D. C. Lowe

A 21st century shift from fossil-fuel to biogenic methane emissions indicated by 13CH4

Getting a rise out of agriculture Methane, a powerful and important greenhouse gas, has been accumulating nearly uninterruptedly in the atmosphere for the past 200 years, with the exception of a mysterious plateau between 1999 and 2006. Schaefer et al. measured methanes carbon isotopic composition in samples collected over the past 35 years in order to constrain the cause of the pause. Lower thermogenic emissions or variations in the hydroxyldriven methane sink caused the plateau. Thermogenic emissions didnt resume to cause the subsequent rise. Instead, the ongoing rise is most likely due to biogenic sources, most notably agriculture. Science, this issue p. 80 The atmospheric methane level has resumed its increase after a plateau between 1999 and 2006. Between 1999 and 2006, a plateau interrupted the otherwise continuous increase of atmospheric methane concentration [CH4] since preindustrial times. Causes could be sink variability or a temporary reduction in industrial or climate-sensitive sources. We reconstructed the global history of [CH4] and its stable carbon isotopes from ice cores, archived air, and a global network of monitoring stations. A box-model analysis suggests that diminishing thermogenic emissions, probably from the fossil-fuel industry, and/or variations in the hydroxyl CH4 sink caused the [CH4] plateau. Thermogenic emissions did not resume to cause the renewed [CH4] rise after 2006, which contradicts emission inventories. Post-2006 source increases are predominantly biogenic, outside the Arctic, and arguably more consistent with agriculture than wetlands. If so, mitigating CH4 emissions must be balanced with the need for food production.

Methane carbon isotope effects caused by atomic chlorine in the marine boundary layer: Global model results compared with Southern Hemisphere measurements

[1]xa0Recent measurements of the apparent kinetic isotope effect (KIE) of the methane (CH4) atmospheric sink in the extratropical Southern Hemisphere (ETSH) have shown the apparent KIE to be larger in magnitude than expected if the sink were the hydroxyl radical (OH•) alone. We present results from simulations using the U.K. Met Offices Unified Model (UM) to evaluate whether atomic chlorine (Cl•) in the marine boundary layer (MBL) could give this effect. We modify the UM to include sources of 12CH4 and 13CH4, soil and stratospheric sinks, and a tropospheric OH• sink. Also included is a Cl• sink in the MBL with a large seasonal cycle and a constant mean value (Cl•mean) in latitude. We show that analysis of the simulated seasonal cycles in CH4 mixing ratio and δ13C give an accurate estimate of the OH• KIE at ETSH midlatitudes. The apparent KIE of the combined OH• and Cl• sink increases in magnitude as Cl•mean increases. The experimentally measured values of apparent KIE in the ETSH midlatitudes of −15‰ in 1994–1996 and −7‰ in 1998–2000 are attained with MBL Cl•mean values of 28 × 103 atoms cm−3 and 9×103 atoms cm−3, respectively (although we consider the latter to be a lower bound). We suggest that 18×103 atoms cm−3 is a reasonable midrange estimate of Cl•mean in the MBL. This value results in a Cl• sink strength of 25 Tg y−1 (range 13–37 Tg y−1) and an enrichment in δ13C of atmospheric CH4 by 2.6‰ (range 1.4–3.8‰). This sink strength is significant but has not yet been included in global CH4 budgets.

14CH4 measurements in Greenland ice: investigating last glacial termination CH4 sources.

Radiocarbon measurements show that wetlands were responsible for the rapid increase of atmospheric methane concentration during the last deglaciation. Methane from Wetlands At the end of the cold climate interval called the Younger Dryas, approximately 11,600 years ago, global temperatures began their final ascent to the warmth of the Holocene, and the concentration of methane in the atmosphere increased rapidly and substantially. There has been much speculation about the cause of that increase, with most recent evidence pointing to wetlands as the source. The most direct proof of that explanation requires the measurement of the radiocarbon content of that methane. Petrenko et al. (p. 506; see the Perspective by Nisbet and Chappellaz) analyzed 1000 kilogramsized samples of Greenland ice, which have sufficient methane to allow measurement of its 14C content. They show that wetland sources indeed must have been responsible for the majority of the rise in atmospheric methane levels at the end of the Younger Dryas. The cause of a large increase of atmospheric methane concentration during the Younger Dryas–Preboreal abrupt climatic transition (~11,600 years ago) has been the subject of much debate. The carbon-14 (14C) content of methane (14CH4) should distinguish between wetland and clathrate contributions to this increase. We present measurements of 14CH4 in glacial ice, targeting this transition, performed by using ice samples obtained from an ablation site in west Greenland. Measured 14CH4 values were higher than predicted under any scenario. Sample 14CH4 appears to be elevated by direct cosmogenic 14C production in ice. 14C of CO was measured to better understand this process and correct the sample 14CH4. Corrected results suggest that wetland sources were likely responsible for the majority of the Younger Dryas–Preboreal CH4 rise.

