The highly forbidden
electric octupole transition in
is a potential candidate for a redefinition of the SI second. We present a measurement of the absolute frequency of this optical transition, ...performed using a frequency link to International Atomic Time to provide traceability to the SI second. The
optical frequency standard was operated for 76% of a 25-day period, with the absolute frequency measured to be 642 121 496 772 645.14(26) Hz. The fractional uncertainty of
is comparable to that of the best previously reported measurement, which was made by a direct comparison to local caesium primary frequency standards.
The regulation of the air: a hypothesis Nisbet, E. G; Fowler, C. M. R; Nisbet, R. E. R
Solid earth (Göttingen),
01/2012, Letnik:
3, Številka:
1
Journal Article
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We propose the hypothesis that natural selection, acting on the specificity or preference for CO2 over O2 of the enzyme rubisco (ribulose-1,5-bisphosphate carboxylase/oxygenase), has controlled the ...CO2:O2 ratio of the atmosphere since the evolution of photosynthesis and has also sustained the Earth's greenhouse-set surface temperature. Rubisco works in partnership with the nitrogen-fixing enzyme nitrogenase to control atmospheric pressure. Together, these two enzymes control global surface temperature and indirectly the pH and oxygenation of the ocean. Thus, the co-evolution of these two enzymes may have produced clement conditions on the Earth's surface, allowing life to be sustained.
Although the heterogeneous nature of the chemical composition of Earth's mantle is now well established, the origin and longevity of the heterogeneities continue to be debated. In order to further ...study early-Earth heterogeneities, we present a set of Sm-Nd, Lu-Hf, Re-Os, and Hf-W isotope and lithophile and siderophile element abundance data for komatiites and basalts from the ∼3.53 Ga Coonterunah, ∼3.34 Ga Kelly, and ∼3.18 Ga Ruth Well and Regal systems of the Pilbara Craton in Western Australia. The Sm-Nd, Lu-Hf, and Re-Os isotope data yield isochrons consistent with the accepted emplacement ages of the respective komatiite-basalt lavas. The mantle sources evolved with long-term 147Sm/144Nd = 0.200 to 0.214 and 176Lu/177Hf = 0.0355 to 0.0395, spanning the entire range of the time-integrated Sm/Nd and Lu/Hf measured in the Archean and Proterozoic komatiite-basalt systems to-date. Unlike with the other early Archean komatiites and basalts, the coupled 143Nd-176Hf isotope systematics of the Pilbara lavas provide no evidence for the involvement of early magma ocean processes in the evolution of their mantle sources. Episodes of variable degrees of partial mantle melting and melt extraction can account for the observed large variations in the time-integrated Sm/Nd and Lu/Hf ratios in the early Archean mantle domains.
In contrast to the highly variable Nd-Hf systematics, the initial γ187Os values vary within a narrow range from +0.9 to −0.4 indicating that the Pilbara mantle sources evolved with chondritic time-integrated Re/Os. The apparent discrepancy between the depletions in incompatible lithophile trace elements and near-chondritic Re/Os observed globally is reconciled via a model whereby early low-degree mantle melting events fractionated Sm from Nd and Lu from Hf, but had little effect on the Re/Os ratio. This in turn would imply early formation and long-term isolation of a basaltic crust highly enriched in incompatible lithophile trace elements.
The calculated total HSE abundances in the komatiite mantle sources range from ∼30% in the Coonterunah to ∼70% in the Regal system, of those in the estimates for the modern BSE, indicative of a 2.4× increase in HSE abundances from 3.53 to 3.18 Ga.
All four komatiite-basalt systems exhibit positive 182W anomalies ranging between +11.4 and +7.7 ppm. The 182W/184W compositions and calculated HSE abundances in the Pilbara komatiite-basalt sources are inversely correlated and are most consistent with grainy late accretion of large differentiated planetesimals. Regression of the combined 182W-HSE data for the komatiite systems allows an estimate of the W isotopic composition of the pre-late accretion BSE of +17 ± 7. This estimate is similar to that of the Moon of +25 ± 5 and lends further support to the notion regarding an initially common W isotopic composition in the Earth-Moon system.
Regression of the available HSE abundance data for komatiite mantle sources worldwide provides an estimate for the time of complete homogenization of late accreted materials within the mantle by 2.5 ± 0.2 Ga. Calculations indicate an average survival time of late accreted planetesimals in the Earth's mantle of 1.9 ± 0.2 Ga, which constrains the average mantle stirring rates for the HSE in the Hadean and Archean.
The distribution of methane sources in Kuwait was mapped through mobile vehicle surveying of methane mole fraction, and by collection of air samples at source for subsequent isotopic analysis. Mobile ...plume identification and isotopic analysis, reveals that by far the largest observed source of methane in Kuwait is from landfill sites (δ13CCH4 of −58‰), with smaller contributions from fossil fuel industry (−50‰), wastewater treatment (−49‰) and ruminant animals (sheep −64‰, cows, −62‰, camels −60‰).
