Abundance and carbon isotopic composition (δ13C) of monocarboxylic fatty acids (n-FA) and n-alkanes in sediments spanning the last 24 kyrs from the southeastern Oki Trough, Japan Sea (core KT96-17, P-2) are presented. The δ13C values of n-alkanes, ranging from –36.9 to –25.6‰, are consistent with those (from –44.9 to –23.3‰) measured in sediments of C-3 and L-3 cores, located ca. 10 km southeast from KT96-17, P-2 core. We employed multiple regression analysis for n-C16–30 FA as a function of latitude and distance from the adjacent landmasses, using the δ13C database obtained in sediments of the northeastern Asia. Calculated latitudes from even n-C20–30 FA ranged from 54 to 37°N over the last 24 kyrs. Although the regression accounts for 59% of the total, the result is strongly supported by other evidences, i.e.: (i) a latitudinal migration from the last glacial period to the Present corresponding to the summer polar front migration observed from loess analysis; (ii) temperature estimated from the latitudes ranged from 2.5 to 15°C, corresponding to those estimated from pollen data; and (iii) C4 plant evolution at ca. 10 ka estimated from their temperatures, which is consistent with historical and archeological studies.
Isotopic anomalies of 54Cr have been reported in bulk chondrites. Stepwise dissolution experiments have suggested the presence of a carrier of the 54Cr anomaly, probably of presolar origin. Although stepwise dissolution experiments and ion microprobe studies of Cr-rich grains have revealed, to some extent, the carrier of anomalous 54Cr, the nature of the carrier is not yet well understood. In this study, we attempted to detect the carrier phases. We performed an in-situ measurement of Cr isotopic compositions of small (~1 μm or less in size) Cr-bearing grains in the Murchison CM2 chondrite using a NanoSIMS 50 ion microprobe. 54Cr has an isobaric interference of 54Fe, which makes the measurement of 54Cr/52Cr isotopic ratios difficult. In order to evaluate the contribution of 54Fe, the abundance of 56Fe was measured. Even after large corrections for 54Fe, δ54Cr values of Cr-rich grains were determined with a precision of ~30‰. No grain out of ~200 Cr-rich grains was confirmed to be a presolar grain with a large 54Cr anomaly. If the carrier of the 54Cr anomaly is Cr-rich, this result implies a low abundance and a huge isotopic anomaly of the carrier, and/or its extremely fine grain size. In the former case, the maximum abundance of the presolar Cr-rich grains is roughly estimated to be ~1 ppm. In that case the minimum isotopic anomaly would be ~90‰.
The ablation protocols for 200 nm ultra violet-femtosecond laser ablation (200FsLA)—inductively coupled plasma mass spectrometry (ICPMS) have been examined using a quadrupole ICPMS and applied to zircon U–Pb age dating coupled to a multiple collector (MC)-ICPMS. As high spatial resolution is often required in small natural zircons, the following ablation protocols were examined: (1) a ~30 μm circular spot crater with 5 Hz repetition rate, and (2) a ~30 μm circular crater formed by rotation raster (a ~20 μm beam rotated along the circumference of a circle with a radius of 8 μm) at 10 Hz. A line raster with a ~30 μm crater at 5 Hz was also examined for comparison. The repetition rates were set in order to achieve the necessary signal intensity for zircon dating. During the spot and the rotation raster analyses there was almost no down-hole fractionation of 207Pb/206Pb, 206Pb/238U, and additionally 232Th/238U. Therefore, precise zircon U–Pb chronology, that does not require a down-hole fractionation correction, is possible using optimized protocols for 200FsLA. However, absolute 206Pb/238U ratios deviated from the natural ratios in zircon by 1.4% for 200FsLA when using synthetic glass as a standard. This indicates that a standard with a matrix that matches the sample or close monitoring of the correction factor is still recommended.
We investigated the summertime formation of secondary organic aerosols (SOA) via the oxidation of isoprene, α/β-pinene and β-caryophyllene in a Quercus crispula and Picea glehnii mast mixed forest located at Hokkaido University Uryu Experimental Forest in Japan. Biogenic SOA tracers and other polar organic compounds (e.g., sugars and aromatic acids) in time-resolved (4 h) aerosol samples (13–15 August, 2001) were characterized using gas chromatography-mass spectrometry. Isoprene SOA tracers including 2-methyltetrols and C5-alkane triols were found to be the most abundant compound class (32–219 ng m–3, average 113 ng m–3), followed by sugars/sugar alcohols. A strong diurnal variation of isoprene oxidation products was observed with higher concentrations during late afternoon-early evening. However, there were no clear trends for α/β-pinene and β-caryophyllene oxidation products. The daytime formation of isoprene SOA correlated well with increased temperature and solar radiation, suggesting a temperature- and/or light-dependent emission of isoprene in the forest followed by photochemical oxidation. Levoglucosan, a biomass burning tracer, showed no correlation with biogenic SOA tracers, indicating that biomass burning contribute little to the formation of biogenic SOA at the sampling site. A significant decrease in the concentrations of biogenic SOA tracers and other polar organic tracers was found during a fog event. Using a tracer-based method, we conclude that the contributions of secondary organic carbon (SOC) from isoprene oxidation products to organic carbon were more significant than those of α/β-pinene and β-caryophyllene oxidation products. The total SOC accounts for 5.7–34% (average 17%) of OC. This suggests that the emission of biogenic volatile organic compounds followed by subsequent oxidation plays an important role in the formation of SOA over the Quercus and Picea mixed forest.
