Late Holocene Neoglacial conditions from the Lesotho highlands, southern Africa: phytolith and stable carbon isotope evidence from the archaeological site of Likoaeng     

Adrian G. Parker  Julia Lee-Thorp  Peter J. Mitchell 《Proceedings of the Geologists' Association. Geologists' Association》2011,122(1):201-211

The late Holocene environmental history of the Lesotho highlands, southern Africa, is poorly understood with few detailed studies to date. At Likoaeng, Senqu Valley, Lesotho, a 3 m stratified sedimentary sequence from an open-air archaeological site records vegetation development for the period 3400-1070 cal. BP. Phytolith analyses and bulk sediment organic matter δ¹³C indicate that C₄ grassland dominated the lower part of the sequence until approximately 2960 cal. BP when there was a switch to C₃ Pooid grassland (2960-2160 cal. BP). Also noted was a change from hunting mainly bovids to a dominance of fishing at the site. The change in grassland type and archaeological subsistence strategies corresponds with an episode of neoglacial cooling and the expansion of Alpine sourgrasses into lower altitudes. From 2160 to 1600 cal. BP grassland became a mix of C₃ and C₄ types and by 1600-1070 cal. BP there was a return to C₄ dominated grassland. During this latter phase there was a reversal from fishing to hunting again (and eventually some keeping of domestic livestock) at the site. These data outline the vegetation response to latitudinal shifts of frontal systems, and relatively strong atmospheric circulation variability, perhaps underpinned by variations of polar water into the Benguela Current during the late Holocene.  相似文献   

Land use change and resource management in the Jnane Mas valley (Khenifra,Middle Atlas,Morocco)     

Brahim Akdim  Ramon Julia  Mohamed Laaouane 《Geography and Natural Resources》2011,32(1):87-94

The purpose of this paper is to bring into light the landscape change and natural resources management in the Moroccan Middle Atlas across centuries. The pastoral way of life prevailed for many centuries. However, following the results of this research, evidence of important socioeconomic changes and resources use is found. Ancient agricultural activities, as well as the exploitation of the forest and even mineral resources have been growing activities over time. The survey shows that the agricultural activity has existed for several centuries as indicated by terraces and encrusted travertine channels used for irrigation. The landscapes and the ways of resources use are therefore in continuous change. The increasing needs in water and agriculture for demographic and climatic reasons (the dry periods in summer or periodic droughts) generated because of intensive over-exploitation of resources. The vegetation degraded facing human pressure (the pastoral activities, the coal of wood making, the lime manufacturing, etc.). Field observations show that irrigation has been used in this domain, for several centuries. Travertine channels, encrusted in the valleys of Jnane Mas and Arougou, illustrate these ancient uses of water resources for agriculture irrigation.  相似文献   

Composition and petrology of HED polymict breccias: The regolith of (4) Vesta     

David W. Mittlefehldt  Jason S. Herrin  Julie E. Quinn  Stanley A. Mertzman  Julia A. Cartwright  Karen R. Mertzman  Zhan X. Peng 《Meteoritics & planetary science》2013,48(11):2105-2134

We have done petrologic and compositional studies on a suite of polymict eucrites and howardites to better understand regolith processes on their parent asteroid, which we accept is (4) Vesta. Taking into account noble gas results from companion studies, we interpret five howardites to represent breccias assembled from the true regolith: Elephant Moraine (EET) 87513, Grosvenor Mountains (GRO) 95535, GRO 95602, Lewis Cliff (LEW) 85313, and Meteorite Hills (MET) 00423. We suggest that EET 87503 is paired with EET 87513, and thus is also regolithic. Pecora Escarpment (PCA) 02066 is dominated by melt‐matrix clasts, which may have been formed from true regolith by impact melting. These meteorites display a range in eucrite:diogenite mixing ratio from 55:45 to 76:24. There is no correlation between degree of regolith character and Ni content. The Ni contents of howardite, eucrite, and diogenites (HEDs) are mostly controlled by the distribution of coarse chondritic clasts and metal grains, which in some cases resulted from individual, low‐velocity accretion events, rather than extensive regolith gardening. Trace element compositions indicate that the mafic component of HED polymict breccias is mostly basalt similar to main‐group eucrites; Stannern‐trend basaltic debris is less common. Pyroxene compositions show that some trace element‐rich howardites contain abundant debris from evolved basalts, and that cumulate gabbro debris is present in some breccias. The scale of heterogeneity varies considerably; regolithic howardite EET 87513 is more homogeneous than fragmental howardite Queen Alexandra Range (QUE) 97001. Individual samples of a given howardite can have different compositions even at roughly 5 g masses, indicating that obtaining representative meteorite compositions requires multiple or large samples.  相似文献   

The X-ray variability of the Seyfert 1 galaxy MCG–6-30-15 from long ASCA and RXTE observations     

Julia C. Lee  †  A. C. Fabian  Christopher S. Reynolds  ‡  W. N. Brandt  Kazushi Iwasawa 《Monthly notices of the Royal Astronomical Society》2000,318(3):857-874

