Halogens in tektites and impact glasses

Abstract— Impact glasses, tektites and some related basement rocks were analyzed for F, Cl, Br and I. The tektite and impact glasses show similar abundance patterns within the groups. Muong Nong-type tektites indicate that the halogens have been depleted in the order I > Br > Cl > F in their melt under oxidizing conditions. For Darwin Glass selective volatilization of F from the melt is a major depleting process. Cl, Br and I are lost to a lesser extent.     

Nonlinearity effects on resonant absorption of surface Alfvén waves in incompressible plasmas

The resonant absorption of small amplitude surface Alfvén waves is studied in nonlinear incompressible MHD for a viscous and resistive plasma. The reductive perturbation method is used to obtain the equation that governs the spatial and temporal behaviour of small amplitude nonlinear surface Alfvén waves. Numerical solutions to this equation are obtained under the initial condition that att = 0 the spatial variation is purely sinusoidal. The numerical results show that nonlinearity accelerates the wave damping due to resonant absorption. Resonant absorption is a more efficient wave damping mechanism than can be anticipated on the basis of linear theory.     

Resonant behaviour of MHD waves on magnetic flux tubes

The resonances that appear in the linear compressible MHD formulation of waves are studied for equilibrium states with flow. The conservation laws and the jump conditions across the resonance point are determined for 1D cylindrical plasmas. For equilibrium states with straight magnetic field lines and flow along the field lines the conserved quantity is the Eulerian perturbation of total pressure. Curvature of the magnetic field lines and/or velocity field lines leads to more complicated conservation laws. Rewritten in terms of the displacement components in the magnetic surfaces parallel and perpendicular to the magnetic field lines, the conservation laws simply state that the waves are dominated by the parallel motions for the modified slow resonance and by the perpendicular motions for the modified Alfvén resonance.The conservation laws and the jump conditions are then used for studying surface waves in cylindrical plasmas. These waves are characterized by resonances and have complex eigenfrequencies when the classic true discontinuity is replaced by a nonuniform layer. A thin non-uniform layer is considered here in an attempt to obtain analytical results. An important result related to earlier work by Hollweg et al. (1990) for incompressible planar plasmas is found for equilibrium states with straight magnetic field lines and straight velocity field lines. For these equilibrium states the incompressible and compressible surface waves have the same frequencies at least in the long wavelength limit and there is an exact correspondence with the planar case. As a consequence, the conclusions formulated by Hollweg et al. still hold for the straight cylindrical case. The effects of curvature are subsequently considered.     

SYMPOSIUM ON EVAPORATION / SYMPOSIUM SUR L'ÉVAPORATION

SUMMARY

The geographical transposition of the results obtained from representative and experimental watersheds is a matter of regressions based upon geomorphological caracteristics of these watersheds. Therefore, the lumped model for the transformation of precipitations into runoff, generally used for small sized basins, must be simplified and standardized in order to be defined, in any case, by the same parameters. Moreover, the elements of the hydrograph, especially the base time, are to be determined by a very strict processus providing with “consistent” estimates. These new necessary conditions lead to see again the methods used till now for scientific purposes in operating the data from small watersheds, to the so-called “standard hydrograph”.     

Dynamics of the turbidity maximum zone in a macrotidal estuary (the Gironde,France): Observations from field and MODIS satellite data

Over a 1-year period, field and satellite measurements of surface water turbidity were combined in order to study the dynamics of the turbidity maximum zone (TM) in a macrotidal estuary (the Gironde, France). Four fixed platforms equipped with turbidity sensors calibrated to give the suspended particulate matter (SPM) concentration provided continuous information in the upper estuary. Full resolution data recorded by the moderate resolution imaging spectroradiometer (MODIS) sensors onboard the Terra and Aqua satellite platforms provided information in the central and lower estuary twice a day (depending on cloud cover). Field data were used to validate a recently developed SPM quantification algorithm applied to the MODIS ‘surface reflectance’ product. The algorithm is based on a relationship between the SPM concentration and a reflectance ratio of MODIS bands 2 (near-infrared) and 1 (red). Based on 62 and 75 match-ups identified in 2005 with MODIS Terra and Aqua data, the relative uncertainty of the algorithm applied to these sensors was found to be 22 and 18%, respectively.Field measurements showed the tidal variations of turbidity in the upper estuary, while monthly-averaged MODIS satellite data complemented by field data allowed observing the monthly movements of the TM in the whole estuary. The trapping of fine sediments occurred in the upper estuary during the period of low river flow. This resulted in the formation of a highly concentrated TM during a 4-month period. With increasing river flow, the TM moved rapidly to the central estuary. A part of the TM detached, moved progressively in the lower estuary and was finally either massively exported to the ocean during peak floods or temporary trapped (settled) on intertidal mudflats. The massive export to the ocean was apparently the result of combined favorable environmental conditions: presence of fluid mud near the mouth, high river flow, high tides and limited wind speeds. The mean SPM concentration within surface waters of the whole estuary showed strong seasonal variations but remained almost unchanged on a 1-year-basis. These observations suggest that the masses of suspended sediments exported toward the ocean and supplied by the rivers were almost equivalent during the year investigated (2005). Results show the usefulness of information extracted from combined field and current ocean color satellite data in order to monitor the transport of suspended particles in coastal and estuarine waters.     

