Paleoenvironmental reconstruction and carbon and sulfur isotopes perturbation in black shales of the Paleocene Bolander member in the N’kapa Formation, Douala Sub-basin Cameroon

Abstract

Black shales deposited across the Paleocene Bolander member of the N’kapa Formation are considered important source rock and shale gas reservoirs in the Douala sub-basin. However, no studies on organic carbon and sulfur isotopes perturbation, accumulation and enrichment of organic matter in black shales exist in the sub-basin. This paper unravels paleoenvironmental reconstruction, the perturbation of organic carbon and pyrite sulfur isotopes record, and mechanism of organic matter enrichment in black shales of the Kompina-Mian and Pendanboko-Kombe areas. Multiple geochemical techniques: organic carbon isotope (δ¹³C_org), pyrite sulfur (δ³⁴S_py), TOC, total sulfur abundance (TS), elemental geochemistry, mineralogy and SEM were used. Paleoclimate proxies indicates that the paleoclimatic condition was mildly-warmer and more humid. Bivariate plots and discriminant functions diagrams to delineate between active and passive tectonic settings such as DF1&2(Arc-Rift-Col)_M1, DF1&2(Arc-Rift-Col)_M2 and DF(A-P)_M revealed that the rocks were sourced from felsic rocks with a passive tectonic setting, with little contribution from active setting. Paleoweathering proxies revealed that the source area experienced moderate chemical weathering as a result of variation in climate and sea-level changes. The proxies for paleoredox and paleosalinity conditions revealed that the sediments were deposited under anoxic sulphidic benthic brackish-water with intermediate salinity. Paleoproductivity parameters like Ba-bio, δ³⁴S_py, δ¹³C_org, TOC, Mo/Al, Cu/Al and S/¹³C_org ratios revealed that climate, brackish-water and anoxia conditions were suitable for high primary productivity and preservation of organic matters. The depleted δ¹³C_org values and negative excursion along the black shale bed results from sulfate-reducing bacteria and chemoautotrophy-mediated carbon cycling under anoxia conditions. While depleted δ³⁴Spy contents are results from biological fractionation during microbial sulfate reduction (MSR), generating pyrite framboids in sediment under strong restrictive conditions with low oxygenation. The mechanism of organic matter accumulation and enrichment demonstrates that δ¹³C_org and δ³⁴Spy was anaerobically recycled by sulphate reducing bacteria and MSR processes, and the caping of benthic black shales by mudstones at shortened exposure duration, caused high primary productivity in anoxic waters. The data of this study demonstrate large climatic and oceanic anoxia fluctuations during the Paleocene influenced peleoweathering, paleosalinity, primary productivity, carbon and sulfur isotopes perturbation, and accumulation and enrichment of organic matter.