Isotope values, organic matter source

Table 9.2 Published ranges of isotope values of potential organic matter sources to estuaries.

Figure 17 Measured (points) and modeled (curves) soil carbonate values for three soils along an elevation (climate) gradient in the Mojave desert/Great Basin of California and Nevada. Modeled carbonate values based on Equation (14) plus the fractionation between CO2 and carbonate ( 10%o). The elevations (and modeled soil respiration rates that drive the curve Ht) are (a) 330 m (0.18 mmol C02m h ) (b) 1,550 m (0.4 mmol CO2 h ) and (c) 1,900 m (1.3 mmol CO2 m h ). The value of soil organic matter (CO2 source) was about — 21%o at all sites. Note that depth of atmospheric CO2 isotopic signal decreases with increasing elevation and biological CO2 production (Quade et al, 1989a) (reproduced by permission of Geological Society of America from...

Stable carbon isotope compositions of organic substances are measured via isotope ratio mass spectrometry. Samples are combusted to convert organic matter into gases (e.g. CO2, N2) that are then ionized and admitted into the isotope ratio-mass spectrometer (IR-MS) where the ratios of the isotopes are quantified (see Chapter 5 for further information). Owing to the low abrmdances of some of the isotopes, the precision and accuracy of the measurements is important (Merritt and Hayes, 1994). One of the most common and informative isotopes is because it is not readily altered by equilibration or degradation processes. Hence, values are most commonly employed as indicators of ultimate plant or geographic sources. In... 

Sr/ Sr ratios for parent waters indicate that calcite formed from fluids with isotopic compositions similar or close to present-day formation fluids. The elevated Sr/ Sr and 5 0 of calcite-forming waters resulted mainly from pervasive dissolution of detrital and diagenetic feldspar prior to calcitiza-tion, and may be in part related to the introduction of oil-accompanying basinal brines. Calcite 5 C values reflect carbon sourced by organic matter and possibly influenced by biodegradation-fermentation processes that affected petroleum emplaced in the reservoir. 

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