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We present varve chronologies for sediments from two maar lakes in the Valle de Santiago region (Central Mexico): Hoya La Alberca (AD 1852–1973) and Hoya Rincón de Parangueo (AD 1839–1943). These are the first varve chronologies for Mexican lakes. The varved sections were anchored with tephras from Colima (1913) and Paricutín (1943/1944) and 210Pb ages. We compare the sequences using the thickness of seasonal laminae and element counts (Al, Si, S, Cl, K, Ti, Mn, Fe, and Sr) determined by micro X-ray fluorescence spectrometry. The formation of the varve sublaminae is attributed to the strongly seasonal climate regime. Limited rainfall and high evaporation rates in winter and spring induce precipitation of carbonates (high Ca, Sr) enriched in 13C and 18O, whereas rainfall in summer increases organic and clastic input (plagioclase, quartz) with high counts of lithogenic elements (K, Al, Ti, and Si). Eolian input of Ti occurs also in the dry season. Moving correlations (5-yr windows) of the Ca and Ti counts show similar development in both sequences until the 1930s. Positive correlations indicate mixing of allochthonous Ti and autochthonous Ca, while negative correlations indicate their separation in sublaminae. Negative excursions in the correlations correspond with historic and reconstructed droughts, El Niño events, and positive SST anomalies. Based on our data, droughts (3–7 year duration) were severe and centred around the following years: the early 1850s, 1865, 1880, 1895, 1905, 1915 and the late 1920s with continuation into the 1930s. The latter dry period brought both lake systems into a critical state making them susceptible to further drying. Groundwater overexploitation due to the expansion of irrigation agriculture in the region after 1940 induced the transition from calcite to aragonite precipitation in Alberca and halite infiltration in Rincón. The proxy data indicate a faster response to increased evaporation for Rincón, the lake with the larger maar dimensions, solar radiation receipt and higher conductivity, whereas the smaller, steeper Alberca maar responded rapidly to increased precipitation.

Byline: Friedrich Lucassen (1,2), Gerhard Franz (2), Rolf L. Romer (1), Dieter Pudlo (2,3), Peter Dulski (1) Keywords: NE-Africa; Intra-plate magmatism; Nd--Pb-Sr isotopes; Upper mantle isotope composition; High-u mantle Abstract: The isotopic composition of mafic small-volume intra-plate magmatism constrains the compositions of the sub-continental mantle sources. The Nd, Pb, and Sr isotope signatures of widespread late Mesozoic to Quaternary intra-plate magmatism in NE Africa (Sudan, South Egypt) are surprisingly uniform and indicate the presence of a high-u (u = 238.sup.U/ [.sup.204]Pb) source in the mantle. The rocks are characterized by small ranges in the initial isotopic composition of Nd, Pb, and Sr and most samples fall within [epsilon] Nd ca. 3--6, 206.sup.Pb/ [.sup.204]Pb ca. 19.5--20.5, 207.sup.Pb/ [.sup.204]Pb ca. 15.63--15.73, 208.sup.Pb/ [.sup.204]Pb ca. 39--40 and 87.sup.Sr/ [.sup.86]Sr ca. 0.7028--0.7034. We interpret this reservoir as lithospheric mantle that formed beneath the Pan-African orogens and magmatic arcs from asthenospheric mantle, which was enriched in trace elements (U, Th, and light REE). Combining our new data set with published data of intra-plate magmatic rocks from the Arabian plate indicates two compositionally different domains of lithospheric mantle in NE-Africa--Arabia. The two domains are spatially related to the subdivision of the Pan-African orogen into a western section dominated by reworked cratonic basement (NE-Africa high-u lithospheric mantle) and an eastern section dominated by juvenile Pan-African basement (easternmost NE-Africa and Arabia moderate u lithospheric mantle). The compositions of the Pan-African lithospheric mantle and the MORB-type mantle of the Red Sea and Gulf of Aden spreading centers could explain the Nd--Pb-Sr isotopic compositions of the most pristine Afar flood basalts in Yemen and Ethiopia by mixtures of the isotopic composition of regional lithospheric and asthenospheric sources. Author Affiliation: (1) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473, Potsdam, Germany (2) Fachgebiet Mineralogie-Petrologie, Technische Universitat Berlin, Sekr. ACK 9, Ackerstr. 71-76, 13355, Berlin, Germany (3) Faculty of Chemistry and Earth Sciences, Institute of Earth Sciences, Friedrich-Schiller-Universitat Jena, Burgweg 11, 07749, Jena, Germany Article History: Registration Date: 22/05/2008 Received Date: 25/07/2007 Accepted Date: 22/05/2008 Online Date: 20/06/2008 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0314-0) contains supplementary material, which is available to authorized users.

We report evidence for the seawater origin of an extremely metal-enriched sulfide- and organic carbon-rich marker bed in a transgressive Early Cambrian black shale sequence along the passive margin of the Yangtze platform. The element concentration pattern in this marker bed suggests that it formed in a sediment-starved, stratified basin with a euxinic water column below an oxic surface layer. Biological activity was high in the surface layer, which was resupplied by communication with oxic oceans. The extremely low terrigenous input and the sulfate-reducing environment in the deeper part of the basin led to exceptionally high metal enrichments by factors of ~[10.sup.7] with respect to modern seawater. The composition of the sulfidic rocks reflects the composition of the Early Cambrian oceans. The molybdenum isotope ratio suggests that during this time Keywords: Cambrian, black shale, anoxic environments, molybdenum isotopes, China.