D. Lowry

Oxygen isotope composition of mantle peridotite

Abstract The oxygen isotopic composition of 76 samples of olivine in spinel-, garnet- and diamond-facies peridotites, and as syngenetic inclusions within diamond has been determined by laser fluorination (LF). δ18OV-SMOW values of olivine are almost invariant, averaging 5.18 ± 0.28‰ (2 S.D.) with an overall range of 4.8–5.5‰. Coexisting clinopyroxenes display positive Δ18Ocpx-olivine fractionations averaging 0.4‰ that are consistent with isotopic equilibrium at normal mantle temperatures. The bulk compositions of spinel-, garnet- and diamond-facies mantle are similar, with a calculated bulk mantle δ18O of +5.5‰. Olivine inclusions in diamonds of Archaean age yield δ18O values that are indistinguishable from olivine in post-Archaean mantle and the LF data do not provide evidence for gross spatial or temporal δ18O variation in peridotitic mantle. There is no evidence that mantle hydration processes impart distinctive oxygen isotope characteristics, other than a subtle isotopic disequilibrium disturbing pyroxenes relative to olivine.

Global atmospheric methane: budget, changes and dangers

A factor of 2.5 increase in the global abundance of atmospheric methane (CH4) since 1750 contributes 0.5 Wm−2 to total direct radiative forcing by long-lived greenhouse gases (2.77 Wm−2 in 2009), while its role in atmospheric chemistry adds another approximately 0.2 Wm−2 of indirect forcing. Since CH4 has a relatively short lifetime and it is very close to a steady state, reductions in its emissions would quickly benefit climate. Sensible emission mitigation strategies require quantitative understanding of CH4’s budget of emissions and sinks. Atmospheric observations of CH4 abundance and its rate of increase, combined with an estimate of the CH4 lifetime, constrain total global CH4 emissions to between 500 and 600 Tg CH4 yr−1. While total global emissions are constrained reasonably well, estimates of emissions by source sector vary by up to a factor of 2. Current observation networks are suitable to constrain emissions at large scales (e.g. global) but not at the regional to national scales necessary to verify emission reductions under emissions trading schemes. Improved constraints on the global CH4 budget and its break down of emissions by source sector and country will come from an enhanced observation network for CH4 abundance and its isotopic composition (δ13C, δD (D=2H) and δ14C). Isotopic measurements are a valuable tool in distinguishing among various sources that contribute emissions to an air parcel, once fractionation by loss processes is accounted for. Isotopic measurements are especially useful at regional scales where signals are larger. Reducing emissions from many anthropogenic source sectors is cost-effective, but these gains may be cancelled, in part, by increasing emissions related to economic development in many parts of the world. An observation network that can quantitatively assess these changing emissions, both positive and negative, is required, especially in the context of emissions trading schemes.

Diamondiferous eclogites from Siberia: Remnants of Archean oceanic crust

We have investigated eight diamond-bearing bimineralic eclogite xenoliths from the Udachnaya Mine, Yakutia, Siberia, in terms of major elements, 87Sr86Sr−, 143Nd144Nd and oxygen isotopic ratios. The β18O-values, measured with the new laser-fluorination technique, are different from mantle values and range between 5.19 and 7.38%. with an average error of 0.08%.. Strontium and neodymium initial isotopic ratios for cpx are between 0.70226 and 0.70699 and 0.51170 and 0.51257, respectively. Chemically and petrographically, the Siberian eclogites are very similar to the South African eclogite suite from Roberts Victor or Bellsbank, the most important similarities being the late Archean age (2.76 Ga) and the δ18O values that deviate from mantle values. However, differences exist in detail, as no samples with δ18O values lower than mantle values have yet been reported from Siberia and the cesium concentrations of the Siberian eclogites are generally lower than those of the Roberts Victor eclogite suite. The data obtained from the studied sample suite are best explained by a model proposing an origin from Archean oceanic crust that was intensely altered prior to subduction to mantle depths. Using oxygen isotopic values, the effects of seawater alteration can be shown and the composition of the unaltered protolith qualitatively estimated. We propose that mantle eclogites from kimberlites were generated by a globally operating subduction process during the late Archean and that differences between samples from different cratons are small compared to their similarities.

