A phytolith index as a proxy of tree cover density in tropical areas: calibration with Leaf Area Index along a forest–savanna transect in southeastern Cameroon

The aim of the study is to calibrate the phytolith index of tree cover density, D/P (the ratio of ligneous dicotyledons phytoliths (D) over Poaceae phytoliths (P)) with Leaf Area Index (LAI) measurements. LAI is the vertically integrated surface of leaves per unit of ground area (m² leaves/m² ground). Modern soil samples from southeastern Cameroon, collected along a continuous forest–savanna transect, have been analyzed for phytoliths. Phytolith assemblages and D/P index clearly record the physiognomy of the forest and savanna communities and of the transition between both of them. A highly significant relationship was obtained between D/P and LAI. The relationship between phytolith data and the vegetation transect is also discussed and compared with existing palynological results obtained along the same transect.     

Late glacial and holocene lake level fluctuations in Lake Biel,western Switzerland

Cores and outcrops from the southern shore of Lake Biel were studied to reconstruct the nearshore environment of the lake between ca. 12000 and 5000 yr BP. Core correlations were established by lithostratigraphical and pollen analytical correlations. From the Allerød to the Preboreal time quiet hydrodynamic conditions favoured the deposition of lake marl in the littoral zone and peat on the shore. Between the Preboreal(?) and the Atlantic the littoral zone shows a higher hydrodynamic environment with allochthonous material, whereas peat and clay layers are recorded from the shore. During the Older Atlantic severe erosional episodes caused the erosion of Boreal, Preboreal and Younger Atlantic layers. The previously described long hiatus between the Allerød and the Boreal time can now be connected with these erosional episodes. From Younger Atlantic to Subboreal time the littoral zone displays quiet conditions again with sedimentation of lake marl. On the basis of these results a lake level curve for Lake Biel is proposed: high lake level stands can be traced during the Allerød, Boreal, Older Atlantic and Younger Atlantic biozones; low lake level stands are found during the Allerød, Younger Dryas, Preboreal and Older Atlantic biozones.     

Filaments,disks, and jets for of and WR stars

It is noted that the spectra of Wolf-Rayet stars can be well described in terms of emission-line formation in a moderate density wind, a ring-like disk, and magnetically supported filaments which link the disk to the central star. The emission-line spectra of Of, O(f), and O((f)) stars likewise can be understood in terms of jets, filaments, and disks. Some examples of Of spectra are described and it is noted that expanding-spherical-wind models do not appear to be able to describe accurately all which is observed. When interpreting the spectra of Wolf-Rayet and O stars, it is useful to think in terms of the type of model which has proved useful for interpreting the emission-line spectra of Herbig Ae/Be and T Tauri stars.     

Quartz fabric evidence for early Pan-African penetrative east-directed shearing in the Itremo Supracrustal Group of central Madagascar

ABSTRACT The Pan-African orogeny left a strong imprint on the basement rocks of Madagascar, which were metamorphosed up to granulite facies conditions. The supracrustal Itremo Group of central Madagascar, comprising quartzites, schists and carbonates of lower metamorphic grade, has to date been described as a folded sedimentary sequence. Despite their fine-grained 'sugary' appearance, most quartzites are plastically deformed tectonites. Quartzite microstructures are mainly of the elongate mosaic type, indicating significant grain boundary migration, and are compatible with dynamic recrystallization under lower amphibolite facies conditions. Consistent asymmetric quartz c -axis fabrics indicate a dominant top-to-the-east shear sense. Hence, the Itremo quartzites bear evidence for a major eastward-directed tectonic event of Pan-African age, possibly resulting from an early Pan-African thrust motion. Younger deformational events, responsible for localized mylonites with top-to-the-WSW sense of motion and N–S-trending folds and shear zones, were superimposed on this first fabric.     

Oxygen isotope analyses of fine silica grains using laser-extraction technique: Comparison with oxygen isotope data obtained from ion microprobe analyses and application to quartzite and silcrete cement investigation

The laser fluorination technique reported here for analyzing the oxygen isotope composition (δ¹⁸O) of fine quartz size fractions 50-20, 20-10, 10-5, 5-2, 2-1 and <1 μm has been validated by comparison with the ion microprobe technique. It yields accurate δ¹⁸O data with an external precision better than 0.15‰. This is a significant methodological improvement for isotopic studies dealing with materials such as soil or biogenic oxides and silicates: particles are often too small and recovered in insufficient amount to be easily handled for ion microprobe analysis. Both techniques were used to investigate δ¹⁸O composition of a Cretaceous quartzite and silcrete sequence from the South-East of France. Quartzite cements average 31.04 ± 1.93‰. They formed from Mid-Cretaceous seawater. Higher in the series, silcretes cements average 26.66 ± 1.36‰. They formed from Upper- or post-Upper-Cretaceous soil water and groundwater. Oxygen isotope data show that the silicification steps from one mineralogical phase to another and from one layer to another (including from an upper pedogenic silcrete to a lower groundwater silcrete) occurred in a closed or weakly evaporating hydrological system.     

Effects of two hurricanes onSyringodium filiforme, manatee grass, within the Loxahatchee River estuary, Southeast Florida

In September 2004, the Loxahatchee River Estuary was affected by Hurricanes Frances and Jeanne, which resulted in a monthly rainfall record of 610 mm and abnormally high freshwater discharges to the system. The occurrence, density, and biomass ofSyringodium filiforme in the Loxahatchee River Estuary declined significantly following the September 2004 storms based on 15 mo of pre-hurricane monitoring and 12 mo of post-hurricane monitoring. Throughout posthurricane monitoring,S. filiforme showed no sign of recovery, thoughHalophila johnsonii increased considerably during the post-hurricane period. Freshwater discharges resulting from the September 2004 hurricanes lowered minimum daily salinity values to near zero and increased standard deviation of daily salinity values to 11‰. Extremely low minimum daily salinity values and high daily salinity fluctuations likely resulted in the observed decline ofS. filiforme. We advise the use of minimum daily salinity values when assessing seagrass habitat suitability or when modeling the effects of alternative water management scenarios.