Igneous graphite in enstatite chondrites

1997 ◽  
Vol 61 (408) ◽  
pp. 699-703 ◽  
Author(s):  
Alan E. Rubin
Keyword(s):  
Large Scale ◽  
Parent Body ◽  
Ultramafic Rocks ◽  
Low Oxygen ◽  
Impact Melt ◽  
C Isotopes ◽  
Melting Event ◽  
C Value ◽  
Enstatite Chondrite ◽  
Enstatite Chondrites

AbstractIgneous graphite, a rare constituent in terrestrial mafic and ultramafic rocks, occurs in three EH and one EL enstatite chondrite impact-melt breccias as 2–150 µm long euhedral laths, some with pyramidal terminations. In contrast, graphite in most enstatite chondrites exsolved from metallic Fe-Ni as polygonal, rounded or irregular aggregates. Literature data for five EH chondrites on C combusting at high temperatures show that Abee contains the most homogeneous C isotopes (i.e. δ13C = −8.1 ± 2.1‰); in addition, Abee's mean δ13C value is the same as the average high-temperature C value for the set of five EH chondrites. This suggests that Abee scavenged C from a plurality of sources on its parent body and homogenized the C during a large-scale melting event. Whereas igneous graphite in terrestrial rocks typically forms at relatively high pressure and only moderately low oxygen fugacity (e.g., ∼ 5 kbar, logfO2 ∼ −10 at 1200°C), igneous graphite in asteroidal meteorites formed at much lower pressures and oxygen fugacities.

scholarly journals Ocean deoxygenation, the global phosphorus cycle and the possibility of human-caused large-scale ocean anoxia

2017 ◽  
Vol 375 (2102) ◽  
pp. 20160318 ◽  
Author(s):  
Andrew J. Watson ◽  
Timothy M. Lenton ◽  
Benjamin J. W. Mills
Keyword(s):  
Chemical Weathering ◽  
Large Scale ◽  
Oxygen Demand ◽  
Subsurface Water ◽  
Phosphorus Cycle ◽  
Low Oxygen ◽  
Positive Feedbacks ◽  
Future Possibility ◽  
Warming World ◽  
Ocean Deoxygenation

The major biogeochemical cycles that keep the present-day Earth habitable are linked by a network of feedbacks, which has led to a broadly stable chemical composition of the oceans and atmosphere over hundreds of millions of years. This includes the processes that control both the atmospheric and oceanic concentrations of oxygen. However, one notable exception to the generally well-behaved dynamics of this system is the propensity for episodes of ocean anoxia to occur and to persist for 10 5 –10 6 years, these ocean anoxic events (OAEs) being particularly associated with warm ‘greenhouse’ climates. A powerful mechanism responsible for past OAEs was an increase in phosphorus supply to the oceans, leading to higher ocean productivity and oxygen demand in subsurface water. This can be amplified by positive feedbacks on the nutrient content of the ocean, with low oxygen promoting further release of phosphorus from ocean sediments, leading to a potentially self-sustaining condition of deoxygenation. We use a simple model for phosphorus in the ocean to explore this feedback, and to evaluate the potential for humans to bring on global-scale anoxia by enhancing P supply to the oceans. While this is not an immediate global change concern, it is a future possibility on millennial and longer time scales, when considering both phosphate rock mining and increased chemical weathering due to climate change. Ocean deoxygenation, once begun, may be self-sustaining and eventually could result in long-lasting and unpleasant consequences for the Earth's biosphere. This article is part of the themed issue ‘Ocean ventilation and deoxygenation in a warming world’.

Experimental study of trace element partitioning between enstatite and melt in enstatite chondrites at low oxygen fugacities and 5GPa

2014 ◽  
Vol 130 ◽  
pp. 167-187 ◽  
Author(s):  
Camille Cartier ◽  
Tahar Hammouda ◽  
Régis Doucelance ◽  
Maud Boyet ◽  
Jean-Luc Devidal ◽  
...  
Keyword(s):  
Experimental Study ◽  
Trace Element ◽  
Low Oxygen ◽  
Trace Element Partitioning ◽  
Element Partitioning ◽  
Enstatite Chondrites

scholarly journals Genomic and Transcriptomic Insight of Giant Sclerotium Formation of Wood-Decay Fungi

2021 ◽  
Vol 12 ◽  
Author(s):  
Shuo Cao ◽  
Yang Yang ◽  
Guiqi Bi ◽  
David Nelson ◽  
Sheng Hu ◽  
...  
Keyword(s):  
Large Scale ◽  
Wood Decay ◽  
Gene Families ◽  
Underlying Mechanism ◽  
Low Oxygen ◽  
Decay Fungi ◽  
Wood Decay Fungi ◽  
Wood Polysaccharides ◽  
Partial Genome ◽  
Sclerotium Formation