Eemian landscape response to climatic shifts and evidence for northerly Neanderthal occupation at a palaeolake margin in northern Germany

The prevailing view suggests that the Eemian interglacial on the European Plain was characterized by largely negligible geomorphic activity beyond the coastal areas. However, systematic geomorphological studies are sparse. Here we present a detailed reconstruction of Eemian to Early Weichselian landscape evolution in the vicinity of a small fingerlake on the northern margin of the Salzwedel Palaeolake in Lower Saxony (Germany). We apply a combination of seismics, sediment coring, pollen analysis and luminescence dating on a complex sequence of colluvial, paludal and lacustrine sediments. Results suggest two pronounced phases of geomorphic activity, directly before the onset and at the end of the Eemian period, with an intermediate period of pronounced landscape stability. The dynamic phases were largely driven by incomplete vegetation cover, but likely accentuated by fluvial incision in the neighbouring Elbe Valley. Furthermore, we discovered Neanderthal occupation at the lakeshore during Eemian pollen zone (PZ) E IV, which is chronologically in line with other known Eemian sites of central Europe. Our highly-resolved spatio-temporal data substantially contribute to the understanding of climate-induced geomorphic processes throughout and directly after the last interglacial period. It helps unraveling the landscape dynamics between the coastal areas to the north and the loess belt to the south.     

Distribution of branched tetraether lipids in geothermally heated soils: Implications for the MBT/CBT temperature proxy

Branched glycerol dialkyl glycerol tetraether (GDGT) membrane lipids occur in soils and peat bogs and are assumed to be produced by anaerobic bacteria. Two indices based on the distribution of these lipids in soils, the Cyclisation of Branched Tetraethers (CBT) and the Methylation of Branched Tetraethers (MBT) index have been shown to linearly relate to pH, and to mean annual air temperature (MAT) and pH, respectively. To directly evaluate the impact of changes in soil temperature on the MBT/CBT proxy, we determined these indices in soils sampled from a transect away from two hot springs in California, which provided a set of soils similar in composition but with different temperatures (12–41 °C). The CBT values of these geothermally heated soils show a good relation with pH (R² 0.76), similar to that of a global MBT/CBT calibration set. Also, the relationship between MBT, soil pH and temperature for the geothermally heated soils is similar to that of a global soil calibration set, although the intercept for the geothermally heated soils is significantly lower, likely because our data set is based on in situ soil temperatures rather than MAT. The results confirm the dependence of the MBT index on soil temperature and pH and support the applicability of the MBT/CBT indices as a proxy for continental palaeotemperatures and past soil pH.     

Distribution of Crenarchaeota tetraether membrane lipids in surface sediments from the Red Sea

The Red Sea represents an extreme marine environment, with high salinity, high temperature and low level of nutrients, complicating the application of standard geochemical palaeotemperature proxies. In order to investigate the applicability of the TEX₈₆ (TetraEther indeX of GDGTs with 86 carbons) proxy for sea surface temperature (SST) in the Red Sea, the distribution of glycerol dialkyl glycerol tetraether membrane lipids (GDGTs) in sediments from the Red Sea and the Gulf of Aden were examined. Against expectations, TEX₈₆ values for the Red Sea do not show a simple linear relationship with SST and deviate from the global core top calibration. In the northern Red Sea, at temperatures between 25 and 28 °C, the values increase linearly with SST, whereas in the southern Red Sea, at temperatures above 28 °C, TEX₈₆ decreases with increasing temperature. Factors like seasonality and depth of production, salinity and nutrient availability, as well as diagenetic overprint or influence of allochtonous terrestrial lipids, cannot explain this pattern. However, the observed TEX₈₆ relationship with SST could be explained by the presence of a hypothetical endemic Crenarchaeota population in the Red Sea with a specific TEX₈₆ vs. SST relationship. In the Southern Red Sea, a two-component mixing model implies an exponential decrease in the endemic population towards the Gulf of Aden. Thus, the application of the TEX₈₆ as a palaeotemperature proxy in the Red Sea is likely only possible for the northern Red Sea area with the specific SST vs. TEX₈₆ relationship determined in this study and potentially for the whole Red Sea basin during glacials, when water exchange with the Indian Ocean was more restricted than today and the endemic archaeal lipid distribution was not affected by transport from the Indian Ocean. Our results suggest that distinct populations of Crenarchaeota in extreme environments such as evaporitic basins may have different membrane composition, necessitating application of another calibration than the global core top calibration for TEX₈₆ palaeothermometry.