Inhalt: Alexander von Humboldt-Forschungsstelle: Ingo Schwarz zum 65. Geburtstag Ottmar Ette: Findung und Erfindung einer Leserschaft. Neuere Editionsprojekte zu Alexander von Humboldt als Grundlage und Herausforderung künftigen Forschens Eberhard Knobloch: Alexandre de Humboldt et le Marquis de Laplace Oliver Schwarz: Alexander von Humboldt als astronomischer Arbeiter, Diskussionspartner und Ideengeber Petra Werner: Innenwelten und bleiche Gärten. Alexander von Humboldt untertage und in der Caripe-Höhle Christian Suckow: Alexander von Humboldt in Ust’-Kamenogorsk Anne Jobst: Neue Briefe Christian Gottfried Ehrenbergs an Alexander von Humboldt Thomas Schmuck: Humboldt, Baer und die Evolution Manfred Ringmacher: Zwei Briefe auf Guaraní in Alexander von Humboldts Handschrift Ute Tintemann: Julius Klaproths Mithridates-Projekt, Alexander von Humboldt und das Verlagshaus Cotta Ulrike Leitner: „Ja! Wenn Berlin Bonn wäre!“ Friedrich Rückerts Berufung nach Berlin Frank Holl: „Zur Freiheit bestimmt“ – Alexander von Humboldts Blick auf die Kulturen der Welt Sebastian Panwitz: Das Humboldt-Mendelssohn-Haus Jägerstraße 22. Ein Quellenfund Laura Péaud: Du Mexique à l‘Oural : l‘expertise humboldtienne au service du politique Bärbel Holtz: „Cicerone“ des Königs? Alexander von Humboldt und Friedrich Wilhelm III. Menso Folkerts: Ein unerwartetes Zusammentreffen in Sanssouci. Alexander von Humboldt und Karl Ludwig Hencke an der Tafel Friedrich Wilhelms IV. Ulrich Päßler: Preußens Mann in Washington. Fünf Briefe Friedrich von Gerolts an Alexander von Humboldt (1858/1859) Bill Roba: German-Iowan Strategies in Celebrating the Centennial of Alexander von Humboldt’s Birth Regina Mikosch: Ingo Schwarz‘ Veröffentlichungen zur Alexander von Humboldt Über die Autoren

Kurz vor der Wahl am 1. Juli steht Mexiko vor großen Turbulenzen: Parteien beschuldigen sich gegenseitig, Mitglieder oder -läufer der organisierten Kriminalität zu sein. Der Krieg gegen die Drogen, der zuvor noch nationale Einheit heraufbeschwor, wird jetzt zum Instrument der Verleumdung des politischen Gegners. Wie kann angesichts brüchiger Staatlichkeit eine sichere Wahl gewährleistet werden?

