Search

We summarize (1) knowledge about the global pre-industrial cycle of CO2 and CH4, and (2) recent reconstructions of the concentration of CO2 and CH4 in the atmosphere (CO2atm and CH4atm) based on ice cores drilled in Antarctica and Greenland. We discuss critically some difficulties in the interpretation of these reconstructions and suggest studies that should permit to better understand the factors controlling CO2atm and CH4atm variations. To cite this article: O. Marchal, J. Chappellaz, C. R. Geoscience 336 (2004). [Copyright &y& Elsevier]

We report the first high-resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented membrane-based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to 6 orders of magnitude with respect to the original prototype (from nmol L -1 to mmol L -1 detection) to fit the range of concentrations at Lake Kivu. The accuracy of the instrument was estimated to ±22 % (2σ) from the standard deviation of eight profiles at 80 m depth, corresponding to ±0.112 mbar of CH4 in water or ±160 nmol L -1 at 25 ∘ C and 1 atm. The instrument was able to continuously profile the top 150 m of the water column within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 %, which at 150 m depth and under thermal conditions of the lake corresponds to 3.5 mmol L -1. Deeper down, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results are in good agreement with discrete in situ measurements conducted with the commercial HydroC® sensor. This fast-profiling feature is highly useful for studying the transport, production and consumption of CH4 and other dissolved gases in aquatic systems. While the sensor is well adapted for investigating most environments with a concentration of CH4 up to a few millimoles per liter, in the future the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and total dissolved gas pressure, for exploring settings with very high concentrations of CH4 such as the bottom waters of Lake Kivu. [ABSTRACT FROM AUTHOR]

We report the first high resolution continuous profile of dissolved methane in the shallow water of Lake Kivu, Rwanda. The measurements were performed using an in situ dissolved gas sensor, called Sub-Ocean, based on a patented, membrane based extraction technique coupled with a highly sensitive optical spectrometer. The sensor was originally designed for ocean settings, but both the spectrometer and the extraction system were modified to extend the dynamical range up to six orders of magnitude with respect to the original prototype (from nmol L-1 to mmol L-1 detection) to fit the range of concentrations at lake Kivu. The accuracy of the instrument was estimated to ±22 % (2 s) from the standard deviation of eight profiles at 80 m of depth, corresponding to ±112 μBar of CH4 in water or ±160 nmol L-1 at 25 °C and 1 atm. The instrument was able to continuously measure the top 150 m of water depth within only 25 min. The maximum observed mixing ratio of CH4 in the gas phase concentration was 77 % at 150 m depth, which at this depth and thermal condition of the lake corresponds to 3.5 mmol L-1. At deeper depth, dissolved CH4 concentrations were too large for the methane absorption spectrum to be correctly retrieved. Results were in good agreement with discrete in situ measurements conducted with the commercial HydroC sensor. The fast profiling feature will be highly profitable for future monitoring of the lake, while the spectrometer could be replaced with a less sensitive analytical technique possibly including simultaneous detection of dissolved CO2 and which would allow to measure at higher concentrations of CH4. [ABSTRACT FROM AUTHOR]

Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ13CH4 and δD(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial. [ABSTRACT FROM AUTHOR]

Abstract: Ancient air trapped inside bubbles in ice cores can now be analysed for methane concentration utilising a laser spectrometer coupled to a continuous melter system. We present a new ultra-high resolution record of atmospheric methane variability over the last 1800yr obtained from continuous analysis of a shallow ice core from the North Greenland Eemian project (NEEM-2011-S1) during a 4-week laboratory-based measurement campaign. Our record faithfully replicates the form and amplitudes of multi-decadal oscillations previously observed in other ice cores and demonstrates the detailed depth resolution (5.3cm), rapid acquisition time (30mday−1) and good long-term reproducibility (2.6%, 2σ) of the continuous measurement technique. In addition, we report the detection of high frequency ice core methane signals of non-atmospheric origin. Firstly, measurements of air from the firn–ice transition region and an interval of ice core dating from 1546–1560 AD (gas age) resolve apparently quasi-annual scale methane oscillations. Traditional gas chromatography measurements on discrete ice samples confirm these signals and indicate peak-to-peak amplitudes of ca. 22 parts per billion (ppb). We hypothesise that these oscillations result from staggered bubble close-off between seasonal layers of contrasting density during time periods of sustained multi-year atmospheric methane change. Secondly, we report the detection of abrupt (20–100cm depth interval), high amplitude (35–80ppb excess) methane spikes in the NEEM ice that are reproduced by discrete measurements. We show for the first time that methane spikes present in thin and infrequent layers in polar, glacial ice are accompanied by elevated concentrations of carbon- and nitrogen-based chemical impurities, and suggest that biological in-situ production may be responsible. [Copyright &y& Elsevier]

Abstract: In relation to Arctic warming, the possible occurrence of methane hydrate degassing events has attracted an increasing interest in recent years. We evaluate the atmospheric impact of rapid and massive emissions of methane and how they are imprinted in ice core records, by combining for the first time models of atmospheric chemistry and trace gas transport in firn. Different emission characteristics as well as climatic conditions (present, pre-industrial, glacial) are considered. The isotopic signatures characterizing stable isotopologues of methane DCH3 and 13CH4 are also analysed. Our results suggest little effect of clathrate degassing on the main methane oxidant: OH radicals. Due to the relatively short atmospheric lifetime of methane, the simulated clathrate-induced perturbations last for less than a century. This time scale is comparable to or shorter than the duration of air bubble closure in polar ice sheets. As a consequence, rapid methane perturbations in the atmosphere are strongly smoothed in ice core records. This smoothing mostly depends on the snow accumulation rate at the site of ice core drilling. We propose a methodology to identify a potential clathrate degassing event in ice core records. Continuous CH4 records from high accumulation rate sites could allow to decipher short time scale events. of CH4 should reveal a typical “lying S” shape at high accumulation rate sites, reflecting the combined effects of the clathrate source signature (negative excursion) and subsequent OH fractionation in the atmosphere (positive excursion). The amplitude ratio of the negative and positive swings recorded in Greenland and Antarctica under similar accumulation rate conditions could also indicate the latitude of a clathrate degassing event. [Copyright &y& Elsevier]

