Effects of weathering on organic matter: I. Changes in molecular composition of extractable organic compounds caused by paleoweathering of a Lower Carboniferous (Tournaisian) marine black shale

Abstract

A detailed bulk and molecular study on paleoweathering of a Lower Carboniferous (Tournaisian) black shale from the Kowala quarry in the Holy Cross Mountains of Poland, revealed significant changes in total organic carbon (TOC), total sulfur (TS) and extract compositions. Paleoweathering resulted in a 97% decrease in TOC and total loss of sulfur, as well as changes in carbonate contents, extract yields and percentage yields of the organic fractions. Pyrite framboids, which are used extensively in paleoecological studies, decreased considerably in the partially weathered zone and totally vanished in the weathered zone. The decrease in TOC is accompanied by a pronounced reduction of organic compound concentrations, but the degradation range differs in the individual weathering zones. Here we show that less stable compounds such as low molecular weight aromatics (e.g. methylnaphthalenes, dibenzofuran, and dibenzothiophene), isorenieratane and its diagenetic products, or maleimides decrease significantly or disappear already in the partially weathered zone, while the more stable polycyclic aromatic hydrocarbons (PAHs) decrease (~ 90%) only in the weathered and highly weathered zones. Besides the organic matter (OM) content, the influence of paleoweathering on the distributions of organic compounds is important in the context of paleoenvironment, source and maturity interpretations. Almost all sterane and triterpane biomarker parameters change their values in the highly weathered zone, but some ratios, e.g. the 2-MeH index, are almost totally resistant to change. The aryl isoprenoid ratio (AIR) values decrease gradually with weathering. This modifies completely the potential interpretation of the nature of the photic zone anoxia. In addition to degradation of OM, some PAHs like benzo[ b]fluoranthene increase in concentration in the partially weathered zone due to their formation from phenyl-derivatives. The correct recognition of paleoweathering in outcrop and drill core samples aids in the proper interpretation of biomarker parameters and contributes to a better understanding of the processes which took place during weathering.