The Earth’s magnetic field (EMF) is generated by convections in the electrically conducting liquid iron-rich outer core, modified by the Earth’s rotation. A drastic manifestation of the dynamics of this fluid body is the occurrence of geomagnetic field reversals in the Earth’s history but also geomagnetic excursions, which are more frequent features of otherwise stable polarity chrons, but often poorly constrained in the geological record. To better understand the origin of the field, we need to know how the field has varied on different geological timescales. This includes not only information about changes in the ancient field’s direction but also about the absolute intensity (palaeointensity) and the age. This palaeointensity record is needed for compiling a full-vector description of the field. A palaeomagnetic and palaeointensity study on lava flows allows gaining insights about the evolution of the EMF through time and space. However, constraining the EMF evolution over different geological timescales remains a difficult objective due to the paucity of available palaeointensity data. One new alternative approach in palaeointensity studies is the recently proposed multispecimen parallel differential pTRM (MS) method, which has potentially several advantages over the commonly used Thellier method, because it is in theory independent of magnetic domain state, less prone to biasing effects, such as thermal alteration and significantly faster to perform in the laboratory. A study of highly active volcanic regions, such as the Trans-Mexican Volcanic Belt, seems promising when attempting a full-vector reconstruction or when looking for field excursions. One aim of this thesis was to gain new information about the occurrence and global validity of geomagnetic excursions from the Brunhes- or Matuyama Chron. For this purpose some 75 lava flows from within the Trans-Mexican Volcanic Belt were sampled for palaeomagnetic analyses. The scatter of virtual geomagnetic poles from lavas younger than 1.7 Ma was used for estimating palaeosecular variation and was found to be consistent with latitude dependent Model G and other high quality palaeomagnetic data from Mexico. The palaeomagnetic mean-vectors of 56 lavas were correlated to the Geomagnetic Polarity Timescale supplemented with information on geomagnetic excursions. On the grounds of their associated radioisotopic ages, four lavas were tentatively correlated with known excursions from marine records. Two lava flows dating of Brunhes Chron were associated with the Big Lost and Delts/Stage 17 excursions, respectively. From further two flows dating of Matuyama Chron, one flow was associated with either the Santa Rosa- or Kamikatsura excursions, while the other could have been emplaced during the Gilsa excursion. The most significant outcome was the finding that both Brunhes excursional flows display nearly fully reversed directions that deviate almost 180°C from the expected normal polarity direction. This observation could indicate that in particular the Big Lost and Delta/Stage17 excursions may represent other short periods during which the field completed a full reversal for a short time, such as was previously found for other older cryptochrons or tiny wiggles. Another focus of this thesis was set on estimating the feasibility of the new MS method for routine palaeointensity determination. This was accomplished by applying the MS method to samples from 11 historical lava flows from Mexico and Iceland from which the actual field intensity was either known from contemporary observatory data, or deduced from magnetic field models. Comparing observed with expected intensity values allowed to test the accuracy of the MS method. It a was found that the majority of palaeointensity estimates after the MS method yielded results that were very close or indistinguishable within the range of uncertainty from the expected values. However, a general trend towards an overestimate in the palaeointensity was also observed, which, on the grounds of corroborating rock magnetic analyses, was associated with multidomain material. This observation was taken as first evidence that the MS method is not entirely independent of magnetic domain state, as was originally claimed. However, a second experiment in which a modification of the most widely used Thellier method was applied to sister samples from 5 Icelandic flows revealed that, in comparison to the MS method, the latter produced more accurate and statistically better defined palaeointensities. Thus, from these first results, the MS method appeared as a viable alternative for future palaeointensity studies. Subsequently it was attempted to corroborate the directional record from Mexican lavas with palaeointensity data. It was possible to acquire palaeointensity estimates for 32 out of 51 investigated lava flows. These new results revealed that the new MS palaeointensities for Mexico are, with a high degree of statistical significance, around 30% higher than expected. The generally high palaeointensities seem to corroborate the results obtained from historical lava flows in this study and other previous studies on synthetic samples where domain state effects were found to cause overestimates in the palaeointensity of up to 30 per cent in the MS method. The primary process that leads to this overestimate is assigned to an asymmetry in the demagnetisation and remagnetisation process. Yet, this overestimate is expected to be no larger than what might be expected from Thellier experiments performed on samples with a given degree of multidomain behaviour.

Literaturbericht Rezensierte Literatur: Nikolaus Werz: Lateinamerika. Eine Einführung, Nomos Verlagsgesellschaft, Baden-Baden 2005, 400 S., ISBN 3-8329-1068-9 Klaus Bodemer/Detlef Nolte/Hartmut Sangmeister (Hrsg.): Lateinamerika Jahrbuch 2004, Vervuert, Frankfurt a.M. 2004, 385 S., ISBN 3-86527-123-5 Peter Imbusch/Dirk Messner/Detlef Nolte (Hrsg.): Chile heute. Politik, Wirtschaft, Kultur, Vervuert, Frankfurt a.M. 2004, 957 S., ISBN 3-89354-590-5 Walther L. Bernecker/Marianne Braig/Karl Hölz/Klaus Zimmermann (Hrsg.): Mexiko heute. Politik, Wirtschaft, Kultur, Vervuert, Frankfurt a.M. 2004, 3. vollständig neu bearbeitete Auflage, 826 S., ISBN 3-86527-140-5 Rafael Sevilla/Andreas Boeckh (Hrsg.): Venezuela – die Bolivarische Republik, Horlemann, Bad Honnef 2005, 322 S., ISBN 3-89502-197-0