Chemical and stable isotopic (δD, δ18O, δ34S) compositions of rivers and groundwaters, mineral constituents of rock samples from wells, and δ34S values of anhydrite in the Izu collision zone and its adjacent eastern area, southern Kanto Plain, central Japan, were analyzed to constrain the water-rock reactions and flow systems of the groundwaters. Inside the accreted Izu–Bonin–Mariana (IBM) basin, a two-dimensional map of the geothermal gradient calculated roughly using the discharge groundwater temperatures and the borehole temperature logging data confirms that the aquifer is recharged by the local meteoric water (LMW) and the high density seawater in the area. The oxygen and hydrogen isotopic compositions reveal that the Ca·Na–SO4 groundwaters in the Tanzawa Mts. and the high Na–Cl groundwaters in the coastal area are of meteoric water and weakly altered fossil seawater origins, respectively. Sulfur in the SO4 rich groundwaters is derived from anhydrite and gypsum based on the sulfur isotopic compositions. The sulfate-type groundwaters were produced by the following process: the LMW infiltrated downward with dissolution of the sulfate minerals from hydrothermal veins in the Tanzawa Group, produced the Ca–SO4 groundwater as a result of Ca2+ exchange partly on Na–smectite layer of mixed-layer chlorite–smectite in the sedimentary rocks of the Tanzawa Group. The Ca–Cl groundwaters in the eastern margin of the Tanzawa Mts. were produced by mixing of LMW with fossil seawater recharged from the surface of the coastal area, and Ca2+ exchange of the mixed-layer mineral in pyroclastic rocks of the Tanzawa Group. Outside the accreted IBM basin, the Na–HCO3 groundwaters in the shallow aquifer were formed by dissolution of authigenic calcite with LMW, and Na+ exchange in the Kazusa Group. The moderate Na–Cl groundwaters in the deep aquifer were formed by mixing of the deep seated fossil seawater with the Na–HCO3 waters in permeable sandstone and conglomerate of the Kazusa Group.
Basalts from Logudoro, Sardinia (Italy), showing geochemical features similar to EM1-type mantle source, have been analyzed for their Li isotopic composition to provide data related to a heretofore unknown reservoir. The Li concentrations of the Logudoro basalts vary between 7.7 and 10.8 ppm, which falls within the common range of Li abundances in OIB, but their Li/Dy ratios are markedly higher than those of most OIB. Based on our calculations, such a feature is ascribed to partial melting processes occurred at higher pressure than those of ordinary OIB, and it is likely to result from the increasing amount of residual garnet present during partial melting. The Li isotopic compositions of Logudoro basalts range from δ7Li = +1.5 to +3.6‰ (δ7Li(‰) = ([7Li /6Li ]sample/[7Li /6Li ]L-SVEC standard – 1) × 1000). These ratios are similar to or slightly lower than those of ordinary mantle materials. The diffusion calculations for recycled subducted oceanic crust and delaminated lower continental crust indicate that the fast diffusion of Li prevents the recycled materials from preserving their original Li isotopic ratio, thus it is difficult to constrain the specific recycled material in OIB source in terms of Li isotopes. Nevertheless, it is likely that the observed Li isotopic ratios have derived from the mantle or lower oceanic crust whose Li isotopic compositions were modified by Li diffusion from a Li-enriched part to Li-poor parts.
Theoretical and experimental aspects of oxygen isotope fractionation in carbonate minerals are critically examined based on a direct comparison of fractionation factors for carbonate-water systems. The results show good agreement between theory and experiment in most cases. In particular, theoretical fractionation factors calculated by the statistico-mechanics method and the increment method are in good agreement with the experimental values for dolomite, siderite, witherite, strontianite and cerussite. These agreements provide corroboration that the two entirely independent approaches of calculation are generally capable of producing thermodynamic equilibrium fractionation factors for the most carbonate-water systems. In particular, the merit of the increment method relative to the statistico-mechanics method is evident for crystalline minerals because it enables the systematic and accurate predictions of oxygen isotope fractionation factors for different structures and compositions of crystalline minerals based only on their crystal chemistry. Thus, the increment method has no limitations to the calculations as commonly encountered by the statistico-mechanics method. Nevertheless, complexity in oxygen isotope fractionations between calcite and the other carbonate minerals can be caused by the oxygen isotope inheritance during polymorphic transformation from aragonite to calcite and by the isotope salt effect on the other carbonates in the presence of aqueous fluids. Therefore, caution must be exercised when interpreting possible disagreements between theory and experiment because of the kinetic effects. In the extreme case, equilibrium oxygen isotope fractionation factors for single carbonate-water systems could be either underestimated or overestimated by one of the theoretical and experimental methods.