Many fungi form persistent and dormant sclerotia with compact hardened mycelia during unfavorable circumstances. While most of these sclerotia are small in size, Wolfiporia cocos, a wood-decay fungus, grows into giant sclerotia, which are mainly composed of polysaccharides of linear (1→3)-β-D-glucans. To explore the underlying mechanism of converting sophisticated wood polysaccharides for biosynthesis of highly homogenized glucans in W. cocos, we sequenced and assembled the genome of a cultivated W. cocos strain (WCLT) in China. The 62-Mb haploid genome contains 44.2% repeat sequences, of which, 48.0% are transposable elements (TEs). Contrary to the genome of W. cocos from North America, WCLT has independently undergone a partial genome duplication (PGD) event. The large-scale TE insertion and PGD occurrence overlapped with an archeological Pleistocene stage of low oxygen and high temperature, and these stresses might have induced the differences in sclerotium due to geographical distribution. The wood decomposition enzymes, as well as sclerotium-regulator kinases, aquaporins, and highly expanded gene families such as NAD-related families, together with actively expressed 1,3-β-glucan synthase for sclerotium polysaccharides, all have contributed to the sclerotium formation and expansion. This study shall inspire further exploration on how fungi convert wood into simple glucans in the sclerotium of W. cocos.

Fabrication of large-scale, layer-deposited, low oxygen-content and uniform silicon nanowires

2007 ◽  
Vol 303 (2) ◽  
pp. 391-394 ◽  
Author(s):  
Mingwang Shao ◽  
Hui Hu ◽  
Huizhao Ban ◽  
Min Li ◽  
Huazhong Gao
Keyword(s):  
Oxygen Content ◽  
Silicon Nanowires ◽  
Large Scale ◽  
Low Oxygen ◽  
Scale Layer

Estimation of the proportion of C3 and C4 plant species in the diet of animals from the ratio of natural 12C and 13C isotopes in the faeces

1979 ◽  
Vol 92 (1) ◽  
pp. 91-100 ◽  
Author(s):  
R. J. Jones ◽  
M. M. Ludlow ◽  
J. H. Troughton ◽  
C. G. Blunt
Keyword(s):  
Time Lag ◽  
Alternative Methods ◽  
C4 Plant ◽  
Advantages And Disadvantages ◽  
C Isotopes ◽  
Tropical Legumes ◽  
C Value ◽  
13C Isotopes ◽  
Definition Of

SummaryThe relation between the ratio of the natural 12C and 13C isotopes of carbon in the feed and resultant faeces of animals was studied to develop a technique for estimating the proportion of C3 species (tropical legumes) and C4 species (tropical grasses) selected by grazing animals.In general, theδ13C values (see text for definition) of faeces from rabbits, sheep, goats and cattle were lower (more negative) than those of the corresponding feeds by from 0·4 to 2·0. This was possibly due to contamination in the gut by tissues or fluids with lower δ13C values. When C4 and C3 feeds were alternated, cattle took about a week to fully achieve the new level (δ13C of – 28·7 on the C3 feed and – 13·1 on the C4 feed) in the faeces. This time lag is associated with the time taken for the feed to move through the digestive tract.When mixed C3 and C4 feeds were fed to rabbits, sheep, goats and cattle there was a negative linear relation between percentage legume (C3) in the feed and the δ13C of the faeces (P < 0·01). A decrease in one unit in the δ13C value was associated with an increase of 7·0–8·5% legume in the diet.Estimation of the percentage legume in the feed from the δ13C value of the faeces and of the C3 and C4 components of the diet, resulted in a consistent over estimation of the legume component because the faeces had lower values than the corresponding feeds. This bias was removed if the prediction was based on the δ13C of the feeds minus 1 unit; the legume percentage in the diets of the sheep, goats and cattle could then be estimated with a precision of about ± 5%.Differences in digestibility between the C3 and C4 components greatly bias the estimations. This bias in the diets fed to rabbits was effectively removed by using in vitro organic matter digestibility values of the two components to correct for the differences. Legume percentage in the diet could then be estimated with a RSD of ± 3%.Advantages and disadvantages compared with alternative methods of estimating the diet of grazing animals are discussed.

scholarly journals Biosequence stratigraphical and palaeoenvironmental findings from the Cretaceous through Tertiary succession, Central Indus Basin, Pakistan

2002 ◽  
Vol 21 (2) ◽  
pp. 115-130 ◽  
Author(s):  
M. I. Wakefield ◽  
E. Monteil
Keyword(s):  
Large Scale ◽  
Age Dating ◽  
Indus Basin ◽  
Arabian Plate ◽  
Integrated Analysis ◽  
Low Oxygen ◽  
Candidate Sequence ◽  
Oceanic Anoxic Event ◽  
Palynological Data ◽  
Sequence Boundaries

Abstract. Integrated analysis of foraminiferal and palynological data from the Duljan-1 well, Central Indus Basin, Pakistan, is used to identify critical surfaces (candidate sequence boundaries (SB) and maximum flooding surfaces (MFS)) and construct a biosequence stratigraphical framework. Within the Barremian through Bartonian–Priabonian? succession 15 depositional sequences have been recognized, each with a candidate MFS. These biosequences are shown to equate with the local lithostratigraphy and tentatively with the ‘global’ large-scale depositional cycles of Haq et al. (1987). Detailed dating has enabled seven candidate MFS to be tentatively equated with MFS identified on the nearby Arabian plate (Sharland et al., 2001). A combination of detailed age dating and palaeobathymetric determinations indicates significant basin uplift and erosion at end Cretaceous and end Eocene times, the latter coinciding with closure of Neo-Tethys. Smaller-scale unconformities are also noted. Multi-disciplinary palaeoenvironmental interpretations enable recognition of detailed changes in water mass conditions. Palynological data suggest these changes result from variations in terrestrial/freshwater input, though evidence of periodically low oxygen bottom water conditions/shallowing of the oxygen minimum zone, possibly ‘Oceanic Anoxic Event-2’ (OAE-2; late Cenomanian–Turonian) is suggested as a further control.

