Mode carbon isotope composition

The carbon isotopic compositions of diagenetic carbonate minerals in Jurassic reservoir sandstones of the northern North Sea vary from 6 C -30 to +10%o, but show a very strong mode around -10%o. 

Deines P. (1992) Mantle carbon concentration, mode of occurrence, and isotopic composition. In Early Organic Evolution Implications for Mineral and Energy Resources (eds. M. Schidlowski, S. Golubic, M. M. Kimberley, D. M. McKirdy, and P. A. Trudinger). Springer, Berlin, pp. 133-146. 

To improve detection of isotope composition in samples with concentrations < 5 mg other techniques have been employed. For example, headspace solid-phase microextraction (HS-SPME) has determined 5 C [37,41] and 5 H [37] of MTBE in aqueous samples, reaching up to one order of magnitude lower detection limits (11 pg L ) for carbon mode [41]. For hydrogen isotope analysis, concentrations down to 1 mg L have been measured [37]. The small isotopic fractionations caused by these extraction techniques were evaluated by Zwank et al. [42] and were negligible and highly reproducible. 

The range in compositions is much narrower in Upper Jurassic submarine fan sandstones than in deltaic Middle Jurassic sandstones, although both have major modes at 6 C -10%o. This difference is due to the exposure of the Middle Jurassic deltaic sediments to a wider range of carbon sources and bacterial isotope-fractionation processes than the... 

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