Low-grade metamorphic black slates were examined for the impact of oxidative weathering after exposure to the atmosphere for well-constrained time periods. Measurements of organic carbon were carried out on roofing slates and alum slate mining waste, and were compared against data obtained from freshly mined material. The results show that the organic-carbon content of roofing slates decreased from initially 0.73–1.7 wt.% to 0.51, 0.42, and 0.50 wt.% after 90, 94, and 100 years, respectively. The organic-carbon content of alum slate mining waste decreased from > 7.5 wt.% at the base (5 m below surface) to < 1 wt.% at the top of a dump during an exposure time of 53 years. The thermal maturity of this black slate organic matter is of anthracite stage. The degradation of organic matter is controlled exclusively by its accessibility within the exposed rock. After intense weathering, different black slate types (alum slate, roof slate) show similar amounts (less than 1 wt.%) of a poorly accessible OM fraction.
The results demonstrate that oxidative weathering of organic matter in black slates is a fast process. A substantial decrease of organic matter occurs within only some decades to a century, rather than over geological time spans.