The first samples from Almahata Sitta showing contacts between ureilitic and chondritic lithologies: Implications for the structure and composition of asteroid 2008 TC3     

Cyrena Anne Goodrich  Michael E. Zolensky  Anna Maria Fioretti  Muawia H. Shaddad  Hilary Downes  Takahiro Hiroi  Issaku Kohl  Edward D. Young  Noriko T. Kita  Victoria E. Hamilton  My E. I. Riebe  Henner Busemann  Robert J. Macke  M. Fries  D. Kent Ross  Peter Jenniskens 《Meteoritics & planetary science》2019,54(11):2769-2813

Almahata Sitta (AhS), an anomalous polymict ureilite, is the first meteorite observed to originate from a spectrally classified asteroid (2008 TC₃). However, correlating properties of the meteorite with those of the asteroid is not straightforward because the AhS stones are diverse types. Of those studied prior to this work, 70–80% are ureilites (achondrites) and 20–30% are various types of chondrites. Asteroid 2008 TC₃ was a heterogeneous breccia that disintegrated in the atmosphere, with its clasts landing on Earth as individual stones and most of its mass lost. We describe AhS 91A and AhS 671, which are the first AhS stones to show contacts between ureilitic and chondritic materials and provide direct information about the structure and composition of asteroid 2008 TC₃. AhS 91A and AhS 671 are friable breccias, consisting of a C1 lithology that encloses rounded to angular clasts (<10 μm to 3 mm) of olivine, pyroxenes, plagioclase, graphite, and metal‐sulfide, as well as chondrules (~130–600 μm) and chondrule fragments. The C1 material consists of fine‐grained phyllosilicates (serpentine and saponite) and amorphous material, magnetite, breunnerite, dolomite, fayalitic olivine (Fo 28‐42), an unidentified Ca‐rich silicate phase, Fe,Ni sulfides, and minor Ca‐phosphate and ilmenite. It has similarities to CI1 but shows evidence of heterogeneous thermal metamorphism. Its bulk oxygen isotope composition (δ¹⁸O = 13.53‰, δ¹⁷O = 8.93‰) is unlike that of any known chondrite, but similar to compositions of several CC‐like clasts in typical polymict ureilites. Its Cr isotope composition is unlike that of any known meteorite. The enclosed clasts and chondrules do not belong to the C1 lithology. The olivine (Fo 75‐88), pyroxenes (pigeonite of Wo ~10 and orthopyroxene of Wo ~4.6), plagioclase, graphite, and some metal‐sulfide are ureilitic, based on mineral compositions, textures, and oxygen isotope compositions, and represent at least six distinct ureilitic lithologies. The chondrules are probably derived from type 3 OC and/or CC, based on mineral and oxygen isotope compositions. Some of the metal‐sulfide clasts are derived from EC. AhS 91A and AhS 671 are plausible representatives of the bulk of the asteroid that was lost. Reflectance spectra of AhS 91A are dark (reflectance ~0.04–0.05) and relatively featureless in VNIR, and have an ~2.7 μm absorption band due to OH^? in phyllosilicates. Spectral modeling, using mixtures of laboratory VNIR reflectance spectra of AhS stones to fit the F‐type spectrum of the asteroid, suggests that 2008 TC₃ consisted mainly of ureilitic and AhS 91A‐like materials, with as much as 40–70% of the latter, and <10% of OC, EC, and other meteorite types. The bulk density of AhS 91A (2.35 ± 0.05 g cm^?3) is lower than bulk densities of other AhS stones, and closer to estimates for the asteroid (~1.7–2.2 g cm^?3). Its porosity (36%) is near the low end of estimates for the asteroid (33–50%), suggesting significant macroporosity. The textures of AhS 91A and AhS 671 (finely comminuted clasts of disparate materials intimately mixed) support formation of 2008 TC₃ in a regolith environment. AhS 91A and AhS 671 could represent a volume of regolith formed when a CC‐like body impacted into already well‐gardened ureilitic + impactor‐derived debris. AhS 91A bulk samples do not show a solar wind component, so they represent subsurface layers. AhS 91A has a lower cosmic ray exposure (CRE) age (~5–9 Ma) than previously studied AhS stones (11–22 Ma). The spread in CRE ages argues for irradiation in a regolith environment. AhS 91A and AhS 671 show that ureilitic asteroids could have detectable ~2.7 μm absorption bands.  相似文献   

A soil moisture monitoring network to assess controls on runoff generation during atmospheric river events     

Edwin Sumargo  Hilary McMillan  Rachel Weihs  Carolyn J. Ellis  Anna M. Wilson  F. Martin Ralph 《水文研究》2021,35(1):e13998

Soil moisture is a key modifier of runoff generation from rainfall excess, including during extreme precipitation events associated with Atmospheric Rivers (ARs). This paper presents a new, publicly available dataset from a soil moisture monitoring network in Northern California's Russian River Basin, designed to assess soil moisture controls on runoff generation under AR conditions. The observations consist of 2-min volumetric soil moisture at 19 sites and 6 depths (5, 10, 15, 20, 50, and 100 cm), starting in summer 2017. The goals of this monitoring network are to aid the development of research applications and situational awareness tools for Forecast-Informed Reservoir Operations at Lake Mendocino. We present short analyses of these data to demonstrate their capability to characterize soil moisture responses to precipitation across sites and depths, including time series analysis, correlation analysis, and identification of soil saturation thresholds that induce runoff. Our results show strong inter-site Pearson's correlations (>0.8) at the seasonal timescale. Correlations are strong (>0.8) during events with high antecedent soil moisture and during drydown periods, and weak (<0.5) otherwise. High event runoff ratios are observed when antecedent soil moisture thresholds are exceeded, and when antecedent runoff is high. Although local heterogeneity in soil moisture can limit the utility of point source data in some hydrologic model applications, our analyses indicate three ways in which soil moisture data are valuable for model design: (1) sensors installed at 6 depths per location enable us to identify the soil depth below which evapotranspiration and saturation dynamics change, and therefore choose model soil layer depths, (2) time series analysis indicates the role of soil moisture processes in controlling runoff ratio during precipitation, which hydrologic models should replicate, and (3) spatial correlation analysis of the soil moisture fluctuations helps identify when and where distributed hydrologic modelling may be beneficial.  相似文献   

