Oxygen isotope values

The 8 N- and 8 0-values of atmospheric N2O today, range from 6.4 to 7.0%c and 43 to 45.5%c (Sowers 2001). Terrestrial emissions have generally lower 8-values than marine sources. The 8 N and 8 0-values of stratospheric N2O gradually increase with altitude due to preferential photodissociation of the lighter isotopes (Rahn and Wahlen 1997). Oxygen isotope values of atmospheric nitrous oxide exhibit a mass-independent component (Cliff and Thiemens 1997 Clifif et al. 1999), which increases with altitude and distance from the source. The responsible process has not been discovered so far. First isotope measurements of N2O from the Vostok ice core by Sowers (2001) indicate large and 0 variations with time (8 N from 10 to 25%c and 8 0 from 30 to 50%c), which have been interpreted to result from in situ N2O production via nitrification. 

Spero H. J., Bijma J., Lea D. W., and Bemis B. (1997) Effect of seawater carbonate chemistry on planktonic foraminiferal carbon and oxygen isotope values. Nature 390, 497-500. 

Stable isotope investigations on plant biominerals. In contrast, only a limited number of studies have investigated the stable isotope composition of plant biominerals as a source of paleoenviron-mental information. Silica extracted from the Eocene paleosols of Axel Heiberg Island in arctic Canada were found to have notably high stable oxygen isotope value, leading to the suggestion that the mineral represented an accumulation of... 

The band of buff-colored, translucent pallisade calcite from 11 mm to 32 mm. Zone C, is relatively event-free, with only a few minor dissolution horizons or detrital drapes (not visible with the naked eye) representing short-term local flood events but not longterm glacial advance flooding. The isotopes show parallel oscillations and are somewhat more enriched than the upper zones of the sample. The oxygen isotopic values lie close to those of modern calcite in the cave, suggesting a moderately cool, wet climate similar to that of today. The oscillations in the carbon isotopic values follow the simple pattern of... 

The isotopic composition of the various calcite types does not vary greatly. Carbon isotope values (5 C) range from -3.0 to -5.5%o pdb, whereas oxygen isotope values (5 0) range from -7.3 to -13.6%o PDB (Fig. 14). 5 C values for nodular, platy, rod-shaped concretions and type 2 tabular units are generally heavier than those of other types... 

Fig. 14. Plot of carbon and oxygen isotope values versus stratigraphic 35Q position. 5 C values exhibit...

The environment of precipitation has a direct effect on carbon and oxygen isotope values for vadose and... 

A generally positive covariation between 6 0 and S C is apparent across the data set as a whole (Fig. 10). However, covariations between 6 0 and 6 C and other parameters, e.g. carbonate content, trace element content or trace element ratios, are absent or very weak. Oxygen isotopic values for both early calcite and early dolomite, combined with temperature constraints inferred from IGVs, are consistent with precipitation from mostly 0-depleted fluids (Fig. 11). Ferroan dolomite and late... 

Cann, 1988). A hydrothermal origin for the dolomites can also be be ruled out. The oxygen isotopic values and lack of non-planar crystal fabrics indicate relatively low preeipitation temperatures (see below), and Carboniferous volcanics at Ballyeastle pre-date deposition of the sandstones. The nearest Permian volcanies reeorded are 70 km north of Ballycastle, between Islay and Jura (Fig. 1) (see Upton et al., 1987 Hitehen et al., 1995) and 60 km to the southeast, where >600 m of lavas are reeorded in the Lame borehole (McCann, 1990). It is generally accepted that there was little hydrothermal activity associated with the Tertiary igneous eentres of western Seotland (Hudson Andrews, 1987 Searl, 1994), and there is no evidenee that the situation in northeast Ireland was any different. 

Table 3. Carbon and oxygen isotope data of the Tirrawarra Sandstone siderite cements. A good match is observed between measured (6 0 5as) and calculated oxygen isotope values determined from image analysis results.

Fig. 15. Cross-plot of carbon and oxygen isotope values for the Tirrawarra Sandstone siderites. The bulk-rock isotope signatures are the cumulative product of the varying proportions of the different siderite cement generations (SI, S2, S3). The pure end-member compositions of the different siderite cement generations can be estimated from samples which are dominated by a single cement generation (circled areas). Only the end-member isotope compositions of the different cement generations should be taken into consideration when evaluating multigeneration siderite cements.

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