Interannual variation of 13C in tropospheric methane: Implications for a possible atomic chlorine sink in the marine boundary layer

[1]xa0We present methane mixing ratio and δ13C time series measured at Baring Head, New Zealand, and Scott Base, Antarctica, over the years 1991–2003. These data demonstrate that the apparent kinetic isotope effect (KIE) of the methane atmospheric sink (derived from the amplitudes of the mixing ratio and δ13C seasonal cycles) is generally much larger than would be expected if the sink were the hydroxyl radical alone and has changed significantly during the observation period on a timescale of ∼3 years. We show using a global transport model that this technique for deriving the KIE should be quite accurate for a single atmospheric sink and that the change with time is unlikely to arise from El Nino–Southern Oscillation transport effects. We infer that a sink in addition to hydroxyl is required. A strong candidate for this extra sink is atomic chlorine in the marine boundary layer (MBL). We derive the amplitude of the chlorine concentration seasonal cycle that would fully account for the apparent KIE. This amplitude ranges from ∼104 atom cm−3 in 1994–1996 to about 3 × 103 atom cm−3 in 1998–2000. If the KIE is enhanced throughout the free troposphere, the seasonal mean concentrations of atomic chlorine required in the MBL would be about 3 × 104 atom cm−3 in 1994–1996 and ∼104 atom cm−3 in 1998–2000.

A new gas chromatograph‐isotope ratio mass spectrometry technique for high‐precision, N2O‐free analysis of δ13C and δ18O in atmospheric CO2 from small air samples

A new gas Chromatograph-isotope ratio mass spectrometry (GC-IRMS) technique for the first N2O-free, high-precision (<0.05‰) isotopic analysis of δ13C and δ18O in atmospheric CO2 from small air samples has been developed. On-line GC separation of CO2 and N2O from a whole air sample is combined with IRMS under elevated ion source pressures. A specialized open split interface is an integral part of the inlet system and ensures a continuous flow of either sample gas or pure helium to the IRMS. The analysis, including all flushing, uses a total amount of 45 mL of an air sample collected at ambient pressure. Of this, three 0.5 mL aliquots are injected onto the GC column, each providing ∼0.8 nmol CO2 in the IRMS source. At this sample size, δ13C precision obtained is at the theoretical shot noise limit. For typical ambient air samples collected in the Southern Hemisphere, demonstrated precisions for δ13C, δ18O, and the CO2 mixing ratio (all measured simultaneously) are 0.02‰, 0.04‰, and 0.4 ppm, parts per million (ppm) respectively. Since these data are achieved from small air samples without contamination by atmospheric N2O or the use of cryogen, the technique will be a valuable tool in global carbon cycle research.

In search of in-situ radiocarbon in Law Dome ice and firn

Abstract Results of AMS radiocarbon measurements on CO and CO2 separated from firn air directly pumped from the ice sheet, and on CO2 separated from air extracted from ice cores by a dry grating technique, are presented. The firn air samples and ice cores used in this study were collected from the region of Law Dome, Antarctica. No evidence of in-situ 14CO2 was found in the firn air samples or the ice core air samples from one site although a slight enhancement of 14CO above expected polar atmospheric concentrations was observed for some firn air samples. A clear in-situ 14CO2 signal for ice pre-dating the radiocarbon bomb pulse was found, however, in air samples extracted from an ice core from a second site. We compare these results and propose an hypothesis to explain this apparent contradiction. The degree to which in-situ 14C is released from the ice crystals during trapping and bubble formation is considered and discussed. The selectivity of the dry grating technique for the extraction of trapped atmospheric gases from ice cores is also discussed and compared with other methods.

Instruments and Methods - A novel method for obtaining very large ancient air samples from ablating glacial ice for analyses of methane radiocarbon

We present techniques for obtaining large (� 100 L STP) samples of ancient air for analysis of 14 C of methane ( 14 CH4) and other trace constituents. Paleoatmospheric 14 CH4 measurements should constrain the fossil fraction of past methane budgets, as well as provide a definitive test of methane clathrate involvement in large and rapid methane concentration ((CH4)) increases that accompanied rapid warming events during the last deglaciation. Air dating to the Younger Dryas-Preboreal and Oldest Dryas-Bolling abrupt climatic transitions was obtained by melt extraction from old glacial ice outcropping at an ablation margin in West Greenland. The outcropping ice and occluded air were dated using a combination of d 15 No f N 2, d 18 Oo f O 2, d 18 Oice and (CH4) measurements. The (CH4) blank of the melt extractions was <4 ppb. Measurements of d 18 O and d 15 N indicated no significant gas isotopic fractionation from handling. Measured Ar / N2, CFC-11 and CFC-12 in the samples indicated no significant contamination from ambient air. Ar / N2, Kr /Ar and Xe /Ar ratios in the samples were used to quantify effects of gas dissolution during the melt extractions and correct the sample (CH4). Corrected (CH4) is elevated over expected values by up to 132 ppb for most samples, suggesting some in situ CH4 production in ice at this site.

