Toyoho Ishimura

Radiocarbon‐based carbon source quantification of anomalous isotopic foraminifera in last glacial sediments in the western North Pacific

A previous study interpreted extremely 13C-depleted excursions of planktonic and benthic foraminifera in last glacial sediments (17,500 to 25,400 cal years B.P.) of the core retrieved from off Shimokita Peninsula and off Hokkaido, Japan, as evidence for periodic releases of methane, arising from the dissociation of methane hydrate. To better understand the formation process of the 13C-depleted excursions, we conducted high-resolution natural radiocarbon measurements and biogeochemical analyses. We found highly depleted 13C excursions ranging from −10.2‰ to −1.6‰ and −6.8‰ to −1.6‰ in planktonic and benthic foraminifera, respectively. Most of the foraminiferal tests in these horizons were brown, most likely as a result of postdepositional alteration, reflecting the formation of authigenic carbonate on the surface of tests. These alterations were also supported by high levels of Mg-calcite and the acid-leaching test for anomalous foraminifera. To evaluate the carbon sources in the altered foraminifera tests, we quantified the relative contributions of 14C-free methane-derived carbon sources to the formation of authigenic carbonates in foraminifera with depleted 13C excursions using a coupled mass balance isotopic model (14C/C and 13C/12C). The radiocarbon ages of both planktonic and benthic 13C-depleted foraminifera were approximately 600 to 2000 years older than those of normal tests from nearby horizons. The relative contributions of authigenic carbonates derived from the methane oxidizing process reached to ∼22 wt% for planktonic foraminifera and ∼15 wt% for benthic foraminifera. The δ13C values of methane calculated from the mass balance model were between −29‰ and −68‰ for planktonic foraminifera and between −40‰ and −108‰ for benthic foraminifera, consistent with δ13C values reported for thermogenic and abiogenic methane in global methane hydrate reservoirs. These data consistently suggest that methane-related drastic environmental change occurred in the horizons that included δ13C anomalies. This study provides important information for interpreting geological records of the methane hydrate instability associated with climate.

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.

Geomicrobiological properties of ultra-deep granitic groundwater from the Mizunami Underground Research Laboratory (MIU), central Japan.

Although deep subterranean crystalline rocks are known to harbor microbial ecosystems, geochemical factors that constrain the biomass, diversity, and metabolic activities of microorganisms remain to be clearly defined. To better understand the geochemical and microbiological relationships, we characterized granitic groundwater collected from a 1,148- to 1,169-m-deep borehole interval at the Mizunami Underground Research Laboratory site, Japan, in 2005 and 2008. Geochemical analyses of the groundwater samples indicated that major electron acceptors, such as NO3− and SO42−, were not abundant, while dissolved organic carbon (not including organic acids), CH4 and H2, was moderately rich in the groundwater sample collected in 2008. The total number of acridine orange-stained cells in groundwater samples collected in 2005 and 2008 were 1.1u2009×u2009104 and 5.2u2009×u2009104u2009cells/mL, respectively. In 2005 and 2008, the most common phylotypes determined by 16S rRNA gene sequence analysis were both related to Thauera spp., the cultivated members of which can utilize minor electron donors, such as aromatic and aliphatic hydrocarbons. After a 3–5-week incubation period with potential electron donors (organic acids or CH4u2009+u2009H2) and with/without electron acceptors (O2 or NO3−), dominant microbial populations shifted to Brevundimonas spp. These geomicrobiological results suggest that deep granitic groundwater has been stably colonized by Thauera spp. probably owing to the limitation of O2, NO3−, and organic acids.

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 nitrogenu2009+u2009ethanol (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.