Naoto Takahata

Volcanic flux of nitrogen from the Earth

Abstract The global flux of nitrogen from subduction zones is estimated by the elemental and isotopic compositions of nitrogen, argon and helium observed in volcanic gases and hydrothermal fluids in island arcs and in back-arc basin basalt (BABB) glasses. The 3He/4He ratios of island arc samples vary from 4.7 Ratm to 7.5 Ratm, indicating a typical subduction signature. The 40Ar/36Ar ratios are consistent with atmospheric values except for a few samples. The δ15N values range from +0.1‰ to +4.6‰, which is generally higher than those of BABB glasses. Taking into account data distribution in the δ15N–N2/36Ar diagram, we distinguish three nitrogen components (mantle-derived, sedimentary and atmospheric nitrogen) for the island arc samples. Contribution of mantle-derived nitrogen is 9–30% in the samples, which is consistent with that of mantle-derived carbon. It is possible to calculate nitrogen flux based on the 3He flux in the literature and N2/3He ratios corrected for elemental fractionation. The nitrogen flux of 6.4×108 mol/year from island arc is comparable with 5.6×108 mol/year from back-arc basin, but smaller than 2.2×109 mol/year from mid-ocean ridges. In detail, island arcs show a large flux of subducted sedimentary nitrogen, while back-arc basins have a relatively small but measurable subduction component. The nitrogen flux of 4.1×106 mol/year from hot spot region is significantly small, which is consistent with the characteristic of global carbon flux from the Earth. Total volcanic flux of nitrogen amounts to 2.8×109 mol/year by taking mid-ocean ridge, hot spot and subduction values. The global nitrogen flux, if it has been constant for the 4.55 billion years of geological time, leads to an accumulation of 1.3×1019 mol in total, which is one order of magnitude smaller than 1.8×1020 mol of the present inventory of nitrogen at the Earths surface.

Carbon and nitrogen assimilation in deep subseafloor microbial cells.

Remarkable numbers of microbial cells have been observed in global shallow to deep subseafloor sediments. Accumulating evidence indicates that deep and ancient sediments harbor living microbial life, where the flux of nutrients and energy are extremely low. However, their physiology and energy requirements remain largely unknown. We used stable isotope tracer incubation and nanometer-scale secondary ion MS to investigate the dynamics of carbon and nitrogen assimilation activities in individual microbial cells from 219-m-deep lower Pleistocene (460,000 y old) sediments from the northwestern Pacific off the Shimokita Peninsula of Japan. Sediment samples were incubated in vitro with 13C- and/or 15N-labeled glucose, pyruvate, acetate, bicarbonate, methane, ammonium, and amino acids. Significant incorporation of 13C and/or 15N and growth occurred in response to glucose, pyruvate, and amino acids (∼76% of total cells), whereas acetate and bicarbonate were incorporated without fostering growth. Among those substrates, a maximum substrate assimilation rate was observed at 67 × 10−18 mol/cell per d with bicarbonate. Neither carbon assimilation nor growth was evident in response to methane. The atomic ratios between nitrogen incorporated from ammonium and the total cellular nitrogen consistently exceeded the ratios of carbon, suggesting that subseafloor microbes preferentially require nitrogen assimilation for the recovery in vitro. Our results showed that the most deeply buried subseafloor sedimentary microbes maintain potentials for metabolic activities and that growth is generally limited by energy but not by the availability of C and N compounds.

Microdistribution of Mg/Ca, Sr/Ca, and Ba/Ca ratios in Pulleniatina obliquiloculata test by using a NanoSIMS: Implication for the vital effect mechanism

