ALBERT

Description: Well defined productivity-precipitation relationships of ecosystems are needed as benchmarks for the validation of land-models used for future projections. The productivity-precipitation relationship may be studied in two ways: the spatial approach relates differences in productivity to those in precipitation among sites along a precipitation gradient (the spatial fit, with a steeper slope); the temporal approach relates inter-annual productivity changes to variation in precipitation within sites (the temporal fits, with flatter slopes). Precipitation-reduction experiments in natural ecosystems represent a complement to the fits, because they can reduce precipitation below the natural range and are thus well suited to study potential effects of climate drying. Here, we analyze the effects of dry treatments in eleven multi-year precipitation-manipulation experiments, focusing on changes in the temporal fit. We expected that structural changes in the dry treatments would occur in some experiments, thereby reducing the intercept of the temporal fit and displacing the productivity-precipitation relationship downward the spatial fit. The majority of experiments (72%) showed that dry treatments did not alter the temporal fit. This implies that current temporal fits are to be preferred over the spatial fit to benchmark land-model projections of productivity under future climate within the precipitation ranges covered by the experiments. Moreover, in two experiments, the intercept of the temporal fit unexpectedly increased due to mechanisms that reduced either water- or nutrient losses. The expected decrease of the intercept was observed in only one experiment, and only when distinguishing between the late and the early phases of the experiment. This implies that we currently do not know at which precipitation-reduction level or at which experimental duration structural changes will start to alter ecosystem productivity. Our study highlights the need for experiments with multiple, including more extreme, dry treatments, to identify the precipitation boundaries within which the current temporal fits remain valid. This article is protected by copyright. All rights reserved.

Description: Dolomite occurs in a wide range of rock compositions, from peridotites to mafic eclogites and metasediments, up to mantle depths of more than 200 km. At low-temperatures dolomite is ordered ( R ), but transforms with increasing temperature into a disordered higher symmetry structure ( R c ). To understand the thermodynamics of dolomite, we have investigated temperature, pressure, kinetics, and compositional dependence of the disordering process in Fe-bearing dolomites. To avoid quench effects, in situ X-ray powder diffraction experiments were performed at 300–1350 K and 2.6–4.2 GPa. The long-range order parameter s , quantifying the degree of ordering, has been determined using structural parameters from Rietveld refinement and the normalized peak area variation of superstructure Bragg peaks characterizing structural ordering/disordering. Time-series experiments show that disordering occurs in 20–30 min at 858 K and in a few minutes at temperatures ≥999 K. The order parameter decreases with increasing temperature and X Fe . Complete disorder is attained in dolomite at ~1240 K, 100–220 K lower than previously thought, and in an ankeritic-dolomite s.s. with an X Fe of 0.43 at temperatures as low as ~900 K. The temperature-composition dependence of the disorder process was fitted with a phenomenological approach intermediate between the Landau theory and the Bragg-Williams model and predicts complete disorder in pure ankerite to occur already at ~470 K. The relatively low-temperature experiments of this study also constrain the breakdown of dolomite to aragonite+Fe-bearing magnesite at 4.2 GPa to temperature lower than ~800 K favoring an almost straight Clapeyron-slope for this disputed reaction.

Description: Although alkali-alkali earth carbonates have not been reported from mantle-derived xenoliths, these carbonates may have a substantial role in mantle metasomatic processes through lowering melting temperatures. On the Na 2 Mg(CO 3 ) 2 –K 2 Mg(CO 3 ) 2 join only the Na-end-member eitelite ( R space group), was reported in nature. The K-end-member ( R m ) readily hydrates even at low temperatures, therefore, only baylissite, K 2 Mg(CO 3 ) 2 ·4H 2 O, has been observed. Because of the role of (K,Na)Mg-double carbonates in mantle metasomatism, we performed high P-T experiments on K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 . Structure refinements were done upon compression of single crystals from 0 to 9 GPa at ambient temperature employing synchrotron radiation. Fitting the compression data to the second-order Birch-Murnaghan EoS resulted in V 0 = 396.2(4), 381.2(5), and 347.1(3) Å 3 and K 0 = 57.0(10), 54.9(13), and 68.6(13) GPa for K 2 Mg(CO 3 ) 2 , (K 1.1 Na 0.9 ) 2 Mg(CO 3 ) 2 , and Na 2 Mg(CO 3 ) 2 , respectively. These compressibilities are lower than those of magnesite and dolomite. The KMg-double carbonate transforms into a monoclinic polymorph at 8.05 GPa; the high- P phase is 1% denser than the low- P polymorph. The NaMg-double carbonate has a phase transition at ~14 GPa, but poor recrystallization has prevented structure refinement. The parameters for a V-T EoS were collected at 25–600 °C and ambient pressure and are α 0 = 14.31(5) x 10 –5 K –1 and 16.73(11) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively. Moreover, fitting revealed an anisotropy of thermal expansion along the a - and c -axis: α 0 ( a ) = 2.84(6) x 10 –5 and 4.78(5) x 10 –5 K –1 and α 0 ( c ) = 10.47(11) x 10 –5 and 8.72(5) x 10 –5 K –1 for K 2 Mg(CO 3 ) 2 and Na 2 Mg(CO 3 ) 2 , respectively.