Emission of methane from plants

It has been proposed that plants are capable of producing methane by a novel and unidentified biochemical pathway. Emission of methane with an apparently biological origin was recorded from both whole plants and detached leaves. This was the first report of methanogenesis in an aerobic setting, and was estimated to account for 10–45 per cent of the global methane source. Here, we show that plants do not contain a known biochemical pathway to synthesize methane. However, under high UV stress conditions, there may be spontaneous breakdown of plant material, which releases methane. In addition, plants take up and transpire water containing dissolved methane, leading to the observation that methane is released. Together with a new analysis of global methane levels from satellite retrievals, we conclude that plants are not a major source of the global methane production.

Arctic methane sources: Isotopic evidence for atmospheric inputs

By comparison of the methane mixing ratio and the carbon isotope ratio (δ13CCH4) in Arctic air with regional background, the incremental input of CH4 in an air parcel and the source δ13CCH4 signature can be determined. Using this technique the bulk Arctic CH4 source signature of air arriving at Spitsbergen in late summer 2008 and 2009 was found to be −68‰, indicative of the dominance of a biogenic CH4 source. This is close to the source signature of CH4 emissions from boreal wetlands. In spring, when wetland was frozen, the CH4 source signature was more enriched in 13C at −53 ± 6‰ with air mass back trajectories indicating a large influence from gas field emissions in the Ob River region. Emissions of CH4 to the water column from the seabed on the Spitsbergen continental slope are occurring but none has yet been detected reaching the atmosphere. The measurements illustrate the significance of wetland emissions. Potentially, these may respond quickly and powerfully to meteorological variations and to sustained climate warming.

Oxygen isotopic composition of hydrous and anhydrous mantle peridotites

Oxygen isotope ratios, determined using the laser fluorination technique, are reported for minerals from anhydrous and hydrous (i.e., amphibole-bearing) spinel lherzolites from Yemen, as well as from hydrous spinel lherzolites and amphibole megacrysts from Nunivak Island, Alaska. Oxygen isotopic compositions of olivine vary from 5.1–5.4%c and of pyroxene from 5.5–6.0%c and no systematic difference exists between minerals in hydrous and anhydrous lherzolites. The oxygen isotopic composition of the amphibole in the peridotites and of the amphibole megacrysts is also very homogeneous and varies from δ18O = 5.3−5.6%o. These results indicate that the metasomatic minerals in the lherzolites are in oxygen isotopic equilibrium with the peridotitic minerals. The only isotopic disequilibria are observed in minerals which have grown in melt-pockets formed by partial melting of amphibole. The homogeneity of the oxygen isotopic ratios of mantle minerals in this study indicate that the fluids circulating in the mantle and precipitating amphibole or mica had the same oxygen isotopic compositions as the mantle protolith or that the fluids had been buffered by the isotopic composition of the olivine, the most abundant mineral, during percolation through the peridotites.

Rising atmospheric methane: 2007-2014 growth and isotopic shift

From 2007 to 2013, the globally averaged mole fraction of methane in the atmosphere increased by 5.7 ± 1.2 ppb yr−1. Simultaneously, δ13CCH4 (a measure of the 13C/12C isotope ratio in methane) has shifted to significantly more negative values since 2007. Growth was extreme in 2014, at 12.5 ± 0.4 ppb, with a further shift to more negative values being observed at most latitudes. The isotopic evidence presented here suggests that the methane rise was dominated by significant increases in biogenic methane emissions, particularly in the tropics, for example, from expansion of tropical wetlands in years with strongly positive rainfall anomalies or emissions from increased agricultural sources such as ruminants and rice paddies. Changes in the removal rate of methane by the OH radical have not been seen in other tracers of atmospheric chemistry and do not appear to explain short-term variations in methane. Fossil fuel emissions may also have grown, but the sustained shift to more 13C-depleted values and its significant interannual variability, and the tropical and Southern Hemisphere loci of post-2007 growth, both indicate that fossil fuel emissions have not been the dominant factor driving the increase. A major cause of increased tropical wetland and tropical agricultural methane emissions, the likely major contributors to growth, may be their responses to meteorological change.

