Elemental carbon in sediments: Determination and isotopic analysis in the presence of kerogen

A procedure has been developed for separate measurement and isotopic analysis of kerogen and elemental C (soot, charcoal) in sedimentary rocks. The carbonaceous residue left after HF-HCl dissolution is oxidized with 0.10 MCr₂O₇⁼ in 2 MH₂SO₄ at 50°C. Kerogen reacts with a half-life of 6-180 hrs, depending on maturity, and thus can be quantitatively removed from elemental C, which reacts more slowly (t_{{1}/{2} ≈ 600-2000 hr}). The amount and isotopic composition of kerogen and elemental C are then obtained by measurement of etched and unetched samples, after applying a correction for the loss of elemental C in the etch.

The rate of the oxidation reaction has been studied in detail for carbon black and for carbonaceous residues from 2 K-T boundary clays, and less extensively for 12 samples of charcoal and kerogen. Most materials obey first-order kinetics to a fair approximation, but carbon black and K-T soot-both consisting of spherules-show a sudden, 4.0 or 1.7-fold increase in the rate after a mass loss of 23 or 82%, respectively. Apparently the interiors of soot spherules are more reactive than the exteriors.