Description: Ionization of the atmosphere due to precipitating solar energetic particles as well as magnetospheric particles is a major source of thermospheric electron density. In this paper we evaluate numerical simulations of the 3-D spatial and temporal electron densities produced by these particle populations through a comparison with incoherent scatter radar observations. The 3-D precipitation patterns are determined with the Atmospheric Ionization Module Osnabrück (AIMOS). We use a version of the general circulation and chemistry model Hamburg Model of the Neutral and Ionized Atmosphere (HAMMONIA) enhanced by ion chemistry to calculate the impact of particle ionization on the electron density. These modeled data are compared to radar observations from European Incoherent Scatter Svalbard and Tromsø as well as the incoherent scatter radar stations at Millstone Hill and Sondrestrom. Particle precipitation is severely affected by geomagnetic disturbance and latitude. Therefore, different locations (inside the polar cap and at auroral latitudes) and geomagnetic conditions are included in the comparison. The main results of the paper can be summarized as follows: (1) as expected, particle forcing will significantly improve modeled electron density in comparison to results of the radar measurements; (2) in particular nighttime comparisons of the electron density are affected; here the particle forcing will account for a boost of 2 to 3 orders of magnitude.

Description: The Mars Science laboratory (MSL) made a successful landing at Gale crater early August 2012. MSL has an environmental instrument package called the Rover Environmental Monitoring Station (REMS) as a part of its scientific payload. REMS comprises instrumentation for the observation of atmospheric pressure, temperature of the air, ground temperature, wind speed and direction, relative humidity (REMS-H), and UV measurements. We concentrate on describing the REMS-H measurement performance and initial observations during the first 100 MSL sols as well as constraining the REMS-H results by comparing them with earlier observations and modelling results. The REMS-H device is based on polymeric capacitive humidity sensors developed by Vaisala Inc. and it makes use of transducer electronics section placed in the vicinity of the three (3) humidity sensor heads. The humidity device is mounted on the REMS boom providing ventilation with the ambient atmosphere through a filter protecting the device from airborne dust. The final relative humidity results appear to be convincing and are aligned with earlier indirect observations of the total atmospheric precipitable water content. The water mixing ratio in the atmospheric surface layer appears to vary between 30 and 75 ppm. When assuming uniform mixing, the precipitable water content of the atmosphere is ranging from a few to 6 precipitable micrometers.

Description: We have developed a probabilistic model to simulate the fate and transport of non-conservative constituents in urban watersheds. The approach implemented here extends previous studies that rely on the Geomorphological Instantaneous Unit Hydrograph concept to include non-conservative constituents. This is implemented with a factor χ that affects the transfer functions and therefore accounts for the loss (gain) of mass associated with the constituent as it travels through the watershed. Using this framework we developed an analytical solution for the dynamics of dissolved oxygen (DO) and biochemical oxygen demand (BOD) in urban networks based on the Streeter and Phelps model. This model breaks-down the catchment into a discreet number of possible flow paths through the system, requiring less data and implementation effort than well-established deterministic models. Application of the model to one sewer catchment in the Chicago area with available BOD information proved its ability to predict the BOD concentration observed in the measurements. In addition, comparison of the model with a calibrated Storm Water Management Model (SWMM) of another sewer catchment from the Chicago area showed that the model predicted the BOD concentration as well as the widely accepted SWMM. The developed model proved to be a suitable alternative to simulate the fate and transport of constituents in urban catchments with limited and uncertain input data. This article is protected by copyright. All rights reserved.

Description: Continuous monitoring of the atmospheric boundary layer (ABL) depth ( z i ) is important for investigations of trace gases with near-surface sources. This study investigates the temporal variability of z i on both diurnal and seasonal time scales over a full year (2011) and relates these changes to the atmospheric 222 Rn concentrations ( C Rn ) measured near the top of a 200 m tower at a rural site (Trainou) in France. Continuous z i estimates were made using a combination of lidar and hourly 4-height carbon dioxide (CO 2 ) profile measurements. Over the diurnal cycle, the 180 m C Rn reached a maximum in the late morning as the growing ABL passed through the inlet height (180 m) transporting upward high C Rn air from the nocturnal boundary layer. During late afternoon, a minimum in the C Rn occurred mainly due to ABL-mixing. We argue that ABL-dilution occurs in two stages: firstly, during the rapid morning growth into the residual layer, and secondly, during afternoon with the free atmosphere when z i has reached its quasi-stationary height (around 750 m in winter, or 1700 m in summer). An anti-correlation (R 2 of −0.49) was found while performing a regression analysis between the daily z i growth rates and the corresponding changes in the C Rn illustrating ABL-dilution effect. We also investigated the numerical proportions of the time within a season when z i remained lower than the inlet height and found a clear seasonal variability for the nighttime measurements with higher number of cases with shallow z i (〈200 m) in winter (67.3 %) than in summer (33.9 %) and spring (54.5 %). Thus, this pilot study helps delineate the impact of z i on C Rn at the site mainly for different regimes of ABL, in particular, during the times when the z i is above the measurement height. It is suggested that when the z i is well below the inlet height, measurements are most possibly indicative of the residual layer 222 Rn; an important issue that should be considered in the mass budget approach.