Fumiko Nakagawa

Production of methane from alasses in eastern Siberia: Implications from its 14C and stable isotopic compositions

[1] The radiocarbon ( 14 C) and stable isotopic ( 13 C and D) compositions of methane and carbon dioxide from alasses (typical landforms in permafrost terrain, consisting of lakes and wetlands) were measured around Yakutsk, eastern Siberia, where few isotope studies have been done. The carbon isotopic compositions of methane and carbon dioxide were used to study the pathways of methane formation in alasses. The mean value of methane and of carbon dioxide in each alass ranged from --63.9 to ∼58.2‰ and from -16.7 to -0.6‰, respectively. In small alasses, where the supply of fresh organic matter from surrounding wetland ecosystems is large, methane was produced almost equally from acetate fermentation and CO 2 reduction pathways. In larger alasses the CO 2 reduction pathway slightly dominates over acetate fermentation, owing to a smaller supply of labile organic matter. The 13 C enrichment in CO 2 in large lakes indicates depletion of methane precursors. Gas-bubble methane is depleted in deuterium (the mean 6D value from lakes is -360 ± 14%o and from shores is -380 ± 20%o), which reflects the deuterium-depleted environmental water (from -136 to -117‰) of alasses. Lake methane relatively enriched in D is interpreted to be the result of depletion in the hydrogen pool in lake sediments. Methane in shallow lakes is somewhat enriched in 14 C relative to modern biogenic methane and is produced from fresh, recently fixed organic matter. The 14 C content of methane from deeper lakes is 10-20% less than that of shallow lake methane, indicating a greater contribution from older methane derived from deeper parts of the sediment. The mean 13 C, D, and 14 C of methane from the alasses in general are estimated to be -61.1 ± 4.4‰, -363 ± 20%o, and 104 ± 6 pMC, respectively. This corresponds to the reported mean isotopic composition of methane from wetlands for δ 13 C, but shows lower value for δD and 14 C content.

Assessment of winter fluxes of CO2 and CH4 in boreal forest soils of central Alaska estimated by the profile method and the chamber method: a diagnosis of methane emission and implications for the regional carbon budget

This research was carried out to estimate the winter fluxes of CO2 and CH4 using the concentration profile method and the chamber method in black spruce forest soils in central Alaska during the winter of 2004/5. The average winter fluxes of CO2 and CH4 by chamber and profile methods were 0.24 ± 0.06 (SE; standard error) and 0.21 ± 0.06 gCO2-C/m2/d, and 21.4 ± 5.6 and 21.4 ± 14 μgCH4-C/m2/hr. This suggests that the fluxes estimated by the two methods are not significantly different based on a one-way ANOVA with a 95% confidence level. The hypothesis on the processes of CH4 transport/production/emission in underlying snow-covered boreal forest soils is proven by the pressure differences between air and in soil at 30 cm depth. The winter CO2 emission corresponds to 23% of the annual CO2 emitted from Alaska black spruce forest soils, which resulted in the sum of mainly root respiration and microbial respiration during the winter based on the δ13CO2 of .22.5‰. The average wintertime emissions of CO2 and CH4 were 49 ± 13 gCO2-C/m2/season and 0.11 ± 0.07 gCH4-C/m2/season, respectively. This implies that winter emissions of CO2 and CH4 are an important part of the annual carbon budget in seasonally snow-covered terrain of typical boreal forest soils.

Stable isotope and radiocarbon compositions of methane emitted from tropical rice paddies and swamps in Southern Thailand

Stable isotopes (δ13C, δD) and radiocarbon weremeasured in methane bubbles emitted from rice paddies and swamps in southernThailand. Methane emitted from the Thai rice paddies was enriched in13C (mean δ13C; −51.5 ±7.1‰ and−56.5 ± 4.6‰ for mineral soil and peat soil paddies,respectively)relative to the reported mean value of methane from temperate rice paddies(− 63 ± 5‰). Large seasonal variation was observed inδ13C(∼32‰) in the rice paddies, whereas variationinδD was much more smaller (∼20‰), indicating that variation inδ13C is due mainly to changes in methane production pathways.Values of δ13C were lower in swamps (−66.1 ±5.1‰)than in rice paddies. The calculated contribution of acetate fermentation fromδ13C value was greater in rice paddies (mineral soils:62–81%, peat soils: 57–73%) than in swamps (27–42%). δDin methane from Thai rice paddies (−324± 7‰ (n=46)) isrelativelyhigher than those from 14 stations in Japanese rice paddies ranging from−362 ± 5‰ (Mito: n=2) to −322 ± 8‰(Okinawa: n=3), due tohigher δD in floodwaters. 14C content in methane produced fromThai rice paddies (127±1 pMC) show higher 14Cactivity compared with previous work in paddy fields and those from Thai swamps(110±2 pMC).

