THE CORREO AND SUWANEE SPRING METEORITES: TWO NEW ORDINARY CHONDRITE FINDS

Two new ordinary chondrites were found about 40 km west of Albuquerque, New Mexico. Correo is an H4 chondrite with distinct chondrules and major olivine (Fo_81.4), orthopyroxene (En_82.3) and plagioclase (An₁₂). Suwanee Spring is an L5 chondrite with few distinct chondrules and a highly recrystallized matrix. Major minerals are olivine (Fo_75.4), orthopyroxene (En_77.7) and plagioclase (An₉). The metallic Ni-Fe phases of both meteorites are typical of slowly-cooled ordinary chondrites.     

Upper limits in the toroidal component of force-free magnetic fields in stellar atmospheres in the context of solar and stellar flares

Doing numerical calculations of axially symmetric force-free fields, Milsom and Wright (1976) have noticed that there seem to be no solutions if the toroidal component of the field exceeds a certain limit. In the present paper this problem is reexamined in the approximation of a plane stellar surface using a very simple analytic approximation. The results of Milsom and Wright (1976) are confirmed but, in contrast to their interpretation, it is shown that these limitations do not indicate the possibility of sudden changes of the topology of the magnetic field. This is because in a stellar atmosphere the toroidal component of the surface magnetic field is no independent quantity but is produced by shearing motions in the star which will prevent the toroidal magnetic field from exceeding its maximum value. To study the possibility of sudden changes in the magnetic field, which could cause stellar flares, the calculations are re-done prescribing the motion of the magnetic footpoints (shear in the stellar surface) instead of the toroidal component of the surface field. Using the same mathematical formalism it is found that no sudden changes can occur for configurations where all field lines connect to the stellar surface but that sudden changes may be possible for a more complicated field topology.     

‘Modelling the Arctic Boundary Layer: An Evaluation of Six Arcmip Regional-Scale Models using Data from the Sheba Project’

A primary climate change signal in the central Arctic is the melting of sea ice. This is dependent on the interplay between the atmosphere and the sea ice, which is critically dependent on the exchange of momentum, heat and moisture at the surface. In assessing the realism of climate change scenarios it is vital to know the quality by which these exchanges are modelled in climate simulations. Six state-of-the-art regional-climate models are run for one year in the western Arctic, on a common domain that encompasses the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment ice-drift track. Surface variables, surface fluxes and the vertical structure of the lower troposphere are evaluated using data from the SHEBA experiment. All the models are driven by the same lateral boundary conditions, sea-ice fraction and sea and sea-ice surface temperatures. Surface pressure, near-surface air temperature, specific humidity and wind speed agree well with observations, with a falling degree of accuracy in that order. Wind speeds have systematic biases in some models, by as much as a few metres per second. The surface radiation fluxes are also surprisingly accurate, given the complexity of the problem. The turbulent momentum flux is acceptable, on average, in most models, but the turbulent heat fluxes are, however, mostly unreliable. Their correlation with observed fluxes is, in principle, insignificant, and they accumulate over a year to values an order of magnitude larger than observed. Typical instantaneous errors are easily of the same order of magnitude as the observed net atmospheric heat flux. In the light of the sensitivity of the atmosphere–ice interaction to errors in these fluxes, the ice-melt in climate change scenarios must be viewed with considerable caution.     

The response of flow duration curves to afforestation

The hydrologic effect of replacing pasture or other short crops with trees is reasonably well understood on a mean annual basis. The impact on flow regime, as described by the annual flow duration curve (FDC) is less certain. A method to assess the impact of plantation establishment on FDCs was developed. The starting point for the analyses was the assumption that rainfall and vegetation age are the principal drivers of evapotranspiration. A key objective was to remove the variability in the rainfall signal, leaving changes in streamflow solely attributable to the evapotranspiration of the plantation. A method was developed to (1) fit a model to the observed annual time series of FDC percentiles; i.e. 10th percentile for each year of record with annual rainfall and plantation age as parameters, (2) replace the annual rainfall variation with the long term mean to obtain climate adjusted FDCs, and (3) quantify changes in FDC percentiles as plantations age. Data from 10 catchments from Australia, South Africa and New Zealand were used. The model was able to represent flow variation for the majority of percentiles at eight of the 10 catchments, particularly for the 10–50th percentiles. The adjusted FDCs revealed variable patterns in flow reductions with two types of responses (groups) being identified. Group 1 catchments show a substantial increase in the number of zero flow days, with low flows being more affected than high flows. Group 2 catchments show a more uniform reduction in flows across all percentiles. The differences may be partly explained by storage characteristics. The modelled flow reductions were in accord with published results of paired catchment experiments. An additional analysis was performed to characterise the impact of afforestation on the number of zero flow days (N_zero) for the catchments in group 1. This model performed particularly well, and when adjusted for climate, indicated a significant increase in N_zero. The zero flow day method could be used to determine change in the occurrence of any given flow in response to afforestation. The methods used in this study proved satisfactory in removing the rainfall variability, and have added useful insight into the hydrologic impacts of plantation establishment. This approach provides a methodology for understanding catchment response to afforestation, where paired catchment data is not available.     