The Mg/Ca ratio within foraminiferal calcareous tests (shells) is widely used to reconstruct past seawater temperature. However, recent studies reported that the organic components within a test affect the Mg/Ca distribution. In this study, we have measured the Mg/Ca, Sr/Ca, and Ba/Ca ratios within the planktonic foraminifera Pulleniatina obliquiloculata by using a NanoSIMS (secondary ion mass spectrometer (SIMS)), which has excellent spatial resolution (∼1 μm) and allows us to compare the distribution of chemical compositions with that of the organic components. Element compositions show banding distributions composed of alternately higher and lower values of those elemental ratios. The Mg/Ca ratios, previously considered to be mainly controlled by calcification temperature, show larger variations than the values expected from the seawater temperature at the habitat depth of P. obliquiloculata. Comparison of the elemental distribution with the test microstructure reveals that the bands of high Mg/Ca and Sr/Ca ratios correspond with layers of the organic components. Such coincidence suggests that the organic components strongly affect the Mg/Ca and Sr/Ca ratios within a test. In spite of the heterogeneous distribution, temperature estimated from the averaged Mg/Ca ratio within a test is consistent with seawater temperature at the habitat depth of P. obliquiloculata, indicating that whole Mg/Ca ratio of foraminiferal test may be useful as paleotemperature proxy. In contrast to the Mg/Ca ratio the heterogenity in Ba/Ca ratio, which previously has been considered to be mainly controlled by the ambient seawater composition, is not fully matched with the layers of the organic compositions. Although the organic components concentrate Ba, other unknown factors appear to also cause heterogenity in Ba incorporation.

Nitrogen recycling in subduction zones

The isotopic composition of nitrogen bears important information concerning the fate of this volatile in the mantle-crust-atmosphere system. Based on the isotopic compositions of mid-ocean ridge basalts and pristine diamonds, a δ 15 N value of about -5±2‰ (where δ 15 N = [( 15 N/ 14 N) sample /( 15 N/ 14 N) air -1] x 1000) is assigned to the upper mantle. In contrast, the origin of nitrogen has not been well documented in subduction zones. We report here δ 15 N values, N 2 / 36 Ar, and 40 Ar/ 36 Ar ratios of a comprehensive suite of samples from along convergent plate boundaries measured using a newly developed static mass spectrometry method. The δ 15 N values and the N 2 / 36 Ar ratios vary significantly from -2.7‰ to 5.0‰ and from 2.28x10 4 to 7.15x10 5 , respectively. Taking a typical δ 15 N value for sediments, assuming that the N 2 / 36 Ar ratio of mid-ocean ridge basalts is representative of the upper mantle, and that later isotopic and elemental fractionation are not significant, it is possible to deconvolve each fraction of nitrogen on the basis of simple mixing equations. About 50% of nitrogen in back-arc basin basalts originated from the upper mantle, whereas island arc samples contain only about 15% of the mantle-derived nitrogen, the major fraction being derived from recycled sedimentary nitrogen.

Past daily light cycle recorded in the strontium/calcium ratios of giant clam shells

The historical record of daily light cycle in tropical and subtropical regions is short. Moreover, it remains difficult to extract this cycle in the past from natural archives such as biogenic marine carbonates. Here we describe the precise analysis of Sr/Ca, Mg/Ca, and Ba/Ca ratios in a cultivated giant clam shell, using a laterally high-resolution secondary ion mass spectrometer with 2 μm resolution. The Sr/Ca ratio exhibits striking diurnal variations, reflecting the daily light cycle. A clear seasonal variation in Sr/Ca is also observed in another longer set of measurements with 50 μm resolution. Light-enhanced calcification and elemental transportation processes, in giant clam and symbiotic algae, may explain these diurnal and annual variations. This opens the possibility to develop the Sr/Ca ratio from a giant clam shell as an effective proxy for parameters of the daily light cycle.

Ten-year helium anomaly prior to the 2014 Mt Ontake eruption

Mt Ontake in central Japan suddenly erupted on 27th September 2014, killing 57 people with 6 still missing. It was a hydro-volcanic eruption and new magmatic material was not detected. There were no precursor signals such as seismicity and edifice inflation. It is difficult to predict hydro-volcanic eruptions because they are local phenomena that only affect a limited area surrounding the explosive vent. Here we report a long-term helium anomaly measured in hot springs close to the central cone. Helium-3 is the most sensitive tracer of magmatic volatiles. We have conducted spatial surveys around the volcano at once per few years since November 1981. The 3He/4He ratios of the closest site to the cone stayed constant until June 2000 and increased significantly from June 2003 to November 2014, while those of distant sites showed no valuable change. These observations suggest a recent re-activation of Mt Ontake and that helium-3 enhancement may have been a precursor of the 2014 eruption. We show that the eruption was ultimately caused by the increased input of magmatic volatiles over a ten-year period which resulted in the slow pressurization of the volcanic conduit leading to the hydro-volcanic event in September 2014.