Disturbance of deep-sea environments induced by the M9.0 Tohoku Earthquake

The impacts of the M9.0 Tohoku Earthquake on deep-sea environment were investigated 36 and 98 days after the event. The light transmission anomaly in the deep-sea water after 36 days became atypically greater (∼35%) and more extensive (thickness ∼1500 m) near the trench axis owing to the turbulent diffusion of fresh seafloor sediment, coordinated with potential seafloor displacement. In addition to the chemical influx associated with sediment diffusion, an influx of 13C-enriched methane from the deep sub-seafloor reservoirs was estimated. This isotopically unusual methane influx was possibly triggered by the earthquake and its aftershocks that subsequently induced changes in the sub-seafloor hydrogeologic structures. The whole prokaryotic biomass and the development of specific phylotypes in the deep-sea microbial communities could rise and fall at 36 and 98 days, respectively, after the event. We may capture the snap shots of post-earthquake disturbance in deep-sea chemistry and microbial community responses.

Sensitive determinations of stable nitrogen isotopic composition of organic nitrogen through chemical conversion into N2O

We present a method for high-sensitivity nitrogen isotopic analysis of particulate organic nitrogen (PON) in seawater and freshwater, for the purpose of determining the aquatic nitrogen fixation rate through the 15N2 tracer technique for samples that contain a low abundance of organisms. The method is composed of the traditional oxidation/reduction methods, such as the oxidation of PON to nitrate (NO3*) using persulfate, the reduction of NO3* to nitrite (NO2*) using spongy cadmium, and further reduction of NO2* to nitrous oxide (N2O) using sodium azide. Then, N2O is purged from the water and trapped cryogenically with subsequent release into a gas chromatography column to analyze the stable nitrogen isotopic composition using continuous-flow isotope ratio mass spectrometry (CF-IRMS) by simultaneously monitoring the NO+ ion currents at masses 30, 31, and 32. The nitrogen isotopic fractionation was consistent within each batch of analysis. The standard deviation of sample measurements was less than 0.3 per thousand for samples containing PON of more than 50 nmolN, and 0.5 per thousand for those of more than 20 nmolN, by subtracting the contribution of blank nitrogen, 8 +/- 2 nmol at final N2O. By using this method, we can determine delta15N for lower quantities of PON better than by other methods, so we can reduce the quantities of water samples needed for incubation to determine the nitrogen fixation rate. In addition, we can expand the method to determine the nitrogen isotopic composition of organic nitrogen in general, such as that of total dissolved nitrogen (TDN; sum of NO3*, NO2*, ammonium, and DON), by applying the method to filtrates.

Grain-scale heterogeneities in the stable carbon and oxygen isotopic compositions of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8)

We determined grain-scale heterogeneities (from 6 to 88 microg) in the stable carbon and oxygen isotopic compositions (delta(13)C and delta(18)O) of the international standard calcite materials (NBS 19, NBS 18, IAEA-CO-1, and IAEA-CO-8) using a continuous-flow isotope ratio mass spectrometry (CF-IRMS) system that realizes a simultaneous determination of the delta(13)C and the delta(18)O values with standard deviations (S.D.) of less than 0.05 per thousand for CO(2) gas. Based on the S.D. of the delta(13)C and delta(18)O values determined for CO(2) gases evolved from the different grains of the same calcite material, we found that NBS 19, IAEA-CO-1, and IEAE-CO-8 were homogeneous for delta(13)C (less than 0.10 per thousand S.D.), and that only NBS 19 was homogeneous for delta(18)O (less than 0.14 per thousand S.D.). On the level of single grains, we found that both IAEA-CO-1 and IAEA-CO-8 were heterogeneous for delta(18)O (1.46 per thousand and 0.76 per thousand S.D., respectively), and that NBS 18 was heterogeneous for both delta(13)C and delta(18)O (0.34 per thousand and 0.54 per thousand S.D., respectively). Closer inspection of NBS 18 grains revealed that the highly deviated isotopic compositions were limited to the colored grains. By excluding such colored grains, we could also obtain the homogeneous delta(13)C and delta(18)O values (less than 0.18 per thousand and less than 0.16 per thousand S.D., respectively) for NBS 18. We conclude that NBS 19, IAEA-CO-1, or pure grains in NBS 18 are suitable to be used as the standard reference material for delta(13)C, and that either NBS 19 or pure grains in NBS 18 are suitable to be used as the reference material for delta(18)O during the grain-scale isotopic analyses of calcite.

Determination of the 15N/14N, 17O/16O, and 18O/16O ratios of nitrous oxide by using continuous-flow isotope-ratio mass spectrometry.

We developed a rapid, sensitive, and automated analytical system to determine the delta15N, delta18O, and Delta17O values of nitrous oxide (N2O) simultaneously in nanomolar quantities for a single batch of samples by continuous-flow isotope-ratio mass spectrometry (CF-IRMS) without any cumbersome and time-consuming pretreatments. The analytical system consisted of a vacuum line to extract and purify N2O, a gas chromatograph for further purification of N2O, an optional thermal furnace to decompose N2O to O2, and a CF-IRMS system. We also used pneumatic valves and pneumatic actuators in the system so that we could operate it automatically with timing software on a personal computer. The analytical precision was better than 0.12 per thousand for delta15N with >4 nmol N2O injections, 0.25 per thousand for delta18O with >4 nmol N2O injections, and 0.20 per thousand for Delta17O with >20 nmol N2O injections for a single measurement. We were also easily able to improve the precision (standard errors) to better than 0.05 per thousand for delta15N, 0.10 per thousand for delta18O, and 0.10 per thousand for Delta17O through multiple analyses with more than four repetitions with 190 nmol samples using the automated analytical system. Using the system, the delta15N, delta18O, and Delta17O values of N2O can be quantified not only for atmospheric samples, but also for other gas or liquid samples with low N2O content, such as soil gas or natural water. Here, we showed the first ever Delta17O measurements of soil N2O.

