Van Bocxlaer, Bert

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Ghent University Academic Bibliography

 The crystallochemical study of phlogopites in cumulates evidences the factors responsible for the signifi cant Ti- and Al-enrichment in these minerals. The Ti-incorporation results from two mechanisms: [6]Ti4+ + 2[4]Al3+ = ([6]Mg2+ , [6]Fe2+) + 2[4]Si4+ and [6]Ti4+ + [6] = 2([6]Mg2+ , [6]Fe2+). The Al-incorporation may be explained by: ([6]Mg2+ , [6]Fe2+) + [4]Si4+ = [6]Al3+ + [4]Al3+. These different substitution mechanisms are related to the crystallisation temperature of phlogopites. A series of spectroscopic analyses (infra-red and Mössbauer) also revealed the presence of Fe3+ and water in the network of these micas and the low abundance of vacancies in octahedral sites.ReferencesDÖRR, W., BELKA, Z., MARHEINE, D., SCHASTOK, J., VALVERDE-VAQUERO, P. & WISZNIEWSKA, J., 2002. U-Pb and Ar-Ar geochronology of anorogenic granite magmatism of the Mazury complex, NE Poland. Precambrian Research, 119: 101-120.LONGHI, J., FRAM, M.S., VANDER AUWERA, J. & MONTHIETH, J.N., 1993. Pressure effects, kinetics, and rheology of anorthositic and related magmas. American Mineralogist, 78: 1016-1030.WISZNIEWSKA, J., CLAESSON, S., STEIN, H.J., VANDER AUWERA, J. & DUCHESNE, J.C., 2002. The north-eastern Polish anorthosite massifs: petrological, geochemical and isotopic evidence for a crustal derivation. Terra Nova, 14: 451-460.Changes in the malacofauna of Lake Malawi since mid-Holocene timesBert VAN BOCXLAERResearch Unit Palaeontology, Ghent University, Krijgslaan 281, S8, B-9000 GhentIt is generally accepted that Lake Malawi, the second largest African rift lake (surface: 29,000 km²), is a prime example of an ʻ ancient lakeʼ, i.e., an extensive lacustrine system that persisted for at least 100,000 years (Gorthner, 1994). The assumption that the modern Malawi fauna is the result of a long-lasting evolutionary process is principally based on the marked diversity and endemism of its modern fi sh and molluscan communities. During the missions of the Hominid Corridor Research Project under the direction of Prof. Dr. Friedemann Schrenk in the nineties, a well preserved fossil mollusc assemblage of mid-Holocene age (14C: 5,845 ± 85 BP) was discovered at the southern outlet of the lake, allowing a glance at the fauna in the near geological past. A morphological and conchyometric study was carried out on the named fossil assemblages, comparing the mid-Holocene shells with series of modern specimens. Differences in community structure were also analysed and the results were correlated with available data on fl uctuations of abiotic factors in the Malawi Basin during the Holocene. The purpose of this study was to assess the evolutionary changes in morphology and community structure that took place in this endemic ʻancient lake  ʼfauna over a period of c. 6,000 years. The modern malacofauna of Lake Malawi consists of a mixture of lake endemics and non-endemics. This in contrast to that of ʻancient lake  ʼ Tanganyika, which is exclusively endemic. At present fi ve gastropod genera occur in Lake Malawi, namely Bellamya represented by a clade of 3 endemic species and 1 non-endemic species, Lanistes (3 endemics/2 non-endemic), Gabbiella (1 endemic), Melanoides (7 or 8 endemics /1 non-endemic) and Bulinus (2 endemics/2 non-endemics) (Mandahl-Barth, 1972; Brown 1994). The mid-Holocene community consists of the same fi ve gastropod genera with the following endemic/non-endemic ratio: Bellamya (0/1), Lanistes (3/2), Gabbiella (1/0), Melanoides (5/0) and Bulinus (2/1). This shows that all gastropod genera, except Gabbiella, underwent marked changes since the mid-Holocene and the comparative morphological study shows that most modern clades of endemics did originate from non-endemic lineages during the last 5,000 to 10,000 years. As to the bivalves, at least one of the three genera with an endemic representative in L. Malawi shows marked morphological changes since the mid-Holocene. The young age of the endemic lineages is corroborated by recent genetic research on the parthenogenetic Melanoides clade, which appears to consist of different, genetically virtually identical, stable morphs (Genner et al., unpublished).Since the mid-Holocene a distinct trend of decrease in shell size and shell thickness in all genera and a shift in dominance in favour of the most opportunistic non-endemic lineage (i.e. M. tuberculata) in the genus Melanoides suggest suboptimal ecological conditions in the modern lake compared to 6,000 years ago.The general conclusion of the study is that most species of the endemic malacofauna of Lake Malawi, supposedly one of the most important ʻancient lakes  ʼin the world, are only 5,000 to 10,000 years old. The persistence of a lake over a period of several hundred thousands of years does hence not imply the persistence of its fauna and therefore major parts of the lacustrine ecosystem. From the present study it would appear that the unstable climate conditions in subequatorial Africa since the Pleistocene caused oscillatory patterns in lake levels and physico-chemical characteristics of the lacustrine ecosystem. These oscillations were of such magnitude that they caused a cyclic pattern of brief extinctions and radiation events in slow moving epi- and endofaunal biota, e.g. the mollusca, that are more vulnerable to such changes than more mobile groups such as fi shes and crabs.ReferencesBROWN, D.S., 1994. Freshwater Snails of Africa and their Medical importance. Revised 2nd Edition. London: Taylor & Francis.GENNER, M.J., MICHEL, E., ERPENBECK, D., WITTE, F., PIECHOCKI, A. & POINTIER, J.P., Unpublished. Molecular evidence uncovers the origins of an endemic Lake Malawi gastropod radiation.GORTHNER, A., 1994. What is an ancient lake? Archiv für Hydrobiologie Beihefte Ergebnisse der Limnologie, 44: 97-100.MANDAHL-BARTH, G., 1972. The freshwater Molluscs of Lake Malawi. Revue de Zoologie et de Botanique africaines, 86(3-4): 257-289.124 JUNIOR MEETING

Changes in the malacofauna of Lake Malawi since mid-Holocene times

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Changes in the malacofauna of Lake Malawi since mid-Holocene times

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