Nobuyuki Nakai

The kinetic isotope effect in the bacterial reduction and oxidation of sulfur

The kinetic isotope effect in the bacterial reduction of sulfate to sulfide and oxidation of native sulfur and sulfide to sulfate has been investigated at the temperature of 32°C. In these studies, mixed culture techniques were used in order to more closely approximate isotopic fractionation as it occurs in nature. In addition, the δS34 compositions of sulfur in coexisting sulfate and sulfide in lake waters and streams were determined for comparison to the laboratory raw culture results.

Direct Detection of Radiocarbon Using Accelerator Techniques and Its Application to Age Measurements

Radiocarbon measurements by the direct detection of 14C atoms with the Tandetron accelerator mass spectrometer were begun in the autumn of 1983 at Nagoya University. The 14C dating of geological and archeological samples is routinely performed with a precision of ±1% (±83 years) using a carbon-silver target containing only 2–5 mg of carbon in measurements lasting 5 hours. The accuracy of a single 14C measurement was estimated by a reproducibility test to be ±1.34%. Radiocarbon ages measured with the Tandetron spectrometer for ten natural samples agreed well with those obtained by radioactivity measurements. The 14C background of the spectrometer limits the measurable age to less than 60000 y BP. The limit will be extended to about 82000 y BP if the 14C background is eliminated.

Applications of environmental 14C measured by AMS as a carbon tracer

AMS techniques have been applied to measure 14C concentrations, or Δ14C values, of annual tree rings (1945-1983) of a Kiso hinoki tree grown in Gifu prefecture, and of acid-insoluble carbonaceous compounds extracted from cored sediments (surface to 30 cm deep) of Lake Biwa in Shiga prefecture. An increase in Δ14C values was found for both tree rings and cored sediments, resulting from 14C artificially produced by nuclear weapon tests. Activities of 210Pb and 214Pb were measured to estimate the sedimentation rate of the Lake Biwa sediments and to provide a temporal control. A primitive model of carbon exchange between the troposphere, surface ocean water, and the biosphere was applied to the annual changes in Δ14C of the hinoki tree rings and also of coral rings from Florida, USA. The times required to transfer bomb-carbon from the troposphere to surface sea water and to the biosphere were estimated to be about 11 and 23 years, respectively.

Measurement of 14C Concentrations of Stratospheric CO2 by Accelerator Mass Spectrometry

In order to measure the concentrations of anthropogenically influenced gases in the stratosphere, we have collected air samples from the lower stratosphere since 1985, by a balloon-borne cryogenic sampling method, developed at the Institute of Space and Astronautical Science (ISAS). Air samples of ~16 liters at STP were collected in the stratosphere at altitudes from 18.6 to 30.4 km, over the northeastern part of Japan (39.5°N,139-142°E), on 1 September 1989. We conducted 14C analyses to study the vertical and horizontal air-mass movement in the stratosphere, and to investigate the air transport mechanism between troposphere and stratosphere. Carbon dioxide (containing a few mg carbon) was separated cryogenically from the air samples, and the 14C concentration of the CO2 was measured by a Tandetron accelerator mass spectrometer, using Fe-graphite targets prepared by reducing CO2 on Fe-powder with hydrogen in a Vycor tube at 650°C. The 14C concentrations, expressed as 14∆C, of CO2 were 267-309‰ at altitudes of 21-30 km, and 134‰ at 19-20 km. The a14C values at 21-30 km were higher than those of the current tropospheric C02, of around 80-200‰. The observed 14C concentrations, higher in the stratosphere than the troposphere, seem to be explained by large bomb-produced 14C inventories and/or high 14C production by cosmic rays, as well as weak vertical mixing of air masses in the stratosphere.

Accelerator mass spectrometry of 14C at Nagoya University

Abstract The Tandetron accelerator mass spectrometer was installed at Nagoya University in 1982. In addition to the investigation of sample preparation, radiocarbon measurements and the application to natural samples, tests have been performed by using graphitized carbon and carbon-silver targets. Of these, graphite targets give an ion beam current as high as 30 μA for 12C− ions and 14C-results with a precision of better than ± 0.4% in measurements of 14C/13C. C-Ag targets are also satisfactory and give a precision in the region of ± 2.0% for 14C/13C. 14C measurements of tree rings, grown from 1950 to 1983, give a maximum 14C concentration during 1964.

Application of 14C-dating to sedimentary geology and climatology: sea-level and climate change during the holocene

Abstract AMS 14 C dating of small sized fossil shells, wood chips and sedimentary humic matter has been applied to the investigation of paleoclimatic and sea-level changes in the Holocene, using three estuarine and brackish lake sediment cores, drilled in Kawasaki city and Lake Hamanako, Japan. Precise and detailed ages at differing depths clarified large transitions of Holocene sedimentation rates, of two orders of magnitude. The vertical variation in δ 13 C and C/N results for sedimentary organic matter, combined with AMS 14 C ages, established continuous climatic and sea-level fluctuation patterns through time and indicated the existence of neoglaciation coincident with the marine regression at 7500 to 7000 yr BP.

Synchronous variations in the content of magnetic minerals and planktonic-foraminiferal δ13C in deep-sea sediments

Abstract A correlation between magnetic mineral contents and δ13C was obtained using two deep-sea sediment cores having distinctly different sedimentation rates, taken from the western equatorial Pacific. It was inferred that the fluctuations in the content of magnetic minerals, predominantly of bacterial origin, are caused mostly by variations in CaCO3 dissolution. The correlation seems to originate from changes in the vertical distribution of nutrients and carbon with isotopic fractionation.

The Radiocarbon Measurement with the Tandem Accelerator at Nagoya University

The Tandetron accelerator mass spectrometer manufactured by General Ionex Corporation in the USA was installed at Nagoya University to measure the low-abundance isotopes in natural samples. In the installation, the apparatus was especially assembled to measure the radiocarbon abundance.