Sulphur geodynamic cycle

Evaluation of volcanic and hydrothermal fluxes to the surface environments is important to elucidate the geochemical cycle of sulphur and the evolution of ocean chemistry. This paper presents S/3He ratios of vesicles in mid-ocean ridge (MOR) basalt glass together with the ratios of high-temperature hydrothermal fluids to calculate the sulphur flux of 100 Gmol/y at MOR. The S/3He ratios of high-temperature volcanic gases show sulphur flux of 720 Gmol/y at arc volcanoes (ARC) with a contribution from the mantle of 2.9%, which is calculated as 21 Gmol/y. The C/S flux ratio of 12 from the mantle at MOR and ARC is comparable to the C/S ratio in the surface inventory, which suggests that these elements in the surface environments originated from the upper mantle.

Continuous monitoring of dissolved gas concentrations in groundwater using a quadrupole mass spectrometer

Abstract An apparatus has been developed for fully automated, continuous measurement of the concentration of selected gases in groundwater using a quadrupole mass spectrometer. The apparatus has several advantages over conventional methods: (1) since a gas-permeable microporous film is attached to the head of a sampling tube, gases dissolved in groundwater are separated from water by putting the sampling tube into water; (2) by continuously introducing gases separated from water into an on line mass spectrometric system, we have successfully made isotopic measurements such as 40 Ar/ 36 Ar in groundwater on a continuous basis; and (3) very short intervals of measurement, less than several minutes, are attained by switching the electric voltage of the mass spectrometer discontinuously from peak to peak of arbitrarily chosen gas species. Some experiments are described to evaluate the basic performance of the apparatus such as the reproducibility in repeated measurements, dependence on the total amount and chemical composition of gases, and the response time to a sudden change of dissolved gas contents. An example is presented of observation of distilled water, which demonstrates that the new apparatus is sufficient for continuous monitoring of selected dissolved gases in water for various purposes.

Intracellular Isotope Localization in Ammonia sp. (Foraminifera) of Oxygen-Depleted Environments: Results of Nitrate and Sulfate Labeling Experiments

Some benthic foraminiferal species are reportedly capable of nitrate storage and denitrification, however, little is known about nitrate incorporation and subsequent utilization of nitrate within their cell. In this study, we investigated where and how much 15N or 34S were assimilated into foraminiferal cells or possible endobionts after incubation with isotopically labeled nitrate and sulfate in dysoxic or anoxic conditions. After 2 weeks of incubation, foraminiferal specimens were fixed and prepared for Transmission Electron Microscopy (TEM) and correlative nanometer-scale secondary ion mass spectrometry (NanoSIMS) analyses. TEM observations revealed that there were characteristic ultrastructural features typically near the cell periphery in the youngest two or three chambers of the foraminifera exposed to anoxic conditions. These structures, which are electron dense and ~200–500 nm in diameter and co-occurred with possible endobionts, were labeled with 15N originated from 15N-labeled nitrate under anoxia and were labeled with both 15N and 34S under dysoxia. The labeling with 15N was more apparent in specimens from the dysoxic incubation, suggesting higher foraminiferal activity or increased availability of the label during exposure to oxygen depletion than to anoxia. Our results suggest that the electron dense bodies in Ammonia sp. play a significant role in nitrate incorporation and/or subsequent nitrogen assimilation during exposure to dysoxic to anoxic conditions.

Early trace of life from 3.95 Ga sedimentary rocks in Labrador, Canada

The vestiges of life in Eoarchean rocks have the potential to elucidate the origin of life. However, gathering evidence from many terrains is not always possible, and biogenic graphite has thus far been found only in the 3.7–3.8 Ga (gigayears ago) Isua supracrustal belt. Here we present the total organic carbon contents and carbon isotope values of graphite (δ13Corg) and carbonate (δ13Ccarb) in the oldest metasedimentary rocks from northern Labrador. Some pelitic rocks have low δ13Corg values of −28.2, comparable to the lowest value in younger rocks. The consistency between crystallization temperatures of the graphite and metamorphic temperature of the host rocks establishes that the graphite does not originate from later contamination. A clear correlation between the δ13Corg values and metamorphic grade indicates that variations in the δ13Corg values are due to metamorphism, and that the pre-metamorphic value was lower than the minimum value. We concluded that the large fractionation between the δ13Ccarb and δ13Corg values, up to 25‰, indicates the oldest evidence of organisms greater than 3.95 Ga. The discovery of the biogenic graphite enables geochemical study of the biogenic materials themselves, and will provide insight into early life not only on Earth but also on other planets.