R.W. Stewart

Abstract

The physical mechanism that causes an interdecadal oscillation in a coarse resolution sector ocean model forced by mixed boundary conditions is studied. The oscillation is characterized by large fluctuations in convective activity and air/sea heat exchange on a decadal timescale. Changes in the subsurface temperature and surface salinity are essential for the existence of the oscillation. It is shown that a large part of these variations can be explained with the hypothesis of a constant ocean velocity field. This may easily lead to the erroneous conclusion that the oscillation is mainly a “nondynamical” phenomenon. In this paper it is demonstrated that the ocean dynamics play an essential role in explaining decadal oscillations.     

Social networks and undocumented Mozambican migration to South Africa

This paper analyses the social networks which facilitate and sustain undocumented migration from Mozambique to South Africa. A key contention is that the migrant social networks are not limited to a spatially bounded area; transcend geography, location and territory; can be considered as spatial conveyors of social capital; and operate transnationally at three different locations: in the sending communities, on borders and in the destination areas. In the sending communities, interpersonal relationships are based on bonds of kinship, and friendship through which the migrants get moral and material support for the movement. At the borders migrants establish connections with border agents, guides, and conveyors who support them in entering South Africa and provide transportation to their preferred destinations. At the destination areas the newcomers have also counted on the bonds of kinship and friendship among former immigrants, who assist them on their arrival with accommodation and food as well as in the process of getting jobs and documentation. In South Africa undocumented migrants were subjected to high levels of xenophobia, exploitation and deportation, structural, sociopolitical forces against which social networks are largely ineffective. However, through the social networks the undocumented Mozambican labour migration to South Africa has become a self-sustained circular process that is difficult to control.     

Sensitivity of an ecosystem model to hydrology and temperature

We tested the sensitivity of a dynamic ecosystem model (LPJ-GUESS) to the representation of soil moisture and soil temperature and to uncertainties in the prediction of precipitation and air temperature. We linked the ecosystem model with an advanced hydrological model (JULES) and used its soil moisture and soil temperature as input into the ecosystem model. We analysed these sensitivities along a latitudinal gradient in northern Russia. Differences in soil temperature and soil moisture had only little influence on the vegetation carbon fluxes, whereas the soil carbon fluxes were very sensitive to the JULES soil estimations. The sensitivity changed with latitude, showing stronger influence in the more northern grid cell. The sensitivity of modelled responses of both soil carbon fluxes and vegetation carbon fluxes to uncertainties in soil temperature were high, as both soil and vegetation carbon fluxes were strongly impacted. In contrast, uncertainties in the estimation of the amount of precipitation had little influence on the soil or vegetation carbon fluxes. The high sensitivity of soil respiration to soil temperature and moisture suggests that we should strive for a better understanding and representation of soil processes in ecosystem models to improve the reliability of predictions of future ecosystem changes.     

The Haughton Impact Structure,Devon Island,Canada: Setting and History of Investigations

Abstract— Although mapped initially as a piercement dome, subsequent discovery of shock metamorphism in clasts of an impact breccia, shatter cones in outcrops of uplifted target rocks and morphological and geophysical characteristics consistent with a complex crater, confirmed a meteorite impact origin for the Haughton structure, Devon Island. Results of three field investigations carried out prior to 1984 defined a complex crater, 20 km in diameter, formed in a lower Paleozoic sedimentary sequence overlying gneisses of the Precambrian basement. The distribution of allochthonous breccia overlying the disturbed target rocks and of the sediments deposited in the crater-filling lake were mapped. A Miocene or possibly Holocene age for the crater was based on paleo-flora and fauna assemblages from the lake sediments. Gravity and magnetic surveys revealed anomalies coincident with the crater, but not interpretable from surface lithologies. Some of the early investigations were of a reconnaissance nature and results and interpretation can only be considered preliminary. Other studies that were carried out in some detail, petrographic investigations in particular, require complementary work for a fuller understanding of their significance. As a result, in 1984 the HISS (Haughton Impact Structure Studies) group carried out a program of detailed geological mapping and sampling, and seismic, gravity, and magnetic surveys in an attempt to improve the definition of the surface and subsurface nature of Haughton, and to formulate a more complete understanding of its formation and subsequent history. Results of these various studies are presented in the eight succeeding papers of this volume.     

A new approach to detecting vegetation and land-use change using high-resolution lipid biomarker records in stalagmites

A hundred-year stalagmite lipid biomarker record from Mechara, southeastern Ethiopia, is presented. The record has been recovered at a 10-yr temporal resolution, marking the first time this has been achieved in stalagmite biomarker work and providing the first opportunity to investigate the relationship between stalagmite lipid records and hydrological transport lags, a vital issue in interpreting palaeoenvironmental signals. Preserved plant-derived n-alkanes and n-alkanols show clear changes in composition over time, relating to known land-use changes in the area, particularly the expansion of agriculture in the early twentieth century. The level of environmental detail provided by this technique, combined with the long-term chronological framework offered by stalagmites, holds significant promise for the investigation of early human environments and their associated climatic and anthropogenic controls.     

Discovery of asteroid 1976 AA

Asteroid 1976 AA was discovered as a result of a continuing systematic search for planet-crossing asteroids. It is the first asteroid to be thoroughly investigated by means of photometry and radiometry on its discovery apparition. It is also the first asteroid found with a semimajor axis and period less than that of the Earth and the first Earth-crossing asteroid which does not cross the orbit of either Mars or Venus. We estimate that there might be several tens of objects to absolute magnitude 18, which are exclusively Earth crossing. Some of these objects might be exceptionally easy to reach by spacecraft.     

Gravity and Magnetic Investigations in the Haughton Impact Structure,Devon Island,Canada

Abstract— The results of a new gravity survey show that the Haughton impact structure is associated with a 24 km diameter negative Bouguer gravity anomaly with a maximum amplitude of ?12 mgal. A local minimum with a half-width of 2 km and an amplitude of ?4 mgal is located at the center of the structure. A positive magnetic total field anomaly with a half-width of 0.6 km and an amplitude of 700 nT coincides with the local central gravity anomaly. The overall negative gravity anomaly is explained by lowered rock densities due to impact-related fracturing in the crater area. The central gravity and magnetic anomalies are believed to be due to highly shocked and heated sedimentary and crystalline basement rocks forming the unexposed peak of the central uplift in the Haughton impact structure.     

The discovery of 1982 DB,the most accessible asteroid known

A small Apollo object was found while photographing the split comet DuToit 2-Hartley on February 27/28, 1982. Designated 1982 DB, this Earth-crossing asteroid passed the Earth at a distance of 4.08 million km about 1 month prior to its discovery. Asteroid 1982 DB has been determined to be the most accessible near-Earth minor planet known. It provides many excellent opportunities for rendezvous and sample return missions, and a rare dual rendezvous mission with 1980 AA as the second target. For a mission to be realized, opportunities to observe 1982 DB during future apparitions must be taken.