Abstract: Reconstructions of past atmospheric concentrations of greenhouse gases provide unique insight into the biogeochemical cycles and the past radiative forcing in the Earth''s climate system. We present new measurements of atmospheric nitrous oxide along the ice cores of the North Greenland Ice Core Project and Talos Dome sites. Using records of several other ice cores, we are now able to establish the first complete composite nitrous oxide record reaching back to the beginning of the previous interglacial about 140,000yr ago. On the basis of such composite ice core records, we further calculate the radiative forcing of the three most important greenhouse gases carbon dioxide, methane and nitrous oxide during more than a full glacial–interglacial cycle. Nitrous oxide varies in line with climate, reaching very low concentrations of about 200 parts per billion by volume during Marine Isotope Stages 4 and 2, and showing substantial responses to millennial time scale climate variations during the last glacial. A large part of these millennial time scale variations can be explained by parallel changes in the sources of methane and nitrous oxide. However, as revealed by high-resolution measurements covering the Dansgaard/Oeschger events 17 to 15, the evolution of these two greenhouse gases may be decoupled on the centennial time scale. Carbon dioxide and methane concentrations do not reach interglacial levels in the course of millennial time scale climate variations during the last glacial. In contrast, nitrous oxide often reaches interglacial concentrations in response to both, glacial terminations and Dansgaard/Oeschger events. This indicates, from a biogeochemical point of view, similar drivers in both temporal cases. While carbon dioxide and methane concentrations are more strongly controlled by climate changes in high latitudes, nitrous oxide emissions changes may mainly stem from the ocean and/or from soils located at low latitudes. Accordingly, we speculate that high latitudes could play the leading role to trigger glacial terminations. [Copyright &y& Elsevier]

Abstract: Isotopic mass spectrometry coupled online with gas chromatography (GC-C-IRMS) permits measurement of relative proportions of gaseous hydrocarbon (CH4 to C4H10) and CO2, and determination of carbon isotope ratio of hydrocarbon molecules. Access to these parameters provides valuable information about the source and the genesis of naturally-occurring gas, as well as on post-formation physico-chemical processes which might have taken place in the geological environment. In particular, it is possible to distinguish hydrocarbon gas of bacterial origin from that of thermogenic origin based on proportion and carbon isotope ratio of methane as measured by GC-C-IRMS. However, in samples containing very low amounts of hydrocarbons (from 1 ppm to 1000 ppm), accurate measurement of isotope ratios is often impossible due to the limitations of conventional GC-C-IRMS techniques using direct sample introduction. A technique was developed to overcome this limitation. It is based on a novel approach allowing pre-concentration of hydrocarbons prior to GC-C-IRMS analysis. The pre-concentration step consists in selective trapping of hydrocarbon molecules on a cold adsorbent phase, and removal of non-adsorbed gases (N2, O2, Ar,…). In a second step, pre-concentrated alkanes are desorbed, and released in an inert carrier gas, focused through a capillary and introduced into the GC-C-IRMS for chromatographic separation and measurement of concentration and carbon isotope composition of each individual carbon molecule. In order to achieve sufficient accuracy, several operating conditions are of prime importance, including sufficient signal intensity, well defined peak shape and low signal/noise ratio. Accurate measurements can be performed on samples as small as 10 cm3 of bulk gas in standard conditions, with concentrations as low as 1 ppm of methane, 0.5 ppm of ethane and 0.3 ppm of propane and butane. Total analytical uncertainty on δ 13C measurements ranges from ±0.2‰ to ±1.5‰, depending on the hydrocarbon molecule. [Copyright &y& Elsevier]

Atmospheric methane is an important greenhouse gas and a sensitive indicator of climate change and millennial-scale temperature variability. Its concentrations over the past 650,000 years have varied between ∼350 and ∼800 parts per 109 by volume (p.p.b.v.) during glacial and interglacial periods, respectively. In comparison, present-day methane levels of ∼1,770 p.p.b.v. have been reported. Insights into the external forcing factors and internal feedbacks controlling atmospheric methane are essential for predicting the methane budget in a warmer world. Here we present a detailed atmospheric methane record from the EPICA Dome C ice core that extends the history of this greenhouse gas to 800,000 yr before present. The average time resolution of the new data is ∼380 yr and permits the identification of orbital and millennial-scale features. Spectral analyses indicate that the long-term variability in atmospheric methane levels is dominated by ∼100,000 yr glacial–interglacial cycles up to ∼400,000 yr ago with an increasing contribution of the precessional component during the four more recent climatic cycles. We suggest that changes in the strength of tropical methane sources and sinks (wetlands, atmospheric oxidation), possibly influenced by changes in monsoon systems and the position of the intertropical convergence zone, controlled the atmospheric methane budget, with an additional source input during major terminations as the retreat of the northern ice sheet allowed higher methane emissions from extending periglacial wetlands. Millennial-scale changes in methane levels identified in our record as being associated with Antarctic isotope maxima events are indicative of ubiquitous millennial-scale temperature variability during the past eight glacial cycles. [ABSTRACT FROM AUTHOR]