Simultaneous determination of δ15N and δ18O of N2O and δ13C of CH4 in nanomolar quantities from a single water sample

We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (delta(15)N, delta(18)O, and delta(13)C) of nanomolar quantities of nitrous oxide (N(2)O) and methane (CH(4)) in water, by combining continuous-flow isotope-ratio mass spectrometry and a helium-sparging system to extract and purify the dissolved gases. Our system, which is composed of cold traps and a capillary gas chromatograph that use ultra-pure helium as the carrier gas, achieves complete extraction of N(2)O and CH(4) in a water sample and separation among N(2)O, CH(4), and the other component gases. The flow path following exit from the gas chromatograph was periodically changed to pass the gases through the combustion furnace to convert CH(4) and the other hydrocarbons into CO(2), or to bypass the combustion furnace for the direct introduction of eluted N(2)O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO(2) (+) and N(2)O(+). The analytical system can be operated automatically with sequential software programmed on a personal computer. Analytical precisions better than 0.2 per thousand and 0.3 per thousand and better than 1.4 per thousand and 2.6 per thousand were obtained for the delta(15)N and delta(18)O of N(2)O, respectively, when more than 6.7 nmol and 0.2 nmol of N(2)O, respectively, were injected. Simultaneously, analytical precisions better than 0.07 per thousand and 2.1 per thousand were obtained for the delta(13)C of CH(4) when more than 5.5 nmol and 0.02 nmol of CH(4), respectively, were injected. In this manner, we can simultaneously determine stable isotopic compositions of a 120 mL water sample with concentrations as low as 1.7 nmol/kg for N(2)O and 0.2 nmol/kg for CH(4).

Stable carbon and oxygen isotopic analysis of carbon monoxide in natural waters.

Techniques have been developed to allow on-line simultaneous analysis of concentration and stable isotopic compositions ((13)C and (18)O) of dissolved carbon monoxide (CO) in natural water, using continuous-flow isotope ratio mass spectrometry (CF-IRMS). The analytical system consisted sequentially of a He-sparging bottle of water, a gas dryer, CO(2)-trapping stage using both Ascarite trap and silica-gel packed gas chromatography (GC), on-line oxidation to CO(2) using the Schütze reagent, cryofocusing, GC purification using a capillary column and measurement by CF-IRMS. Each sample analysis takes about 40 minutes. The detection limit with delta(13)C standard deviation of 0.5 per thousand is 300 pmol and that with delta(18)O deviation of 1.0 per thousand is 750 pmol. Analytical blanks associated with these methods are 21+/-9 pmol. The procedures are evaluated through analyses of temporally varying concentration and isotopic compositions of CO in an artificial lake on the university campus. The delta(13)C and delta(18)O values of CO showed wide variation in accordance with diurnal variation of CO concentration, probably due to significant isotopic effects during photochemical production and microbial oxidation of CO in the aquatic environment. The delta(13)C and delta(18)O values of CO should be a useful tool in studies of the mechanism and pathways of CO production and consumption in natural waters.

Stable hydrogen isotopic analysis of nanomolar molecular hydrogen by automatic multi-step gas chromatographic separation

We have developed a new automated analytical system that employs a continuous flow isotope ratio mass spectrometer to determine the stable hydrogen isotopic composition (δD) of nanomolar quantities of molecular hydrogen (H(2)) in an air sample. This method improves previous methods to attain simpler and lower-cost analyses, especially by avoiding the use of expensive or special devices, such as a Toepler pump, a cryogenic refrigerator, and a special evacuation system to keep the temperature of a coolant under reduced pressure. Instead, the system allows H(2) purification from the air matrix via automatic multi-step gas chromatographic separation using the coolants of both liquid nitrogen (77 K) and liquid nitrogen + ethanol (158 K) under 1 atm pressure. The analytical precision of the δD determination using the developed method was better than 4‰ for >5 nmol injections (250 mL STP for 500 ppbv air sample) and better than 15‰ for 1 nmol injections, regardless of the δD value, within 1 h for one sample analysis. Using the developed system, the δD values of H(2) can be quantified for atmospheric samples as well as samples of representative sources and sinks including those containing small quantities of H(2) , such as H(2) in soil pores or aqueous environments, for which there is currently little δD data available. As an example of such trace H(2) analyses, we report here the isotope fractionations during H(2) uptake by soils in a static chamber. The δD values of H(2) in these H(2)-depleted environments can be useful in constraining the budgets of atmospheric H(2) by applying an isotope mass balance model.