Biomarker and compound-specific δ<Superscript>13</Superscript>C evidence for changing environmental conditions and carbon limitation at Lake Koucha,eastern Tibetan Plateau

A sediment core from Lake Koucha (eastern Tibetan Plateau) was investigated using organic biomarkers and their stable carbon isotope signatures. The correlation between TOC content, total amount of aquatic macrophyte-derived n-alkanes (e.g. nC₂₃) and δ¹³C values of TOC and nC₂₃ indicates that Lake Koucha was macrophyte-dominated before 8 cal ka BP. Shortly after the lake turned from a saline to a freshwater system at 7.2 cal ka BP, a variety of algal and bacterial markers such as hopanoids and isoprenoids emerged, of which phytane, pentamethylicosene (PMI), moretene and diploptene are particularly abundant. Phytane and PMI show different isotopic signals (≈−18 and ≈−28‰, respectively), which indicates that they originated from different sources. Phytane may have been derived from cyanobacteria, while methanogenic archaea may be the source of PMI. The isotopic depletion of diploptene and moretene (≈−60‰) indicates the presence of methanotrophs. After 6.1 cal ka BP, the saturated C₂₀ highly branched isoprenoid (HBI) became the dominant constituent of the aliphatic hydrocarbon fraction. Such dominance has rarely been reported in lacustrine environments, and indicates a strong presence of algae (most likely diatoms) or cyanobacteria. At 4.7 cal ka BP, the appearance of an unsaturated C₂₅ HBI, which is a specific biomarker for diatoms, was noted. Furthermore, the level of nC₁₇-alkane was observed to increase in abundance in the uppermost two samples. These results suggest that the lake was phytoplankton-dominated during the last 6.1 ka. Relatively low biomarker concentrations and δ¹³C values at 6.0, 3.1 and 1.8 cal ka BP indicate the occurrence of cool periods, which is in agreement with inferences from other locations on the Tibetan Plateau. The δ¹³C values of nC₂₃ range from −23.5 to −12.6‰, with high values at the peak of macrophyte abundance at ca. 11 cal ka BP and at the phytoplankton maximum between ca 6.1 and 2.8 cal ka BP. Thus, aquatic macrophyte-derived mid-chain n-alkanes have been found to be excellent indicators of carbon-limiting conditions, which lead to the assimilation of isotopically-enriched carbon species. The limitation of carbon sources could be a localized phenomenon occurring in dense plant stands (as in the older section of the core), or it may be induced by high primary productivity (as in the younger section). Since the δ¹³C value of the inorganic carbon source may vary, the offset between the δ¹³C values of nC₂₃ and TIC could serve as a more precise proxy for carbon-limiting conditions in lacustrine environments, which could in turn be interpreted with respect to lacustrine paleo-productivity.     

Modelling the impact of agroforestry on hydrology of Mara River Basin in East Africa

Land‐use change is one of the main drivers of watershed hydrology change. The effect of forestry related land‐use changes (e.g. afforestation, deforestation, agroforestry) on water fluxes depends on climate, watershed characteristics and spatial scale. The Soil and Water Assessment Tool (SWAT) model was calibrated, validated and used to simulate the impact of agroforestry on the water balance in the Mara River Basin (MRB) in East Africa. Model performance was assessed by Nash–Sutcliffe Efficiency (NSE) and Kling–Gupta Efficiency (KGE). The NSE (and KGE) values for calibration and validation were: 0.77 (0.88) and 0.74 (0.85) for the Nyangores sub‐watershed, and 0.78 (0.89) and 0.79 (0.63) for the entire MRB. It was found that agroforestry in the watershed would generally reduce surface runoff, mainly because of enhanced infiltration. However, it would also increase evapotranspiration and consequently reduce baseflow and overall water yield, which was attributed to increased water use by trees. Spatial scale was found to have a significant effect on water balance; the impact of agroforestry was higher at the smaller headwater catchment (Nyangores) than for the larger watershed (entire MRB). However, the rate of change in water yield with an increase in area under agroforestry was different for the two and could be attributed to the spatial variability of climate within the MRB. Our results suggest that direct extrapolation of the findings from a small sub‐catchment to a larger watershed may not always be accurate. These findings could guide watershed managers on the level of trade‐offs that might occur between reduced water yields and other benefits (e.g. soil erosion control, improved soil productivity) offered by agroforestry. Copyright © 2016 John Wiley & Sons, Ltd.     

Evolution of X-ray cavities

A wide range of recent observations have shown that active galactic nuclei (AGN) driven cavities may provide the energy source that balances the cooling observed in the centres of 'cool-core' galaxy clusters. One tool for better understanding the physics of these cavities is their observed morphological evolution, which is dependent on such poorly understood properties as the turbulent density field and the impact of magnetic fields. Here, we combine numerical simulations that include subgrid turbulence and software that produces synthetic X-ray observations to examine the evolution of X-ray cavities in the absence of magnetic fields. Our results reveal an anisotropic size evolution of the cavities that is dramatically different from simplified, analytical predictions. These differences highlight some of the key issues that must be accurately quantified when studying AGN-driven cavities, and help to explain why the inferred pV energy in these regions appears to be correlated with their distance from the cluster centre. Interpreting that X-ray observations will require detailed modelling of effects, including mass entrainment, distortion by drag forces and projection. Current limitations do not allow a discrimination between purely hydrodynamic and magnetically dominated models for X-ray cavities.     