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The brecciated texture of polymict eucrites: Petrographic investigations of unequilibrated meteorites from the Antarctic Yamato collection     

Christian Vollmer  Stella Rombeck  Julia Roszjar  Adam R. Sarafian  Stephan Klemme 《Meteoritics & planetary science》2020,55(3):558-574

We report on the petrography and mineralogy of five Yamato polymict eucrites to better constrain the formation and alteration of crustal material on differentiated asteroids. Each sample consists of different lithic clasts that altogether form four dominant textures and therefore appear to originate from closely related petrological areas within Vesta′s crust. The textures range from subophitic to brecciated, porphyritic, and quench‐textured, that differ from section to section. Comparison with literature data for these samples is therefore difficult, which stresses that polymict eucrites are extremely complex in their petrography and investigation of only one thick section may not be representative for the host rock. We also show that sample Y‐793548 consists of more than one lithic unit and must therefore be classified as polymict instead of monomict. The variety and nature of lithic textures in the investigated Yamato meteorites indicate shock events, intense post‐magmatic thermal annealing, and secondary alteration. These postmagmatic features occur in different intensities, varying from clast to clast or among coexisting mineral fragments on a small, local scale. Several clasts within the eucrites studied have been modified by late‐stage alteration processes that caused deposition of Fe‐rich olivine and Fe enrichment along cracks crosscutting pyroxene crystals. However, formation of these secondary phases seems to be independent of the degree of thermal metamorphism observed within every type of clast, which would support a late‐stage metasomatism model for their formation.  相似文献   

The Allende multicompound chondrule (ACC)—Chondrule formation in a local super‐dense region of the early solar system          下载免费PDF全文

Addi Bischoff  Gerhard Wurm  Marc Chaussidon  Marian Horstmann  Knut Metzler  Mona Weyrauch  Julia Weinauer 《Meteoritics & planetary science》2017,52(5):906-924

In Allende, a very complex compound chondrule (Allende compound chondrule; ACC) was found consisting of at least 16 subchondrules (14 siblings and 2 independents). Its overall texture can roughly be described as a barred olivine object (BO). The BO texture is similar in all siblings, but does not exist in the two independents, which appear as relatively compact olivine‐rich units. Because of secondary alteration of pristine Allende components and the ACC in particular, only limited predictions can be made concerning the original compositions of the colliding melt droplets. Based on textural and mineralogical characteristics, the siblings must have been formed on a very short time scale in a dense, local environment. This is also supported by oxygen isotope systematics showing similar compositions for all 16 subchondrules. Furthermore, the ACC subchondrules are isotopically distinct from typical Allende chondrules, indicating formation in or reaction with a more ¹⁶O‐poor reservoir. We modeled constraints on the particle density required at the ACC formation location, using textural, mineral‐chemical, and isotopic observations on this multicompound chondrule to define melt droplet collision conditions. In this context, we discuss the possible relationship between the formation of complex chondrules and the formation of macrochondrules and cluster chondrites. While macrochondrules may have formed under similar or related conditions as complex chondrules, cluster chondrites certainly require different formation conditions. Cluster chondrites represent a mixture of viscously deformed, seemingly young chondrules of different chemical and textural types and a population of older chondrules. Concerning the formation of ACC calculations suggest the existence of very local, kilometer‐sized, and super‐dense chondrule‐forming regions with extremely high solid‐to‐gas mass ratios of 1000 or more.  相似文献   

On Flare-CME Characteristics from Sun to Earth Combining Remote-Sensing Image Data with <Emphasis Type="Italic">In Situ</Emphasis> Measurements Supported by Modeling     

Manuela Temmer  Julia K. Thalmann  Karin Dissauer  Astrid M. Veronig  Johannes Tschernitz  Jürgen Hinterreiter  Luciano Rodriguez 《Solar physics》2017,292(7):93

We analyze the well-observed flare and coronal mass ejection (CME) from 1 October 2011 (SOL2011-10-01T09:18) covering the complete chain of effects – from Sun to Earth – to better understand the dynamic evolution of the CME and its embedded magnetic field. We study in detail the solar surface and atmosphere associated with the flare and CME using the Solar Dynamics Observatory (SDO) and ground-based instruments. We also track the CME signature off-limb with combined extreme ultraviolet (EUV) and white-light data from the Solar Terrestrial Relations Observatory (STEREO). By applying the graduated cylindrical shell (GCS) reconstruction method and total mass to stereoscopic STEREO-SOHO (Solar and Heliospheric Observatory) coronagraph data, we track the temporal and spatial evolution of the CME in the interplanetary space and derive its geometry and 3D mass. We combine the GCS and Lundquist model results to derive the axial flux and helicity of the magnetic cloud (MC) from in situ measurements from Wind. This is compared to nonlinear force-free (NLFF) model results, as well as to the reconnected magnetic flux derived from the flare ribbons (flare reconnection flux) and the magnetic flux encompassed by the associated dimming (dimming flux). We find that magnetic reconnection processes were already ongoing before the start of the impulsive flare phase, adding magnetic flux to the flux rope before its final eruption. The dimming flux increases by more than 25% after the end of the flare, indicating that magnetic flux is still added to the flux rope after eruption. Hence, the derived flare reconnection flux is most probably a lower limit for estimating the magnetic flux within the flux rope. We find that the magnetic helicity and axial magnetic flux are lower in the interplanetary space by ～?50% and 75%, respectively, possibly indicating an erosion process. A CME mass increase of 10% is observed over a range of \({\sim}\,4\,\mbox{--}\,20~\mathrm{R}_{\odot }\). The temporal evolution of the CME-associated core-dimming regions supports the scenario that fast outflows might supply additional mass to the rear part of the CME.  相似文献   

