Browsing by Author "Hall, Grant"

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    A 1000-year carbon isotope rainfall proxy record from South African baobab trees (Adansonia digitata L.)
    (Public Library of Science, 2015) Woodborne, Stephan; Hall, Grant; Robertson, Iain; Patrut, Adrian; Rouault, Mathieu; Loader, Neil J; Hofmeyr, Michele
    A proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab ( Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.
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    A background investigation into the feasibility of heavy stable isotopes (87Sr/86Sr) as source tracers of early hominids
    (1995) Hall, Grant; Sillen, Andrew; Armstrong, R
    Heavy stable strontium isotope ratios (⁸⁷Sr/⁸⁶Sr) may be used to characterize the habitat choices of early hominids. The basis for this approach is that different geological substrata may have a characteristic range of ⁸⁷Sr/⁸⁶Sr ratios in the available soil strontium pool. This range may be represented in the plants growing on these various geologies and thus into the bones of animals feeding in these areas. Through an examination of strontium isotope ratios for isotope and soils from 10 different geologies in the Sterkfontein Valley and immediate vicinity, it was possible to obtain some tentative indication of the possible range in ⁸⁷Sr/⁸⁶Sr ratios in an area most likely to cover favoured foraging habitats of early hominids, such as Australopithecus robustus. On the basis of isotopic data from extensive dolomite sampling, it is clear that there is a wide range of variation in strontium isotope ratios for dolomite substrata around Swartkrans. This may be as a result of topography, water action and atmospheric pollution. However, in spite of such variation, it is possible to distinguish isotopically different habitats, such as riparian from slopes. Such variation will complicate the interpretation of early hominid home ranges, but does provide alternative avenues for behavioural interpretations and also emphasises the necessity for the collection of background isotopic data.