Ultrahigh pressure metamorphic rocks from the Chinese Continental Scientific Drilling Project: I. Petrology and geochemistry of the main hole (0–2,050 m)

The main hole (MH) of the Chinese Continental Scientific Drilling Project (CCSD) in southern Sulu has penetrated into an ultrahigh-pressure (UHP) metamorphic rock slice which consists of orthogneiss, paragneiss, eclogite, ultramafic rock and minor schist. Recovered eclogites have a UHP metamorphic mineral assemblage of garnet + omphacite + rutile ± phengite ± kyanite ± coesite ± epidote. Ultramafic rocks contain garnet + olivine + clinopyroxene + orthopyroxene ± Ti-clinohumite ± phlogopite. Gneisses and schists contain an amphibolite-facies paragenesis, but their zircons have coesite, garnet, omphacite (or jadeite) and phengite inclusions, indicating that eclogites and gneisses have been subjected to in situ UHP metamorphism. Using available geothermobarometers, P–T estimates of 3.1–4.4 GPa and 678–816°C for eclogites were obtained. If surface outcrops and neighboring shallow drill holes are considered together, we suggest that a huge supracrustal rock slab (> 50 km long × 100 km wide × 5 km deep) was subducted to a depth > 100 km and then exhumed to the surface. The depth interval (0–2,050 m) of the CCSD-MH can be divided into six lithological units. Unit 1 consists of alternating layers of quartz-rich and rutile-rich eclogites, with thin interlayers of gneiss and schist. Eclogites of unit 1 are characterized by Nb, Ta, Sr and Ti depletions, low Mg number and general LREE enrichment. Unit 2 comprises rutile- and ilmenite-rich eclogite and minor “normal” eclogite and is characterized by high TiO₂, total Fe, V, Co and Sr, and very low SiO₂, alkali, Zr, Ba, Nb, Ta and total REE contents, and LREE-depleted REE patterns with slightly positive Eu anomalies. Unit 3 contains ultramafic rock and minor MgO-rich eclogite. Protoliths of UHP rocks from units 1, 2 and 3 represent a layered mafic to ultramafic intrusion at crustal depth. Units 4 and 6 consist of interlayered eclogite and paragneiss; the eclogites are characterized by Th, U, Nb, Ta and Ti depletion and K enrichment and LREE-enriched REE patterns. Paragneisses show Nb, Ta, Sr and Ti depletions and LREE-enriched REE patterns occasionally with slightly negative Eu anomalies, indicating that their protoliths represent metamorphic supracrustal series. Unit 5 consists mainly of orthogneisses, showing distinct Nb, Ta, Sr and Ti depletions, and LREE-enriched REE patterns with pronounced negative Eu anomalies, suggesting granitic protoliths. In conclusion it is proposed that the southern Sulu UHP belt consists of a series of meta-supracrustal rocks, a layered mafic–ultramafic complex and granites.     

Spatial and seasonal variability of polygonal tundra water balance: Lena River Delta, northern Siberia (Russia)

The summer water balance of a typical Siberian polygonal tundra catchment is investigated in order to identify the spatial and temporal dynamics of its main hydrological processes. The results show that, besides precipitation and evapotranspiration, lateral flow considerably influences the site-specific hydrological conditions. The prominent microtopography of the polygonal tundra strongly controls lateral flow and storage behaviour of the investigated catchment. Intact rims of low-centred polygons build hydrological barriers, which release storage water later in summer than polygons with degraded rims and troughs above degraded ice wedges. The barrier function of rims is strongly controlled by soil thaw, which opens new subsurface flow paths and increases subsurface hydrological connectivity. Therefore, soil thaw dynamics determine the magnitude and timing of subsurface outflow and the redistribution of storage within the catchment. Hydraulic conductivities in the elevated polygonal rims sharply decrease with the transition from organic to mineral layers. This interface causes a rapid shallow subsurface drainage of rainwater towards the depressed polygon centres and troughs. The re-release of storage water from the centres through deeper and less conductive layers helps maintain a high water table in the surface drainage network of troughs throughout the summer.     