Climate control on the evolution of Late Pleistocene alluvial‐fan and aeolian sand‐sheet systems in NW Germany     

Janine Meinsen  Jutta Winsemann  Julia Roskosch  Christian Brandes  Manfred Frechen  Stefan Dultz  Jürgen Böttcher 《Boreas: An International Journal of Quaternary Research》2014,43(1):42-66

The Late Pleistocene was characterized by rapid climate oscillations with alternation of warm and cold periods that lasted up to several thousand years. Although much work has been carried out on the palaeoclimate reconstruction, a direct correlation of ice‐core, marine and terrestrial records is still difficult. Here we present new data from late Middle Pleniglacial to Lateglacial alluvial‐fan and aeolian sand‐sheet deposits in northwestern Germany. Records of Late Pleniglacial alluvial fans in central Europe are very rare, and OSL dating is used to determine the timing of fan aggradation. In contrast to fluvial systems that commonly show a delay between climate change and incision/aggradation, the small alluvial‐fan systems of the Senne area responded rapidly to climatic changes and therefore act as important terrestrial climate archives for this time span. The onset of alluvial‐fan deposition correlates with the climate change from warm to cold at the end of MIS 3 (29.3±3.2 ka). Strong fan progradation started at 24.4±2.8 ka and may be related to a period of higher humidity. The vertical stacking pattern of sedimentary facies and channel styles indicate a subsequrent overall decrease in water and sediment supply, with less sustained discharges and more sporadic runoffs from the catchment area, corresponding to an increasing aridity in central Europe during the Late Pleniglacial. Major phases of channel incision and fan aggradation may have been controlled by millennial‐scale Dansgaard–Oeschger cycles. The incision of channel systems is attributed to unstable climate phases at cold–warm (dry–wet) or warm–cold (wet–dry) transitions. The alluvial‐fan deposits are bounded by an erosion surface and are overlain by aeolian sand‐sheets that were periodically affected by flash‐floods. This unconformity might be correlated with the Beuningen Gravel Bed, which is an important marker horizon in deposits of the Late Pleniglacial resulting from deflation under polar desert conditions. The deposition of aeolian sand‐sheet systems (19.6±2.1 to 13.1±1.5 ka) indicates a rapid increase in aridity at the end of the Late Pleniglacial. Intercalated flash‐floods deposits and palaeosols (Finow type) point to temporarily wet conditions during the Lateglacial. The formation of an ephemeral channel network probably marks the warm‐cold transition from the Allerød to the Younger Dryas.  相似文献   

Diurnal variations in,and influences on,concentrations of particulate and dissolved arsenic and metals in the mildly alkaline Wallkill River,New Jersey,USA     

Julia L. Barringer  Timothy P. Wilson  Zoltan Szabo  Jennifer L. Bonin  Jeffrey M. Fischer  Nicholas P. Smith 《Environmental Geology》2008,53(6):1183-1199

Diurnal variations in particulate and dissolved As and metal concentrations were observed in mildly alkaline water from a wetlands site on the Wallkill River in northwestern New Jersey. The site, underlain by glacial sediments over dolomite bedrock, is 10 km downstream from a mined area of the Franklin Marble, host to Zn ores, also As and Mn minerals. In mid-September 2005, maxima and minima in dissolved-oxygen-concentration and pH, typically caused by photosynthesis and respiration, occurred at 2000 and 0800 hours. Concentrations of dissolved As (1.52–1.95 μg/L) peaked at dusk (2000 hours), whereas dissolved Mn and Zn concentrations (76.5–96.9 and 8.55–12.8 μg/L, respectively) were lowest at dusk and peaked at 1000 hours. These opposing cycles probably reflect sorption and desorption of As (an anion), and Mn and Zn (cations) as pH varied throughout the 24-h period. Doubly-peaked cycles of B, Cl, SO₄, and nutrients also were observed; these may result from upstream discharges of septic-system effluent. Both recoverable amd particulate Al, Fe, Mn, and Zn concentrations peaked between 0200 and 0600 hours. The particulate metals cycle, with perturbations at 0400 hours, may be influenced by biological activity.  相似文献   

3D mineralogical mapping of the Kovdor phoscorite–carbonatite complex (Russia)     

Julia A. Mikhailova  Andrey O. Kalashnikov  Victor A. Sokharev  Yakov A. Pakhomovsky  Nataly G. Konopleva  Victor N. Yakovenchuk  Ayya V. Bazai  Pavel M. Goryainov  Gregory Yu. Ivanyuk 《Mineralium Deposita》2016,51(1):131-149

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