Pyroxenite-rich mantle formed by recycled oceanic lithosphere: Oxygen-osmium isotope evidence from Canary Island lavas

Plate tectonic processes result in recycling of crust and lithosphere into Earths mantle. Evidence for long-term preservation of recycled reservoirs in the mantle comes from the enriched isotopic character of oceanic island basalt (OIB) lavas. Although recycled constituents can explain much of the geochemical variation in the OIB-source mantle, it has been shown that direct melting of these components would lead to magmas with evolved compositions, unlike OIB. Instead, it has been argued that either metasomatic pyroxene-rich peridotite that has inherited the trace element and isotopic character of subducted materials, or high-temperature intramantle metasomatism of lithosphere can explain OIB compositions. To test these models, we present new oxygen and osmium isotope data for lavas from the Canary Islands of El Hierro and La Palma. These islands have distinct 18O/16O and 187Os/188Os compositions that can be explained through melting of pyroxenite-enriched peridotite mantle containing <10% recycled oceanic lithosphere. We also assess O-Os isotope systematics of lavas from Hawai‘i and the Azores and show that they also conform to addition of distinct recycled oceanic components, including lithosphere and pelagic sediment. We conclude that enriched isotopic signatures of some OIBs are consistent with pyroxenite-rich mantle sources metasomatized by recycled components.

Antiquity of the biological sulphur cycle: evidence from sulphur and carbon isotopes in 2700 million-year-old rocks of the Belingwe Belt, Zimbabwe.

Sulphur and carbon isotopic analyses on small samples of kerogens and sulphide minerals from biogenic and non-biogenic sediments of the 2.7 × 109 years(Ga)-old Belingwe Greenstone Belt (Zimbabwe) imply that a complex biological sulphur cycle was in operation. Sulphur isotopic compositions display a wider range of biological fractionation than hitherto reported from the Archaean. Carbon isotopic values in kerogen record fractionations characteristic of rubisco activity, methanogenesis and methylotrophy, and possibly anoxygenic photosynthesis. Carbon and sulphur isotopic fractionations have been interpreted in terms of metabolic processes in 2.7Ga prokaryote mat communities, and indicate the operation of a diverse array of metabolic processes. The results are consistent with models of early molecular evolution derived from ribosomal RNA.

London methane emissions: Use of diurnal changes in concentration and δ13C to identify urban sources and verify inventories

Diurnal air sampling campaigns at the Royal Holloway site on the western fringe of London, United Kingdom, have been used to: (1) test the validity of using carbon isotopes to identify local methane sources, (2) determine the isotopic signature of overnight build-up profiles, in order to estimate regional emissions, and (3) verify statistical estimates of emissions. For CH4 an overall London δ13C source mix of −48.7±0.3‰ has been calculated from gradual overnight methane buildup in air masses moving from the east. Isotopic characterization of specific methane peaks shows them to be derived either from natural gas leaks (δ13C −33‰ to −35‰) or waste treatment emissions (δ13C −51‰ to −53‰). While landfill/waste emissions dominate, gas distribution losses represent ∼20% of the bulk local source. Various estimates of total London methane emissions for 1996 were made, using diurnal excess, isotopic data and trajectory movement across London to the sampling station. The results are in the range 240–312 kt/yr, higher than the London estimate in current U.K. greenhouse gas inventory assessments of emissions but within error of earlier statistical estimates for 1996. The results show that it is possible to use atmospheric concentration, isotopic, and meteorological data together to verify statistical estimates testing them for internal consistency and using better constrained data to calibrate more poorly known source fluxes. Importantly, atmospheric data can place constraints on poorly constrained landfill emission estimates for the region. These quasi-independent methods for verification of greenhouse gas emissions will contribute in assessing compliance with the Kyoto agreement.