold bare soil well saturated with water, the surface resistance is about 10 s cm^-1 (maximum deposition velocity of about 0.1 cm s^-1). The highest resistances obtained are for transfer to the surface of Lake Michigan, yielding values near 90 s cm^-1 for resistance (0.01 cm s^-1 for deposition velocity).Work supported by the U.S. Dept. of Energy and the U.S. Environmental Protection Agency.