Oribatid mite assemblages across the tree-line in western Norway and their representation in lake sediments     

Arguitxu de la Riva-Caballero  H. John B. Birks  Anne E. Bjune  Hilary H. Birks  Torstein Solh?y 《Journal of Paleolimnology》2010,44(1):361-374

Little is known about the relationships between fossil oribatid mite assemblages in lake and mire sediments and the composition, abundance, and richness of their living communities. Because oribatid mites are a relatively new area of palaeolimnological study, there is a great lack of knowledge about the taphonomic processes that might affect fossil mite assemblages. The DOORMAT (Direct Observation Of Recent Macrofossils Across Tree-line) project was designed to study the transport and deposition of oribatid mites and plant remains in the tree-line area of western Norway. The present study also compares modern oribatid assemblages with fossil oribatid assemblages in a Holocene lake-sediment sequence from the nearby Trettetj?rn, and considers the optimal location for studying fossil oribatid mites within a lake basin. Seven novel terrestrial traps (50 × 80 cm Astroturf doormats) were placed at major vegetational transitions along an altitudinal transect crossing the tree-line ecotone from 633 to 1,120 m a.s.l. at Upsete, west Norway. Three sediment traps were placed in Trettetj?rn (810 m a.sl.) at the inlet, the middle, and near the outlet. In each terrestrial trap, the oribatid assemblage was found to be characteristic of the surrounding habitat. The lake-trap analyses showed that aquatic or moist-habitat species had the highest chance of being incorporated into the lake sediments; the number of terrestrial species decreased considerably from both outlet and inlet traps to the central trap in the deepest water. The area adjacent to the inlet of Trettetj?rn would therefore be the optimal location for a sediment core for oribatid analysis. However, this conclusion is not supported when the modern trap results are compared with the Trettetj?rn sequence from the lake centre.  相似文献   

marcos, a numerical tool for the simulation of multiple time-dependent non-linear diffusive shock acceleration     

Gilles Ferrand  Turlough Downes  Alexandre Marcowith 《Monthly notices of the Royal Astronomical Society》2008,383(1):41-56

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Are fossil assemblages in a single sediment core from a small lake representative of total deposition of mite, chironomid, and plant macrofossil remains?     

Marianne Presthus Heggen  Hilary H. Birks  Oliver Heiri  John-Arvid Grytnes  H. John B. Birks 《Journal of Paleolimnology》2012,48(4):669-691

How representative of the whole-lake fossil assemblage are analyses from a single sediment core taken in the centre of a small lake? This question was addressed in five shallow Norwegian lakes that ranged in location from low-altitude, boreal-deciduous forest to mid-alpine environments. Surface-sediment samples were taken from the deepest part of each lake and in two transects running from the lake centre to shore, and analysed for mites, chironomids, and plant remains. Ordination techniques summarised patterns of variation between and within lakes. Correlations between whole-lake assemblages and water depth and sediment organic content (loss-on-ignition) were investigated. Representativeness of each sample of the whole-lake assemblage was determined by comparing Principal Components Analysis scores of the original data with those of Monte Carlo-simulated data sets, using the actual data as constraints in the simulations. The majority of samples are representative of the whole-lake assemblages. Littoral samples, however, are most frequently unrepresentative or poorly representative samples. Water depth is an important controlling variable. A sediment core from the lake centre has the highest probability of representing the whole-lake assemblage. It may, however, also yield the lowest concentrations of terrestrial remains. A sediment core from the slope is slightly more likely to be unrepresentative of the total plant macrofossil assemblage, but generally has higher concentrations of terrestrial remains. These site differences should be considered when choosing a core location. Overall, the three fossil types are deposited in similar patterns. Therefore they can be satisfactorily analysed using a single core.  相似文献   

Rapid and Intense Phosphate Desorption Kinetics When Saltwater Intrudes into Carbonate Rock     

Hilary Flower  Mark Rains  David Lewis  Jia-Zhong Zhang 《Estuaries and Coasts》2017,40(5):1301-1313

It is important to understand how phosphate sorption dynamics of coastal carbonate aquifers are affected by seawater intrusion, because many coastal aquifers are composed of carbonate rocks and subject to an increase in saltwater intrusion during relative sea-level rise. Twelve carbonate rock and unconsolidated sediment specimens were acquired from a test corehole spanning the full thickness of the Biscayne aquifer in southeastern Florida. All 12 samples exhibit low phosphorus content but variable contents of iron. Column leaching experiments were conducted with two carbonate aquifer samples, alternating between freshwater and saltwater flow. With the first influx of saltwater, phosphate concentration in leachate increased rapidly from a freshwater value of approximately 0.2 μM to peaks of between 0.8 and 1.6 μM. The phosphate concentration began to diminish as saltwater continued to flow, but sustained desorption continued for over 2 h. Overall, seawater drove sorption behavior much more than chemical composition for the aquifer rocks and sediment from the seven rock samples for which we did isotherm sorption experiments. Our results indicate that an immediate and intense pulse of phosphate desorption from carbonate rock and sediment with low phosphorus content occurs in response to an influx of seawater and that the duration of desorption will vary by layer within a single aquifer.  相似文献