『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.
Keywords: Paleoweathering; Organic matter; Pyrite framboids; Black
shale; Biomarkers; Maleimides; PAHs』
1. Introduction
2. Materials and methods
2.1. Samples
2.2. Vitrinite reflectance
2.3. Total organic carbon
2.4. Pyrite framboid diameter analysis
2.5. Extraction and separation
2.6. GC-MS
2.7. GC-MS-MS
2.8. Quantification and identification
3. Results
3.1. General geochemical characteristics
3.2. Molecular composition
3.2.1. N-alkanes and isoprenoids
3.2.2. Steranes and hopanes
3.2.3. Isorenieratane and aryl isoprenoids
3.2.4. Polycyclic aromatic compounds
3.2.5. Maleimides
4. Discussion
4.1. Time of weathering
4.2. Molecular changes caused by paleoweathering
4.3. Paleoenvironmental and OM source implications
5. Conclusions
Acknowledgements
Appendix A. Supplementary materials
References