Environmental influences on light and dark CO2 fixation by Kalanchoe daigremontiana

1975 ◽  
Vol 53 (11) ◽  
pp. 1132-1138 ◽  
Author(s):  
E. Nalborczyk ◽  
L. J. Lacroix ◽  
R. D. Hill
Keyword(s):  
Co2 Fixation ◽  
Red Light ◽  
Day Length ◽  
Low Oxygen ◽  
Long Day ◽  
C Value ◽  
Warm Night ◽  
Dark Co2 Fixation ◽  
Kalanchoë Daigremontiana ◽  
Kalanchoe Daigremontiana

The influence of day length, light quality, temperature, drought, and oxygen concentration on gas exchange of Kalanchoe daigremontiana was investigated. The ratio of photosynthesis to dark CO2 fixation with plants under a long-day and short-warm-night regime was 2.0 and under a short-day and long-cool-night regime, 0.2. With drought conditions this value may be less than 0.02. Under low oxygen concentrations high photosynthetic rates of about 20 mg CO2 dm−2 h−1 were observed with a compensation point of about 20 ppm. Restricting CO2 supply to dark periods produced plants with a δ13C value of −10.6‰. Restricting CO2 supply to light periods gave a δ13C value of −25.9‰, whereas no control of CO2 gave δ13C values of −15‰. The data obtained suggest that variations in δ13C values in Kalanchoe result from changes in the proportion of light and dark CO2 fixation.Far-red light and oxygen promoted the release of CO2 from Kalanchoe after the plants had undergone a period of dark CO2 fixation. Maximum release occurred within about 1 h. The effect could be reversed by removal of either far-red light or oxygen. Decreases in the acidity of the plants accompanied CO2 release, which indicated that decarboxylation of a C4 acid was the source of the CO2.

Origin of Iron Meteorite Groups IAB and IIICD

1980 ◽  
Vol 35 (8) ◽  
pp. 781-795 ◽  
Author(s):  
John T. Wasson ◽  
John Willis ◽  
Chien M. Wai ◽  
Alfred Kracher
Keyword(s):  
Parent Body ◽  
Iron Meteorite ◽  
Iron Meteorites ◽  
Impact Melt ◽  
Equilibrium Crystallization ◽  
Ni Content ◽  
Oxide Phases ◽  
Host Metal ◽  
Impact Melts ◽  
The Impact

AbstractSeveral low-Ni iron meteorites previously assigned to group IAB are reclassified IIICD on the basis of lower Ge, Ga, W and Ir concentrations and higher As concentrations; the low-Ni extreme of IIICD is now 62 mg/g, that of IAB is 64 mg/g. The resulting fractionation patterns in the two groups are quite similar. It has long been established that, in contrast to the magmatic iron meteorite groups, IAB and IIICD did not form by fractional crystallization of a metallic magma. Other models have been proposed, but all have serious flaws. A new model is proposed involving the formation of each iron in small pools of impact melt on a parent body consisting of material similar to the chondritic inclusions found in some IAB and IIICD irons, but initially unequilibrated. These impact melts ranged in temperatures from ~ 1190 K to ~ 1350 K. The degree of equilibration between melt and unmelted solids ranged from minimal at the lowest temperature to moderate at the highest temperature. The lowest temperature melts were near the cotectic in the Fe-Ni-S system with Ni contents of ~ 12 atom %. Upon cooling, these precipitated metal having ~ 600 mg/g Ni by equilibrium crystallization. The Ni-rich melt resulted from the melting of Ni-rich sulfides and metal in the unequilibrated chondritic parent. Low-Ni irons formed in high temperature melts near the composition of the FeS-Fe eutectic or somewhat more metal rich. We suggest that the decreasing Ge, Ga and refractory abundances with increasing Ni concentration reflect the trapping of these elements in oxide phases in the unequilibrated chondritic material, and that very little entered the Ni-rich melt parental to the Oktibbeha County iron. The remaining elements tended to have element/Ni ratios in the melts that were more or less independent of temperature. The remarkable correlation between I-Xe age of the chondritic inclusions and Ni content of the host metal is explained by a detailed evolution of (mega)regolith in which these groups originated. The most Ni-rich melts could only be generated from an unequilibrated chondrite parent; as the continuing deposition of impact energy produced increasingly higher grades of metamorphism, the maximum Ni content of the impact melts (and their subsequently precipitated metal) gradually decreased.

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