Regular weekly air samples were collected over two years from three sites in Kuwait, one NW of the city, one to the SE and one in the city from the rooftop of the Kuwait College of Science. Associated with higher mole fraction is a consistent depletion in 13C of methane, pointing to a national source mix with δ13CCH4 of −55.9‰. This is significantly different from calculations using EDGAR v.5 inventory that suggest a source mix of −52.8‰. Diurnal campaigns from a city location confirm that the sources are dominantly biogenic (−59 to −56‰) in and around the urban areas.
The EDGAR 5.0 inventory suggests that the dominant sources of methane in Kuwait are leaks from gas flaring and fossil fuel distribution, with additional smaller emissions from landfills, sewage (wastewater) treatment and ruminant animals, but with a waste component increasing relative to fossil fuel emissions in recent decades. Measurements during 2015 and 2016 suggest that for all but the far south and SW of the country which is not in the meteorological footprint of fixed site or mobile measurement, there is a dominant waste source and much smaller observed proportion from fossil fuel activities.
This research demonstrates for the first time in Kuwait that continuous mobile measurements for plume identification coupled with high-precision isotopic analysis using CF-GC-IRMS (Continuous Flow Gas Chromatography-Isotope Ratio Mass Spectrometry) is an effective way of identifying methane sources and understanding their relative contributions. To unambiguously quantify relative contributions will require further work, particularly in the SW region of the country, utilizing aircraft and/or satellite retrievals. The results of this research will contribute to understanding the methane budget of this poorly studied region.
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•Weekly sampling and methane isotopic analysis for 3 Kuwait sites over 2 years.•Mobile CH4 measurement campaign for 7 days with subsequent GIS mapping.•Characterization of methane sources into fossil fuel, waste and ruminant categories.•Diurnal sampling in Kuwait City suburb identifies landfill as main methane source.•Isotopes identify waste as main source, not fossil fuels as in EDGAR inventory.
A switch from a fossil fuel to a hydrogen‐based energy system could cause significant changes in the magnitude and composition of anthropogenic emissions. Model simulations suggest the most ...significant impact of these emission changes would occur in the troposphere, affecting OH. This impact is dependent upon the magnitude and nature of trade‐offs in changing fossil fuel use. In the stratosphere, changes in water vapour resulting from expected increases in surface molecular hydrogen emissions via leaks occurring during production, transport and storage, are found to be significantly smaller than previous estimates. We conclude that the expected increase in molecular hydrogen emissions is unlikely to have a substantial impact on stratospheric ozone, certainly much smaller than the ozone changes observed in the last two decades.
Measurements of the stable isotopic composition (δD(H2) or δD) of atmospheric molecular hydrogen (H2) are a useful addition to mixing ratio (χ(H2)) measurements for understanding the atmospheric H2 ...cycle. δD datasets published so far consist mostly of observations at background locations. We complement these with observations from the Cabauw tall tower at the CESAR site, situated in a densely populated region of the Netherlands. Our measurements show a large anthropogenic influence on the local H2 cycle, with frequently occurring pollution events that are characterized by χ(H2) values that reach up to ≈1 ppm and low δD values. An isotopic source signature analysis yields an apparent source signature below −400‰, which is much more D-depleted than the fossil fuel combustion source signature commonly used in H2 budget studies. Two diurnal cycles that were sampled at a suburban site near London also show a more D-depleted source signature (≈−340‰), though not as extremely depleted as at Cabauw. The source signature of the Northwest European vehicle fleet may have shifted to somewhat lower values due to changes in vehicle technology and driving conditions. Even so, the surprisingly depleted apparent source signature at Cabauw requires additional explanation; microbial H2 production seems the most likely cause. The Cabauw tower site also allowed us to sample vertical profiles. We found no decrease in χ(H2) at lower sampling levels (20 and 60 m) with respect to higher sampling levels (120 and 200 m). There was a significant shift to lower median δD values at the lower levels. This confirms the limited role of soil uptake around Cabauw, and again points to microbial H2 production during an extended growing season, as well as to possible differences in average fossil fuel combustion source signature between the different footprint areas of the sampling levels. So, although knowledge of the background cycle of H2 has improved over the last decade, surprising features come to light when a non-background location is studied, revealing remaining gaps in our understanding.
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•Multi-year time series of H2 and δD(H2) were measured at a regional tall tower site.•The dataset contains the first measured δD height profiles in the boundary layer.•The features of the time series are consistent with large anthropogenic influences.•The apparent δD source signature is much lower than fossil fuel combustion estimates.•Both source signature and profiles suggest microbial H2 production around the tower.
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