A Simple Procedure for Evaluating Global Cosmogenic 14C Production in the Atmosphere Using Neutron Monitor Data

Radiocarbon ( 14 C) produced by cosmogenic processes in the atmosphere reacts rapidly with atomic oxygen to form 14 CO. The primary sink for this species is oxidation by the OH radical, the single most important oxidation mechanism for pollutants in the atmosphere. Hence, knowledge of the spatial and temporal distribution of 14 CO allows important inferences to be made about atmospheric transport processes and the distribution of OH. Because the chemical lifetime of 14 CO against OH attack is relatively short, 1-3 months, its distribution in the atmosphere should show modulations due to changes in 14 C production caused by variations in the solar cycle. In this work we present a simple methodology to provide a time series of global 14 C production to help interpret time series of atmospheric 14 CO measurements covering the whole of solar cycle 23. We use data from neutron monitors, a readily available proxy for global 14 C production, and show that an existing 6-year time series of 14 CO data from Baring Head, New Zealand, tracks changes in global 14 C production at the onset of solar cycle 23.

Modeling the effects of methane source changes on the seasonal cycles of methane mixing ratio and δ13C in Southern Hemisphere midlatitudes

[1]xa0The apparent kinetic isotope effect (ɛA) inferred from measurements of methane (CH4) and δ13CH4 in the extratropical Southern Hemisphere (ETSH) is significantly larger than expected if the sink were caused by hydroxyl radical (OH•) alone. If the OH• sink were the primary driver for the observed seasonal cycles in CH4 and δ13CH4 in the ETSH, published laboratory data suggest an ɛA of the order −3.9‰. However, a value of the order −11‰ has been inferred from measurements in the background troposphere. Atomic chlorine (Cl•) in the marine boundary layer (MBL) could explain the difference, but an alternative hypothesis is that the large ɛA in the ETSH is due to seasonally varying CH4 emissions. We test this by sequentially omitting or doubling CH4 emissions from each seasonally varying source in the UMeth model (Unified Model with Methane) and evaluating the corresponding effects on ɛA at Baring Head (41.40°S). If only the OH• sink is included, the maximum change in ɛA is about 1.7‰. When both OH• and a mean Cl• concentration of 18 × 103 atoms cm−3 in the MBL are included, the maximum change in ɛA is about 1.1‰. For realistic changes in the CH4 sources over a few years (∼10% of the simulated changes), we estimate that changes in ɛA are of the same order as the uncertainties in the derivation of ɛA from time series of CH4 and δ13CH4 over a comparable period. We conclude that realistic changes in seasonally varying CH4 sources do not significantly affect ɛA or previously derived estimates of the MBL Cl• sink using ɛA.

A New Method for Analyzing 14C of Methane in Ancient Air Extracted from Glacial Ice

We present a new method developed for measuring radiocarbon of methane (14CH4) in ancient air samples extracted from glacial ice and dating 11,000-15,000 calendar years before present. The small size (~20 µg CH4 carbon), low CH4 concentrations ((CH4), 400-800 parts per billion (ppb)), high carbon monoxide concentrations ((CO)), and low 14C activity of the samples created unusually high risks of contamination by extraneous carbon. Up to 2500 ppb CO in the air sam- ples was quantitatively removed using the Sofnocat reagent. 14C procedural blanks were greatly reduced through the construc- tion of a new CH4 conversion line utilizing platinized quartz wool for CH4 combustion and the use of an ultra-high-purity iron catalyst for graphitization. The amount and 14C activity of extraneous carbon added in the new CH4 conversion line were determined to be 0.23 ± 0.16 µg and 23.57 ± 16.22 pMC, respectively. The amount of modern (100 pMC) carbon added during the graphitization step has been reduced to 0.03 µg. The overall procedural blank for all stages of sample handling was 0.75 ± 0.38 pMC for ~20-µg, 14C-free air samples with (CH4) of 500 ppb. Duration of the graphitization reactions for small (<25 µg C) samples was greatly reduced and reaction yields improved through more efficient water vapor trapping and the use of a new iron catalyst with higher surface area. 14C corrections for each step of sample handling have been determined. The resulting overall 14CH4 uncertainties for the ancient air samples are ~1.0 pMC.