Sensitivity of European glaciers to precipitation and temperature – two case studies

A nonlinear backpropagation network (BPN) has been trained with high-resolution multiproxy reconstructions of temperature and precipitation (input data) and glacier length variations of the Alpine Lower Grindelwald Glacier, Switzerland (output data). The model was then forced with two regional climate scenarios of temperature and precipitation derived from a probabilistic approach: The first scenario (“no change”) assumes no changes in temperature and precipitation for the 2000–2050 period compared to the 1970–2000 mean. In the second scenario (“combined forcing”) linear warming rates of 0.036–0.054°C per year and changing precipitation rates between −17% and +8% compared to the 1970–2000 mean have been used for the 2000–2050 period. In the first case the Lower Grindelwald Glacier shows a continuous retreat until the 2020s when it reaches an equilibrium followed by a minor advance. For the second scenario a strong and continuous retreat of approximately −30 m/year since the 1990s has been modelled. By processing the used climate parameters with a sensitivity analysis based on neural networks we investigate the relative importance of different climate configurations for the Lower Grindelwald Glacier during four well-documented historical advance (1590–1610, 1690–1720, 1760–1780, 1810–1820) and retreat periods (1640–1665, 1780–1810, 1860–1880, 1945–1970). It is shown that different combinations of seasonal temperature and precipitation have led to glacier variations. In a similar manner, we establish the significance of precipitation and temperature for the well-known early eighteenth century advance and the twentieth century retreat of Nigardsbreen, a glacier in western Norway. We show that the maritime Nigardsbreen Glacier is more influenced by winter and/or spring precipitation than the Lower Grindelwald Glacier.     

Multiplier hysteresis and image motion errors in limb darkening scans obtained in April 1981 (and later)

In response to the comment of P. Foukal (1989) we show that in our limb darkening observations multiplier hysteresis and image motion (seeing) can be ruled out as major sources of error.     

Verdreifachte Achsenbilder an den Magnesitkristallen von Asturreta in Spanien

Zusammenfassung Die nach einer Scheinbasis tafeligen Magnesitkristalle von Asturreta (und ebenso ähnliche Dolomitkristalle aus den Magnesitlagerstätten Sunk, Oberdorf und Lanersbach) liefern bei Einbettung in den meisten üblichen Immersionsmitteln bei konoskopischer Beobachtung nicht ein, sondern drei einachsige Achsenbilder. Die Erscheinung wird nicht etwa durch Verzwillingung verursacht, sondern, da die Scheinbasis aus zahlreichen winzigen, streng parallelen r aufgebaut ist, durch einen Summeneffekt der symmetriebedingten -Flächenlagen in Abhängigkeit vom Einbettungsmittel. Das konnte durch Rechnung und Messung bewiesen werden.Mit 4 Textabbildungen.     

On the influence of non-Gaussianity on turbulent transport

Non-Gaussianity effects, first of all the influence of the third and fourth moments of the velocity probability density function, have to be assessed for higher-order closure models of turbulence and Lagrangian modelling of turbulent dispersion in complex flows. Whereas the role and the effects of the third moments are relatively well understood as essential for the explanation of specific observed features of the fully developed convective boundary layer, there are indications that the fourth moments may also be important, but little is known about these moments. Therefore, the effects of non-Gaussianity are considered for the turbulent motion of particles in non-neutral flows without fully developed convection, where the influence of the fourth moments may be expected to be particularly essential. The transport properties of these flows can be characterized by a diffusion coefficient which reflects these effects. It is shown, for different vertical velocity distributions, that the intensity of turbulent transport may be enhanced remarkably by non-Gaussianity. The diffusion coefficient is given as a modification of the Gaussian diffusivity, and this modifying factor is found to be determined to a very good approximation by the normalized fourth moment of the vertical velocity distribution function. This provides better insight into the effect of fourth moments and explains the varying importance of third and fourth moments in different flows.     

Water Budgets of Two Watersheds in Different Climatic Zones under Projected Climate Warming

A deterministic monthly runoff model (MINRUN96)was applied to watersheds with substantially differentclimates. One watershed is in the north-central U.S.(Minnesota) and is heavily timbered. The other is inthe south-central U.S. (Oklahoma) and is mainlycovered with pastures and agricultural crops. Runoffwas simulated for past historical climate and twoprojected 2 × CO₂ climate scenarios. The output ofGeneral Circulation Models (GCMs) was used to specifythe two 2 × CO₂ climate scenarios. One GCM is theGoddard Institute of Space Studies (GISS) model andthe other is from the Canadian Center of ClimateModelling (CCC). In the northern watershed morerunoff is projected to occur in winter under a warmerclimate and less runoff in spring. About 80%increase in fall runoff and 20% decrease in soilmoisture in June and July is projected for thesouthern watershed. When runoff simulations for the2 × CO₂ climate scenarios were compared to pastrunoff, it was apparent that the change in runoffdepended on both the season and the magnitude of theprecipitation change. An increase in springprecipitation caused a significant increase in directrunoff, whereas an increase in fall precipitationcaused only a slight increase in total runoff. Alsothe runoff-precipitation relationship in the warm andseasonally dry southern watershed is very differentfrom that in the temperate and humid climate of thenorth. Therefore, runoff responses to projectedclimate change are substantially different in the tworegions.