Acapulcoite-lodranite meteorites: Ultramafic asteroidal partial melt residues

Acapulcoites (most ancient Hf-W ages are 4,563.1?±?0.8 Ma), lodranites (most ancient Hf-W ages are 4,562.6?±?0.9 Ma) and rocks transitional between them are ancient residues of different degrees of partial melting of a chondritic source lithology (e.g., as indicated by the occurrence of relict chondrules in 9 acapulcoites), although the precise chondrite type is unknown. Acapulcoites are relatively fine- grained (～150–230?μm) rocks with equigranular, achondritic textures and consist of olivine, orthopyroxene, Ca-rich clinopyroxene, plagioclase, metallic Fe,Ni, troilite, chromite and phosphates. Lodranites are coarser grained (540–700?μm), with similar equigranular, recrystallized textures, mineral compositions and contents, although some are significantly depleted in eutectic Fe,Ni-FeS and plagioclase- clinopyroxene partial melts. The acapulcoite-lodranite clan is most readily distinguished from other groups of primitive achondrites (e.g., winoanites/IAB irons) by oxygen isotopic compositions, although more than 50% of meteorites classified as acapulcoites currently lack supporting oxygen isotopic data. The heat source for melting of acapulcoites-lodranites was internal to the parent body, most likely ²⁶Al, although some authors suggest it was shock melting. Acapulcoites experienced lower temperatures of ～980–1170?°C and lower degrees of partial melting (～1–4?vol.%) and lodranites higher temperatures of ～1150–1200?°C and higher degrees (～5?≥?10?vol.%) of partial melting. Hand-specimen and thin section observations indicate movement of Fe,Ni-FeS, basaltic, and phosphate melts in veins over micrometer to centimeter distances. Mineralogical, chemical and isotopic properties, Cosmic Ray Exposure (CRE) ages which cluster around 4–6 Ma and the occurrence of some meteorites consisting of both acapulcoite and lodranite material, indicate that these meteorites come from one parent body and were most likely ejected in one impact event. Whereas the precise parent asteroid of these meteorites is unknown, there is general agreement that it was an S-type object. There is nearly total agreement that the acapulcoite-lodranite parent body was <～100?km in radius and, based on the precise Pb–Pb age for Acapulco of 4555.9?±?0.6 Ma, combined with the Hf/W and U/Pb records and cooling rates deduced from mineralogical and other investigations, that the parent body was fragmented during its cooling which the U/Pb system dates at precisely 4556?±?1 Ma. Hf-W chronometry suggests that the parent body of the acapulcoites-lodranites and, in fact, the parent bodies of all “primitive achondrites” accreted slightly later than those of the differentiated achondrites and, thus, had lower contents of ²⁶Al, the heat producing radionuclide largely responsible for heating of both primitive and differentiated achondrites. Thus, the acapulcoite-lodranite parent body never experienced the high degrees of melting responsible for the formation of the differentiated meteorites, but arrested its melting history at relatively low degrees of ～15?vol.%.     

Characterizing Major Controls on Spatial and Seasonal Variations in Chemical Composition of Surface and Pore Brine of Maharlu Lake,Southern Iran

Maharlu Lake with Na–Cl water type is the terminal point of a closed basin in southern Iran. A total of 10 water samples from two rivers discharging to the lake and 78 water samples of surface and pore brine of Maharlu Lake have been collected from different depths (surface, 20, 50 and 100 cm) of four sampling stations along the lake during a period of lake water-level fluctuation (November 2014–July 2015). To investigate chemical interaction between lake surface water and shallow pore water and to understand the major factors governing chemical composition of Maharlu brine, concentrations of major and minor (boron, bromide and lithium) solutes, pH and total dissolved solids have been measured in collected water samples. Saturation indices of evaporite minerals in collected water samples have been also calculated. The chemical behavior of dissolved solutes and evaporative evolution of the lake brine during a hydrological period have been simulated using PHREEQC. The results of our investigations indicated that chemical composition of lake surface water and pore brine of Maharlu Lake are mainly connected with lake water-level fluctuations and distance from input rivers (and depth), respectively. Hydrochemical investigations and statistical analysis showed that the brines chemistry of Maharlu is mainly controlled by three processes: brine evaporative evolution, dissolution–precipitation and diagenetic evolution of secondary carbonates.