Euan G. Nisbet

Escape of methane gas from the seabed along the West Spitsbergen continental margin

More than 250 plumes of gas bubbles have been discovered emanating from the seabed of the West Spitsbergen continental margin, in a depth range of 150-400 m, at and above the present upper limit of the gas hydrate stability zone (GHSZ). Some of the plumes extend upward to within 50 m of the sea surface. The gas is predominantly methane. Warming of the northward-flowing West Spitsbergen current by 1°C over the last thirty years is likely to have increased the release of methane from the seabed by reducing the extent of the GHSZ, causing the liberation of methane from decomposing hydrate. If this process becomes widespread along Arctic continental margins, tens of Teragrams of methane per year could be released into the ocean.

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.

Constraining the potential temperature of the Archaean mantle: A review of the evidence from komatiites

The maximum potential temperature of the Archaean mantle is poorly known, and is best constrained by the MgO contents of komatiitic liquids, which are directly related to eruptive temperatures. However, most Archaean komatiites are significantly altered and it is difficult to assess the composition of the erupted liquid. Relatively fresh lavas from the SASKMAR suite, Belingwe Greenstone Belt, Zimbabwe (2.7 Ga) include chills of 25.6 wt.% MgO, and olivines ranging to Fo93.6, implying eruption at around 1520°C. A chill sample from Alexo Township, Ontario (also 2.7 Ga) is 28 wt.% MgO, and associated olivines range to Fo94.1, implying eruption at 1560°C. However, inferences of erupted liquids containing 32–33 wt.% MgO, from lavas in the Barberton Greenstone Belt, South Africa (3.45 Ga) and from the Perseverance Complex, Western Australia (2.7 Ga) may be challenged on the grounds that they contain excess (cumulate) olivine, or were enriched in Mg during alteration or metamorphism. Re-interpretation of olivine compositions from these rocks shows that they most likely contained a maximum of 29 wt.% MgO corresponding to an eruption temperature of 1580°C. This composition is the highest liquid MgO content of an erupted lava that can be supported with any confidence. The hottest modern magma, on Gorgona Island (0.155 Ga) contained 18–20% MgO and erupted at circa 1400°C. If 1580°C is taken as the temperature of the most magnesian known eruption, then the source mantle from which the liquids rose would have been at up to 2200°C at pressures of 18 GPa corresponding to a mantle potential temperature of 1900°C. These temperatures are in excess of the mantle temperatures predicted by secular cooling models, and thus komatiites can only be formed in hot rising convective jets in the mantle. This result requires that Archaean mantle jets may have been 300°C hotter than the Archaean ambient mantle temperature. This temperature difference is similar to the 200–300°C temperature difference between present day jets and ambient mantle temperatures. An important subsidiary result of this study is the confirmation that spinifex rocks may be cumulates and do not necessarily represent liquid compositions.

Clinopyroxene composition in mafic lavas from different tectonic settings

Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na2O vs MnO vs TiO2, SiO2 vs TiO2 and SiO2 vs Al2O3 provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.

Oceanic nickel depletion and a methanogen famine before the Great Oxidation Event

It has been suggested that a decrease in atmospheric methane levels triggered the progressive rise of atmospheric oxygen, the so-called Great Oxidation Event, about 2.4 Gyr ago. Oxidative weathering of terrestrial sulphides, increased oceanic sulphate, and the ecological success of sulphate-reducing microorganisms over methanogens has been proposed as a possible cause for the methane collapse, but this explanation is difficult to reconcile with the rock record. Banded iron formations preserve a history of Precambrian oceanic elemental abundance and can provide insights into our understanding of early microbial life and its influence on the evolution of the Earth system. Here we report a decline in the molar nickel to iron ratio recorded in banded iron formations about 2.7 Gyr ago, which we attribute to a reduced flux of nickel to the oceans, a consequence of cooling upper-mantle temperatures and decreased eruption of nickel-rich ultramafic rocks at the time. We measured nickel partition coefficients between simulated Precambrian sea water and diverse iron hydroxides, and subsequently determined that dissolved nickel concentrations may have reached ∼400 nM throughout much of the Archaean eon, but dropped below ∼200 nM by 2.5 Gyr ago and to modern day values (∼9 nM) by ∼550 Myr ago. Nickel is a key metal cofactor in several enzymes of methanogens and we propose that its decline would have stifled their activity in the ancient oceans and disrupted the supply of biogenic methane. A decline in biogenic methane production therefore could have occurred before increasing environmental oxygenation and not necessarily be related to it. The enzymatic reliance of methanogens on a diminishing supply of volcanic nickel links mantle evolution to the redox state of the atmosphere.

A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992

Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983–1991 was (11.6±0.2) ppbv yr−1, but the increase in 1992 was only (1.8±1.6) ppbv. In the Southern Hemisphere, the average increase (1983–1991) was (11.1±0.2) ppbv yr−1, and the 1992 increase was (7.7±1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by mans activities.

Methane on the Rise—Again

Atmospheric concentrations of the greenhouse gas methane are rising, but the reasons remain incompletely understood. Roughly one-fifth of the increase in radiative forcing by human-linked greenhouse gases since 1750 is due to methane. The past three decades have seen prolonged periods of increasing atmospheric methane, but the growth rate slowed in the 1990s (1), and from 1999 to 2006, the methane burden (that is, the total amount of methane in the air) was nearly constant. Yet strong growth resumed in 2007. The reasons for these observed changes remain poorly understood because of limited knowledge of what controls the global methane budget (2).

Boninites, komatiites and ophiolitic basalts

The rare Phanerozoic rock type boninite petrographically resembles Archaean basaltic komatiites and the range of boninite compositions overlaps that of basaltic komatiites. Quench amphibole and hydrous glass in the groundmass of boninites confirm earlier ideas that they may be the products of partial melting of peridotite in hydrous conditions, whereas basaltic komatiites have been assumed to result from fractionation of dry melts. Where field relationships are known, boninites are found either in ophiolites or a fore-arc tectonic environment.

Archean Greenstone Belts Are Not Oceanic Crust

Archean greenstone belts with a preponderance of mafic volcanic rocks, often preserved in tectonically complex sequences, are obvious candidates in the search for remnants of Archean ocean crust. We review the tectonic setting and stratigraphy of a number of greenstone sequences previously interpreted as Archean ophiolites and conclude, on the basis of basal unconformities, presence of xenocryst zircons, geochemical and isotopic evidence for crustal contamination, intrusive relationships with older basement and their internal stratigraphy, that none of these examples is derived from Archean oceanic crust. Thermal, tectonic, and isostatic constraints imply that Archean oceanic basins did exist and were covered with several kilometers of water. We consider it possible, indeed probable, that relict Archean oceanic crust is preserved in granite-greenstone terrains or other Archean tectonic settings but that an unequivocal Archean ophiolite has not yet been recognized.

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.